Effects of post-weld heat treatment on microstructure and mechanical ...

Effects of post-weld heat treatment on microstructure and mechanical properties of friction stir welded joints of 2219-O aluminium alloy J. C. Feng, Y. C. Chen* and H. J. Liu Post-weld heat treatment (PWHT) of 2219-O aluminium alloy friction stir welding joints was carried out at solution temperatures of 480, 500 and 540uC for 32 min followed by aging at 130uC for 9 h. The effects of PWHT on the microstructure and mechanical properties of the joints were investigated. Experimental results show that PWHT causes coarsening of the grains in the weld, and the coarsening degree increases with increasing solution temperature. The tensile strength of the heat treated joints increases with increasing solution temperature. The maximum tensile strength can reach 260% that of the base material at the solution temperature of 540uC. PWHT has a significant effect on the fracture locations of the joints. When the solution temperature is lower than 500uC, the joints fracture in the base material; when the temperature is higher than 500uC, the joints fracture in the weld. The change of the fracture locations of joints is attributed to the presence of precipitate free zones beside the grain boundaries and coarsening equiaxed grain structures in the weld. Keywords: Friction stir welding, Aluminium alloy, Microstruture, Mechanical properties, Post-weld heat treatment

Introduction As a solid state joining process, friction stir welding (FSW) has been widely used in difficult to weld heat treatable aluminium alloys to obtain high quality joints.1 However, many studies on the mechanical properties of the FSW joints of heat treatable aluminium alloys such as 2014-T651,2 2017-T351,3 2024-T6,4,5 2195-T8,6 2519T87,7 6061-T6,8,9 6063-T510 and 7075-T65111 have indicated that FSW gives rise to softening of the joints, and results in the degradation of mechanical properties. Therefore, more studies are necessary to search some effectively remedial measures to further improve the mechanical properties of FSW joints. A general method used in this area is post-weld aging heat treatment. Some studies12–14 show that the mechanical properties of FSW joints can be improved by this process, while others show that the mechanical properties are not improved,15,16 or even degraded.17,18 These results show that post-weld aging heat treatment process has limitations to further improving the mechanical properties of the FSW joints. However, if the base material (BM) is in O condition, the post-weld solution and aging heat treatment (PWHT) can restore the mechanical properties of the joints successfully. Another National Key Laboratory of Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China *Corresponding author, email [email protected]

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ß 2006 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 14 May 2005; accepted 27 July 2005 DOI 10.1179/174328406X79298

advantage is that when welding is carried out in O condition, the post-weld forming operation can be much more easily performed.19,20 However, studies in this area have shown that the fine grains in the weld nugget zone are not stable during solution treatment, which results in undesirable coarse grain structures.21,22 Observations of grain growth (GG) phenomenon in the FSW AA2195 showed that the GG initiated on the surface at ,485uC within 2 min, and dominated ,30% of the weld nugget at ,540uC in 3 min.23,24 Multipass FSW of AA7050 exhibited GG at 400uC.25 Hassan et al.22 examined the stability of fine grains in the stir zone of FSW AA7010 during solution heat treatment. The GG initiated at the bottom of the stir zone at 475uC. Sato et al.26 examined the stability of fine grains in the stir zone of FSW 1100 aluminium alloy during solution heat treatment. The GG occurred only when the solution temperature was higher than the maximum temperature during FSW. Studies mentioned above show that the GG are related not only to alloy types, but also to solution temperatures. Studies on the effects of the solution temperatures on the microstructure and mechanical properties of FSW joints are important for us to evaluate the feasibility of bringing solution heat treatment process into the field of FSW. In the 2000 series heat treatable aluminium alloys, 2219 aluminium alloy has many potential applications owing to its high specific strength, good fracture toughness and excellent stress corrosion resistance.27,28 Therefore, a profound study of its

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Friction stir welded joints of 2219-O aluminium alloy

friction stir weldability is useful. The present study aims to demonstrate the effects of PWHT technology on the microstructure and mechanical properties of the FSW joints of 2219-O aluminium alloy.

