Learning Objectives
By the end of this section, students will be able to:
- Describe the range of engineered metallic materials available for AM.
- Describe the means of preparing and processing those materials for use in AM applications.
Over 100 different alloys are either commercially available or have been proven in the laboratory as suitable for AM. These alloys cover all the primary classes, with many being available for multiple processes, as summarized in Table 3.5. The majority of alloys currently used in AM were originally developed for casting, welding, or wrought (sheet, plate, forging, extrusion) processes for two reasons:
- Using an alloy with a known chemistry and characteristics reduces the risk of unforeseen metallurgical issues, such as brittleness or corrosion issues.
- It can be difficult to build a material supply chain for a new alloy. Hence, most of the alloys are similar or identical to existing ones, and are already produced as a wire, powder, or foil for another market.
This situation will change in the future as the AM industry grows and more alloys are developed. Note that the mechanical properties, cost, and other characteristics for a given alloy class can range widely; consulting a current comparison guide will be necessary. Relative cost is often expressed in terms of per kg, so in some cases the use of a lower density but higher cost alloy can be less expensive, keeping in mind that metal feedstock cost may only represent 10%-50% of the final part cost.
While most alloys are characterized by their composition, density, and mechanical properties, two other characteristics of importance to AM are weldability and heat treatment. Weldability is important because many AM processes are fusion processes, that involve melting the alloy and fusing it to the previous layers through solidification. Fusion based AM processes are often described as either ‘making the part by welding’ or ‘making the part by micro-welding’. Heat treatment is important because certain alloys require a specific heat treatment to obtain optimum properties.
Heat treatment
These primary types of heat treatment are below, with the percent of melting point referring to the absolute melting point in K:
- Stress-Relief – This is used for most alloys which undergo a fusion process to eliminate residual stresses that can lead to distortion, decreased fatigue lives, or reduced corrosion resistance. It generally takes place between 30% and 40% of the melting point. This is often performed on parts made using fusion AM prior to removing it from the build plate.
- Annealing – This is used in austenitic (3xx series) stainless steels, copper alloys, and a variety of Al, Co, Ni, Co, Fe, and refractory alloys. The primary goal is to provide a more stable microstructure for a better balance of mechanical properties and corrosion resistance. Annealing temperatures are generally between 50% and 60% of the melting point
- Austenize, Quench, and Temper – This is primarily used in alloy steels, tool steels, martensitic (4xx series) stainless steels, and some titanium alloys. It involves heating the material to around 60% of its melting point (austenize), rapidly cooling it (usually a water or liquid quench), and then reheating to around 300C (tempering) to remove brittleness.
- Solution Anneal, Quench, and Age – This is most commonly used for aluminum alloys, PH (Precipitation Hardening) stainless steels, Ni alloys, some Cu alloys, Mg alloys, and some Ti alloys. It involves heating an alloy to around 90% of its melting point (Solution Anneal), rapidly cooling (Quench), and aging at around 25% of melting point to precipitate out phases that strengthen alloys, hence why it is referred to as precipitation hardening.
The actual temperatures, times, heating and cooling rates for any alloy often need to be quite closely controlled. Since the overwhelming number of alloys used in AM were previously developed for casting, wrought, and welding applications, the optimum heat treatment for the same alloy used for AM may be different. It should also be noted that the rapid (generally liquid) cooling in some of the heat treatments can induce residual stresses, distortion, and even cracking.
Another thermal treatment often used in conjunction with AM is hot isostatic pressing (HIP). This consists of heating the material to around 50% of its melting point while under extremely high pressure (100MPa – 200MPa) in Ar gas for 2 to 8 hours, then cooling back to room temperature. The purpose is to close any discontinuities (pores, lack of fusion, or cracks) in the interior of the part. HIP is not effective for discontinuities that are open to the surface. Also, the presence of an inert gas in a pore (such as that which may become trapped during AM processing in and Ar atmosphere) may also keep it from fully healing. This is either because as the pore shrinks, the internal gas pressure will increase until it matches the HIP pressure, arresting further shrinkage and healing. In some instances, especially if the part is treated to a high-temperature solution annealing after HIP, the gas and the pore may then expand, as the temperature heats the gas and raises the pressure, while simultaneously lowering the flow stress of the metal. The overall relationship between pore size, gas content, alloy, pressure, time, and temperature is very complex with examples where HIP is successful and where it is not successful in the literature, with the current consensus being that it does provide a benefit.
