6082-T6铝合金高铁结构件双丝MIG焊连接技术基础研究
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摘要
中国人口众多、内陆深广，解决大规模人口流动问题，最安全、快捷、经济、环保和可靠的交通方式是高速铁路。而高速列车发展的方向就是安全与轻量化。铝合金结构车体具有轻量化、增加荷载，减少制动距离和减少运输能耗等优点；同时，在严寒季节和高纬度地区无低温脆性，吸收冲击能量强，在发生事故时，更能保障人员的安全，在高速列车车体制造中铝合金具有其它材料不可替代的特点。高速列车铝合金车体制造关键技术之一是高强铝合金的焊接。高速列车铝合金车体结构件的焊接难点在于焊接工程量大，焊接距离长，构件对几何精度、化学成份和材料处理等方面要求非常严格。本文结合高速列车铝合金车体的生产，开展了6082-T6铝合金高铁结构件双丝MIG焊连接技术基础研究。
双丝MIG焊焊接速度快和熔敷量大，焊接6082-T6铝合金焊缝固态相变后的组织比单丝MIG焊接6082-T6铝合金焊缝组织要复杂的多，随匹配焊接材料的比例、与母材混合后的化学成分和冷却条件的不同，可出现不同的组织。双丝MIG焊接6082-T6铝合金接头由焊缝、热影响区和母材三部分构成，焊接中厚板能保证焊缝一次成型。焊缝中心区由等轴晶和柱状晶组成，透射电镜研究揭示，焊缝相组成为α-Al和β-Mg2Si。焊丝中添加Mn和Zr在焊接过程中易形成呈弥散状分布的析出相，析出相能够钉扎晶界，抑制晶粒长大，提高铝合金焊接接头的力学性能。热影响区与母材原始组织比较，相组成未发生变化，但组织形貌发生较大的变化，该区组织晶粒相对粗大，发生再结晶，部分晶界局部熔化，部分低熔共晶聚集在三角晶界处，保留冷轧的带状痕迹。双丝MIG焊接6082-T6铝合金接头的抗拉强度和延伸率分别为238.33MPa和7.87%，断裂位置发生在接头的热影响区，得到的接头抗拉强度与延伸率达到母材的60～66%与45～50%。
双丝MIG焊接6082-T6铝合金除了应对进行单丝焊时所考虑的各种焊接参数外，还要兼顾双丝焊的相关因素对焊缝的影响。双丝MIG焊接6082-T6铝合金工艺参数要依据接头尺寸形状、坡口形式以及焊缝成形的要求，同时也必须考虑对裂纹、气孔和热影响区软化的影响来确定。结果表明，双丝MIG焊接6082-T6铝合金采用双脉冲电流方式与前脉冲后直流和前直流后脉冲两种电流方式比较，焊接过程中电弧稳定，飞溅较少，焊缝成形美观，接头热影响区较窄，焊后结构件变形小。双脉冲电流方式、前脉冲后直流和前直流后脉冲三种电流方式得到的接头抗拉强度分别为238.33、187.04和207.02MPa，延伸率分别为7.87%、2.48%和2.33%。焊接电流是决定焊缝熔深的主要因素。增大焊接电流可提高生产率，但随着电流的增加，焊缝组织逐渐粗化，二次枝晶间的距离增加，焊接热影响区宽度增大，并易产生过热组织，降低接头韧性。焊接速度的确定受制于焊接电流，焊接速度较小时，熔融的金属位于电弧前面填充焊缝坡口，形成虚焊现象；焊接速度较大时，熔融的金属无法填满熔池，易造成咬边缺陷。当主机焊接电流I＝205A、焊接速度v＝120cm/min和接头采用V形坡口时，焊缝组织最为细小，接头熔透，熔宽为9.41mm，余高为0.37mm，接头的抗拉强度为248.01MPa，延伸率为8.20%。
铝合金的化学活泼性极强，线膨胀系数大，导热性好，作者经过多年对高速列车铝合金车体焊接工艺的研究，通过对生产现场收集的焊接缺陷进行分析，发现双丝MIG焊接6082-T6铝合金，接头存在裂纹、气孔以及高速列车铝合金车体在运行过程出现腐蚀现象等缺陷，并较为系统地研究了双丝MIG焊接6082-T6铝合金接头缺陷的形貌特点和形成原因，为预防接头裂纹和气孔的出现以及腐蚀的发生提供生产与技术依据。双丝MIG焊接6082-T6铝合金，得到接头组织相对粗大，组织的不均匀性可以使接头各部位的电极电位产生不均匀性，也要影响接头的耐腐蚀性。通过动电位极化曲线、腐蚀速率、腐蚀产物和表面形貌的分析，研究了6082-T6铝合金双丝MIG焊焊接接头的腐蚀过程与行为及其对焊缝与母材力学性能的影响。焊缝发生腐蚀时晶界处的Mg2Si强化相构成连续的阳极链状通道，脆性相的析出是降低焊缝耐腐蚀性的主要原因之一。
