Solidification Fundamentals

Solidification Fundamentals

• Casting is a manufacturing process for creating near-net shape components by solidifying molten material within a mold
• It is one of the oldest manufacturing techniques, tracing back to approximately 4000 BC
• Casting remains relevant for creating complex shapes or facilitating high-volume production
• Approximately 97% of metal products use at least one solidification process during manufacturing
• Solidification involves a phase transition from liquid to solid
• This process requires overcoming an energy barrier, which means metals do not freeze precisely at their theoretical freezing point
• Undercooling involves cooling a liquid below its equilibrium freezing point
• It is essential for overcoming the energy barrier during solidification
• Significant undercooling is required for homogeneous nucleation which rarely happens outside controlled lab conditions
• Homogeneous nucleation involves the formation of a solid phase from a pure liquid without external influence
• Critical undercooling is needed to overcome the high energy barrier to make a stable solid nucleus within the liquid
• Heterogeneous nucleation involves forming a solid phase on a pre-existing surface
• Examples of pre-existing surfaces include impurities or the mold wall
• Less undercooling is needed as the existing surface reduces the energy barrier for nucleation
• Most real-world solidification involves heterogeneous nucleation
• Nucleation refers to the initial formation of a stable solid phase within a liquid, referred to as a "seed"
• This is a vital step in solidification because it provides a site for further crystal growth
• Crystal growth is how atoms attach to the existing solid nucleus in a repeating pattern, forming a crystal structure
• The rate of crystal growth depends on factors like temperature gradients and impurities
• A unit cell denotes the smallest repeating unit of a crystal structure
• Simple cubic, body-centered cubic, and face-centered cubic structures are examples of unit cells
• The arrangement of atoms within determines many of the material's properties
• Grain structure involves the arrangement of crystals (grains) in a solidified material
• Grain size and shape (morphology) affect the mechanical properties of the casting significantly
• Columnar grains are long and aligned
• Equiaxed grains are roughly equal in all dimensions
• Equiaxed grains are generally preferred due to superior mechanical properties
• Grain refiners are additives to promote heterogeneous nucleation and refine grain structure
• These additives provide numerous nucleation sites, leading to a finer and more uniform grain structure

Sand Casting

• Sand casting is a casting process using molds made of sand
• Sand molds are inexpensive for low-volume production though they offer a poor surface finish
• They are unsuitable for high-precision castings
• A pattern is a replica of the final casting used to create the mold cavity
• Patterns are typically made of wood, metal, or plastic
• Cope and drag constitute the two halves of a sand mold, the cope on top and the drag on the bottom
• The parting line is the surface separating the cope and drag
• A core denotes a sand insert for internal cavities or features in the casting
• Sprue, runners, and gates refer to channels to direct molten metal into the mold cavity
• A riser is a reservoir of molten metal that feeds the casting during solidification, compensating for shrinkage
• Risers are crucial for preventing shrinkage porosity
• Shakeout is the process of removing the casting from the sand mold
• Sand casting offers low capital investment and the feasibility of realizing complex shapes or large components
• Sand casting features high unit costs, poor surface finish, and it cannot make thin sections

Die Casting

• Die casting uses a reusable mold
• It enables high-volume production of high-precision castings
• There is significant capital investment for the dies
• A die is a reusable metal mold used in die casting, typically made of hardened steel
• High-Pressure Die Casting (HPDC) injects molten metal into the die using significant pressure
• HPDC enables production of thin-walled components with high dimensional accuracy
• There is potential for gas mixing into liquid metal
• Pressure is typically 15-30 MPa and cycle times are short (200-300/hr)
• This is restricted to low melting point alloys to prevent die wear
• Low-Pressure Die Casting (LPDC) uses low pressure to fill the die
• LPDC produces castings with fewer defects compared to HPDC but is not able to produce thin parts
• This is due to risk of freezing before the mold fills
• Hot chamber die casting involves molten metal held in a chamber within the machine
• Cold chamber die casting involves molten metal is ladled into the machine's injection chamber
• Die casting offers very low unit cost, high definition and surface finish, and excellent dimensional accuracy
• Cold metal molds give fast solidification for a fine grain structure
• Die casting involves large capital investment, difficult microstructure control, and it is limited to low melting point alloys
• It cannot be used for complex shapes or large castings, and alloys must have a lower melting point than the mold

