Study On Microstructures And Properties Of Laser Welding Joints Of Automotive Dual-Phase Steel DP780

With the advantages of low cost, high work hardening rate, low yield ratio, good ability of continuouse yielding and excellent performance of stamping and forming, dual phase steel has been widely applied in the field of automobile industry. As the laser welding of dual phase steel has some characteristics such as strong energy concentration, narrow soft zone and high efficiency, it has become a main method for dual phase steel welding. In the practical application of dual phase steel products, the structure constituent and properties of welding joints should meet the requirements of actual production. So the research on the microstructures and properties of laser welding joints of dual phase steel will benefit for the application of dual phase steel laser welding.Based on laser penetration butt welding process, a disc Yb:YAG laser with an output power of 4000W was used to join the advanced high strength steel DP780 with a thickness of 1.2mm. The micro-structural characteristics and influences of laser welding parameters on microstructures and mechanical properties of joints were investigated by means of optical microscope, scanning electron microscopy, transmission electron microscopy and other analytical tests.Observing with the scanning electron microscope(SEM), the results showed that the weld mainly included lath martensite, low-carbon bainite and retained austenite, the presence of lath martensite greatly increased the microhardness of the weld. To determine the content of martensite in the weld, firstly color metallography technique was used to determine the volume fraction of martensite and retained austenite, and then the content of retained austenite was further measured through micro-area X-ray Diffraction(XRD) analysis, finally the volume fraction of martensite in weld was determined. We can find that the content of martensite can be increased by increasing the welding speed, because higher cooling rate provides a greater degree of super-cooling which makes more low-carbon phase change into lath martensite. The morphology of martensite, bainite and retained austenite were also observed by transmission electron microscope(TEM), which showed that the increasing of the welding speed could reduce the width of lath martensite and bainite.The microhardness of the weld is 1.6 times higher than that of the base metal. There is heat affected soft zone in the welded joint. The formation of the soft zone is due to the effect of welding thermal cycle. Double phase steel saved a lot of distortion energy in the processing of controlled rolling. The distortion energy can be released from the occurrence of deformation recovery and recrystallization. In the tensile strength test, the fracture happened at the base metal instead of the soft zone because the soft zone is narrow and the microhardness is only about 1% softer than that of the base metal. During erichsen test, fractures happened vertically in the weld at the laser power 2000W and welding speed 2m/min,4m/min. The increasing of the weld's microhardness was the main reason for decreasing of the joint's shaping ability. When the laser power was 4000W and the welding speed was 2m/min, the tearing occurred along the soft zone. In the condition the soft zone was so wide that the shaping ability of the joints became poor.A slab CO2 laser with an output power of 3500W was used to do lap welding experiments protected by double gas nozzle. The results show as the welding speed is increased, the weld's width becomes narrower, the maximum shear stress of welded joint is decreased, but the shear strength is increased. The reason is that the content of bainite with low toughness is decreased in the weld with the increasing of welding speed. The maximum shear strength of welded joints can be increased by optimizing the form of lap welding.