March_2022_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A R C H 2 0 2 2 2 5 market trends as they prioritize which materials to qualify next. AN EXPENSIVE JOURNEY TOWARD CERTIFICATION OF AM MATERIALS Beyond adherence to specifica- tions, flight certification is the holy grail for L-PBF parts in manned aircraft. Up to now, the major commercial aircraft manufacturers (and the larger suppli- ers that serve them) have gone it alone, developing proprietary internal materi- al specifications for producing AM parts certified for flight. Part of the reason that L-PBF has been limited to major aerospace manufacturers is due to the massive cost of material characteriza- tion; the other part is limitations in the regulations. For example, FAA 14 CFR 21.1 only permits airframe, engine, and propeller type certificate holders to create their own material specs and allowables. For everyone else there are no FAA-recog- nized specs or allowables. Regardless of the fidelity of the data provided, MRO providers and smaller aviation-related companies encountered significant cost and certification challenges that held back the widespread adoption of AM. However, a robust data set— MMPDS Volume 2 for Additive Materials — is soon to be released, enabling com- panies of all sizes to embrace AM with- out going to such lengths of time and expense. The FAA, DOD, and NASA all recognize MMPDS (Metallic Materials Properties Development & Standard- ization) as an industry-based source for design allowables referencing a ma- terial spec, typically AMS (Aerospace Material Specifications, from SAE In- ternational), that can be used for parts and repairs. The initial alloy to be certi- fied for additive applications by MMPDS is Alloy 718 (an Inconel), the workhorse material in jet engines that’s proved its worth in traditional manufacturing for years. (Next up are likely titanium and aluminum.) The first L-PBF equipment maker to have submitted initial data for Alloy 718 to MMPDS consideration is Velo3D. The list of material properties that is being characterized is comprehensive and includes tensile and compres- sive strength (including stress-strain curves), elongation, as-printed fatigue, bearing strength, shear, and creep/ stress-rupture. For tensile, modulus, and fatigue properties, these will not only be at room temperature, but up to the typical maximum operating tem- peratures. This will create a data set comparable to what exists in MMPDS Volume 1 for wrought Alloy 718. EXPLORING THE VALUE OF MATERIALS FOR AM AHEAD OF FORMAL CERTIFICATION Although this level of exactitude is desirable for any application involv- ing flight, unmanned rocket, drone, and satellite-launch companies are already enthusiastic about the robustness of the parts they’re seeing emerge from the more-advanced L-PBF systems. Ahead of MMPDS certification, a vari- ety of materials are being successfully qualified and employed by these users: Titanium 6Al-4V (in drone engines for light weight and strength), Aluminum F357 (ideal for heat exchangers), and Hastelloy X (Hast-X, employed in com- bustion-zone gas turbine engines to re- sist high-temperature stress-corrosion cracking and oxidation) (Fig. 2). For example, SpaceX, which owns numerous Sapphire AM systems from Velo3D, produces multiple 3D-printed parts for the Raptor rocket engines that will power its flagship launch vehicle, the Starship. The Starship is planned to deliver supplies to the International Space Station, launch the Starlink con- stellation, and carry missions to the Moon and Mars. Boom Supersonic, a project that’s reinvigorating supersonic jet travel at a more technically sophisticated level than the previous Concorde aircraft, is 3D printing vanes, ducts, louvers, and more from titanium. And back on planet Earth, Purdue University’s Zucrow Laboratories are using 3D-printed parts in a giant exper- imental burner that’s creating a hyper- sonic-flowenvironment in a ground-test cell. Researchers are proving-out a vari- ety of injector geometries for a combus- tor made out of Hast-X, one of the few high-strength, high-temperature super- alloys that can withstand the extremes of the hypersonic environment. THE ALUMINUM F357 EXAMPLE Developing an optimum L-PBF compatible material for such exploits can take some doing, as demonstrated by the journey that L-PBF has taken over the years to arrive at Aluminum F357. Some of the original success in 3D printing aluminum came with AlSi 12 , an alloy that is 12% silicon, a fairly high proportion. The Si serves to increase the flowability of the AM meltpool (where the laser meets the metal pow- der), and also to decrease the amount Fig. 2 — Whole and cut views of Sierra Turbines’ microturbine, 3D printed in Hastelloy X.