Powder Bed Fusion Processes Chapter 5 PDF

Chapter 5 Powder Bed Fusion Processes  Powder Bed Fusion Processes Objectives: – Discuss on: Powder bed fusion processes SLS process description Materials – Polymers and composites; Metals and composites; Ceramics and ceramic...

Chapter 5 Powder Bed Fusion Processes  Powder Bed Fusion Processes Objectives: – Discuss on: Powder bed fusion processes SLS process description Materials – Polymers and composites; Metals and composites; Ceramics and ceramic composites Powder fusion mechanisms – Solid-state sintering; Chemically induced sintering; Liquid-phase sintering and partial melting (distinct binder and structural materials: separate particles. Composite particles, and coated particles; and indistinct binder and structural materials); and full melting Part fabrication – Metal parts; Ceramic parts Process parameters and modeling – Process parameters; Applied energy correlations and scan patterns Powder handling – Powder handling challenges; powder handling systems; and powder recycling PBF process variants and commercial machines – Polymer laser sintering; Laser based systems for metals and ceramics; Electron beam melting; Line-wise and Layer-wise PBF processes for polymers  Powder Bed Fusion Processes Process benefits and drawbacks Conclusions Assignment: – Read Chapter 5, Pages 107 - 145 Homework: – Exercises: 1-6  Powder Bed Fusion Processes Powder Bed Fusion (PBF) Processes: – Among the first commercialized AM processes – Developed at the University of Texas at Austin – Selective Laser Sintering (SLS) – first PBF process commercialized – SLS method of operation – Fig. 5.1. – Other PBF processes modify this basic approach in one or more ways to enhance machine productivity, enable different materials to be processed, and/or to avoid specific patented features – All PFB processes share a basic set of characteristics: One or more thermal sources for inducing fusion between powder particles A method for controlling powder fusion to a prescribed region of each layer Mechanisms for adding and smoothing powder layers LS process originally developed for producing plastic prototypes using a point-wise laser scanning technique This approached has been extended to metal and ceramic powders – additional thermal sources have been utilized and variants for layer–wise fusion of powdered materials are being commercially introduced Range of materials: polymers, metals, ceramics, and composites Increasingly being used for direct digital manufacturing of end-use products, as materials properties are comparable to many engineering grade polymers, metals, and ceramics  Powder Bed Fusion Videos PBF that uses lasers – LS = Laser Sintering; pLS = Polymer Laser Sintering; mLS= Metal Laser Sintering; Electron Beam and other thermal sources for sintering; Non-laser thermal sources Design Dictionary: Powder Bed 3D Printing https://www.youtube.com/watch?v=kBHsfNDsbCs Selective Laser Sintering (SLS) Technology https://www.youtube.com/watch?v=9E5MfBAV_tA Oak Ridge - Animation of Additive Manufacturing with Electron Beam Melting https://www.youtube.com/watch?v=BxxIVLnAbLw  pLS Process pLS Process: – Described as a paradigm approach to which the other PBF processes will be compared – Fuses thin layers of powder – typically 0.075 - 0.1 mm thick – which have been spread across the build area using a counter rotating powder leveling roller – The part building process takes place inside an enclosed chamber filled with nitrogen gas to minimize oxidation and degradation of the powdered material – The powder in the build platform is maintained at an elevated temperature just below the melting point and/ or glass transition temperature of the powdered material – Infrared heaters placed above the build platform to maintain an elevated temperature around the part being formed – CO2 laser is directed on to the powder and is moved using galvanometers to thermally fuses the material to form the slice cross-section – After completing a layer, the build platform is lowered by one layer thickness and a new layer of powder is laid and leveled using the counter-rotating roller – The beam scans the subsequent slice cross-section – The process repeats until the part is built – A cool-down is typically required to allow the parts to uniformly come to a low-enough temperature that can be handled and exposed to ambient temperature and atmosphere  Materials Polymers and composites – Polymers: Thermoplastic or thermoset – Thermoset polymers typically not processed using PBF into parts – they degrade and do not melt – Thermoplastic: Amorphous polymers have a random molecular structure, with polymer chains randomly intertwined; Crystalline polymers have a regular molecular structure, but this is uncommon – Commonly used in PBF: Thermoplastic polymer – commonly known as nylon Metals and composites – Several types of steel: stainless steels and tool steels, titanium and its alloys, nickel based alloys, some