Abstract: Butt welding of 2A12 aluminum alloy with thickness of 30 mm was conducted by friction stir welding, the microstructure and impact performance of the joints were analyzed at different thickness positions. The results showed that with the decrease of the heat input, the grain size in the upper part of the weld nugget zone (NZ) was coarsening compared to the lower part of NZ. Causing the impact toughness in the upper area of the joint lower than that in the bottom. The grain size was the smallest in the NZ, a little bit coarser in the thermo-mechanically affected zone (TMAZ), and the coarsest in the heat affected zone (HAZ). There existed difference in impact toughness of the welded joint. The impact toughness of NZ was higher than that of HAZ, The impact toughness of TMAZ was the lowest due to the existence of micro cracks. The grain size of the upper part of NZ was smaller than that of the bottom part of TMAZ, but the impact toughness of these two parts was just the opposite, because the number of the second phase (Al2CuMg) was larger in NZ. In conclusion the change of impact toughness of welded joints was not only related to the microstructure variation, but also the amount of the second phase in the region.
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