Study On Tensile Deformation Behavior Of Fiber Laser Welding Joint Of Twinning Induced Plasticity Steel For An Auto-Body

With the development of automobile industry,the requirements for automobile lightweight and service safety are constantly improved.The application and development of advanced high strength steel provides an important way for these requirements.Twinning Induced Plasticity(TWIP)steel not only has high tensile strength and hardening rate,but also has good plasticity,toughness and formability.It can reduce the body weight significantly,maintain the high energy absorption and impact resistance in the form of the thin gauge sheet steel.It has become one of the important development directions of the new generation of ductile high strength steel.Welding technology is unavoidable in automobile manufacturing technology.The welding quality and the performance of the welding joint of the automobile's body will directly affect the life of the automobile and the service reliability and safety.Among the welding technologies,laser welding has gained many advantages,such as low heat input,fast welding speed,high welding efficiency and high matching accuracy,which is widely used in the joining of automotive structural materials.In particular,laser tailor welded technology can achieve the "first welding and then stamping forming" production mode of tailor-welded blanks with different materials and thickness,showing great advantages in improving joint strength,body structure stiffness and structural design flexibility.Both of the structural components production and vehicle crash processes belong to the dynamic deformation process under the high strain rate.The strain rate of the forming process is about 1?10 s^-1,while during the high speed crash,the local strain rate can be as high as 1000 s^-1.As an important part of the body parts,the laser welded structure will also bear the effect of dynamic loading under the high strain rate.Therefore,the research of deformation behavior and fracture mechanism of TWIP steel and its laser welded joints under different strain rate and analysis of mechanical properties and deformation behavior considering the effect of strain rate is particularly important for design and application of vehicle structures to ensure the service reliability and safety under the high strain rate dynamic loading.In this study,high power fiber laser system was used to butt welding 1.6 mm thick TWIP950 steel to obtain the welded joints with good welding quality.The effect of welding process parameters on the surface morphology,microstructure and microhardness distribution of the welded joint was characterized by means of laser scan confocal microscope,optical microscope,scanning electron microscope,transmission electron microscope,X-ray diffracto-meter and Vickers microhardness tester.The tensile tests at different strain rates were carried out for the base metal and welded joints of TWIP950 steel using material testing machine and high speed tensile testing machine at room temperature.The effects of laser welding and strain rate on the mechanical properties of TWIP950 steel was examined.Comprehensive analysis of the above experimental results,and combined with tensile fracture location,fracture morphology and deformation microstructure,the deformation behavior and fracture mechanism of base metal and welded joint of TWIP950 Steel during tensile tests at different strain rates were analyzed.The results show that the full-penetration welded joints of TWIP950 steel with good surface formability and without welding defects can be fabricated using the high power fiber laser system.The welding parameters have great effects on the surface morphology,microstructure and microhardness distribution of the welded joint of TWIP950 steel.The optimized laser welding parameters of TWIP950 steel are laser power of 3000 W,welding speed of 3 m/min,defocus of+2 mm,argon shielding gas flow rate of 15 L/min.The welded joint contained the FZ with the microstructure of austenite dendrite,the coarse HAZ-1 with the austenite grain size larger than that of the BM,the HAZ-2 with the austenite grain size close to that of the BM but microhardness higher than that of the BM and the BM.The mechanical properties of both of TWIP950 steel and its laser welded joint have sensitivity to strain rate.After laser welding,the ultimate tensile strength,plasticity and product of strength and elongation of the welded joint decreased significantly compared with that of the base metal of TWIP950 steel,while the yield strength decreased slightly.With the increase of the strain rate,the yield strength of the base metal and welded joint increased,showing the positive sensitivity of the strain rate.The ultimate tensile strength of the base metal and welded joint decreased first and then increased with increasing strain rate with the critical transition strain rate of 88 s^-1 and 1 s^-1,respectively.The changing trend of product of strength and elongation is basically consistent with that of the elongation,which appeared first decreasing,then increasing and finally decreasing as the strain rate increased.With the increase of the strain rate,the fracture position of the laser welded joint of TWIP950 steel had not changed obviously,and all of the tensile specimens were broken in the fusion zone.The increase of strain rate did not change the ductile fracture mechanism of the TWIP950 steel and the laser welded joint.The dislocation slip,TWIP effect and TRIP effect are coordinated during the plastic deformation process of TWIP950 steel.The adiabatic temperature rise was also affected on the microstructure evolution and plasticity of TWIP950 steel at high strain rate.The deformation twinning density of the laser welded joint of TWIP950 steel increased with the increase of the strain rate.