| In recent years, the requirement of the car’s safety and comfort has become higher and higher, and further enhance the car’s performance and improve vehicle safety has become very urgent.Therefore, light weight, low energy consumption, good safety cars have got extensive attention by big automobile manufacturers.Laser welding is an advanced welding method, which has the advantages of low distortion, high efficiengy, high quality of welding joint, and also has obvious advantages compared with the traditional welding method. Therefore, the laser welding is widely used in automobile industry. Advanced high strength steel not only has good comprehensive mechanical properties, but also can meet the development requirements of lightweight and safety of automobile industry. It has become a very important direction in the research and application of material used for the automobile. Therefore, the combination of advanced high strength and laser welding has been more and more researched and utilized.In this paper, a type of YLS-5000 optical fiber laser is used for the laser welding of advanced high strength TRIP590 steel. The effects of micro structure, microstructure, microhardness, tensile property and welding speed on microstructure and properties of the welded joints were studied. The fatigue properties and fatigue damage mechanism of TRIP590 laser welded joints were studied.When the laser power is 3KW, the left power amount is 0, and the welding speed is 3m/min,4m/min and 5m/min respectively, the welding quality is good, the weld is full, and there are no cracks, holes and other defects. When the welding speed reaches 5m/min, the welding speed is relatively fast, and the welding pool is unstable and produces a small amount of flying. The weld width decreases with the increase of the welding speed.TRIP590 steel laser welded joint can be divided into three prats:base metal, heat affected zone(HAZ) and the weld.The width of the weld and HAZ is very narrow, and decreases with the increase of the welding speed. The microstructure of weld is manily plate strip martensite, the micro structure of HAZ near the base area is ferrite, bainite and retained austenite, the microstructure of HAZ near the welding is ferrite and bainite, martensite and retained austenite. With the increase of welding speed, the microstructure of the welding is tiny, the content of martensite is higher, and the dense slender of martensite lath also increases. Because the microstructure of the welding joint is different, the distribution of microhardness is uneven, the weld has the highest hardness, about 492.5 HV.TRIP590 laser weld tensile specimens are broken in the base metal, the tensile strength of welded joint is higher than base metal. The content of residual austenite before tensile is 11.58%, and the content of residual austenite of welded joint after tensile are 0.85%,0.98% and 0.85% respectively, with a corresponding welding speed 3m/min, 4m/min and 5m/min respectively, showes that transformation induced effect during the tensile deformation is significant, improving the plasticity as well as the strength. Under different welding speed, the fiber area of the tensile fracture exist a lot of shaft toughening, manifesting the characteristic of ductile fracture.The data of fatigue test of laser welded joints is fitted. The results show that the relationship of stress and fatigue life are△σ/2=4197(2Nf)-0.2473,△σ/2=11386.8(2Nf)-0.3601,△σ/2=450.9(2Nf)-0.06392, with the speed of 3m/min,4m/min,5m/min respectively.Under the condition of 106 cycles, when the welding speed is 3m/min,4m/min and 5m/min, the fatigue limit of the samples is 223.1MPa,122.6MPa and 334.9MPa respectively, showing the samples at a welding speed of 5m/min can get a higher fatigue life at the same stress level.Different cyclic times of hysteresis loop is not coincidence. The initial stage stable hysteresis loop position is different, when the maximum stress is different. With the decrease of the maximum stress, hysteresis loop reach a steady state faster. With the increase of cycling times, samples produce cyclic hardening. The greater of the maximum stress is, the greater of plastic deformation of the first cycle is, the variation of the strain range is also greater, and the cyclic hardening is mor obvious.In stress control mode, with the increase of cycling times, plastic deformation increase, TRIP590 welded joint produce a ratchet effect. When the maximum stress is 100% of the yield strength, the evolution of the ratchet effect includes initial stage and failure stage. When the maximum stress is less than the yield strength and the life of sample reach 106 cycle without fracture, the evolution of the ratchet effect include the initial stage and steady stage. When the maximum stress is less than the yield strength and life does not reach 106 times, the ratchet effect evolution of most sample containes the initial stage, stable stage and failure stage. With the decrease of the maximum stress, the ratcheting strain rate exists a stable state, and a lower stress level can reach to a steady state faster, which showes the stress level has a great influence on ratchet effect.The fatigue fracture of TRIP590 laser welded joints can be divided into three parts: fatigue source area, fatigue expanding area, and the transient zone. Fatigue source locates at the surface of the specimen, the fatigue extension zone has many river-like patterns and secondary cracks, and the transient zone has a lot of toughness, which reflects the characteristics of ductile fracture. When the fatigue life is same, the TRIP effect is more significant with a greater stress level. Under the 106 cyclic time, fatgiue source originate in welded joint firstly, and then expand in the sample to fracture. Sag of the weld formed in welding process change the stress state and make the stress concentration, which have a great influence on the formation of fatigue crack and the fatigue life. |
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