Friction stir welding between CPTi and AZ31B magnesium alloy: effect of rotating tools on the welding strength of the joint

摘要

In this study, butt-welding was carried out by the friction stir process between pure titanium sheets for industrial use and AZ31B magnesium alloy sheets, to investigate various factors affecting the weld strength. The conclusions obtained are as follows. The weld strength in the friction stir welding of pure titanium sheets for industrial use and AZ31B magnesium alloy sheets is affected by Ti fragments in Mg. The probe with thread showed tensile strength 79 percent higher at maximum than the cylindrical probe. The weld strength of the joints produced with the probe with right-hand thread and the one with left-hand thread was nearly the same. Two reasons could be assumed for the difference in the weld strength between the cylindrical probe and the probe with thread. One is: The area with plastic flow of Mg was enlarged owing to the diffusion of Ti fragments into Mg and induced the increase in the amount of Mg pushed towards the newly formed surface of the Ti side. The other is: The downward plastic flow reduced the likelihood of the occurrence of voids near the weld interface. In the element analysis of the weld interface, Al contained in Mg was observed to have diffused into the Ti side. The reduction in the tensile strength owing to the decrease in Al content in Mg is considered to be one of the reasons why fracture was initiated from the vicinity of the interface. As a result of conducting the X-ray diffraction at the vicinity of the interface on the Mg side, the bottom surface of Mg crystals were found to line up with a certain angle to the weld interface, forming a crystal texture. This formation of a crystal texture had a compound effect combined'with the reduction of the tensile strength of Mg caused by the diffusion of Al dissolved in Mg into Ti , and resulted in the weld strength not reaching the tensile strength of Mg base material.