Experimental investigation of laser scan strategy on the microstructure and properties of Inconel 718 parts fabricated by laser powder bed fusion

Abstract

Inconel 718 (IN718) is a nickel-based superalloy which exhibits excellent tensile and impact resistant properties along with good corrosion resistance at high temperatures. However, due to the high toughness and work hardening, the machinability of this superalloy is low. Therefore, the selective laser melting (SLM) process has been adopted as an efficient technique to fabricate IN718 parts as it overcomes the problems associated with conventional manufacturing of superalloys. In the SLM process, various process parameters like scan strategy, laser power, scan speed, and energy density are defined for the fabrication to regulate the microstructure and thus control the mechanical properties like tensile strength, yield strength, impact strength, and hardness. In the SLM process, the thermal cycles induce residual stress into the part. These residual stresses can be detrimental to the microstructure and hence mechanical properties of the part. Residual stresses lead to warping of the part during the fabrication process, thereby leading to failure of the component. Although each process parameter has an independent and definitive effect on the overall mechanical and metallurgical properties, scan strategy is an independent process parameter which directly affects the level of residual stresses, microstructure, and mechanical properties of the SLM part, as the heat zones in part can be shifted from one location to another by varying the scan strategy. This variation in the area of the heat-affected zone determines the grain size ranging from equiaxed to elongated. Hence, the scan strategy is the only parameter that is varied for this study. The various scan strategies adopted here are chess, striped, flow-optimized, and customized scan strategies. In this study, the microstructural evaluation was deduced, followed by compositional analysis and hardness tests on the SLM fabricated parts. The residual stress of the parts was then determined using the hardness method. This effort was undertaken to identify the effect of scan strategy on residual stress and to discuss the metallurgical interactions between the mechanical and microstructural properties within the IN718 superalloy.