Dissertation > Industrial Technology > Metallurgy and Metal Craft > Welding, metal cutting and metal bonding > Welding process > General approach > Welding defects and quality inspection

Basic Study on the Technology of Twin Wire MIG Welding Applied To6082-T6Aluminum Alloy Components of the High Speed Railway

Author RuanYe
Tutor QiuXiaoMing
School Jilin University
Course Materials Processing Engineering
Keywords Alloy components of the high speed railway Twin wire MIG welding Microstructures Corrosion Active flux
CLC TG441.7
Type PhD thesis
Year 2012
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China has a large population and its inland is profound. The safest, the most efficient,the most economical, the most environmentally friendly and the most reliable means oftransportation is the high-speed railway to solve the problem resulted by large-scalepopulation flow. The developing direction of high speed trains is safety and lightweight.Aluminum alloy car body structures with lightweight have the advantages such as increasingthe load, shortening the braking distance and reducing energy consumption and so on intransportation. At the same time because of aluminum alloys without low temperaturebrittleness in the cold season or at high latitude areas, their train cars can absorb huge impactenergy to protect persons in the accident. Also aluminum alloys have features that othermaterials do not have in the high-speed train car manufacturing. High strength aluminumalloy welding is one of the key techniques in manufacturing high speed train cars ofaluminum alloys. During the manufacturing the welding difficulty lies in the large amountsof welding engineering, long distance of welding, very strict requirements for the geometricaccuracy, chemical compositions, material treatments and other aspects. In this paper thebasic study was carried out on the technology of twin wire MIG welding applied to6082-T6aluminum alloy components of the high speed railway combined with the production of highspeed train bodies of aluminum alloys.For the high welding speed and the large deposition amount of filler materials of twinwire MIG welding, the weld microstructures of twin wire MIG welded joints after solidphase transformation are more complex than those by single wire MIG welding, in whichdifferent types of microstructures may occur with the deposited proportions of matchedwelding materials, the weld chemical compositions after mixing with the base metal anddifferent cooling conditions.The twin wire MIG welding joints of6082-T6aluminum alloys were composed of threeparts: weld seam, heat affected zone and base metal. And twin wire MIG welding formedium-thick plates could be finished in one pass. The center of the weld seam wasconsisted of equiaxed and columnar grains and phases of α-Al and Mg2Si were found there.Fine intermetallic compounds were formed in the weld leading to finer grains and improvingthe mechanical properties of aluminum alloy joints by adding Mn and Zr to the weldingwire.Compared to the original microstructures of base metal, the phase compositions in HAZwere not changed but their morphologies changed greatly. The grains became coarser andcrystallization took place there. Moreover, partial grain boundary melted locally and somelow melting point eutectic gathered together in the triangle grain boundary. On the whole thefeatures of cold rolled strips were retained in HAZ. Thus HAZ was the weakest position ofthe welded joints because fracture occurred there. The tensile strength and elongation rate oftwin wire MIG welded joints of6082-T6aluminum alloys were238.33MPa and7.87% respectively, which reached6272%and4550%corresponding values of the basematerials.Welding parameters of twin wire MIG welding of6082-T6aluminum alloys must beconsidered not only including those in single wire MIG welding, but also factors related totwin wire MIG welding process being taken into account. Process parameters in twin wireMIG welding of aluminum alloy6082-T6was formulated not only by the joint geometric,groove type and weld forming requirements, but considering the effect of cracks, gas poresand HAZ softening on welding quality. The results showed that more stable arc, less spatters,good weld appearance, narrow heat affected zone and small deformation after welding wereachieved by the double pulse current mode in twin wire MIG welding of aluminum alloy6082-T6, compared with the other two current modes like pulse plus DC current or viceversa.The tensile strength of joints with double pulse current mode, pulse plus DC currentmode and DC plus pulse mode were respectively238.33,187.04and207.02MPa. And theelongation rates were respectively7.87,2.48and2.33%. The welding current was the keyfactor to determine the weld penetration. Increasing welding current could raise productivity.But the weld microstructures got coarser and overheat microstructures appeared in HAZ. Atthe same time, the distance between secondary dendrites and width of HAZ became bigger.All these could deteriorate the joint toughness.Welding speed was selected depending on the welding current. When welding speedwas slow, the molten metal filled the weld groove in the front of arc and resulted in falsefusion of base metal. On the other hand, when welding speed was too fast, the molten metalcould never fill in the welding pool and the undercut defect easily appeared. Under thecondition of the host current of I=205A and welding speed of ν=120m/min, the weldmicrostructures were finest. The complete penetration was gained, weld width of the jointwas9.41mm and the reinforcement height was0.37mm. In that case, the tensile strengthand elongation rate of the welded joint were248.01MPa and8.20%respectively.Aluminum alloy has many features, such as extremely high chemical reactivity, biglinear expansion coefficient and good thermal conductivity. After the research on the weldingprocedure specification of high-speed train cars of aluminum alloys for many years, theauthor found cracks and pores of twin wire MIG joints of6082-T6aluminum alloy bygathering the welding defects at production sites and corrosion defects during service. Thendefect appearance characteristics and the influencing reasons were systematically analyzedand productive and technical base were put forward to prevent cracks, pores and corrosion.As microstructures of twin wire MIG joints of6082-T6aluminum alloys wererelatively coarse, the microstructural inhomogeneity resulted in the electrode potentialinhomogeneity of different parts in welded joints. The corrosion resistance was affected aswell. The corrosion process and behavior and their effects on the mechanical properties ofweld seam and base metal were studied through analyzing of dynamic potential polarizationcurves, corrosion rates, corrosion products and surface morphologies of twin wire MIGjoints of6082-T6aluminum alloys. Continuous chain-like channels in anode were formed bystrengthening phase of Mg2Si in grain boundaries. So coarse grains and precipitation ofbrittle phase were the main reasons reducing the corrosion resistance of the welded joints.The penetration of6082-T6aluminum alloy joints gained by twin wire MIG welding was still relatively shallow. When welding thick plates, grooves must be prepared andmultipasses with high currents have to be carried out. In that case the microstructures andproperties in both weld and HAZ were affected greatly and welding deformation was liableto occur. Active fluxes were researched to enhance the penetration of twin wire MIG weldedjoints of the aluminum alloy.The results showed that the ingredients of active fluxes affected significantly thepenetration of twin wire MIG joints of6082-T6aluminum alloy, welding arc stability andwelding seam forming. The adhesive strength of the flux with joint surface and grain sizes ofthe weld microstructures were influenced by particle sizes of the active flux. Activity effectdepended on the quantities of the active flux on per unit area. But excess fluxes sprayed onjoint surface would bring about welding defects such as inclusions in weld, bad fusion andlack of penetration, etc.By applying orthogonal design, the ingredients of SiO2-Cr2O3active flux and theprocedure were optimized. The best levels of each factor were achieved as follows. The masspercentage of SiO2is88%in SiO2-Cr2O3active flux, the average particle size is48μm andthe spraying amount is0.9mg/cm2. Under the same condition, the weld penetration was upto5.78mm, which was increased by84.08%compared to the one without flux.

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