Effects of reduced ambient pressure on spattering during the laser beam welding of mild steel

When welding with high-power disk and fiber lasers, some process phenomena can occur to such an extent that they can severely limit the full exploitation of the technological advantages of these lasers. One of them is the development of strong spattering. In order to identify the mechanisms leading to spattering and, in particular, to clarify their dependence on ambient pressure, welds on the bead with mild steel were carried out at ambient pressures between 1 and 0.01 bar and welding speeds between 1 and 20 m/min. As a result, the measured weight loss due to a spattered material shows a characteristic dependence on the welding speed with a clear maximum at all pressure levels. At lower pressures, however, the maximum is drastically reduced and occurs at lower speeds. Measurements of the geometric data of the seam cross section confirm the fact that the seam depth increases with decreasing pressure at low speeds. At high speeds, however, a decrease in the weld penetration depth was observed, while the cross-sectional area of the weld seam remains more or less constant, indicating that the amount of energy coupled in is independent of ambient pressure. These data were used to investigate the role of moving steps at the keyhole front and the influence of metal vapor on the momentum and energy transfer to a separating melt element and the droplet expelled.