It is now well accepted that narrow bead welding, as in the case of the electron-beam application, and production of weldments with high precision and high quality can be achieved by the use of high-power laser beams. In this work, a continuous wave high-power CO2 laser was applied for molybdenum. First, effects of welding conditions such as laser power, welding speed and kind of shield gas were investigated. Secondly, ductile-to-brittle tansition behaviour of the laser-beam-welded joint obtained in the most optimum condition was evaluated and was compared with that of the electron-beam-welded joint.
The material used for welding was sintered molybdenum sheets having lower levels of impurities such as oxygen. The laser source was a continuous wave CO2 laser with a nominal power of 2.5-7 kW. The welding was performed by a melt-run technique with the welding speed of 1.7-83 mm/s and the shield gases of He and Ar. Ductile-to-brittle transition behaviour, which represents the ductility of a material, was evaluated by a three-point bend test. After the test, the fracture surfaces were examined by a scanning electron microscopy.
The results are summarized as follows:
(1) Sound and full penetration of a molybdenum sheet of 1 mm thickness was achieved under limited welding conditions (power: 7 kW, welding speeds: 42-50 mm/s and shield gas: He) as compared with electron-beam welding.
(2) Transition of the penetration mechanism from a thermal conducting mode to a key-hole mode occurred under the welding conditions in which the power was 7 kW and the welding speed was lower than 50 mm/s.
(3) Ductile-to-brittle transition behaviour of the laser-beam-welded joint in an optimum condition was approximately the same as that of the electron-beam-welded joint.
