Learning Objectives
By the end of this section, students will be able to:
- Differentiate several feedstocks of metallic materials, and how they relate to AM processes.
- Describe the relationships between metallic feedstocks, AM processing, AM post-processing, and the resultant AM part.
Aluminum Alloys
Currently, the overwhelming use of aluminum alloys in AM is for L-PBF, with DED, ME, MJ, CS, and SL all seeing their initial introductions. The difficulty in sintering low-density materials with adherent oxides presents challenges for BJP and Polymer ME/MJ. Aluminum alloys have a wide range of properties, falling into several different categories. All of the precipitation hardened alloys require quenching in either water or an engineered quenchant, so the potential for distortion and residual stresses must be addressed.
The most commonly used alloys at this time are Al-10Si-Mg and Al-7Si-Mg. These are essentially the AM counterparts of A360 (a die casting alloy) and D356 (a sand, permanent mold, and investment casting alloy). The Si content provides good weldability, and hence is favorable for AM. The presence of Mg with Si enables them to be precipitation hardened to a reasonable strength:weight ratio. Related to these alloys are the 4xxx welding and 6xxx (usually 6061) wrought alloys. The 4xxx welding alloys are available in wire form, hence are readily available for wire-based processes. They contain Si for weldability with some variants having Mg. Variants with Mg can be precipitation hardened, while those without are only annealed. Since 6061 is a wrought alloy, it is readily available as bar for ME, but is also available in wire (for DED), powder (for L-PBF and Cold Spray), and foil form (SL). It can be precipitation hardened to slightly higher properties than the casting-based alloys.
Because of their weldability and excellent corrosion resistance, 5xxx alloys are primarily used for architectural and marine use, but the inability to heat treat them limits their mechanical properties. Common product forms would be bar, wire, and sheet/foil. Because 5xxx alloys respond well to cold-working, cold-worked foils and SL or bar and ME would provide higher properties.
The 2xxx series alloys were history’s original precipitation hardened system and were the primary ones used to build in aircraft from the 1920s through the 1970s, with advanced variants still used. They can be heat treated to higher properties than the 3xx, 4xxx, and 6xxx alloys. The high copper content, however, generally gives them poor weldability due to hot cracking. Welding grades such as 2219 are the exception, and are available in wire, although bar is also available. A20X® is an AM variant of a casting alloy that offers 2xxx mechanical properties and is available in powder.
The high Zn and Cu content in 7xxx series alloys also leads to hot cracking problems, so they have generally only been considered usable for solid-state processes such as ME and SL. The attraction of the 7xxx system is the very high strength that can be achieved with precipitation hardening. The development of a 7xxx alloy that is suitable for AM has been the subject of considerable research, with the first variant, 7A77.50 being registered and in the process of commercialization.
Scalmalloy® is one of very first alloys specifically designed for AM, whose unique Al-Mg-Sc chemistry, takes advantage of the rapid solidification in L-PBF to develop precipitates that impart very high properties without the need for precipitation hardening. The ability to achieve very high properties without needing to solution anneal and quench can provide additional design freedom as it eliminates the concerns over residual stresses and distortion from quenching, although the Sc content makes it a relatively expensive alloy. It also provides for very good thermal stability at elevated temperatures.
The typical process flow for both a precipitation-hardened alloy and Scalmalloy® using L-PBF, starting with initiation of the build program will illustrate the unique processing aspects of precipitation hardened and non-precipitation-hardened alloys.
- Build part.
- Remove part, build plate, and excess powder from machine.
- Analyze log file from machine. Unless this is automatically performed at the end of the cycle, it will take place after part removal so that machine is available for the next build.
- Perform rough finishing. Depending on the supply chain, the actual order of these operations can be quite flexible, with the exception that some form of stress relief should be performed prior to build plate removal to prevent movement and distortion of the part during build plate removal. As mentioned above, HIP can serve as the stress relief operation. Depending on the type of supports, HIP vessel operators may require removal of the supports to ensure that there is no entrapped powder that could escape and cause damage to the internals of the HIP vessel, especially the heating elements.
- Stress relief
- HIP
- Build plate removal
- Support removal
- Solution anneal, quench, and age. This is almost always performed after build plate removal to prevent the build plate and part from cooling at different rates during quenching and distorting the part.
