Processing of monolithic bulk metallic glass using sinking electrical discharge machining

Bulk metallic glasses (BMGs) are usually classified as difficult-to-cut materials due to their high hardness, high strength, and metastable atomic structures. How to process BMGs with good surface finish is crucial before having widespread structural-applications. In this work, the sinking EDM performance of a monolithic Zr₅₇Cu₂₀Al₁₀Ni₈Ti₅ (at.%) BMG was examined and compared with partially crystalized BMG, industrial pure zirconium and TC4 alloys under different processing modes (rough and refined conditions), dielectrics (EDM oil and kerosene), and varying processing parameters. The findings have shown that the BMG workpiece materials are more easily removed during sinking EDM than conventional alloys, resulting in relatively higher MRR, which is attributed to the lower melting point and thermal conductivity of BMGs. For example, under rough conditions, the average MRR of Zr57 BMG (1.23 mm³/min) was about twice the value of Zr702 (0.66 mm³/min). The EDMed surface roughness was affected by the processing mode, processing parameters, processing medium, and atomic structures, where the lowest surface roughness (Ra) of 0.58 μm was obtained. Carbonization occurred on the EDMed surfaces of BMGs due to the formation of ZrC, where crystallization and losses of parent elements were affected by the processing parameters, processing medium, and the partial crystallization of the workpiece materials. By changing the processing medium from EDM oil to kerosene brought in smoother surface roughness, however, more significant carbonization effect also occurred due to the decomposition of kerosene. Based on the EDM performance under varying processing conditions, the sinking EDM mechanisms were illustrated and discussed. The present findings have shown that although the metastable atomic structures of monolithic BMGs are affected by the discharge energy during sparking, the crystallization and carbonization effects can still be minimized under appropriate processing conditions. The present findings not only confirm the feasibility to process BMGs using sinking EDM technique but also provide a comprehensive understanding on the EDM performance of BMGs to achieve good surface quality.