DM308 Part 1 Materials PDF

DM308 Production Techniques 2 Lecture 1 – Structural properties of materials Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Introduction Course outline Lecture 1 – Structural propert...

DM308 Production Techniques 2 Lecture 1 – Structural properties of materials Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Introduction Course outline Lecture 1 – Structural properties of materials Lecture 2 – Crystals, defects and interfaces Lecture 3 – Phases and microstructures Lecture 4 – Properties and processing of ceramics Lecture 5 – Properties and processing of glasses Lecture 6 – Properties and processing of light alloys Lecture 7 – Properties and processing of high-temperature alloys Lecture 8 - Properties and processing of composites Useful texts Useful books: 1. “Materials science and engineering” – Callister 2. “Engineering materials” – Ashby and Jones 3. “Materials: Engineering, Science, Processing and Design” – Ashby and Shercliff 4. “Introduction to dislocations” – Hull and Bacon 5. “Phase transformations in metals and alloys” – Porter and Easterling 6. “Light alloys” – Polmear 7. “Superalloys” – Reed 8. “Introduction to composite materials” – Clyne and Hull 9. “Composite Materials and Structures for Engineering Students” - Grove Course assessment 1 written exam (2 questions set by me) - if still social distancing – exam will be online and open book 2 pieces of coursework (one of which is set by me) The coursework format is TBC (likely to be essay or group presentation) The coursework submission is due by the end of Week 7 Coursework topic will not be in the exam (= less revision). Please use the MyPlace forum If you have any questions about the lecture content or assessment for my part of DM308: Do not email the lecturer directly Post your question to the Discussion Forum under Section 1 This will help fellow students who have a similar query Check the forum for existing answers to your questions For general module enquiries, please get in touch with the module registrar – Prof. Yi Qin Materials engineering recap. Materials in an ATM Which materials would you use to build the ATM components? Why would you chose them? FRONT BACK Illuminated sign LCD screen CPU Printer Network adapter Pin entry device Rear service LCD Card reader Cash dispenser Cash cassettes Cabinet What is materials engineering? Materials Science and Engineering is a hybrid discipline specialising in the development of new materials through a holistic understanding of their behaviour: 1. How structure of materials controls their properties 2. How processing determines the material structure 3. How properties determine performance in a given application Central to the discipline is the scientific study of materials using experimental and theoretical techniques, known as characterisation Why is it important? Make the impossible possible Prevent things from going wrong Failed solder joint Material families Metals and Alloys metallic bonding crystalline conducting mostly break in a ductile manner Ceramics ionic or strong covalent bonding crystalline insulating or semi-conducting break in a brittle manner Glasses Polymers covalent bonding non or partly-crystalline mostly insulating can be brittle or plastic covalent bonding non-crystalline insulating break in a brittle manner Composite materials Metal matrix composites Ceramic matrix composites ` MMCs Matrix is ductile Matrix is strong in tension Typically reinforced with ceramics Electrically conducting Polymer matrix composites PMCs Matrix is very tough Matrix can have low density Typically reinforced with ceramics CMCs Matrix is hard and wear resistant Matrix has thermal resistance Reinforced with ceramics/metals Natural composites Nature is smart Typically PMCs Parts of plants and animals The original engineering materials Material properties Engineers have less control over: Density Cost Scarcity Toxicity Engineers have more control over: Structural properties (we focus on these for this course) Functional properties Functional properties Dielectric constant Refractive index Resistivity Piezoelectric constant Magnetic permeability Ferromagnetism Ferroelectricity Piezoelectricity Thermal conductivity N.B. Functional properties will not be covered in this course. Structural properties Stiffness Strength Hardness Ductility Fracture Toughness Wear resistance Environmental Resistance Thermal expansion Thermal shock resistance Structural properties of materials Part 1 – Revisiting Hooke’s law Revisiting Hooke’s law Remember the spring constant experiment from school? The force F is proportional to the extension x, related by a proportionality constant k, which is a property of the spring. It would be more useful to derive a version of Hooke’s law that is independent of the spring shape and only depends on the spring material. See next slide. Stress and strain Stress F = applied force A0 = initial cross-section area Strain l = length l0 = initial length Tension and compression In tension, In compression, so the strain and is +ve is –ve By convention tensile stresses are +ve and compressive stresses are –ve Thus the modulus E is always positive Stress-strain testing Let us look at a typical uniaxial testing machine. These can either be hydraulically actuated or driven by a servo motor, like the one shown below. Column (houses electrical lead screw drive or hydraulic actuator) Crosshead (moves up and down) Grips (hold specimen in place and transmit force to it) Controller/Computer (controls test and logs data) Load cell (measures applied force) Material specimen (the thing you test) Measuring strain accurately We need to measure the material’s extension accurately to calculate the strain. Using the crosshead position is inaccurate due to slack and elasticity of the system. So the extension must be measured directly from the deforming specimen gauge. strain gauge crosshead Measuring strain electrically A circuit known as a Wheatstone bridge offers a possible solution. Strain gauge www.doitpoms.ac.uk Extensometer Measuring local strain optically Sometimes it is important to know how strain is localised in complex parts. Digital image correlation (DIC) is a contactless method for measuring strain. A computer is used to analyse a speckled pattern painted on the material surface. Achilles tendon Speckle pattern applied Strain mapping Luyckx et al., Journal of Experimental Orthopaedics, 2014, 1:7 Stress vs. strain in brittle materials Ideal brittle materials exhibit purely elastic behaviour before fracture. Elastic behaviour is when all of the strain is recovered when the material is unloaded. In the elastic regime there is a linear dependence between stress and strain. (i.e. Hooke’s Law) The proportionality constant, or elastic gradient, is known as the elastic modulus. (Young’s modulus). The stress at which the material fractures is known as the tensile, compressive or shear strength depending on the deformation mode. Stress vs. strain in ductile materials Consider a ductile material deformed in tension. Up to the elastic limit, the deformation is linear and reversible. (elastic) If stress is increased further, deformation is non-linear and permanent. (plastic) The transition stress between elastic and plastic behaviour is known as the yield strength. The maximum stress sustained by the material is the Ultimate Tensile Strength,. Real stress vs. strain curves The Young’s modulus and yield stress are not easy to determine. Typically one takes the overall gradient up to a certain stress to obtain E. Using this gradient, the intersection with graph at 0.2% strain is usually taken for. Elasticity and bond energy The elastic modulus is actually related to the strength of the bonds between the atoms/ions in a material. Below we see a binding energy curve between two atoms. (spring analogy) repulsion attraction The curve is a superposition of energies arising from the repulsive and attractive forces. These forces are related to the distance between the atoms. U0 is the bond energy and r0 is the equilibrium bond length. Shear In addition to tension and compression there is a third mode of deformation known as shear. Shear strain Shear stress F = applied traction force A = area subjected to traction Shear modulus Different stress scenarios simple tension simple compression biaxial tension pure shear hydrostatic pressure Ashby and Jones What are the stress states? 3D stress and strain We can define matrices to fully describe the stress-strain state in a material: 3D Hooke’s Law i.e. tensor summation notation This means that elasticity is a complex directional property. Finite element analysis The 3D description of Hooke’s law can be used in conjunction with Finite Element Analysis to investigate stresses within complex components and assemblies. Stress concentration in turbocharger impeller (red-yellow areas) Stresses in a pressure vessel (red ≥ yield stress) Structural properties of materials Part 2 – Beyond Hooke’s law Hardness Hardness is the ability of a material to resist scratching or indentation. In ductile materials the hardness correlates with the tensile yield strength. Thus a hardness test is a cheaper, less wasteful method to measure YS. Several tests exist, but the Vickers Hardness Test is the most widely used. Diamond indenter Diamond Pyramid Hardness indent in polycarbonate Specimen Vickers Harndess Number: ebatco.com Toughness Toughness = ability of a material to absorb energy during deformation. The energy per unit volume stored or dissipated by a material is given by the area under the stress vs. strain graph. Stored elastic component and in 3D But toughness is the total energy Measuring fracture toughness The Charpy impact test way is a method of comparing the fracture toughness of different materials. It relies on standardised test specimens with a machined notch, that are broken by the swing of a pendulum hammer. End of Swing Material Specimen Impact Hammer Anvil Specimen Start of Swing Specimen Scale Ductile vs brittle Charpy ductile ductile brittle brittle hevvypumps.com hevvypumps.com` Ductile to brittle transition Some materials may undergo a ductile-to-brittle transition as the temperature is changed. The transition temperature is abbreviated DBTT. It is important to take this into account during design and manufacturing stages, carefully considering all possible end use scenarios. brittle Case study: Titanic The low-grade iron rivets were used in the bow of the ship. The impurities and slags caused them to become brittle in the icy waters of the Atlantic. This led to the catastrophic hull tear during the iceberg collision and ultimately the loss of the ship and death of 1,517 people. Case study: Liberty ships The ships featured a welded (rather than riveted) hull to aid massproduction during WW2. Welds underwent a ductile-brittle transition at low temperatures causing ships to break apart mid-ocean. Cyclic loading In some cases a component may be subjected to cyclic loading. This may place additional demands on the material selected for manufacture. tension-compression tension-tension The material may fail even if cycled within the elastic limit. If this is not taken into account, the consequences can be disastrous. Case study: De Havilland Comet Stress concentration at window corners led to catastrophic crack propagation and loss of three aircraft. Case study: Aloha Airlines 234 1988 1978 Stress concentration at rivet holes exacerbated by high number of flight cycles caused fuselage panel to unzip mid-flight. Stress intensity factors Stress intensity factors K tell us how much the effective stress is increased at the tip of a crack or initiation site. They depend strongly on the following: 1. Crack opening mode (I, II or III) 2. Crack geometry 3. Specimen/material geometry A critical stress intensity factor (e.g. KIC) is the value at which the specimen is fully fractured due to the propagation of the crack. For Mode I, the stress intensity at an internal crack in an infinite plate is given by: (Y = correction factor) The critical strain energy release rate is a good measure of fracture toughness: Fatigue and (S-N curves) Fatigue is the damage done to a material by a cyclic load and can result in catastrophic failure, as we have seen on the previous slide. The S-N curve shows how the number of cycles