Because of the high temperature and long time, HIP often doubles as a stress relief or an annealing operation. If the HIP temperature is sufficiently high and the cooling rate is relatively fast and controlled, it is possible to also use this as the solution annealing and quench step for precipitation hardened alloys that do not require a rapid quench. This is known as a combined cycle, and it is also possible after the quench to hold the part at an elevated temperature to age as well. Since HIP vessels are significantly more expensive to purchase and operate than heat treat furnaces, however, economics may still recommend performing these operations in different cycles.
Metallic Feedstock Types and Processes
Because AM by definition makes parts in layers, the metallic feedstocks used are small or thin, meaning powders, wires, or foils. While powder bed fusion (PBF) by definition uses powders, and sheet lamination (SL) by definition uses sheets or thin foils; directed energy deposition (DED) processes can use powder or wire, while binder jetting (BJP), material extrusion (ME), Material Jetting (MJ) and cold spray (CS) use powders. Typical feedstock types and sizes, along with their associated processes, are provided in Table 3.5. The different ways in which these feedstocks are manufactured has an impact on their subsequent behaviors in AM.
| Feedstock | Process | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Type | Size | Laser Powder Bed Fusion | Electron Beam Powder Bed Fusion | Small Puddle DED | Large Puddle DED | Binder Jet and Polymer material Extrusion/Jetting | Material Extrusion | Material Jetting | Cold Spray | Sheet Lamination |
| Powder | <15𝜇m | X | ||||||||
| 15𝜇m - 45𝜇m | X | X | ||||||||
| 45𝜇m - 105𝜇m | X | X | X | X | X | |||||
| 105𝜇m - 150𝜇m | X | X | X | |||||||
| >150𝜇m | X | X | ||||||||
| Wire | <1.5 mm diameter | X | X | |||||||
| >1.5 mm diameter | X | X | X | |||||||
| Foil | ~250𝜇m | X | ||||||||
Wire Manufacturing
Regardless of the alloy system, wire is made in generally the same way. A cylindrical ingot, or cast simple shape suitable for further processing, of the alloy is cast in a melt shop, and then progressively reduced in diameter in a series of forging, followed by bar mills, until the bar is relatively small in diameter, with 25mm being near the top end. This bar is then pulled through a series of dies to progressively smaller diameters, known as drawing, until the final wire diameter is achieved. Because drawing is a cold-working process that progressively strengthens a metal and reduces its ductility, intermediate anneals are performed to reduce the strength and restore the ductility. Other steps involve the chemical removal of lubricants and oxide coatings. An operation done for some arc welding (generally Gas Metal Arc or MIG) wire is to add a thin Cu coating to conduct electricity from the weld electrode to the wire. Because of this, care must be taken when ordering wire if a Cu coating is not wanted.
Highly workable, non-reactive alloys such as mild and austenitic stainless steels will have an annealing station as part of a large, continuous wire drawing line that may produce thousands of kilometers of wire in a single day. Alloys that are more difficult to work, especially reactive materials, such as titanium alloys, will be drawn to the extent of their ductility, annealed in a vacuum furnace, and then further reduced as an iterative operation until the final diameter is achieved. Additionally, surface oxidation may need to be removed, which further reduces the amount of sellable wire. This means that in some alloys, small variations in diameter can result in significant increases in the unit cost of the wire.
Some variants on this process, especially for low melting point alloys like Al and Cu, will make the bar using continuous casting methods which can range from 10mm diameter to up to 75mm diameter. Another variant, generally for metals with very high melting points, known as refractory metals, such as tungsten, molybdenum, niobium, tantalum, etc, will press and sinter powders into a billet, which will then be fully densified using HIP, forging, or extrusion. This approach would also be used for metal matrix composites and alloys that cannot be cast, such as some nickel-based superalloys.
