On understanding the specific cutting mechanisms governing the workpiece surface integrity in metal matrix composites machining

Machining of metal matrix composites (MMCs) is always challenging due to the mismatch of mechanical and thermal properties between the soft matrix and the abrasive reinforced particles, which causes rapid tool wear and severe surface damage. This paper investigates the effects of cutting regimes on surface integrity in machining of MMCs to understand the (sub) surface damage mechanism induced by thermo-mechanical loads in accordance with the evaluation on particle behaviours and matrix metallurgical transformation. For the first time, it is observed that two different cutting regimes (semi-brittle and ductile cutting) occur in machining of MMCs depending on the feed rate/uncut chip thickness. The machined surface morphology greatly depends on these two cutting regimes wherein various particle removal modes (i.e. push-in, crack and pullout) are generated due to the different cutting mechanisms. The semi-brittle cutting regime tends to happen under low uncut chip thickness and lead to obvious damaged surface morphology (high density of fractured particles), while matrix plastic deformation associated with high cutting temperature and built-up heat is found on the machined surface. Furthermore, the semi-brittle cutting regime further leads to an interesting phenomenon within the superficial surface: (i) a layer of broken SiC particles and (ii) the plastic flow of matrix around the hard particles which act as local barriers. Also, the aggregation of fractured particles and strain hardening of matrix can cause an increased hardness at the near-surface area. An additional cutting experiment on matrix material as a comparison revealed that the brittle fracture of reinforced particles plays a key role in the specific mechanism of MMCs under very low uncut chip thickness, which can cause cutting force increase, flank wear accelerate and the formation of surface damage.