Reduction mechanism of porosity in tandem twin-spot laser welding of stainless steel

Abstract

Porosity is easily formed in a keyhole-type of deeply penetrated laser weld beads. In this study, therefore, high power CO2 laser welding utilizing a single spot or a tandem twin-spot beam was performed on thick plates of Type 304 steel with the objectives of investigating feasibility of porosity prevention and establishing the optimum conditions in tandem twin-spot beam welding. Porosity formation tendency was first investigated in a partial penetration weld made with a single-spot CO2 laser beam at low speed. Consequently, it was confirmed that many pores were present near the bottom parts of the weld beads produced without and with assist gas at a power exceeding 10 and 15 kW, respectively. On the other hand, it was revealed that the number of pores was drastically reduced by twin-spot laser beam welding. Especially, some weld beads produced at 30 kW indicated the best grade of the inspection test result according to JIS Z3106. Subsequently, the reason for the reduced porosity was investigated by using the microfocused X-ray transmission in-situ observation system with a high speed video camera. During high-power laser welding of Type 304 with the single-spot beam, it was confirmed that the keyhole fluctuated up and down and consequently a lot of bubbles were generated from the keyhole tip near the bottom part of the molten pool, leading to the formation of porosity. On the other hand, when an enlarged keyhole was produced by a twin-spot beam, few bubbles were generated from the keyhole tip. Moreover, even if the bubbles were formed, most of them did not move away from the formation location and were soon absorbed in the keyhole. This is interpreted by considering that no strong melt flow was induced near the bottom part of the molten pool just behind the tip of the widened keyhole. Consequently, the difference of porosity formation between single-spot and twin-spot laser beam welding is attributed to the differences of their bubble formation mechanisms, liquid flow near the bottom part of the molten pool, and the bubble disappearance via the keyhole.