Research On Resistance Spot Welding Technology Of Al-Si Coated Hot Stamping 22MnB5 And Galvanized DP590

In order to reduce the weight of automobiles and achieve the goal of energy saving and emission reduction,the application of advanced high strength steels is more and more widely.Due to its good formability and high ductility,22MnB5 steels have become first choice for hot stamping parts.To meet the requirements of surface quality and dimensional accuracy of hot stamping parts,Al-Si coated 22MnB5 steels for hot stamping has emerged.The DP590 steels is also used in large quantities on the body parts,DP590 sheet after galvanized surface treatment has excellent corrosion resistance.In existing joining process,resistance spot welding is still the primary method of joining body parts.The spot weld performance of the two lap joints determines the NVH(noise,vibration,and harshness)characteristics of the car body.Al-Si coated 22MnB5 after hot stamping process,the reaction between the coating and matrix creates the Fe-Al-Si intermetallic which was hard and brittle.The differences between two materials in the matrix composition,coating hardness,melting point,resistivity,resulting in spatter easily during the resistance spot welding process.The spatter will lead to narrow welding process window and poor mechanical properties of welded joints.In view of the above problems,this article studies the Al-Si coated hot stamping 22MnB5/galvanized DP590 resistance spot welding process and the mechanical properties of welded joints,as well as the influences of Fe-Al-Si coating thickness on the resistance spot welding process.In this paper,two kinds of high-strength Al-Si hot-stamping 22MnB5 and zinc-coated DP590 high-strength steels,which are used in a domestic automobile factory,are studied.Designing a reasonable resistance spot welding process to solve the problem of welding spatter and improve the mechanical properties.The main contents of the thesis are as follows:(1)Designing the double pulse resistance spot welding process based on the existing resistance spot welding technology.The first pulse applies a small current for a long time to clean the zinc-lap interface,and the second pulse applies a high current for a short-time to obtain the ideal nugget size.The longer the dwell time between two pulses,the larger the resistance spot welding process window.(2)During the welding process,the metal in the nugget zone melts first and then solidifies.Elemental segregation easily occurs around the fusion line because of the fusion line in the area of the solid-liquid interface.Forming the high-carbon martensite as carbon elements segregated.Meanwhile,Carbon and manganese are segregated to form residual austenite.Among the residual austenite,the hardness of high-carbon martensite is higher than the hardness of the nugget zone,which easily becomes the stress concentration zone and leads to the brittleness of the nugget.After the end of the second pulse,the descending time is added to promote the diffusion of carbon,it can inhibit the nugget brittle fracture and improve the mechanical properties of the welded joints.(2)Fe-Al-Si intermetallic compounds with different thicknesses were obtained by different heating regimes,and the spot welding process windows of different thickness intermetallic compounds were explored.The thicker the intermetallic compounds,the narrower the process window.