Effect of tool geometry on friction stir lap welding of magnesium alloy to aluminum alloy

摘要

Dissimilar friction stir lap welding of magnesium alloy AZ31 and aluminum alloy 6061 was conducted by two different tools with threaded and unthreaded probes. The formation of liquid and intermetallic compounds were investigated by means of optical microscopy, electron probe microanalysis (EPMA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS). A relatively large volume of intermetallic compounds and higher hardness was observed in the stir zones using both tools. The results indicated that Al_(12)Mg_(17) was the main intermetallic compound formed, and are likely a result of eutectic reaction during cooling of incipient liquid phase. The eutectic microstructure morphology at the joining interface was different by employing each tool. The threaded tool formed thin lamellar eutectic structure, while the unthreaded one resulted in a localized eutectic structure. The eutectic width for lamellar microstructure was smaller than localized eutectic structure. Furthermore, the lamellar microstructure extended from the retreating side up to the tool tip, but the localized eutectic located close to the pin tip. The microstructure studies suggest that the eutectic microstructure morphology is controlled by the interaction of the tool with material in the stir zone.