| With the rapid development of high-speed railway,aluminum alloy is widely used in high-speed train manufacturing.Welding quality of aluminum alloy directly affects the manufacturing cost and safety.However,there are many problems in fusion welding of the aluminum alloy,such as porosity,inclusion,welding heat crack and joint softening,etc.These problems have limited the application of aluminum alloy in train manufacturing to some extent.Friction stir welding(FSW)is a new type of solid phase bonding technology,which is considered as one of the most effective methods to solve welding problem for aluminum alloy welding,especially in the welding aluminum alloy thick plate.6xxx or 7xxx large aluminum alloy extrusion profiles is mainly used in the high speed train.Therefore,it is of great practical significance to study the microstructure and properties of 7xxx aluminum alloy FSW joints.This study used OM,SEM,SEM-EDS,TEM and layered microstructure observation and tensile performance test methods,focuses on the relationship between microstructure and properties of 40mm thick 7B05-T5 aluminum alloy double FSW joints,especially the uniformity of microstructure and properties along the thickness direction of the joint.The study found:1.Microstructure of friction stir weld joint of aluminum alloy 7B05 consists of nugget zone,thermo-mechanically affected zone(TMAZ),the heat affected zone(HAZ)and base material.The microstructure of nugget zone is greatly refined due to dynamic recrystallization.Crystal grain becomes small;Crystal grains of TMAZ show an elongated shaped due to severely plastic deformation during FSW;The grain structure in the HAZ region,which has not been disturbed mechanically by FSW,is similar to that of the base metal,but it is larger than that of the base metal;2.Microstructure of nugget zone along the thickness direction is affected by welding temperature gradient.The grain size of the surface is larger and central layer grain size is small;3.Microhardness distribution curves are typical of the "W" distribution characteristic.The hardness of the nugget zone is lower,and the softening zone is located in the heat affected zone.The lowest values of the hardness values are in the left softening zone of the joint.The hardness of the middle and lower position in the nugget zone is decreased due to the heat affected by the post welding;4.The change of mechanical properties is related to the features of the strengthening phase of the joint in the process of FSW.In the parent material,the TEM micrographs revealed a very fine distribution of intragranular precipitates.In addition,coarser precipitates were evident in both high-angle grain and subgrain boundaries.In the HAZ,the TEM micrographs shows a representative micrograph of intragranular and grain boundary precipitation.Precipitates inside the grains remain homogeneously distributed.Compared to the parent-material microstructure,the strengthening precipitates have coarsened severely.In the NZ,only a small amount of very fine in homogeneously distributed precipitates were detected inside the grains with much finer and less spaced grain boundary precipitates;5.Tensile properties show that the joint efficiency reached more than 85%.Fault location is located in heat affected zone,and the region of softening hardness test determined;6.Mechanical properties along the thickness direction through the hierarchical method show that the tensile strength of the specimen is relatively low in lower layer,The fracture locations of all samples are located in the heat affected zone,and the softening region determined by the microhardness surface distribution is the same;7.For the small tensile samples with different depth in the NZ,the fracture position of the specimen does not fracture along "Z" line,which indicated that the "Z" line was not the weak area of mechanical properties. |
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