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Study of friction stir welding of 01420 aluminum–lithium alloy
Materials Science and Engineering A 452–453 (2007) 170–177
Study of friction stir welding of 01420 aluminum–lithium alloy Shitong Wei, Chuanyong Hao ∗ , Jichun Chen Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning 110016, People’s Republic of China Received 18 January 2006; received in revised form 16 October 2006; accepted 17 October 2006
Abstract The 01420 Al–Li alloy plates were friction stir welded at different welding parameters. The effect of welding parameters on the quality of the 01420 Al–Li friction stir welded joints was investigated. As a result, the optimized combinations of various welding parameters (pin rotation speed, welding speed and welding pressure) were identified. By comparison of the heat inputs during welding process at different parameter combinations, the relationship between the grain size in the stirred zone (SZ) and the welding parameters was established. Furthermore, the mechanical properties of the defect-free joints under different heat inputs were investigated. The experimental results showed that the maximum ultimate strength of the joints is equivalent to 86% that of the base material and the maximum bending angle of the joints can reach 180◦ . © 2006 Elsevier B.V. All rights reserved. Keywords: Friction stir welding; Welding parameter; Microstructure; Mechanical property
1. Introduction Friction stir welding (FSW) is a new solid-state welding method developed by the Welding Institute (TWI) in 1991 [1]. The weld is formed by the deformation of the material at temperatures below the melting point. There is no melting of the material, so FSW has several advantages over the commonly used fusion welding techniques [2–8]. For example, there are no voids and cracking in the weld, there is no distortion of the work-pieces, and there are no needs of filler materials, shielding gases and costly weld preparation during FSW process. FSW is perhaps the most remarkable and potentially useful welding technique [9–16]. However, during FSW process using inappropriate welding parameters can cause defects in the joint and deteriorate the mechanical properties of the FSW joints. The previous studies are mostly based on the joints welded with fixed welding parameters [17–23]. Only a small number of studies have involved the effects of welding parameters on the defects, microstructures and mechanical properties of the joints [24–28]. Although FSW consistently gives high quality welds, proper use of the process and control of a number of parameters is needed to achieve this. To produce the best weld quality, these parameters have to be determined individually for each new component and alloy [29]. The quality of friction stir welded
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joint is controlled by three welding parameters when using definite pin surface profile, these are pin’s rotation speed, welding speed and welding pressure. Pin’s rotation speed and welding speed can be controlled easily. But, welding pressure is of utmost importance and difficult to be controlled. For this relatively new method, there is still a lack of the optimal combinations of various welding parameters for different materials, different thickness, etc. The current work was conducted to study the FSW process on the 2-mm thick 01420 Al–Li alloy with the focus on the appropriate combinations of various welding parameters. In addition, the influence of these parameters on the quality, microstructures and mechanical properties of the FSW joints is studied. 2. Experimental procedure The base material used in the present study was 2-mm 01420 aluminum–lithium alloy, whose nominal chemical composition (wt%) was 1.9–2.2 Li, 4.8–5.2 Mg, 0.08–0.15 Zr, 0.1 Fe, 0.2 Si, balance Al. The mechanical properties of the base material are listed in Table 1. The plate was cut and machined into rectangular welding samples of 100 mm long by 60 mm wide. The welding tool is made from 1Cr18Ni9Ti stainless steel. The tool consisted of a flat shoulder and a frustum-shaped pin. The diameter of the shoulder is 9 mm. The upper and root diameters of the pin are 2 and 2.6 mm, respectively. The pin with a length is just slightly shorter than the plate thickness. Schematic illustration of the welding tool is shown in Fig. 1. During the welding
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Table 1 Mechanical properties of 01420 Al–Li alloy Tensile strength (MPa)
Yield strength (MPa)
Elongation (%)
Bending angle (◦ )
481
341
8
32
Fig. 3. Appearance of the weld: (a) upper surface; (b) root surface.
Fig. 1. Schematic illustration of the welding tool.
process, the plates are clamped firmly to a tool steel backing plate. The tool was tilted 2◦ from the vertical, and rotated in the clockwise direction. The welding parameters used were rotation speed of 480, 600, 930, 1370 and 1960 rpm, welding speed of 23.5, 39.1, 54.5, 72 and 85.7 mm/min, welding pressure was changed between 1000 and 7000 N with different rotation and welding speed. Following welding, the joints were cross-sectioned transversely perpendicular to the welding direction for metallographic analysis using an electrical-discharge cutting machine, and then were polished and etched with Keller solution, consisting of 250 ml water, 5 ml nitric acid, 6 ml hydrochloric acid, and 2 ml hydrofluoric acid, and were applied for about180 s, then observed by optical microscopy. The surface and cross-section of the welds welded at different process parameters were examined. The effects of various parameters on the joints quality were analyzed. The proper welding pressure for given rotation and welding speed was confirmed. Then the heat inputs under different parameter combinations were compared. The relationship between the grain size in the SZ and the welding parameters were established. The hardness measurements were taken along the centerline on the transverse cross-section of the welds at a spacing of 0.25 mm by using a Vickers hardness tester at a 1 N load for 10 s. The dimensions of the specimens for transverse tensile test of FSW joints are shown in Fig. 2. Tensile tests
Fig. 2. Tensile test specimen of the FSW weld.
