Cracking in dissimilar Mg alloy friction stir spot welds

Cracking during dissimilar friction stir spot welding of thixomoulded AM60 and AZ91 sheet materials is investigated. Liquation cracking occurs in the thermomechanically affected zone (TMAZ) region during friction stir spot welding when AZ91 material is the lower sheet in the dissimilar AM60/AZ91 sandwich. A TMAZ microstructure comprising elongated alpha-Mg grains and aligned Mg17Al12 is created during the tool penetration stage in spot welding and melted eutectic film formation is facilitated since the temperature in the stir zone (504 degrees C) is much higher than the alpha-Mg+Mg17Al12 eutectic temperature (437 degrees C). There is no evidence of liquation cracking in the stir zone or the TMAZ region of AM60 sheet when it is the lower sheet in the dissimilar sandwich. However, liquation cracking is observed in the upper sheet material (AZ91) in the location beneath the tool shoulder close to its periphery.