Effect Of Microstructure On Mechanical Properties And Fracture Behavior Of Friction Stir Welded Joints Of 7050 Aluminum Alloy

Under the action of thermal and mechanical stirring,the microstructures of friction stir welded joints of aluminum alloys have changed in different regions,resulting in mechanical heterogeneity.The heterogeneity of mechanical properties of welded joints will have a great influence on its service.By means of metallographic photographs and hardness curves,the region of friction stir welding joint of aluminum alloy was clearly divided.After that,micro mechanical tensile specimens were taken in all areas of welded joints and tensile tests were carried out.At the same time,the surface strain data of the specimen were obtained by digital image correlation method.For the fractured specimens,the coarse second phase,crack initiation and propagation on the surface and side of the fracture were observed under field emission electron microscopy.At last,the microstructures of the welded joint specimens before and after tensile fracture were observed by transmission electron microscopy?TEM?and electron backscatter diffraction?EBSD?.Based on the above research and analysis,the results show that:1)Compared with base metal,the tensile properties of different areas of friction stir welded joints decreased in varying degrees.The most serious decrease was in heat affected zone.The yield strength of advancing and retreating side decreased 51.23%and 50.58%respectively compared with base metal.The performance of the stir zone decreases the smallest,and the thermal mechanical affected zone is located between the stir zone and the heat affected zone.The strain begins to concentrate in the heat affected zone as soon as the welded joint specimen enters the yield stage in the tensile process.There is no obvious difference between the advancing side and the retreating side.After the stress reaches the tensile limit,the strain in the heat affected zone of the advancing side suddenly increases until the fracture occurs.2)From base metal to heat affected zone,thermal mechanical affected zone and stir zone,the precipitate phase gradually dissolves and coarsens under the action of heat,and the grain boundary precipitate free zone gradually widens,which is the main reason for the decrease of welded joint performance.The grains in the thermal mechanical affected zone are bent and deformed under the mechanical action,which leads to the introduction of a large number of dislocations,and continuous dynamic recrystallization occurs in the stir zone to refine the grains.The above reasons make the performance of these two regions recover.The main reason for the tensile fracture of welded joint specimens occurring in the heat affected zone on the advancing side is that the rotation direction of the stirring needle is the same as that of the welding on the advancing side,while the retreating side is the opposite.This will cause the temperature of the advancing side to be slightly higher than that of the retreating side,the dissolution and coarsening of the precipitate phase and the broadening of the precipitate free zone on the advancing side will be slightly more serious than on the retreating side.Finally,there is a slight difference in macro performance.3)The major coarse second phase of 7050-T7451 aluminum alloy was identified by transmission electron microscope.Furthermore,according to the distribution of the coarse second phase near the fracture surface and the crack initiation and propagation,the order of influence on fracture is Al₂₃CuFe₄,Al₂CuMg,Mg₂Si and Al₇Cu₂Fe.Statistical analysis of the area ratio of the coarse second phase on the surface and side of the fracture revealed that the crack propagated preferentially along the area where the coarse second phase is densely distributed.This trend is enhanced as the strength of the matrix decreases.At the same time,it can be seen that as the temperature rises in the joint,the coarse second phase also begins to coarsen.The coarse second phase of the stir zone is broken due to mechanical agitation,and the size is relatively small.