Fabrication of Metal Matrix Composites by Friction Stir Processing.

摘要

This thesis focuses on fabrication of aluminum matrix composites using friction stir processing. Al 5059 alloy was selected as the matrix alloy, and the effect of processing parameters on microstructure and mechanical properties of this alloy was studied in detail.;Grain size measurement was performed on the samples to investigate the effect of process parameters on recrystallization and grain growth in the stir zone. Mechanical properties were also obtained by microhardness and tensile tests. A tool with 3 flats in combination with a rotation speed of 454 RPM provides slightly higher grain refinement and subsequent mechanical properties. It is shown that friction stir processing reduces the fraction of elongated precipitates that form on the grain boundaries of the base material, and this resulted in approximately a 10% improvement in elongation to failure in friction stir processed Al 5059 samples. Particle fragmentation also appeared to increase the rate of work hardening, which also likely contributed to enhanced ductility.;A multi-pass multi-tool FSP technique using different processing parameters was proposed for fabrication of Al 5059/Al2O3 and Al 5059/CNT composites. A uniform distribution of reinforcing particles was achieved in both cases with a uniform hardness profile through the stir zone. The microhardness of the composites was significantly higher than the original aluminum alloy. However, it was noted that the thermo-mechanical cycles during processing of the Al/CNT composites have destroyed the tubular structure of the CNTs.;Friction stir processing was conducted using three different tools with different rotation speeds. Microstructural characterization was carried out by optical microscopy, SEM and TEM. It was shown that fine grains form in the stir zone as a result of dynamic recrystallization and these fine grains do not grow during the cooling cycle because of the effect of magnesium on reducing grain boundary mobility. TEM analysis also showed that the microstructure contained Al6(Mn,Fe) particles with two different morphologies. It was confirmed that refinement of these particles can more effectively pin the grain boundaries and suppress grain growth.