Fundamental studies of friction -stir welding (FSW) of magnesium alloys to 6061-T6 aluminum and FSW of dissimilar magnesium alloys
This study has primarily explored the specificities of the process used for the friction-stir welding (FSW) of the magnesium (Mg) alloys (both SSC and wrought) to themselves and to 6061-T6 aluminum (Al), as well as the microstructural analysis of the resultant welds in order to understand the fundamental mechanisms involved in the mixing of these metals. Dissimilar Mg alloy systems included the FSW of AZ91D with AM60, and the FSW of AZ91D with AZ31B-H24. Both Mg AZ91D and AZ31B-H24 alloys were welded to 6061-T6 Al. Dissimilar Al alloy welds included the FSW of Al 6061-T6 to Al 5052-H34. Dynamic recrystallization was observed in the weld region as well as in the transition region (HAZ), with a clear decrease in the grain size from the base material through the transition zone and into the FSW zone. The welds were free of porosities. The FSW zone in the welds of Mg alloys (AZ31B and AZ91D) to 6061-T6 A1 showed unique dissimilar-weld characteristics such as complex intercalated microstructures with lamellar-like bands of Mg-rich and Al-rich regions. EDX analysis of the weld zones revealed bands with equal parts of Mg and Al, as well as unique recrystallized bands with predominance of either material were observed. The transition from the HAZ to the FSW zone consistently shows a sharp demarcation on either side of the weld. The transition from the Mg AZ31B HAZ to the FSW zone reveals a demarcation band region that uniquely characterizes all Mg AZ31B-Al 6061-T6 welds. In the case of the FSW of Mg AZ91D-Al 6061-T6, the demarcation band was revealed to form in the retreating side of the weld. Vickers microhardness testing performed on the weld cross-sections provided microhardness profiles that revealed the compensation of the normal degradation of 6061-T6 Al in the HAZ. It was also noticed that all the Mg-AI welds showed very high and erratic microhardness values in the weld zone, in comparison to the base material. The dissimilar Mg alloy welds revealed a homogenous, equi-axed, fine-grain structure in the FSW zone, along with complex intercalated microstructures. A sharp demarcation was seen on the advancing side (AZ91D) and a fairly diffuse flow was observed on the retreating side (AM60B) for both sets of solid fractions. Vickers microhardness testing on the dissimilar Mg alloy systems revealed no degradation of residual microhardness of the material in the FSW zone or the transition zone. Limited TEM studies of these welds revealed dense dislocation structures. Considering the Hall-Petch relationship, both the presence of dense dislocation structures and the decrease in weld zone grain sizes might be contributing to the lack of degradation observed in the weld region. The FSW of dissimilar Al alloys show a sharp advancing side demarcation, a rather diffuse retreating side, and an intercalated weld zone microstructure. The interfaces within this weld system are also unique in that a very fine boundary layer demarcates the lamellar bands of recrystallized material within the weld zone. The transitioning of the elongated grains into the weld zone, and the widely varying grain structures across the advancing side interface are also confirmations of general weld characteristics.
Somasekharan, Anand Chandrika, "Fundamental studies of friction -stir welding (FSW) of magnesium alloys to 6061-T6 aluminum and FSW of dissimilar magnesium alloys" (2005). ETD Collection for University of Texas, El Paso. AAI3167941.