- Lot acceptance testing. Excise and test metallographic and mechanical test (usually tensile) coupons. This may also include removal of features built on the part to prevent distortion during HIP, solution annealing, or quenching.
- Surface smoothing. This includes processes such as tumble de-burr, bead blasting, etc. Integrated finishing machines for AM parts are being introduced to the market, with some of the machines incorporating support removal in the same operation. This can also include manual operations, such as manual removal of support rash (protuberances on the part where supports were attached).
- Interface machining. This is generally machining of higher-tolerance features for mating with other parts, although non-mating regions with high tolerances would also be machined here.
- Part acceptance testing. Depending on the industry, a part acceptance testing can range from simple checks with a gage, to highly detailed measurements on a coordinate measuring machine (CMM), to extensive nondestructive testing methods. This will be more fully discussed in 6.2 Production Acceptance Testing.
- Radiographic nondestructive testing. This can range from simple film radiography to digital radiography to elaborate computed tomography (CT) testing. One benefit with aluminum is that its low density allows for lower, more sensitive X-ray energies, thicker parts, or a combination of the two. As AM parts become more complex, CT testing may sometimes be required to validate the locations and dimensions of internal cavities.
- Dimensional inspection
- Penetrant inspection. Penetrant inspection is highly dependent on surface roughness. That found in typical L-PBF parts that have not been smoothed will not allow for high-sensitivity fluorescent penetrant testing used in aerospace, and even makes lower-sensitivity dye penetrant testing challenging due to the high number of false positive indications. Aerospace grade penetrant inspection of aluminum requires a pre-penetrant etch of machined surfaces, which can require a unique chemical solution, depending on the alloy.
- Chemical treatments (paint, anodize, etc.) - Chemical treatments for higher Si alloys (Al-10Si-Mg, Al-7Si-Mg) can be significantly different than ones with lower Si contents, and different non-Si alloys have different chemical treatments.
- Build part.
- Remove part, build plate, and excess powder from machine.
- Analyze log file from machine.
- Perform rough finishing. Scalmalloy® requires an aging cycle that has different parameters than the HIP cycle, while a 5xxx alloy would not require anything except a stress relief.
- Stress relief
- HIP
- Build plate removal
- Support removal
- Lot acceptance testing.
- Surface smoothing.
- Interface machining.
- Part acceptance testing.
- Chemical treatments.
Carbon and Maraging Steels
As the most widely used material system on the planet, there are thousands of steel grades, with hundreds of them available in powder and wire form. Carbon, in a range of content from 0.08% up to 1% provides the strengthening agent, with austenize, quench, and temper as the primary heat treatment to provide strength. The use of a liquid quench brings in complications for distortion and residual stresses. The other alloying elements are generally used to provide hardenability (the ability to fully harden thick sections) or other property enhancements such as elevated temperature properties, corrosion resistance, toughness, etc. Carbon steels can be divided into 4 main categories:
Mild steels generally have under 0.08% carbon and relatively low properties. In wrought form, these are by far the most widely used class of metal on the planet because of their low cost, formability, weldability, predictable mechanical properties, and wide range of product forms. Because even ‘high-cost’ carbon steels are very low cost, and can have superior properties, mild steels are rarely used for AM. This could change in the future as AM moves into industries such as construction and shipbuilding.
Low alloy steels will tend to have around 1% alloying elements by weight and under 0.4% carbon. They can provide very high properties with heat treatment in thin sections, with alloy 4130 being a prime example. Their low cost and the broad experience in the powder metallurgy industry makes them attractive for BJP and polymer ME. Their lower hardenability is not a tremendous liability in AM, as section thicknesses tend to be relatively low. Higher carbon contents can result in cracking in fusion AM processes.
High alloy steels (around 3% alloying elements and up to 0.4% carbon, as in 4340) and tool steels (around 10% alloying elements and up to 1% carbon) are very much like low alloy steels in terms of the impact of carbon on fusion AM processes. Higher alloy and tool steels would be selected based on the need for enhanced properties, or occasionally for hardenability. One of the benefits of AM for making tooling is the ability to put in conforming cooling channels, shown in Figure F03_02_Conformal, so it can be imagined that the high hardenability of tool steels may become less necessary for AM tools. Both types of steel are available in wire and powder form, although the choice is not as wide as for wrought and cast products.
