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 increas ing blade rotation speed and cutting load in the early period and then appr oach constant values at higher blade rotation speeds and cutting loads. The se features are ascribed to the effects of strain hardening, fatigue harden ing and high strain-rate hardening. The Buehler 4217 emery blade being alwa ys composed of new cutting edges of SIC and Al2O3 powders, exhibits a bette r cutting rate than a diamond blade to cut TiNi SMAs. A high-speed steel dr ill coated with TiN film exhibits a better drilling ability than a high-spe ed 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 ar ise 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 defor mation 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 chara cteristics than Ti49Ni51 SMA. (C) 2000 Elsevier Science B.V. All rights res erved.