Investigation and optimization of machining peculiarities of heat treatment and non-heat treatment X40CrMoV51 alloy by powder-suspended electro-discharge machining in final-finishing operation

X40CrMoV51 alloy in the heat-treated (HT) and non-heat-treated (NHT) states processed by electro-discharge machining (EDM) with suspended tungsten compound powder has been rarely investigated. Hence, this study explored and compared the effect of control parameters, including peak-current (I p ), pulse-on time (T on ), and powder amount (A p ) on material removal rate (MRR), tool wear rate (TWR), and surface roughness (R a ) of the two material states, and finds an optimum combination of control parameters for each material state for improving the MRR, R a , and TWR. With this goal, the predictive models for MRR ht , MRR nht , TWR ht , TWR nht , R aht , and R anht were instituted, and also evaluated by the analysis of variance (ANOVA). The Grey Relational Analysis (GRA), Non-Dominated Sorting Genetic Algorithm II with Evaluated by an Area-based Method of Ranking (NSGA II-EAMR, and multi-objective particle swarm optimization with Evaluated by an Area-based Method of Ranking (MOPSO-EAMR) algorithms were executed for enhancing the MRR, R a , and TWR. As a results, the influence of control parameters on MRR, TWR, and R a for each material state is dissimilar. Regarding optimization results, the GRA produced the highest MRR value (MRR ht = 0.0236 g/min and MRR nht = 0.0223 g/min). The NSGA II-EAMR offered the lowest TWR ht and R aht , with a reduction in TWR ht of 41.37% and R aht of 14.86% compared to GRA, and with a decrease in TWR ht of 1.01% and R aht of 2.01% compared to MOPSO-EAMR. Meanwhile, the MOPSO-EAMR suggested the lowest TWR nht and R anht , with a reduction in TWR nht of 45.49% and R anht of 10.16% compared to GRA, and with a decrease in TWR nht of 4.23% and R anht of 0.44% compared to NSGA II-EAMR. Additionally, the surface metallurgical features of two material states obtained from the optimum control parameters of NSGA II-EAMR and MOPSO-EAMR were explored. The results showed that the surface defects, micro-crack acreage percentage (MCAP), composition, content, and distribution of chemical elements, distribution and density of W 2 C phase, surface topography, thickness of recast layer (TRL) in two material states are different.