Influence of ambient pressure on spattering and weld seam quality in laser beam welding with the solid-state laser

Presently, a research project with respect to laser beam welding in a vacuum is carried out at the Institute of Joining and Welding in Braunschweig, together with the two industrial enterprises Trumpf Laser-und Systemtechnik and pro-beam. For this purpose, the 1.5 m³ vacuum chamber of the former electron beam plant was reconstructed so that the laser beam may be coupled into the chamber via quartz glass. By means of this test set-up, reproducible pressures of up to 10-3 hPa could be set. The research was done with a 6 kW disc laser on mild steel of the quality S355 in thick sheet metal up to 15 mm. The result showed that a reduction of the ambient pressure causes major changes in the generation of the metal vapor plume above the joining zone as well as a major impact on the welding seam geometry. In the case of identical welding parameters, a narrower welding seam develops and the achievable penetration depth rises significantly. Thus, by a slight decrease of the ambient pressure by approximately 10 hPa, spattering could be suppressed in a way that optically excellent welding seams could be achieved which, in this seam depth, qualitatively equal those of an electron beam weld seam. With metal sheets of 10 mm thickness, penetration welds were achieved at a feed rate of 2.0 m/min which, in this excellent quality of atmosphere, cannot even be obtained with a CO2 laser. An excellent seam surface with regular flaking and a slight over-fill without weld seam collapse was formed. At the same time, an excellent seam root without any irregularities occurred. Due to this tremendous improvement in quality of laser beam welding in a vacuum, new fields of application and markets may be opened to this procedure which so far has not been industrially established.