Effect of laser focus shift on surface quality and density of Inconel 718 parts produced via selective laser melting

Selective laser melting (SLM) is a method of laser powder bed fusion additive manufacturing (AM) currently being pursued in numerous industries, including space launch and space flight. In this study we performed an extensive parameter development investigation to better understand the effect of laser parameters on surface roughness, density, and porosity of SLM Inconel 718 parts. Laser energy density was varied via laser focus shift, and the effects on porosity in both as-printed and post-HIP treated states were analyzed. Tensile testing was also conducted to investigate the effect of processing conditions on the mechanical properties of SLM 718. It was found that for these laser parameters, while the material met ultimate tensile strength and yield strength requirements per AMS 5662, the strain-to-failure was reduced with negative focus shift due to increases in porosity levels. It was also found that while correlations were observed between surface roughness, density, and porosity within the laser focus shift range investigated, porosity measurement appears to be the clearest indicator of build quality for AM processed 718.