Microstructure and machinability evaluation in micro milling of selective laser melted Inconel 718 alloy

Abstract

Selective laser melting (SLM) of Inconel 718 (IN718) alloy is common these days. The microstructure and surface integrity of fabricated components are highly depend on SLM parameters and post-processing processes. Micro milling is a promising solution to achieve good surface finish. However, the machinability of SLM fabricated IN718 is different than the wrought IN718 due to their significant differences in microstructure and properties, thus technical parameters applicable for wrought IN718 are unsuitable to SLM fabricated IN718. Therefore, the microstructure and machinability of SLM fabricated IN718 during micro milling processes were evaluated in this work, and comparisons with wrought IN718 were made. Micro milling experiments with varied spindle speed and feed rate were performed and comparisons between tool wear, surface roughness, residual stress and micro hardness were made. Further, the microstructure of SLM fabricated and wrought IN718 before and after machining were compared in terms of grain geometry and crystallographic texture. SLM fabricated IN718 shows slight grain coarsening and texture weaken behaviors, which is different with the obvious grain coarsening and slight texture strengthen phenomena of wrought IN718. The SLM fabricated component has lower compressive residual stress as compared with wrought alloy, and the tool wear, surface roughness and micro hardness values are higher while micro milling wrought IN718 alloy. The reasons leading to these differences were analyzed from the perspectives of technological characteristics and material properties. Observed phenomena and clarified mechanisms can be used as basis to determine suitable technological parameters of micro milling SLM fabricated IN718 alloy.