J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 2 2, 2 0 0 3, 41 – 43
Tensile properties and fracture locations of friction-stir welded joints of 1050-H24 aluminum alloy H . L I U ∗, M . M A E D A , H . F U J I I , K . N O G I Joining and Welding Research Institute, Osaka University, Osaka 567-0047, Japan E-mail:
[email protected]
Friction stir welding (FSW) is a promising welding process that can produce low-distortion, high-quality and low-cost joints of aluminum alloys [1–3]. Recent studies have indicated that different types of aluminum alloys have different weldabilities to FSW. Concerning the heat-treatable aluminum alloys, FSW produces a softened region in the joints because of the dissolution and growth of strengthening precipitates during the thermal cycle of welding [4–11]. With respect to the non-heat-treatable aluminum alloys, FSW does not bring about softening in the joints of non-strainhardened alloys [12–14], but a softened region can be produced in the friction-stir welded joints of strainhardened alloys because of the decrease in the dislocation density in the weld and heat-affected zone (HAZ) [14–16]. However, the aforementioned studies are mostly based on the joints welded with definite welding parameters. Only a small number of studies have involved the effects of welding process parameters on the microstructures and mechanical properties of the joints [6, 7, 15] or the detailed results are not published because of the restriction of proprietary rights [8, 9, 16]. This letter aims to study the FSW of a 1050-H24 aluminum alloy, and the focus is placed on the tensile properties and fracture locations of the joints welded with different welding parameters. The base material used in this study was a 1050-H24 aluminum alloy plate of 5 mm thick, strain-hardened and then partially annealed in the as-received state, and its chemical compositions and mechanical properties are listed in Table I. The plate was cut and machined
into rectangular welding samples of 300 mm long by 80 mm wide, and the samples were butt-welded longitudinally using an FSW machine. The diameter of the tool shoulder was 15 mm, and the diameter and length of the tool pin were 6 mm and 4.7 mm, respectively. The tool was tilted 3◦ from the vertical, and rotated at 1500 rpm and traveled at 100–800 mm/min during the welding. That is to say, the revolutionary pitch was 0.07–0.53 mm/r. All the specimens used for the metallographic analyses and tensile tests were cross-sectioned perpendicular to the welding direction from the joints using an electrical-discharge machine. The cross-sections of the specimens for metallographic analyses were polished with an alumina suspension, etched with Keller’s reagent, and observed by optical microscopy. The configuration and size of the transverse tensile specimens were prepared according to JIS Z2201. Prior to the tensile test, the Vickers hardness profiles across the weld, HAZ and partial base material were measured along the centerlines of the cross-sections of the tensile specimens according to JIS Z2244-HV0.1, and the Vickers indents with a spacing of 1 mm were used to determine the fracture locations of the joints. The tensile tests were carried out at room temperature at a crosshead speed of 1 mm/min using a screw-driven test machine, and the tensile properties of each joint were evaluated using five tensile specimens cut from the same joint. Fig. 1 shows the tensile properties and fracture locations of the joints friction-stir-welded at different
Figure 1 Tensile properties and fracture locations of the joints welded at different revolutionary pitches: (a) tensile properties and (b) fracture locations. ∗ Visiting scholar from National Key Laboratory of Advanced Welding Production Technology, Harbin Institute of Technology, Harbin 150001, People’s Republic of China. C 2003 Kluwer Academic Publishers 0261–8028
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T A B L E I Chemical compositions and mechanical properties of 1050-H24 aluminum alloy Chemical compositions (wt%)
Mechanical properties
Al
Si
Fe
Cu
Mg
V
Ti
Tensile strength
0.2% proof strength
Elongation
Hardness
99.58
0.04
0.32
0.02
0.01
0.01
0.02
115.5 MPa
60.4 MPa
29.2 %
40.2 HV
revolutionary pitches. It can be seen from Fig.1a that the tensile properties of the joints are all lower than those of the base material (see Table I), and the 0.2% proof strength increases as the revolutionary pitch increases. When the revolutionary pitch is smaller than 0.3 mm/r, the tensile strength increases with the revolutionary pitch, and the elongation is kept at a comparatively high level. While the revolutionary pitch is greater than 0.3 mm/r, the tensile strength decreases with the increase in the revolutionary pitch, and the elongation dramatically decreases to a comparatively low level. This result means that the FSW parameters have significant effects on the tensile properties of the joints, and the optimum FSW parameters can be determined from the relations between the tensile properties and the welding parameters. For example, the revolutionary pitch of 0.27 mm/r corresponding to the rotation speed of 1500 rpm and the welding speed of 400 mm/min is optimum. In Fig.1b, the fracture location is expressed by the distance between the fracture surface and the weld center, and the distance will be marked as minus if the fracture occurs on the retreating side of the joint. When the revolutionary pitch is smaller than 0.3 mm/r, the joint is fractured on the advancing side of the joint, and the fracture location approaches the weld center as the revolutionary pitch increases. While the revolutionary pitch is greater than 0.3 mm/r, the joint is fractured on either the retreating side or the advancing side, and the fracture location is gradually estranged from the weld center as the revolutionary pitch increases. This result indicates that the FSW parameters also have clear effects on the fracture locations of the joints. The variation in the tensile properties and fracture locations of the joints based on the FSW parameters is, to a large extent, related to the hardness profiles and welding defects in the joints. Fig. 2 shows the typical cross-sections of the joints welded at different revolutionary pitches, and Fig. 3 shows the hardness profiles and stress-strain relations in the joints welded at different revolutionary pitches. When the revolutionary pitch is smaller than 0.3 mm/r, FSW produces defect-free
