Microscopic Observations of Friction Stir Welded 6061 Aluminum Alloy

Abstract

As the automotive industry trends towards increased use of aluminum, the friction stir welding process offers many potential benefits for joining of aluminum. In this study, the microstructure in friction stir welded 6061 aluminum alloy was observed by metallographic technique, electron backscatter diffraction pattern (EBSD) and optical microscopy. The microstructure in the heat affected zone (HAZ) was significantly different from that in a thermo-mechanically affected zone (TMAZ). EBSD indicated that many more low-angle grain boundaries in TMAZ, i.e., subgrains with a recovered granular structure, were observed than in HAZ. Friction heating and plastic flow during friction stir welding created fine recrystallized grains and recovered grains in the TMAZ. The friction stir welding process produced a softened region in the 6061 Al welded alloy. In the stir zone, equiaxed grains were created and the grain size was smallest in the bottom area.

Microstructural Evolution in Friction Stir Welded 1050 Aluminum and 6061 Aluminum Alloy

“…In the left of the orientation map, a misorientation angle histogram for the right image is shown, the number fraction of low angle boundaries in the total number of boundaries is about 68%. This is distinctly different from that of base material in the 6061 aluminum alloy treated by T6 condition, 6) in which there are few low angle boundaries (number fraction only about 10%). Figure 3(b) shows a plastic flow in the TMAZ.…”

“…1, a jar shaped microstructure is formed in the joint portion which is called for in a SWZ, the interior of the SWZ is deformed as a comparatively uniform microstructure, not like the wedgeshaped microstructure as shown in our previous work 6) on 6061 aluminum alloy. This is a remarkable difference in microstructure between the friction stir welded 1050 aluminum and 6061 aluminum alloy.…”

“…Compared with the hardness in friction stir welded 6061 aluminum alloy, as shown in ref. 6 with the value of 60 HV, it is far low.…”

“…Figure 3(b) shows a plastic flow in the TMAZ. It is not a significant plastic flow like stir welded 6061 aluminum alloy 6) because of strong viscosity of 1050 aluminum. The grains that are delineated by the thick black lines incline in one direction and the grains were elongated, suggesting that grains were lightly deformed by stirring and partly recovered.…”

“…3) Further more, Salem and Rhodes et al examined the evolution of the fine grains found in friction stir processed 2095 and 7050 aluminum and presented that the thermal and mechanical effect associated with the particle size and hardness. 4,5) In our previous work, 6) we have investigated a dynamic recrystallization and a microstructure formed during friction stir welding of 6061 aluminum alloy by electron backscatter diffraction pattern(EBSD) and reported that the subgrains formed in thermo-mechanically affected zone(T-MAZ) are rotated along an axis as for the grain plastic flow. As mentioned above, many researches on FSW in 6000, 7000, 5000 and 2000 systems have been related with precipitates and microstructure.…”