were carried out at room temperature using an Instron-type testing machine with a crosshead speed of 1.67 × 10−2 mm/s. The fractography of the tensile fracture surface was examined with a scanning electron microscope (SEM). Face-bend test was performed on all welds. Samples for bend test with dimensions of 60 mm × 10 mm × 1.8 mm. The bend radius used was 5.4 mm. 3. Results and discussion 3.1. Appearance of the weld The typical surface appearances of the defect-free joints are shown in Fig. 3. It can be seen that the joints were almost without distortion because of the less heat input and low temperature. Fig. 4 displays the surface defects appear during FSW process. For the fixed welding speed and rotation speed, the welding pressure between the tool shoulder and the work-piece increases from 1000 to 7000 N gradually. At first, groove-type defect appears on the surface of the weld. With the increase of welding pressure, the groove-type defect disappears gradually. But when the pressure exceeds one value, the welding tool would sink into the weld gradually, resulting in the depression of the weld surface and wavy burrs at the edge of the
Fig. 4. Surface defects often appear during friction stir welding process.
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defect would appear in the weld as shown in Fig. 5(b). Further increasing the welding speed or decreasing the welding pressure would cause extension of the tunnel defect up to the weld surface and forming of the groove-type defect. It is noticeable that the tunnel defect is often observed near the bottom of the weld at the advancing side [37]. From these observations, FSW could be considered as a process where a cavity formed behind the welding tool is later filled by a plasticized material flowing from the front of the pin to its rear. If the cavity is not filled, then a cavity will stay in the weld [38]. 3.3. Welding pressure
Fig. 5. Cross-sections of the joins: (a) without defects; (b) with cavity.
weld region. When the rotation speed is fixed and the welding speed is quite slow, the welded material would be overheated causing local melting of the back side of the weld. As the welding speed increases, the local melting phenomenon vanishes and then good surface appearance would be obtained. When the welding speed continues to increase, the surface quality becomes bad gradually, and the semicircle streaks appear clearly, even the groove-type defect could be observed. For the constant welding speed, when the tool rotation speed is too low, the material gets less heat input, causing insufficient metal flow during welding process. As a result, it is easy to bring groovetype defect, the material only attains local bonding. Whereas when the rotation speed is too high, it can cause severe depression of the weld surfaces and appearance of wavy burrs. The types of defects mentioned above are general for all materials when using inappropriate welding parameters during FSW process. 3.2. Macrostructure of the weld The joint of FSW can be divided into four zones, the stirred zone (SZ), the thermo-mechanically affected zone (TMAZ), the heat affected zone (HAZ) and the base material (BM). The simultaneously rotational and translational motions of the welding tool create an asymmetry between the two sides of the joint. On one side, where the two motions of the welding tool are on the same direction, this side can be called advancing side (AS). But on the other side, the directions of the two motions are opposite and it can be called the retreating side (RS) [30–35]. Fig. 5 shows the typical macrographs on cross-sections of the joints. The shape of the weld zone may depend on the welding parameters, configuration of the tool and thermal conductivity of the material [36]. When the welding parameters are appropriate, FSW can produce defect-free joints like the one shown in Fig. 5(a). If choosing the inappropriate welding parameters, such as the faster welding speed or the lower pressure, the tunnel
It can be found from the results that the downward pressure used during the welding process has a great influence on the weld quality. For the fixed welding speed and rotation speed, the pressure has an approximately constant value. When the pressure exceeds that value, it will cause depression and waving burrs, and deteriorate the surface quality of the joints. But when the pressure is lower than the value, tunnel and groovetype defects will appear. They all deteriorate the quality of the joint. Thus, choosing the proper pressure is extraordinary significant. For getting defect-free joints, the pressure is very crucial for the given rotation speed and welding speed. With the increase of the welding speed or the decrease of the rotation speed, the pressure should be increased to a corresponding value. From the results of many experiments, the optimal welding pressures those can get defect-free joints for various combinations of rotation and welding speed were confirmed. They were shown in Table 2. 3.4. Heat input During the FSW process, the main heat source is generally considered to be the friction between the rotating tool shoulder and the surface of the welded plates [39]. So the size of the tool shoulder has significant influence on the surface appearance of the weld. When the size of the tool shoulder is too small, the frictional heat input is extremely low. The plastic material cannot flow sufficiently. So the bonding cannot be achieved. However, when the size of the tool shoulder is too large, the heat generation is too high, it can widen the weld. To a large extent, the quality of the weld depends on the heat inputs during the welding process. Once the parameters of a FSW process are chosen, such as the material, welding pressure and the size of the welding tool, the Table 2 Optimal welding pressures for the given rotation and welding speeds R (rpm)
1960 1370 930 600 480
P (N) (mm/min) V = 23.5
V = 39.1
V = 54.5
V = 72.0
V = 85.7
1463.4 1958.4 2646.0 3598.2 3951.0
1958.4 2557.8 3123.0 3951.0 4586.4
2417.4 3123 3598.2 4269.6 5221.8
2646 3299.4 3916.8 4744.8 5715.0
2962.8 3598.2 4093.2 5221.8 6244.2
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total energy input per unit time, E, is determined by [40] E = πμPR
r12 + r1 r2 + r22 45(r1 + r2 )
In the current work, the size of the tool is fixed, so r1 , r2 and μ are invariable. Thus, a constant C can be defined as (1) C = πμ
where r1 is the radius of the tool shoulder, r2 the root radius of the pin, μ denotes friction coefficient between the tool shoulder and the work-pieces, P expresses the downward pressure on the shoulder, and R presents the rotation speed of the tool. Then, the energy input per unit length of the weld can be expressed as e = πμPR
r12
+ r1 r2 + r22
45V (r1 + r2 )
where V is the welding speed.
