Development of Analytical Force Model for End Milling of Magnesium Matrix Composites

Metal matrix composites are being increasingly used in the automotive industry due to their excellent mechanical and physical properties. Specifically, magnesium matrix composites exhibit enormous potential in a wide range of applications in biomedical, aerospace, and automobile industries. Metallic matrix is typically reinforced with rigid ceramic particles leading to increased strength, modulus and wear resistance. The reinforcement particles that enhance the mechanical properties of the composites generate higher cutting forces, cause rapid tool wear and defects on the surface during machining, in comparison to an unreinforced matrix. The cutting forces generated in a machining process reflect the integrity of the surfaces. Therefore, the development of a predictive force model would enable a time saving and cost-effective method to predict the machined surface integrity. In this study, an analytical approach to predict the cutting forces in peripheral milling of magnesium metal matrix composite, AZ91/SiCp/15% is presented. The model was developed by segmenting the cutting edge into smaller elements and by considering the events that occur when each element encounters the magnesium composite. Three distinct events have been identified, 1) particle fracture may occur when the element encounters just the ceramic particles, 2) when the ductile matrix is encountered, it undergoes plastic deformation, and 3) when the element comes in contact with both the particle and matrix, particle debonding occurs due to mismatch in coefficient of thermal expansion. The size of the elements and the probability of the events occurring during the engagement of the cutting tool were estimated based on the concentration, and size distribution of SiC particles in the magnesium matrix. The stresses and forces experienced by the elements during the three events are calculated using Griffith’s equation for brittle fracture, Altintas force model for end milling of ductile materials and Eshelby’s model for non-dilute systems for particle debonding, respectively. The total forces experienced by the cutting edge is calculated by considering the cumulative forces experienced by all the elements that are engaged with the composite. The predicted forces are then compared with forces measured experimentally using a rotating type dynamometer, for feed rates ranging from 0.2 to 1 mm/rev and spindle speeds from 1000 to 4000 RPM.  The predicted feed forces and the measured forces are in good agreement for all the cutting conditions. However, the predicted axial and radial thrust forces were found to diverge from the measured forces at higher feed of 1 mm/rev. The plausible reason for this deviation is also discussed.