Investment Casting (Lost-Wax Casting)

• Investment casting utilizes a wax pattern to create a ceramic mold
• Investment casting can produce intricate parts with excellent surface finish from high-melting-point metals
• It is more expensive than sand casting since the mold is non-reusable
• A wax pattern defines a pattern made of wax used to create the mold cavity
• A ceramic shell represents the mold created around the wax pattern
• The mold is coated with fine ceramic slurry, repeated until the shell has developed desired wall thickness
• De-waxing defines a process of removing the wax pattern from the ceramic mold
• It occurs by heating the mold to 90 ~ 175°C
• Harden mould involves firing the mould at 650 ~ 1050°C to dry the mould and bond the ceramic particles
• Investment casting allows high-melting-point metals to be cast and complex shapes are possible
• Investment casting enables good surface finishes
• Investment casting is expensive, as the mold is not reusable, and time-consuming

Defects in Castings

• Process induced defects within castings
• These defects impact part quality
• Shrinkage porosity defines voids formed due to volume reduction during solidification, as solid metals typically have lower volumes than their liquid states
• Gas porosity defines voids formed due to trapped gases in molten metal
• Hot tears are cracks due to tensile stresses during solidification
• These stresses arise from shrinkage of the metal while constrained by the mold
• Inclusions denote foreign materials trapped in the casting
• Examples of inclusions are foreign materials like oxides, slag, or sand
• Misruns denote incomplete filling of the mold cavity
• Cold shut denotes a discontinuity that forms when two streams of molten metal fail to fuse properly

Factors Affecting Fluidity

• Fluidity determines the ability of molten metal to flow and fill a mold cavity, being essential for casting
• Viscosity: Higher viscosity reduces fluidity
• Surface Tension: Higher surface tension reduces fluidity
• Freezing Range: Shorter freezing ranges promote higher fluidity
• Inclusion Content: Inclusions increase viscosity and reduce fluidity
• Mold Materials: Higher thermal conductivity of the mold reduces fluidity
• Mold Design: Sprue, runner, and riser design affects fluidity
• Superheating: Higher superheating increases fluidity
• Pouring Rate: Higher pouring rates increase fluidity
• Heat Transfer: Higher heat transfer reduces fluidity

Comparison of Casting Processes

Casting Facts

• Casting: A near-net shape manufacturing process using the solidification of molten material
• Solidification needs energy to overcome an energy barrier, causing undercooling
• Homogeneous nucleation requires significant undercooling, rare in practice
• Heterogeneous nucleation happens on pre-existing surfaces, more common
• Nucleation marks stable solid nucleus formation, starting crystal growth
• Grain structure (columnar vs. equiaxed) significantly changes mechanical properties
• Grain refiners promote heterogeneous nucleation, creating smaller grain structures
• Sand casting utilizes sand molds which is low cost plus poor surface finish
• Die casting employs reusable metal molds, so high-volume production of precise parts
• Investment casting utilizes wax patterns plus ceramic molds, creating intricate parts with excellent surface finish
• Common casting defects involve shrinkage porosity, gas porosity, hot tears, and inclusions
• Fluidity is the capacity of molten metal to flow into a mold cavity
• Viscosity, surface tension, freezing range, and inclusions influence fluidity
• Superheating plus pouring rate affects fluidity
• Risers supply auxiliary molten metal that compensates for shrinkage while solidifying
• Casting process choice depends on production volume, part complexity, surface finish, plus material properties
• Understanding nucleation plus grain structure is essential for managing the mechanical properties of castings
• Proper mold design is crucial for defect prevention and ensuring castings are successful
• Diverse casting processes feature individual benefits with drawbacks regarding expenditure, output speed, complexity, plus exterior quality