173
(2)
r12 + r1 r2 + r22 45(r1 + r2 )
(3)
Then Eq. (2) can be changed to e=
CPR V
(4)
Using Eq. (4) to the proper combinations of the welding pressure, rotation speed and welding speed those can get defect-free joint, the values of heat inputs could be gotten. They were shown in Table 3. From the table we can see that with the increase of the welding speed or the decrease of the rotation speed,
Fig. 6. Microstructures: (a) BM, (b–f) SZ at different rotation and welding speeds; (b) 480 rpm, 85.7 mm/min; (c) 600 rpm, 72 mm/min; (d) 930 rpm, 54.5 mm/min; (e) 1370 rpm, 39.1 mm/min; (f) 1960 rpm, 23.5 mm/min.
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Table 3 Heat inputs at different welding parameters R (rpm)
1960 1370 930 600 480
e/C (mm/min) V = 23.5
V = 39.1
V = 54.5
V = 72.0
V = 85.7
122,058 114,174 104,706 91,872 80,694
98,172 89,622 74,286 60,624 56,304
86,940 78,498 61,398 46,998 45,990
72,036 62,784 50,598 39,546 38,106
67,752 57,528 44,424 36,558 34,974
the heat input for forming the defect-free joints reduces. The heat inputs have the reverse trend compared with the welding pressure. 3.5. Microstructure The microstructure of the base metal and the variation of microstructures in the center of the SZ associated with the welding conditions were shown in Fig. 6. The base metal was composed of grains elongated along the rolling direction of the work-piece, as shown in Fig. 6(a). The SZ consisted of a fineequiaxed recrystallized grain structure (Fig. 6(b–f)). The grain sizes in the center of the SZ in all the welding conditions are smaller than that of the base metal. The size of the equiaxed grain in the SZ increased when the rotation speed was increased or the welding speed was decreased, it had the same trend as the heat input. This can be explained by the equation of grain growth relations [41] Q D2 − D02 = At n exp − (5) RT where D is the instantaneous recrystallized grain size, D0 the initial recrystallized grain size, A a constant, t the time, n a constant usually taken as unity, Q the corresponding activation energy for grain growth, R the gas constant, and T is the absolute temperature. T is proportional to the amount of the heat input, D0 is assumed to be an invariable. Therefore, it is apparent that as the heat input increases, the recrystallized grain size will increase. Fig. 7 shows the relationship between the heat inputs and the grain size in the SZ.
Fig. 7. Relationship between the heat inputs and the grain size in the SZ.
in Fig. 8, which shows that the microhardness of the advancing side of the SZ is retained steadily to the edge of the SZ, and has a sharp decrease to the TMAZ, but the microhardness of the retreating side of the SZ decreases from the weld center to the edge of the SZ continuously. The SZ boundaries shown in Fig. 9 also display this asymmetry. The advancing side boundary of the SZ and TMAZ is typically sharp and readily discernable. Whereas the retreating side boundary is much more diffuse [42]. Any distinct differences in the hardness in the TMAZ and HAZ compared with that of the BM remained unclear. The variation of the maximum hardness values in the SZ is shown in Fig. 10. The microhardness distribution in the friction stir weld shows a dependence on the welding parameters, such as welding speed, tool rotational speed and welding pressure. The hardness tends to decrease with an increase in heat input. This was mainly due to the different thermal effect with welding conditions. More energy was inputted when the welding speed was slower, or the tool rotation speed and welding pressure were higher. Therefore, the grain size might increase at these conditions. FSW can be considered as a hot-working process in which a large amount of deformation is imparted to the work-piece through the rotating pin and the shoulder. Such deformation gives rise to subgrains with dislocation tangles at the boundaries [43]. When the dynamic recrystallization happens, the original base material grain structure will be completely eliminated and
3.6. Mechanical properties 3.6.1. Microhardness Fig. 8 demonstrates the microhardness variations along the centerline on a cross-section of the weld at a tool rotation speed of 480 rpm and welding speed of 54.5 mm/min. It can be seen from the figure that the SZ had a higher hardness than the base metal. This is due to the generation of extremely fine grains in the SZ. The variations in microhardness can be directly correlated to locations in the weld. In the SZ, the hardness values are consistently higher from the retreating side to the center of the weld, and then fluctuate slightly to the advancing side of the SZ. The data from this microhardness map can be used to evaluate the asymmetry of the weld. The characteristic asymmetry is evident
Fig. 8. Hardness profile on a cross-section of the weld with the rotation speed 480 rpm and welding speed 54.5 mm/min.