to failure changes with the stress amplitude. Below a certain amplitude (fatigue limit) the material will have an infinite service life. HCF and LCF High cycle fatigue (HCF) 1. High frequency 2. Low stress amplitude 3. Elastic deformation 4. Large no. cycles to failure We can plot the log of strain amplitude vs. the log of no. of cycles to failure Nf Low cycle fatigue (LCF) 1. Low frequency 2. High stress amplitude 3. Some plasticity 4. Low no. cycles to failure There is no district boundary between HCF and LCF behavior, and the transition occurs depending on the ductility of the material. It is usually in the region of 103 cycles, but can this can vary substantially. Fatigue crack growth rate It is important to predict the rate at which a crack grows during load cycling. A crack that does not lead to fracture is known as a sub-critical. The Paris’ law model predicts that a sub critical crack growth rate exhibits a power law relationship with the stress intensity factor range. The constants C and m depend on the properties of the material, environment and stress ratio. Creep Creep is when a material loaded within its elastic limit exhibits plastic deformation over a long period of time. Creep becomes particularly problematic at higher temperatures. ε Rupture burst steam pipe failed turbine blades in a jet engine = const. = creep rate reep rate minimum c Primary tr atsb.gov.au bearinc.com I εr III II Secondary (Steady State) Tertiary log t Stages of creep Incubation period – no measurable deformation is observed as the defects needed for plastic deformation (called dislocations) to occur must first be generated in sufficient numbers. Not all materials exhibit an incubation stage. Primary creep – defect density continues to increase and an initially high rate of deformation is observed. The deformation rate slowly degreases as the dislocations defects entangle and slow down. Secondary creep – the rate of dislocation entanglement is the same as the rate of dislocation generation and a steady deformation rate is observed for a while. Tertiary creep – when the material reaches a certain level of plastic deformation new types defects (voids) start to form, grow and coalesce. The deformation rate begins to increase progressively. Failure – finally the material is unable to sustain more deformatio and breaks. Plastic deformation and time How much plasticity a material exhibits usually depends on the strain rate. Given enough time and sufficient force even brittle materials can flow. At the same time, very rapid deformation can limits the extent of plasticity. Geological flow Creep Lab test Very high strain rate 10-8-10-6 s-1 10-4-102 s-1 106 s-1 Materials and the environment One important aspect of materials design and selection is the need to consider the operating environment. (How is the material protected?) 1. Will the material react chemically with its environment ? (e.g. corrosion, oxidation, chemical attack, dissolution) 2. Will the temperature of the environment affect the material properties? (e.g. high temperature oxidation and corrosion, DBTT, glass-rubber transition, creep) 3. Will the material be affected by sudden changes in the environment? (e.g. thermal shock) Stress corrosion crack of titanium rocket fuel tank Oxidation of aero engine turbine blades Thermal shock failure of ceramics E.g. electronics are no exception Corrosion is a problem Thermal shock testing of electronics Gold plating to prevent oxidation of contacts Wrapping up Summary Materials properties are and important aspect of design Properties are categorised as: Structural or Functional You should now understand what the different structural properties mean for a material Structural properties define the operational limits of a material If the limits are exceeded the materials fail Fatigue properties are important if there’s cyclic loading Over time, creep may be an issue in ductile materials It is important to consider the operating environment when selecting materials (i.e. risk of oxidation, corrosion, thermal degradation) DM308 Production Techniques 2 Lecture 2 – Crystals, defects and interfaces Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] What is a crystal? A crystal is a combination of a lattice (a spatial grid of points) and a basis (object or objects that sit/s on those points). The two elements comprise a crystal structure. Basis Lattice Crystal + a The crystal is formed by placing atom(s) or molecule(s) (the basis) on each lattice point. The unique minimal spacings between the lattice points are known as the lattice parameters. Not all materials are crystalline Some materials are only partly crystalline (semi-crystalline) while others have completely no crystal structure at all and are amorphous. Amorphous e.g. glasses e.g. bulk metallic glasses Semi-crystalline e.g. polymers/elastomers such as polyethylene (Pd43Cu27Ni10P20) www.tf.uni-kiel.de/matwis www.chm.bris.ac.uk Polymer crystallites Organic (carbon-based) polymers can form semi-crystalline structures known as spherulites. This greatly increases the polymer density, opaqueness and strength. Semi-crystalline lamella Amorphous polymer between lamellae Spherulites in polyester and polylactic acid viewed under cross-polarised light Ivanov and Rosenthal, Polymer Crystallization II pp 95-126 polymerdatabase.com Unit cells Crystals are usually described using repeat elements that can be tessellated in space. These elements are known as unit cells. The smallest possible unit cell is known as a primitive unit cell. How many 3D lattices are there? There are 14 non-equivalent Bravais lattices that can fill the entirety of space when repeatedly translated. These are the simplest unique lattice structures from which more complex ones can be formed. They are described by their lattice parameters a, b and c as well as their unit cell axis angles a, b, and g. Example crystal structures Face-centred cubic e.g. Cu, Al, Ni, Au, g-Fe Rock salt e.g. NaCl, TiC, TiO Body-centred cubic e.g. Cr, Nb, Ta, W, a-Fe , b-Ti Hexagonal close-packed e.g. Zn, Co, Mg, a-Ti Diamond Perovskite e.g. diamond, Si, Ge e.g. BaTiO3, CaTiO3, SrTiO3 Close packed crystals When arranging spheres/circles in two dimensions we the closest packing is obtained with a hexagonal arrangement. We can then stack these hexagonal layers in two possible ways: Hexagonal close-packed (HCP) Stacking alternates every 2 layers (…ABABAB….) Face-centred cubic (FCC) Stacking alternates every 3 layers (…ABCABC….) Both arrangements give the highest possible packing fraction for a crystal of 74%. This means that only 26% is empty space. i.e. cubic unit cell https://chem.libretexts.org Crystallographic directions Directions in a crystal are indexed using the corresponding vectors using indices [uvw]. Square brackets are used to denote crystallographic directions. Crystallographic planes Planes are usually described by their normal vector. (i.e. one of the two directions that are perpendicular to the plane) To determine the indices (hkl) of a plane, we need to see where it intercepts each of the three coordinate axes and take the reciprocal of the values. Round brackets are used to denote planes. Crystallographic defects A crystallographic defect is a location in a crystal where the periodicity (or symmetry) is disrupted. The following types of defects are possible: Point defects (“0-dimensional”) Line defects (“1-dimensional”) Planar defects (“2-dimensional”) A crystal that has no defects is called perfect or ideal. Defects can play a significant role in determining the properties of a crystalline material, both functional and structural. The effect of defects can be both detrimental and beneficial to the properties of a material. Therefore, it is important to understand them and hence control their effect on the material properties. Point defects Defects featuring only atoms from the original crystal are called intrinsic, whilst defects from impurity atoms are called extrinsic. E.g. semiconductor doping Small proportion of dopant impurities can be used to control the electronic properties of semiconductors. P and N-type doping is used to increase the number of free charge carriers (electrons or holes) in these materials. pure silicon n-type p-type E.g. solid oxide fuel cells Solid oxide fuel cells are reactors in which a fuel (e.g. hydrogen or hydrocarbons) is oxidised to produce an electric current. The devices rely on ionic conductivity of a solid oxide ceramic, which is usually heated to around 1000 Celsius. At this temperature the oxygen vacancies in the ceramic lattice are very mobile and allow the ionisation and transport of oxygen from the air in the cathode. mobile oxygen ion + fuel = oxidised fuel + oxygen vacancy + 2 electrons oxygen atom + oxygen vacancy + 2 electrons = mobile oxygen ion Line defects - dislocations Dislocations are by far the most common type of line defect, which can be viewed as lines of extra or missing atoms. There are three possible types: edge, screw and mixed dislocations. A dislocation is described by its Burgers vector (b)– the displacement vector that is needed to close a circuit around the dislocation line. respectively. (Edge = b ┴ dislocation line , Screw = b || dislocation line) Dislocation glide When a shear stress is applied to the dislocation, it is able to move through the crystal by the breaking and re-forming of inter-atomic bonds. This phenomenon is known as dislocation glide and is responsible for the plastic deformation of crystalline material. N.B. Plastic deformation in crystals is also possible via other less-common mechanisms. t t Glide/slip plane Slip step The minimum stress required to initiate the glide of a dislocation is called the critical resolved shear stress (CRSS). It ultimately determines the yield strength of a material. E.g. dislocations in FCC crystals The most likely dislocations to form will have their Burgers vector aligned with one of the close-packed directions in a crystal. These dislocations will move along a glide plane in which the Burgers vector lies. In an FCC crystal there are 4 unique (non-equivalent) close-packed {111}-type planes. Each of these planes hosts three unique close-packed slip directions linking the lattice sites. These are of ½-type directions. Thus, there are at least twelve possible lattice dislocations that can form. As a result, FCC crystals are comparatively plastic due to this high number of slipsystems. Stress field at dislocation core Let us create an edge dislocation by inserting an extra half-plane of atoms. This compresses the crystal above the dislocation line. It also stretches the crystal below the dislocation line. This creates tensile and compressive stress fields around the dislocation core. These stress fields can interact with other stress fields in the crystal, such as those around other dislocations, solute atoms, interfaces, etc. Dislocation interactions The stress fields around dislocations can give rise to attractive and repulsive forces. Like Burgers vectors repel and opposite ones attract. When dislocations with opposing Burger’s vectors meet, they annihilate. +i.e. The net Burgers vector becomes zero. Dislocation sources A crystal may contain intrinsic dislocations, from when it formed. A dislocation segment in which the ends are immobilised (“pinned”) will act a source for multiple new dislocations via the Frank-Read mechanism. The segment bows out under applied stress until it forms a dislocation loop and a newly reformed segment. The cycle repeats generating more loops. Dislocation sources Electron microscope image of a Frank-Read source in silicon Computer simulation of a Frank-Read source pierrehirel.info Dislocation glide = plasticity The generation and multiplication of dislocations is responsible for the plastic deformation of a material. (i.e. dislocations are the carriers of plastic deformation) If a material is capable of sustaining dislocation glide, it will be ductile. The stress will continue to gradually rise after the yield of some materials. This is known as work hardening or strain hardening and occurs because the dislocations that are continually generated act as obstacles to one another. As their number increases with progressive deformation, so does the number of obstacles. v Elasticity = stretching of bonds by the applied force Plasticity = breaking and reforming of bonds by dislocation movement due to an applied force Strain hardening Work hardening can therefore be exploited to strengthen ductile materials. Unloading the material after yielding recovers the elastic strain, and the yield strength is increased to approximately the highest sustained plastic stress. However, this strengthening occurs at loss in the overall ductility. Yielding and plastic deformation increases the dislocation number density. The retained dislocations strengthen the material the next time it is loaded. Planar defects Interfaces between dissimilar crystals Regions with different crystal structures and chemical composition can co-exist in the same material. These regions with distict chemistry and crystal structure are known as phases. Three types of inter-phase interfaces are possible: Coherent The two lattices mesh perfectly, but the difference in lattice parameters results in a misfit strain. Semi-coherent The two lattices mesh well, but some dislocations are formed between coherent areas. Incoherent The two lattices do not mesh and the interface between them comprises a dense network of geometrically necessary dislocations. Solute strengthening If a dissolved impurity (i.e. solute) atom has a different size to the atoms of the host lattice, a stress field will be generated. This stress field will interact with the stress field around a dislocation and will inhibit its glide through the lattice. This is another example of beneficial point defects. The critical resolved shear stress due to a dissolved impurity is given by: shear strength of solute square of atom/particle radius solute concentration shear modulus of solute E.g. solute hardening in Al alloys Below we see an example of solution strengthening in aluminium. By alloying aluminium with 5 weight % magnesium, its strength is significantly improved relative to pure aluminium. Order strengthening A dislocation moving through and chemically ordered crystal may create an antiphase boundary. The creation of energetically unfavourable “wrong” bonds acts to oppose the motion of the dislocation. Thus effect acts to increase the strength since dislocations have to travel in pairs – one that creates the APB and one that removes it from the crystal. The increase in the CRSS to move the dislocation is given by: gAPB – is the APB energy per unit area b – is the magnitude (length) of the Burgers vector Precipitation hardening Some materials are processed in a such way that numerous fine particles (precipitates) with a stronger/harder crystal structure are formed within the prior parent crystal. Such particles can also act as obstacles to dislocation glide. Depending on the particle size and volume fraction, the dislocations must either cut through the particles or bypass them via the Orowan bowing mechanism. Either way, this increases the strength of the material. Precipitate cutting Bypass by Orowan bowing Precipitation hardening Precipitates sheared by dislocations in Al alloys Bowing and Orowan loops in a Ni-based alloy Polycrystalline materials Most manufactured materials are polycrystalline. (i.e. they consist of multiple crystals (or grains) which are mis-oriented relative to one another) This is not only because single crystals can be difficult to produce, but because polycrystalline materials are stronger. Grains in an aluminium alloy Grain boundaries in a bubble raft viewed under polarised light Crystal orientation map in a titanium alloy >Note the microstructural and volume changes during the production stages. Processing of technical ceramics Powder Material preparation Raw materials Pulverising Mixing Filtering Granulating Plasticising Spray drying Moulding /forming Dry pressing Injection moulding Extrusion Isostatic pressing Green machining Milling Turning Drilling Cutting Sintering Binder removal Sinter firing Finishing Grinding Lapping Polishing Honing Metallising Glazing Assembly Quality control Optical examination Dimensional check Crack detection Strength testing Technical ceramic production relies on the same principles as traditional ceramic production, but is usually more refined and involves a greater number of production stages with a greater degree of control. The quality of the raw material preparation is paramount. Processing of technical ceramics Coarse material preparation Jaw crusher Gyratory crusher Roller crusher Hammer mill Fine powder preparation Ball milling Roller milling Impact grinding Fine ceramic powder can be produced from crushed minerals using one of the above methods. The resulting powders can then be sieved into different size fractions. Ceramic additives There are four main types of additives that can be mixed in with the ceramic powder in order to adjust the properties of the raw: Binder – ensures the cohesion of the ceramic powder in the green parts. Dispersant – decreases the viscosity of the mixture and increases the volume concentration of ceramic particles. Plasticiser – modifies the flow behaviour of the binder and hence the raw ceramic mixture. Lubricant – minimises friction and adhesion between the mixture and the mixture parts. Sintering When ceramics are fired, the powder particles within undergo a process known as sintering, whereby the particles bond and coalesce over time, the porosity decreases and thus density of the ceramic increases. Stages of sintering During sintering necks begin to form between particles that are touching. At this stage the ceramic is still very porous. As sintering progresses the necks thicken to form defined grain boundaries while the network of pores gradually closes. Eventually a dense granular ceramic structure is obtained with some minor distribution of closed pores at the the prior particle boundaries Continuing the sintering process can further reduce this porosity and obtain a very high density ceramic product. Sintering mechanism The free surfaces of the powder particles have an interracial energy and sintering acts to minimise the overall surface area and hence overall surface energy. Material is transported from convex regions with high vapour pressure to concave neck regions with low vapour pressure where it condenses. Within the particles the opposing diffusion of atoms and vacancies helps close the pores. Plastic flow can also help transport material during sintering, especially if an external stress is applied during the firing of the ceramic. Ceramic forming processes Powder compaction: axial pressing, cold isostatic pressing, hot isostatic pressing Plastic forming: extrusion, injection moulding Casting: slip casting, tape casting, gelcasting, freeze casting, slurry dipping Coating: plasma spraying, physical vapour deposition, chemical vapour deposition Powder compaction Dry and semi-dry axial pressing Comparatively simple shapes can be made simply by pressing a dry or semi-dry ceramic powder to form the green product. This simply involves pressing it into a die or mould with sufficient pressure. Cold isostatic pressing (CIP) During isostatic pressing the ceramic powder is placed into flexible or deformable mould. The mould is then placed in a pressure chamber which is filled with an inert gas (usually argon) at a very high pressure. The very large nearhydrostatic pressure compresses the powder uniformly into a “green” preform. The possible geometries are limited with this process due to the flexible mould required, so additional machining may be necessary. Flexible elastomer moulds Hot isostatic pressing (HIP) During hot isostatic pressing the pressure chamber heated, allowing for simultaneous sintering. This allows for a very high degree of powder consolidation. However the mould material has to withstand the high temperatures and further limits the possible geometries. The requirement for high temperatures and pressures means that particular attention must be paid to ensuring safety. N.B. HIP-ing is a very good method for reducing the porosity of products produced using additive manufacturing techniques. Casting Slip casting In slip casting, a liquid ceramic mixture called “slip” is poured into a plaster mould. The plaster absorbs moisture from the adjacent slip making it more viscous than the original mixture. Pouring out the slip leaves behind the viscous layer which is allowed to dry further to a “leather dry” state. The green ceramic is then removed from the mould for subsequent trimming and firing. Slip casting allows mass production of ceramics with very complex relief. Tape casting Tape casting allows the production of thin ceramic tapes or sheets from a ceramic slurry. The slip flows from a hopper onto a carrier film and the thickness of the ceramic tape is adjusted by the height of the doctor blade. The tape is then dried and spooled up for further shaping and subsequent sintering. Gel-casting In gel-casting the ceramic powder is mixed with water and a monomer binder that undergoes a polymerisation reaction. The polymer forms a gel which binds the ceramic particles. This gel can be used to fill a variety of complex moulds. The gel is then dried into a solid and burned out prior to sintering. Gel-casting Gel-casting is suitable for highly advanced technical ceramic forming and allows a high degree of precision and complex geometries. Sol-gel processing Freeze casting Freeze casting uses a suspension of ceramic particles in water. The water is then frozen in a carefully controlled manner so that the ice grows as regularly spaced lamellae. The suspended ceramic particles settle between and around the ice lamellae. The ice is then allowed to sublimate at low pressure leaving a porous ceramic pre-form behind. The preform then undergoes sintering. Freeze casting allows the production of highly porous ceramics for filters, catalysts and biological/medical applications. Plastic forming Plastic forming Clays and other plastic ceramic mixtures can be extruded, injection moulded and jiggered to form a variety of geometric shapes. Jiggering Extruded ceramics Extrusion Injection moulded part Coating Physical vapour deposition Physical vapour deposition can be used to coat ceramics onto substrate materials. PVD is carried out in a vacuum, whereby the ceramic is heated so that it evaporates and is transported onto the substrate where it condenses. This allows for very thin coatings, several nanometres thin if desired. PVD can also achieve low porosity and a refined surface finish. Plasma sprtaying Plasma spraying can also be used to make ceramic coatings. The ceramic powder is fed through plasma created by passing an electric arc through an jet of high pressure argon. The particles melt in the plasma and the jet transports them onto the substrate. It is important that the substrate is able to withstand the high temperatures involved. The molten droplets form splats on the substrate surface and solidify. Plasma spraying is faster than PVD. but gives a much rougher and more porous coating. Properties of Ceramics Melting points Technical ceramics have comparatively high melting points which generally surpass most alloys and all polymers. Maximum use temperatures Technical ceramics offer the highest maximum use temperatures, due greater resistance to high-temperature oxidation/corrosion. Stiffness and thermal expansion Most technical ceramics have comparable or greater stiffness and lower thermal expansion than most metal alloys. Hardness and wear resistance Lower is better Most technical ceramics are considerably harder than metal alloys and polymers. They also offer excellent wear resistance. ← Higher is better Strength and fracture toughness Ceramic offer reasonable strength and fracture toughness. However, this often implies compressive loading and high-quality manufacturing. Density Technical ceramics have low density and as a result have the highest specific stiffness and highest specific strength. Note: CFRP is a ceramic-reinforced polymer. Thermal and electrical conductivity Ceramics are mostly electrical insulators. They have a high to moderate thermal conductivity. Properties summary Good: Good high temperature capability High hardness High stiffness Good strength Electrically insulating (some exception exist) Good fracture toughness High wear resistance Bad: Brittle failure mode Notch sensitive (surface defects can lower toughness and tensile/bending strength) Can be difficult