Manufacturing Challenges Associated with the Use of Metal Matrix Composites in Aerospace Structures

Metal Matrix Composites (MMCs) are materials which consist of a metal alloy reinforced with ceramic particles or fibers. These materials possess a very high strength to weight ratio, good resistance to impact and wear, and a number of other properties which make them attractive for use in aerospace and defense applications. For example, MMCs have being extensively used for structural tubing in the space shuttle orbiter, the antenna mast of the Hubble Space Telescope, control surfaces and propulsion systems for aircraft, and tank armors. However, difficulties arise when joining those materials with fusion welding and impose limitations on the size of MMC components. Melting of the material leads to formation of an undesirable phase when molten Aluminum (Al) comes into contact and reacts with the reinforcement. This phase forms a strength depleted zone along the jointline. Friction Stir Welding (FSW) is a relatively joining technique, developed at The Welding Institute (TWI) in 1991. Because FSW occurs below the melting temperature of many metal alloys, it precludes formation of deleterious phases and results in a more favorable welded microstructure that is closer to that of the parent material. At NASA, this process was first applied to weld the super lightweight external tank for the space shuttles program. Today FSW is employed to join structural components in Delta IV, Atlas V, and Falcon IX rockets as well as NASA’s Orion Crew Exploration Vehicle and Space Launch System. Currently, FSW researchers are interested in extending the application of the process to new materials which are difficult to weld using conventional fusion techniques, such as MMCs. Rapid wear of the welding tool in FSW of MMCs is a consequence of the large discrepancy in hardness between the steel tool and the reinforcement material. This chapter summarizes the challenges encountered when joining MMCs to themselves or to other materials in structures. Specific attention is paid to the influence of the process variables for FSW on the wear process. A phenomenological model of the wear process was established based on the rotating plug model of FSW. The effectiveness of tool materials with high hardness (e.g. Tungsten Carbide, high speed steel, and tools with diamond coatings) in resisting abrasive wear is also considered. In-process force, torque, and vibration signals are analyzed to determine the feasibility of in situ monitoring of tool shape changes as a result of wear. One advantage of this model is that its successful implementation would eliminate the need for off-line evaluation of tool condition during joining. Monitoring, controlling, and reducing tool wear in FSW of MMCs are critical to full application of these materials in aerospace structures where they would be of most benefit. The work presented in this chapter can be further extended for machining of MMCs, where the wear of the tool materials is also a limiting factor.

Keywords: Advanced manufacturing, Friction Stir Welding, Materials joining, Metal Matrix Composites, Tool wear.