Microstructures in friction-stir welded dissimilar magnesium alloys and magnesium alloys to 6061-T6 aluminum alloy

Abstract

Wrought Mg alloy AZ31B-H24 and semisolid-cast Mg alloy AZ91D (approximately 3% primary solid fraction) were friction-stir welded to Al alloy 6061-T6. Semisolid-cast (approximately 3% and approximately 20% primary solid fractions) Mg alloys AZ91D and AM60B were also joined using the same technique, with AZ91D on the advancing side. Numerous welds were made with the Mg alloys and the 6061-T6 Al alloy in alternating advancing and retreating sides. Light optical metallography was used to observe and confirm the weld zone characteristics unique to dissimilar welds. Dynamic recrystallization (DRX) was observed in the weld region as well as in the transition region, with a clear decrease in the grain size from the base material through the transition zone and into the weld zone. The welds were free of porosities. The welds between the dissimilar Mg alloys revealed a homogeneous, equiaxed, fine-grained structure in the weld zone. The weld zone in the welds of the Mg alloys to Al alloy 6061-T6 showed unique dissimilar weld, flow characteristics, such as complex intercalated microstructures with recrystallized lamellar-like shear bands rich in either Mg or Al. Elemental analysis performed on the weld region showed bands with equal parts of Mg and Al, as well as unique recrystallized bands with a predominance of either material. Vickers microhardness testing on all the welds revealed a lack of degradation of residual microhardness of the materials in the weld zone or the transition zone, with some unusual, erratic hardness spikes exhibiting hardness values as much as three times that of the base material hardness.