Friction stir welding of AZ31 magnesium alloy thin sheets using “pin” and “pinless” tools

The present investigation aims at studying the friction stir welding of thin AZ31 magnesium alloy sheets (1.5 mm in thickness). Different tool geometries and process parameters were investigated. In particular, two properly designed tools, with different values of the shoulder diameter, have been used; each of them has been manufactured both with and without the pin (“pin” and “pinless” tool). It was shown that, as the tool characterised by the shoulder with lower diameter (8 mm) is used, the presence of the pin leads to the obtaining of peak stress and ductility values higher than those provided by the “pinless” tool. A strong beneficial effect on both ductility and strength of the joint is obtained by increasing the shoulder diameter (19 mm) even though the “pin” tool configuration is characterized by a very narrow welding window. An investigation has been also carried out in order to evaluate the micro-hardness distributions and the microstructure of the welded joints. A different metal flow can be clearly observed depending on the presence/absence of pin. Moreover, a more homogeneous microstructure is obtained using the “pinless” tool.