Study On Mechanism And Process In Cold Metal Transfer Spot Welding Of Aluminum Alloy And Zinc Coated Steel

In order to save energy and reduce exhaust emission,lightweighting of vehicle bodies becomes the trend of automotive industry development.Aluminum alloy use in the automotive industry is increasing to realize the lightweighting target with the advantages of low density and high strength.However,in terms of the cost and manufacturing difficulties of aluminum alloy,some manufacturers use a range of materials with an approach of using the right material for the right application at the current stage.This strategy then translates to a mainstream reality of dissimilar material application in the car body which poses a challenge for conventional joining processes.On account of the large melting point,thermal conductivity and electrical resistivity differential between aluminum and steel and the formation of hard and brittle Fe-Al intermetallic compound（IMC）during fusion welding process,it is difficult to make a weld with traditional joining process.Therefore,it appears that there is a need for the development of new aluminum to steel joining process which is flexible and relatively low cost.In this study,cold metal transfer spot welding of aluminum and steel was proposed based on CMT（Cold Metal Transfer）welding system.The mechanical performance,macro morphology,microstructure,hardness of the welds,arc shape and metal transfer process were studied.The growth of IMC layer at the faying interface was further investigated in regards to the welding heat input.The correlation of IMC layer-weld size-weld strength was systematically analyzed.The influence of galvanic corrosion on the properties of the welds were also studied.Aluminum and steel CMT weld-bonding process was proposed based on the corrosion mechanism of dissimilar materials weld.A reliable joining process of aluminum and steel process was developed for dissimilar materials car body.This dissertation consists of four parts:1)Study of welding mode of CMT spot weldingThree different welding modes,e.g.direct welding（DW）mode,plug welding（PW）mode,and edge plug welding（EPW）mode,were investigated and compared in terms of mechanical properties,macro-and microstructures,and IMC properties of the joint.The following conclusions were obtained:The DW mode provides the easiest path for implementation,however,substantial heat input is required to melt the top aluminum sheet.The large,and concentrated heat input resulted in a severe tearing fracture,shrinkage voids,and uneven IMC layer along the faying surface leading to poor joint properties.Welding with the PW mode could significantly reduce the heat input requirement.However,because of the intense heat input at the center of the hole,severe solidification shrinkage voids were typical the IMC layer was uneven.The zinc vapor trapped in the hole led to severe instability of the welding arc,metal transfer process,and thus poor joint quality.The EPW mode homogenized the heat input into the hole making the welding process significantly more stable since the zinc vapor could easily escape from the hole.A uniform and thin intermetallic compound was obtained and leading to the highest strength joints,4.084 k N peak strength,with low welding distortion.2)Study of fracture mechanism of aluminum and steel CMT spot weldingIn order to study the fracture mechanism of the welds,micro hardness,metallographic analysis of the weld grain structure,as well as microstructural and chemical composition analysis of the IMC layer were further studied.Specifically,the correlation of IMC layer-weld size-weld strength was systematically analyzed.The characteristic weld at a wfs of 4.2 m/min and 5.6 m/min produced a zinc-rich zone at the weld root and a continuous and compact IMC layer in the middle portion along the faying surface.As expected,joint strength increased as the weld nugget size increased and the joint strength,4.1 k N,could be obtained at a wire feed speed,wfs,of 5.6 m/min.Fracture initiated at the root and propagated into the weld nugget of the aluminum sheet in the through-thickness direction.The propagation within the weld nugget followed the boundary between columnar and equiaxed grains.The significant increase of IMC layer thickness which was over 8μm at higher large input caused the weld to fracture completely along the brazed interface through the thick and hard IMC layers.3)Thermal distribution at the faying interface and growth mechanism of IMC layerThe temperature at some thickness in the steel was measured by infrared camera and the heat distribution at the faying interface was calculated by extrapolation method.The growth behavior of the IMC phase was investigated in regards of thermodynamics and dynamics and the thickness of the IMC layer was calculated.The temperature at the faying interface could be obtained precisely by infrared measurement combined with extrapolation method and the highest heat accumulation was reached at the torch path angle of 270°.The IMC phase formed at the interface possibily was Fe Al、Fe₂Al₅and Fe Al₃ and the kinetic coefficient for growth of Fe₂Al₅was the largest which is the most prone to form at the interface.Parabola relationship between IMC layer thickness and heat input was obtained which can predict the IMC layer growth and ultimate IMC thickness.The best welding parameters for EPW mode was that the wire movement angle was 22.5°and the wire alignment to the hole edge was 0 mm.4)Corrosion mechanism study of aluminum and steel CMT spot weldOpen circuit potential（OCP）of base materials and different portions of the CMT spot weld from top to the bottom were measured first.The welds with and without e-coating after exposure for 20 and 63 cycles were investigated and compared in terms of joint morphology,mechanical properties,macro-and microstructure.A weldbonding process of aluminum and steel was proposed based on CMT welding process.The influence of adhesive on the arc welding process,microstructure characteristics which including intermetallic compounds layer,mechanical performance and corrosion resistance of the weld were investigated.The OCP of the zinc coating on the steel substrate and zinc rich zone at the notch root were the most negative which were most prone to corrode.The OCP of Fe-Al IMC layer in the interface mixed with zinc was also very low which will start to corrode after the corrosion of zinc rich area.The OCP of aluminum alloy and steel substrate were the most positive indicating the strongest corrosion resistance.Then the IMC layer will start to corrode after the corrosion of zinc rich area.20 cycles of exposure did not produce obvious effect on weld mechanical property.However,after exposure for 63 cycles,the welds degraded significantly and the strength dropped to almost 62%.E-coating has a very positive influence on corrosion property of the welds.A closer distance between adhesive layer and the joint center resulted in adhesive’s more serious burnout,more welding defects in the joints and poor mechanical properties,however a farther distance resulted in a good appearance and satisfied mechanical properties.Adhesive can effectively enhance the corrosion resistance of the CMT welding joint.There was only 7.36%decline of the strength of the weldbonding joint after three weeks’immersion corrosion test and the result satisfies the anticipation.