to process (due to high temperatures and high hardness) Oxides Alumina - Al2O3 High hardness Moderate strength Electrical insulator Thermal insulator Bioinert Uses: Refractories and crucibles Abrasives and cutting tools Automotive spark plug insulators Bioceramics for orthopaedic implants Ballistic protection (armour plates) High temperature engineering components Zirconia - ZrO2 Moderate hardness High strength High toughness Thermal insulator Bioinert Uses: Crucibles and refractories Heat engine components Dies and punches Dental implants and prostheses Ceramic knives Thermal barrier coatings Zirconia phase transformations Zirconia can have one of three crystallographic structures depending on the temperature. The resulting volume changes can lead to processing problems when manufacturing precision components. It also complicates the use of zirconia at high temperatures. Stabilised zirconia Zirconia can be blended with other oxides to stabilise a particular crystallographic structure. Yttria (Y2O3) is often added to stabilise zirconia in its cubic form. This form is known as yttriastabilised zirconia or YSZ. It is essentially a ceramic alloy. Zr:O ratio in ZrO2 is 1:2 Y:O ratio in Y2O3 is 1:1.5 Adding yttria therefore creates oxygen vacancies in the YSZ. At high temperatures YSZ can conduct oxygen vacancies (=electric current). YSZ is thus used for fuel cells and oxygen sensors. (See lecture 2) Uranium dioxide (UO2) Most nuclear reactors use uranium dioxide as the fuel material rather than metallic uranium. The first reason is that UO2 has a higher melting point than metallic uranium. (i.e. safer to use) Secondly it does not burn since it is already oxidised. (i.e. safer to use) Uranium dioxide is compressed and sintered into ceramic pellets which are packed into neutron-transparent zirconium alloy tubes (fuel pins), which then form the larger fuel assemblies. Carbides Tungsten carbide - WC Very high hardness (2600HV) High melting point (2870°C) Electrically conductive Alloys with cobalt to tune properties Prone to high-temperature oxidation Uses: Cutting tools and dies for machining Mining and foundation drills Walking pole tips and tire inserts Balls for ballpoint pens Surgical equipment Jewellery Silicon carbide - SiC High-temperature strength Forms protective oxide (SiO2) at high temperatures Sublimates rather than melts (2700°C) Good wear resistance Semicoducting Uses: Abrasive and cutting tools Ballistic protection (armour plates) Foundry crucibles High performance brake discs Furnace heating elements LEDs, diodes, transistors Titanium carbide - TiC Extreme hardness (3200HV) High melting point (3160°C) Outstanding wear resistance Currently only used as coatings or cermet composites, i.e. no bulk form Uses: Hard surface coatings for tools Coatings for watch parts TiC Cermet composites E.g. rock crusher hammers on the left. Nitrides Boron nitride - BN Has multiple crystalline forms: cubic (c-BN), hexagonal (hBN) and others. c-BN has second-highest hardness after diamond (7750HV) Good heat conductor Uses: h-BN high-temperature/vacuum lubricants h-BN crucibles heatsinks/heat spreaders c-BN grinding wheels and abrasives Boron nitride nanotubes Boron nitride can be synthesised into nanotubes (BNNTs) with a similar structure to carbon nanotubes (CNT). Unlike CNTs, BNNTs do not burn in air. BNNTs also exhibit some interesting defect healing properties. Thus BNNTs open some interesting possibilities for novel composites. Silicon nitride - Si3N4 Si3N4 offers moderate properties at a moderate cost, which can often be the deciding factor for selecting it for a given application over other ceramics and/or metals. Hardness (1600HV) Decomposes above 1900ºC Uses: Bearings High-temperature structural parts ^(e.g. automobile/rocket parts) Bioceramics for orthopaedic implants (e.g. spinal fusion) Metalworking cutting tools (cheaper than WC) Titanium nitride - TiN High hardness (2100HV) Low friction properties Non-toxic / Biocompatible Nice gold colour Combinable with other nitrides Uses: Wear-resistant tool coatings ^(good edge retention and low wear) Surgical tool and bone pin coatings Motorcycle suspension forks Aesthetic coatings in plumbing Ceramic alloys Clays Clays have been used by people for millennia for buildings, pottery and objects of art. Clays are very fine-grained rock material that typically combine a range of minerals and some organic matter. A commonly occurring clay mineral is Kaolinite - Al2(OH)4Si2O5 though several others exist. Clays also contain some amounts of quartz (SiO2) and other metal oxides. (E.g. terracotta contains red iron oxide Fe2O3) When clay is fired, the clay minerals decompose into silica Al2O3 and alumina SiO2, which can form a range of alloys. (See phase diagram on next slide). Open pit clay mine Kaolinite Alumina-silica phase diagram Liquid Temperature (°C) L+Mullite L+SiO2 L+Al2O3 Liquid+Mullite Mullite+Al2O3 SiO2+Mullite Mullite Composition (wt.% Al2O3) 3Al2O3·2SiO2 Porcelain microstructure The variability of clay composition can produce a wide variety of microstructures after firing. Note the three mullite morphologies below. Mullite, porcelain and mullite ceramics Mullite is also known as porcelanite and is largely responsible for the properties of porcelain. Kaolinite clays form mullite when fired. If the processing and composition are right, the mullite will form a microstructure of interlocking needles, which imparts considerable strength to the porcelain. Mullite-containing ceramics don’t have to be made from clay, but can be produced using pure silica and alumina. Typical products include fire bricks and other refractories. Porcelain is also used as the outer layer on zirconia dental implants. AlON AlON ? AlON High hardness (2100HV) High melting point (2150ºC) Transparent when polished Polishing improves fracture toughness considerably Uses: Currently ALON ceramics are used predominantly in military applications such as transparent armour, military aircraft sensor windows and missile domes. Perhaps future uses can be found in the civil sector too? 1.6 inches of AlON offers more protection than 3.7 inches of glass/polymer laminate armour. This allows considerable weight savings. SiAlONs High hardness (1700HV) Maximum use temperature (1200ºC) High chemical stability and resistance to thermal shock Phosphorescent in UV light when doped Uses: Tubes and ladles for non-ferrous metal processing Cutting tools / lathe inserts Tube forming rollers Coloured phosphors Hydrocyclones for filtering out dust/sand from oil and gas Summary Ceramics are an important family of materials with some outstanding properties: specifically high hardness and good hightemperature properties. Processing of ceramics usually involves: powder preparation, forming/shaping, drying, firing/sintering and finishing. Ceramic forming processes are divided into: powder compaction, plastic forming, casting and coating. Ceramics can be used both in their pure form as well as used to make alloys. Typical applications include: refractories and crucibles, machining tools and dies, abrasives, high temperature structural components, biomedical implants. DM308 Production Techniques 2 Lecture 5 – Properties and processing of glasses Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Historic use of glass Glass occurs in nature within a number of minerals, including obsidian (volcanic), moldavite (meteor impact) and fused sand (lightning strike). Some of these can be knapped like flint and have been used by since prehistoric times by people to make tools and artefacts. First man made glass was produced as early as 3000BCE probably as a by product of metal-working (slags). It remained a luxury material until the its more widespread adoption in the Roman Empire. The middle ages saw more widespread the use of glass as a building material for stained glass widows in religious buildings and palaces. Full glass windows only became commonplace with the advancement of modern plate glass technologies. Silicon dioxide The building block of glass is silicon dioxide SiO2, also known as silica. Silicon can bond with 4 other atoms (tetravalent) and oxygen can bond with 2 other atoms (bivalent). As a stand-alone molecule SiO2 would have double covalent bonds between the silicon and oxygen atoms. These bonds are quite weak and molecular silica rarely exists in nature. Silicon dioxide tends to form larger structures. Silicon-oxygen tetrahedra Instead, silicon-oxygen tetrahedra are formed. Each silicon atom is bonded with four oxygen atoms and each oxygen atom is bonded with two silicon atoms. Thus there are no double Si=O bonds, only single Si-O bonds. There are many ways in which the tetrahedra can connect with one another. Quartz Silica can have a range of possible crystal structures. Their stability depends on the pressure and temperature. The most common crystalline form of silica is Quartz. It has a diamond-like structure. All the oxygen atoms act as bridges between silicon atoms. Thus, these oxygen atoms are termed “bridging oxygens”. Quartz There are two forms / polymorphs of quartz, which differ in the way that the silicon-oxygen tetrahedra are connected. ` a-quartz b-quartz (trigonal) (hexagonal) Quartz uses Quartz occurs naturally in many minerals including sand and rock crystal. Historically rock crystal has been carved to produce decorative art objects. Today quartz is value more for its piezoelectric properties. (straingenerated voltage and voltageinduced strain). It is used to make quartz resonators for timing electronic signals (inc. wathches, computers). It can also be used to make very sensitive balances – quartz crystal microbalances. Silica phase diagram Consider the phase diagram below for phase transitions in silica. Atmospheric pressure is 1 bar. (Close to 0 kbar) Before melting, a quartz undergoes several phase transitions. This limits the applicability of pure quartz as a hightemperature material. Alloying is usually required to stabilise a particular polymorph. (e.g. with Al2O3) Structure of Glass Glassy silica does not have the long-range order of quartz. There is no definable crystal structure of lattice+basis that obeys long range translational symmetry (tessellation). Glassy silica without additives is called Fused Silica. Glass Quartz Fused silica - Near zero thermal expansion - Exceptional resistance to thermal shock - Chemically inert - Good UV transparency - Low dielectric constant and dielectric loss Uses: High-temperature lamp envelopes Lenses and mirrors in variable temperature environments Raw material for optical communication fibres Refractories Glass transition Unlike crystalline silica, amorphous glass has no defined melting point. Instead, it gradually transitions from a stiff brittle glassy state to a soft rubbery state whereby it flows under applied stress. Specific volume Liquid Supercooled liquid Shrinkage due to freezing Glass Crystalline solid Tg Temperature Tm Modifying glass properties Fused silica has a high glass transition temperature (1600ºC) and high viscosity that make it difficult to work with. Additives are thus frequently introduced to alter the properties of glasses to facilitate their processing and to alter their properties. Network formers – are additives that help form the molecular network in the glass. Network modifiers – break up the molecular network of glass by creating more “non-bridging oxygen” atoms. Network formers and modifiers Silica-based glasses readily incorporate oxides of other elements as either network formers or network modifiers. - network former - network modifier - intermediate behaviour Soda-lime glass Approximately 90% of all manufactured glass is soda-lime glass. Consists of 70-75 wt% silica. Soda, Na2O, is added as a network modifier to lower the glass transition temperature. Addition of Na2O makes the glass soluble in water. Lime, CaO, is added to improve the chemical stability and makes the glass insoluble in water. The glass transition is ~570°C, allowing for convenient forming. Uses: construction, containers Borosilicate glass Soda-lime glass has poor thermal shock resistance and cracks if the temperature differential exceeds 37°C. Borosilicate glass can withstand thermal stresses of 165°C. Boric oxide, B2O3, is added as a network former. A smaller amount of Na2O is also added. The glass