| The relatively poor wear resistance of aluminum alloys has limited their applications in aerospace and automobile industries. The incorporation of hard ceramic particles into aluminum alloys to form aluminum matrix composites can improve their wear resistance. However, those composites also suffer from a great loss in ductility and toughness due to the presents of ceramic particles. Since wear failure of components is mainly determined by their surface properties, if the ceramic particles would be added only to the surface layers, the wear resistance could be improved without sacrificing the bulk properties. In order to meet the requirements of wear, friction stir processing (FSP) technology was used to fabricate particle reinforced composite layer on the surface of aluminum alloy, with significant economic benefits.In this paper, nano-sized ZrO2particles, Al2O3particles and AlN particles were selected as reinforcements, respectively. Particles reinforced surface composite layers were fabricated via FSP on6061-T6aluminum alloy. The tool and technological parameter were optimized by experimental method and numerical simulation method. Furthermore, the microstructures, phases, particle distribution, micro-hardness and wear resistance were systematically investigated.The optimal tool for this research was the one with big shoulder and middle pin. The diameters of its shoulder and pin were18mm and6mm, with a ratio of3. The optimal technological parameters were900rpm of rotation speed,60mm/min of traverse speed and6of stirring number. On these optimal conditions, the content of reinforcements in the fabricated surface composite layer was max, and the distribution of particles was most uniform.The FLUENT software was used to simulate the steady-state FSP process. The simulated results showed that the temperature distribution was non-uniform, while the velocity distribution was uniform. Most plastic material flowed past the tool from retreating side, with possible circular flow. Shapes and sizes of tool have great effects on plastically deforming area. The heat production during FSP would increase with higher rotation speed, slower traverse speed or bigger stirring number. Comparing with traverse speed, the effect of rotation speed on flow field was more significant.Due to occurrence of dynamic re-crystallization process, the surface composite layer consisted of fine and equiaxed grains. Content of particles in A16061/ZrO2composite was max with the best distribution, and then Al6061/Al2O3composite. Al6061/AlN composite had minimum particles with the worst distribution.Because of the strengthening of fine grains and particles, micro-hardness values of three composite layers were much, higher than those of base metal. Hardness values of A16061/ZrO2composite were biggest, about2.1times of base metal.Owing to higher hardness values on surface and the existence of ceramic particles, three composite layers had higher wear resistance compared to the base metal. Wear resistance of three composite layers was diverse at different sliding speed:at a low sliding speed, A16061/ZrO2composite had the best wear resistance, and then Al6061/Al2O3composite and Al6061/AlN composite; however, at a high sliding speed, the wear resistance of Al6061/Al2O3composite was the best, and then Al6061/AlN composite, while that of Al6061/ZrO2composite was relatively poor. The differences of wear resistance were connected with thermal conductivity of particles and composite layers.During FSP, no reaction occurred between Al2O3or AlN particles and Al matrix, while ZrO2particles and Al matrix reacted to Al3Zr andAl2O3. It was an exothermic reaction with two steps:(1)4Al+3ZrO2→2Al2O3+3[Zr] and (2)3Al+[Zr]→Al3Zr. The total reaction equation was3Al+13ZrO2→3Al3Zr+2Al2O3. Moreover, the in-situ reaction in Al-ZrO2system had low initiation temperature of reaction and high reaction rate during FSP. |
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