Study On Microstructure, Mechanical Properties And Interface Behavior Of Laser Welding Steel And Aluminium With Powder Addition

Lightweight has become the inevitable trend of the development of theautomobile industry. The use of light materials of the vehicle body plays an importantrole in the lightweight vehicle. When light materials are considered to reduce vehicleweight, the connection of steel and aluminum must be taken into account. However,aluminum and steel have vastly different crystal structure and physical, chemicalproperties. It is difficult to combine steel with aluminum and is easy to generatebrittle intermetallic compounds. Compared to conventional fusion welding with alarge amount of heat input and difficult to control, laser welding has many advantages(such as heat concentration, easy to control, large aspect ratio of welding seam, etc).It is an ideal method of welding steel and aluminum. Therefore, the technicalproblems to realize the combination of the interface of steel and aluminum and reducethe brittle impact of FeAl intermetallic compounds are urgent to be resolved.6016aluminum alloy and zinc-coated steel which are used in vehicles body are taken as anobject of study in this paper. The experimental scheme of laser lap welding steel andaluminum with or without powder additon is designed. On the basis of thecombination of the first-principle method based on the density functional theory andexperiment, using optical microscopy, scanning electron microscopy, X-raydiffraction, tensile test and Castep, Dmol commercial software, the microstructure,fracture morphology, interface elements distribution, shape and thickness of interfacelayer of intermetallic compounds, phase structure stability, the microscopicmechanism of Fe/Al interface and mechanical properties of the joint are studied in thelaser welding steel and aluminum with and without powder addition. The results canprovide a new thinking, important theoretical guidance and technical supporting forpromoting the application of laser welding of steel and aluminum.The bulk properties of Fe and Al are tested. Using the layer technology, the laserwelding of Fe/Al interface model is established through the calculation of the layerdistance of Fe/Al and lattice mismatch. The interface bonding strength is evaluated bythe work of fracture. The energy and electronic structure of Fe/Al interface model aretheoretical calculated before and after microalloying. Based on revealing Fe/Alinterface microcosmic mechanism, Sn powder with low melting point, high boilingpoint and good convergence with steel and aluminum material is determined as filler powder which is used in the laser welding of steel and aluminum.The laser welding test is carried out on the thickness of1.4mm DC56D+ZFgalvanized steel and the the thickness of1.2mm6016aluminum alloy. With Snaddition, the welding seams surface is continuous, forming uniform, non-obviousholes, cracks and other defects. Grain size is fine. The fluidity of the molten poolmetal is improved. The interface layer is composed of FeAl and FeAl3intermetalliccompound. The thickness of layer is about10-13μm. Because of the change of thesteel and aluminum interface elements distribution with Sn podwer addition, itpromotes the combination of the steel and aluminum interface and decreases thethickness of the intermetallic layer. The new formation of FeSn intermetalliccompound which has good ductility, structure stability at high temperature reduces theformation of Fe-Al intermetallic compound. The mechanical properties of weldingjoints are improved with Sn addition.