Study On The Behavior And Mechanism Of Adhesive In Steel/aluminum Weld Bonding

Automobile lightweight has become the frontier and hotspot of automobile technology in twenty-first century, the using of lightweight materials such as high strength steel, aluminum alloy plays an important role for vehicle lightweight. And laser welding is the ideal method that the connection of steel/aluminum can be realized, while for dual phase steel and aluminum alloy welding, because it's easy to form crack defects or the organization appear softening phenomenon during the dual phase steel welding and aluminum alloy with high reflectivity, it's very difficult to realize the dual phase steel/aluminum alloy high quality welding. To explore a new method of steel/aluminum connection, this paper selecs DP590 high strength steel and 6016 aluminum alloy as the research object, the experimental scheme of laser lap welding of steel and aluminum with or without adhesive addition and plasma spectrum are designed. By using horizontal metallographic microscope, scanning electron microscope with EDS and EBSD, electron backscatter diffraction, energy dispersive spectroscopy, micro hardness tester, microcomputer control electronic universal testing machine, high speed cameras and micro spectrometer and other means to study the microstructure of welded joint region, fracture morphology, distribution and content of phase, grain size, interface combining situation, hardness, mechanical properties of welded joints, the morphology of welding plasma and the spectral information of plasma.To explore the mechanism of adhesive addition on the basis of the contrast analysis about plasma's shape, element characteristic spectral lines, electron density, weld formability, microstructure and performance and the research results are expected to provide a new thought of research for automotive dual phase steel/aluminum alloy welding and provide important theoretical and experimental basis for promoting the automotive lightweight.Analyzing the plasma's shape, element characteristic spectral lines and electron density found during the laser welding the plasma of steel and aluminum is substantially perpendicular to the board, while during laser weld bonding, the plasma often form a certain angle with the surface of welding plate.Thus, the angle formed will decrease plasma's absorption of laser energy and the particle that adhesive layer decomposed is conducive to improving the high reflectivity of aluminum alloy. In addition, with the help of NIST spectral information database, the author finds that whether the adhesive exists or not, the Fe and Mn elements, especially the Fe, show obvious characteristic of spectral line in the wavelength range of 515 nm ~ 545 nm. Compared the relative intensity of spectrum and electron density of plasma before and after adding layer and found that more laser energy is absorbed by the specimen in the process of laser weld bonding.Analyzing weld joint formability, microstructure and performance of steel and aluminum found that adding adhesive will get good formability of the surface of weld specimen and no softening tissue formation of weld zone, and compared with the specimen without adhesive, the average shear strength is increased by about 1. 44 times. Adding adhesive will increase the absorption of laser energy for aluminium alloy, accelerate the cooling rate of dual phase steel, lead to the proliferation from Al to steel side restrained, make the content of Al on both sides of weld width increase, and reduce the content of Al in the molten pool.Adding adhesive increased the effective contact area of steel and aluminum alloy and improved the bonding, steel / aluminum interface generated iron rich Fe-Al compounds with ductile, the content of ferrite and martensite phase of weld zone is basic quite with dual phase steel base metal, adding layer doesn't deteriorate the upper phase composition of dual phase steel and the grains are uniform distribution, and adhesive with high elastic modulus and small deformation resistance can share shear force with weld, what's more,the uniform distribution of grains helps external force that specimen bears uniformly distributed in the welding joint overlapping region, improving the mechanical properties of welded joints.