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Fig. 11. Tensile strength of the joints under different heat inputs.
welding conditions. As mentioned above, it is clear that the size of recrystallized grain varies with heat input. Therefore, it can be concluded that the generation of additionally refined grains and the higher dislocation density in the SZ are the main factors that caused the hardness increase associated with decreasing heat input [28].
Fig. 9. (a and b) Microstructures in SZ–TMAZ on the RS and AS, respectively.
replaced by a very fine-equiaxed recrystallized grain structure in the SZ. Meanwhile, the dislocation density is decreased. But the dislocation may not be eliminated completely. Some previous studies [43–45] have actually shown that such dynamic recrystallization often leaves some grains with a high density of dislocations in the SZ. The recrystallized grain size increases with the heat input, and which causes the further decrease of the dislocation density. It is considered that the variation of hardness is related to the microstructural changes in the SZ induced by
Fig. 10. Variation of maximum hardness in the SZ as heat input increases.
3.6.2. Tensile and bend properties Fig. 11 shows the transverse tensile strength of the defect-free joints under different heat inputs. From the figure we can know, the heat input has little influence on the joint strength. The maximum tensile strength of the joints can reach 86% that of the base material. Moreover, the fracture locations of joints are near the interface between the SZ and the TMAZ on the advancing side, as shown in Fig. 12. It is the same location where sharp hardness (Fig. 8) and microstructure (Fig. 9) changes happen. These are due to the remarkable difference in the internal structure between the SZ and the TMAZ on the advancing side. The SZ is composed
Fig. 12. Fracture of the tensile specimen near the interface between the SZ and the TMAZ on the advancing side: (a) photograph of the fractured tensile sample; (b) micrograph of the precise fracture location.
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Fig. 15. Macroscopic view of the bended weld.
Fig. 13. SEM microfractographs of tensile fracture surfaces of the joint with low magnification (a) and enlargement of the rectangle (b).
of fine-equiaxed recrystallized grains, while the TMAZ consists of coarse deformed grains. Therefore, the interface between the SZ and TMAZ on the advancing side becomes the poorest location in the tensile test [37,46]. But maybe it can be improved by changing the pin geometry [37]. During the tensile test, the tensile fracture initiates at the root side of the interface between the SZ and TMAZ on the advancing side, then extend to the
Fig. 14. Bending angles of the joints at different heat inputs.
upper side along the interface, finally resulting in the damage of the joint. The fracture surfaces were examined with a SEM and the pictures are presented in Fig. 13. The fracture surface shows a dimple pattern. Face-bend test was used as an important tool to understand about the ductility and toughness of the friction stir welds. In the case of forming defect-free joints, the bending angles of the joints under different heat inputs are displayed in Fig. 14. In this figure we can see that with the increase of the heat input, the bending angle increases gradually. When the heat input exceeds 97,200 C, the bending angle reaches 180◦ . Macroscopic view of the bended weld is indicated in Fig. 15. As above mentioned, the reason is that as the heat input increases the dislocation density decreases. Therefore, it can cause less stress concentration during the bend test and get a bigger bending angle. So the bending angle increases with heat input. 4. Conclusions The effects of welding parameters on the quality and mechanical properties of the 01420 Al–Li friction stir welded joints were determined. There is an optimized welding pressure for given rotation and welding speeds. With the increase of the welding speed or the decrease of the rotation speed, the welding pressure was expected to increase correspondingly. Additionally, the heat inputs during welding process at different parameters combinations were compared. It was found that the heat input for forming the defect-free joints reduces with the increase of the welding speed or the decrease of the rotation speed. The relationship between the grain size in the SZ and the heat inputs were also established. The recrystallized grain size in the SZ increased with the heat inputs. The hardness values within the SZ were higher than those of the BM, increased with decreasing heat input. The heat input had little influence on the transverse tensile strength of the joints. The maximum ultimate strength of the joints is equivalent to 86% that of the base material. The bending angle increases gradually with the heat inputs. When the heat input of the welding process exceeds a definite value, the bending angle of the joints can reach 180◦ .
