Study On Microstructures And Mechanical Properties Of Dissimilar Aluminum Alloy Joints By Friction Stir Welding

Nowadays the aluminum alloys are widely used in the manufacture of Highspeed trains. However, the traditional welding methods used for aluminum alloys such as MIG or TIG, lead to gas pores, cracks and other welding defects easily. As a solidstate welding method, FSW solve those problems fundamentally and is becoming the worldwide development trend of welding methods and research focus on the high-speed train manufacturing. 6N01 and 7N01 aluminum alloys are widely used in the manufacture of CRH, so the welding between them is inevitable in some structures such as corbels and so on. Because of the difference of chemical constitution and the precipitation behaviors between those two aluminum alloys, it makes high demands on welding consumables and welding procedures when tradition fusion welding methods are used. For FSW, the welding consumable is unnecessary and joint properties are better. Therefore, it has great significance to do the research on microstructures and properties of dissimilar FSW joints of the 7N01-T4/6N01-T5 aluminum alloys, which can reduce manufacturing costs and improve the reliability of the train.FSW joint can be divided into advancing side(AS) and retreating side(RS) which are asymmetry, so when the base metal of two different aluminum alloys are located on the different sides, there are two types of 7N01/6N01 aluminum alloy joints. Those two types of joints may have different mechanical properties. Firstly, the characteristics of microstructures of those dissimilar joints were studied, the results showed dissimilar joints were composed of welding nugget zone(WNZ), thermomechanically affected zone(TMAZ), heat-affected zone(HAZ) and base metal(BM). The microstructure in WNZ was fine equiaxed grains. Elongated grains were discovered in TMAZ as a result of the hit by plastic material during welding and the density of grain dislocations was higher in AS. The grains in HAZ are seriously coarsened as a result of the effect of welding thermal cycle. At the 6N01 aluminum alloy side, a great number of needle-shaped β’’ precipitates were discovered in BM whose size was between 20 nm and 40 nm. Precipitates in WNZ and TMAZ dissolved during FSW. The original β’’ precipitates coarsened or transformed into rod-shaped β’ phase in HAZ and the degree of transformation from β’’ phase into β’ phase was different in two types of dissimilar joints.Studies on the mechanical properties of two types of dissimilar joints showed that the side of 6N01 aluminum alloy was the weakness of the joint,. At that side, the hardness decreased from BM to HAZ and then went up from HAZ to WNZ. At the same time, that side was the major area where tensile fracture and fatigue fracture of the joints occurred. The setting of two types of aluminum alloys had effects on the mechanical properties of the joints. When 6N01 aluminum alloy was located on AS, The strength of the joint was improved by 5% and impact toughness of the joint would be twice, and it had a weaker decrease of hardness in HAZ and narrow softened zone, but its fatigue property was worse, which decreased by 15.5MPa. Both of two types of dissimilar joints had great bending property.The strength would be the best when the β’’ phases were the major precipitates in 6N01 aluminum alloy. The HAZ at 6N01 aluminum alloy side was the weakness of the joint due to a large number of β’ precipitates which contributed less to dispersion strengthening. The hardness in BM was the highest because of the effects of dispersion strengthening brought by a great number of β’’ phases. The solid solution strengthening effects and the fine grain strengthening effects was great in WNZ, and in TMAZ, good solid solution strengthening effects and dislocations strengthening effects existed, so the hardness in those areas of the joints were higher than that in HAZ. The fatigue failure of two types of the dissimilar joints occurred in HAZ. When 6N01 aluminum alloy was located on RS, it could be observed from fatigue fractures that the surface was rougher, which contributed to roughness-induced crack closure. Meanwhile, β’ phases were coarsened which contributed to bridging effect to delay the fatigue crack propagation rate, so that type of joints had a better fatigue property.