Characteristics of cutting forces and chip formation in machining of titanium alloys

Abstract

Chip formation during dry turning of Ti6Al4V alloy has been examined in association with dynamic cutting force measurements under different cutting speeds, feed rates and depths of cut. Both continuous and segmented chip formation processes were observed in one cut under conditions of low cutting speed and large feed rate. The slipping angle in the segmented chip was 55°, which was higher than that in the continuous chip (38°). A cyclic force was produced during the formation of segmented chips and the force frequency was the same as the chip segmentation frequency. The peak of the cyclic force when producing segmented chips was 1.18 times that producing the continuous chip. The undeformed surface length in the segmented chip was found to increase linearly with the feed rate but was independent of cutting speed and depth of cut. The cyclic force frequency increased linearly with cutting speed and decreased inversely with feed rate. The cutting force increased with the feed rate and depth of cut at constant cutting speed due to the large volume of material being removed. The increase in cutting force with increasing cutting speed from 10 to 16 and 57 to 75 m/min was attributed to the strain rate hardening at low and high strain rates, respectively. The decrease in cutting force with increasing cutting speed outside these speed ranges was due to the thermal softening of the material. The amplitude variation of the high-frequency cyclic force associated with the segmented chip formation increased with increasing depth of cut and feed rate, and decreased with increasing cutting speed from 57 m/min except at the cutting speeds where harmonic vibration of the machine occurs.