Experimental Investigation and Optimization of Wire Electrical Discharge Machining for Surface Characteristics and Corrosion Rate of Biodegradable Mg Alloy

Wire electrical discharge machining (Wire EDM) is a spark erosion process that modifies the surface characteristics by creating overlapped craters and oxides on the machined surface. This oxide and metamorphic layer can be advantageous for Mg alloy in terms of improved corrosion resistance and osteoblast activities. In the current work, face-centered central composite design has been used to carry out the experiments on Wire EDM to study the influence of process parameters and to generate the correlation between input parameters and performance characteristics of ZM21 Mg alloy. Performance characteristics chosen for Mg alloy machining are cutting speed, surface roughness and corrosion rate. To identify the important input parameters and a numerical model that fits the response characteristics, analysis of variance has been used. Using a scanning electron microscope (SEM), it has been found that Wire EDM resulted in overlapped craters and formation of µ-cracks on the machined surface, influenced by discharge energy developed across the electrodes. SEM and XRF (x-ray fluorescence) analysis confirm the formation of a metamorphic layer on the machined surface which leads to corrosion resistance improvement as compared to polished ZM21 Mg alloy. Under response surface methodology, desirability function was utilized to obtain the optimal solutions for multi-response characteristics which were validated experimentally and the sample machined at optimal setting shows improved surface morphology with an oxide layer having uniform nanoscale structure. Electrochemical impedance spectroscopy analysis (for 7 days) shows an increasing trend of corrosion resistance for machined samples, which supports the application of Wire EDM for Mg alloy implants.