Liquation of Mg Alloys in Friction Stir Spot Welding

The use of friction stir spot welding (FSSW) to join Al alloys in automobiles (e.g., by Mazda Motors) is expected to extend to Mg alloys in view of their increasing use to further reduce the vehicle weight. FSSW is considered a solid-state welding process, but liquation (liquid formation) and cracking have been reported recently in Mg alloys in FSSW In the present study, liquation in Mg alloys was investigated using as-cast AZ91E Mg (similar to Mg-8.6Al-0.6Zn, a widely used Mg alloy) as an example and 6061-T6Al(similar to Al-1Mg-0.6Si, an Al alloy widely welded by FSSW) as a reference for comparison. With the same welding schedule, liquation occurred in AZ91E Mg but not 6061 Al. A simple test with an augmented torque to amplify the difference in the liquation susceptibility between different alloys was demonstrated. It showed no liquation in 6061 Al but severe liquation and cracking in AZ91E Mg, including formation of eutectic liquid films, cracking along liquated grain boundaries, removal of liquated material by the tool, and a mirror-like weld top surface. The microstructural evolution leading to liquation in FSSW was presented. Liquation made the torque fluctuate and fail to show a clear peak as the rotating tool shoulder reached and penetrated the workpiece surface. The heat input, determined from the torque and the axial force, was much less with AZ91E Mg than 6061 Al and thus not why AZ91E Mg liquated more severely. A method to explain the liquation susceptibility was thus proposed. The curves of temperature vs. fraction solid (T-f(S)) during solidification were calculated. They indicated little liquation-causing constituent in 6061 Al but much more in AZ91E Mg, AM60 Mg, and AZ31 Mg and with a much lower liquation temperature. Most Mg alloys including these three have Al as the major alloying element, thus providing gamma (a Mg-Al compound) to react with a (the Mg-rich phase) and cause liquation by the eutectic reaction alpha + gamma -> L at a low temperature <= 437 degrees C). The effect of heat treating before welding on liquation in FSSW was discussed.