The Increase Of Weld Depth And The Suppression Of Weld Defects For The High-power Laser Welding Of Stainless Steel

AISI 304 stainless steel is widely used in the nuclear power industry for its excellent corrosion resistance and machinability.It is a key technique to join the thick 304 stainless steel plate during the nuclear power equipment manufacturing process.The welding deformation is large and the efficiency is low using the traditional arc welding method.The high-power laser welding of thick plate has been applied in the large equipment manufacture including the nuclear power industry with the development of 10kW level laser.The laser deep welding has many advantages such as large depth and narrow bead width,lower heat affected zone and welding deformation,and high welding efficiency.Especially,high-power fiber laser has been receiving great attention in the welding area due to their excellent advantages such as higher beam quality and higher operating flexibility.The weld depth was greatly influenced by the laser energy loss during high-power laser welding process of thick 304 stainless steel plates.In addition,the weld appearance and quality was also influenced by the formation of weld defects such as the spatters,pores and weld hump.Based on the background and the occurring issues,the energy efficiency and loss of the high-power laser welding of thick plates was estimated and calculated,and the influence of absorbed energy on the weld dimensions was analyzed.The techniques to improve the laser absorption and weld depth were developed and the relative mechanism was revealed.The weld defects were studied and the suppression of weld defects was investigated.This study could provide an applied and theoretical guidance for implementing a stable welding process and improving the weld quality.First,the laser welding process of 304 stainless steel,which could change from the laser heat conduction welding mode to laser deep welding mode,was studied.The joint depth and weld plume under different welding mode was investigated.It was found that the threshold from the laser heat conduction welding to laser deep welding was 0.97kW/mm.The formation and maintance of deep keyhole was studied.The mechanism of invariable keyhole penetration during the welding process was revealed.The physical properties of weld plume and the laser energy loss by the plume was considered.The results have shown that the base material was melted and vaporized under the effect of laser beam,which could reunite and produce the nanometer particles during the cooling process.The plume temperature was about 5000 K,the ionization degree was below 1%,and the particle density was high up to 10²⁰cm^-3.The energy utilization and loss efficiency of the high-power laser welding of thick plates was calculated,and the influence of weld depth by the keyhole absorption was discussed.The influences of laser power,weld speed,laser defocused distance and laser beam angle on the weld depth,the variation of the plume and keyhole were investigated.The energy utilization and loss efficiency was calculated under different welding conditions and the mechanism was revealed.The relationship between the joint depth,the energy utilization and loss,the plume behavior and the interaction of the laser beam and the deep keyhole was carefully explained.The specific energy utilization and loss efficiency of the high-power laser welding of thick plate was analyzed at a laser power of 10kW and a welding speed of 1m/min.Among them,the melting efficiency of the weld metal was 53%,the overheating efficiency of the weld pool was 16%,the heat conduction efficiency of the base metal was 14%,and the laser energy loss by the plume was 19%.Thus,the utilization efficiency could be increased by the suppression of the plume.The variation of keyhole absorption rate with the increase of the keyhole penetration was summarized.The relationship between the welding behavior and the energy utilization under different weld penetration was deeply explained.Second,according to the above analysis,the laser weld depth and the keyhole absorption rate was improved by developing new techniques.The new equipment with a small diameter of assisted gas jet nozzle was employed to straightly blow the keyhole inlet.The plume was reduced greatly,the keyhole inlet was enlarged and becoming stable and the movement of weld pool melt changed using this method.The laser absorption increased and the maximum improvement of weld depth could reach 37%.In addition,a specific groove was fabricated on the plate surface,and the laser welding process was conducted in the middle of the groove.The plume was suppressed and the melt movement was changed.The laser absorption improved and the maximum improvement of weld depth could reach 25%.It was remarkable that this welding technique has been used in the nuclear industry successfully.Additionally,the pre-set gap was made before welding,and the penetration was improved greatly due to the reduction of the plume and the change of welding process.The maximum improvement of weld depth could reach 80%.Third,the weld defects,such as the porosity,the spatter and the back weld hump were discussed and analyzed.The characteristics and formation patterns were investigated under different welding condition.Bubbles were produced in all the laser welds.Many pores were retained in the 304L stainless steel laser welds made in Ar.Almost no pores were found in the laser welds made in N₂ or no shielding gas.The dissolution of N₂ bubbles in the liquid melt pool lead to the elimination of the pores in the 304L laser welds.The thermodynamics and kinetics models were established to explain this process.Also,for the defects of the spatter and back weld hump,the formation mechanisms were analyzed,and the characteristics and formation patterns were also observed under different welding condition.The measures to reduce or suppress the defects were proposed.