transition is ~820°C Low thermal expansion Max. use temperature ~500°C Uses: lab-ware, cookware, telescope mirrors, low-refraction optics, medical equipment, 3D printing platforms, lamp envelopes, nuclear waste immobilisation, solar heat collectors. Lead glass Lead glass is also know as “lead crystal” and is made with the addition of lead oxide PbO. PbO lowers the melt viscosity Easier to form when hot High refractive index and light dispersal Absorbs radiation Evidence of toxicity Poor thermal shock resistance Uses: glassware, jewellery, decorative arts, optics, radiation shielding (e.g. hospital x-ray booths, hot cells, electron microscopes) Bioglass Bioglass is a glass composition designed for medical applications that require repair of damaged bone tissue – i.e. manufacture of bioactive scaffolds. The formulation of bioglass is such that it is water soluble and chemically similar to hydroxyapatite – the main mineral in bone. Bioglass- 45% SiO2, 24.5% CaO, 24.5% Na2O, 6.0% P2O5 by weight. (Hydroxyapatite is Ca10(PO4)6(OH)2) Bioglasses dissolve in the body and their minerals are used in growth of new bone tissue. Glass ceramics Materials containing both crystalline and amorphous phases can be produced using controlled crystallisation. The crystalline phase has a negative thermal expansion coefficient. Controlling the phase proportions allows for materials with net zero thermal expansion. The also materials have very low thermal conductivity and can be made transparent to infra-red radiation. Glass ceramic systems: LAS – Li2O-Al2O3-nSiO2 MAS – MgO-Al2O3-nSiO2 ZAS – ZnO-Al2O3-nSiO2 Uses: cooktops, fire doors, fireplaces, cookware, high temperature applications Uranium glass and vitrification Glass made with uranium oxide was produced by glass-blower before the onset of the nuclear age and the resulting tight control of uranium. It was yellow-green in appearance and was mostly used in artistic glass-blowing. It also fluoresces under UV light. Today glass is used to immobilise highly radioactive nuclear waste. (Glass incorporates oxides and most nuclear fuel is in uranium oxide). This process is known as vitrification. A highly-stable water-insoluble formulation of glass is used and sealed in stainless steel containers. The waste is still highly-radioactive and very hazardous to humans for centuries. There are no long-term studies to confirm the safety. (Fingers crossed it works.) Float glass process The Pilkington float glass process is used to produce uniformly flat glass sheet. It relies on floating the molten glass on top of a bath of molten tin, which has higher density than the glass. Float glass process stages 1. Raw materials are weighed and loaded automatically into the furnace. Water is added to improve mixing and minimise airborne dust hazards. 2. (I) Raw materials are melted at 1600°C (II) Molten glass is homogenised and bubbles are removed (III) Glass melt is cooled to 1100°C to give correct viscosity for float bath. 3. Glass flows out over a smooth bath of molten tin leaving the bath at ~500°C. The tin bath is in a protective atmosphere of nitrogen and hydrogen. 4. Glass is annealed within a long cooling tunnel (Lehr) where it is gradually cooled to room temperature. This avoids residual stresses in the glass. 5. The glass ribbon is electronically inspected and cut into sheets. 6. The glass sheets are packed and loaded for offline coating and shipment. Fusion forming process Fusion forming is also known as “overflow and down-draw method”. Melting Powder Refining Flow control It is another process for producing flat glass sheet. It is another process for producing flat glass sheet. Forming Melt glass The molten glass overflows from a wedge shaped tank, flowing down its sides and fusing when the two sides meet. Since there is no contact with molten tin, the end product is flatter and its thickness is more uniform. Very thin glass sheet can be produced. Uses: electronic display panels Annealing Cutting Blow moulding Blow moulding is used to manufacture containers such as bottles and jars, as well as light-bulb envelopes. The glass is inflated using compressed gas inside a forming mould. Often, a two-step “blow and blow” process is used for production. The first step is used to make a blank from a molten glass gob, and the second step forms the blank into the final product. Glass fibre production Glass can be made into fibres that may be woven into yarns and fabrics. These find practical uses either as thermal insulation or in glass fibre reinforced polymer composites (GFRP). Nozzle drawing is a process that is widely used to manufacture glass fibres. Liquid glass is fed into a tank with a perforated bottom plate. The glass flows out of the perforations as filaments, which further thinned by gravity. Water is sprayed to cool the fibres and they are spooled up. A separate nozzle may spray the fibres with binders or other surface treatments. Glass wool production Glass wool production uses a similar principle to produce fibrous glass. The molten glass is fed into a fiberiser machine that rotates at high speed. The centrifugal forces cause the glass to scape radially through numerous holes in the spinner wall. A jet of hot air blows the escaping fibres downwards, where they are cooled with sprayed water. The fibres are also sprayed with a binder on their way down. The fibres land on the conveyor, which feeds them into a furnace to cure the binder. Slicing and packing follows. Optical communication fibre production 1 preform An optical communications fibre must have a core with a low refractive index that is clad by material with a high refractive index. A special glass tube is fixed in a lathe and rotated. Special reagent gases are fed through the rotating tube wile a burner traverses its entire length. The hot gases react to form a glassy soot on the inside of the tube. Once the inner wall of the tube is completely coated it is pulled apart to make a preform with a pointed end. Optical communication fibre production 2 Rod drawing is the process used to manufacture high quality optical communication fibres. A specially made preform is lowered into a furnace at 2000°C. A droplet called a “gob” is formed at the end of the preform. The gob drops away under gravity and thus drawing out a single fibre from the preform. The outer preform material forms the optical fibre cladding, and the silicon dioxide soot inside forms the core. Fibre dimensions are checked electronically and a protective coating is applied. Up to 300km of continuous fibre can be produced this way. preform Precision glass forming Precision glass forming allows the shaping of glass preforms into components without the need for grinding and polishing. It is used to produce optical lenses with complex profiles for digital cameras and other imaging instruments. It also has other applications for shaping custom glass parts for the transportation industry. A glass gob or sheet is placed into the dies/moulds and heated using infrared radiation. The travel of the moulding dies is then carefully controlled to produce glass products with a high-quality surface finish. Quality of die materials and their surface smoothness are also very important. Tolerances must be very high. Thermally toughened glass Glass can be toughened thermally by heating and rapid quenching. This creates a compressive stress at the surface and tensile stress in the centre. The surface compression acts to close any microcracks and lowers tensile stresses (hopefully to zero or below) which would otherwise propagate cracks. Thermally toughened glass The process can be used to create either thermally strengthened glass or tempered glass. Tempered glass has much higher surface compression and hence higher internal tension. When tempered glass breaks it shatters into many small pieces rathaer than large dangerous shards. Chemically toughened glass Glass can be toughened chemically by placing it in a molten salt bath. Exchange of ions takes place between the glass and the salt. For soda-lime glass, potassium ions can be used which are larger than the sodium ions. Potassium ions are larger than sodium ions. This creates compressive stresses at the surface, closing any microcracks and strengthens the glass. Summary Glasses are a versatile materials family that has a variety of uses: construction, optics, mass communications, containers and biomaterials. Glass is an amorphous form of silicon dioxide SiO2 (silica), which is built up of interconnected silicon-oxygen tetrahedra. Glasses do not have a melting point, but instead have a softening temperature known as the glass transition temperature (Tg). Below Tg they behave as brittle solids and immediately above Tg glasses behave like supercooled liquids and will flow under applied stress. The properties of glasses can be adjusted by changing the molecular structure using network formers and network modifiers. These additives are oxides of other chemical elements. Network modifiers break up the glass structure, increasing the number of non-bridging oxygens, resulting in a lower T g. Soda-lime glasses are the most widely used formulation and have a low Tg that facilitates processing. Glasses can be processed using a variety of methods: float glass process, fusion forming, blow-moulding, extrusion, rod-drawing, precision forming. Glasses can be toughened using surface treatments that create compressive stresses in their outer surfaces. DM308 Production Techniques 2 Lecture 6 – Aluminium alloys and their processing Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Property control in Al alloys Element no. 13 abundance Aluminium is the third most abundant element in the Earth’s crust. Aluminium alloys are the second most widely used alloy system today. Oxygen 46.60% Silicon 27.72% Aluminium 8.13% Iron 5.00% Calcium 3.63% Sodium 2.83% Potassium 2.59% Magnesium 2.09% Data: USGS Report 1953 Energy consumption to process Al Importance of recycling Given the high cost of aluminium reduction, large scale recycling is attractive from a commercial standpoint. The cost of recycling aluminium is only ≈5% of the cost of extracting aluminium from ore. Recycled aluminium forms 1/3 of the global Al consumption and 1/2 of Al consumption in the European Union. Demand for recycled aluminium exceeds supply. Uses of aluminium alloys Electrical systems Transportation White goods Machinery Construction Food packaging What can we alloy Al with? Most metals can alloy with aluminium. Not many have sufficient solid solubility in Al to be major alloying additions. Element Temperature (ºC) Zinc Maximum solid solubility (wt.%) (at.%) 443 82.8 66.4 Silver 566 55.6 23.8 Magnesium 450 17.4 18.5 Lithium 600 4.2 16.3 Germanium 424 7.2 2.7 Copper 548 5.65 2.40 Silicon 577 1.65 1.59 Manganese 658 1.82 0.90 Titanium 665 1.3 0.74 Chromium 661 0.77 0.40 Solid solution strengthening of Al Annealed high purity Al has a very low yield strength of 7-11MPa. Manganese and Copper have a strong solid solution hardening effect, but start to precipitate intermetallic phases above 0.2-0.3%. Magnesium is the best solid solution strengthening element on a relative weight basis, with high solid solubility. (175MPa yield with 6% Mg) Zinc gives little solution strengthening, despite high solubility in Al. Work hardening and annealing Al alloys fall into two groups: 1). Mechanical properties are controlled by work hardening 2). Mechanical properties are controlled by annealing e.g. cold rolled alloy N.B. White blobs show nucleation sites of recrystallized grains. deformation/transition band shear band e.g. cold worked alloy with intermetallic particles Work hardening of Al Deformation structures in strain hardened Alloy 6060 - AlMgSi0.5 Work hardening characteristics Introducing strain by coldforming (such as coldrolling) can substantially increase the strength of an aluminium alloy. Here we see increases in both in the UTS (top) and the 0.2% yield stress (middle) as a function of the amount of strain introduced by cold rolling (bottom). Temperature and work hardening Cold working temperature has a very pronounced effect on the resulting strength. At greater temperatures the rate of dynamic recovery processes is higher. This means dislocations are either annihilated or rearranged to form a cellular structure. Cryogenic deformation slows dynamic recovery allowing a greater increase in strength at the expense of ductility. Data shown for alloy 1100 Alloying and work-hardening Alloying can also be used to improve strain hardening. Here the solution hardening effect of Mg is also exploited to slow down the rate of dynamic recovery compared to pure Al. If a cellular structure is desired in Al-5%Mg the alloy would have to be worked at a higher temperature. E.g. hot rolling. Annealing of Al alloys The pre deformation properties can be partially or fully restored by annealing at elevated temperature. Below: annealing behaviour of commercially pure Al. Age hardening 1. Solution heat treatment at high temperature in single phase region. 