Experimental procedure The BM used in the present study was a 5 mm thick 2219-O aluminium alloy plate, with its chemical composition and mechanical properties listed in Table 1. The welding samples, 260650 mm, were longitudinally butt welded using an FSW machine. The welding tool size and welding parameters are listed in Table 2. After welding, the samples were cut into two parts, one part for PWHT and the other for as welded examinations. The PWHT technology included solution heat treatment and aging treatment. The solution heat treatment was carried out at temperatures of 480, 500 and 540uC, respectively, for 32 min. After the solution heat treatment, the samples were quenched in water at 25uC. Aging treatment was carried out at 130uC for 9 h. After the heat treatment, all the joints, including the as welded joints, were cross-sectioned perpendicular to the welding direction for metallographic analyses and tensile tests. The cross-sections of the metallographic specimens were polished with a diamond paste, etched with Tucker’s reagent (15 mL hydrochloric acid, 5 mL nitric acid, 5 mL hydrofluoric acid and 8 mL water) and observed by optical microscopy. For transmission electron microscopy (TEM) investigations, thin foil disk specimens, 3 mm in dia., were cut from the weld zones of the as welded joints and heat treated joints using an electron discharge machine, and were prepared by means of double jet electropolishing using a solution of 30% nitric acid in methanol (18 V and –35uC). These TEM specimens were observed using a CM12 at 120 kV. Vickers hardness profiles across the weld zone (WZ), heat affected zone and partial BM were measured under a load of 4.9 N for 10 s along the centrelines of the cross-sections of the tensile specimens using a microhardness tester, and the spacing between the adjacent measured points was 1 mm. The room temperature tensile test was carried out at a crosshead speed of 1 mm min21 using an Instron-1186 testing machine. The marked length of each specimen was 50 mm, and the tensile properties of each joint were evaluated using three tensile specimens cut from the same joint.

1 Cross-sections of a as welded joints, b and c heat treated joints

welded joint, the grains of the heat treated joints are coarsened. Moreover, the grain size of the joint solutionised at 480uC is smaller than that of the joint solutionised at 540uC. This indicates that the coarsening degree of grains increases with increasing solution temperature. In addition, the grains in the BM have an orientation along the rolling direction, while the ones in the WZ are equiaxed. The difference of the microstructure in different zones of the joints has a significant effect on the mechanical properties of the joints. Detailed discussion about this is seen below. Figure 2 shows the microstructure of the as welded joint and heat treated joint in the WZ. It can be seen

Experimental results Figure 1 shows the cross-sections of joints before and after heat treatment. Compared with the grains of the as

Table 1 Chemical composition and mechanical properties of 2219-O aluminium alloy plate Chemical compositions, wt-%

Mechanical properties

Al

Cu

Mn

Fe

Ti

V

Zn

Si

Zr

Tensile strength

0.2% proof strength

Elongation

Bal.

6.48

0.32

0.23

0.06

0.08

0.04

0.49

0.2

159 MPa

114 MPa

17.5%

Table 2 Tool size and welding process parameters used in experiments Tool size, mm

Welding parameters

Shoulder diameter

Pin diameter

Pin length

Rotation speed

Travel speed

Tool tilt

15

6

4.8

800 r min21

200 mm min21

2.5u


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a as welded joints; b heat treated joints at 540uC 2 Microstructure of WZ in joints

from Fig. 2a that the microstructure of the as welded joint in the WZ is equiaxed and the average grain size is ,3–4 mm. Figure 2b shows the microstructure of the heat treated joint in the WZ. The grains are coarsened and the precipitate free zones (PFZ) beside the grain

3 Tensile properties temperatures

of

joints

at

different

solution

boundaries can be seen clearly. The disk precipitates reprecipitate during PWHT. Figure 3 shows the tensile properties of the joints before and after heat treatment. As seen from this figure, the tensile strength of the heat treated joints is significantly higher than that of the as welded joints and the value increases with increasing solution temperature. The maximum tensile strength obtained at solution temperature of 540uC is 410 MPa, which is equivalent to 260% that of the BM. Figure 4 shows the fracture locations of the joints in tensile testing before and after the heat treatment. As seen from this figure, the as welded joints fracture in the BM (Fig. 4a), but the fracture locations of the heat treated joints changed with solution temperature (Fig. 4b–d). When the solution temperature is lower than 500uC, the joints fracture in the BM of the joint (Fig. 4b and c); when the solution temperature is higher than 500uC, the joints fracture in the WZ (Fig. 4d). This means that PWHT has a significant effect on the fraction locations of joints. Figure 5 shows the cross-section

4 Fracture locations of a as welded joints, b, c and d heat treated joints

a before tensile test; b after tensile test 5 Cross-sections of fracture locations of joints treated at 540uC

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a as welded joints; b heat treated joints 6 Microhardness distributions in joints

details of the fracture location of the heat treated joints during tensile test.

