Defect Formation Mechanism and Reduction Procedure in 10 kW High Power Fiber Laser Welding of Stainless Steel
Yousuke KAWAHITO, Masami MIZUTANI, Seiji KATAYAMA
A 10 kW power fiber laser welding is supposed to produce sound deep penetrations, but to occasionally generate welding defects. The objectives of this research are to investigate penetration, to clarify welding defect mechanism and to develop defect prevention procedure in bead-on-plate welding of Type 304 austenitic stainless steel plates with a 10 kW fiber laser beam. The penetration depth reached 18 mm at the maximum. However, underfilling and humping weld beads were formed at 6 m/min or higher welding speeds under the conventionally-focused and tightly-focused conditions, respectively. At 3 m/min or lower welding speeds, porosity was generated at any fiber laser spot diameter. With respect to the underfilling, spatter generation was influenced mainly by a strong shear force of a laser-induced plume and was widely reduced by controlling direction of the plume blowing out of a keyhole inlet. The humping formation was decided by several dynamic or static factors such as melt volume, melt flow, surface tension and solidification. Its suppression was effective in expansion of the bead width at defocused-laser positions or reduction of melt volume out of keyhole inlet under full-penetration conditions. Concerning porosity, X-ray transmission observation images demonstrated that pores were formed not only at the tip of the keyhole but also at the middle part. The keyhole behavior was stabilized by using nitrogen shielding gas, which led to the porosity prevention. Consequently, a 10 kW laser power could produce sound welds of 18 mm depth, but the reduction procedures of welding defects were required for the production of high-quality welds on the basis of understanding their formation mechanism in 10 kW high-power fiber laser welding.