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Porosity reduction in CO2 laser welding of aluminium alloys ‐ Influence of penetration, joint, oxygen gas and oxide films

Owing to the recent intensified awareness towards protection of the earth's environment, the use of aluminium alloys has been increasingly extended, having drawn attention in the fields in which their lightness and recyclability can be exploited. In terms of welding execution, this expansion of their applications has prompted the need for an improvement in weld quality and welding speed, and hence expectation has been growing for laser welding processes which are presumed to improve the weld quality and the welding speed. Compared with iron and steel materials, however, aluminium alloys are difficult to fuse and inclined to the occurrence of cracking and porosity at the welded joint. Therefore, the actual applications of laser to the welding of aluminium alloys are very limited. For this reason, it is very important to investigate the conditions in which defects can be reduced, so as to improve the weld quality. By executing CO2 laser welding tests for various aluminium alloys, our Institute has been studying the effect of laser beam modes and light deflecting directions on fusibility, and the influence of laser irradiation conditions and shielding gas conditions on fusibility and porosity formation. It has also been aiming at establishing higher quality laser welding processes by investigating the method of monitoring fusibility through the observation of welding plasma, and the laser welding processes with the use of wires. In the previous report, a study was made of the effect of the conditions of shielding gas and laser irradiation on porosity formation. In this report, studies will be made of how much porosity formation is influenced by the state of the butt-welded surface, the oxygen concentration in the Ar shielding gas and the anodised oxide films when penetration depth is even deeper and when penetration welding is executed. Incidentally, the word 'Poroshiti (porosity)', which is often used as a synonym to 'Kiko (pore)', is a nominal form of 'porous' and in a real sense the word to express the condition in which small holes exist ('Takosei'). Therefore, in this paper, the word 'Poroshiti (porosity)' will be used in this sense, whilst the word 'Kiko (pore)' will be used when individual holes or the number of holes are discussed. Apparatus and methods used in the experiment