Study On Microstructure And Mechanical Properties Of Dual Beam Laser Welding Brazing Al/Steel Dissimilar Materials

In order to achieve energy saving and emission reduction,the joining of high strength steel and aluminum alloy has been applied in automobile manufacturing to reduce the weight of car body.Laser welding technology is widely used in car body manufacturing process due to the requirements of high speed,precision and automation.However,the great difference in thermophysical properties between aluminum and steel makes it difficult to ensure the reliability of the joint,especially the poor metallurgical compatibility between aluminum and iron makes it easy to form brittle intermetallic compound(IMC)during the welding process,which would be harmful to the mechanical properties of the welding joint.Based on that,how to effectively control the formation of IMC is essential to improve the mechanical properties of Al/steel welding joint.So it is in great need to carry out the study on the welding technology of Al/steel dissimilar materials and do research about the IMC formation mechanism.Therefore,in this paper,the research on dual beam laser welding brazing of DP590high strength steel and AA7075 aluminum alloy is carried out.The relationship between welding brazing parameter and joint forming is obtained.The formation mechanism and influencing factors of IMC at Al/steel interface is revealed.The mechanism of IMC morphology and distribution on mechanical properties of joints is clarified.This paper provides experimental data and theoretical support for obtaining high-quality welded joints of Al/steel dissimilar alloys.Firstly,welding brazing of Al/steel dissimilar materials experiment is conducted.By changing the laser power,welding speed and beam energy distribution ratio,it is found the forming quality increase with the increase of heat input.The wetting angle decrease and the wetting distance first increase and then decrease with the decrease of main beam energy ratio.By observing the cross section of the joint,it is found there are IMC layers leaving Al/steel interface and migrating into welding brazing seam.The migration of IMC layer only shows in the areas where laser beam directly irradiates.Numerical simulation result shows that the flow field in the molten pool is the driving force for the migration behavior of the IMC layer.The forming quality of the joint obtained by laser welding brazing with filler wire is worse than that of the joint obtained by dual beam laser welding brazing.However,because of changing of the flow field,the driving force of the IMC layer migration is eliminated,so the migration of the IMC layer doesn't show in the joint obtained by laser welding brazing with filler wire.Secondly,the microstructure of the welding brazing joint is characterized.Thick IMC layer shows at middle Al/steel interface while the IMC layer is much thinner at the root and toe of Al/steel interface.The thickness of the IMC layer in the middle of the Al/steel interface is about8?12?m under different welding process parameters and the thickness of IMC layer increase with the increase of heat input.There are two different morphologies in the IMC layer.Needle-like IMC composed of Fe Al₃(?phase)shows at welding brazing seam side and flatten IMC layer composed of Fe₂Al₅(?phase)shows at steel side.Coarse columnar crystals grow in flatten IMC layer due to the oriented diffusion of aluminum and iron atoms.For migrated IMC layers,the migrated IMC layers near Al/steel interface composed of?phase have the similar morphology with the flatten IMC layer at Al/steel interface;the migrated IMC layers far away from Al/steel interface composed of?phase and?-Al have loosen morphology..There is a newly formed IMC layer at the Al/steel interface below the migrated IMC layer,which composed of?phase.The thickness of newly formed IMC layer is much thinner and the morphology is uneven.Finally,the mechanical properties of Al/steel welded joints under different welding process parameters are tested.In the tensile test,the joint failing at seam is ductile fracture mode with relatively high strength;the joint failing at the Al/steel interface is cleavage fracture mode with poor strength.The best welding parameter is laser power of 1.3 k W,welding speed of 0.6 m/min and laser beam energy distribution ratio of 8:2.The tensile strength of the joint using this welding parameter can reach 130.1N/mm.The wetting distance less than 2.2 mm or IMC thickness at Al/steel interface exceeding 12?m will risk the joint failing at the interface.The observation and analysis of the cross section of the tested tensile specimen indicate that the migration of the IMC layer would weaken the strength of the Al/steel interface below it.Therefore,the occurrence of the IMC layer migration should be avoided as much as possible.