MICROSTRUCTURE DEPENDENCE OF AL6061 SURFACE COMPOSITE ON TOOL ROTATION SPEED DURING FRICTION STIR PROCESSING

Aluminium (Al) based alloys are used in aerospace and automotive industries due to their high specific stiffness and high specific strength. To enhance their performance, often their mechanical properties are improved by making composites via incorporation of ceramic particles as reinforcement. For requirements, such as high wear resistance, high surface hardness is essential, and therefore making of their ‘surface composites’ suffices. Friction stir process (FSP) is an effective technique to produce surface composites. By varying tool rotation speed, microstructure can be controlled to achieve high hardness. In this work, aluminium alloy Al6061 based surface composites containing silicon carbide and alumina microparticles were made by FSP method. Surface composites were produced at three tool rotation speeds (rpm: 600, 800, 1000). Composites were characterized for their microstructure, i.e. grain size, at four distinct zones, namely, nugget zone (stir zone), heat affected zone, thermo-mechanically affected zone and base metal. Microhardness was measured for the composites at their nugget zone (stir zone) and for the base metal. Hardness of the composites was higher than the base metal, due to recrystallized microstructure i.e. reduction in grain size, and uniform distribution of ceramic particles and their strengthening mechanisms. With increase in tool rotation speed, the grain size in the composites decreased and consequently their hardness increased, such that, at the highest speed (1000 rpm), the grain size at the stir zone was smaller by an order of magnitude and the hardness was three times higher, compared to those of the base metal. Dependence of grain size (and concomitant increase in hardness) on tool rotation speed provides an effective route for microstructure control and hardness enhancement during processing of surface composites, without resorting to post-fabrication secondary processes.

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