Machining of metal matrix composites: Forces, tool wear and attainable surface quality

Metal Matrix Composites (MMCs) are new generation engineering materials that possess superior physical and mechanical properties compared to non-reinforced alloys. This makes them attractive for a wide range of applications in the automotive, aerospace and defense industries. However, the presence of abrasive ceramic reinforcements in the ductile matrix causes rapid tool wear and premature tool failure. Cracking and debonding of the reinforcement particles are the major damage modes that directly affect the tool performance and surface integrity of the machined workpiece. This leads to an increase in machining cost, production time and poor quality of machined components. Most studies on machinability of MMCs have been based entirely on experimental results and very few analytical models have been developed. Also, it is imperative to elucidate the causes of premature tool failure and subsurface damage during cutting MMCs. Thus, a better understanding of the cutting mechanism of MMC is required. In particular, the cutting tool wear and surface integrity of the machined surface are important issues which require further research.;A systematic methodology for predicting the tool flank wear progression during orthogonal cutting of MMCs is presented. The model was developed based on the power required for machining and incorporated the tool/workpiece properties and cutting parameters. The geometry of the cutting tool was considered in 2D orthogonal machining. The effect of average particle size and volume fraction on flank wear progression was also investigated at different cutting speeds. The model was validated by conducting orthogonal tests on different MMC materials. It was concluded that the particle size and volume fraction has a major effect on the progress of tool wear.;A novel approach for prediction of tool wear progression during 3D bar turning of aluminium-based particulate-reinforced metal matrix composites is presented. The wear volume loss models were developed based on the two body and three body abrasion mechanisms. The flank wear rate is quantified by considering the tool geometry in 3D bar turning. The model also takes into account of the particulate volume fraction and average size along with the tool nose radius. Validation of the developed model was done by conducting turning experiments under a wide range of cutting conditions.;Surface integrity of the machined parts was also investigated. This investigation focused on the microhardness alterations, as well as surface and subsurface machining-induced defects. SEM and TEM analysis were carried out to study the effect of cutting parameters and particulate properties (volume fraction and average particulate size) on the microhardness variations of the aluminium matrix beneath the machined surface. The tribology of cutting MMCs was also studied. Experiments were conducted under dry and wet cutting conditions. Tool wear mechanisms, cutting forces, chip morphology, and the integrity of the machined surface and subsurface were investigated in detail.;In this thesis, an energy based analytical force model has been developed for orthogonal cutting of MMCs. In this approach, the total specific energy for deformation has been estimated, along with the energy consumed for debonding as function of average particle size, volume fraction and material properties. The developed model was validated by conducting orthogonal cutting tests on different MMC materials. It was found that plastic deformation of the matrix consumes the largest portion of the total specific energy. Microstructural investigations beneath the machined surface were conducted using a transmission electron microscope (TEM) to clearly understand the role played by the aluminium matrix during cutting. The density of dislocations generated in the aluminium matrix as a result of cutting was estimated and correlated with the generated cutting forces.;The wet cutting test results indicated that the machining forces are insensitive to the application of coolant at lower cutting speed, but decreases considerably at higher cutting speed compared to dry cutting. Since the tool wear was accelerated mainly due to abrasive wear mechanisms, it is desirable to have a cutting fluid that can form a lubricating layer/film and can reduce the friction effects at the flank contact region. The surface roughness was slightly deteriorated due to the application of cutting fluid.