Printable Alloys: Which Metals Work in AM?

## What Makes an Alloy Printable In laser or electron beam powder bed fusion, the alloy experiences cooling rates of 10⁴ to 10⁶ degrees C per second — orders of magnitude faster than casting. This extreme thermal cycle creates unique metallurgical challenges: **Solidification cracking**: Alloys with a wide freezing range (large gap between liquidus and solidus) are prone to hot tearing as the last liquid to solidify is pulled apart by thermal contraction. This is the primary failure mode for many conventional alloys in AM. **Residual stress cracking**: The rapid cooling generates steep thermal gradients that create high residual stresses. Alloys that form hard, brittle phases (such as martensite in high-carbon steels) may crack under these stresses during the build. **Reflectivity and absorptivity**: Highly reflective metals (copper, gold, silver) absorb laser energy poorly at the 1070 nm wavelength of standard fiber lasers. Green (515 nm) and blue (450 nm) lasers have been developed specifically to process copper. ## Well-Established AM Alloys ### Titanium: Ti-6Al-4V The most printed metal alloy in the world. Ti-6Al-4V responds well to PBF because its narrow freezing range minimizes solidification cracking, and the rapid cooling produces a fine acicular alpha-prime martensite that, after stress relief at 600-800 degrees C, transforms to a fine alpha-beta microstructure with mechanical properties matching or exceeding wrought material. As-built L-PBF Ti-6Al-4V typically achieves tensile strength of 1050-1200 MPa and elongation of 6-10%, meeting ASTM F3001 requirements for surgical implants. ### Nickel Superalloys: Inconel 718 and Inconel 625 Inconel 718 is the second most printed alloy. Its relatively low aluminum and titanium content (compared to gamma-prime strengthened alloys) gives it good weldability and, by extension, good printability. Post-build heat treatment (solution + double age) produces tensile strengths above 1200 MPa. Inconel 625 is a solid-solution-strengthened alloy that prints reliably with minimal cracking. It is used for chemical processing, marine, and oil and gas AM components. ### Stainless Steels: 316L and 17-4PH 316L is one of the easiest alloys to print by any AM process. Its fully austenitic structure resists cracking, and as-built parts achieve >99.5% density routinely. Mechanical properties of L-PBF 316L (UTS 600-700 MPa, elongation 30-50%) exceed wrought annealed 316L due to the fine cellular-dendritic solidification structure and dislocation networks. 17-4PH (precipitation-hardened martensitic stainless) prints well but the as-built microstructure is a mixture of austenite and martensite (unlike the fully martensitic wrought condition) due to the rapid cooling. Solution treatment at 1040 degrees C followed by aging at 480 degrees C (H900 condition) produces UTS of 1200-1300 MPa. ### Aluminum: AlSi10Mg and A357 AlSi10Mg is the dominant aluminum AM alloy. The near-eutectic Al-Si composition provides a narrow freezing range and excellent fluidity, minimizing cracking. As-built L-PBF AlSi10Mg achieves UTS of 350-450 MPa (significantly higher than die-cast A360 at 317 MPa) due to the ultrafine silicon network formed by rapid solidification. The trade-off is limited elongation (3-8%). Scalmalloy (Al-Mg-Sc-Zr), developed by Airbus APWorks, uses scandium-zirconium nano-precipitates to achieve UTS above 500 MPa in the aged condition, making it the strongest aluminum AM alloy available. ### Cobalt-Chrome: CoCr (ASTM F75) CoCrMo alloys print well by both L-PBF and EB-PBF. The primary application is dental prosthetics (crowns, bridges, partial denture frameworks) where AM has almost entirely replaced lost-wax casting due to superior fit, faster turnaround, and lower cost per unit. ## Challenging Alloys ### High-Strength Aluminum: 7075, 2024, 6061 The most commonly used wrought aluminum alloys (7075-T6, 2024-T3, 6061-T6) are extremely difficult to print by fusion processes. Their wide freezing ranges and high thermal contraction cause severe solidification cracking. Multiple research groups have demonstrated crack-free printing by adding grain-refining nanoparticles (TiC, ZrH₂) to the powder, but these modified alloys are not yet widely commercially available. 6061 is slightly more tractable than 7075, and some machine manufacturers now offer parameter sets, but cracking remains a risk on larger cross-sections. ### Nickel Superalloys: CM247LC, IN939, Hastelloy X Gamma-prime strengthened superalloys with high aluminum and titanium content (CM247LC: 5.6% Al, 1.0% Ti) crack extensively during L-PBF due to strain-age cracking in the solidifying and re-heating layers. EB-PBF, with its high preheat, can print these alloys successfully because the slower cooling reduces residual stress below the cracking threshold. Hastelloy X was initially difficult to print due to hot cracking along grain boundaries, but optimized parameter sets with reduced energy density and modified scan strategies have made reliable L-PBF production possible. ### Tool Steels: H13, M2 H13 hot-work tool steel can be printed by L-PBF but requires immediate tempering or platform preheating (200-400 degrees C) to prevent cracking from the hard as-quenched martensite. M2 high-speed steel, with 0.85% C and high alloy content, cracks severely during L-PBF without preheating and is better suited to binder jetting followed by sintering. ### Pure Copper Copper's high reflectivity at 1070 nm means standard fiber lasers couple poorly with the powder. Green lasers (515 nm, offered by Trumpf) and high-power infrared lasers (>500 W) have made pure copper printing practical, achieving >99.5% density and electrical conductivity above 95% IACS. Applications include induction coils, heat exchangers, and rocket engine combustion chambers. ## Alloy Development for AM The AM industry is moving beyond adapting existing wrought and cast alloys toward designing alloys specifically for AM thermal cycles. Design principles include narrow freezing ranges, grain-refining additions (Zr, Sc for aluminum; carbon, boron adjustments for nickel), and compositions that produce ductile as-built microstructures without mandatory post-processing.