Estimation of wire strength based on residual stresses induced during wire electric discharge machining

Abstract

Frequent breakages of wire electrode during wire electric discharge machining (WEDM) adversely affect the productivity and surface quality of the finished products. Uncontrolled thermal loads and large thermal stresses generated during the discharge phenomenon reduce the strength of the wire which eventually leads to wire rupture. Thermal residual stresses retained in the wire can cause the formation of microcracks, voids and pits which increases the probability of wire rupture. Thus, it is essential to predict and prevent the wire breakage phenomenon for better machining productivity. This paper presents an approach to estimate the wire safety index based on thermal residual stresses that generate on the wire electrode by using a finite element method based model. A three-dimensional nonlinear transient thermo-mechanical model for a molybdenum wire used during WEDM has been developed to compute the temperature and the induced stresses. The influence of the wire velocity has been incorporated. The computational ability of the developed model has been verified by conducting systematic experiments. The experimental values of the residual stresses were obtained by using X-ray diffraction (XRD) technique. The stresses produced on the wire were observed to reach values beyond the yield stress (600 MPa) of the molybdenum wire at certain process conditions, which causes the reduction in strength of the wire. The wire safety index was evaluated by calculating a ratio (X) between the maximum value of residual stress induced and the yield stress of the wire. Formation of microcracks on the wire surface was considered as an indicator of wire breakage to estimate the threshold value of wire strength beyond which the wire is considered to be unsafe for machining.