Effect of Multipass on Microstructure and Impact Toughness of As-Cast Al–20Si Alloy via Friction Stir Processing

Abstract

In this work, the effect of multipass friction stir processing (FSP) on the microstructure and impact toughness of an as-cast Al–20Si alloy was investigated. FSP resulted in the breakage of coarse primary Si particles and acicular eutectic Si particles, elimination of porosity, and better homogeneity of Si particles. The average size and aspect ratio of Si particles decreased from 86.0 to 1.4 μm and from 3.42 to 1.48 after seven-pass FSP, respectively. The impact toughness measured by Charpy impact testing significantly increased with the increase of the number of passes and remained stable at the range of 7.3–7.7 J/cm2 after three-pass FSP. Improvement of impact toughness was primarily attributed to the microstructural refinement and to the content of ultra-fine Si particles during multipass FSP. In addition, the fracture mode of impact-test specimens changed from brittle cleavage fracture to ductile fracture after FSP. However, little difference in both microstructure and impact toughness can be observed between three-pass and seven-pass FSP due to the limit breakup effect on Si particles.