A Guide to Metals Used in Additive Manufacturing
Metal Additive Manufacturing Processes
Powder bed fusion (PBF) is an additive manufacturing (AM) process in which ultra-fine layers of powdered metal are sequentially spread across a build plate before being melted by a laser. The laser is directed by an STL file derived from CAD data. As each layer cools, it binds to the preceding layer. The process yields a 3D-printed object which faithfully represents the information in the CAD file.
PBF systems either utilize a laser, multiple lasers or a beam of electrons to selectively melt thin layers of build material about 20 to 100 microns thick. For comparison purposes, consider that a human hair is approximately 75 microns in diameter.
There are a number of different powder bed systems, including Direct Metal Laser Sintering (DMLS), Direct Metal Laser Melting (DMLM) and Electron Beam Melting (EBM). In DMLM, Concept Laser’s patented LaserCUSING technology allows for the tool-free, economic production of complex parts, even in small quantities. Arcam’s EBM Multibeam technology maintains several melt pools simultaneously to improve productivity.
Parts consolidation is a key advantage of PBF systems. Because the printing process allows the inclusion of intricate and complex designs, it is often possible to combine multiple parts into one complex design, reducing manufacturing, assembly and machining costs. Thin-walled components, fine meshes and integrated conformal cooling channels are all possible.
Traditional manufacturing methods like investment casting require tooling that is only cost-effective when the economies of scale are realized. Since AM processes allow for direct manufacturing from CAD files, it is possible to economically produce customized components and smaller quantities of parts.
While casting and other traditional manufacturing methods restrict freedom of design, PBF processes allow for complex geometries and last-minute design modifications. Designers can also optimize the use of valuable build materials to simultaneously reduce weight, retain structural strength and cut costs.
Traditional manufacturing methods often require time-consuming subtractive machining that generates excess waste. However, when traditional manufacturing methods are well-suited to certain aspects of production, it is possible to simply integrate AM into the process where it is particularly advantageous.
In numerous applications, 3D printing reduces the number of steps from object design to finished product. This often cuts the time-to-market, enhancing an enterprise’s competitive position.
Metal Powder Production
High-quality metal powder is very important for successful powder bed fusion. In such processes, build material flow rates are optimized through the use of closely packed, spherical metal particles of similar size. Consistent metal particles also optimize object density. ASTM International continues to work toward standardizing specifications for AM metal powders.
For optimum performance, metal powders are precisely fabricated via gas atomization or plasma atomization. In the gas atomization process, molten metal shooting from a nozzle is dispersed and solidified by a stream of nitrogen or argon gas. Spherical metal particles drop to the bottom of the enclosure where they are subsequently collected. Although less commonly used, plasma atomization is used with reactive metals that have very high melting points, like titanium alloys. Wire filament is melted with a plasma torch, resulting in the production of spherical particles as the molten metal cools.
Stainless steel exhibits a number of mechanical properties favored in a variety of automotive, industrial, food processing and medical applications, including hardness, tensile strength, formability and impact resistance.
A variety of stainless steel metal powders are used in AM processes like DMLM, including 316L (low-carbon), 17-4PH, hot-work and maraging steel.
Stainless steel 316L is an appropriate selection for non-magnetic applications. It is more ductile than 17-4PH and it offers higher corrosion resistance. 316L stainless steel offers somewhat better weldability than 17-4PH. It delivers a higher build rate, although it is not heat-treatable.
The composition of this low-carbon stainless steel makes it ideal for use in food and chemical processing. Due to its high corrosion resistance, 316L is also commonly used in marine applications. It is also used in a variety of medical instruments, particularly when small quantities or customized configurations are specified.
Although Stainless Steel 17-4PH delivers a slightly lower build rate than 316L, it does offer several advantages. It is magnetic, heat-treatable and harder than 316L. Stainless steel 17-4PH is used to fabricate functional prototypes, automotive parts and industrial parts.
Hot-work stainless steels are high-load metals ideal for use in the production of parts used in high-volume injection molding. Maraging stainless steels are preferred for the fabrication of tool inserts with conformal cooling that are used in investment casting and injection molding.
Lightweight aluminum alloys are traditionally used in many industrial, aerospace and automotive applications. They possess high strength-to-weight ratios, and they also demonstrate good resistance to metal fatigue and corrosion. One key advantage of aluminum alloy powders is that they typically offer better build rates than other metal powders used in PDF processes.
Due to the geometrically complex structures possible with additive manufacturing, further weight reduction is often possible with little or no compromise in strength and overall performance. Aluminum alloys possessing fine-grained microstructures with grains roughly equal in size are typically as strong as their wrought counterparts. Excellent fusion characteristics make aluminum alloys particularly well-suited for use in 3D printing.
Low-density aluminum alloys like AlSi10Mg and AlSi12 have the hardness and tensile strength to resist high loads. They also demonstrate high thermal and electrical conductivity, and they are easy to rework. AlSi10Mg is appropriate for use in numerous aerospace and automotive applications, while AlSi12 is used in medical, aerospace and automotive applications. Durable AISi7Mg0,6 is a low-weight aluminum alloy with good mechanical properties used in a variety of ways, including in high-voltage applications.