Maraging steels, which are carbon-free Iron nickel alloys, were developed to provide very high strength steels without the need for rapid quenching and the inherent cracking and distortion risks. They can also provide higher toughness than some tool steels. They use a variant of precipitation hardening known as Maraging. Maraging steels are of interest in AM because of the ability to provide high strength parts without the high carbon content that can cause cracking in fusion AM processes. Their higher alloying element content (up to 25%), however, means that the feedstock will be significantly more expensive in relative terms than low and high alloy steels. The rapid cooling present in most fusion AM processes also opens possibilities to avoid a high-temperature heat treatment after build and proceed to a stress-relief/aging cycle.
Processing considerations for medium and higher carbon steels made using a wire DED process can be quite different from L-PBF of aluminum alloys, as described below.
- Build part. This may have to be stopped from time to time and the part sent off for stress relief to prevent cracking of any martensitic regions.
- Remove part and build plate from machine.
- Analyze log file from machine.
- Perform rough finishing. Similar flexibility to L-PBF, with the exceptions that while supports are less common in DED processing, they would be larger, and would probably be retained until interface machining.
- Austenize, quench, and temper.
- Lot acceptance testing.
- Surface smoothing. While tooling surfaces often need to be smoother than parts, this will often be done during interface machining, with the remainder of the surfaces left as built.
- Interface machining.
- Part acceptance testing. The higher density of steels makes radiographic inspection more difficult, while very high strength steels do not require pre-penetrant etch. Additionally, steels can be inspected using magnetic particle inspection.
- Chemical treatments. While there are significant efforts to develop alternatives to Cr plating, the process still exists, and may be required for some applications. Additionally, some chemical treatments of high-strength steels present the risk of hydrogen embrittlement, so a low-temperature bake out may be required.
Stainless Steels
Their combination of weldability, corrosion resistance, and low relative cost make stainless steels one of the principal alloy systems used in AM. The widespread use of stainless steels in powder metallurgy, MIM, and welded chemical, medical, and food industry applications means widespread availability of powder, wire, and foils. The stainless steels used in AM fall into four primary classes based on chemical composition and method of hardening.
Austenitic (3xx) and Ferritic (4xx) stainless steels are relatively low-strength steels. Cold working in wrought versions can double the yield strength, but they are not strengthened by heat treatment. The difference between austenitic and ferritic is that former contains a significant (>8%) amount of Ni which gives the alloy a more formable cubic structure while making it nonmagnetic. These alloys are ideal for a wide variety of applications where resistance from oxidation or corrosion is desired, but high strength is not.
Martensitic stainless steels have a low (<0.2%) carbon content and are commonly used for molds and dies where resistance against corrosion is desired, especially where the part produced in the mold has a high surface finish requirement. Designs with conformal cooling channels would be an ideal AM application. Precipitation hardened stainless steels also obtain their properties by heat treatment and would be used where one would use an austenitic or ferritic stainless, but where higher mechanical properties are desired. Because the required cooling rate for PH stainless steels is lower than for martensitic stainless steels, there are fewer concerns with residual stresses and distortion.
- Build green part.
- Remove green part from machine and excess powder.
- Analyze log file from machine.
- Remove binder using thermal or chemical methods, resulting in a brown part
- Sinter brown part to target density, checking furnace log file and taking simple dimensions as a quick cycle check.
- Optional HIP if full density is desired. No final heat treatment is required.
- Lot acceptance testing.
- Surface smoothing.
- Interface machining. This would include supports and excess material needed for support during sintering.
- Part acceptance testing. If required, non-magnetic austenitic steels would require penetrant testing.
- Chemical treatments. Passivation of the surfaces to properly form a protective oxide is often performed.
Titanium Alloys
Titanium alloys are highly valued by multiple industries due to their combination of high strength, low density, toughness, and corrosion resistance. This combination of properties comes at a steep price, as shown in Table 3.5, which is why titanium alloys were one of the first systems where AM was implemented. Not only are titanium alloys costly, but their toughness also makes machining expensive.
