Study On The Joint Formation Control And Mechanical Property Of Twin-spot Laser Welding-Brazing Of DP590/AA7075 Dissimilar Metals

Substitution of the traditional steels with light materials aluminum alloys with low density and high specific rigidity and application of advanced manufacturing technologies are effective methods to meet the lightweighting requirement for the automotive body-in-white manufacturing.However,in order to ensure the safety of automobiles,the joining of steel and aluminum brought by the partial replacement of high-strength steels by aluminum alloys became an inevitable problem.The brittle intermetallic compound（IMC）are easy to form during the thermal joining process due to the large difference in the thermophysical properties of steel and aluminum,which causes the deterioration of the joint mechanical properties.Based on the weldability of steel and aluminum,this study proposes a novel welding-brazing technology using a twin-spot laser beam to join DP590 galvanized high strength steel and AA7075 high strength aluminum alloy without using flux or filler metal.The weld formation and its influence factors of dissimilar metal welding-brazing joints were systematically studied and the morphology of intermetallic compounds were systematically characterized.The weld formation and IMC evolution mechanism at the interface were clarified using the established three-dimensional numerical fluid dynamics model of steel/aluminum dissimilar metal twin-spot laser welding-brazing process.The mechanical property of steel/aluminum weld-brazing joint and its internal relevance with IMC morphology,distribution and thickness were clarified to provide guideline for the connection of steel/Al dissimilar metals,IMC formation mechanism and weld quality control in the body-in-white manufacturing industry.Firstly,the influence of the twin-spot configuration,the main spot energy percentage,spot distance,laser power and welding velocity on the weld formation were based on the quantitative analysis of the forming characteristics of the steel-aluminum dissimilar metal twin spot laser welding-brazing joint.The results showed that the sould weld appearance,with wetting angle about 22.6°and spreading length about 3.5 mm,was obtained when the deflected beam was located on the steel plate and tangent to the edge of the Al plate,the main spot energy percentage is 80%,the twin spot distance is 1.2 mm,the welding velocity is 0.7 m/min and the laser power is 1200 W.Based on the calculated temperature distribution and fluid flow field,the intrinsic reason for the improvement of joint formation quality by the twin spot beams was explained.It was found that the mechanism of the wettability improvement by the deflected beam was to vaporize the Zn coating by preheating which avoided the formation of surface oxides and accelerated the spreading of molten Al.After the spreading of molten Al,the front of molten pool liquefied the Zn layer,reducing the interfacial tension and the wetting angle.Meanwhile,the diffusion and dissolution of Fe and Al atoms into the liquid Zn layer promoted the further spreading of the molten pool by reactive wetting.Under the molten pool,the high temperature made the interface Zn coating vaporized after spreading of liquid Al.The weld pores and unwetting zone formed at the weld root area due to the zinc evaporation.After the spreading of molten Al,the deflected beam irradiated the molten pool surface,accelerating the fluid flow and mass transfer,increasing the peak temperature and existence time of the molten pool and effectively improving the spreading of the molten pool.When the twin-spot configuration was closed to the Al side or the main spot energy percentage was higher,the amount of molten Al increased.However,heating area of the deflected beam was decreased,resulting in decreased wetting area and the spreading difficulty of the molten pool.When the deflected spot energy percentage was 50%,the temperature of steel surface was higher and the evaporation zone of zinc coating was larger.Because of the lower energy of main spot,the amount of molten Al was smaller,the molten Al was not able to spread in time and the steel surface may be oxidized before the liquid aluminum was spread,which was not conducive to the wetting of the molten Al.When the twin spot distance increased from the overlapping case（D=0.6 mm）to the non overlapping case（D=1.5 mm）,the width of molten pool increased.However,the low temperature in the middle of the twin spot reduced the fluidity of molten pool and shorten the spreading length.Next,the different IMCs morphologies and phase microstructure of the joint were characterized.The migration behavior of IMC layer into the weld metal was observed and its formation mechanism was explained based on the calculated temperature distribution and fluid flow field.It was found that the interface IMCs were mainly composed of theη-Fe₂Al₅ phase with dense plate-like morphology and theθ-Fe Al₃ phase with three types of morphologies according to different formation mechanism,namely the acicular,sparse plate-like and the irregular morphology.For the migrated IMC layer,with the increase of the distance from the interface,the morphology of the plate-like IMC layer chaged from dense to sparse and theη-Fe₂Al₅ phase inside the dense IMC layer changed into the mixed microstructure ofθ-Fe Al₃ phase and Al inside the sparse IMC layer.The migration position corresponds to the beam position and the maximum distance of migration located under the spot center.The formation of migrated IMC layer was affected by the fluid flow behavior.The downward fluid flow due to the dual vortexes caused by the twin spot continuously impacted the primary formed IMC layer,causing the broken and detachment of the IMC layer.Then the lifted IMC layer was migrated into the molten pool by the fluid flow.The lifted IMC layer reacted with the Al atoms in the liquid molten pool to form aθ-Fe Al₃ phase and the morphology of the IMC layer changed to sparse.At the interface far away from the twin spot center,the downward flow of the molten pool was weak,and theη-Fe₂Al₅ phase formed at different positions along the interface gradually grew up,connected with each other,forming a flat IMC and finally became dense morphology.During the cooling process,the acicularθ-Fe Al₃ phase grew on the dense IMC to the weld metal through the reaction of Al and Fe atom.Finally,the influence mechanism of the IMCs on the mechanical properties of the steel/Al welding-brazing joint was analyzed.It was found that,with the increase of main spot energy percentage,twin spot distance,laser power and welding velocity,the tensile-shear load of the joint increased first and then decreased.Under optimized process parameters with laser power 1400 W,welding velocity 0.7 m/min,twin spot distance1.2 mm and main spot percentage 80%,the tensile-shear load reached its highest level,about 1.33 k N.The failure mode could be divided into WM,IF and WM+IF modes according the joint fracture locations.For the IF fracture mode,the crack propagated along the interface with lower tensile-shear load and brittle fracture characterization,which was caused by the thick and irregular IMC at the interface.For the WM mode,the thickness of the interface IMC layer was thin,which enhanced the interfacial strength and deflected the crack propagation into the weld metal,resulting in higher tensile-shear load.The joint strength with migrated IMC layer depends on the new-formed IMC after migration.When the new-formed IMC was thin（2.9-4.6μm）,the plastic deformation was produced in the surrounding weld metal and steel,which hindered the crack opening and propagation.When the new-formed IMC was too thin（below 1.7μm）and uneven,the interface bonding strength was weakened.When the new-formed IMC was thicker（12.0-12.5μm）,the plastic deformation was produced mainly inside the IMC layer during the crack opening process.The crack would propagate longer distance inside the thicker IMC layer even if the crack propagation energy was lower,resulting in lower tensile-shear load.