Development of Cylindrical Wire Electrical Discharge Machining Process and Investigation of Surface Integrity and Mechanical Property of EDM Surface Layers.

Abstract

The cylindrical wire Electrical Discharge Machining (EDM) process was developed to generate precise cylindrical forms on hard, difficult-to-machine materials. A precise, flexible, and corrosion-resistant underwater rotary spindle was designed and added to a conventional two-axis wire EDM machine to enable the generation of free-form cylindrical geometries. A detailed spindle error analysis identified the major source of error at different frequencies. The mathematical models for material removal rate and surface finish were derived. Experimental results indicated that higher maximum material removal rate might be achieved in the cylindrical wire EDM than the 2D wire EDM. Effects of some key process parameters, wire feed rate, pulse on-time and part rotational speed, on the surface finish and roundness are explored. For WC-Co parts, an arithmetic average surface roughness and roundness as low as 0.68 and 1.7 mm, respectively, can be achieved. Surfaces of the cylindrical EDM parts were examined using Scanning Electron Microscopy (SEM) to identify the macro-ridges and craters on the surface. Cross-sections of the EDM parts are examined using the SEM to quantify the recast layer and heat-affected zone under various process parameters. This study also used nanoindentation to investigate the influence of cylindrical wire EDM process on the mechanical properties of WC-Co composite. Multiple indents were conducted on the cross-section of the recast layer, heat-affected zone, and bulk material. The SEM micrographs were used to correlate the individual nano-indent to the measured hardness and modulus of elasticity. The experimental results showed that the heat-affected zone had more compact microstructure less indentation cracking. The recast layer had lower hardness and modulus of elasticity than the original material and heat-affected zone. EDS X-ray and X-ray diffraction were used to analyze the material compositions of the heat-affected zone and recast layer and to understand the effects of wire EDM process on the machined WC-Co surface. The micro-blasting process has been applied to improve the surface integrity of the part machined by wire EDM. The surface roughness of WC-Co parts could be reduced significantly using micro-size SiC abrasives. The surface finish improvement was also shown by the SEM micrographs of the blasted surfaces and their cross-sections. The undesired recast layer, which has poor mechanical properties and irregular features, was eventually removed by micro-blasting. Compressive residual stress introduced by micro-blasting could increase the surface wear resistance. Experiments with different process parameters were conduced to study the efficiency and effectiveness of micro-blasting for surface property control.