Abstract
The 2A14-T4 Al-alloy T-joints were prepared by stationary shoulder friction stir welding (SSFSW) at different welding speeds. A smooth T-joint surface was obtained with the optimized welding process parameters. The results show that the microstructure of weld nugget zone (WNZ) consists of fine equiaxed grains caused by full dynamic recrystallization. The average grain size of the second weld nugget zone (WNZ2) is the largest, that of the weld nugget overlap zone (WNOZ) is in the middle, and that of the first weld nugget zone (WNZ1) is the smallest. The recrystallization mechanism in WNZ is mainly the geometric dynamic recrystallization accompanied by partial continuous dynamic recrystallization. The weak {100}<001> texture is formed in the WNOZ after two times of stirring, and the weak {111}<110> texture appears in WNZ1 and WNZ2. The thermo-mechanically-affected zone (TMAZ) undergoes plastic deformation, while the heat-affected zone (HAZ) is only affected by weld thermal cycles. No plastic deformation or the dynamic recrystallization of grains occurs in HAZ. The hardness of WNZ is high, and the zone with the lowest hardness is located in HAZ which is close to TMAZ. With increasing the welding speed, the ultimate tensile strength is firstly increased and then decreased. The mixed brittle/ductile fracture dominates the fracture mode of skin and stiffener plates.
Science Press
T-joints of aluminum alloys are widely used in aerospace, petrochemical, automobile, and agricultural equipmen
Stationary shoulder friction stir welding (SSFSW) invented by TWI was firstly used to weld titanium alloys with low thermal conductivity and good welding quality was obtaine
The microstructure and mechanical properties of SSFSWed T-joints are greatly affected by tool design, temperature field distribution, and welding parameters. However, few researches discuss the new SSFSW process for T-joint fabrication. Therefore, further investigation for SSFSWed T-joints of aluminum alloy is needed. This research performed welding tests of SSFSWed T-joints of 2A14-T4 Al alloy and analyzed the temperature field distribution, the forming quality on the weld surface, the evolution of microstructures, and the related mechanical properties of alloys prepared at different welding speeds. The results provide the engineering application of 2A14-T4 Al alloy and the development of SSFSW process.
The 2A14-T4 Al alloy was used as the base material (BM). The length, width, and thickness of skin plate were 300, 200, and 8.5 mm, and those of the stringer plate were 300, 100, and 8.5 mm, respectively. The chemical composition and mechanical properties of 2A14-T4 Al alloy are presented in
A YXR33 tapered stirring pin with right-handed screw threads of ultra-high strength, high ductility, and excellent corrosion resistance was used to improve the material plasticity. Its length, root diameter, and front diameter were 92, 10.6, and 7.4 mm, respectively (
Fig.1 Appearances of SSFSW tools for T-joint fabrication: (a) stirring pin, (b) stationary shoulder, and (c) welding fixture assembly; schematic diagram of welding fixture assembly (d)
A gantry-type friction stir welding equipment was used in the experiment because of its easy operation. The main stationary angle was 0.5°.
The metallographic specimen was cut from the welded joint by a wire-cut electric discharge machine and then eroded for 25~30 s by the standard Keller's reagent. The microstructures of different regions in cross section of T-joint were analyzed by an Olympus-DSX510 optical microscope (OM). The cross section of the metallographic specimen after corrosion was analyzed by an ARTCAN-300SSI-C microhardness tester (
Fig.2 Schematic diagram of microhardness test for T-joints of 2A14-T4 alloy
The distribution of the temperature field in SSFSWed T-joints was used in the commercial finite element analyzer software ABAQUS. Considering the complex thermodynamic variation during the welding process, the plastic deformation heat of materials and the latent heat of phase change during the structural transformation were ignored, and only the friction heat was taken into consideratio
To optimize the computations, several different types of meshes were adopted. The to-be-welded material was meshed using 8-node linear-heat-transfer hexahedral elements DC3D8. Fine structural meshes were generated near the weld zone, sparse meshes were generated far away from the weld zone, and transitional meshes with the size from 1 mm×1 mm to 5 mm×5 mm were generated in the middle. In general, 61 143 mesh elements in total were generated in the to-be-welded material. Besides, uniform meshes were generated in the stationary shoulder using 10-node secondary-heat-transfer tetrahedron elements DC3D10, considering that no heat was produced during the welding process. 1098 meshes in total were generated in the shoulder part.
Fig.4 Mesh generation results of geometrical model: (a) to-be-welded material and (b) stationary shoulder
Fig.5 shows the comparison between the simulated thermal cycle curves and the measured results of SSFSWed T-joints prepared at a rotation speed of 2000 r/min and a welding speed of 50 mm/min. Only the temperature distribution field in the first pass is considered, because the second pass has a nearly identical temperature distribution pattern with the first welds. The simulated peak temperatures on the advancing side (AS) and the retreating side (RS) exceed the measured data by approximately 5 °C, suggesting that the simulation results fit well with the measured data in terms of temperature variation rate and temperature value.
