When you think of casting metals, you might imagine vats of molten metals and foundry workers in stuffy protective gear. Today, finished parts might instead be 3D-printed layer by layer from metallic powders with powder bed fusion additive manufacturing (PBFAM), a process that bypasses many typical foundry hazards. Thanks to PBFAM, precise metal parts can be turned out faster and more efficiently than standard methods.
“However, at present PBFAM mainly uses conventional alloying systems. To develop alloys for PBFAM, materials must first be formed into pre-alloyed powders via gas and plasma atomisation before their printability can be tested,” said Pan Wang, a Research Scientist at A*STAR’s Singapore Institute of Manufacturing Technology (SIMTech).
“Current alloy development processes are inefficient and unsustainable, using much costly raw material for low outputs,” Wang added. “To accelerate the development of advanced alloys for PBFAM, we need a different approach.”
With that goal in mind, Wang and his SIMTech colleagues teamed up with materials scientists from the National University of Singapore. They proposed pairing thermophysical calculations with an in situ alloying method within electron beam powder bed fusion (EBPBF), a common form of PBFAM. With EBPBF, high-powered electron beams melt spots on a metal powder bed to manufacture alloys layer-by-layer, giving researchers unprecedented control and speed in the process.
"In situ alloying minimises the material lost and supports sustainability,” said Wang, who led the study. He added that in situ alloying also optimised the alloy development process, turning multiple steps into a one-step affair that ensures the alloys have desirable printing environments during PBFAM. This also removed the operational costs for testing their printability.
Wang’s team tested their approach using a five-metal high entropy alloy (HEA) doped with increasing amounts of titanium. Using a computational tool called CALPHAD, they mapped how variations in titanium content influenced the resulting alloy’s physical properties.
“The HEA with the lowest titanium content had the best structural properties,” said Wang, adding that a tiny addition of titanium boosted the HEA’s hardness by around 200 percent.
According to Wang, the team’s biggest achievement was showing their new approach could easily 3D-print novel high-strength HEAs in intricate shapes. By combining in situ alloying with computational science techniques, the team also found they could speed up alloy discovery while reducing material waste: they successfully used the same batch of titanium powder to fabricate and test several different alloy formulations in their study.
The team’s ongoing work aims to realise the full potential of their HEA’s structural applications, with the goal of one day replacing similar but more costly alloys. This effort includes testing how their HEAs behave under harsh environmental conditions such as high temperatures or corrosive exposure.
The A*STAR researchers contributing to this research are from the Singapore Institute of Manufacturing Technology (SIMTech).