2. Rapid quenching to obtain a supersaturated solid solution (SSSS). 3. Controlled decomposition of SSSS to form finely dispersed precipitates by heat treatment in the multiphase region at intermediate temperatures 1 3 2 Precipitation in the Al-Cu system GP Zones in Al-2.5%Cu SSSS decomposition occurs in stages, via formation of metastable intermediate precipitates. Initially small clusters of atoms may form and act as nuclei for Guinier-Preston (GP) zones. GP zones are ordered solute-rich groups of atoms 1or 2 atom planes in thickness. They are coherent with the matrix, but usually have a coherency strain (lattice misfit). Precipitation in the Al-Cu system Eventually much larger intermediate precipitates form that have a more defined chemical composition. Such precipitates are usually only partly coherent with the matrix. Precipitates can nucleate from or at the site of a GP zone. However heterogeneous nucleation can also take place at lattice defects such as dislocations. Al2Cu Maximum hardening requires an optimum dispersion of precipitates. (i.e. size + volume fraction) Precipitate-free zones No PFZ PFZ PFZ in Al-4Zn-3Mg Al-4Zn-3Mg + 0.3% silver All precipitation hardened alloys can form precipitate depleted zones adjacent to the grain boundaries. Solute diffusion is faster at the grain boundaries and GBs are also good heterogeneous nucleation sites for precipitates. Solute atoms are therefore leached from the matrix surrounding the grain boundaries as they diffuse to form precipitates there. PFZ are thus devoid of strengthening precipitates and act as stress concentrators. The width of the PFZ can be controlled through heat treatment and alloying. Combining ageing and deformation Al-5.3Cu-1.3Li-0.4Mg-0.4Ag-0.16Zr without (left) and with 6% cold work (right) prior to the ageing heat treatment. The microstructure produced during ageing can be controlled by employing cold-work in conjunction with the heat treatment. This allows a coarser but more uniform precipitate distribution. Corrosion of Al Aluminium is a very reactive metal, but it is quite resistant to corrosion because it is covered in a protective oxide film. However, when in contact with an electrolyte and a less reactive metal it is highly susceptible to galvanic corrosion. The aluminium acts as a sacrificial anode and can corrode quite rapidly. Certain alloying additions can adversely affect the corrosion resistance. Al-Cu alloys have poor corrosion resistance since copper is less reactive than aluminium. (Cu has the worst effect of all alloying elements.) Single-phase solid solutions are the most corrosion-resistant form of alloys. They are less susceptible to localised attack (pitting) than multi-phase alloys. Anodising Aluminium can be further protected from corrosion by anodising. Anodising increases the thickness of the protective oxide film. The aluminium is used as an anode in an acid electrolyte bath. Application of a direct electric current breaks down the water in the electrolyte down releasing oxygen. The oxygen reacts with the anode surface to form Al a porous oxide. The very regular pore structure readily accepts dyes. Wrought Al alloys Wrought aluminium products Sheet/Plate Foil Wire Tube/Rods 75-80% of aluminium is used for wrought products These products are produced from cast ingots Complex Extrusions The structures of the ingots are altered significantly by various mechanical working processes and heat treatments. Vertical direct chill (DC) casting Vertical DC casting is the most commonly used process for ingot production Molten alloy is poured into one or more fixed water cooled moulds with a retractable base. Solidification is a two stage process: (1) solidification of alloy adjacent to chilled mould wall, (2) solidification of the remainder during submould heat removal by spray cooling. Vertical DC casting is a semicontinuous process capable of producing ingots up to 15 tonnes in mass. Problems with DC casting The problem with DC casting methods is the rippled surface finish they produce. This occurs due to stick-slip contact as they move past the sides of the mould when solidification first occurs This can lead to surface tears and microstructural inhomogeneities such as inverse segregation in the surface regions. The surfaces of DC ingots therefore have to be machined or scalped prior to rolling or extrusion, which adds to the cost. Improving DC ingot quality The surface quality can be improved by reducing the rate of heat transfer from solidifying ingot to the mould. One method is to direct high pressure air along the metal/mould interface. Alternatively an electromagnetic casting mould where a water-cooled induction coil repels the liquid metal from the mould. Thus metal solidification only occurs when the metal is leaving the mould to be spray cooled. This not only improves the surface finish dramatically, but also refines the grain structure of the ingot. Eliminating the need for scalping. A smaller grain structure is a result of: (1) fast cooling provided by direct spray cooling, (2) convection currents induced in the liquid metal by the electromagnetic field. Horizontal DC casting Horizontal DC has a number of advantages over vertical methods: All machines are on the same level – lower capital investment costs Better conjunction with other manufacturing processes (e.g. raw material feeding, rolling, heat treatment, product storage) More convenient operation for the personnel Graphite moulds: low wettability, high CTE, high thermal conductivity, high thermal shock resistance, self-lubricating, easily machinable Horizontal DC casting + Travelling mould casting Single-roll caster Twin-roll caster Twin-belt caster Travelling moulds have revolutionized casting of lower strength Al alloys. Mould is “endless” – i.e. zero relative movement between the mould and casting surfaces. Castings have a surface with a low defect density. This allows casting of thin rod, slab and sheet. The castings may be coiled and/or subjected to further processing (e.g. rolling) without the need for surface machining. Travelling mould casters Principal alloy families Al Mn Al-Mn Al-Mg-Mn Si Al-Mg Non age hardening alloys Zn Al-Zn-Cu-Mg Cu Mg Al-Zn-Mg Al-Mg-Si Age hardening alloys Al-Cu-Mg Alloy nomenclature e.g. Al 7075 T6 Temper designations 4 digit designation Major alloying elements 1xxx 99.00% Al min. F – as fabricated 2xxx Cu O – annealed wrought products 3xxx Mn 4xxx Si 5xxx Mg 6xxx Mg,Si 7xxx Zn 8xxx Others 1. First digit defines alloy group 2. Second digit indicates modifications to the alloy 3. Last two digits identify the aluminium alloy or indicate the aluminium purity Suffix letter 1st suffix digit H – cold worked (strain hardened) 1 – cold worked only 2 – cold worked & partially annealed 3 – cold worked & stabilised T – heat treated (stable) 1 – partial solution + natural ageing 2 – annealed cast only 3 – solution + cold work 4 – solution + natural ageing 5 – artificial ageing only 6 – solution + artificial ageing 7 – solution + stabilising 8 – solution + cold work + artificial ageing 9 – solution + artificial ageing + cold work 2nd suffix digit 2 – ¼ hard 4 – ½ hard 6 – ¾ hard 8 – hard 9 – extra hard Non heat-treatable alloys The NHT family encompasses wrought compositions that do not respond to strengthening by heat treatment. They comprise of three chief sub-groups: pure aluminium grades, alloys with Mn or Mg as the main alloying element. Approximately 95% or rolled aluminium products are made from these alloys belong to the above groups. Major uses of NHT alloy sheet are: packaging, construction and transportation Selection criteria: (1) structural capability, (2) formability and (3) surface quality. 1xxx series Electrical conductors Lithographic plates This group includes super-purity (SP) aluminium (99.99%) It also includes commercial-purity (CP) grades CP grades contain up to (1%) impurities or alloying additions Tensile properties are low (e.g. 7-11MPa proof stress in SP aluminium) Chief uses: electrical conductors (best electrical conductivity) Other uses: chemical process equipment, foil, decorative architectural products and lithographic plates (i.e. good surface finish and no preferential chemical attack on secondary phases) 3xxx series - Al-Mn and Al-Mn-Mg 3003 – foil, cooking utensils, roofing sheet 3004 – beverage cans Moderate strength (3003 - 110 MPa, 3004 – 180MPa) High ductility Excellent corrosion resistance Mn forms fine Al6Mn particles that contribute to strengthening Mg added as a solid solution strengthener 5xxx series - Al-Mg Pressure vessels Boat hulls Liquid transport tanks 5xxx alloys comprise the bulk of all Al sheet products 0.8-5% Mg can be used - good solid solution strengthening A range of strengths (e.g. 40MPa yield in 5005 vs. 500MPa in 5456 H19) The softer alloys usually exhibit good ductility (up to 25% elongation) Good corrosion resistance Uses: welded applications (liquid transport tanks, pressure vessels, boat hulls, armour) Heat treatable alloys  phase precipitates in Al-4wt%Cu Cu containing precipitates in Al-Mg-Si-Cu These are wrought alloys that respond to strengthening by heat treatment. They rely on precipitation of secondary and tertiary intermetallic phases in the alpha solid solution matrix phase. There are three series: 1. 2xxx (Al-Cu and Al-Cu-Mg) 2. 6xxx (Al-Si-Mg) 3. 7xxx (Al-Zn-Mg and Al-Zn-Mg-Cu) The alloys can also be divided into two groups based on properties: 1. Medium strength weldable (Al-Mg-Si and Al-Zn-Mg) 2. High strength with limited weldability (Al-Cu, Al-Cu-Mg and Al-Zn-Mg-Cu) 2xxx series - Al-Cu and Al-Cu-Mg Al-Cu-Mg alloys like Duralumin (Al-3.5Cu-0.5Mg-0,5Mn) have been widely used by the aircraft industry since the era of the Zeppelin airships, where they were used for the rigid airframe. Their use quickly spread to aeroplanes. Today Duralumin has been superseded by newer derivative systems like 2014 and 2024 with 0.2% proof stresses as high as 320 and 490MPa respectively. However, a close derivative of Duralumin, 2017, is still used in aircraft rivets. 2xxx alloys are typically produced as sheet roll clad with Al-1Zn for corrosion resistance In addition to high strength, 2xxx also possess good creep resistance for Al alloys. This led to development alloys like 2618 (Al-2.2Cu-1.5Mg-1Ni-1Fe) for supersonic aircraft. 6xxx series - Al-Mg-Si Medium strength Al-Mg-Si alloys are used for the majority of Al extrusions Good weldability Corrosion resistance Immunity to stress corrosion cracking The strength is proportional to Mg and Si content Precipitation processes in Al-Mg-Si alloys are recognised as the most complex. Clustering of silicon atoms can occur before GP zone formation. Precipitates that nucleate on these clusters tend to be coarser which adversely affects strength. Therefore, it is important to control the clustering process by alloying. 