aluminum alloys, and cobalt-chromium have been processed and commercially available Ceramics and ceramic composites – Ceramic materials are generally described as compounds that consists of metal oxides, carbides, and nitrides and their combinations. – Several ceramic materials available include aluminum oxide and titanium oxide – Biocompatible materials have been developed for specific applications. Example: calcium hydroxyapatite, a material very similar to human bone has been processed using pLS for medical applications  Powder Fusion Mechanism There are 4 different fusion mechanisms – Solid-state sintering – Chemically induced binding – Liquid-phase sintering – Full melting Most commercial processes utilize primarily liquid-phase sintering and melting Solid-state sintering – Sintering indicates the fusion of powder particles without melting (i.e. in their slid state) at elevated temperature – This occurs at one of the absolute melting temperature and the melting temperature – The driving force for solid-state sintering is the minimization of the total energy, Es, of the powder particles – The mechanism for sintering is primarily diffusion between powder particles Es = s X SA , where: Es is the surface energy, s is the surface energy per unit area for a particular material, atmosphere, and temperature, and SA is total particle surface area – When particles fuse at elevated temperatures, the total surface area decreases, and thus surface energy decreases (Fig. 5.2)  Solid-State Sintering Solid-State Sintering: – As the total surface area of the powder bed decreases, the rate of sintering slows – To achieve very low porosity level, long sintering times or high sintering temperatures are needed – As total surface area in a powder bed is a function of particle size, the driving force for sintering is directly related to the surface area to volume ratio for a set of particles – The larger the surface area to volume ratio, the greater the free energy driving force. Thus, smaller particles experience a greater driving force for necking and consolidation, and thus, smaller particles sinter more rapidly and initiate sintering at lower temperature than larger particles – As the diffusion rate exponentially increase with temperature, sintering becomes increasingly rapid as temperatures approach the melting temperature, which can be modeled using the form of the Arrhenius equation – There are three secondary ways sintering affects a build: If the loose powder within the build platform is held at an elevated temperature, the powder bed particles will begin to sinter to one another As a part is being formed in the build platform, thermally-induced fusing of the desired cross- sectional geometry causes that region of the powder bed to become much hotter than the surrounding lose powder (Fig. 5.3) Rapid fusion of a powder bed using a laser or other heat source rarely results in 100% dense, porosity-free parts.  Arrhenius Equation The Arrhenius equation is important because it states that the rate constant is a function of temperature. Applications involving the Arrhenius consist of determining creep rates, crystal vacancies, and thermally induced processing rates. Arrhenius Equation  Implications As the activation energy decreases, the rate constant increases since less energy is required for the reaction to occur As temperature increases, the rate constant increases The activation energy of an un-catalyzed reaction is much larger than a catalyzed reaction  Exercise 1 Find the rate constant if the temperature is 289K, Activation Energy is 200kJ/mol and pre- exponential factor is 9 M-1s-1  Solution 1 lnk = -(200 X 1000) / (8.314)(289) + ln9 k = 6.37X10-36 M-1s-1  Exercise 2 Find the new temperature if the rate constant at that temperature is 15M-1s-1 while at temperature 389K the rate constant is 7M-1s1, the Activation Energy is 600kJ/mol  Solution 2 Apply the equation ln(k1/k2)=-Ea/R(1/T1-1/T2) ln(15/7)=-[(600 X 1000)/8.314](1/T1 - 1/389) T1 = 390.6K  Chemically-induced Sintering Involves the use of thermally-activated chemical reactions between powders and the atmospheric gases to form a by-product which binds the powder together This fusion mechanism is primarily utilized for ceramic materials Examples of reactions between powders and atmospheric gases: – Laser processing of SiC in the presence of Oxygen, whereby SiO2 forms and binds together a composite of SiC and SiO2; – Laser processing of ZrB2 in the presence of Oxygen, whereby ZrO2 forms and binds together a composite of ZrB2 and ZrO2; and – Laser processing of Al in the presence of N2, whereby AlN forms and binds together the Al and AlN particles One common characteristic of chemically-induced sintering is part porosity. As a result, post-process infiltration or high-temperature furnace sintering to higher densities is often needed to achieve properties that are useful for most applications The post-process infiltration may involve other reactive