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Study of friction stir welding of 01420 aluminum–lithium alloy Shitong Wei, Chuanyong Hao ∗ , Jichun Chen Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning 110016, People’s Republic of China Received 18 January 2006; received in revised form 16 October 2006; accepted 17 October 2006
Abstract The 01420 Al–Li alloy plates were friction stir welded at different welding parameters. The effect of welding parameters on the quality of the 01420 Al–Li friction stir welded joints was investigated. As a result, the optimized combinations of various welding parameters (pin rotation speed, welding speed and welding pressure) were identified. By comparison of the heat inputs during welding process at different parameter combinations, the relationship between the grain size in the stirred zone (SZ) and the welding parameters was established. Furthermore, the mechanical properties of the defect-free joints under different heat inputs were investigated. The experimental results showed that the maximum ultimate strength of the joints is equivalent to 86% that of the base material and the maximum bending angle of the joints can reach 180◦ . © 2006 Elsevier B.V. All rights reserved. Keywords: Friction stir welding; Welding parameter; Microstructure; Mechanical property
1. Introduction Friction stir welding (FSW) is a new solid-state welding method developed by the Welding Institute (TWI) in 1991 [1]. The weld is formed by the deformation of the material at temperatures below the melting point. There is no melting of the material, so FSW has several advantages over the commonly used fusion welding techniques [2–8]. For example, there are no voids and cracking in the weld, there is no distortion of the work-pieces, and there are no needs of filler materials, shielding gases and costly weld preparation during FSW process. FSW is perhaps the most remarkable and potentially useful welding technique [9–16]. However, during FSW process using inappropriate welding parameters can cause defects in the joint and deteriorate the mechanical properties of the FSW joints. The previous studies are mostly based on the joints welded with fixed welding parameters [17–23]. Only a small number of studies have involved the effects of welding parameters on the defects, microstructures and mechanical properties of the joints [24–28]. Although FSW consistently gives high quality welds, proper use of the process and control of a number of parameters is needed to achieve this. To produce the best weld quality, these parameters have to be determined individually for each new component and alloy [29]. The quality of friction stir welded
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joint is controlled by three welding parameters when using definite pin surface profile, these are pin’s rotation speed, welding speed and welding pressure. Pin’s rotation speed and welding speed can be controlled easily. But, welding pressure is of utmost importance and difficult to be controlled. For this relatively new method, there is still a lack of the optimal combinations of various welding parameters for different materials, different thickness, etc. The current work was conducted to study the FSW process on the 2-mm thick 01420 Al–Li alloy with the focus on the appropriate combinations of various welding parameters. In addition, the influence of these parameters on the quality, microstructures and mechanical properties of the FSW joints is studied. 2. Experimental procedure The base material used in the present study was 2-mm 01420 aluminum–lithium alloy, whose nominal chemical composition (wt%) was 1.9–2.2 Li, 4.8–5.2 Mg, 0.08–0.15 Zr, 0.1 Fe, 0.2 Si, balance Al. The mechanical properties of the base material are listed in Table 1. The plate was cut and machined into rectangular welding samples of 100 mm long by 60 mm wide. The welding tool is made from 1Cr18Ni9Ti stainless steel. The tool consisted of a flat shoulder and a frustum-shaped pin. The diameter of the shoulder is 9 mm. The upper and root diameters of the pin are 2 and 2.6 mm, respectively. The pin with a length is just slightly shorter than the plate thickness. Schematic illustration of the welding tool is shown in Fig. 1. During the welding
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Table 1 Mechanical properties of 01420 Al–Li alloy Tensile strength (MPa)
Yield strength (MPa)
Elongation (%)
Bending angle (◦ )
481
341
8
32
Fig. 3. Appearance of the weld: (a) upper surface; (b) root surface.
Fig. 1. Schematic illustration of the welding tool.
process, the plates are clamped firmly to a tool steel backing plate. The tool was tilted 2◦ from the vertical, and rotated in the clockwise direction. The welding parameters used were rotation speed of 480, 600, 930, 1370 and 1960 rpm, welding speed of 23.5, 39.1, 54.5, 72 and 85.7 mm/min, welding pressure was changed between 1000 and 7000 N with different rotation and welding speed. Following welding, the joints were cross-sectioned transversely perpendicular to the welding direction for metallographic analysis using an electrical-discharge cutting machine, and then were polished and etched with Keller solution, consisting of 250 ml water, 5 ml nitric acid, 6 ml hydrochloric acid, and 2 ml hydrofluoric acid, and were applied for about180 s, then observed by optical microscopy. The surface and cross-section of the welds welded at different process parameters were examined. The effects of various parameters on the joints quality were analyzed. The proper welding pressure for given rotation and welding speed was confirmed. Then the heat inputs under different parameter combinations were compared. The relationship between the grain size in the SZ and the welding parameters were established. The hardness measurements were taken along the centerline on the transverse cross-section of the welds at a spacing of 0.25 mm by using a Vickers hardness tester at a 1 N load for 10 s. The dimensions of the specimens for transverse tensile test of FSW joints are shown in Fig. 2. Tensile tests
Fig. 2. Tensile test specimen of the FSW weld.
