| In order to achieve the lightweight,aluminum alloy materials are widely used in varieties of parts of high-speed train.7N01 aluminum alloy is the ideal medium-strong weldable materials,mostly used for the end beam,the buffer,the side frame,the corbel,etc.At present,the ER5356 wire is widely used for MIG welding of 7N01 aluminum alloy.The filler metal ER5356 belongs to the Al-Mg system,which can not be strengthened by heat treatment.However,a large number of previous tensile tests show that tensile specimens are broken in the weld zone,while the strength is significantly improved with the natural aging.It indicated that the weld zone occurred natural aging.Therefore,It is reasonable to conclude that the precipitated strengthening phase forming element Zn in the base metal necessarily diffuse to the weld zone in some way and form a supersaturated solid solution during the welding or cooling process,which causes the weld zone occurs natural aging.In addition,previous experiments have found that the natural aging strengthening ability of 7N01 aluminum alloy MIG welded joints is obviously unstable.So,it is important to study the diffusion behavior of Zn element and its effect on the propeties of welded joint so as to reveal the basic factors which affect the natural aging strengthening ability of 7xxx aluminum alloy welded joint.And it is meaningful for evaluating the performance and life expectancy of aluminum alloy welded joint for high-speed train body.In this study,10mm thick 7N01S-T5 aluminum alloy profiles from high-speed train pillow beams were subjected to X-shaped groove machining,and ER5356 wire was used for double-sided four-pass MIG welding.The microstructures and the distribution of Zn elements of welded joint were investigated by OM,SEM,TEM,EPMA and so on.Besides,Combining with the results of the tensile properties and microhardness of the welds which were measured in different natural aging times,the main factors influencing the natural aging behavior of 7N01S-T5 aluminum alloy welded joints and its influence on the mechanical properties of welded joints were studied.The tensile specimens of 7N01S-T5 aluminum alloy welded joint are broken in the weld zone,and the microhardness of the weld zone is also the lowest hardness of the welded joint,indicating that the weld zone is the weak zone of the joint.According to the microstructural characteristics of the welded joints,the weld zone is a typical casting structure with large grain size,and the number of precipitated strengthening phases is low,so the strength and hardness are low in the weld zone.The fusion zone near the base metal side is partially melted and a small equiaxed crystal is formed under rapid cooling conditions with high strength and hardness.The fine second-phases in the quenched zone are re-dissolved in the matrix during the welding process.The η’ phases are gradually precipitated during the natural aging process,and the strength and hardness are higher than those in the weld zone.However,the precipitation is not complete,and the hardness of the quenched zone is lower than the base metal’s.Both the strength and the hardness of the welded joints are improved after the natural aging.During the welding process,the Zn elements in the base metal near the fusion zone have diffused to the weld zone and form GP zone in the natural aging process.After the artificial aging,a small amount of η’ phases are formed in the weld zone.These precipitates increase the strengthening of the weld zone.However,the aging strengthening ability of the weld zone is limited due to the small amount of precipitated phases.So the microhardness of the weld zone has been stabilized at about one month after the natural aging.The Zn concentration in the quenching zone of the heat affected zone is higher than that in the weld zone,and the natural aging strengthening ability is stronger.After the artificial aging,the precipitated phase precipitates more completely.During the welding process,the base metal near the molten pool and the filler metal participate the formation of the molten pool.And the "dilution" effect of the base metal on the filler metal determines the Zn concentration in the weld zone,which influencing the mechanical properties of the welded joint.Due to the X-shaped groove characteristics of the welding test plate,the "dilution" effect of the base metal on the different welding passages of the weld zone is different.And the "dilution" effect of the first weld metal is obvious,which means that the mixing ratio of the base metal is high in the first weld metal.So,Zn concentration is higher and the natural strengthening ability is stronger than other weld metal’s.It is the reason that the tensile strength of the weld metal at the center layer is higher than that of the upper surface and the lower surface.And the microhardness of the is higher than that of other weld zones.In addition,the strength and microhardness of the first weld metal is improved more obviously after natural aging.Therefore,The proportion of the first weld metal in the weld zone is a key factor affecting the ability of the natural aging of the welded joint. |
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