elements, forming new chemical compounds after infiltration The cost and time associated with post-processing have limited the adoption of chemically-induced sintering in commercial machines  Liquid-Phase Sintering and Partial Melting Liquid-Phase Sintering (LPS): – LPS is the most versatile mechanism for PBF. LPS is a term used extensively in the powder processing industry to refer to the fusion of powder particles when a portion of constituents within a collection of powder particles becomes molten, while other portions remain solid – The molten constituents acts as the glue which binds the solid particles together – The high temperature particles can be bound together without needing to melt or sinter those particles directly – LPS is used in traditional powder metallurgy to form, for instance, cemented carbide cutting tools where Co is used as the lower-melting-point constituent to glue together particles of WC – In AM, LSP can be utilized as a fusion mechanism. Kruth et al has formed the basis for the distinctions as shown in Fig. 5.4 Distinct binder and structural materials: separate particles, composite particles, coated particles Instinct binder and structural materials  Full Melting Full Melting – Most commonly associated with PBF processing of engineering metal alloys and semi-crystalline polymers – In these materials, the entire region of material subjected to impinging heat energy is melted to a depth exceeding the layer thickness – Thermal energy of subsequent scans of a laser or electron beam is typically sufficient to re-melt a portion of the previously solidified solid-structure – This type of full melting is very effective at creating well-bonded, high density structures from engineering metals and polymers – The most common material used in PBF processing is nylon polyamide. As a semi-crystalline material, it has a distinct melting point. In order to produce parts with the highest possible strength, these materials should be fully melted during processing – Engineering alloys utilized: Ti, Stainless Steel, Co Cr, etc. – Fig. 5.5 summarizes the various binding mechanisms which are utilized in PBF  Metal and Ceramic Part Fabrication Metal parts – Four common approached in the creation of complex metal components: Full melting: A metallic powder material is fully melted using a high-power laser or electron beam; Liquid-phase sintering: A mixture of two metal powders or a metal alloy is used where a higher-melting-temperature constituent remains solid and a lower-melting- temperature constituent melts; Indirect processing: A polymer coated metallic powder or a mixture of metallic and polymer powder are used for part construction. Figure 5.6 shows the steps involved in indirect processing of metal powders; Pattern methods: In this approach, the part created in the PBF process is a pattern used to create the metal part: investment casting patterns or sand-casting molds Ceramic parts – Processes utilized Direct sintering, Chemically-induced sintering, indirect processing, and pattern methods.  Process Parameters and Modeling Laser processing and parameters – Models discussed can also be applied to other thermal energy sources, such as electron beams or infrared heaters Process parameters – Lumped into four categories – Laser related parameters: laser power, spot size, pulse duration, pulse frequency etc. – Scan related parameters: scan speed, scan spacing, and scan pattern – Powder related parameters: particle shape, size, and distribution, powder bed density, layer thickness, material properties etc. – Temperature related parameters: powder bed temperature, powder feeder temperature, temperature uniformity, etc. Most of these parameters are strongly interdependent and are mutually interacting The required laser power typically increases with melting point of the material and lower powder bed temperature, and also varies depending upon the absorptivity characteristics of the powder bed, which is influenced by material type and powder shape, size, and packing density. – A typical PBF machine includes two galvanometers (one for the x-axis and the other for the y-axis motion). Scanning often occurs in two modes, contour mode and fill mode (Fig. 5.7) In contour mode the outline of the part cross section for a particular layer is scanned. It is typically done for accuracy and surface finish reasons around the perimeter The rest of the cross section is scanned using the fill pattern – randomized scanning is sometimes used – there is no preferential direction for residual stresses induced by the scanning Applied Energy Calculations and Scan Patterns  Applied Energy Calculations and Scan Patterns Fig. 5.8: Balling tendency at various power, P, and scan speed U combinations – 5 typical types of tracks which are formed at various process parameter combinations.  