were carried out at room temperature using an Instron-type testing machine with a crosshead speed of 1.67 × 10−2 mm/s. The fractography of the tensile fracture surface was examined with a scanning electron microscope (SEM). Face-bend test was performed on all welds. Samples for bend test with dimensions of 60 mm × 10 mm × 1.8 mm. The bend radius used was 5.4 mm. 3. Results and discussion 3.1. Appearance of the weld The typical surface appearances of the defect-free joints are shown in Fig. 3. It can be seen that the joints were almost without distortion because of the less heat input and low temperature. Fig. 4 displays the surface defects appear during FSW process. For the fixed welding speed and rotation speed, the welding pressure between the tool shoulder and the work-piece increases from 1000 to 7000 N gradually. At first, groove-type defect appears on the surface of the weld. With the increase of welding pressure, the groove-type defect disappears gradually. But when the pressure exceeds one value, the welding tool would sink into the weld gradually, resulting in the depression of the weld surface and wavy burrs at the edge of the
Fig. 4. Surface defects often appear during friction stir welding process.
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defect would appear in the weld as shown in Fig. 5(b). Further increasing the welding speed or decreasing the welding pressure would cause extension of the tunnel defect up to the weld surface and forming of the groove-type defect. It is noticeable that the tunnel defect is often observed near the bottom of the weld at the advancing side [37]. From these observations, FSW could be considered as a process where a cavity formed behind the welding tool is later filled by a plasticized material flowing from the front of the pin to its rear. If the cavity is not filled, then a cavity will stay in the weld [38]. 3.3. Welding pressure
Fig. 5. Cross-sections of the joins: (a) without defects; (b) with cavity.
weld region. When the rotation speed is fixed and the welding speed is quite slow, the welded material would be overheated causing local melting of the back side of the weld. As the welding speed increases, the local melting phenomenon vanishes and then good surface appearance would be obtained. When the welding speed continues to increase, the surface quality becomes bad gradually, and the semicircle streaks appear clearly, even the groove-type defect could be observed. For the constant welding speed, when the tool rotation speed is too low, the material gets less heat input, causing insufficient metal flow during welding process. As a result, it is easy to bring groovetype defect, the material only attains local bonding. Whereas when the rotation speed is too high, it can cause severe depression of the weld surfaces and appearance of wavy burrs. The types of defects mentioned above are general for all materials when using inappropriate welding parameters during FSW process. 3.2. Macrostructure of the weld The joint of FSW can be divided into four zones, the stirred zone (SZ), the thermo-mechanically affected zone (TMAZ), the heat affected zone (HAZ) and the base material (BM). The simultaneously rotational and translational motions of the welding tool create an asymmetry between the two sides of the joint. On one side, where the two motions of the welding tool are on the same direction, this side can be called advancing side (AS). But on the other side, the directions of the two motions are opposite and it can be called the retreating side (RS) [30–35]. Fig. 5 shows the typical macrographs on cross-sections of the joints. The shape of the weld zone may depend on the welding parameters, configuration of the tool and thermal conductivity of the material [36]. When the welding parameters are appropriate, FSW can produce defect-free joints like the one shown in Fig. 5(a). If choosing the inappropriate welding parameters, such as the faster welding speed or the lower pressure, the tunnel
It can be found from the results that the downward pressure used during the welding process has a great influence on the weld quality. For the fixed welding speed and rotation speed, the pressure has an approximately constant value. When the pressure exceeds that value, it will cause depression and waving burrs, and deteriorate the surface quality of the joints. But when the pressure is lower than the value, tunnel and groovetype defects will appear. They all deteriorate the quality of the joint. Thus, choosing the proper pressure is extraordinary significant. For getting defect-free joints, the pressure is very crucial for the given rotation speed and welding speed. With the increase of the welding speed or the decrease of the rotation speed, the pressure should be increased to a corresponding value. From the results of many experiments, the optimal welding pressures those can get defect-free joints for various combinations of rotation and welding speed were confirmed. They were shown in Table 2. 3.4. Heat input During the FSW process, the main heat source is generally considered to be the friction between the rotating tool shoulder and the surface of the welded plates [39]. So the size of the tool shoulder has significant influence on the surface appearance of the weld. When the size of the tool shoulder is too small, the frictional heat input is extremely low. The plastic material cannot flow sufficiently. So the bonding cannot be achieved. However, when the size of the tool shoulder is too large, the heat generation is too high, it can widen the weld. To a large extent, the quality of the weld depends on the heat inputs during the welding process. Once the parameters of a FSW process are chosen, such as the material, welding pressure and the size of the welding tool, the Table 2 Optimal welding pressures for the given rotation and welding speeds R (rpm)
1960 1370 930 600 480
P (N) (mm/min) V = 23.5
V = 39.1
V = 54.5
V = 72.0
V = 85.7
1463.4 1958.4 2646.0 3598.2 3951.0
1958.4 2557.8 3123.0 3951.0 4586.4
2417.4 3123 3598.2 4269.6 5221.8
2646 3299.4 3916.8 4744.8 5715.0
2962.8 3598.2 4093.2 5221.8 6244.2
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total energy input per unit time, E, is determined by [40] E = πμPR
r12 + r1 r2 + r22 45(r1 + r2 )
In the current work, the size of the tool is fixed, so r1 , r2 and μ are invariable. Thus, a constant C can be defined as (1) C = πμ
where r1 is the radius of the tool shoulder, r2 the root radius of the pin, μ denotes friction coefficient between the tool shoulder and the work-pieces, P expresses the downward pressure on the shoulder, and R presents the rotation speed of the tool. Then, the energy input per unit length of the weld can be expressed as e = πμPR
r12
+ r1 r2 + r22
45V (r1 + r2 )
where V is the welding speed.
