Friction Stir Welding of Metal Matrix Composites: Predictive Process Modeling

Metal Matrix Composites (MMCs) are strong, lighweight materials consisting of a metal matrix (usually an aluminum alloy) reinforced with ceramic particles or fibers.  Because of their high strength to weight ratio, temperature resistance, and hardness, these materials are excellent candidates for use in aerospace and defense applications.  Melting of the matrix alloy during fusion welding is accompanied by reactions between the molten alloy and the reinforcement material, resulting in the formation of deleterious phases linked to a degradation in joint strength.  These conglomerates are absent in friction stir welded MMC joints since the process occurs below the melting point of the workpiece material.  FSW of MMCs is complicated by rapid and sever wear of the tool pin, a consequence of contact between the tool and the much harder reinforcement material.  Harder tool materials (which would be resistant to abrasion from these inclusions) are often too brittle to withstand the stresses associated with FSW.

This presentation provides an overview of the author's investigations into tool wear in FSW of MMCs.  The contribution of each major process variable to tool wear is quantified and used to develop a regression model which can predict the amount of wear based on these variables.  The use of a dimensionless number (derived from the process parameters) as a classifier for tool condition is evaluated. The effects of reinforcement size, the inclusion percentage, and the hardness of the tool relative to the reinforcement material on wear are also assessed.  Together, this experimental data is used to inform construction of a physics-based model of tool wear in FSW of MMCs.  The feasibility of sensing wear in-process by monitoring force and torque signals is also explored.