Formation Mechanism And Fracture Behavior Of Laser Spot Welded Third Generation High-Strength Steel

The demand for lightweight automobiles has promoted the development of advanced materials and their connection technologies.As an emerging non-contact connection technology,laser spot welding has the characteristics of low heat input,fast welding speed,and flexible joint arrangement.It is expected to replace resistance spot welding and has a very broad application prospect in the automobile manufacturing industry.Quenching and Partitioning(Q&P)steel is one of third-generation advanced high-strength steel to achieve mass production.It has both high strength and high plasticity.It has broad applications in key structural components such as automotive Apillars and B-pillars.Q&P steel is composed of high-density dislocation martensite,ferrite,and metastable austenite.The complex structure characteristics determine that it is easily transformed during welding thermal cycles,resulting in joint morphology during spot welding.Difficult to control,there are defects such as small joint nugget diameter and serious collapse,and complicated microstructure evolution law,which in turn leads to low mechanical properties and poor stability of joints,which limits the application of Q&P steel in the automobile industry and is an urgent need for the automobile manufacturing industry.Aiming at the difficulties of Q&P steel welding,this paper compares and studies the influence of new laser dynamic defocusing spot welding technology,laser spiral spot welding technology and traditional laser fixed spot welding technology on joint formation and microstructure.The results show that the diameter of the nugget of laser fixed-point welding joints is difficult to continuously increase,and the joints are prone to collapse and spatter,and the joint performance is poor;the laser dynamic off-focus welding technology adjusts the energy density in the thickness direction of the plate in real time to obtain the surface For well-formed joints,the nugget diameter is increased by about 28.4% compared with laser fixed spot welding,and the joint performance is improved by about 79.4%;the laser spiral spot welding technology uses the spiral path to obtain a stable keyhole that moves with the beam,and realizes the precise shape of the joint Through the control,the evolution law of the joint morphology with the change of laser power was obtained,and the nugget diameter and joint performance were equivalent to those of laser dynamic off-focus welding,but the total spot welding time was reduced from 3s to 0.8s,and the process efficiency was high.In addition,a technical solution to solve the problem of poor welding stability of Q&P steel with side blowing gas was proposed,and the influence of different side blowing states on the shape of laser spiral spot welding joints was studied,and optimized blowing methods and parameters were obtained.Improve the stability of the laser spot welding process.Based on the above research,the shear performance of the joint under different process conditions is analyzed and the failure mode is determined.A high-speed camera was used to obtain the dynamic behavior and evolution of the photo-induced plasma plume,molten pool,and keyhole during the welding process.Among them,the plasma plume state reflects the proportion of laser energy absorbed by the molten pool,while the dynamic behavior of the molten pool and keyhole is Directly affect the quality of the joint.Studies have shown that with the progress of the spot welding process,the continuous input of laser energy causes the fluctuation of the molten pool to increase,the cavity appears in the keyhole,and the plasma plume area on the lower surface of the joint increases,so the laser energy loss is serious.As a result,the size of the joint failed to continuously increase and collapse defects occurred;with the increase of the defocus amount,the laser dynamic off-focus welding reduced the power density,weakened the fluctuation of the molten pool,increased the keyhole area,and reduced the plasma plume on the lower surface of the joint Therefore,the laser energy absorbed by the molten pool increases,reducing the burning loss of the molten metal,increasing the size of the joint and suppressing the collapse defect;the laser spiral spot welding of the same laser power is due to the stirring effect of the laser spiral path,the molten pool and the spoon The hole stability is increased,and there is no plasma plume on the lower surface of the joint,so the molten pool fully absorbs the laser energy,thereby obtaining a joint with a large size and no collapse defect.The side-blowing assisted laser spot welding improves the stability of the molten pool and reduces the plasma plume fluctuations on the lower surface of the joint.At the same time,the plasma plume and molten pool state behavior of different process parameters are analyzed,and the morphology of the laser spiral spot welded joint is established.The relationship with the plasma plume area and the orifice diameter of the molten pool.The response surface method was used to establish a regression model between the shear performance and process parameters of laser spiral spot welding,and the process parameters corresponding to the best mechanical properties were obtained.Through the internal and external crack growth methods of the joint,the regression model obtained the best mechanical properties of the joint and the crack initiation and propagation of the poor joint fracture process are compared.The results show that the joint tension-shear fracture process is divided into four stages: crack initiation stage,Expansion phase I along the circumferential direction of the joint,phase II expansion along the circumferential direction of the joint,and instantaneous fracture stage.According to the analysis of the fracture process,the peak load of the joint appears in the expansion phase I along the circumferential direction of the joint,and the crack initiates and propagates along the boundary between the molten zone and the coarsegrained heat-affected zone.In this stage,the crack propagation length of the joint with the best mechanical properties is less than that of the mechanical Poor performance joints.Using Digital Image Correlation Technology(DIC)to observe the changes of strain and displacement during surface fracture,the strain before fracture is concentrated on the base material to delay the initiation of cracks.Through microstructure analysis,the large-angle grain boundary of the coarse-grain heataffected zone of the joint is compared with the molten zone The proportion of heataffected zone and other heat-affected zones is the smallest,which is good for cracks to propagate from the edges of the two plates.At the same time,the large-angle grain boundary content of the coarse-grain heat-affected zone of the joint with the best mechanical properties is about 10% higher than that of the joint with poor mechanical properties,indicating that the grain is refined Contribute to the improvement of the fracture toughness of the joint.Based on the in-depth understanding of laser spot welding forming rules and welding forming conditions in this article,laser spiral spot welding technology is used to achieve high-quality welding without spatter and pores on the galvanized sheet.The effect of zinc vapor on the welding process is explained,and the mechanism of zinc vapor escape is revealed.The results show that the evolution state of the keyhole and molten pool metal is the key factor affecting the escape of zinc vapor.Through the analysis of the influence of process parameters on the quality of joints,the use of The method of increasing the size of the keyhole and improving the stirring ability of the molten pool metal provides a stable channel for the escape of zinc vapor,inhibits the generation of welding pores,and improves the stability of the welding state.Innovatively proposed laser spot welding technology that combines laser dynamic defocusing and spiral scanning,which further promotes the escape of zinc vapor and realizes high-quality connection of three-layer galvanized sheets.In summary,this article reveals the internal reasons that affect the performance of laser spot welded joints through in-depth analysis of the state characteristics,microstructure and fracture process of the welding process,expounds the joint forming mechanism,fracture behavior and influencing factors,and proposes to pass The innovative technology of laser action mode control to optimize the morphology and mechanical properties of the joint,controls the absorption mode of the laser energy in the molten pool,optimizes the state of the keyhole and the flow of the molten pool,and realizes the precise size/morphology and microstructure of the joint Controlled and obtained high-strength,high-stability Q&P steel laser spot welded joints.In addition,it reveals the escape mechanism of zinc vapor,and innovatively proposes a laser spot welding technology that combines laser dynamic defocusing and spiral scanning,which effectively suppresses the pore defects caused by zinc vapor and realizes high-quality connection of galvanized sheets.The related achievements of the thesis fill the research blank of Q&P steel laser spot welding technology,and provide theoretical and technical support for the application of laser spot welding technology and Q&P steel in the automotive industry.