Cobalt Chrome Alloys
3D-printed parts fabricated from cobalt chrome alloys like ASTM F75 CoCr when excellent resistance to high temperatures, corrosion and wear is critical. It is an appropriate selection where nickel-free components are required, such as in orthopedic and dental applications. Medical implants produced from cobalt chrome metal powder possess the hardness and bio-compatibility necessary for long-term performance.
Articulating surfaces of 3D-printed orthopedic knee and hip implants are finely polished in post-processing. ASTM F75 CoCr is often used where powder bed temperatures exceed 850 degrees C. to reduce internal stresses. In the Arcam Q10 plus system, 70-micron layer thicknesses optimize the balance between resolution and build speed.
Auto racing teams use cobalt chrome alloys when they want to quickly fabricate race engine components. Cobalt chrome alloys are also used to 3D-print parts for everything from jet engines to industrial gas turbines. The use of CoCr in AM processes is often more cost-effective than when it is used in traditional investment casting.
Parts printed from cobalt chrome alloys often benefit from hot isostatic pressing (HIP), which combines high temperatures and pressures to induce a complex diffusion process that strengthens grain structures, producing fully dense metal parts.
In general, titanium alloys are used to produce a wide range of industrial components, including blades, fasteners, rings, discs, hubs and vessels. Titanium alloys are also used to produce high-performance race engine parts like gearboxes and connecting rods. Like cobalt chrome, titanium’s biocompatibility makes the metal a viable option for medical applications, particularly when direct metal contact with tissue or bone is a necessity.
In 3D printing, the mechanical properties of titanium alloys Ti6Al4V and Ti6Al4V (ELI) make them popular choices. Ti6Al4V is a titanium alloy that is 6 percent aluminum and 4 percent vanadium. It maintains its high tensile strength even at extreme temperatures. It is a grade-5, alpha-beta alloy widely used in the fabrication of titanium parts. It is also weldable and heat-treatable. This low-density, high-strength metal also offers excellent corrosion resistance.
Due to rapid cooling of the metal melt pools, Ti6Al4V parts exhibit a fine-grained, dense microstructure that often exceeds that found in investment cast parts.
Another titanium alloy used in PBF processes is Ti6Al4V ELI, which contains less nitrogen, oxygen, iron and carbon. The reduced presence of these interstitials further enhances ductility and fracture resistance. Ti6Al4V ELI is commonly used in offshore oil and gas extraction applications, where the metal alloy’s extreme resistance to stress corrosion cracking in salt water is an advantage. The advanced metal alloy is also used in extremely demanding cryogenic and aerospace applications.
Unalloyed, commercially pure titanium is available in grades one through four. All grades exhibit extreme corrosion resistance, ductility and weldability, although Grade One is relatively more formable than Grades Two, Three and Four. Grade Four is the strongest.
Titanium Grade Two is a metal offering a desirable balance between formability and strength. It is used to create a wide variety of industrial parts, including those used in condenser tubing, heat exchangers, jet engines, airframes and marine chemical applications. Titanium Grade Two is also used in orthopedic prostheses and implants.
Nickel chromium super-alloys like Inconel 718 and Inconel 625 produce strong, corrosion-resistant metal parts. These alloys are often used in high-stress, high-temperature aeronautical, petrochemical and auto racing environments. Inconel 718 performs at temperatures as low as minus 423 degrees F and as high as 1,300 degrees F.
The mechanical properties of Inconel 625 are considerably enhanced by the use of significant amounts of nickel, chromium and molybdenum in the metal. It resists pitting and cracking when exposed to chlorides. As a result, Inconel 625 is frequently used in metal parts used in marine applications. Due in part to its high resistance to both alkalis and acids, Inconel 625 is also used in components used in oil and gas production. It also resists oxidation at extremely high temperatures.
718 is an age-hardened version of 625. The hardening process generates precipitates that better secure metal grains in place. Although Inconel 718 is approximately twice as strong as Inconel 625, the latter offers somewhat better corrosion resistance and long-term high-temperature resistance.
Inconel 718 is a metal that is also highly resistant to the corrosive effects of hydrochloric acid and sulfuric acid. It also demonstrates excellent tensile strength and good weldability.
Inconel 718 is used to produce metal parts used in gas turbines, jet engines, cryogenic storage tanks and petrochemical applications. The mechanical properties of Inconel 718 also make it an ideal choice for the 3D metal printing of race engine parts that must perform well in high-temperature, high-stress environments.
Tested and validated
GE Additive has access to a validated powder supply chain with full traceability of every powder batch delivered. All powders are extensively tested before delivery to customers. This includes ensuring that the EBM and LaserCUSING parameter settings (process themes) are optimized to work well with the metal powder used.
And with AP&C’s proprietary Advanced Plasma Atomization (APA™) Technology, the melting wire never comes into contact with any solid surface, preventing contamination and ensuring a high purity product. The APA uses plasma torches to melt and atomize the metal wire feedstock, allowing for an accurate feeding rate with excellent control over powder size distribution and batch-to-batch consistency.