A study on the machining characteristics of TiNi shape memory alloys

The machining characteristics of TiNi shape memory alloys (SMAs) have been studied. The time needed to cut TiNi SMAs is found to decrease with increasing blade rotation speed and cutting load in the early period and then approach constant values at higher blade rotation speeds and cutting loads. These features are ascribed to the effects of strain hardening, fatigue hardening and high strain-rate hardening. The Buehler 4217 emery blade being always composed of new cutting edges of SIC and Al2O3 powders, exhibits a better cutting rate than a diamond blade to cut TiNi SMAs. A high-speed steel drill coated with TiN film exhibits a better drilling ability than a high-speed steel drill for the TiNi SMAs because the TiN film has high hardness and excellent wear resistance, further, a tungsten-carbide drill exhibits the best drilling ability. The drilling forces for TiNi SMAs can exceed 5000 N which are higher than those for many commercial alloys. This feature may arise from the high toughness and viscosity, and the unique pseudoelasticity of TiNi SMAs. Wavy tracks appearing on the drilled surface are ascribed to sliding wear between the blunt drill and the drilled surface of TiNi SMAs. These wavy tracks will increase the frictional force, promote the vibration of the drilling machine and cause damage of the twist drill. Plastic deformation occurs during drilling and hence the specimen's hardness near to the drilled holes can reach 310 and 370 Hv for the Ti50Ni50 and Ti49Ni51 SMAs, respectively. At the same time Ti50Ni50 SMA exhibits better drilling characteristics than Ti49Ni51 SMA. (C) 2000 Elsevier Science B.V. All rights reserved.