双丝MIG焊接6082-T6铝合金，焊缝的熔深仍较浅，对厚板结构件需要开坡口和大电流进行多道焊接，对接头热影响区和焊缝组织与性能的影响较大，接头易产生焊接变形。研发了提高双丝MIG焊接铝合金焊缝熔深的活性剂。结果表明，活性剂的成分显著影响双丝MIG焊接6082-T6铝合金的焊缝熔深、焊接过程电弧稳定性和焊后焊缝成型。活性剂的颗粒尺寸影响其与接头的附着强度以及焊缝组织的晶粒大小。单位面积活性剂的量影响活性剂的活性效果，表面喷涂活性剂的量过大，会产生焊缝夹杂、熔合不良和未焊透等焊接缺陷。采用正交设计试验方法，对SiO_2-Cr_2O_3活性剂成分和工艺进行优化，SiO_2-Cr_2O_3活性剂中SiO_2质量分数为88wt%，平均颗粒度为48μm，喷涂量为0.9mg/cm_2时，在相同焊接参数和接头不开坡口的条件下，未喷涂活性剂得到的焊缝熔深为3.14mm，喷涂活性剂得到的焊缝熔深为5.78mm。喷涂活性剂与未喷涂活性剂得到的焊缝比较，熔深提高84.08%。
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 reached62~72%and45~50%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 SiO_2-Cr_2O_3active flux and theprocedure were optimized. The best levels of each factor were achieved as follows. The masspercentage of SiO_2is88%in SiO_2-Cr_2O_3active flux, the average particle size is48μm andthe spraying amount is0.9mg/cm_2. Under the same condition, the weld penetration was upto5.78mm, which was increased by84.08%compared to the one without flux.
双丝MIG焊焊接速度快和熔敷量大，焊接6082-T6铝合金焊缝固态相变后的组织比单丝MIG焊接6082-T6铝合金焊缝组织要复杂的多，随匹配焊接材料的比例、与母材混合后的化学成分和冷却条件的不同，可出现不同的组织。双丝MIG焊接6082-T6铝合金接头由焊缝、热影响区和母材三部分构成，焊接中厚板能保证焊缝一次成型。焊缝中心区由等轴晶和柱状晶组成，透射电镜研究揭示，焊缝相组成为α-Al和β-Mg2Si。焊丝中添加Mn和Zr在焊接过程中易形成呈弥散状分布的析出相，析出相能够钉扎晶界，抑制晶粒长大，提高铝合金焊接接头的力学性能。热影响区与母材原始组织比较，相组成未发生变化，但组织形貌发生较大的变化，该区组织晶粒相对粗大，发生再结晶，部分晶界局部熔化，部分低熔共晶聚集在三角晶界处，保留冷轧的带状痕迹。双丝MIG焊接6082-T6铝合金接头的抗拉强度和延伸率分别为238.33MPa和7.87%，断裂位置发生在接头的热影响区，得到的接头抗拉强度与延伸率达到母材的60～66%与45～50%。
