| Research Abstract |
The formation and movement of bubble leading to pores or porosity, and keyhole behavior were observed during high power YAG laser welding of aluminum alloy A5083 and stainless steel Type 304 in vacuum in addition to in inert-gas shielding through microfocused X-ray transmission imaging system. In particular, the effect of the vacuum on penetration increase and porosity reduction was investigated. Obtained results are as follows : (1)Under the conditions using a normal shielding gas in any alloys, a keyhole behaved itself up and down violently, and many bubbles were generated from the bottom part of the keyhole, and moved from the bottom to the upper part of the molten pool, and some disappeared above the surface and into the keyhole, but the rest resulted in pores or porosity. (2)The liquid flow at the bottom of Type 304 molten pool from the keyhole tip was stronger in YAG laser welding than in CO_2 laser welding(3)In vacuum YAG and CO_2 laser welding of A5083 and Type 304, it was conf
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irmed that the penetration was deeper with an decrease in the pressure, and a sound weld could be produced at the pressure of about 0.3 kPa or less. (4)In vacuum welding, a keyhole was swollen at the bottom, but no bubbles were generated from the keyhole tip. This was revealed to be the reason for no porosity in vacuum welding. (5)In YAG laser welding using a shielding gas, a strong liquid flow was observed from the upper part to the tip around the keyhole, and moreover along the bottom of the molten pool from the keyhole tip to the rear surface of the molten pool. (6)On the other hand, in vacuum laser welding, a strong stream was present upwards along the back wall of the keyhole. Namely, it was found that the liquid flow was completely different between in general laser welding using a shielding gas and in vacuum welding. Such liquid flow was considered to exert the formation of porosity.
As mentioned above, the formation situations of bubble and porosity in addition to keyhole behavior were revealed in vacuum as well as in shielding gas, and the effect of vacuum on porosity prevention was confirmed. A lot of fundamental knowledge was obtained for the production of high quality and high speed welds. Less
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