Prediction of crater induced failure of coated wires during wire EDM of Ti-6Al-4V alloy

Coated wires are widely used in wire electric discharge machining (WEDM) due to their better performance in terms of material removal, cutting speed and excellent sparking abilities as compared to uncoated wires. Prediction and restriction of the wire breakage phenomenon are however crucial in order to achieve sustainable and efficient machining. In the present work, a three-dimensional thermo-mechanical finite element model (FEM) of spark generation on a coated wire tool has been developed by considering realistic assumptions like moving heat source characteristics of spark plasma, Gaussian heat source, current and discharge duration based spark radius equation. The numerical model was well validated with experimental results conducted in a High-speed wire EDM machine with reciprocating wire travel movement. The temperature and stresses generated in the wire at different sets of process conditions are computed. The craters generated due to a variety of combination of process parameters were thoroughly analysed and a methodology has been developed to predict the failure of coated wires. The wire strength is estimated based on the amount of material removed from the zinc coated brass wire tool. It was noted that the wire experiences failure when the crater volume exceeds a threshold value during the machining operation. This value of crater volume was considered as the threshold limit or the wire safety factor to avoid unwanted wire failure. In the end, parametric studies to investigate the effects of input parameters on the temperature, residual stresses and crater volume generated in the wire were carried out.