Observation of a unique wear morphology of cBN inserts during machining of titanium metal matrix composites (Ti-MMCs); leading to new insights into their machinability

Titanium-based metal matrix composite (T-MMC) is a relatively new material with increasing demand in different industries particularly in aerospace. Yet, there is a significant lack of knowledge regarding its machinability. Tool wear mechanisms of cBN inserts during turning of TiMMCs including adhesion, abrasion, chemical interactions, and their effects on the wear surface morphology were investigated. A unique wear morphology was observed on both rake and flank surfaces, consisting of different distinct zones. The mechanisms leading to this morphology were discussed for the first time, which has led to new understandings and insights into the machinability of Ti-MMCs and more generally into machinability of titanium-based materials. High-magnification imaging revealed a discontinuity within the adhered material to the tool that appeared as a black zone surrounding the contact area. The brittle fracture in this black zone suggested embrittlement of the adhered material in this region. Further investigation confirmed that a very small amount of magnesium in the workpiece material, which is an impurity left over from the production of the titanium alloy, has a key role in this regard. Thus, it could have a profound effect on the machinability of this class of material. Furthermore, magnesium oxidation was found as the main root cause of fire hazard which is known as one of the main issues associated with machining of titanium-based alloys.