| Owing to the excellent corrosion resistance and low-temperature toughness,aluminum alloy thick plate is widely used in the liquefied natural gas（LNG）storage tanks and shipbuilding industry.However,due to its fast thermal conductivity,low surface characteristics,keyhole-induced porosity may occur during the welding,the keyhole-induced porosity formation is closely related to keyhole collapse caused by keyhole instability,and the presence of the keyhole-induced porosity would strongly weaken the joint strength.Research shows that the dynamic keyhole behavior and flow field in weld pool have a great influence on the formation of keyhole-induced porosity.However,some useful information is hard to be obtained entirely relied on experiment,such as the temperature distribution and the fluid flow field,which are closely related to the porosity formation and amalgamation.This research adopts both experiment and simulation to study the formation of keyhole-induced porosity in laser welding of aluminum alloy,which provides a theoretical basis for the effective technological measures to suppress the porosity.Through the exploration of the welding process,the laser arc hybrid welding（LAHW）process produces joints with a better bead formation,less porosity and higher tensile strength for butt joint of 10 mm-thick aluminum alloy plate,which is successfully guided 2 passes of 30 mm-thick aluminum alloy.Conclusion can be made that the double-side hybrid laser-GMAW process established in this research is promising in industrial application.The experimental results are showed as follows:Firstly,a three-dimensional mathematical model considering the Fresnel absorption and multi-reflection of laser beam is developed to study porosity formation based on the method of VOF.Auxiliary adoption of high-speed camera,the keyhole and weld pool behaviors during the laser welding of aluminum alloy and steel are analyzed,and the keyhole collapse and porosity formation mechanism are also revealed.In laser welding of aluminum alloy,the more violent fluid flow behind the keyhole,the larger depth to width ratio can cause the keyhole more unstable,and the bulge formed at the rear wall of the keyhole will continue to flow forward and contact with the front wall of the keyhole,causing the keyhole collapse.The instability of the keyhole and bubble being captured by solidification interface are the main factors that affect the formation of porosity in laser welding of both aluminum alloy and steel.Also,the differences in porosity formation between aluminum alloy and steel laser welding are also illuminated.In laser welding of aluminum alloy,the much larger solidification rate and longer escaping distance make the bubble easily captured by the solidification interface,and easily combined.All of these reasons contribute to the formation of larger number and size of porosity in laser welding of aluminum alloy.The numerical and experimental results are in good agreement.Then,the research indicates that the elimination of keyhole-induced porosity is mainly by improving the stability of the keyhole and making the bubble difficult captured by the solidification interface.Based on numerical simulation with comprehensive mathematic model and experimental study,the effect of the preheated,welding speed and full penetration condition on porosity formation are focused.The results show that the keyhole collapse frequency of preheated is less than that of the non-preheated,i.e.,46 Hz vs.70 Hz,and the bubble is uncomfortably captured by the solidification interface.With the increasing welding speed,the stability of the keyhole increases and the bubble easily escapes from the weld pool.When the keyhole is at full penetration condition during fiber laser welding of 5083 aluminum alloy,the frequency of keyhole collapse increases,but two flow patterns exist in the rear part of the keyhole,which makes bubble formation caused by instability of the keyhole difficult to occur.Based on the high-speed camera to observe the keyhole and weld pool behaviors,the effect of the type of shielding gas,the different flow rate and nozzle position on porosity formation are analyzed.According to the experimental results,there is no significant difference in the frequency of keyhole collapse when pure argon and nitrogen gas are used as shielding gas,but from the X-ray analysis results,the porosity number reduces when pure nitrogen gas is used as shielding gas.The reason for less porosity volume is that the solubility values are about 2.72 wt%-3.58 wt%,and the nitrogen content in the weld metal has increased by 3.5×10-3 wt.%,so the N2 bubbles can quickly dissolve into the liquid 5083 aluminum alloy melt pool in a short period of time.A proper flow rate can suppress the porosity,and 20 L/min is the most proper flow rate.The less fluctuation of the weld pool,less spatters and less keyhole collapse frequency are the three effective factors for eliminating of porosity in the flow of 20L/min.The nozzle placed at the trailing position can stabilize the keyhole,and thus significantly reduce the porosity number.Afterwards,the vaporization of volatile alloying elements such as Mg is an important issue during laser welding process,and laser welding experiments of 1050,6061,2A12,5754,5083 and 5A06 aluminum alloys with transparent glasses are performed to directly observe the shape of weld pool and the fluctuation of keyhole by a high-speed video camera.Based on experimental investigation and numerical study,the relationship between the Mg content in the weld and the keyhole-induced porosity formation mechanism is revealed in detail,and the quantity and distribution tendency of keyhole-induced porosity formation are also analyzed in laser welding of various aluminum alloys.As the Mg content increases,the weld depth increases in the welding direction,and longer time is required to reach the quasi-steady state;besides,the keyhole is more stable,and less keyhole-induced porosity is formed.Based on pressure balance acting on the keyhole wall,the high recoil pressure contributes to the stability of the keyhole,resulting in the suppression of porosity.Higher density of porosity is formed at the middle and bottom of the weld due to the easier collapse of keyhole in the respective regions when laser welding of aluminum alloys has higher Mg content.Finally,the influence of the laser-GMAW welding process on the porosity is studied,the LAHW and arc laser hybrid welding（ALHW）are used to analyze and compare the porosity formation and microstructure,and high-quality joint for laser-GMAW process of 10 mm and 30 mm thick aluminum alloys can be welded.The results show that the stability in laser-GMAW process is significantly increased and the bubbles easily flee from the weld pool,which contributes to the suppression of porosity.Compared with ALHW,fewer air involved in the weld pool,better stability of the keyhole and the easier escape of bubbles from the weld pool are the three ways to suppress the porosity.In both ALHW and LAHW processes,the columnar grains are found in the fusion boundary,while equiaxed grains are observed in the weld center.In LAHW process,although the grain size is larger,the strength of the joint is higher,which is attributed to less porosity formation,the slight loss of Mg and more second-phase particles with uniform distribution.30 mm thick 5083 aluminum alloy plates can be welded by LAHW process,and the tensile strength of the joints is 263MPa,whose strength is 88.0%of the base metal’ strength. |
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