By: Xudong Zhang, Eiji Ashida

Hitachi Materials Laboratory, Hitachi Ltd., Japan

Shoh Tarasawa

Hitachi-GE Nuclear Energy, Ltd., Japan

Seiji Katayama

Joining and Welding Research Institute, Osaka University, Japan

Gas jet laser welding can increase the fusion depth of laser welded joints by 35% compared with conventional laser welding technology (Fig.1). It becomes possible to weld 50 mm thick steel plates by using a 10 kW laser, even no filler wire is added.

Laser welding is expanding into modern industries and new applications because of its many advantages like deep penetration and minimizing heat inputs, which results in high welding efficiency and less distortion. Even in welding of thick section materials which are widely used in industrial fields such as power plants, ship building and pipe lines, laser welding is as well being considered to stead multi-pass arc welding for some structures and components.

In conventional laser penetration welding, the laser beam is focused onto the surface of the material which will be welded, so that the material irradiated by the extremely high power intensity (higher than 10 kW/mm2) is vaporized in a very small area (less than 0.3mm2) to form a deep keyhole. Then the metal around the keyhole is melted and solidified to form a weld bead. The thickness to be welded is usually depended on the keyhole depth. Thus, in order to weld thick section materials, high laser power and low welding speed is preferred to get deeper penetration. For high power laser welding at low welding speed, however, the molten pool becomes very wide and forms wine-cup weld bead shape, which results in excessive wide weld width but the penetration depth is insufficient, as shown in Fig.1 (b).

In order to increase the penetration depth and reduce the bead surface width, a new deep penetration welding method-gas jet laser welding-was developed. This technology uses a gas jet to form a deep and big cavity at the upper of the keyhole, and simultaneously to suppress the metal plume injecting from the keyhole inner (Fig.1(a)). In this case, the laser beam directly points at the bottom of the cavity so that deeper penetration is obtained. The main effect of the gas jet is (1) Deepen and stabilize the keyhole, (2) Suppress metal plume injecting from the keyhole, (3) Suppress spatter and porosity.

Preliminary test of bead-on-plate laser welding with gas jet showed that the penetration depth was increased about 35%, from 18.2mm (Fig. 1(b)) to 24.5mm (Fig. 1(c)), at 10 kW laser power and 0.3 m/min welding speed. It was observed as well that the metal plume is considerably suppressed during welding. Then verification experiments for welding of 40mm~50mm thick stainless steel plates by both sides were performed without filler wire. It was found that butt-joint welding of 40 mm plate could be performed at 0.3 m/min welding speed without porosity and other welding defects. As for 50 mm thick plate, good weld bead could be obtained with bead-on-plate welding at 0.2 m/min welding speed (Fig.1 (d)).

Fig.1  Scheme and welded results of gas jet laser welding (Laser power : 10kW)

(a)  Scheme of gas jet laser welding

(b)  Conventional laser welding

(c)   Gas jet laser welding

(d)  Result of gas jet laser welding of 50 mm thick stainless steels (without filler wire)

Reference

(1)   X. Zhang, E. Ashida, S. Tarasawa, Y. Anma, M. Okada, S. Katayama, M. Mizutani. Welding of thick stainless steel plates up to 50 mm with high brightness lasers. ICALEO2009, paper 1605

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