7xxx series - Al-Zn-Mg & Al-Zn-Mg-Cu The Al-Zn-Mg alloys have the greatest potential for age hardening, allowing a wide variety of thermo-mechanical processing routines to optimise the properties. Alloys based on this system can have a UTS as high as 600MPa (7075-T6, 7178-T6). They have the highest strength to weight ratio of all Al alloys. While the alloys are very strong, they tend to be readily susceptible to stress corrosion cracking (SCC). Some aerospace manufacturers reverted to heavier Al-Cu-Mg system. Copper is added to reduce the propensity for SCC. However, alloys without Cu, have a much better weldability. SCC resistance is improved by careful processing and microalloying. Major uses: Transportation industry Superplastic alloys Processing route to obtain very fine grained 7475 Superplastically formed car tailgate panel Hot superplastic forming in action Superplasticity is the ability of a material to accommodate abnormally large extensions (as great as 1000%) without necking or fracturing. An alloy has to satisfy two requirements: (1) consist of two stable phases in roughly equal proportions (e.g. eutectoid), (2) have a very fine grain size (1-2m). Some aluminium systems exhibit superplasticity: 5085 SPF (Al-4.5Mg-0.7Mn-0.15Cr), Al6Cu-0.5Zr, but have limited applications. Current emphasis is on developing superplastic behaviour in high-strength 7xxx series alloys via thermo-mechanical processing (e.g. 7475). The alloy can accommodate elongations in excess of 500%, but forming strain rates are very low, the forming temperature must be suitably high, and the alloy is expensive as a consequence. Cast Al alloys Anodising Engine Block Transmission Casing Cookware The ratio of cast aluminium to wrought is constantly increasing This is mainly due to more castings used in automobile industry 70% of all Al castings in Western Europe are for automotive In 2004 the Cast:Wrought ratio in USA automotive use was 1:2 Castings divided into “primary” prepared from new metals and “secondary” which use recycled stock and have more impurities Most common systems are: Al-Si, Al-Si-Mg and Al-Si-Cu When to use Al castings? Advantages Low density Low melting point Low gas solubility (except H) Better fluidity than wrought compositions Good surface finish Allow control of grain size Allow artificial ageing Complex product shapes (in fewer steps) Superior creep properties to wrought alloys Disadvantages Inferior mechanical properties to wrought alloys Low ductility Susceptible to casting defects High hydrogen solubility High shrinkage (difficult to control geometry) Importance of Al stock Aluminium is supplied to foundries in the following stock forms: en.wikipedia.org Dross from Al smelting/casting. www.atherm.com 1. Primary metal ingots from a smelter supplied as 99.50-99.85% purity Al or pre-alloyed. 2. Secondary metal ingots from recycled aluminium alloy products with composition adjusted according to needs 3. Molten metal delivered directly to foundry from an external smelter facility. 4. In-house scrap returns. Increasing the use of secondary Al increases the loss of metal to dross – a mass of solid impurities/metal oxides floating on or dispersed in the melt. Transportation of molten aluminium by road. However, due to low cost of secondary aluminium, foundries may implement de-gassing and filtering of the melts prior to casting. What defines castability? The castability of an alloy depends on a combination of factors: Fluidity Mould filling ability Volume shrinkage Succeptibility to hot tearing Porosity forming characteristics Surface quality Dross forming characteristics The melt temperature will affect some of these factors and is therefore an important variable to consider. In addition to alloy castability, it is also important to consider the castability of a component’s geometric design. E.g. Vc/Vb where Vc and Vb are volume of component and volume of smallest box it fits in. (Smaller ratio is easier to cast.) Fluidity Fluidity is usually measured using a spiral mould and measuring the length run out by the molten metal. Fluidity depends strongly on the composition since it affects: viscosity, surface tension, freezing range and solidification mode. Impurities in Al reduce fluidity, by widening the freezing range, forming intermetallics, and changing planar front solidification to mushy modes. Fluidity is greatest at eutectic compositions. Left: Fluidity of a metal matrix composites based on alloy LM6 containing different fractions of silicon carbide particles. The carbide particulates act to decrease the fluidity. Precipitation of intermetallic inclusions during solidification has similar effects. American Journal of Materials Science 2012, 2(3): 53-61 DOI: 10.5923/j.materials.20120203.04 Volumetric shrinkage Total volumetric shrinkage is strongly dependent on composition. Each phase present has its own density characteristics. Pure aluminium exhibits 6% shrinkage whereas hypereutectic Al-Si exhibits 1-2%. Shrinkage may only be manifested as an evident contraction in the solid. It can also result in a number of defects: surface defects such as pipes (a), large isolated voids a.k.a. gross porosity (b), interconnected porosity, such as centerline (c) and interdendritic microporosity (d). Porosity Shrinkage micropore in alloy 319 https://www.nde-ed.org Radiogram showing entrapped gas porosity. Most casting processes have some associated porosity. Turbulent flow can result in entrapment of gases, forming bubbles that freeze as pores. Entrapped gases can also be a result of moisture, dissolved hydrogen or combustible lubricants. Evacuating the mould can overcome entrapped gas porosity problems. Porosity also arises from volume shrinkage. Moulds are designed to concentrate porosity in risers or to redistribute it uniformly as less harmful micropores. Hot tearing Hot tearing occurs when the stresses generated within a casting as it solidifies are too great. It is evident as tearing or cracking in the casting. The phenomenon occurs at locations where the shrinking casting is physically restrained. Alloys with wide freezing ranges are more susceptible. Alloys with low volume fraction of eutectics are also more susceptible since the eutectic composition liquid can fill and repair hot-tears as they form. Al-Si based alloys have the greatest resistance. Al-Cu and Al-Mg systems are most prone. Die soldering Die soldering is a particular problem in die casting of Al alloys. The molten Al alloys can react with steel dies causing the casting to solder to the die surface. Iron and molten aluminum have a high affinity for one another. Rapid interatomic diffusion can result in formation of aluminide intermetallics at the die surface, which facilitate bonding. This makes automatic ejection from the die impossible and castings must be removed manually, causing delays. To minimise likelihood of soldering: (1) add iron to the alloy (2) use lowest possible melt injection temperature (3) use die surface coatings (e.g. boron nitride) Relative castability Below are the relative castabilities of Al alloys ranked according to their main alloying additions: Best Worst 3xx > 4xx > 5xx > 2xx > 7xx Si+Cu/Mg Si Mg Cu Zn Nomenclature (USA) >99.00% Al 1xx.x Cu 2xx.x Si with added Cu or Mg 3xx.x Si 4xx.x Mg 5xx.x Zn 7xx.x Sn 8xx.x Other 9xx.x Unused series 6xx.x The first digit identifies major alloying element or alloy group. In 1xx.x alloys the next two digits indicate minimum Al percentage. E.g. 150.x is 99.5% Al. In 2xx.x to 9xx.x the 2nd two digits only serve to identify individual alloys in the group The last digit to the right of the decimal point indicates the product form: 0 = casting and 1 = ingot. The system used to denote temper is the same as that for wrought alloys. Al-Si alloys Alloys with Si as the major addition are the most important casting alloys. Si addition substantially increases the “fluid life” of the melt. This is because Si has a much higher heat of fusion than Al, (1810 vs. 395kJ/kg). Thus, the significant latent heat keeps the metal molten for longer. Other advantages: Corrosion resistance Good weldability Reduced CTE Reduced shrinkage Disadvantages: Difficult to machine due to hard Si particles Uses of binary alloys: castings where strength is not a prime consideration: cookware, pump casings, manifolds. Al-Si alloys microstructure The eutectic composition is 12.6 wt.% Si. Hypoeutectic alloys are more commonly used. “Fluid life” is better in hypereutectic alloys. Al-9.8wt.%Si Al-19wt.%Si Slow solidification (e.g. sand casting) produces a very coarse microstructure comprising large silicon plates in an Al solid solution matrix. Rapid cooling (e.g. permanent mould casting) produces a more refined microstructure. The silicon adopts a fibrous morphology, improving ductility and strength. Alloy modification can also be used to refine the eutectic. Modification of Al-Si Light Alloys – I. Polmear A357 (a) Unmodified, (b) 300 ppm Sr, (c) 2400 ppm Sr. Light Alloys – I. Polmear Condition UTS (MPa) Elongation (%) Hardness (Rockwell B) Sand cast 125 2 50 Modified sand cast 195 13 58 Chill cast 195 3.5 63 Modified chill cast 220 8 Mechanical properties of Al-13%Si alloys. 72 Addition of very small amounts of alkali metals, such as Na or Sr, is used to refine the eutectic microstructure. Mechanical properties are thus improved. It is likely that modification alters the nucleation and growth of silicon during solidification. Excessive modification does revert the microstructure to a coarser morphology. Intricate castings possible with Al-Si The high fluidity of Al-Si alloys allows very intricate thin-walled castings to be produced. Al-Mg-Si alloys Microstructure of Al-12.7Si-0.7Mg Arrows show  phase in alloy 357 together with smaller eutectic Si particles. Some  particles have cracked. Small additions of Mg induce a significant age hardening effect. This is due to precipitation of disperse intermetallic phases (e.g. Mg2Si). 356 (Al-7Si-0.3Mg) and 357 (Al-7Si-0.5Mg) are examples of alloys widely used for sand and permanent mould castings. The T6 temper of these alloys have yield strengths that are double those than the equivalent alloys without magnesium. Higher Mg contents tend to lead to formation of undesirable phases such as  (Al9FeMg3Si5) which are brittle and crack preferentially. Uses of Al-Mg-Si Light Alloys – I. Polmear Precision cast fighter aircraft pylon. Due to their mechanical strength alloys such as 356 and 357 are precision cast to produce premium castings for the aerospace industry. The Al-Si-Mg alloys also have excellent corrosion resistance, due to a low galvanic potential between the elements. Fatigue performance, however, is insensitive to ageing treatment since crack nucleation preferentially occurs at casting defects. Al-Si-Cu alloys More complex compositions have been developed for specific applications. A332 (Al-12Si-1Cu-1Mg-2Ni) – has improved high-temperature strength arising from Ni-containing intermetallic precipitates. A390 (Al-17Si-4Cu-0.55Mg) is a wear-resistant alloy used to produce allaluminium automotive cylinder blocks. This has allowed to move away from cast iron cylinder blocks. A lightweight engine has been designed by BMW using an A390 cylinder block core in a magnesium alloy jacket that is sand cast around it. BMW N52 engine block. Other castable alloys Al-Cu Al-Mg Al-Zn Age hardenable Corrosion resistant Age hardenable Prone to hot tearing Machinable Natural ageing Not age hardenable Largely obsolete Troublesome casting Silver additions have allowed production the strongest possible castings using premium techniques UTS of 550MPa and elongations of 10%. Expensive Ag used only in military A20X aerospace casting E.g. hydrogen evolution during sand casting Prone to SCC Used in sacrificial anodes Poor casting characteristics High eutectic melting point allows brazing of castings. Sand casting Sand casting is a low-cost method for producing aluminium castings. A solid pattern is encased in sand mixed with a special binder. Removing the pattern creates the cavity into which molten metal is cast. When the molten metal has solidified, the compacted sand is removed to release the finished casting. The method does not allow for complex casting geometry. The final surface finish of the castings is also comparatively rough and depends on the coarseness of the sand. Permanent mould casting Also known as gravity die casting, this method employs a re-usable mould for higher-volume production of castings. Metal fills the mould under the force of gravity, which limits geometric complexity of castings. A finer surface finish is possible with this method. Pressure die casting To improve the molten metal’s die-filling ability, it can be fed into the die under pressure. This is usually achieved using a piston or a screw to feed the molten metal into the die cavity. Using cooled metal dies and mechanical ejectors allows for high-volume production output. The pressure exerted on the alloy allows more complex moulds to be filled. The metal die allows for a smooth finish in the castings. Squeeze casting Improvements in mechanical performance of 7010 subjected to squeeze