Study of Machinability of Metal Matrix Composites and its Effect on Surface Integrity

Thesis (Ph.D.)--University of Washington, 2020In recent years there has been growing interest in developing metal matrix composites, owing to their superior mechanical properties that can be tailored based on requirements. However, the reinforcement particles that strengthen the composite also lead to poor machinability. Often, these reinforcement particles are harder than the cutting tool material causing rapid tool wear which in turn, results in poor surface quality. Therefore, in order to avail the outstanding potential of metal matrix composites for structural components, it is imperative to study their machinability and fatigue performance. Specifically, in this study, the machinability and surface integrity of functionally gradient SiCp/Al and SiCp/Mg were investigated through peripheral milling process. The functionally gradient aluminum composites are typically used in disk brake applications that require gradient mechanical properties along the radius, while uniformly reinforced magnesium composites are finding applications in the aerospace and biomedical industries. The cutting forces, acoustic emissions, tool wear, and surface quality generated were analyzed as a function of feed rate and spindle speed. Statistical design of experiments approach was used, and the analysis of variance was performed to identify the significant parameters for these responses. Feed rate was observed to be the dominant parameter for both types of composites. The surface integrity of the machined composite was evaluated through fatigue performance of milled coupons. The machining induced damage was found to have a significant effect on the fatigue life. In addition to the surface roughness parameters, the interaction between the reinforcement particles and the cutting edge were observed through SEM micrographs of the machined surface. Particle fracture, matrix cracking, voids due to particle debonding and particles pushed into the matrix were observed on the machined surface. The presence of voids and cracks resulted in poor surface finish and fatigue life. Further, the events identified through SEM micrographs of the machined surface were used to develop a physics-based model to predict cutting forces in the peripheral milling of metal matrix composites. Three major events were considered, occurrence of particle fracture, plastic deformation of the metallic matrix and particles debonding due to mismatch in coefficient of thermal expansion. The probability of these events was calculated based on particle concentration and size in the matrix and was used to evaluate the contribution of each event to the total force experienced by the cutting edge. The predicted forces were in good agreement with the measured forces for lower feed rates from 0.1 to 0.765 mm/rev. A major contribution of this study is to identify the effect of the presence of hard reinforcement particles dispersed in a ductile matrix in an intermittent cutting process like milling. Although, machinability of metal matrix composites has been studied for continuous cutting process like turning, the effect of repeated impact of cutting edge on the composite has not been reported extensively. In addition to the analysis of signals obtained during milling, a simple quantification technique was developed to measure the tool wear along the helical cutting edge using optical micrographs and adaptive thresholding technique. The cutting mechanism and machining induced surface defects were identified, and these observations were used to develop a physics-based model to predict the cutting forces generated based on the probability of each event. Although secondary manufacturing processes like machining were found to have a significant effect on the fatigue life of composites, only the effects of primary manufacturing processes have been reported in the literature. In this study, the effect of machining induced surface damages was further investigated by evaluating the sub-surface damage and its effect on fatigue performance. Additionally, the fatigue crack initiation and propagation mechanism through the reinforcement particles, ductile matrix and particle-matrix interface has been investigated