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Acta Metall Sin  2015, Vol. 51 Issue (11): 1391-1399    DOI: 10.11900/0412.1961.2015.00099
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Shude JI1(),Quan WEN1,Lin MA1,Jizhong LI2,Li ZHANG1
1 Faculty of Aerospace Engineering, Shenyang Aerospace University, Shenyang 110136
2 Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024
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As a solid state technology, friction stir welding (FSW) has been used to join titanium alloys for avoiding the fusion welding defects. So far, many previous studies have attempted to elucidate the microstructure characteristics and evolution during the FSW process of titanium alloy, but few are about the mechanism of microstructure transformation along the thickness direction of joint. For solving this problem, in this work, 2 mm thick TC4 titanium alloy is successfully welded by FSW. On the basis of numerical simulation, the effects of temperature distribution on the microstructure along the weld thickness direction and the tensile strength of welding joint were investigated. The results show that the peak temperatures of material close to weld surface exceed b phase transus temperature under the rotational speed of 300 r/min and the welding speed of 50 mm/min. With the increase of distance away from the weld surface, the peak temperature decreases. The peak temperature of weld bottom near the backing board is difficult to be higher than b phase transus temperature owing to quick heat radiation. The region, where the peak temperature is higher than b phase transus temperature, consists of primary a, lath-shape a and residual b phases. The size of lath-shape a inside the weld is larger than that near the weld surface. Primary a and b phases with smaller size are attained in the weld bottom owing to the dynamic recrystallization, and the distribution of b phase on primary a matrix is more homogeneous. When the rotational speed reaches 350 r/min, the area where the peak temperature is higher than b phase transus temperature becomes wider along the thickness direction, which makes the size and quantity of lath-shape a phase increase and then the lath-shape a clump appears. Lath-shape a phase with different orientations hinder the propagation of crack and be beneficial for the tensile strength of FSW joint.

Key words:  friction stir welding      TC4 titanium alloy      peak temperature      microstructure      tensile strength     
Fund: Supported by National Natural Science Foundation of China (No.51204111) and Natural Science Foundation of Liaoning Province (Nos.2013024004 and 2014024008)

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Fig.1  Schematic of dimension of tensile specimen (unit: mm; RS—retreating side, AS—advancing side)
Fig.2  Mesh generation used in simulation
Fig.3  Relationship between temperature (T) and thermal properties of TC4 titanium alloy
Fig.4  Relationship between temperature and yield strength of TC4 titanium alloy
Fig.5  Schematic illustration of boundary conditions of scattering heat used in simulation
Fig.6  Simulated and NiCr-NiSi thermocouple temperature measurement of experimental thermal cycle curves of measurement poins during friction stir welding (FSW) for TC4 titanium alloy at rotational speed of 300 and 350 r/min (l—distance from weld center)
Fig.7  Cross section temperature distributions of TC4 titanium alloy weld joints under rotational speeds of 300 r/min (a) and 350 r/min (b)
Fig.8  Macrostructures (a, c) and cross section morphologies (b, d) of TC4 titanium alloy weld joints at rotational speeds of 300 r/min (a, b) and 350 r/min (c, d) (HAZ—heat affected zone, SZ—stir zone, BM—base metal, SAZ—shoulder affected zone)
Fig.9  SEM image of base material of TC4 titanium alloy
Fig.10  SEM images of TC4 titanium alloy weld joints with distances from weld surface d=0.25 mm (a), d=0.75 mm (b), d=1.25 mm (c) and d=1.75 mm (d) along thickness direction at rotational speed of 350 r/min
Fig.11  Schematic illustrations of microstructural evolution mechanism of TC4 titanium alloy weld joint

(a) initial stage (b) welding process (c) slow cooling stage (d) final stage

Fig.12  SEM images of TC4 titanium alloy weld joints with distances from weld surface d=0.25 mm (a), d=0.75 mm (b), d=1.25 mm (c) and d=1.75 mm (d) along thickness direction at rotational speed of 300 r/min
Fig.13  Fracture locations of TC4 titanium alloy joints under rotational speeds of 300 r/min (a) and 350 r/min (b)
Fig.14  SEM images of fracture morphology of TC4 titanium alloy weld joints under rotational speed of 300 r/min (a) and enlarged views of area 1 (b), area 2 (c) and area 3 (d) in Fig.14a
Fig.15  SEM images of fracture morphology of TC4 titanium alloy weld joints under rotational speed of 350 r/min (a) and enlarged views of area 1 (b), area 2 (c) and area 3 (d) in Fig.15a
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