Experimental study on surface integrity of Ti-6Al-4V machined by LS-WEDM

Titanium alloys have been widely used in the aerospace industry due to its outstanding properties. However, the poor thermal conductivity and high chemical reactivity impair titanium alloys machinability in conventional machining, which make it to be one of the typical difficult-to-machine materials. Electrical discharge machining (EDM) becomes the best alternative to machine titanium alloys. This paper focuses on investigating surface integrity of low-speed wire electrical discharge machining (LS-WEDM) in machining Ti-6Al-4V (TC4) by multiple cuts namely one main cut (MC) followed by trim cuts (TC). The machining parameter levels of multiple cuts and offsets were modified aimed at TC4 material, and the surface roughness of 0.67 μm was obtained after one MC and three TC. In addition, scanning electron microscopy (SEM) was used to observe the surface microstructure, which is characterized by an uneven fusing structure, spherical droplets, irregular droplets, craters, cracks, and micro-void; Moreover, it can be observed that cracks usually began with the edge of micro-voids, and the debris on the machined surface were deformed fragments due to the low thermal conductivity makes TC4 material be ejected or solidified before completely melting. Furthermore, the foreign elements of Cu and Zn were detected in the white layer by energy dispersive spectrograph (EDS) integrated in SEM; it also can be found that the crater edge has more Cu and Zn elements than crater center. The white layers were predominantly nonuniform and discontinuous due to poor penetration hardening of TC4 material. After the third TC, the white layer has become more continuous and the thickness was reduced to 2.7 μm, which was nearly invisible. The hardness of the white layer was almost the same as the base material. Finally, the blueviolet film was observed on the TC4 workpiece surface due to the electrolysis making the surface oxidation. By using X-ray diffraction (XRD), it is confirmed that TiO2, Ti2O3, and TiO existed in the oxidation film. The technique and knowledge that this study proposed could provide a significant contribution to electrical discharge machining surface improvement.