173
(2)
r12 + r1 r2 + r22 45(r1 + r2 )
(3)
Then Eq. (2) can be changed to e=
CPR V
(4)
Using Eq. (4) to the proper combinations of the welding pressure, rotation speed and welding speed those can get defect-free joint, the values of heat inputs could be gotten. They were shown in Table 3. From the table we can see that with the increase of the welding speed or the decrease of the rotation speed,
Fig. 6. Microstructures: (a) BM, (b–f) SZ at different rotation and welding speeds; (b) 480 rpm, 85.7 mm/min; (c) 600 rpm, 72 mm/min; (d) 930 rpm, 54.5 mm/min; (e) 1370 rpm, 39.1 mm/min; (f) 1960 rpm, 23.5 mm/min.
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Table 3 Heat inputs at different welding parameters R (rpm)
1960 1370 930 600 480
e/C (mm/min) V = 23.5
V = 39.1
V = 54.5
V = 72.0
V = 85.7
122,058 114,174 104,706 91,872 80,694
98,172 89,622 74,286 60,624 56,304
86,940 78,498 61,398 46,998 45,990
72,036 62,784 50,598 39,546 38,106
67,752 57,528 44,424 36,558 34,974
the heat input for forming the defect-free joints reduces. The heat inputs have the reverse trend compared with the welding pressure. 3.5. Microstructure The microstructure of the base metal and the variation of microstructures in the center of the SZ associated with the welding conditions were shown in Fig. 6. The base metal was composed of grains elongated along the rolling direction of the work-piece, as shown in Fig. 6(a). The SZ consisted of a fineequiaxed recrystallized grain structure (Fig. 6(b–f)). The grain sizes in the center of the SZ in all the welding conditions are smaller than that of the base metal. The size of the equiaxed grain in the SZ increased when the rotation speed was increased or the welding speed was decreased, it had the same trend as the heat input. This can be explained by the equation of grain growth relations [41] Q D2 − D02 = At n exp − (5) RT where D is the instantaneous recrystallized grain size, D0 the initial recrystallized grain size, A a constant, t the time, n a constant usually taken as unity, Q the corresponding activation energy for grain growth, R the gas constant, and T is the absolute temperature. T is proportional to the amount of the heat input, D0 is assumed to be an invariable. Therefore, it is apparent that as the heat input increases, the recrystallized grain size will increase. Fig. 7 shows the relationship between the heat inputs and the grain size in the SZ.
Fig. 7. Relationship between the heat inputs and the grain size in the SZ.
in Fig. 8, which shows that the microhardness of the advancing side of the SZ is retained steadily to the edge of the SZ, and has a sharp decrease to the TMAZ, but the microhardness of the retreating side of the SZ decreases from the weld center to the edge of the SZ continuously. The SZ boundaries shown in Fig. 9 also display this asymmetry. The advancing side boundary of the SZ and TMAZ is typically sharp and readily discernable. Whereas the retreating side boundary is much more diffuse [42]. Any distinct differences in the hardness in the TMAZ and HAZ compared with that of the BM remained unclear. The variation of the maximum hardness values in the SZ is shown in Fig. 10. The microhardness distribution in the friction stir weld shows a dependence on the welding parameters, such as welding speed, tool rotational speed and welding pressure. The hardness tends to decrease with an increase in heat input. This was mainly due to the different thermal effect with welding conditions. More energy was inputted when the welding speed was slower, or the tool rotation speed and welding pressure were higher. Therefore, the grain size might increase at these conditions. FSW can be considered as a hot-working process in which a large amount of deformation is imparted to the work-piece through the rotating pin and the shoulder. Such deformation gives rise to subgrains with dislocation tangles at the boundaries [43]. When the dynamic recrystallization happens, the original base material grain structure will be completely eliminated and
3.6. Mechanical properties 3.6.1. Microhardness Fig. 8 demonstrates the microhardness variations along the centerline on a cross-section of the weld at a tool rotation speed of 480 rpm and welding speed of 54.5 mm/min. It can be seen from the figure that the SZ had a higher hardness than the base metal. This is due to the generation of extremely fine grains in the SZ. The variations in microhardness can be directly correlated to locations in the weld. In the SZ, the hardness values are consistently higher from the retreating side to the center of the weld, and then fluctuate slightly to the advancing side of the SZ. The data from this microhardness map can be used to evaluate the asymmetry of the weld. The characteristic asymmetry is evident
Fig. 8. Hardness profile on a cross-section of the weld with the rotation speed 480 rpm and welding speed 54.5 mm/min.
