| C24S is a newly developed Al-Li alloy by A1COA. 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 high-efficiency, high-quality, envirementally friendly, effective, solid-state welding process, and in addition it is an attractive technique to weld Al-Li alloys. Through optimizing the welding process, we obtained defect-free weldments with good quality.With the help of micro-hardness measurement, tensile test, fatigue and corrosion behavior test, the properties of the joints were evaluated. And combined approaches were used to get detailed microstructure features within the weld (e.g. Optical Microscope, Transmission Electron Microscope (TEM), Scanning Electron Microscope(SEM), Differential Scanning Caborimetry(DSC),3Dimentional Atom Probe (3DAP) et al), The following conclution were obtained:(1) Through optimizing the FSW process, the favorable weld process parameters were identified without obvious internal defects. The selected welding condition is as follows:a rotational speed of2400rev/min and a translational speed of100mm/min with in-process water cooling. In-process water cooling plays an important role in the quality of the weldment.(2) Under the optimized welding process, tensile properties of the joint are as follows:The yield strength σ0.2=357MPa, ultimate tensile strength σb=435MPa, elongation δ=4.8%. The fracture locations of joints are near the interface between the NZ and TMAZ at the advancing side, where may be weakest place along the weld. The fracture surface shows clear banded morphology, which may be attributed to the materials flow and the formation of semi-cylindrical band during the process of FSW.(3) Under different welding process parameters, the micro hardness distribution along the weld shows the same tendency, and the micro hardness within the weld nugget zone is the same as well. he hardness of the base materials is about～165HV, while in the nugget zone the hardness is lower than BM and averages～135HV. The lowest hardness is found at TMAZ or near the interface between TMAZ and NZ, where several locations with very low hardness (below～120HV) were identified. From the TMAZ to the base material, the hardness increased.(4) The base metal consisted of large pan-caked grain structure which was elongated along the rolling direction. The weld nugget zone (WNZ) shows an asymmetrical’basin’shape. In the nugget the grain structure is fine recrystallized equiaxed grains with an average grain size of2.3μm.The grain boundaries in the nugget zone are mainly of high angle large than (>15°). The grain structure of the thermol-mechanically affected zone (TMAZ) is different from the base material, with distorted and deflected grains.(5) The Al-Li base material consists of complex precipitates, including T1,θ’ and S’ precipitates. The elevated heat input and severe plastic deformation during FSW resulted in precipitation variation across the weld:in the nugget zone and TMAZ, both the primary strengthening T1and secondary0’and S’precipitates dissolved, while small amount of cubic shaped precipitates were found in TMAZ. Within heat affected zone, the precipitate type is the same as the base material. However, due to the variation of the hardness value in HAZ, it is suggested that HAZ experienced dissolution and overaging of precipitates. The hardness variation within the weld can be explained by the dissolution and coarsening sequences of precipitates.(6) After immersion in EXCO solution for96h, the base material showed severe exfoliation corrosion, but the weld nugget zone, thermol-mechanically affected zone, and a portion of the heat affected zone which is near to the WNZ were not susceptive to the exfoliation corrosion. After immersion in IGC solution for6h, severe pits developed within the base material, where no intergranular corrosion was found. Within weld nugget zone, intergranular corrosion and small pits were found. Slight pit corrosion and a small number of intergranular corrosion were developed within the heat affected zone.(7) C24S alloy was susceptible to the stress corrosion in 3.5%NaCl+0.5%H2O2solution. Its tensile stress and elongation in the solution is lower than that in air. While the FSW joints shows similar result in the solution and air, indicating that C24S FSW joint possesses low stress corrosion susceptiblity.(8) The fatigue tests were carried out on MTS858with R=0.1at a frequency of30Hz. The fatigue limit of the base material and FSW joint is a σmax=185MPa, and105MPa, respectively. |
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