Research On Microstructure And Properties Of Laser Welding-brazing Of Galvanized Steel And 6061aluminum

With the air pollution and energy crisis growing serious,the demand for lightweight of car bodies is rapidly increasing in automotive industry.A great of researches on joining aluminum to steel have been carried on to satisfy the necessities of many industrial applications.Because of their different melting point,density and coefficient of linear expansion made the crack easy to produce in the joint.In addition,the formation of brittle Fe-Al intermetallic phase at the interface that severely affect the properties of the welded joints.Recently,considerable work has been reported on the controlling of the intermetallic compounds and improving the mechanical properties.Laser welding has many virtues,such as high welding speed,high energy density,small heat-affected zones,low distortion,and high levels of automation,which can limit the thickness of intermetallic phase layer.CMT,because of its precise control and coordination between the wire motion and metal transfer were greatly suitable to join the dissimilar metal.This paper utilizes laser welding and CMT to join the same thickness of the 6061 aluminum and zinc steel in lap setup.IPG5000 of laser processing equipment was used for laser welding with the AlSi₅ filler.The welding process parameters:laser power was 3000W,welding speed was 7mm/s,focal length was 308mm.And the wire feed speed was 2.5m/min,3.0m/min and 3.5m/min.Argon gas shielding was provided at a flow rate of19l/min.The cross sections of the joint were analyzed by OM,SEM.There were Fe₂Al₅Zn_0.4,Fe₃(Al,Si)_0.5 and Fe₃(Al,Si)_0.7 phases were found at the steel side in the filler/steel fracture surface by XRD.The result of the micro-hardness of the joint shows that the micro-hardness in the interface layer was highest.The tensile test shows that the fracture mainly occurred at the filler/steel interface.The strength of the joint wire feed speed 2.5mm/s was 30.5MPa,with the wire feed speed 3.0mm/s was 61.3MPa,with wire speed 3.5mm/s was 70.7MPa.Fronius RCU5000i arc welding equipment was used with AlSi₅ filler.The welding current was 60A,welding voltage was 14.4V.Analyzed the weld seam shaping and the microstructure of the cross joint.The tensile test shows the strength with welding speed of3.5mm/s(1#)was 97.11MPa,with welding speed of 4.4mm/s(2#)was 113.12MPa and with welding speed of 5.5mm/s(3#)was 137.63MPa.The fracture location of 1#sample and2#sample occurred across the heat-affected zone,the fracture surface was in vertical with the direction of tensile force,which was called tensile stress fracture.There were cracks were found at the weld root both the 1#sample and 2#sample.The crack grow up along the heat affected zone and fusion zone.The fracture of 3#sample occurred across the interface between filler and steel.The fracture surface was in parallel with direction of tensile force,which was called shear fracture.Low-cycle fatigue experiments were performed on the specimen welded by CMT at home temperature,a stress ratio R of 0,a frequency of 10Hz on Zwick/Roell Fatigue testing machine.Specimens reaching 1 million cycles were stopped and considered run-outs.Processing the test dates and fatigue fracture to analyzed fatigue properties.The result of the fatigue test shows that 1#sample at a maximum stress?_max of 22.5MPa has a life time of 1 million cycles.Sample of 2-1#at a maximum stress?_max of 25MPa has a life time of 74 cycles.The fracture occurred at the heat-affected zone.Sample of 2-2#at a maximum stress?_max of 22.5MPa has a life time of 59800 cycles.The fracture also occurred at the heat-affected zone.The relationship between the stress and strain was analyzed by the hysteresis loop.The properties of cyclic hardening and cyclic softening were analyzed by the curve of strain amplitude and cycle life times.The fracture morphology were observed by SEM.The fatigue fracture of the aluminum/galvanized steel joints can be divided into three phases such as:(a)fatigue source area,(b)fatigue expanding area,(c)the fatigue final rupture area.There were many fatigue source areas in the fatigue area and many river pattern and second micro-crack at the fatigue strip.Presence of dimples in the final failure location indicated ductile fracture.