Discussion Grain coarsening in the WZ is similar to grain growth phenomenon reported in friction stir processed 7475 aluminium alloy.25 Therefore, the reasons for grain growth put forward by Charit25 are cited here to explain the observation of grain coarsening in the WZ. It is stated that grain growth is largely due to the imbalance between thermodynamic driving forces for grain growth and the pinning forces impeding grain boundary migration. The pinning forces decrease with the increase in the solution temperature because of the dissolution of the precipitate phase during solution heat treatment. Because the pinning forces decrease, the balance mentioned above breaks down and the grains are coarsened. The higher the solution temperature, the greater the dissolvability of the precipitate. Therefore, degree of grain coarsening increases with increasing solution temperature. The tensile properties of the joints are related to the microhardness profiles in the joints.3,5,8,29 Typical

microhardness distributions in the joints before and after heat treatment are shown in Fig. 6. Before heat treatment, the microhardness values in the WZ are higher than those in the BM (Fig. 6a), which attributes to the grain refinement in the WZ by FSW process (Fig. 2a). Therefore, the joints fracture in the BM and the tensile strength of the joints equal to that of the BM. After heat treatment, the microhardness in the joints significantly increases (Fig. 6b), therefore resulting in a significant increase in the tensile strength of the joints, which is due to the reprecipitation of strengthening precipitates during PWHT (Fig. 2b). Moreover, the microhardness of the joints increases with increasing solution temperature. Therefore, the tensile strength of the heat treated joints increases with increasing solution temperature. The fracture locations in the heat treated joints change with solution temperature, which can be explained by the inner structure of the joints. Figure 7 shows the microstructure of the heat treated joints. It can be seen from this figure that the grains in the WZ are equiaxed (Fig. 7a, b and d), but the ones in the BM are pancake shaped (Fig. 7c). With increasing solution

a A region in Fig. 1b, b B region in Fig. 1b ; c, C region in Fig. 1 b, d D region in Fig. 1c 7 Microstructure of joints


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temperature, the coarsening degree of the grains increases (Fig. 7b and d). Moreover, the PFZ emerge beside the grains boundaries in the WZ after heat treatment. Because the PFZ is soft, the strain concentration will be extreme.20 The crack initiation is liable to occur in the PFZ during tensile test. In addition, the width of the PFZ increases with increasing solution temperatures.20 The higher the solution temperature, the wider the width of the PFZ. In other words, with the increase in solution temperature, the presence of the PFZ and the coarsening equiaxed grains structure make the WZ the weak part of the joints and have a decisive effect on the fracture locations. Therefore, the joints fracture in the BM at lower solution temperature, while fracturing in the WZ at higher solution temperature. All in all, the higher the solution temperature, the higher the tensile strength of the joints. On the other hand, high temperature leads to an increase in the coarsening degree of grains and the width of the PFZ. Therefore, one must compromise on the two different effects.

Conclusions The effects of PWHT on the microstructure and mechanical properties of the FSW joints of 2219-O aluminium alloy have been investigated. The main results are as follows. 1. PWHT results in coarsening of the grains in the WZ, and coarsening degree increases with increasing solution temperature. Moreover, the PFZ appear beside the grains boundaries in the weld after PWHT. 2. The tensile strength of heat treated joints increases with increasing solution temperature. The maximum tensile strength of joints can reach 260% that of the BM at solution temperature of 540uC. 3. PWHT has a significant effect on the fracture locations of joints. When the solution temperature is lower than 500uC, joints fracture in the BM of the joint. On the other hand, when the solution temperature is higher than 500uC, joints fracture in the WZ. The change of the fracture locations of joints is attributed to the presence of the PFZ beside grain boundaries and the coarsening equiaxed grain structures in the weld.

References 1. C. J. Dawes and W. M. Thomas: Weld. J., 1996, 75, 41–45. 2. M. G. Dawes, S. A. Karger, T. L. Dickerson and J. Przyoatek: Proc. 2nd Int. Symp. on ‘Friction stir welding’, Gothenburg, Sweden, June 2000, TWI Ltd, Paper S2-P1. 3. H. J. Liu, H. Fujii, M. Maeda and K. Nogi: J. Mater. Proc. Technol., 2003, 142, 692–696.

