Thermo-hydraulic-metallographic modeling and study of the thermotropic metamorphic layer in EDM of Ti-6Al-4V

Abstract An in-depth understanding of microstructure evolution of thermotropic metamorphic layer is the basis for effectively suppressing surface and subsurface defects in electrical discharge machining (EDM) of Ti-6Al-4V. In this work, the thermo-hydraulic-metallographic coupling model with successive pulse was established for the first time, based on which the visual simulation of time-variant phase transformation in recast layer and heat-affected zone (HAZ) was achieved. Further, the microstructure distribution characteristics in thermotropic metamorphic layer was investigated from the perspective of experimental observation. The results show that with the accumulation of successive pulse discharges, the temperature on processed surface gradually increases while the overall temperature distribution tends to be relatively uniform. The microstructure of EDMed surface is characterized by a layered distribution, in which the recast layer undergoes complete α′-Ti (martensitic phase) transformation, and HAZ is composed of upper layer with α′-Ti and lower layer with α+β+α′ mixture. Meanwhile, it was revealed that the microstructure in β grain of HAZ changed from staggered distribution of secondary α-Ti phase to parallel distribution of acicular α'-Ti phase. A transition layer composed of cellular martensite was observed between the recast layer and HAZ, and the internal grain showed a change from parallel arrangement to staggered arrangement as the peak current increased to a relatively larger level. The martensite volume fraction and microhardness of thermotropic metamorphic layer have also been verified to correlate to the peak current. The combined study of thermo-hydraulic-metallographic coupling model and insights into microstructure characteristics were expected to provide theoretical reference for predicting and controlling the surface integrity in EDM of Ti-6Al-4V.