Low-speed machining of a Zr-based bulk metallic glass

Abstract

The cutting performance of a Zr57Cu20Al10Ni8Ti5 (at.%) bulk metallic glass (Zr57 BMG) at low-speed machining was studied, as compared with a Zr48Cu47.5Co0.5Al4 (at.%) bulk metallic glass composite (Zr48 BMGC), commercial industrial pure zirconium (Zr702), titanium alloy (TC4) and 45 steel. The effect of cutting speed, cutting depth, and feed rate on the cutting forces and surface characteristics was examined. Despite with high hardness and strength, Zr57 BMG demonstrated smaller cutting force (Fz) than Zr48 BMGC and Zr702 during machining, resulting from thermal softening. The morphology, roughness, and crystallization on machined surfaces were also investigated. The findings show that Zr57 BMG exhibited excellent machinability at low-speed cutting with excellent surface roughness and no crystallization on machined surface. Scale-like patterns on the machined surface were observed, due to the plastic flow of Zr57 BMG. And it can be effectively suppressed by reducing cutting speed and depth or increasing feed rate. Both spiral and C-shaped chips were formed during the machining of Zr57 BMG, and the proportion of C-shaped chips gradually decreased with the increase of cutting depth and feed rate. A large number of shear bands were found on the back surface of chips, and the C-shaped chips were formed by catastrophic fracture of shear banding during the expansion of spiral chips. Within each shear layer of C-shaped chips, secondary shear bands were observed, which was mainly due to shear localization and catastrophic fracture. Based on orthogonal experiments, the cutting parameters for Zr57 BMG were further optimized and the surface roughness (Ra) was reduced to 0.24 μm, which is comparable to the results obtained by precision machining.