HAZ development and accelerated post-weld natural ageing in ultrasonic spot welding aluminium 6111-T4 automotive sheet

By careful hardness measurements, supported by electron microscopy, it has been shown that there is a clearly observable heat-affected zone (HAZ) when ultrasonic spot welding (USW) aluminium automotive alloys like AA6111-T4, the severity of which depends on the welding energy. In contrast, it has been previously reported that because of the low-energy input with USW no HAZ can be detected. Immediately after welding, softening is seen in the weld zone relative to the normal T4 starting condition. However, this is rapidly recovered by natural ageing, which masks the presence of a HAZ, and the weld strength over long natural ageing times significantly exceeds that of the parent T4 material. This behaviour is caused by dissolution of the solute clusters and Guinier–Preston zones present in the T4 sheet due to the high weld temperatures, which were recorded to reach over 400 °C at the weld interface, followed by accelerated post-weld natural ageing. Interestingly, the weld zone was found to naturally age faster, and to a more advanced state, than the parent material. Modelling has been used to demonstrate that a large excess vacancy concentration can potentially be generated by the high-strain-rate dynamic deformation in USW, which can accelerate the natural ageing response seen in the weld.