Room Temperature Creep Of2219T87Aluminum Alloy And Its Welded Joints

With2219aluminum alloy widely used in the aerospace and other means of transportation, room temperature creep research on aluminum alloy received much attention.To explore room temperature creep mechanism and long-term creep properties of2219T87aluminum alloy, as well as properties evolution after creep, the next four aspects were mainly studied in this paper:(1)300hours creep experiments of2219T87aluminum alloy before and after welding;(2)prediction about the deformation characteristics of2219T87alloy104hours by fitting experimental data;(3)microstructure research on2219T87welded joints after room temperature creep;(4)mechanical properties of2219T87aluminum alloy before and after creep tests. The results were as follows.Room temperature creep behavior of2219T87aluminum alloy and its welded joints in300hours under constant load which is within the yield strength is typical. The creep rate was large in initial creep stage and then gradually decreased over time. The creep curves of300hours creep experiment showed a steady-state creep stage after100hours. The creep deformation was increased as the load increasing. When same load was applied, both the creep rate and deformation of tensile creep were larger than compression creep. Creep deformation of welded joints by friction stir welding (FSW) was larger than base material under the load of200MPa. Meanwhile, creep deformation of TIG welded joints under the load of150MPa was even larger than base material which is under the load of200MPa. These results indicate that the creep resistance of welded joints is lower than base material. It is found that the creep resistance of FSW weld nugget microstructure is similar to the base material.Long-term creep data was fitted by Origin. The fitted curves and data points of300hours creep experiment showed great matching degree, conforming logarithmic law. However, the creep curves under low stress were relatively flat, showing unobvious creep. The104hours deformation characteristics of2219T87aluminum alloy were calculated by extrapolating the fitted curves. It was found that creep value after104hours of base material and welded joints is less than1%under constant load which is within the yield strength. The creep value of the first300hours is about70%of the whole creep value in104hours. These results indicated that creep deformation in the later stage was slow, but continued in time. The phenomenon was rather obvious in the welded joints under constant load.Microstructure of2219T87aluminum alloy was investigated by scanning electron microscope (SEM), showing the lath structure overall. The grain was slender with length of about300μm and width of about25μm. Al2Cu phase was diffusely distributed on grain boundary, strengthening the Al matrix. Precipitation phases were fined because of dislocation movement cutting particles. Plastic deformation of the material may provide a driving force for the precipitation phase re-dissolution, resulting in precipitates content decreased. FSW welded joint consisted of weld nugget zone and HAZ. Grain size in the weld nugget zone was about10μm and significant defects were not observed in the HAZ. TIG weld joints were divided into weld nugget zone, fusion region and HAZ. Grain size of TIG weld nugget zone was about100μm and pore defects existing in HAZ reduced the property of the weld joints.Mechanical properties of2219T87, such as strength, hardness and fracture were also investigated in this paper. It was found that the mechanical properties of base material were superior to welded joints. The strength of material increased slightly and the elongation decreased after creep experiment. HAZ (heat affected zone) of FSW welded joint has the minimum hardness, which is about90HV. Hardness in weld seam center of TIG weld joint is about75HV, which is also the minimum. Appearance of2219T87fracture which was scanned by SEM consisted of dimples with tear ridge, showing a significant ductile fracture characteristic. The fracture of welded joints by TIG and FSW appeared in the HAZ. The ductile fracture characteristic of specimens weakened after creep test.