Characteristics Of Laser Beam Oscillating Welding-brazing Of Al/steel Dissimilar Materials

The joining of Al/steel dissimilar materials has always been one research topic with high value.With broad application prospects of Al/steel welded joints in aerospace and automotive manufacturing,one important purpose of studying Al/steel connection is to improve the morphology and mechanical properties of welded joints.So far,there have been many researches on the welding of Al/steel connections,but with high welding difficulties,the results are not satisfactory.Based on existing problems in current researches and urgent needs in industrial production,this paper aims to solve the problems of porosity defects and uneven interface layers of Al/steel welded joints and study the welding technology of laser beam oscillating in Al/steel materials to improve the interface structure,welding defects and the mechanical properties of the joints.Firstly,the welding process of 2mm Al/steel material is systematically studies,different forms of laser beam oscillating are contrasted and analyzed,which mainly focuses on the process characteristics of infinite oscillating.With focusing mode,laser beam oscillating improves the energy utilization,and achieves better weld formations under a larger and more uniform heat source,which solves the problem of non-wetting on the back of the weld.In addition,due to the short existence of the molten pool in the single laser welding,porosity is prone to generate before the gas escapes,which can be effectively addressed by laser beam oscillating.Besides,due to the high energy density at the edges,welding splashing is prone to occur in the horizontal and longitudinal oscillating,while the weld formation is significantly improved with the circular and infinite oscillating,and the tensile property of joints is 186 MPa with infinite oscillating.On that basis,the 5mm Al/steel material was further studied with infinite oscillating.The weld formation improves significantly compared to that with single laser welding with the mechanical property of 62 MPa.Secondly,high-speed photographies of the oscillating filling wire welding under different parameters are observed and analyzed.Affected by the plasma,the single laser filler wire welding performs poor in stability.And in the horizontal and longitudinal oscillating welding splashing is prone to occur due to the nonuniform energy distribution with large energy input at the edge.While ring and infinite oscillating shows excellent stability and no splashing with uniform energy input and stable-sized keyhole.In terms of molten pool flow,in the horizontal oscillating filling wire welding,the front end of the molten pool is squeezed,which accelerates the liquid flow and expands the molten pool to increase its existence time.While for the ring oscillating,an ring vortex appears inside the molten pool and moves forward smoothly.In infinite oscillating welding,the liquid flows faster and the heat transfers more uniform with complicated oscillating and violent stirring on the molten pool.Finally,the micro-structures of the Al/steel joints is studied and analyzed to get the weld structures and interface layers under single laser and different oscillating modes.Results shows that the weld structure under oscillating welding has no obvious defects,and the phase composition of the IMC layers at the brazing interface of the steel side performs different under different heat input conditions.With high welding heat input,there are three main phases including Fe2Al5 layer,Fe Al3 layer,and Al8Fe2 Si layer.While with moderate input,there are the Fe Al3 layer and Al8Fe2 Si layer.The thickness of the upper and lower parts differs greatly with single laser welding,while with all modes of oscillating welding,interface layers can be more uniform through adjusting the interface thermal field distribution.Among them,the infinite oscillating welding performs the most continuously uniform with the average thickness of 4.6 ?m.Through the FEM(finite element method)simulation,the thermal cycle curve data at the steel-side interface is obtained,and the Gibbs free energy data of each phase are calculated accordingly.By comparing these data at different temperatures,sequence of these phases generation is obtained to determine the compositions of these interface phases.