Engineering Metals - Atomic Structure and Bonding in Metals

• Metallic bonding determines an electrostatic attraction across positively charged metal ions
• There is also a "sea" of delocalized electrons unlike covalent or ionic bonds that localize electrons between specific atoms
• Characteristics of metallic bonding are high electrical and thermal conductivity, and malleability
• Electrical and thermal characteristics are both due to mobile electrons
• Malleability is due to ability to deform plastically
• Advanced concepts include free electron model and band theory to describe electron activity
• Crystal Structures are a regular, repeating three-dimensional arrangement of atoms in a crystalline solid
• The smallest repeating unit is called the unit cell
• Body-Centered Cubic (BCC) refers to atoms at the corners plus one at the cube's center
• Examples include iron, chromium, and tungsten, under certain temperatures
• BCC structures typically display low ductility plus high strength compared to FCC structures
• Face-Centered Cubic (FCC) refers to atoms occupying the corners and the center of each face
• Examples include aluminum, copper, and nickel
• FCC structures commonly display lower strength plus higher ductility compared to BCC structures
• Hexagonal Close-Packed (HCP) is a complex configuration with atoms within a hexagonal arrangement
• Examples include titanium, magnesium, and zinc. HCP structures exhibit anisotropy
• Anisotropy is different properties across varying directions
• Advanced concepts cover crystallographic directions and planes, namely, Miller indices, together with point group symmetry, and space groups
• Precise description of crystal orientations and defects requires all of these concepts

Engineering Metals - Crystal Defects and Strengthening Mechanisms

• Point defects denote imperfections involving a few atoms like vacancies plus interstitials
• Vacancies are missing atoms
• Interstitials are extra atoms in the lattice
• Vacancies raise diffusion rates, while interstitials trigger lattice distortion, impacting mechanics
• Formation energy of point defects plus concentration impacts material properties
• Line defects (dislocations) represent one-dimensional defects, commonly edge dislocations plus screw dislocations
• Edge dislocations are extra half-plane of atoms
• Screw dislocations are spiral ramp of atoms
• Dislocations prove pivotal for plastic deformation, as their movement permits planes of atoms to slide past each other when stressed
• Higher presence of dislocations facilitates easier material deformation when there is stress
• Grain boundaries denote interfaces across crystals featuring differing orientations
• Grain boundaries serve as barriers for dislocation motion, leading to augmented strength with lower ductility
• Smaller grain size corresponds with additional grain boundaries, contributing to greater strength, a phenomenon described in the Hall-Petch relationship
• Strengthening mechanisms are methodologies employed to augment a metal's strength by impeding dislocation mobility
• Decreasing grain size increases the grain boundaries, impeding dislocation movement
• Plastic deformation introduces enhanced dislocations that intertwine together impeding motion
• Alloying ingredients added generate lattice distortions, also hindering dislocation motion
• Creating small, scattered precipitates serves against dislocation mobility
• This entails solution treatment, quenching plus aging

Alloys

• Phase diagrams represent equilibrium phases across alloy systems through both temperature and composition
• The iron-carbon diagram is essential for understanding steel heat treatment
• Examples of phases are austenite, ferrite and cementite
• The diagram plots stability intervals along with the transformations that occur when heated or cooled
• Heat Treatments: Processes involving heating and cooling to alter the microstructure and properties of steel
• Common heat treatments include annealing, normalizing, quenching, and tempering
• Annealing means heating for high internal stress relief, plus ductility improvement through decreasing the cooling rate
• Normalizing involves air cooling after heating to increase grain size for increased strength
• Quenching involves high cooling for transforming austenite into martensite (a tough, brittle phase)
• Tempering serves martensite that’s been quenched, heating it later to reduced brittleness though retaining some strength
• Cast irons are Iron-carbon alloys that feature a carbon content surpassing 2%
• Graphite or cementite form impacts properties
• Gray Cast Iron, which features graphite flakes, exhibiting brittleness though presenting elevated damping abilities
• Ductile Cast Iron refers to spherical graphite, exhibiting heightened strength plus ductility compared to gray cast iron
• White Cast Iron refers to pure cementite, thus featuring great hardness yet brittle property
• Aluminum alloys show elevated strength-to-weight ratios coupled with resistance to corrosion and sufficient machinability
• Alloying elements comprise copper, magnesium, silicon, plus zinc
• In aluminum alloys, heat treatment through precipitation hardening proves possible
• Magnesium alloys denote structural metal which stands out for lowest weight, though present worse strength with poorer resistance to corrosion than the aluminum
• Titanium alloys exhibit exceptional strength-to-weight proportions, elevated corrosion resistance, matched by biocompatibility
• These alloys show expensive qualities though face machining challenges