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Fig. 11. Tensile strength of the joints under different heat inputs.
welding conditions. As mentioned above, it is clear that the size of recrystallized grain varies with heat input. Therefore, it can be concluded that the generation of additionally refined grains and the higher dislocation density in the SZ are the main factors that caused the hardness increase associated with decreasing heat input [28].
Fig. 9. (a and b) Microstructures in SZ–TMAZ on the RS and AS, respectively.
replaced by a very fine-equiaxed recrystallized grain structure in the SZ. Meanwhile, the dislocation density is decreased. But the dislocation may not be eliminated completely. Some previous studies [43–45] have actually shown that such dynamic recrystallization often leaves some grains with a high density of dislocations in the SZ. The recrystallized grain size increases with the heat input, and which causes the further decrease of the dislocation density. It is considered that the variation of hardness is related to the microstructural changes in the SZ induced by
Fig. 10. Variation of maximum hardness in the SZ as heat input increases.
3.6.2. Tensile and bend properties Fig. 11 shows the transverse tensile strength of the defect-free joints under different heat inputs. From the figure we can know, the heat input has little influence on the joint strength. The maximum tensile strength of the joints can reach 86% that of the base material. Moreover, the fracture locations of joints are near the interface between the SZ and the TMAZ on the advancing side, as shown in Fig. 12. It is the same location where sharp hardness (Fig. 8) and microstructure (Fig. 9) changes happen. These are due to the remarkable difference in the internal structure between the SZ and the TMAZ on the advancing side. The SZ is composed
Fig. 12. Fracture of the tensile specimen near the interface between the SZ and the TMAZ on the advancing side: (a) photograph of the fractured tensile sample; (b) micrograph of the precise fracture location.
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Fig. 15. Macroscopic view of the bended weld.
Fig. 13. SEM microfractographs of tensile fracture surfaces of the joint with low magnification (a) and enlargement of the rectangle (b).
of fine-equiaxed recrystallized grains, while the TMAZ consists of coarse deformed grains. Therefore, the interface between the SZ and TMAZ on the advancing side becomes the poorest location in the tensile test [37,46]. But maybe it can be improved by changing the pin geometry [37]. During the tensile test, the tensile fracture initiates at the root side of the interface between the SZ and TMAZ on the advancing side, then extend to the
Fig. 14. Bending angles of the joints at different heat inputs.
upper side along the interface, finally resulting in the damage of the joint. The fracture surfaces were examined with a SEM and the pictures are presented in Fig. 13. The fracture surface shows a dimple pattern. Face-bend test was used as an important tool to understand about the ductility and toughness of the friction stir welds. In the case of forming defect-free joints, the bending angles of the joints under different heat inputs are displayed in Fig. 14. In this figure we can see that with the increase of the heat input, the bending angle increases gradually. When the heat input exceeds 97,200 C, the bending angle reaches 180◦ . Macroscopic view of the bended weld is indicated in Fig. 15. As above mentioned, the reason is that as the heat input increases the dislocation density decreases. Therefore, it can cause less stress concentration during the bend test and get a bigger bending angle. So the bending angle increases with heat input. 4. Conclusions The effects of welding parameters on the quality and mechanical properties of the 01420 Al–Li friction stir welded joints were determined. There is an optimized welding pressure for given rotation and welding speeds. With the increase of the welding speed or the decrease of the rotation speed, the welding pressure was expected to increase correspondingly. Additionally, the heat inputs during welding process at different parameters combinations were compared. It was found that the heat input for forming the defect-free joints reduces with the increase of the welding speed or the decrease of the rotation speed. The relationship between the grain size in the SZ and the heat inputs were also established. The recrystallized grain size in the SZ increased with the heat inputs. The hardness values within the SZ were higher than those of the BM, increased with decreasing heat input. The heat input had little influence on the transverse tensile strength of the joints. The maximum ultimate strength of the joints is equivalent to 86% that of the base material. The bending angle increases gradually with the heat inputs. When the heat input of the welding process exceeds a definite value, the bending angle of the joints can reach 180◦ .
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