AMT Tech Report Selection by Stephen LaMarca:
The University of Washington recently published a study concluding that Ti6Al4V AM powder can be reused up to 30 times before significantly increasing in porosity. “Powder reuse has become a central issue in the pursuit to industrialize metal Additive Manufacturing (AM). Furthermore, build porosity is a critical concern to component reliability and damage tolerance of the metal.However, the contributions of powder reuse to metal porosity has received very limited attention.” Reuse good; porosity bad.
In a study recently published by a team of researchers at the University of Washington, the porosity resulting from powder bed fusion-electron beam melting (PBF-EB) AM of Ti6Al4V was characterized over a series of 30 build cycles (consisting of ~ 480 h cumulative build time) using X-ray Micro Computed Tomography (CT). The study showed that the AM powder can be reused up to 30 times with no significant increase in porosity.
Powder reuse has become a central issue in the pursuit to industrialize metal Additive Manufacturing (AM). Furthermore, build porosity is a critical concern to component reliability and damage tolerance of the metal. However, the contributions of powder reuse to metal porosity has received very limited attention.
The study investigated the volume fraction of pores (i.e. the porosity), as well as the pore size, shape, and spatial distribution. The most prevalent pores identified were: those originating from the gas atomized powder, and those caused by incomplete fusion of the melt pool. Although there was a minor reduction in the pore size distribution with powder reuse, the overall average volumetric porosity was 0.10 ± 0.02% and there was no significant change with increasing reuse.
Pore density maps within the cylindrical specimens for metal of (a) b1, (b) b10, (c) b20 and (d) b30, respectively.
In addition, an inverse relationship between pore diameter and sphericity was found, with large pores posing greater effective stress concentration. Whereas the greatest pore density was located at the transition between the contour and melt beams, approximately 0.5–1.0 mm adjacent to the surface of parts, the largest pores were located within the interior hatching region.
Hence, despite progressive deformation of the particles and an increase in oxidation, there was no discernable change in metal porosity with powder reuse in PBF-EB AM of Ti6Al4V. While the exact powder quality and porosity requirements vary significantly according to the specific applications, with medical and aerospace applications often implementing very strict regulations, the truth is that there is still not that much literature on powder quality after reuse and the overall assumption that it cannot be remelted and reused several times sometimes appears dogmatic rather than based upon proven facts.
Reusing AM powders, especially highly spherical powder necessary for PBF processes, would be a key element in reducing overall part costs as the AM industry transitions towards serial production.