Abstract
This paper presents the effects of titanium addition and heat treatment on microstructure of 2A12 aluminum alloy joints by CO₂ laser-metal inter gas hybrid welding. The 5 mm thick 2A12 aluminum alloy plates were welded by hybrid welding with and without titanium addition, respectively. The results showed that the oriented columnar dendrites were formed near the fusion boundary without titanium addition. However, the Al₃Ti particles are formed to provide excellent nucleation sites for α-Al grains after titanium addition. Furthermore, a chill zone consisting of fine spherical grains was formed to prevent the growth of columnar dendrites between the base metal and fusion zone. Moreover, the grain size of the chill zone was much smaller than that of the weld center region. After heat treatment, the microstructure transited from columnar dendrites to fine equiaxed grains and the eutectics are believed as a-A1, θ phases, and some other inclusions.

Key words
laser welding, hybrid welding, aluminum alloy

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