Powder Handling Powder handling challenges: – Several different systems for powder delivery in PBF have been developed. – The lack of a single solution for powder delivery – counter-rotating roller was first used in PBF. – Any powder delivery system for PBF must meet at least four characteristics: A powder reservoir of sufficient volume to build to the maximum build height Transporting the correct volume of powder from the powder reservoir to the build platform The powder must be spread to form a smooth, thin, repeatable layer of powder The powder spreading must not create excessive shear forces that disturb the previously processed layers – In addition any powder delivery system must be able to deal with these universal characteristics of powder feeding: As particle size decreases, interparticle friction and electrostatic forces increase When the surface area to volume ratio of the particle increases, its surface energy increases and become more reactive When handled, small particles have a tendency to become airborne and float as a cloud of particles Smaller powder particle sizes enables better surface finish, higher accuracy, and thinner layers Various types of powder feeding systems are shown in Fig. 5.10.  PBF Process Variants and Commercial Machines Polymer laser sintering – Designed for directly processing polymers and for indirect processing of metals and ceramics – The machines are commonly called either Selective Laser Sintering (SLS) or Laser Sintering (LS) machines – 3D Systems’ low-temperature machines are designed to run a large variety of powdered material types. Due to the use of CO2 lasers and nitrogen atmosphere with approximately 0.1-3.0% oxygen, pLS machines are incapable of directly processing pure metals or ceramics. Nylon polyamide materials are the most popular pLS materials – these processes can also be used many other types of polymer materials as well as indirect processing of metals and ceramic powders with polymer binders – Fig. 5.11 shows melting and solidification characteristics for an idealized polymer DSC curve for polymer laser sintering – Fig. 5.12 shows a schematic of an EOS machine: laser sintering powder delivery and processing for foundry sand Laser-based systems for metals and ceramics – Fig. 5.13: Nd-YAG laser in pLS resulted in a much better absorptivity for metal powders – Fig. 5.14: 3D Micromac Powder Feed System – the build platform is located between two powder feed cylinders – Fig. 5.15: 3D Micromac parts made from aluminum oxide powders  Electron Beam Melting Electron Beam Melting (EBM) – Has become a successful approach to PBF – In contrast to laser-based systems, EBM uses a high-energy electron beam to induce fusion between metal powder particles – Table 5.1: Difference between EBM and SLM – Fig. 5.16: Schematic of an EBM apparatus – Electron beams are inherently different from laser beams, as electron beams are made up of stream of electrons moving near the speed of light, whereas laser beams are made up of photons moving at the speed of light. – When an electron beam is passed through a gas at atmospheric pressure, for instance, the electrons interact with the atoms in the gas and are deflected – In contrast, a laser beam can pass through a gas unaffected as long as the gas is transparent at the laser wavelength – Thus, EBM is practiced in a low-partial-pressure vacuum environment – Fig. 5.17: CoCrMo mLM microstructure – The presence of beam traces in the final microstructure is process parameter and material dependent. For titanium alloys contiguous grain growth across layers even for mLM is not uncommon. For other materials with high melting pint, the layering may be more prevalent.  Line-wise and Layer-wise PBF Processes Fig. 5.18: Three different approaches to line- and layer-wise PBF processing: – Mask-based sintering – Printing of an absorptivity-enhancing agent in the part region – Printing of a sintering inhibitor outside the part region  Process Benefits and Drawbacks PBF can process wide variety of materials in contrast to many other AM processes During part building, loose powder is a sufficient support material for polymer PBF. This saves sufficient time during part building and post-processing Accuracy and surface finish of powder-based AM processes are typically inferior to liquid based processes. They are influenced by the operating conditions and the powder particle size. Finer particle sizes produce smoother, more accurate parts but are difficult to spread and handle. Larger particle size facilitates easier powder processing and delivery, but hurt surface finish, minimum feature size and minimum layer thickness; The build materials typically exhibit 3-4% shrinkage, which can lead to part distortion; Materials with low thermal conductivity results in better accuracy. Total part construction time can take longer than other additive manufacturing processes because of preheat and cool-down cycles are involved The future for PBF remains bright, and it is likely that PBF processes will remain one of the most common types of AM technologies for the foreseeable future