Research On Laser Welding Of High Strength Steel/Cold Rolled Steel With Different Thickness

High strength steel with high strength and high toughness, has been successfully found application in automotive body stiffeners and energy-absorbing parts. With the aid of laser welding technology in the automotive manufacturing, the amount of parts and mould can be decreased, the material consumption be optimized and the structure performance be improved significantly. Numerical simulation of temperature field distribution,experimental processing and post-welding micro-structure on laser welding of high strength steel/cold rolled steel sheet with different thicknesses are investigated in this thesis.Firstly, based on the heat transfer mode of laser welding, the numerical model on laser welding of high strength steel/cold rolled steel with different thickness is established. The numerical model is simulated with Finite Element Method(FEM) and the influence to temperature distribution of the specimen under differential welding parameters is analyzed. The result indicate that the width and depth of the pool increase with the decreasing of welding power or the increasing of welding speed. Under a certain welding speed range, with the increasing of laser power, the vapor starts to appear on the surface of the specimen, which is the critical state of thermal conduction welding transforming into deep penetration. The relationship between the laser welding speed and critical power is obtained by numerical simulation.Secondly, an experimental platform for laser welding of high strength steel/ cold rolled steel with different thickness is set up and experimental study is carried out. Based on welding temperature field simulation and orthogonal experiment, the welding parameters are optimized on the influence study of laser power, welding speed and defocusing distance of welding quality. Under the condition of appropriate welding process parameters, the mechanical performance of welding joint is good and the tensile strength is superior to the base material. Meanwhile, hardness tests are carried out on the cross-section of welded specimen. The results indicate that the hardness of the pool area and heat affected zone are higher than that of base material and without softening occured.Finally, the microstructure transformation and heat affected zone grain growth law of butt welding joint are observed. By metallographic observation, the needle martensite, lath martensite, upper bainite and lower bainite etc. are mainly found in the specimen. Based on thermal cycle theory, in the grain growth of the integration interval(Ac3,Ar3), the kinetic equation of the grain growth in the high strength steel and cold rolled steel at the welding heat affected zone(HAZ) are established. The theoretical calculation and experimental results indicate that the weld heat affected zone(HAZ) grain growth phenomenon occurs during laser welding of high strength steel and the cold rolled steel, and the welding heat input has a direct influence on the grain growth. Within a certain range, the more the linear energy input,the faster the grain grow.