Investigation of through thickness microstructure and mechanical properties in friction stir welded 7N01 aluminum alloy plate

An on-going problem in friction stir welded (FSW) joints used in the high-speed train sector is that the microstructure and mechanical properties can significantly vary in thick sections. Because inhomogeneous properties can reduce weld efficiency and degrade service performance, it is of some interest to understand how inhomogeneous properties can develop in FSW welds made from precipitation hardening alloys such as 7N01. In the current study, butt welds were made using 12 mm thick plates and then sectioned perpendicular to the weld line. Five 2.2 mm thick slices were cut from a section and used to measure tensile properties access the weld thickness. The microstructure was characterized using scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) to measure variation in grain size and second phase particle distributions over the weld zone. Results showed that, with the exception of the top slice, yield strength (σy) and ultimate tensile strength (σUTS) obtained from the slices were fairly consistent and comparable to values from the full weld. Elongation (δ) was maximum at mid-thickness decreased significantly towards the crown. Although significantly reduced elongation was found at the top of the weld, and likely limits overall weldment ductility, elongation of the full FSW weld was improved over that of the base metal and can be attributed to enhanced post-necking straining. The finest grains and second phase particles were observed at mid-thickness. In comparison, the coarsest grains were observed at the top of the nugget. This microstructural variation can be understood by considering the temperature and strain field gradients that are generated in the plasticized zone. It is expected that the findings will help to promote a better understanding of post-weld microstructure development and mechanical properties of thick plates.