In order to apply aluminum alloys to structural components, they should be joined with higher strength than that of the original materials. However, blowholes are apt to form in aluminum welding, because the process of melting and solidifying the material is unstable. Aluminum alloys were welded using 2-kW and 3-kW continuous wave Nd:YAG lasers with the aim of obtaining a stable welding process. Two beams were delivered by optical cables 0.6 mm in diameter and focused on the surface of the specimens as dual spots. At a shorter beam distance of 0.36 mm, the weld bead surface was humped, making it unacceptable in terms of quality. Sound weld beads were obtained at beam distances of 0.6 mm and 1.0 mm. X-ray observation was carried out in order to investigate the mechanism of process stabilization. A large keyhole opening that was observed at a large beam distance is thought to result in a stable welding process.
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