Chip Fracture Behavior in the High Speed Machining of Titanium Alloys

Machining of titanium alloy is a severe fracture procedure associated with localized adiabatic shearing process. Chip segmentation of titanium alloy is usually characterized with adiabatic shear band (ASB) and localized microfracture evolution process. ASB has been recognized as the precursor of fracture locus due to its sealed high strain intensity. Besides strain intensity, stress triaxiality (pressure-stress states) has also been identified as a significant factor to control fracture process through altering critical loading capacity and critical failure strain. The effect of stress triaxiality on failure strain was traditionally assessed by dynamic split Hopkinson pressure bar (SHPB), quasi-static tests of tension, compression, torsion, and shear for finite element (FE) analysis. However, the stress triaxiality magnitudes introduced by these experiments were much lower than those generated from the high speed machining operation due to the fact that ASBs in chip segmentation are usually involved in much higher strain, high strain rate, high stress, and high temperature associated with phase transformation. However, this aspect of fracture evolution related with stress triaxiality and phase transformation is not well understood in literature. This paper attempts to demonstrate the roles of stress triaxiality and phase transformation in chip segmentation especially in the high speed machining of titanium alloy in FE framework. Johnson-Cook (JC) failure model is calibrated by addressing the characteristics of stress triaxiality and phase transformation associated with high speed machining. This research confirms that the selection of failure criterion parameters incorporated the effects of stress triaxiality and the alpha-beta phase transformation is indispensible to successfully predict fracture behavior during chip segmentation process in the high speed machining of titanium alloys.