Machining of Bulk Metallic Glass

Abstract

The turning and drilling of Zr52.5Ti5Cu17.9Ni14.6Al10 metallic glass (BMG) are evaluated in this study. The mechanics of machining and chip formation and characterization are investigated. In the lathe turning of BMG, above a threshold cutting speed, the low thermal conductivity of BMG leads to chip temperatures high enough to cause the chip oxidation and associated light emission. The high temperature produced by this exothermic chemical reaction causes crystallization within the chips. Oxide layer, amorphous region, fully crystalline region, and crystalline-amorphous transition region are observed in the cross-section of the chips. The x-ray diffraction peaks match the pattern for monoclinic ZrO2. Turning chips morphology suggests that increasing amounts of viscous flow control the chip-removal process. Moreover, viscous flow and crystallization can occur during the machining of the bulk metallic glass, even under the high temperature gradient and strain rate. For the BMG chip without light emission, the serrated chip with adiabatic shear band and void formation was observed. High cutting speed significantly reduced the forces for BMG machining due to thermal softening. Roughness of machined BMG surfaces is generally better than that of Al6061-T6 and SS304. Tool wear is a problem for BMG turning. Chipping and thermal softening on the lathe tool cutting edges can be observed. Drilling of BMG shows that holes with precision geometry and good surface roughness can be efficiently produced in BMG using the high speed steel and WC-Co drills at spindle speed that does not exceed the limit for chip light emission. Morphology of BMG drilling chip are classified and analyzed. The thermal conductivity of tool material and cutting speed are concluded as two critical factors that triggered the chip exothermic oxidation and light emission. The chip light emission has profound impact on the drill wear, as shown by the experimentally measured thrust force and torque. This study concludes the precision machining of BMG is possible with the selection of feasible tools and process parameters.