casting. Squeeze casting allows us to overcome problems associated with shrinkage during solidification. Pressures around 200MPa are applied to the melt. Flow of melt into incipient shrinkage pores is facilitated. Entrapped gases are forced to remain in solution. Direct squeeze casting = a moving die is used to exert pressure. Indirect squeeze casting = a piston is used to exert pressure. Allows casting of more complex alloys traditionally limited to wrought processing. Can be used for MMC manufacture by adding fibres or particulates. Semi-solid processing Cast dendritic structure. The casting is usually a near-finished product and cannot be mechanically worked. Grain refiners are an option but can lower the mechanical properties. One option is to break up the dendrites during solidification creating more grain nuclei in the melt. This is semi-solid processing. Fluidity persists with up to 60% solid. Cast globular structure. How does one refine the grain size of an aluminum casting? Semi-solid slurries are thixotropic – i.e. their viscosity decreases with stirring. Commercialisation is still at an early stage. Semi-solid processing Two established methods exist: rheocasting and thixoforming. Thixoforming requires specialised billets to be produced which is expensive. Rheocasting alows the use of existing alloys. They slugs are heated electromagnetically to a mushy Liquid+Solid state before high-pressure die casting. Summary Aluminium alloys are the second most used alloy system after steels. Aluminium is the most abundant metal in the earth’s crust, but requires considerable energy to extract from the ore. Recycling is thus very important. The alloys have a comparatively low melting point and are thus relatively easy to process which, contributes to their widespread use. They are quite a representative metallic system and provide a good introduction to the processing of engineering alloys. Aluminium alloys show a good combination of physical properties. Particularly, they have a comparatively high strength-to-weight ratio. The mechanical properties are either controlled by deformation (work-hardening and grain refinement) or by ageing (precipitation of strengthening phases). Age-hardenable aluminium alloys are typically stronger than strain-hardenable aluminium alloys but are less weldable. There are two families of aluminium alloys: wrought and cast. Wrought alloys start as castings, but are made into their final products using plastic deformation. Cast aluminium alloys are cast into the final product geometry. The castability of an alloy refelcts its suitability for cast applications and is a combination of a number of factors: viscosity, fluidity, shrinkage... DM308 Production Techniques 2 Lecture 7 – Metallurgical processing of high-temperature superalloys Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] High-temperature superalloys Jet engine operation Suck→ Squeeze→ Bang→ Blow Combustion Chamber Exhaust Nozzle Compressor mVaircraft mVjet Shaft Turbine Faster, hotter, cheaper A 200°C rise in turbine entry temperature (TET) gives a 5% saving in fuel burn. A 1% decrease in fuel burn for a Boeing 777 reduces operating costs by about £1,000,000 over a 15 year period. If the weight of an aero engine can be reduced by 50kg this gives a 0.5% saving in fuel burn. Weight ↓ Temperature ↑ Fuel Efficiency ↑ Cost ↓ Life ↑ Evolution of turbine entry temperature 1728K (1455°C) – Melting point of Ni There is a persisting drive to increase the turbine entry temperature (TET) of aero engines. This is the temperature of the gas entering the first stage of the high-pressure (HP) turbine as it leaves the combustion chamber. The Carnot cycle dictates that increasing in the TET would increase the thermal efficiency of the engine, which is the ratio of the useful work to the work of combustion. The TET of any engine is always going to be limited by the hightemperature capability of the materials used to manufacture the turbine components. While work is always ongoing to develop new materials that can withstand greater operating temperatures, other technologies have allowed the TET to exceed the melting points of the superalloys that are presently used. Intricate networks of cooling channels are machined into the interior of HP turbine blades, ensuring that their temperature is within the limits of the material. The HP turbine components are also protected by ceramic thermal barrier coatings (TBCs). Materials in a jet engine Below we see the typical distribution of materials in a modern engine: Nickel base superalloys Superalloy components The major superalloy components and key material requirements: Turbine Blades Turbine Discs Creep resistance Oxidation resistance Castability Coatability Guide Vanes High strength @ temp. Creep resistance Fatigue resistance Oxidation resistance Corrosion resistance Processability Combustors Thermal stress resistance Oxidation resistance Castability Coatability LCF fatigue resistance Oxidation resistance Weldability Ni-Al binary system The Ni-Al binary system is the basis of nickel base superalloys:  ’  Microstructure of superalloys A typical superalloy microstructure comprises a g phase matrix populated by numerous g’ precipitates, which provide strengthening (precipitation hardening).  ’  ’ Crystal structure of g and g’ phases The g and g’ phases both have a face-centred cubic crystal structure, but g’ has an ordered structure. This gives rise to order strengthening due to the formation of antiphase boundary defects when dislocations glide though the g’ precipitates during plastic deformation. g Al Al Ni Al Al Ni Ni Ni Ni Al Al Ni Al ≈ 8-22 at.% Al in Ni chemically disordered Al Ni3Al chemically ordered Properties of the g’ phase The special properties of the g’ phase constitute a major contribution to the high-temperature strength of superalloys: 1. Ductile compared to most intermetallics. 2. Strength increases with temperature (Yield Stress Anomaly). 3. Precipitates from the  phase via short-range ordering. 4. Has a low lattice parameter mismatch with the parent  phase and forms coherent interfaces with it.  ’ Strengthening in superalloys Superalloys exhibit a combination of beneficial strengthening mechanisms that result in outstanding high-temperature strength: Solution strengthening Order strengthening Strength of Superalloys Precipitate strengthening Coherency strengthening PX vs SX superalloys Superalloy components generally fall into two categories singlecrystals (SX) and polycrystalline (PX): Turbine Blades and Guide Vanes Small Cast to near net shape Single crystals High volume fraction of ’ Expensive (due to element choice) Turbine Discs and Combustors Large Wrought into shape Polycrystalline (PX) Smaller volume fraction of ’ Less expensive Structure of a turbine blade Single crystal turbine blades have complex geometries with elaborate networks of internal cooling channels and holes. On the outside, thermal barrier coatings act as heat shields protecting the base metal from the extreme temperatures. Production of such sophisticated components requires a combination of state-of-the-art manufacturing techniques. Cooling air Cooling air Single pass Multi-pass Thermal Barrier Coating Investment casting Investment casting using the “lost wax” process is used to manufacture the turbine blades with their intricate inner structure. It allows complex geometries to be cast and can produce a fine surface finish. Investment casting of turbine blades A wax model of the blade known as a template is first prepared. Inside the template is a ceramic core that has the shape of the cooling channels. Multiple templates are joined using wax runners and dipped into ceramic slurry. The wax is burned out in a furnace to leave behind a fired ceramic mould. Molten alloy is then poured into the mould and allowed to solidify. The ceramic ores are removed using chemical or vibrational methods. Wax template/model Finished investment mould ceramic core Finished castings Investment casting of turbine blades Cutaway of investment mould showing intricate ceramic core that will form the cooling channels. Automated coating of wax template assembly in ceramic slurry. Why cast single crystal blades? Lost wax investment casting is only part of blade manufacture. Conventional casting only produces polycrystalline blades with fine and uniform grains. At high-temperatures the grains can slide during creep. The creep life can be improved significantly using directional solidification which produces oblong columnar grains and reducing overall grain boundary area. Eliminating grain boundaries altogether in single crystal castings gives the longest creep life. Why cast single crystal blades? Below we see a grain structure comparison between conventionally cast, directionally solidified and single crystal turbine blade castings. The directional solidification process used to produce the latter two is shown on the following slide. conventionally cast directionally solidified single crystal Directional solidification Water Melt chamber separator valve Crucible and charge Melting chamber Water cooled induction melting coil Vacuum Graphite heating element Furnace chamber Mould Mould withdrawal chamber Water cooled chill and ram Mould chamber separator valve Water Growing single crystals The high thermal gradient created by the directional solidification process forces the liquid to solidify as tree-like structures called dendrites. Dendrites aligned parallel to the solidification direction grow faster than the rest. As they grow, they branch out to form secondary and tertiary arms/branches which block the growth of slower misaligned dendrites. Single crystal A spiral grain selector helps accelerate the elimination process by continually changing the growth direction. Eventually only the dendrites of a single crystal orientation should remain. Grain selector Poly crystal Possible casting defects Though directional solidification is a state-of-the-art process, it is still not perfect and a number of casting defects may be possible that would result in a blade being scrapped: Secondary Grains Freckles Low-angle Grain Boundaries Recrystallised Grains Ceramic Core ”Kissout” Slivers Bicrystal (from starter) Scrap rates Below we see that up to a third of all blade castings have to be scrapped and recycled due to the presence of casting defects: Second generation alloys (up to 3wt.% Re) Third generation alloys (up to 6wt.% Re) Heat treatment of turbine blades The blades are then subjected to a series of heat treatments to obtain the optimum microstructure. Solution treatment dissolves all g’ precipitates and homogenises the composition. Ageing heat treatments are used to grow the desired distribution of g’ precipitates to obtain optimum creep srength. Superalloy CMSX-4 Temperature Solution heat treatment ≈ 1320°C / 16 hours Primary ageing treatment 1080°C / 4 hours Secondary ageing treatment 870°C / 16 hours Time Post-casting operations The heat-treated blades still have to undergo a number of further finishing and quality control procedures: 1 Machine blade to shape and perform NDT 2 Apply bond coat 3 Deposit TBC 4 Laser drill effusion (cooling) holes 5 Heat treat to optimise microstructure 6 Re-Perform NDT Neutron radiograph of turbine blade showing retained fragment of ceramic core. Superalloys in harsh environments Some superalloys operate in very harsh environments that may accelerate high-temperature oxidation and corrosion. Therefore, they must be adequately protected to prevent premature failure. Component failure Below are examples of turbine blades that failed due to hightemperature oxidation and/or corrosion. Coatings can be used to help protect the blades from these undesirable degradation phenomena. Forming protective oxides Oxide-metal volume ratio is one way of judging whether an oxide will be protective. Alumina and chromia are good candidates because oxygen diffusivity in these oxides is low. Chromia volatilises at higher temperatures, but provides good corrosion resistance. Types of turbine blade coatings Thermal barrier coatings Overlay coatings Diffusion coatings Diffusion coatings Al-rich b phase Interdiffusion zone Al-rich d phase Overlay coatings Type I MCrAlX Ni or Co Reactive rare earth metal Y, Zr, Hf Type II NiCrAl Type III Plasma spraying Powder Cathode P