Metals in Additive Manufacturing
Although thermoplastics drew the most attention in the early days of 3D printing, material options continue to grow. Additive manufacturing (AM) now uses metals, ceramics, glass, composites, graphene-embedded plastics, paper, concrete, food, yarn and bio-inks - used to create artificial organs and soft tissues - among others. In 2017, NASA announced the successful testing of an igniter made of multiple metal alloys. This bi-metallic capability could reduce rocket engine costs by one-third and manufacture time by one-half. In myriad applications, metals and metal alloys have been revolutionary in AM and 3D printing.
Research continues to provide advances in AM, addressing challenges that define which materials can be used, including high melting points, layer thickness, print speed and production capacity. Researchers at Lawrence Livermore National Laboratory used AM to print composite silicone objects that exhibit shape memory characteristics - and this may only be the start.
Metal Additive Manufacturing Materials
High-quality metal powder and wire feedstock are very important for successful powder bed fusion in AM. A number of different metals are available in powdered form to suit exact processes and requirements. Titanium, steel, stainless steel, aluminium, and copper, cobalt chome, titanium and nickel-based alloys are available in powdered form as well as precious metals like gold, platinum, palladium and silver.
Wire feedstock options are also wide-ranging; steel and stainless steel alloys as well as pure metals like titanium, tungsten, niobium, molybdenum and aluminium are all available as wire feedstock.
Unalloyed, commercially pure titanium has many uses in additive manufacturing. It is available in grades one through four and its applications vary, depending on its grade. 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.
Waste reduction is a key advantage with expensive titanium and other alloys. Lead times are often reduced, and limited production runs are far more feasible. Popular Mechanics reports that Boeing estimates it could save $3 million per 787 Dreamliner if the potential of 3D-printed titanium parts is fully achieved. GE has built a functioning miniature 3D-printed jet engine with titanium components. A demonstrator engine tested by NASA sustained liquid hydrogen's minus 423 degrees F temperature, as well as the 6,000 degrees F temperature of the burning fuel.
In general, titanium alloys are used in additive manufacturing 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.
Stainless steel used in additive manufacturing 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. EBM technology uses powdered stainless steel to produce dense, super-strong, waterproof parts for extreme environments like jet engines, rockets and even nuclear facilities. For example, in 2016, a feasibility study explored the viability of using low-carbon stainless steel in EBM machines to produce nuclear pressure vessels. The 316L steel was selected because it is weldable, corrosion-resistant and extremely strong.
Aluminum is sintered in the Direct Metal Laser Sintering (DMLS) process or melted in the Selective Laser Melting (SLM) process. Fine detail down to 25 microns and wall thicknesses of as little as 50 microns are possible when aluminum is used. Parts typically have a textured, matte surface which distinguishes them from traditional milled aluminum parts. Due to its low weight, 3D-printed aluminum is used for automotive and racing parts.
Lightweight aluminum alloys for additive manufacturing 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.
It is possible to sinter powdered gold, silver, platinum and palladium for additive manufacturing in DMLM machines. Extremely fine metal powder is partially melted to create jewelry. At the conclusion of the process, the object is removed from the remaining metal powder, akin to an archaeological dig. Unique and beautiful pieces of jewelry feature interlocking or interwoven designs only possible with additive manufacturing. Forbes profiled a jeweler creating one-of-a-kind items with six-figure valuations, customized to customer preferences. To commemorate a Parisian honeymoon, one client requested and received a charm featuring a 3D-printed Eiffel Tower perched atop a pearl.
Cobalt Chrome Alloys
3D-printed parts are 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. Cobalt chrome alloys are used in additive manufacturing to print parts that 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.
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. The mechanical properties of nickel-based alloys used in additive manufacturing, such as 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. 718 is an age-hardened version of 625. The hardening process generates precipitates that better secure metal grains in place. 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.