joints (see Fig. 2a). In this case, the tensile properties and fracture locations of the joints are dependent only on the hardness profiles in the joints. It can be seen from Fig. 3a that a softened region has been produced in the friction-stir welded joints of the 1050-H24 aluminum alloy, thus resulting in the tensile properties lower than the base material. There is a lowest-hardness zone on the advancing side of each joint, therefore the joint is not fractured on the retreating side but on the advancing side. Moreover, the lowest hardness value of the joint increases and the distance between the lowest hardness zone and the weld center decreases as the revolutionary pitch increases, consequently the tensile strength of the joint increases and the fracture location of the joint approaches the weld center. On the other hand, when the revolutionary pitch is greater than 0.3 mm/r, a crack-like defect occurs in the lower half-part of each joint because the friction heat is not sufficient to fulfill the requirements of softening and plastic flow of the weld metal (see Fig. 2b and c). In this case, the tensile properties and fracture locations of the joints are significantly affected by the defects in the joints. According to a curve in Fig. 3b and the real tensile procedure, the joint is fractured through two stages: (1) the joint is partially fractured, from the back surface of the joint to the crack-like defect, at the original interface between the two welding samples; and (2) the residual part of the joint is fractured from the tip of the crack-like defect to the top surface of the joint. It is because of the two-stage fracture that the elongation of the joint is at a low level. It can be also seen from Fig. 2b and c that the crack-like defect slopes slightly from the retreating side to the advancing side, so the joint tends to fracture on the retreating side. As mentioned above (see Fig. 3a), however, the lowest-hardness zone is on the advancing side of each joint, consequently some joints are also fractured on the advancing side. In addition, the length of the crack-like defect increases and the distance between the top surface of the joint and the defect tip on the retreating side decreases as the revolutionary pitch increases, therefore the tensile strength of the joint decreases and the fracture
Figure 2 Optical graphs of the cross sections of the joints welded at different revolutionary pitches: (a) 0.07 mm/r, (b) 0.4 mm/r and (c) 0.53 mm/r.
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Figure 3 Hardness profiles and stress-strain relations in the joints welded at different revolutionary pitches: (a) hardness distributions and (b) stressstrain relations.
location of the joint is gradually estranged from the weld center. As a result, the tensile properties and fracture locations of the friction-stir welded joints of the 1050-H24 aluminum alloy are significantly affected by the welding parameters, and there are optimum values in the FSW parameters, e.g. an optimum revolutionary pitch of 0.27 mm/r corresponding to the rotation speed of 1500 rpm and the welding speed of 400 mm/min. When the welding parameters are deviated from the optimum values, the tensile properties of joints deteriorate and the fracture locations of joints change remarkably. All of the experimental results can be explained by the hardness profiles and welding defects in the joints.
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7. T . H A S H I M O T O , S . J Y O G A N , K . N A K A T A , Y . G . K I M and M . U S H I O , in Proceedings of the 1st International Symposium on Friction Stir Welding, California, USA, June 1999, Paper No. S9P3. 8. Y . S . S A T O and H . K O K A W A , Metall. Mater. Trans. A 32 (2001) 3023. 9. M . W . M A H O N E Y , C . G . R H O D E S , J . G . F L I N T O F F , R . A . S P U R L I N G and W . H . B I N G E L , ibid. 29 (1998) 1955. 10. L . M A G N U S S O N and L . K A L L M A N , in Proceedings of the 2nd International Symposium on Friction Stir Welding, Gothenburg, Sweden, June 2000, Paper No. S2-P3. 11. Y . N A G A N O , S . J O G A N and T . H A S H I M O T O , in Proceedings of the 3rd International Symposium on Friction Stir Welding, Kobe, Japan, September 2001, Paper No. Post-12. 12. Y . S . S A T O , S . H . C . P A R K and H . K O K A W A , Metall. Mater. Trans. A 32 (2001) 3033. 13. L . E . M U R R , G . L I U and J . C . M C C L U R E , J. Mater. Sci. Lett. 16 (1997) 1801. 14. O . V . F L O R E S , C . K E N N E D Y , L . E . M U R R , D . B R O W N , S . P A P P U , B . M . N O W A K and J . C . M C C L U R E , Scripta Mater. 38 (1998) 703. 15. Y . S . S A T O , M . U R A T A , H . K O K A W A and K . I K E D A , in Proceedings of the 7tht International Welding Symposium, Kobe, Japan, November 2001, p. 633. 16. H . J I N , S . S A I M O T O , M . B A L L and P . L . T H R E A D G I L L , Mater. Sci. Technol. 17 (2001) 1605.
Received 24 June and accepted 9 September 2002
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