Investigation On Microstructure And Mechanical Properties Of Laser Fusion Welding-Brazing Aluminum Alloy To Galvanized Steel

Today,the issue of energy conservation and emission attracts much attention.The automotive industry has proposed the concept of lightweight car,using high-strength aluminum alloy instead of traditional steel to form a steel-aluminum integrated frame structure.Aluminum and steel have large differences in physical and chemical properties,and both of them will undergo metallurgical reaction during the welding process,generating brittle intermetallic compounds and reducing the mechanical properties of the joints.Therefore,the key point of developing the automobile bodies is how to achieve the reliable joint between Al and steel.Laser welding is widely used in the automotive industry because of the advantages of small heat input,fast cooling speed and high automated degree.Laser fusion welding-brazing is considered to be a feasible method,which can control the generation of intermetallic compound to some extent.However,in laser fusion welding-brazing aluminum alloy to steel,the morphology and growth of the interface structure,mechanical properties of joints and fracture characteristics remain to be further studied.Laser fusion welding-brazing 6061 aluminum alloy to galvanized steel without filler metal was investigated by full-factor experiments,and this article established a welding process window.The effects of energy input?E?on the forming,microstructure and mechanical properties of joints were studied in detail.With the increase of the energy input,the weld became poorer and the content of Fe near the interface increased obviously,which made the microstructure of aluminum alloys become tree-whisker shape.Besides two common intermetallic compounds?lamellar IMC and acicular IMC?which were reported before,other two new intermetallic compounds?IMC?,namely stellate IMC and skeleton IMC were found at the joint under high energy input.The chemical composition and possible phase types of different IMCs observed were determined according to energy dispersive spectroscopy?EDS?results.The temperature history during welding were simulated based on a finite volume model to further investigate the influence of energy input on diffusion process which was thought to be the decisive factor for IMC growing.The growth orientation and phase morphology were determined by temperature histories,local iron atom fraction and constitutional supercooling.It could be concluded that the increasing energy input was harmful for mechanical properties because it increased thickness of lamellar IMC and brought skeleton IMC which led to some microcracks.Further,Laser fusion welding-brazing 6061 aluminum alloy to galvanized steel with different kinds of filler metal(CuSi₃,AlSi₁₂ and ZnAl₁₅)were investigated separately in order to improve joint performance.The welding processing window was optimized and microstructure of the weld joints was analyzed in detail.The joints filled CuSi₃ wire had a special copper/aluminum transition region composed of Cu₉Al₄ and CuAl₂,which was thought to be a weak area of the joint.When laser fusion welding-brazing Al to steel with AlSi₁₂ wire,The interfacial IMCs of with an average thickness of 2-10?m had lamellar IMC and acicular IMC,and the interface phase was composed of Fe₂?Al,Si?₅,Fe?Al,Si?₃and Al₈Fe₂Si,with the addition of Si into the filler metal.However,the brazing interface between Al and steel filled with ZnAl₁₅ wire only had lamellar IMC.The interfacial intermetallic compounds with an average thickness of 12?m mainly composed of Fe2Al5Zn0.4.At last,the mechanical properties of Al-steel welds with different kinds of filler metal were tested,and the fracture characteristics were emphatically analyzed.The welds filled CuSi₃ wire were fractured at coarse skeleton area of the Cu-Al transition region,and the maximum tensile force of joints was 1.23kN.When filling AlSi₁₂ wire,there are many kinds of fracture locations.No fusion at the weld root in the aluminum alloy,the porosity of zinc,no brazing partially,microcracks at the weld toe and hard and brittle IMC phase,all of this would be the sources of cracks and induced joint fractures.The maximum tensile force of joints was improved to 2.48kN.When filling ZnAl₁₅ wire,there are still many kinds of fracture locations but no obvious weakened areas,indicating that the brazing performance of aluminum alloys was improved.When the fracture occurred in the base metal of the aluminum alloy,the tensile strength of the joint reached 3kN.The study found that filling ZnAl₁₅ wire was the best way to improve the mechanical properties of joints.