Electrochemical surface smoothing of spark erosion treated Zr-based bulk metallic glasses in NaCl-ethylene glycol electrolyte

So far, many investigations have reported that Zr-based bulk metallic glasses (BMGs) could be precisely and flexibly shaped by electrical discharge machining (EDM). However, EDM induced crystallization and roughness is still a bottleneck. In this research, bench-mode electrochemical surface smoothing (ESS) in the NaCl-ethylene glycol (EG) electrolyte has been proposed to post-process the spark erosion treated Zr-based BMGs in order to solve the abovementioned problems. The utilization of the NaCl-EG electrolyte overcomes the difference in electrochemical dissolution mechanisms of multiple components contained in Zr-based BMGs. Taking a sort of Zr-based BMGs, Vit1 (Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5), as an example, voltammetry and chronoamperometry methods have been performed on the BMG surface before and after wire-EDM, respectively, in the NaCl-EG electrolyte to reveal the steady and transient electrochemical behaviors. In order to achieve high-efficiency and high-quality ESS of Zr-based BMGs, a 3D printed setup with lateral flushing has been developed, combined with the micron sized inter-electrode gap (IEG) and the pulse current. The surface morphologies after ESS have been compared to evaluate the effects of the initial surface morphologies, dwell time, and pulse parameters. The surface material properties and mechanical performance before and after ESS have also been investigated by EDS, XRD, XPS, and nanoindentation. The results suggests that the proposed ESS method is capable of quickly eliminating the crystalline layer induced by the thermal shock and reducing roughness caused by the discharge craters, which has been finally validated by fast ESS of two Zr-based BMG molds with ruled surfaces in the NaCl-EG solution.