On Influence of Processing Parameters on Surface Roughness and Microstructure of Hastelloy X Produced by Selective Laser Melting (SLM)

Selective laser melting (SLM) technology is a layer-wise powder-based additive manufacturing method capable of building 3D components from their CAD models. This approach offers enormous benefits for generating objects with geometrical complexity. However, due to the layer-wise nature of the process, surface roughness is formed between layers, thus influenced by layer thickness and other processing parameters. In this study, systematic research has been carried out to study the influence of processing parameters on surface roughness and microstructure in Hastelloy X alloy and samples used were manufactured using EOSINT M 280 machine. Laser power, scan speed, layer thickness and sloping angle of a surface were systematically varied to understand their effects on surface roughness. The arithmetic average roughness, Ra, was measured by surface roughness tester and an optimum condition for achieving the lowest roughness for upper surface (slope angle>90°) and down surface (slope angle<90°) has been obtained, respectively. The formation mechanism for the roughness on these two types of surfaces has been studied. Computer simulation was also used to understand thermal profiles at those two surfaces and their resultant influence on surface roughness. The simulated result has been found to be consistent with the measured. Microstructural characteristics (molten pool morphology, grain size and texture) were examined using optical and scanning electron microscopy. Some samples have been subjected to post heat treatment or post HIPping (hot isostatic pressing). A distinctive bimodal grain structure as well as grain recrystallization were observed for heat-treated samples while homogeneous microstructure with all porosity closed was achieved following HIPping. Tensile tests suggested that the yield and ultimate tensile strengths of the as-fabricated samples (650 and 830 MPa, respectively) were higher than those of the heat-treated (470 and 780 MPa) and HIPped samples (360 and 740 MPa).