Prediction of changes in the surface layer during copy-piercing electrical discharge machining

Introduction. This paper presents the results of theoretical and experimental studies of the thickness, continuity and number of defects in the white layer formed during the copy-piercing electrical discharge machining, using low-alloy steel 0.4 C-Cr and medium-alloy steel 0.35 C-Cr-Mn-Si as an example. The purpose of the work is to theoretically and experimentally investigate the defective surface layer formed during copy-piercing electrical discharge machining. Research methods. Mathematical models of a single discharge pulse impact on the surface being machined were obtained using the finite element method. An Electronica Smart CNC copy-piercing electrical discharge machine was selected as the equipment for copy-piercing electrical discharge machining of samples made from 0.4 C-Cr and 0.35 C-Cr-Mn-Si steels. Simulating and experiments were carried out in two modes. The operating parameters were: pulse-on time (Ton, μs), voltage (U, V), and current (I, A). Metallographic studies were performed using an OLYMPUS GX 51 optical microscope. Results and discussion. Mathematical models were developed to simulate the impact of a single discharge pulse on the machined surface during copy-piercing electrical discharge machining. These models allow predicting the thickness of the white layer depending on the processing modes and the properties of the material being processed. The theoretical values of the white layer thickness vary in the range of 20–25 μm during copy-piercing electrical discharge machining in the minimum mode and vary in the range of 60–80 μm in the maximum mode. It was established that the experimental values of the white layer thickness vary in the range of 20–25 μm during copy-piercing electrical discharge machining in the minimum mode and vary in the range of 55–85 μm in the maximum mode. The deviation of the theoretical values of the white layer thickness from the experimental ones differs by no more than 5 %, which confirms the accuracy of the obtained models. It was found that during copy-piercing electrical discharge machining in the minimum mode, the continuity of the white layer is on average two times greater than when machining in the maximum mode. The continuity of the white layer of 0.4 C-Cr steel is 10 % higher compared to 0.35 C-Cr-Mn-Si steel during copy-piercing electrical discharge machining in the maximum mode and 17 % higher during copy-piercing electrical discharge machining in the minimum mode. It was revealed that when machining in the maximum mode, the number of microcracks is more than 2 times greater than when machining in the minimum mode. The number of cracks in the white layer when processing chromium steels 0.4 C-Cr and 0.35 C-Cr-Mn-Si is comparable, the difference does not exceed 10 %.