Effects of powder recycling on stainless steel powder and built material properties in metal powder bed fusion processes
Gregor Jacob, Christopher U. Brown, M A. Donmez, Stephanie S. Watson
A series of 11 identical laser based powder bed fusion (PBF) builds were completed with varying amounts of virgin and recycled nitrogen gas atomized S17-4 PH stainless steel powder following a specific powder recycling strategy that simulates industrial practice. Mechanical properties of parts were evaluated using tensile and hardness tests. Recycled powder properties, such as particle size distribution, flowability, chemical composition, and microstructure were evaluated. The recycled powder showed no significant changes in its particle size and particle shape but apparent density and powder bed density increased while flow time improved. Recycling the powder in a nitrogen atmosphere caused a slight increase of the martensite phase in the predominately austenitic S17-4 PH powder. PBF fabricated austenitic-/ martensitic S17-4 PH showed a ratio of approximately 50 % to 50 % between austenite and martensite. Tensile tests on the stress relieve heat treated specimens did not show any dramatic change in the tensile properties with recycling up to 11 times. The fine dendritic, austenitic / martensitic microstructure of the stress relieved heat treated S17-4 PH reached a tensile strength of YS0.2 above 520 MPa, and an elongation of 28 %. Mechanical and material properties from specimens fabricated from powder recycled up to 11 times were similar to specimens fabricated from virgin powder.