| Titanium and its alloys have excellent performance and technology features, andthat have been widely applied in the aerospace industry. The application of fusionwelding techniques to titanium alloys resulted in severe distortion, formation of pores orinclusions, coarse microstructure, thus solid-state joining technologies would appear tobe more suitable for joining titanium alloys. In this paper, a thickness of2mm TC4titanium alloy sheet was joined by friction stir welding. Influence of process parameterson weld appearance were analyzed, and the optimal welding parameters were obtained.Microstructural features and mechanical properties of the joints were examined.The results shown that plunge depth of the shoulder, tilt angle and rotation speedof the tool and other welding parameters have impact on weld formation. Surfacegrooves, cavities and other defects could be found when improper welding parameterswere used. Good surface appearance and defect-free welds could be obtained with therotation speed of475r/min, welding speed of23.5m/min and the welding tilt angle of3°.The weld cross-section seems like ‘basin-shape’, the thermo-mechanicallyaffected zone (TMAZ) is not found, thus the weld only consists of shoulderdeformation zone, stir zone (SZ) and heat affected zone (HAZ). The interface betweenthe SZ and HAZ in the advancing side (AS) is very clear, on the contrary, more vaguein the retreating side (RS). Banded texture can be observed clearly at the bottom of theSZ. There was a thermal gradient through the thickness direction of the welds. Peaktemperature of the shoulder deformation zone, which closer to the top surface of theweld, was above the the β-transus temperature, thus β→α+β transformed during thecooling stage of the welding process and alternate lamellar α+β were formed. However,the SZ is composed of primary α and transformed β which suggested that the peaktemperature in the SZ of the weld below the β-transus temperature. The grain size ofthe SZ is much finer than that of the base metal (BM).The average transverse tensile strength of as-welded joint is940.5MPa when therotation speed of475r/min and welding speed of23.5m/min was used, reaching morethan92%that of the BM. The tensile strength decrease when rotation speed increasingto600r/min, and the globular oxide could be observed on the fracture surfacemorphology which reduced the strength of the joints. Vickers hardness profiles acrossthe thickness of the welds are different, the upper of the welds has the highest hardness value, and there are no difference between the middle and the bottom; Hardnessdifferences also existed between the advancing side and the retreating side, and thisphenomenon is associated with the differences of the flow state of the plastic materialsbetween the both sides during the welding process. The abnormal high hardness valuein the SZ was caused by falling debris from the wear and tear of the tool.After vacuum annealing, TC4titanium alloy has undergone a certain degree ofsoftening, microhardness and tensile strength have declined. The morphology of theFSW joints has changed, and the zigzag-curves also can be observed. Transverse tensiletest results shown that tensile strength of FSW joints are lower than1000MPa, and themaximum is998.2MPa, reaching90%that of the BM in the same state. Microcrack canbe found at the junction of the SZ and HAZ as well as at the bottom of SZ in theadvancing side. Under the stretching force, the crack growth and eventually fracturedthe joint, and thus forming a ‘trench’ in the macroscopic morphology of the fracture bySEM. The existence of the microcrack decreased the mechanical properties of the joint. |
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