Study on dynamic chip formation mechanisms of Ti2AlNb intermetallic alloy

Ti2AlNb alloy is a relatively newly developed high-temperature-resistant structural material due to its excellent mechanical properties, which is also regarded as one of the most difficult-to-cut materials. The aim of this study is to characterize the chip formation during the machining of Ti2AlNb intermetallic alloy due to its critical role in studying optimization of machining process variables. A geometry model of chip formation was established to analyze the influence of strain rate hardening effect and thermal softening effect on the flow stress in the shear zone. A series of orthogonal cutting tests and quick-stop cutting tests were conducted with varying cutting speeds and cutting depths to validate the evolutions of flow stress inside the shear band. Results associated with cutting forces, cutting temperature, chip morphology, and microhardness are discussed and compared with those of most commonly used Ti6Al4V alloy. It is shown that the cutting forces and cutting temperature of Ti2AlNb alloy are higher than those of Ti6Al4V alloy by 35.3 and 20%, respectively. The chip morphologies of Ti2AlNb alloy show very different characteristics with different cutting conditions (i.e., some very small sawtooth on the free surface of chips under low cutting speed, then transform into irregular serrated chips with increasing cutting speed, and to serrated chip with adiabatic shear band under high cutting speed), which correlate well with the evolutions of microhardness distribution. The chip morphologies and chip formation characteristics are dependent largely on the dynamic flow stress, which is sensitive to the competition of the thermal softening effect with strain rate hardening effect.