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4. S. Benavides, Y. Li, L. E. Murr, D. Brown and J. C. Mcclure: Scripta Mater., 1999, 41, 809–815. 5. T. Hashimoto, S. Jyogan, K. Nakada, Y. G. Kim and M. Ushio: Proc. 1st Int. Symp. on ‘Friction stir welding’, CA, USA, June 1999, TWI Ltd, Paper S9-P3. 6. T. U. Seidel and A. P. Reynolds: Metall. Mater. Trans. A, 2001, 32A, 2879–2884. 7. P. S. Pao, E. Lee, C. R. Feng, H. N. Jones and D. W. Moon: Proc. 4th Int. Symp. on ‘Friction stir welding’, UT, USA, May 2003, TWI Ltd, 8. 8. H. J. Liu, H. Fujii, M. Maeda and K. Nogi: J. Mater. Sci. Lett., 2003, 22, 1061–1063. 9. G. Liu, L. E. Murr, C. S. Niou, J. C. Mcclure and F. R. Vega: Scripta Mater., 1997, 37, 355–361. 10. Y. S. Sato, H. Kokawa, M. Enomoto and S. Jogan: Metall. Mater. Trans. A, 1999, 30A, 2429–2437. 11. M. W. Mahoney, C. G. Rhodes, J. G. Flintoff, R. A. Spurling and W. H. Bingel: Metall. Mater. Trans. A, 1998, 29A, 1955– 1964. 12. M. Cabibbo, E. Meccia and E. Evangelista: Mater. Chem. Phys., 2003, 81, 289–292. 13. H. Hori, S. Makita, T. Minamida, S. Watanabe, E. Anzai and H. Hino: Proc. 3rd Int. Symp. on ‘Friction stir welding’, Kobe, Japan, September 2001, TWI Ltd, Paper S5-P2. 14. C. Juricic, C. D. Donne and U. Drebler: Proc. 3rd Int. Symp. on ‘Friction stir welding’, Kobe, Japan, September 2001, TWI Ltd, 5. 15. K. V. Jata, K. K. Sankaran and J. J. Ruschau: Metall. Mater. Trans. A, 2000, 31A, 2181–2192. 16. M. W. Mahoney, C. G. Rhodes, J. G. Flintoff, R. A. Spurling and W. H. Bingel: Metall. Mater. Trans. A, 1998, 29A, 1955– 1964. 17. J. Backlund: Proc. Conf. TWI, NALCO-7, Cambridge, April 1998, 171. 18. K. Lindner, Z. Khandkar, J. Khan, W. Tang and A. P. Reynolds: Proc. 4th Int. Symp. on ‘Friction stir welding’, UT, USA, May 2003, TWI Ltd, Paper S09A-P2. 19. M. Kumagai, S. Tanaka, H. Hatta, H. Yoshida and H. Sato: Proc. 3rd Int. Symp. on ‘Friction stir welding’, Kobe, Japan, September 2001, Paper S6-P2. 20. K. N. Krishnan: J. Mater. Sci., 2002, 37, 473–480. 21. Ø. Frigaard, Ø. Grong, J. Hjelen, S. Gulbrandsen-dahl and O. T. Midling: Proc. 1st Int. Symp. on ‘Friction stir welding’, CA, USA, June 1999, TWI Ltd, Paper S11-P2. 22. K. A. A. Hassan, A. F. Norman, D. A. Price and P. B. Prangnell: Acta Mater., 2003, 51, 1923–1936. 23. W. J. Arbegast and P. J. Hartley: Proc. 5th Int. Conf. on ‘Trends in welding research’, Pine Mountain, GA, USA, June 1998, ASM International, 541–546. 24. W. J. Arbegast: 13th Annual ‘Advanced aerospace materials and processes’ Conf. and Symp., Orlando, FL, USA, June 2002, ASM International. 25. I. Charit, R. S. Mishra and M. W. Mahoney: Scripta Mater., 2002, 47, 631–636. 26. Y. S. Sato, H. Watanabe, S. H. C. Park and H. Kokawa: Proc. 5th Int. Symp. on ‘Friction stir welding’, Metz, France, September 2004, TWI Ltd, Paper S04A-P1. 27. G. Albertini, G. Bruno, B. D. Dunn, F. Fiori, W. Reimers and J. S. Wright: Mater. Sci. Eng. A, 1997, 224, 157–165. 28. A. C. Nunes: Weld. J., 1984, 63, 27–35. 29. H. J. Liu, H. Fujii, M. Maeda and K. Nogi: Sci. Technol. Weld. Joint, 2003, 8, 450–454.