Abstract? By: Lin Zhao1, Susumu Tsukamoto1, Goro Arakane1, Tomohiro Sugino2

Consortium of JRCM (The Japan Research and Development Center for Metals)

1 National Institute for Materials science, 1-2-1, Sengen, Tsukuba 304-0047, Japan

2 IHI Corp.,1, Shin-Nakahara-Cho, Isogo-ku, Yokohama 235-8501, Japan

It is well known that oxygen can significantly increase the penetration depth in arc welding. It is caused by change in the Marangoni convection direction from outward to inward with increasing the oxygen content.  Increase in the penetration depth by oxygen was reported also in laser welding. Some researchers suggested that it was caused by the same mechanism as arc welding. However, narrow and deep keyhole is formed during welding and the penetration depth should be mainly determined by the keyhole depth. Thus, the mechanism of increase in the penetration depth by oxygen has not been clarified yet. In the present study, the effect of oxygen on the keyhole and fluid flow behaviour has been investigated to understand the mechanism of different weld geometries for various oxygen contents in fibre laser welding.

Partial penetration bead on plate welding was carried out on 20 mm thickness 0.10C-0.30Si-1.33Mn steel under the constant laser power of 7 kW and welding speed of 1.0m/min. To elucidate the effect of oxygen content on the penetration depth, the oxygen content in He-O2 shielding gas was varied from 0 to 20%.

The penetration depth increases and the weld width decreases with increasing the oxygen content as shown in Fig. 1. In high oxygen content, the fluid flows from the rear pool end to the keyhole on the pool surface and it flows down just behind the keyhole. This means the inward Marangoni convection is promoted by oxygen. However, the keyhole depth observed from the X-ray transmission image coincides well with the penetration depth in the transverse section both in high and low oxygen contents as shown in Fig. 2. This indicates that increase in the penetration depth by oxygen is not caused by the fluid flow. Deeper keyhole formed in high oxygen shielding increases the penetration depth.

The most possible mechanism is formation of CO. There are a lot of oxide films on the pool surface in 0% O2 shielding, whereas few oxide films exist in 10% O2 shielding. This indicates that some oxides enter into the keyhole by inward Marangoni convection. As the keyhole wall temperature is quite high, these oxides can be decomposed. CO is more stable than the other oxides such as FeO, MnO and SiO2, at high temperature. Then, CO formation is possible in the keyhole. If CO is formed in the keyhole, it can expand the keyhole, resulting in reduction in the keyhole surface temperature. As a result, the keyhole depth increases by adding oxygen.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Paper 1802