Mechanisms of green foam water on the sustainability of upward sinking electrical discharge machining

An upward sinking electrical discharge machining (USEDM) technology based on green foam water is proposed, which adopts green foam water made of deionized water and air as the working fluid. USEDM has the advantages of high machining efficiency, no fire hazards compared with the current commonly used kerosene-based conventional sinking electrical discharge machining (CSEDM). In this paper, experimental and computer numerical simulation analysis methods were used to investigate the mechanisms of air microbubble size and air content in green foam water on the sustainability metrics and inter-polar dielectric breakdown characteristics. The results revealed that compared with pure deionized water, using green foam water with 50% air and air microbubble diameters of 20–40 μm for USEDM machining resulted in a 3.56x increase in material removal rate (MRR), a 63.40% reduction in relative electrode wear rate (REWR), a 51% decrease in maximum surface height difference (MSHD), and a 108% increase in relative energy utilization rate (REUR). Numerical simulations demonstrate that within a certain range of air content, the maximum inter-polar electric field strength increases as bubble diameter decreases and air content increases. This facilitates the breakdown of foam water within the machining gap. However, once air content exceeds a certain threshold, the foam water gradually transitions from liquid-phase-dominated to gas-phase-dominated, leading to a significant reduction in breakdown distance and USEDM performance. Machining Inconel 718 using green foam water as working fluid is not only friendly to the working environment, but also the waste liquid can be discharged harmlessly with simple filtration. The research results have important theoretical and engineering application value to promote the sustainable development of USEDM.