The creation of surface metal matrix composites through friction stir processing

Tungsten carbide - aluminum surface metal matrix composites were created on 7050 aluminum substrates via friction stir welding. Several processing parameters were studied and optimized to produce a uniform distribution of the WC particles in the aluminum matrix. The best results were achieved when a convex-shaped shoulder and threaded pin were used at a plunge depth 0.2 mm - 0.7 mm deeper than the length of the pin. A groove application technique for introduction of the WC particles was found to be better than a slurry technique. Other parameters such as rotational and transverse speeds and lead angle had a lesser effect on incorporation. When identical parameters were used, incorporation patterns were found to be similar, showing that this process could produce repeatable results. The depth of the surface composite layer was found to be dependent on the application technique, the tool used and the plunge depth. With some notable exceptions, the volume fraction of tungsten carbide was not high enough to produce an increase in hardness over the base material or improve the wear properties. Heat treating the processed samples caused a possible reaction between the tungsten carbide and aluminum. As a result, the original hardness and wear properties of the aluminum matrix were restored but no additional benefit was observed from the tungsten carbide. Since there was no reaction present in the as-processed samples, FSP may be capable of incorporating thermodynamically unstable reinforcements in MMCs --Abstract, page iii