Additive manufacturing (AM) of metal gears is in its nascency, but Integrated Computational Materials Engineering (ICME) technologies have served an integral role in understanding, evaluating and designing material microstructures and heat treatments specifically tailored for the unique processing conditions of 3D-printed gears. Applying their Materials by Design methodologies, QuesTek Innovations has expanded their ICME framework under US Army Small Business Innovation Research (SBIR) funding to adapt their high-performance Ferrium C64 gear steel to AM processes to demonstrate printability across multiple systems, achieve AMS minimum tensile properties, and observe a positive response to heat treatment.

As a wrought and forged product, Ferrium C64 steel exhibits a unique combination of high strength, high toughness, high hardness, high fatigue life, and high-temperature resistance, and it has been aerospace-qualified and adopted as a proven material for demanding gear and transmission components. This alloy is beginning to replace common gear steels such as 9310 and Alloy 53, allowing for lightweighting, increased power density and improved thermal stability.

Additively manufactured Ferrium C64 single tooth bending fatigue test gears. (Courtesy of QuesTek)

Starting in 2016, QuesTek began to evaluate and demonstrate Ferrium C64 in AM processing. It has been successfully demonstrated in laser powder bed fusion additive manufacturing equipment, and printed components have exhibited improved performance over AM forms of 17-4PH, maraging, and tool steels. It has been atomized several times in >500 lb batches, and test pieces have been repeatedly printed on EOS machines for mechanical testing and microstructural evaluation.

The strength and elongation have been found to be nearly the same as C64 forged bar, and while there is a slight debit in fracture toughness, initial axial (ASTM E466) and single-tooth bending fatigue test results are comparable to the forged stock, making atomized Ferrium C64 steel one of the most advanced alloys in AM for gears.

QuesTek is evaluating and demonstrating AM Ferrium C64 for aerospace gears and fatigue-driven applications as well as racing transmission gears, gears with integral bearing races, and other power trans­mission components where durability, compactness, lightweighting, high-tem­perature resistance and/or high surface fatigue resistance are the top priorities.

Technology readiness levels (TRLs). (Courtesy of NASA)

When Gear Technology asked about the technology readiness level (TRL) of Ferrium C64 for AM, Jeff Grabowski, manager of business development at QuesTek, said, “I would put AM C64 at TRL 5 (several industrial-scale atomizing runs; prints at three different vendors and growing; printed and tested actual gear components; measured quite a bit of mechanical property data); and by mid-2023, we should be at a TRL 6.”

QuesTek does not manufacture powder, but they have worked on more than 75 projects in metallic AM to resolve technical and metallurgical issues that are known in the industry. The major issues are cracking of commodity alloys upon rapid cooling, as well as the modeling and fine-tuning of unique microstructures (both beneficial and detrimental phases) that subsequently form. They have used physics-based models and ICME technologies to optimize the composition and thermomechanical processing steps to design entirely new alloys across the metallic alloy spectrum including high-strength steel, stainless steel, aluminum, titanium, nickel, copper, magnesium, and tungsten. These efforts aim to combine alloy printability with performance.

The following properties have been achieved with AM Ferrium C64 that was heat-treated: 200 ksi yield strength, 230 ksi ultimate tensile strength, 18% elongation, 58 ksi-√in fracture toughness and surface hardness of 64 HRC.

The promise of AM, in general, has included the potential to manufacture complex geometries such as internal cooling or lubrication channels; reduce gear system inertia through the use of advanced designs that are difficult to manufacture conventionally; improve durability by the use of multiple, optimized materials in a single part; change the cost of manufacturing by only placing material where it’s needed; reduce product development time and time to market; improve safety and repeatability; and assist humans with aids and tools.

For the Army, lead time for manufacturing gears to test in Science and Technology (S&T) prototype demonstrators could sometimes take 18 months and required costly special tooling. Thus, the Army needed a way to develop a new or improved AM process for prototyping aerospace gears. Commonly available additive steels for printing are not good candidates for gears.

The particular promise of AM Ferrium C64 has been its far-reaching implications for aerospace and other industries. These developments promise either the ability to enable those improved designs and/or foster rapid prototyping to reduce lead times in the qualification and deployment of future systems. Ferrium C64 is currently licensed out to Carpenter Technology, which produces and sells the alloy in its forged and atomized forms.

While most of QuesTek’s effort has been on laser powder bed fusion processes, Grabowski said, “We have a new funded effort  where we are doing DED/powder-blown, binder jet, and wire AM trials in the next 6–12 months. We are currently achieving nearly the same properties in AM as we do in a forged bar, and are in the process of doing trials on C64 of a slightly modified chemistry to take advantage of the AM process for even further improved performance.”