By: Susumu Tsukamoto1, Lin Zhao1, Goro Arakane1, Tomohiro Sugino2
1 National Institute for Materials Science, 2IHI Corp.
High power and high brightness solid state lasers such as fibre and disc lasers have a great potential to weld heavy section plate members. In this case, one of the major problems is formation of some weld defects such as porosity and hot cracking. Especially, the porosity is easily formed in deep partial penetration laser welds. From this point of view, prevention of the porosity in partial penetration fibre laser and fibre laser-GMA hybrid welding has been attempted in the present study.
Most attractive characteristic of the laser welding is narrow and deep weld penetration. It is attributed to formation of narrow and long keyhole by evaporation of the materials as shown in Fig.1. However, the keyhole is basically unstable, if the length is larger than the circumference. Then, the keyhole tends to be closed during deep penetration laser welding. If the keyhole is closed near the root, the separated keyhole tip forms the bubble and remains as a porosity. Thus, the porosity is formed by keyhole instability. Stabilisation of the keyhole should be effective to prevent the porosity.
In the previous paper, we revealed that the porosity was successfully prevented by periodical change in the laser power (laser power modulation) in deep penetration CO2 laser welding. It is because the laser power modulation can stabilise the keyhole at optimum frequency and waveform. Then, we applied this technique to fibre laser and hybrid welding at first. The results indicate that the power modulation can effectively prevent the porosity in fibre laser welding, but a lot of porosities are still remained in the hybrid welds. On the other hand, the porosity is effectively suppressed with a small amount of oxygen addition in the shielding gas both in fibre laser and hybrid welding as shown in the x-ray radiographs of Fig.2. Observation of dynamic keyhole behaviour using an in-situ x-ray transmission imaging system confirmed that the keyhole was stabilised by a small amount of oxygen addition. Spatter generation can also be reduced by oxygen in fibre laser welding due to stabilisation of the keyhole. Thus, oxygen can stabilise the laser welding process and then prevent the weld defects.
Fig.1 Formation of keyhole in laser welding.
Fig.2 X-ray radiographs of hybrid welds with or without oxygen in the shielding gas.
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