On Heat Treatment and Surface Characterization of Spark Eroded Nickel-Based Superalloy Developed by Additive Manufacturing

In this study, a nickel-based superalloy Inconel 718 material was developed through direct metal laser sintering process. To enhance the mechanical properties, printed material was heat-treated at two different conditions in the combination of solution and ageing. The transformations of microstructure from original anisotropic phase were studied using optical microscope. The changes in mechanical properties were evaluated. The hardness of the heat-treated samples obtained is 35HRC and 39HRC for the heat treatment plan at 1100 °C + 845 °C and 980 °C + 720 °C (solution + ageing), respectively. Further, micro-holes were machined on these two material samples using spark erosion micro-machining or micro-electric discharge machining (µEDM) process. Applied voltage, capacitance and material hardness were three major variable parameters. Experiments were performed with hollow tungsten carbide rod of 300 µm in diameter as a drill tool rotating at a speed of 1500 rpm for a constant machining time of 30 min. Optical and scanning electron micrographs were used to analyse the hole quality and therefore the performance of µEDM. Perfect concentric drill hole was produced with 35HRC sample than the other. Weld spatters and metal drops were found around the surface of 39HRC sample. The range of drill hole varied to a maximum of ϕ385 µm in low-hardness and ϕ400 µm in high-hardness samples. The depth of drill hole was maximum with high hard sample at 100 V and 100 nF process condition. Light spectroscope-based roughness characterization revealed that the minimum surface roughness measured was 6.946 µm in low hard and 9.829 µm in high hard material.