| Aluminum-lithium alloy is a head of new structural materials with low-density and high-performance. Along with the rapid development of the aerospace industry, aluminum-lithium alloys has also been increasingly widely used, which can reduce the structural quality 10% to 15%, the stiffness increased by 15% to 20% when compared with conventional aluminum alloy. With a wide range of applications of the aluminum-lithium alloy in the aerospace industry, the increased welding research has been taken seriously.The introduction of welded structures into aerospace engineering promises further weight and cost benefits by replacing riveting and mechanically fastened joints. Friction stir welding (FSW) is a new solid-state joining process in which a rotating tool, consisting of a shoulder and a probe, is plunged into the joint and traversed along the joint line to form a weld, and in addition it is an attractive technique to weld Aluminum-lithium alloys. Through optimizing the welding process, we obtained defect-free weldments with good quality.With the help of macroscopic deformation tests, micro-hardness measurement, tensile test, and the properties of the joints were evaluated. Combined approaches were used to get detailed microstructure features within the weld, e.g. Optical Microscope, Scanning Electron Microscope(SEM), Transmission Electron Microscope (TEM), The following conclusion were obtained:(1) Through optimizing the FSW process of 2 series of aluminum-lithium alloy with the thicken of 1.5mm, the favorable weld process parameters were identified without obvious internal defects. The selected welding condition is as follows:the double ring rotating tool with shoulder diameter is 10mm, the rotational speed of 800 rpm and a translational speed of 200 mm/min. Then the better performance and optimize the welding process window has been attained.(2) Under the optimized welding process, tensile properties of the joint are as follows:The yield strength Rpo.2=324MPa,ultimate tensile strength Rm=439MPa, elongation δ%=9.04%,the strength coefficient can reach 85.7%.The fracture locations of joints the NZ, where may be weakest place along the weld, Which presents ductile-brittle hybrid pattern fracture.(3) Under different welding process parameters, the micro-hardness distribution along the weld shows the same tendency, which "U" shape appears and the micro-hardness within the base material (BM) is greater than other ones, the hardness is about 170 HV, while in the nugget zone(NZ)or between TMAZ(thermo mechanically-affected zone) and NZ,the hardness is lowest than BM and averages 110 HV. The hardness in HAZ(heat-affected zone) is lower than BM but higher than NZ.(4) The base material consisted of large flakes grain structure which was elongated along the rolling direction. The weld nugget zone(WNZ) shows an asymmetrical’bowl’shape, which has obvious display material flow patterns of the "onion ring" shaped plastic flow lines. In the nugget the grain structure is fine recrystallized equiaxed grains.The grain structure of the thermol-mechanically affected zone (TMAZ) is different from the that of base material, grains has large deformations and was stretched. While the grains of heat-affected zone is coarse and the deformation is small.(5) The Al-Li base material consists of complex precipitates, including Ti (A2CuLi)、 θ’(Al2Cu) and S’etc. Therefore, the substrate strength, hardness and other mechanical properties are better. The elevated heat input and severe plastic deformation during FSW resulted in different effects across the weld: in the nugget zone, both the primary strengthening T1 and secondary θ’and S’ precipitates dissolved. The secondary θ’and S’ precipitates dissolved and the strengthening T1 large in size were found in TMAZ, where hardness declines. Within heat affected zone, the precipitate type is the same as the base material. However, hardness decrease is primarily due to the strengthening relative to the size of T1 changes and amount of dissolved θ’ precipitates. |
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