双丝MIG焊接6082-T6铝合金除了应对进行单丝焊时所考虑的各种焊接参数外，还要兼顾双丝焊的相关因素对焊缝的影响。双丝MIG焊接6082-T6铝合金工艺参数要依据接头尺寸形状、坡口形式以及焊缝成形的要求，同时也必须考虑对裂纹、气孔和热影响区软化的影响来确定。结果表明，双丝MIG焊接6082-T6铝合金采用双脉冲电流方式与前脉冲后直流和前直流后脉冲两种电流方式比较，焊接过程中电弧稳定，飞溅较少，焊缝成形美观，接头热影响区较窄，焊后结构件变形小。双脉冲电流方式、前脉冲后直流和前直流后脉冲三种电流方式得到的接头抗拉强度分别为238.33、187.04和207.02MPa，延伸率分别为7.87%、2.48%和2.33%。焊接电流是决定焊缝熔深的主要因素。增大焊接电流可提高生产率，但随着电流的增加，焊缝组织逐渐粗化，二次枝晶间的距离增加，焊接热影响区宽度增大，并易产生过热组织，降低接头韧性。焊接速度的确定受制于焊接电流，焊接速度较小时，熔融的金属位于电弧前面填充焊缝坡口，形成虚焊现象；焊接速度较大时，熔融的金属无法填满熔池，易造成咬边缺陷。当主机焊接电流I＝205A、焊接速度v＝120cm/min和接头采用V形坡口时，焊缝组织最为细小，接头熔透，熔宽为9.41mm，余高为0.37mm，接头的抗拉强度为248.01MPa，延伸率为8.20%。
铝合金的化学活泼性极强，线膨胀系数大，导热性好，作者经过多年对高速列车铝合金车体焊接工艺的研究，通过对生产现场收集的焊接缺陷进行分析，发现双丝MIG焊接6082-T6铝合金，接头存在裂纹、气孔以及高速列车铝合金车体在运行过程出现腐蚀现象等缺陷，并较为系统地研究了双丝MIG焊接6082-T6铝合金接头缺陷的形貌特点和形成原因，为预防接头裂纹和气孔的出现以及腐蚀的发生提供生产与技术依据。双丝MIG焊接6082-T6铝合金，得到接头组织相对粗大，组织的不均匀性可以使接头各部位的电极电位产生不均匀性，也要影响接头的耐腐蚀性。通过动电位极化曲线、腐蚀速率、腐蚀产物和表面形貌的分析，研究了6082-T6铝合金双丝MIG焊焊接接头的腐蚀过程与行为及其对焊缝与母材力学性能的影响。焊缝发生腐蚀时晶界处的Mg2Si强化相构成连续的阳极链状通道，脆性相的析出是降低焊缝耐腐蚀性的主要原因之一。
双丝MIG焊接6082-T6铝合金，焊缝的熔深仍较浅，对厚板结构件需要开坡口和大电流进行多道焊接，对接头热影响区和焊缝组织与性能的影响较大，接头易产生焊接变形。研发了提高双丝MIG焊接铝合金焊缝熔深的活性剂。结果表明，活性剂的成分显著影响双丝MIG焊接6082-T6铝合金的焊缝熔深、焊接过程电弧稳定性和焊后焊缝成型。活性剂的颗粒尺寸影响其与接头的附着强度以及焊缝组织的晶粒大小。单位面积活性剂的量影响活性剂的活性效果，表面喷涂活性剂的量过大，会产生焊缝夹杂、熔合不良和未焊透等焊接缺陷。采用正交设计试验方法，对SiO_2-Cr_2O_3活性剂成分和工艺进行优化，SiO_2-Cr_2O_3活性剂中SiO_2质量分数为88wt%，平均颗粒度为48μm，喷涂量为0.9mg/cm_2时，在相同焊接参数和接头不开坡口的条件下，未喷涂活性剂得到的焊缝熔深为3.14mm，喷涂活性剂得到的焊缝熔深为5.78mm。喷涂活性剂与未喷涂活性剂得到的焊缝比较，熔深提高84.08%。
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 reached62~72%and45~50%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 SiO_2-Cr_2O_3active flux and theprocedure were optimized. The best levels of each factor were achieved as follows. The masspercentage of SiO_2is88%in SiO_2-Cr_2O_3active flux, the average particle size is48μm andthe spraying amount is0.9mg/cm_2. Under the same condition, the weld penetration was upto5.78mm, which was increased by84.08%compared to the one without flux.
引文
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