It can also be observed that during the welding process, the stirring pin is subjected to the material resistance in the forward process, and the heat produced in friction accumulates rapidly. As the stirring pin moves closer to the thermocouple-measured region, its heat grows faster than the heat of insertion phase does. When the stirring point moves to the closest position to the temperature-meter-measured region, the peak temperature can be obtained. Afterward, the stirring pin continues to move forward, getting further away from the thermocouple-measured region. No sufficient heat can be transferred from the stirring pin to the measured region for sustaining the heat dissipation in the thermocouple-measured region, thereby leading to a temperature drop. The peak temperatures at the AS and RS are not completely symmetrical. Specifically, the peak temperature at the AS is 80.6% of that at the RS. This is because the material on the front side is subjected to shear extrusion and gradually rotated to the rear side, causing the material accumulation and higher peak temperature on the RS. Meanwhile, the inconsistency of heat dissipation condition also accelerates the non-uniformity of the peak temperature distribution on two sides.
Fig.6 Distributions of temperature fields in stable welding process using different welding speeds: (a) 25 mm/min, (b) 50 mm/min,
(c) 75 mm/min, and (d) 100 mm/min
Fig.7 Surface morphologies of SSFSWed T-joints after 1st and 2nd passes at different welding speeds
The appearance and inverse pole figures (IPFs) with the information of misorientation angle distributions and mean grain size distributions of SSFSWed T-joint prepared at a rotation speed of 2000 r/min and a welding speed of 50 mm/min are shown in
Fig.8 Cross section of SSFSWed T-joint prepared at rotation speed of 2000 r/min and welding speed of 50 mm/min
It is evident from Fig.9 that the material in WNZ is subjected to violent stirring by the stirring pin and the original structure undergoes an intense thermo-mechanical action, leading to the recovery and recrystallization of WNZ grains. Moreover, WNZ consists of fine equiaxed grains. The mean grain sizes in WNZ1, WNOZ, and WNZ2 are 4.01, 4.39, and 5.80 μm, respectively. The deformation in the first pass reduces the heat conduction efficiency of the stirring pin during the second pass, which thereby fails to provide sufficient heat in time and significantly weakens the recrystallization degree of grains in this region after welding. Furthermore, WNZ is dominated by high-angle grain boundaries (HAGBs) with a high proportion of approximate 78%, meeting the condition of geometric dynamic recrystal-lization (GDRX
Furthermore, TMAZ is subjected to a shear action induced by the material plastic flow around the stirring pin and by the weld thermal cycles to a certain degree during the welding process. Consequently, the grains become elongated and are deformed along the direction of the maximum shear force. The mean grain size in TMAZ is 14.44 μm. The proportion of HAGBs in TMAZ is only 47.6%, which is significantly decreased, compared with that in WNZ. Therefore, it can be inferred that the distortion energy in TMAZ is too low for recrystallization.
Grains in HAZ is coarsened to a certain degree under weld thermal cycles. A large number of low-angle grain boundaries (LAGBs) appear in HAZ. Therefore, the grain thickness begins to decrease and the sub-structures start to appear in grains. The mean grain size and the proportion of HAGBs are 50.40 μm and 92.1%, respectively. As HAZ is only subjected to weld thermal cycles during the welding process, the energy is gradually increased and it promotes the dislocation propagation and entanglement. Subsequently, the LAGBs constantly absorb the surrounding dislocations and form HAGBs. Grains in HAZ are flat with a high rotation density of sub-structures. No dynamic recrystallization occurs in the HAZ region.
The texture characteristics of SSFSWed T-joints prepared at a rotation speed of 2000 r/min and a welding speed of 50 mm/min are presented in
Fig.10 PFs of different zones in SSFSWed T-joint prepared at rotation speed of 2000 r/min and welding speed of 50 mm/min:
(a) WNZ1, (b) WNOZ, (c) WNZ2, (d) TMAZ, and (e) HAZ
Fig.11 displays the microstructures and mean grain sizes of T-joints at different welding speeds. It is noticeable that the mean grain sizes in WNZ and TMAZ are decreased with increasing the welding speed. In WNZ, as the welding speed increases, the material is subjected to a lower stirring action, reducing the deformation rate of the material, weakening the recrystallization degree, and resulting in grain coarsening. Moreover, the increasing welding speed leads to a decline of the thermal input. Thus, the deformation temperature drops significantly and the grain growth is suppressed. These two actions occur simultaneously as the welding speed is increased from 25 mm/min to 100 mm/min. However, the thermal input plays a key role in determining the final grain size. As the thermal input drops steadily, the grain growth in WNZ becomes increasingly difficult and the mean grain size in WNZ gradually decreases. During the stirring action, TMAZ is elongated. Moreover, the increasing welding speed weakens the stirring action and the elongation of grains. The reduction of the thermal input also decreases the deformation temperature and the mean grain size.