Foil Manufacturing
Foil manufacturing is much like wire manufacturing except that instead of round bar being drawn through a die, flat sheets are reduced in thickness using a rolling mill. Even without cold-working, the pressure needed to roll an alloy increases as it gets thinner. As a result, foils are produced in specialized mills with relatively small rollers that are backed with other rolls, known as Sendzimer Mills. Like wire, intermediate anneals are often necessary.
Powder Manufacturing
Whereas wire and foil are made using variants of the same process, a wide array of processes are used to make powder.
Plasma
These processes use a plasma to melt solid metal or alloy of the desired composition to form spherical particles. Barring the pick-up of undesired gases, the composition of the powder is the same as the feedstock. Generally, whatever you can get in bar, wire, or particle stock you can make into spherical powder. There are three processes in this category:
- Plasma Rotating Electrode (PREP) uses a plasma torch to melt a cylindrical bar of the desired alloy. The rapid (>10,000 RPM) spinning of the bar causes the molten metal to fly away from the bar and form spherical particles. A combination of the alloy surface tension and the radial velocity of the molten alloy as it departs the arc determine the powder particle size. Attributes of powder from this process are a relatively coarse size distribution, nearly perfect spheres, almost no satellites (small powder particles adhered to larger ones), and almost no porosity.
- Plasma Atomization (PA) uses a wire feedstock that is melted by plasma torches in place of forcing a molten metal and gas through an orifice. The wire feedstock is significantly more expensive than the billet or ingot feedstock used in gas atomization. The primary benefit of plasma atomization is that it has lower satellite and porosity content than gas atomization produces.
- Spheriodization is a way to convert existing irregularly shaped or porous particles into spherical powder particles. In this process, the particles to be converted into spheres fall through a plasma. While they are falling due to gravity, the particles re-solidify into spheres. A combination of alloy surface tension and the initial particle size determine the powder particle size, thus making the initial particle size key to the powder size distribution. This powder can be finer than PREP but is generally coarser than atomized. It will have more satellites and pores than PREP but will generally be better in both categories than typical atomized powder.
Atomization
Atomization generally involves mixing a liquid metal stream with flowing fluid and forcing the mixture through an orifice to scatter the liquid, which then solidifies into generally spherical powders. Key variables that influence the size distribution of the powder are the size of the molten stream, the metal:gas ratio, their velocities, and additional gas flows, and the size of the orifice. Additional characteristics of the powder are sphericity, satellites and porosity. Depending on the variables and degree of control, a wide range of characteristics is possible.
Inert Gas Atomization (IGA) and Electrode Inert Gas Atomization (EIGA) are the most common processes for reactive and high-value alloys, such as titanium (reactive), aluminum (reactive), magnesium (reactive) nickel-base (high-value), and higher value tool and stainless steels. This is because the inert gas (almost always Ar) does not react with the base metal or any oxidation-sensitive alloying elements. It should be noted that using an inert gas provides protection from oxidation but is no guarantee of a powder with good flow and packing characteristics or homogeneity. Standard IGA melts the alloy in a crucible and pours it into a gas stream, while EIGA melts a bar with an electrode, with the molten metal impacted by the stream.
Gas Atomization (GA) is primarily used for non-reactive, non-ferrous alloys as well as lower-grade tool, stainless, and carbon steels. The typical gas used is nitrogen, but sometimes air is used. This process is not used for reactive metals, as the result would be catastrophic.
Water Atomization uses water as the fluid in place of the gas. This actually allows for more rapid cooling of the particles. Like GA, it is not used for reactive or higher-grade powders. This is the lowest cost process and produces a relatively coarse particle size distribution and irregular powders.
Ground/Machined
Ground and Machined processes take existing large stock (bar, plate, granules, etc) and use mechanical energy by grinding or machining to make powders. By adjusting the machining and grinding parameters, a wide range of shapes and sizes can be achieved. One of the benefits of using solid-state methods to make the powder is the absence of melt-related defects, such as gas pores, although care must be taken to avoid contamination by the cutting media.
