Effect of Microstructure and Tunnel Defect on Quasi-Static Fracture of Friction Stir Welded (FSW) AA5083-H111

Abstract

Friction Stir Welding (FSW) is a solid state process that utilizes a rotating tool, comprising a shoulder and pin, pushed in to stir the material while the tool moves along the jointline. Better mechanical properties, environmental friendliness and lower cost make it the fastest evolving welding technology . AA5083 was selected owing to its enhanced superplasticity, high strength and superior corrosion resistance, making it particularly effective for manufacturing ship and automobile bodies, and pressure vessels of space ships and aircraft . This work examined the role of defects and microstructure on the fast fracture mechanism in welded 5083-H111 aluminum alloy. The sample was welded at 400rpm tool rotation speed and 50mm/minute transverse speed. Optical microscopy images across the weld were taken. The sample was subjected to tensile test and the fracture surfaces were viewed with the SEM in SE mode. (did fast fracture occur during the welding pass or was it mechanically tested post-weld? FSW resulted in the creation of the following microstructural zones (Figure 1): a) weld nugget, which underwent superplastic forming by the mechanical stirring, b) the flow arm zone, exhibiting finer grains as a result of more intense stirring, c) thermo-mechanically affected zone (TMAZ), the region touching the weld nugget, and d) heat affected zone (HAZ), which is typical in all welds. Failure was initiated at the sharp corners of the triangular weld tunnel (from stress concentration), and propagated upwards, bending through the TMAZ, HAZ and the flow arm zone, and downwards at the tool marks into the weld root (Figure 1). SEM-SE images revealed smaller dimples slightly elongated downwards, co-existing with cleavage facets in the root area in Figure 2, indicating mixed failure modes. In the TMAZ and HAZ in Figure 3, large elongated dimples can be seen which sometimes contain precipitates. EDX analysis of these precipitates revealed a composition of 93.1%Al, 6.4%Mg, 0.2%Si and 0.2%Fe (wt%). Smaller dimples on cleavage facets were found at the top in the flow arm zone which underwent quasi-cleavage fracture, shown Figure 4. The sizes of dimples and the lack of cleavage facets indicated that the material in the TMAZ/HAZ was much more ductile than that in the flow arm zone and the root, which did exhibit the presence of cleavage facets. Therefore it can be determined that the difference in failure mechanisms through the weld is owing to different microstructural zones that differ in grain size and precipitates.

Keywords: FSW, fracture & failure analysis, material flow, microscopy, tunnel defects,