Eutectic Al–Si piston alloy surface transformed to modified hypereutectic alloy via FSP

In the present study, friction stir processing was used to transform the surface of eutectic LM13 Al–Si alloy into a modified hypereutectic alloy by addition of 10% volume fraction Si particles. Particle distribution, microstructure, hardness and tensile properties of the stirred zone were studied as functions of tool geometry, FSP pass number, and rotational and traverse speeds. Friction stir processing increases the tensile strength, elongation and hardness of the as-cast metal via elimination of porosities and dendritic branches, and also breakup of needle-like eutectic silicon. Threaded pin results in a better particle distribution while the square pin leads to lower Si particle size. Increase in pass number causes better particle distribution, lower Si particle size, higher strength and elongation, and lower hardness. Increasing the rotational speed improves particle distribution, reduces Si particle size, and increases strength, elongation and hardness. Increment in traverse speed results in worse particle distribution, larger Si particles size and lower elongation but higher hardness which having an insignificant effect on the tensile strength.