Milling/Turning: In some cases, this is as simple as purchasing chips or turning scrap from a machine shop and subjecting them to additional grinding or cutting operations to make powder. In other cases, an engineered process to deliberately make powders may be used. A benefit of this is that almost any wrought alloy can be converted into powder without the complexities of melting and oxidation.
Hydride – Dehydride (HdH): This process is limited to titanium and its alloys, due to the unique combination of hydrogen absorption and embrittling characteristics. In this process titanium or titanium alloy particles are put in a hydrogen atmosphere at around 700C. After a period of time, a significant (>1%) amount of hydrogen is absorbed into the titanium, and embrittles it. The particles are cooled, removed from the furnace and ground to the desired particle size. After this, they are put into a vacuum furnace at around 700C, and the hydrogen evaporates out of the titanium or titanium alloy, leaving behind smaller, but irregularly shaped titanium or titanium alloy particles of the same composition as the initial feedstock. A variety of feedstocks can be used, such as machining chips (alloy), sponge (pure Ti), or even an alloy made using a direct reduced process. This powder can be used as-is, or spheriodized to make a spherical powder. In this case, the size of the powder is determined by the initial particle size and the time used to grind it. Porosity levels would reflect the initial feedstock. Impurity levels would also reflect the initial feedstock, so if machining chips are used, they must be well cleaned in advance to remove carbides and steel from any cutting tools; along with any cutting fluids.
Direct Reduced
Direct reduced powders range start with an oxide or other metal-containing compound and use reacting gases, liquids, or thermal decomposition to produce a metal powder. While they are generally used to make pure metals, some variants can produce alloys. The original processes were the use of hydrogen to reduce refractory metal oxides into metal powders, and thermal decomposition of carbonyl iron into iron powder. Some processes also use electrochemical methods. Multiple processes for making titanium alloys are in development, with some in the initial stages of commercialization. Most of these powders are irregular, and some can be spongy with a very low (<20%) tap density (density of a compacted container of powder, expressed as a percentage of fully dense alloy), and would require secondary processing, such as spheriodizing, to achieve a higher tap density and better flow characteristics.
Powder Attributes and Characterization
In addition to the chemistry and way of making powder, a variety of powder attributes impacts their use in AM. The attributes include overall size, the distribution of size, surface oxide, individual particle chemistry.
Not only do different processes use different types of feedstock, but the size of the feedstock varies as well. The size of the powders is generally specified as a range, where the lower number indicates the size that 10% of the particles are smaller than (called D10), and the larger indicates the size that 90% of the particles are smaller than (called D90). Some specifications place requirements on the powder size distribution (PSD), such as D2, D50, and D98. The size ranges provided in Table 2 are commonly used, but many processes and users specify powders in other size ranges. While spherical powder is currently preferred and is usually specified for PBF and DED applications, the ability to flow and spread evenly are the most important requirements. Sphericity is less important for the other processes, and in the case of BJP and polymer ME/MJ powders that don’t flow as well are preferred, as they have better green (condition after printing) strength and brown (condition after binder removal) strength to better retain their shape during sintering.
When powder is manufactured, the overall PSD covers a range that resembles a Gaussian distribution that may range from under 5µm to over 250µm. The powder manufacturer will then sieve the powder into the specified size ranges. The challenge for the powder manufacturer is to align the powder they produce, the price they can charge, and the customers they have to maximize the revenue they obtain from each lot of powder they produce. Much like the lumber industry where heartwood costs more than bark (excepting cork), some PSDs have a significantly higher market price than others, even though the unit cost to produce each size range (grow the tree) is the same.