The mean grain size in HAZ is firstly increased and then decreased with increasing the welding speed. The thermal input of HAZ only drops slightly and is still high as the welding speed is increased from 25 mm/min to 50 mm/min, compared with that of WNZ. On the contrary, the strain rate drops noticeably, resulting in the partial grain growth. As the welding speed further increases, the thermal input drops greatly, which seriously suppresses the grain growth, thereby decreasing the mean grain size.
Fig.12 displays the microhardness change trend along the directions of the skin plate and the stiffener plate. The overall microhardness distribution along the skin plate is W-shaped. The zone with the lowest microhardness is located in HAZ which is close to the TMAZ boundary. Recovery and incomplete dynamic recrystallization occur in TMAZ. Consequently, the elongated grains of different sizes are formed. Thus, the microhardness of TMAZ is higher than that of HAZ. Moreover, HAZ is only affected by the welding temperature field, and grains are coarsened with an obvious decrease of microhardness. The overall microhardness distribution in the stiffener plate is N-shaped. The zone with the lowest microhardness is also located in HAZ which is close to the TMAZ boundary.
Precipitation is the main strengthening factor of the 2A14-T4 Al alloy. Fig.13 displays the temperature change trends in the WNZ and TMAZ at different welding speeds. As the welding speed is increased from 25 mm/min to 100 mm/min, the peak temperature in the WNZ is decreased from 578.2 °C to 481.6 °C; the peak temperature in TMAZ is decreased linearly from 340.3 °C to 267.5 °C at a significantly lower temperature decreasing rate than the value in WNZ. The peak temperature of WNZ is higher than the solid-solution temperature of precipitations. The original CuAl2 phase is fully recovered, whereas the α solid-solution is partly recovered. In the subsequent cooling process, atoms are reorganized into reprecipitated phases. The microhardness of WNZ1 is slightly lower than that of WNZ2, which can be attributed to the growth and coarsening of precipitated phases in WNZ1 by the thermal input during 2nd welding process. The welding peak temperatures of TMAZ and HAZ are lower than the solid-solution temperature of precipitations. Under the effect of thermal cycling, the precipitates grow and become coarse. Therefore, the minimum microhardness of HAZ can be obtained near TMAZ. No prominent difference can be noticed in the microhardness distributions along the skin plate direction or the stiffener plate direction with increasing the welding speed. With increasing the welding speed, the microhardness is increased slightly along both the directions of skin and stiffener plates. This is because the heat input decreases and the content of precipitated phase in the T-joints relatively increases, which leads to the slight increase in microhardness.
The ultimate tensile strength (UTS) of BM and SSFSWed T-joints along the skin plate and the stiffener plate directions is presented in Fig.14. With increasing the welding speed, UTS of the SSFSWed T-joints is firstly increased and then decreased along both the skin and stiffener plate directions. This is because with increasing the welding speed from 25 mm/min to 50 mm/min, the heat dissipation effect of the welding zone is increased obviously, while the degree of plastic deformation of material near the pin is also increased. So the proportion of plastic deformation increases, leading to the improvement of the mechanical properties of the welding area. However, when the welding speed is too fast, such as 75~100 mm/min, the heat input of the weld per unit length is decreased, the plastic deformation of the material becomes worse, and the plastic material has no time to backfill the weld, resulting in groove defects on the weld surface and the decline in UTS of the T-joint
The fracture morphologies along the skin plate direction at different welding speeds are shown in
Fig.15 Fracture morphologies of SSFSWed T-joints along skin plate direction at different welding speeds: (a1, a2) 25 mm/min, (b1, b2) 50 mm/min, (c1, c2) 75 mm/min, and (d1, d2) 100 mm/min
The fracture morphologies along the stiffener plate direction at different welding speeds are presented in
Fig.16 Fracture morphologies of SSFSWed T-joints along stiffener plate direction at different welding speeds: (a1, a2) 25 mm/min, (b1, b2) 50 mm/min, (c1, c2) 75 mm/min, and (d1, d2) 100 mm/min
1) T-joints of 2A14-T4 aluminum alloy after stationary shoulder friction stir welding (SSFSW) show a smooth surface without shoulder marks or flashes under the optimized welding process parameters. The surfaces of T-joints prepared at different welding speeds are quite different.
2) The SSFSWed T-joints have an open-dumbbell morphology with two wide ends and a narrow middle zone. The texture of weld nugget zone (WNZ) is weak because both the first weld nugget zone (WNZ1) and the second weld nugget zone (WNZ2) have the {111}<110> shear texture; whereas the texture type of the weld nugget overlap zone (WNOZ) is transformed into a {100}<001> cubic texture after two welding passes.
3) The microhardness change trends can be characterized as “W” and “N” shapes along the skin plate and stiffener plate directions, respectively. The zone with the lowest microhardness is located in heat-affected zone (HAZ) which is close to the thermo-mechanically-affected zone (TMAZ) boundary, resulting in an annealing effect of the second weld on the previous weld.
4) The maximum ultimate tensile strength (UTS) is firstly increased and then decreased with increasing the welding speed. The fracture morphology consists of deformed dimples of different sizes, and the second phase particles can be found in these dimples.
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