Effect of post-weld heat treatment on double-sided friction stir welded joint of 120 mm ultra-thick SiCp/Al composite plates

Abstract

Friction stir welding (FSW) is a very promising and efficient technique in joining aluminum matrix composites (AMCs) without clear drawbacks, and post-weld heat treatment (PWHT) is a viable option to improve the mechanical properties of friction stir welded joint for heat treatable AMCs. In this work, 120 mm ultra-thick plates of 16 vol% SiCp/2014Al composites were successfully joined by double-sided FSW and subsequently friction stir welded joints were subjected to four different PWHTs (T6–25, T6–70, T4–25 and T4–70, denoting T6 or T4 heat treatment and being quenched in water at 25 °C or 70 °C after solution during heat treatment), respectively. The effect of PWHT on the distributions of microstructures and mechanical properties along the thickness direction in the nugget zone (NZ) of friction stir welded joint is investigated. For as-FSW joint, the tensile strength first decreases gradually along the thickness direction in the shoulder-affected and pin-affected regions, and then increases in the overlapping region. For T6–25 joint, the tensile strength does not vary a great deal in the shoulder-affected and pin-affected regions, and then increases slightly in the overlapping region. After T6–25 PWHT, the finer precipitates θ' are detected, aluminum grains grow up, and the tensile strength is obviously improved both in the NZ and base material attributing to the precipitation strengthening effect from finer precipitates θ'. The distributions of microstructures and mechanical properties along the thickness direction in the NZ are changed from heterogeneous distribution for as-FSW joint to nearly homogeneous distribution for T6–25 joint except in the overlapping region. After different PWHTs, the T6–25 joint shows the highest Vickers hardness in the NZ, followed by T6–70, T4–25 and T4–70 joints, and the as-FSW joint shows the lowest Vickers hardness. A comprehensive understanding can be provided for the effect of PWHT on the double-sided friction stir welded joint of thick or ultra-thick metal matrix composite plates.