Another consideration in powders, especially reactive alloys (Al, Mg, and Ti), is that the surface of the powder particles is covered with a thin oxide layer that is generally of a constant thickness. Thus, the smaller the powder diameter, the higher the ratio of surface area (and oxide) to volume. This means that the smaller PSDs are generally higher in oxygen than the larger ones. This is important when purchasing powders to make sure that one is not trying to get very small powders with a very low oxygen content, which can result in either excessive cost or poor availability. This comes into play in processes that require recovery and re-use of powders, as each run through the AM and recovery processes can result in oxygen pick-up. If the starting oxygen level is high relative to the maximum allowed amount, re-use can result in the powder quickly going above the maximum amount of oxygen, requiring scrappage and excessive costs. A final consideration in handling and mixing powders is that many specifications prohibit blending powder that is above the maximum allowed oxygen (or any other element, for that matter) with powder below the maximum allowed oxygen to create a blend that is within specification.
In powder bed fusion, the diameter of the powder is limited by the layer thickness, as having individual powder particles larger than the layer thickness can interfere with the recoater and impact the homogeneity of the layer. At the same time, powder particles that are too small can experience displacement by the melt pool. Electron Beam has traditionally used a larger powder size, but both processes are capable of using sizes in the 15µm – 105µm range, or even larger for very thick layers. Likewise, in DED, powder particles and wire diameters that are too close to the powder diameter can negatively impact the stability of the puddle, hence the reason small (generally <3mm) puddle processes use smaller diameter wire than large (>6mm) puddle processes.
Binder jet and polymer material extrusion/jetting (ME/MJ) can use a wide range of powder sizes, with small sizes being preferred for resolution, surface finish, and sintering time, and larger sizes preferred for cost, shrinkage control, and handling safety. A blend of sizes is sometimes preferred to optimize for sintering time and shrinkage. Material Extrusion (commercially known as the MELD® process) uses bar, large diameter wire or very coarse powder, often referred to as granules. One variant of Material Jetting (Xerox/Vader®) uses wire. The foil size used in sheet lamination is a trade-off of having a flexible foil (thinner is better), layer thickness (thicker is faster), and alloy type (higher-temperature and harder alloys are more difficult to ultrasonically weld, hence thinner is better).
A final way of characterizing powders is whether each particle is of the target chemistry (pre-alloyed) or if the powder particles are each a pure metal, and the ratio of different powders is the target chemistry (blended elemental). While these represent the extreme ends, blends of pure metals and alloys are also used, with an example being a mixture of 90% pure Ti and 10% Al/V master alloy (with a ratio of 60% Al and 40% V) being used to make Ti-6Al-4V. Each has its own advantages and disadvantages. Like the example above on oxygen content, the benefit of pre-alloyed powder is that the risk of chemical inhomogeneity in the final part is minimized, since one does not have to worry about the powder segregating during handling or passage through the AM machine and recovery/re-use processes. As a result, almost all of the fusion AM processes use pre-alloyed powder. Blended elemental powders and their offshoots, on the other hand, have been successfully used in the powder metallurgy industry for decades. Some of the benefits of blended elemental powders are lower cost, reduced inventory for a facility processing multiple closely related alloys, higher green/brown strength and faster sintering. Because of this, blended elemental alloys will more likely be used in BJP or polymer ME processes. A final potential use of blended elemental powders and AM is the ability to change the composition of from one region to another of a single part. These are often referred to functionally gradient materials and have been demonstrated in both DED and BJP processes.
Feedstock Safety and Cleanliness
Other feedstock factors relate to safety and handling. Smaller powders are more of a fire and inhalation safety issue, with reactive alloys of most concern. Because contamination is a concern with powders as well, powder handling is moving towards always keeping them in a sealed container throughout the entire process chain (‘protect the operator from the powder and the powder from the operator’), especially as powder bed fusion and binder jetting require recovery and re-use of powders. Safety and cleanliness are less of a concern in polymer material extrusion because the powder is encased in a filament with >40% polymer and a low surface area. In the case of polymer material jetting, the powder particles are also encased in a polymer. Additionally, polymer material jetting tends towards small, precise parts so that there is little powder in a given area. The coarse powder used for Material Extrusion is of little safety concern. Foil and wire have little or no safety issues, and while cleanliness is the primary handling issue, even this is reduced since wire-based AM processes are once through, with no recovery and re-use. Larger diameter wires are more difficult to handle but have less surface area for contamination.