Like wrought and cast product forms, Ti-6Al-4V is the primary alloy used for AM. Its weldability and ability to absorb its oxide in vacuum means that it can be used in nearly every AM process. This includes both aerospace and medical applications. Some of the first implementations of Ti-6Al-4V in aerospace use large-puddle DED processing to add features to a piece of plate, which is then stress relieved, ultrasonically inspected, and machined to the final configuration. The benefit of using AM is in reducing the ratio of raw material used needed to make a finished part (referred to as the buy:fly ratio in aerospace). The reduction in the high costs of procuring and machining Ti-6Al-4V more than make up for the additional costs for the DED processing. It should be noted that one challenge in fusion AM of Ti-6Al-4V and similar alloys is their high strength to stiffness ratio, which can result in excessive distortion, residual stresses, and even a higher propensity to crack during build. Thoughtful design and processing, however, can minimize this. While it is possible to heat treat Ti-6Al-4V to obtain maximum properties, the need for a rapid quench and the residual stress issues make this relatively uncommon. It is generally stress relieved or HIPped.
Commercially pure (CP) titanium is primarily used in the chemical industry due to its excellent corrosion resistance, formability, and significantly lower cost than Ti-6Al-4V, especially in sheet products. Its use in AM would also be primarily in the chemical industry for valve and connector components. A variety of Near-Alpha, Near-Beta, and Alpha titanium alloys are beginning to see use in applications where higher strength or higher temperature capability then Ti-6Al-4V is needed, without moving to higher-density Ni-based alloys. Most of these alloys use a post-build heat treatment to obtain optimum properties, which may differ significantly from the standard wrought products. The combination of high temperature capability and design complexity from AM makes some of these alloys ideal for heat shields and ducts.
Processing considerations for Ti-6Al-4V made using a large puddle wire DED process differ slightly from that for carbon steels, as described below.
- Build part. This may have to be stopped from time to time and the part sent off for stress relief to minimize distortion or prevent cracking.
- Remove part/build plate from machine.
- Analyze log file from machine.
- Perform final anneal and/or HIP (if required)
- Lot acceptance testing.
- Pre-machining for ultrasonic inspection. This generally consists of simple, 3-axis milling to remove the crowns from the top of the part, unless inspection can be performed through the base plate. Locating features for finish machining may also be machined in at this point to reduce set-up time.
- Ultrasonic inspection
- Final machining. As can be seen in M4, all surfaces of the part are machined.
- Part acceptance testing. Since ultrasonic inspection was already performed, this is limited to penetrant (with a different etching solution than used for aluminum alloys) and dimensional inspection.
- Chemical treatments.
Nickel-Based Alloys
Nickel-based alloys are often used where titanium alloys don’t quite have enough strength, stiffness, or temperature capability to meet requirements. They can also perform better than titanium alloys or pure titanium in some corrosive environments. While they are significantly lower in cost than titanium alloys, they are often more costly to machine, and their higher density often means more material mass is needed for the same application. Like titanium, use of AM for buy:fly and cost reductions will apply for some geometries.
Where lower mechanical properties are acceptable, 6xx (Ni-Fe-Cr) alloys are generally used where high temperature oxidation resistance is desired, and Ni-Cu (and Cu-Ni variants) alloys are used where corrosion resistance is more desired. Both of these alloy systems have excellent ductility and weldability, so lend themselves well to a variety of AM processes. Variants of these alloys will have either better temperature capability, oxidation resistance, or improved corrosion resistance.
Where either higher strength or higher strength at elevated temperature is desired, 7xx alloys are used. These use precipitation hardening with a relatively low cooling rate like PH stainless steels for significantly higher mechanical properties, as well as elevated temperature properties. Because they also have excellent corrosion resistance, variants of them are used in the chemical and petroleum industry in hot, corrosive environments. When even higher temperature properties are needed, superalloys containing Ni, Co, and other high-temperature elements are used. While these alloys sacrifice room temperature strength, they will have excellent resistance to deformation over long times at the elevated temperatures (creep) found in rocket and turbine engines. Some of the higher temperature grades, however, have lower weldability that can impact the ability to use for fusion AM processes.
Processing considerations for Ni-base alloys using a small puddle powder DED process differ slightly from that described above for Ti-6Al-4V.