Computer-Aided Manufacturing (CAM)

• CAM software translates design data into instructions for manufacturing equipment
• CNC machines are an example of this form of equipment
• CAM advances precision augmented through productivity and automation within production
• Specialized application software and corresponding expertise plus initial investment costs define CAM

Metal Properties

Metals Facts

• Metallic bonding: Electrostatic adhesion across positive ions, increased conductivity
• BCC: Typically elevated strength reducing ductility
• FCC: Heightened ductility decreasing strength
• HCP: Commonly anisotropic behaviour
• Point imperfections: Vacancies, interstitials, affect diffusion, properties
• Dislocations: Crucial for plastic deformation
• Grain boundaries: Hinder dislocation motion, increase strength
• Hall-Petch relationship: Smaller grain size leads to higher strength
• Work hardening: Introducing dislocations increases strength, reduces ductility
• Alloying agents: Reduce dislocation motion
• Precipitation hardening: Precipitates hinder dislocation motion
• Iron-carbon diagram: Reveals (austenite, ferrite, cementite) according to temperature plus composition
• Heat treatments: Alter microstructure and properties by annealing, normalizing, quenching, tempering
• Martensite: Formed by quenching austenite
• Cast irons: Classified as gray, ductile, white type
• Aluminum features strength-to-weight aspect, and resists corrosion
• Magnesium offers lowest weight traits, however, demonstrate depressed strength-to-corrosion resistance
• CAM defines computer-aided manufacturing, which translates the design document as manufacturing specifications
• Lever rule proves effective when determining phase proportions across two-phase segment featuring on diagram

Manufacturing Processes - Machining Fundamentals

• Machining forms a process creating preferred shapes with controlled material removal
• Subtractive production enables exceptional precision with creation on interchangeable parts
• Turning is the process of rotating a workpiece and removing material with a stationary tool
• Milling forms a process removing material by a revolving tool from the stationary workpiece
• Drilling creates holes on material through use regarding revolving drill bits
• Grinding involves abrasive wheels to perform material removal
• Machining functions by a shearing process, inducing localised shear stress leading to chip generation
• Rake angles define the angle in the midst of tool outline paired by the direction in order for trimming, side angles define tools by appearance against machined exterior impact on cutting, chipping
• Tool material covers high-speed steel, carbides and ceramics
• Option hinges over object by desired performance
• Machining brings high heat
• Advanced cutting-edge techniques work through extremely difficult to machine substance combined regarding cryogenic process

Machining Challenges

• Machining poses difficulties when processing high-strength materials such as titanium alloys which results and requires high temperatures coupled to chemical reactivity exist
• Chatter indicates self-excited vibration throughout machining which increases wear and tear on low surfaced objects
• Tools prove prone to malfunctioning resulting out of specific mechanisms, which will include aggressive put on

Sheet Metal Forming Processes

• Sheet Metal Forming describes a production method involving a given formation with flat sheet metal to achieve preferred three-dimensional forms through plastic deformation
• Key Sheet Metal Forming Processes:
• Shearing
• Bending
• Stretch Forming
• Deep Drawing
• Stamping
• Multi-point Forming
• Incremental Sheet Forming

Surface Treatment Techniques

• Modifying the superficial characteristics regarding one compound boosts its operational elements
• A few of these consist with the accompanying
• Mechanical Surface Treatments
• Thermal Surface Treatments
• Thermo-chemical Surface Treatments
• Plating, Coating techniques
• Ion implantation

Manufacturing Elements

• Factors describing the element can cause it's choice when manufacturing for material properties, it should show great strength, ductility (high)
• High resistance (High resistance)
• Material processing properties impacts producing
• Tools impact forming processes

Cost of Manufacturing

• Machining a low-volume item is slow through most substances
• Sheet Metal Forming is different with how fast metals take after
• Surface treatment modifications involve wear plus other objects.

Manufacturing Facts

• Primary metal removal forms contain turning, milling, as well grinding
• Cutting processes include shearing producing localised stress
• Tool materials form through HSS, carbides ceramic
• Machining comes from heat that coolants prevent
• Sheet metals structure a few plastic sheets
• Shearing takes metal utilizing a die
• Bending metals works in angles
• Surface treatment alters surfaces for better use though can impact the volume
• Hardness through materials impact properties