Study On The Welding Technology And Microstructure Of Unequal Thickness Laser Welding Tailored Blanks

With the application and development of laser welding tailored blanks(LWTB), so many manufacturer of automotive paid their attention to this technology because it can reduce the automotive weight and the production costs, and improve the security. The microstructure of welded seam on the ST14 steels and High Strength Steels(HSS) LWTBs was analyzed in this paper, as well as its influence on the LWTBs' formability, the welding property of LWTBs with unequal thickness was also discussed. It is wished that all of this can give the theoretical and technical support for the development of LWTBs' quality.The height and width of laser welding seam were affected by laser power and welding speed. The shielding gas can not only prevent the oxygenization and pore, but also restraint the laser-induced plasma. The defect of incomplete fusion would be avoided when the width of divided edge was less than 1/10 thickness of parent material. The pore and undercut may appear in the joints when galvanized steel sheet was welded.Two types of ST14 LWTBs with equal thickness, which is 1.5mm and 0.8mm, were experimented in cup test. The cup test value was compared between welded seam's and parent metal's. Then, the tailor-welded blanks with unequal thickness were examined in tensile test. The split part was inspected after the LWTBs were tensile tested. The microstructure of welding seam and the changes of its hardness were discussed. The tailor-welded blanks' formability was investigated when the laser welding parameter changed. Conclusions are stated. Characteristics of weld line for deep drawing is lower than parent metal, furthermore, the cup test values of LWTBs were influenced by the welding speed, the values were raised when the welding speed grew up. But the weld line's tensile strength is greater than parent metal. The LWTBs' formability will be improved if the spiculate ferrite is yielded when the welding speed and the cooling speed raised. Two kinds of high-strength-steel(HSS) sheets, HSA340(2.0mm, 1.5mm) and H340LAD+Z(0.7mm), were introduced in this study. They were arranged in pairs and welded together by laser welding to be tailored blanks on similar strength but unequal thickness principle. The special cross-section of welded joints was chosen for metallographic examination and hardness measurement. The microstructure of weld and the changes of its hardness were analyzed. The studied results showed, in the welding process, the microstructure of weld will have great diversity at local region dosing each parent metal because of the different cooling condition of each parent metal. The width of heat affected zone(HAZ) in LWTBs is narrow, the remarkable boundary divides the HAZ with the parent meal and weld seam, not only the grain size but also the microstructure in HAZ are fine. The hardness of weld and the HAZ are higher than parent metal. The joints of tailored blanks will be obviously improved when the incident position and incident angle of laser beam were set rationally...