- Build part. While it is likely that fewer stress relief operations compared to other alloys due to the lower strength/stiffness ratio and lack of brittle phases will be needed, this is quite design dependent.
- Remove part/build plate from machine.
- Analyze log file from machine.
- Perform heat treatment, which may include HIP and a solution anneal, quench and age for PH alloys. Large, thin, complex parts may require a fixture to support them during high-temperature heat treatments.
- Lot acceptance testing.
- Surface smoothing, generally a grit blast.
- Final machining.
- Part acceptance testing.
- Chemical treatments.
Cobalt-Based Alloys
Outside of superalloys, cobalt-based alloys have seen most of their AM use in the medical industry, with some applications now appearing in aerospace due to their combination of weldability, ease of heat treatment, and corrosion resistance. These fit in between 6xx and 7xx nickel-based alloys in terms of properties, with generally lower material cost and a simpler heat treatment than required for 7xx alloys. In general, the process path for L-PBF CoCr parts is similar to other alloys, with the exception that current specifications for F75 medical alloy require a high-temperature homogenization cycle. This is an example of a heat treatment for a cast alloy that may not need to be as long for a part produced using AM with the very rapid cooling cycle.
Other Alloys
The wide range of processes and the geometric freedom provided by AM has found applications using a wide variety of other alloys as well. These include copper alloys, refractory metals, magnesium, metal matrix composites, and intermetallic compounds.
In addition to the Cu-Ni alloy system, copper alloys are often used for their high electrical or thermal conductivity. Precipitation hardened Cu alloys generally require liquid quenching.
Since the SL process for the heat exchanger takes place near room temperature and is a hybrid manufacturing process without melting, the process path is the shortest of all.
- Build part.
- Remove part/build plate from machine.
- Analyze log file from machine.
- Lot acceptance testing.
- Final machining.
- Part acceptance testing. This could include ultrasonic inspection.
- Chemical treatments.
The process path for the cold spray propeller would be quite short, especially since Al bronze is a non-heat treatable alloy.
- Build part.
- Remove part/build plate from machine.
- Analyze log file from machine.
- Lot acceptance testing.
- Surface smoothing and polishing.
- Final machining.
- Part acceptance testing.
- Chemical treatments.
Refractory metals (W, Mo, Ta, Nb, etc) are being investigated for very high temperature applications. Because refractory metals are often brittle at room temperature and have high oxygen sensitivity, careful consideration must be taken, especially for fusion AM applications.
The low density, adherent oxide, and flammability of Mg have limited its potential in AM, although newer alloys with reduced flammability are being investigated.
Metal matrix composites (metals with either oxide, carbide, or boride reinforcements) are beginning to emerge from the laboratory into applications research. The high cooling rates of fusion AM could potentially allow for the fabrication of net-shape parts with a lower risk of the reinforcements agglomerating to the detriment of mechanical properties. Finally, AM has enabled the first widespread use of titanium aluminides, which have excellent creep resistance combined with low density. The rapid solidification and high, stable build environment of EB-PBF are enabling the fabrication of turbine blades. The process path would be similar to L-PBF of Scalmalloy®.
Future Alloys
Multiple organizations have initiated efforts over the last 5 years to develop alloys whose composition and processing are more compatible with AM, especially fusion processes. Much like Scalmalloy® does, these will take advantage of the rapid solidification in fusion AM processes to use chemistries with properties that one could not achieve using conventional ingot metallurgy. One concept would be to develop a precipitation hardened alloy where the rapid solidification and cooling in AM would leave alloying elements in solution, but where the stress relief would cause them to precipitate out, providing high strength without the need for a high-temperature heat treatment or rapid quench. While it typically takes 20 years for a new alloy to enter service, advances in computational modeling and the reduced scale-up needed for AM processes could result in these alloys being available sooner.
An ideal processing path for an alloy designed for L-PBF would be:
- Build part.
- Remove part, build plate, and excess powder from machine.
- Analyze log file from machine.
- Perform rough finishing and heat treatment.
- HIP
- Build plate removal
- Support removal
- Lot acceptance testing.
- Surface smoothing.
- Interface machining.
- Part acceptance testing.
- Chemical treatments.