| Al-Zn-Mg-Cu(7XXX)series aluminum alloys as super-duralumin alloy developed with aerospace technologies since 1940 s,which is characterized by higher strength to weight ratio,good mechanical properties,good corrosion resistance,recyclable advantage etc.7 Series aluminum alloys has become competitive structural material widely applied in aerospace industry,national defense,rail vehicles and automobile industry.Super-hard aluminum alloy mainly formed currently by extrusion and die forging,seldom formed by arc welding.Hot crack,microscopic pores and low strength of super-duralumin alloy joint are prone to occur with traditional welding method,and FSW solid phase joining is used only.Double-pulsed MIG has unique high-low frequent pulse-coupled stirring role,which made the Al-Zn-Mg-Cu series aluminum alloy fusion welding possible.6.35 mm rolled super-duralumin alloy of AA7075-T651 welding are investigated in the paper,based on DP-MIG welding method and adopted “under-matched” ER5356 and ER5556 welding wires and V-groove butt welding.Experiment scheme: Firstly DP-MIG welding parameter optimation.Analyse double-pulsed arc shape,droplet transition,research low-fenquency pulses effects on microstructures and mechanical properties.Then based on research results above,which is applied mechanical vibration and determine optimum parameter of vibration frequency and amplitude through expriment.Finally applied post-weld heat treatment(PWHT)on the optimal weld joint,then research on weld joint mechanical and fatigue properties of different wires,nanometer microstructure of weld joint various positions such as crystal boundary,twin crystal boundary and segregation etc.And also observed aging strengthening separated phase size,shape and distribution,found influence mechanisms of heat treatment T6 scheme on microstructure and mechanical properties.Optimal DP-MIG welding parameters and combined “double-pulsed MIG-mechanical vibration” is used for AA7075-T651 aluminum alloys welding,and processed heat treatment.The paper draws the main conclusions as follows:(1)AA7075-T651 super-duralumin alloy is welded with ER5356 and ER5556 welding wires by DP-MIG process.Optimum process parameters are as follows: the welding average current I=190A,welding velocity V=0.78 cm·s-1,welding low-frequency impluse F=3Hz,arc stiffness is-6,shield gas flow rate is 18L/min,and the wire extension is 12 mm.It draws to the following conclusion: The weld joint tensile strength used with ER5356 welding wire is 292.5MPa,reaches to 52.0% of base material tensile strength,the minimun hardness of the weld joint is 80.8HV,elongation is 3.5%.While the weld joint tensile strength used with ER5556 welding wire is 342.5MPa,reaches to 60.9% of base material tensile strength,the minimun hardness of the weld joint is 90.6HV,elongation is 8.5%.Compared with ER5356 welding wire,the tensile fracture surface of weld joint with ER5556 has obviously characteristic of tear rige,deep and uniform dimple with many second phases,namely microvoid coalescence fracture pattern.While the weld joint of ER5356 has characteristic of little tear rige,shallow and big dimple with little second phases,presents mixed type fracture pattern,namely transcrystalline fracture and intergranular fracture coexisted.(2)The optiaml process prameters of “ Double pulsed MIG-mechanical vibration ” mixed technology described below: the welding current I=190A,welding velocity V=0.78 cm cm·s-1,vibration amplitude A=0.02 mm,vibration frequency F=40Hz.The combination technology revealed well interaction of welding parameters and vibration parameters.With vibration,the tensile strength of ER5356 weld joint is 352.6MPa,the joint strength coefficient is 0.627.The columnar fine grain appears in the vicinity of the weld zone,oriented to columnar grain growing direction.Fine precipitated phase reveals dense distribution,the joint central zone is fine uniaxial dentrite and cellular dendrite oriented with vibraction direction.Obvious porosity and tiny crack have not seen under microscope,and the joint texture with vibration is finer compared with no vibration,combined DP-MIG high-low pulsed mixing with vibration,could obviously reduce hot crack and accelerate gas overfolw.(3)After post-weld heat treatment(solution treatment(480?×50 min)+ water quench at indoor temperature + artificial ageing(80?×24h)+ artificial ageing(120?×24h)),Comparative test of non-vibration joints follows,ER5356 joints tensile strength is 445.7MPa,the joints strength coefficient increased from 0.520 to 0.793,ER5556 joints tensile strength is 490.0MPa,the joints strength coefficient increased from 0.609 to 0.872.Using “double-pulsed MIG-mechanical vibration ” mixed technology,ER5356 joints tensile strength is 490.2MPa,the joints strength coefficient increased from 0.558 to 0.872.(4)PWHT enhances AA7075-T651 super-duralumin alloy weld joints mechanical property fundamentally owing to the types,numbers and distribution of aging precipitated phase.TEM shows that aging precipitated phase MgZn2 uniformly distributes with differently size and shape among weld joint zone,which increases mechanical property and improves “soften”region.low-cycle fatigue shows joint fatigue life of ER5556 is longer than ER5356,ER5356 joint has high cyclic deformation resistance with 0.3-0.5% strain amplitude,while ER5556 joint has high cyclic deformation resistance with 0.6-0.7% strain amplitude.Based on DP-MIG welding of AA7075-T651 aluminum alloy,vibration and post-weld heat treatment are both effective ways for reducing welding defects and improve welding quality.The research provides the theory base and guidance for MIG method welding super-duralumin alloy. |
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