Influence of processing innovations on joint strength improvements in friction stir welded high strength copper alloys

Abstract

One of the major challenges in welding the Cu–Cr–Zr alloys is that the high welding temperature often causes the coarsening of the precipitates, resulting in lower joint strength. Various strategies were studied to reduce the welding temperature below the aging temperature for limiting the precipitate coarsening. The results showed that great challenges existed in reducing the friction stir welding (FSW) temperature below the peak-aging temperature of Cu–Cr–Zr alloy (420–480 °C) without using coolant. Through softening the Cu–Cr–Zr alloys to the supersaturated solid solution state prior to FSW, the peak temperature during FSW of Cu–Cr–Zr alloy under water was reduced to ∼481 °C, which is close to the aging temperate of the Cu–Cr–Zr alloy. After post-weld aging heat treatment, the stir zone exhibited higher hardness than the peak-aged Cu–Cr–Zr alloy due to the combination of the high density of precipitates 10.5 nm in diameter and refined grains 0.65 μm in diameter, enabling the achievement of high joint strength equal to the peak-aged base metal. In addition, the FSW zone also exhibited a high electrical conductivity of 88.9% IACS, which is higher than that of the peak-aged base material (81.0% IACS).