Elucidation of high-power fibre laser welding phenomena of stainless steel and effect of factors on weld geometry

Abstract

The fibre laser has been receiving great attention due to its advantages of high efficiency, high power and high beam quality, and is expected to be one of the most desirable heat sources for high-speed and deep-penetration welding. In this study, therefore, in bead-on-plate welding of Type 304 stainless steel plates with 6 kW fibre laser, the effects of laser power, power density and welding speed on the formation of sound welds were investigated with four laser beams of 130, 200, 360 and 560 µm in spot diameter, and their welding phenomena were clarified with high-speed video cameras and an x-ray transmission real-time imaging system. The weld beads showed a keyhole type of penetration at any diameter, and the maximum penetration of 11 mm in depth was obtained at 130 µm spot diameter and 0.6 m min−1 welding speed. It was found that the laser power density exerted a remarkable effect on the increase in weld penetration at higher welding speeds, and sound partially penetrated welds without welding defects such as porosity, underfilling or humping could be produced at wide process windows of welding speeds between 4.5 and 10 m min−1 with fibre laser beams of 360 µm or 560 µm in spot diameter. The high-speed video observation pictures and the x-ray images of the welding phenomena at 6 m min−1 welding speed and 360 µm spot diameter show that a sound weld bead was formed owing to a long molten pool suppressing and accommodating spattering and a stable keyhole generating no bubbles from the tip, respectively.

Investigation of weld defects in friction-stir welding and fusion welding of aluminium alloys

“…Their research showed that keyhole-induced porosity can be avoided by using effective welding parameters and vacuum conditions. The results substantiated the work of Kawahito et al (2007), who stated that processing parameters and surface conditions are responsible for porosity formation but can be effectively controlled by optimisation.…”

Effects of laser focusing properties on weldability in high-power fiber laser welding of thick high-strength steel plate

“…For welding speeds greater than 50 mm/s, partial penetration welds were produced, and humps were formed on the top surface of the welds. The humps were generated at a small spot diameter of 200 lm and higher welding speeds, as reported by Kawahito et al 10,11 Moreover, the weld width became narrower and the penetration depth became shallower with the increase in the welding speed, and the X-ray transmission analysis showed that porosities in the longitudinal welds were present around the tip of the keyhole. Figure 5 shows the top surface appearance, weld beads, and X-ray transmission results for the cross sections of the welds obtained by fiber laser welding with optics system B with a 4 mm focus depth.…”

“…However, high-power fiber laser welding of a thick metal plate, such as stainless steel, steel, and aluminum alloys, has been performed to investigate the effects on weldability of laser-welding parameters, such as laser power, focus distance, welding speed, shielding gas, and beam spot diameter, and of material parameters, such as gap tolerance, surface preparation, and filler materials. [10][11][12][13][14][15][16][17] For example, Kawahito et al 10 investigated the effects of laser power, power density, and welding speed on the formation of sound welds between thick stainless steel plate by high-power fiber laser welding with spot diameters of 130, 200, 360, and 560 lm. The laser power density had a strong effect on the increase in weld penetration depth as welding speed increased, and sound partially penetrated welds with no weld defects, like underfill, undercut, humps, and porosity, were produced at welding speeds from 75 to 167 mm/s and spot diameters of 360 or 560 lm.…”

Keyhole-induced porosity in laser-arc hybrid welded aluminum

“…The form and size of the keyhole varies according to external conditions including laser power, travelling speed, and material properties [19]. Studies have shown that the excessive pressure generated during keyhole welding induces temporal fluctuation and spatial instability of the keyhole, resulting in keyhole collapse and the entrapment of metal vapor and gases [10,12,13,21]. It was observed that the size and shape of the keyhole fluctuated violently and, under this condition, large bubbles formed intermittently at the bottom of the keyhole and were trapped during solidification of the weld metal [13].…”