Effect of Surface Oxides on the Melting and Solidification of 316L Stainless Steel Powder for Additive Manufacturing

Abstract

Surface oxidation of metallic powders may significantly affect their melting and solidification behavior and limit their service life in the additive manufacturing (AM) process. In the present work, three levels of surface oxide concentration were prepared on AM-grade 316L stainless steel powders, and their melting and solidification behavior was systematically studied through in-situ observation, advanced characterization, phase-field modeling, and theoretical analysis. Si, Mn, and Cr participated in the oxidation reaction in powder with low and medium oxygen contents, whereas Fe was involved in the oxidation reaction for the powder samples with high oxygen content. A higher full melting temperature is observed to lead to an integrated melt pool in the melting of the highly oxidized powder, which is due to the reduced permeability produced by the oxide cage effect. For the droplet samples prepared from high oxygen powders, the inclusion with increased volume fraction and coarsened size is attributed to the agglomeration of inclusion particles with the residual oxide in the melt. In the high oxygen powder fusion scenario, an undesired coarse columnar grain structure with a high aspect ratio is formed in the current nonequilibrium solidification process, and a consistent microstructure is predicted using solidification conditions with a high cooling rate and high thermal gradient similar to the conventional AM process. In contrast, fine equiaxed grains in the experiment and slim columnar grains with a small aspect ratio in the phase-field simulation are obtained for the low oxygen powder condition. This study illustrates the effect of powder oxide from a processing aspect and provides insight into the importance of improving the service life of powder feedstock by effectively reducing the surface oxidation process on the powder surface.

Influence of the AlSi7Mg0.6 Aluminium Alloy Powder Reuse on the Quality and Mechanical Properties of LPBF Samples

“…The hypothesis based on the literature data about other powder materials [ 32 , 33 , 34 ] was that AlSi7Mg0.6 powder would degrade with each cycle: its surface would be oxidised and PSD would increase toward bigger particles [ 35 ]. Therefore, it was expected that the laser absorption would change due to the surface oxidation and PSD, impacting the process conditions and the final sample properties.…”

Towards Effective Powder Reuse in "Large Format Laser Aided Additive Manufacturing" (LAAM)

“…Apart from tight process control, the feed powders must be of high purity for successful implementation. This requirement tends to limit the reuse of powders collected after the 3D printing as its changes and impurities, such as oxides, can increase porosity of printed parts and impact the mechanical properties [3][4][5][6][7][8]. One commonly used approach is to monitor the powder use cycles, optionally combined with mechanical measurements on test parts made from the recycled powders [9,10].…”