Machining of long ceramic fibre reinforced metal matrix composites – How could temperature influence the cutting mechanisms?

Metal matrix composites (MMCs) offer a unique set of properties due to the ductile-brittle combination produced by the matrix and the reinforcements. Conventional MMCs are usually particle-reinforced, and their cutting mechanisms have been thoroughly studied, showing that they tend to follow traditional cutting theory as the particles roll within the surface/chip or are pushed in/pulled out of the machined surfaces. However, while the enforcement mechanism is quite unique in fibre reinforced MMCs, very little is known about the cutting mechanisms of this kind of materials. These materials are distinguished for having a, roughly, one-to-one scale alternation of the ductile (i.e., matrix) and hard/brittle (i.e., ceramic fibres) phases; key characteristic that is likely to heavily influence the material removal mechanism. Further, there is an open question on how the (temperature-dependent) stiffness of the matrix would affect the cutting mechanism when considering the hybrid machining process (e.g., heat assisted/cryogenic machining) to improve their machinability. To elucidate these aspects, here, by means of cutting a SiC_f/Ti-6Al-4V MMC, the following particularities/peculiarities of the cutting mechanism of these structures are reported: (1) the chip formation includes, up to now unobserved, extrusion of the ductile component of the MMC (Ti-6Al-4V matrix) between the fractured hard phase (SiC); (2) the properties and deformation mechanisms of the matrix (adjusted by temperature control: −180 °C; 24 °C; 400 °C) will affect the crack initiation of the SiC hard/brittle fibre which is manifested underneath the machined surface. Thus, this work is unique in its approach as it opens the understanding of how these complex and heterogeneous structures could be “activated” (e.g., by thermal means to change the stiffness of a particular phase) for improved cutting conditions.