Microstructure evolution of adiabatic shear bands and mechanisms of saw-tooth chip formation in machining Ti6Al4V

The formation of saw-tooth chip is one of the primary characteristics in high-speed cutting and in the machining of difficult-to-cut materials, such as titanium and its alloys, hardened steels. The saw-tooth chips obtained from Ti6Al4V turning were examined geometrically and metallurgically. The segment spacing, adiabatic shear band (ASB) width and the degree of segmentation were determined by micrographic observations. The microstructure evolution of ASB in terms of cutting speed was analyzed. The mechanism of saw-tooth chip formation was discussed depending on the adiabatic shear sensitivity of workpiece materials when processed at high strain and strain rates. Experimental results show that the evolution of microstructure inside the ASBs is: deformed band → deformed band + transformed band → transformed band with cutting speed increasing. As for those workpiece materials possessed high adiabatic shear sensitivity, the catastrophic instability resulting in the formation of saw-tooth chip is thermoplastic instability; as for those workpiece materials that is insensitive to shear localization, the instability in primary shear zone is periodic cracks originated at free surface ahead of tool; as for those workpiece materials in which adiabatic shear sensitivity is situated between the above two kind of materials, the interaction of thermoplastic instability and periodic cracks results in saw-tooth chip formation.