Effect of powder characteristics and layer thickness on the densification of Selective Laser Melted CoCrMo

Abstract

Selective laser melting (SLM) is a metal additive manufacturing (AM) process that fabricates parts according to a CAD design in a layer-by-layer fashion by melting metal powder. There are a vast number of powder-related parameters in SLM, including the powder size, shape, distribution, and layer thickness. These parameters have a direct impact on the laser-powder interaction during melting and can have an influence on the final part quality. The aim of the study is to investigate the effect of extrinsic powder characteristics and layer thickness on selective laser melted CoCrMo parts. The research investigates the effect on final part density by firstly, using two CoCrMo powders with different particle size distributions, secondly, varying layer thickness with constant laser energy densities and lastly, measuring the consistency of part density at different locations on the build plate. In this study, the laser processing parameters were kept constant (laser power (LP), hatch distance (HD), and scan speed (SS)) whilst two layer thicknesses were used. The two CoCrMo powders used were comprehensively studied in terms of their extrinsic powder characteristics and their influence on final part density. The powder was measured across the build plate at various locations for each build to investigate the consistency of the powder across the build plate. The powder consistency was evaluated through powder bed density (PBD) samples at each build location, and from the samples, the particle packing efficiency and extrinsic powder characteristics were identified. The part density consistency was investigated through positioning cuboid samples at various locations on the build plate. From determining the consistency of the powder characteristics and part density across the entire build plate, correlations between the two results were identified. The results show that there is a difference in final part density when different powder grades were used at consistent SLM processing parameters. Potential reasons for the effect of build location on part density were discussed. The powder with a wider particle size distribution due to having more fine particles showed an increased packing efficiency. It was found that the finer powder deposited a more consistent powder layer across the build plate, which led to a more consistent part density. Spatter was found to have an influence on the powder characteristics and part density at various build locations. When the layer thickness increased, the influence of the powder characteristics on the final part density was exaggerated, and an increase in porosity within the final parts was identified. The findings of this study highlight the importance of powder-related parameters in SLM on the final part density. A valuable understanding of how powder at specific locations on the build plate can influence the part density was established. Based on the findings it is recommended to investigate multiple test samples at various build locations to accurately determine the part quality per build.