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.
