There is increasing application of additive manufacturing of metal parts for aerospace, where control of defects is essential for part quality. In laser powder bed fusion, metal powder is melted locally – with the melt pool around 200 microns wide – to build the part from layers that are around 30 microns thick. My focus will be on two types of defects that can form as the part is built: pores caused by incomplete melting, and oxide inclusions. A simple geometric model – based on overlap of the melt pools – successfully predicts lack-of-fusion pores. Quantifying the boundary for full melting enables identification of process conditions that give faster building without porosity; an example will be shown with approximately double the build rate compared with standard build conditions. Formation of oxide inclusions is inevitable, because of residual oxygen in the build chamber. The oxygen pressure in the chamber is far above the equilibrium pressure for oxide formation, with the result that the oxidation rate is proportional to the oxygen pressure. While formation of oxide inclusions is inevitable, the concentration of oxides can be managed down by changing the melting conditions. Fatigue tests on samples of the AlSi10Mg alloy confirm that conditions that give fewer oxide inclusions do improve fatigue resistance.
About the Speaker:
Petrus Christiaan (Chris) Pistorius is POSCO Professor and Co-Director of the Center for Iron and Steelmaking Research in the Department of Materials Science and Engineering at Carnegie Mellon University. His research is mainly in the areas of pyrometallurgy and solidification; current interests include decarbonization of steelmaking and metals additive manufacturing. Previously he was an associate professor (1991-1996) and professor (1997-2008) in the Department of Materials Science and Metallurgical Engineering, University of Pretoria, South Africa, and served as the head of that department from May 2002 to June 2008. Chris has a Master's degree in Metallurgical Engineering from the University of Pretoria, and a PhD from the University of Cambridge, United Kingdom.