Observations on chip formation and acoustic emission in machining Ti-6Al-4V alloy

Orthogonal cutting tests were undertaken to investigate the mechanisms of c hip formation for a Ti-6Al-4V alloy and to assess the influences of such on acoustic emission (AE). Within the range of conditions employed (cutting s peed, upsilon (c)=0.25-3.0 m/s, feed, f=20-100 mum), saw-tooth chips were p roduced. A transition from aperiodic to per iodic saw-tooth chip formation occurring with increases in cutting speed and/or feed. Examination of chips formed shortly after the instant of tool engagement, where the undeformed chip thickness is slightly greater than the minimum undeformed chip thickne ss, revealed a continuous chip characterised by the presence of Line lamell ae on its flee surface. In agreement with the consensus that shear localisa tion in machining Ti and its alloys is due to the occurrence of a thermo-pl astic instability, the underside of saw-tooth sequents formed at relatively high cutting speeds, exhibiting evidence of ductile fracture. Chips formed at lower cutting speeds suggest that cleavage is the mechanism of catastro phic failure, at least within the tipper region of the primary shear zone. An additional characteristic of machining Ti-6Al-4V alloy at high cutting s peeds is the occurrence of welding between the chip and the tool. Fracture of such welds appears to be the dominant source of AE. The results are disc ussed with reference to the machining of hardened steels, another class of materials from which saw-tooth chips are produced. (C) 2001 Elsevier Scienc e Ltd. All rights reserved.