OBRABOTKAMETALLOV Vol. 27 No. 1 2025 technology Currently, the formation process of the surface modified layer, as well as the extent of its continuity, and the mechanisms of microcracks formation on the surface of workpieces made of difficult-to-machine materials during copy-piercing electrical discharge machining (CPEDM) are not fully studied. The range of materials for which the white layer formation process during machining has been studied is very limited. An urgent task is to develop theoretical dependencies that allow predicting the transformation of the surface layer during processing, as well as the thickness, structure, and properties of the resulting modified layer. The aim of this work is a theoretical and experimental investigation of the defective surface layer formed during copy-piercing electrical discharge machining. Tasks: 1) to develop a theoretical model of a single pulse in the CPEDM, applicable to various materials, to predict the white layer thickness; to obtain theoretical values for the white layer thickness for chromiumcontaining steels 0.4 C-Cr and 0.35C-Cr-Mn-Si. 2) to conduct experimental studies of the white layer thickness formed during CPEDM to verify the developed models, using chromium-containing steels 0.4 C-Cr and 0.35C-Cr-Mn-Si as examples. 3) to conduct experimental studies of the continuity of the white layer after CPEDM of chromiumcontaining steels 0.4 C-Cr and 0.35C-Cr-Mn-Si. 4) to conduct experimental studies of the influence of CPEDM modes on the number of visible defects on the processed surface of chromium-containing steels 0.4 C-Cr and 0.35C-Cr-Mn-Si. Research methodology The experiments were conducted at the a Center for Collective Use “Additive Technologies Center” within the Department of Innovative Mechanical Engineering Technologies of the Perm National Research Polytechnic University. As part of the study, a unit discharge pulse on the processed surface was simulated using the finite element method (FEM). The model consists of three parts: 1) determining the temperature field in the part due to the action of a distributed heat flow; 2) modeling the formation of a pit; 3) determining the temperature field in the part during inter-pulse cooling. All tasks were solved using the finite element method (FEM) with an 8-node element in the ANSYS Mechanical APDL package. The following assumptions and hypotheses were adopted for the solution: isotropy of the workpiece material being processed; temperature-independence of the workpiece material properties; constant convective heat transfer coefficient; negligible energy losses due to changes in the aggregate state of the material; and a constant inter-electrode gap. The effect of a distributed heat flow on the workpiece-electrode surface is considered. When modeling a thermal pulse, the effect of a distributed heat flow on the workpiece-electrode surface is also considered. The heat conduction equation in an axisymmetric formulation for a transient (non-stationary) problem has the form: 2 2 1 T T T c k r t r r r z ∂ ∂ ∂ ∂ ⋅ ρ ⋅ = − ⋅ ⋅ ⋅ + ∂ ∂ ∂ ∂ (1) where r is the current radius; z is the current height. The ABCD region represents the area of action of the working pulse (Fig. 1). On the boundary AB, the condition of axial symmetry for the heat problem applies. The boundaries DE and EF are cooled by the working fluid (WF). As a first approximation, these boundaries are modeled as convective heat exchange surfaces, and the convective heat exchange coefficient is assumed to be constant. The condition on these boundaries takes the form: , ( ) DE EF T k h T T n ∞ ∂ − = − ∂ . (2)
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