Materials Science and Engineering B 148 (2008) 82–87 A ys ya S ihog st 200 Abstract The prese simil by friction s n mi of the welde zone periodic cha ents. for the joint ted o © 2007 Else Keywords: Fr struc 1. Introdu Friction invented at The Welding Institute (UK) in 1991 [1,2]. The joint- ing process proceeds in a solid-state where temperature during welding is relatively less than the melting point of welded metal [3–5]. The heat generation caused by the friction between the welding tool and weld metal makes the surrounding material around the joint line [5 are require and chemic Althoug lar joining no informa microstruc The presen speed and fi distribution 7075-T6 A ∗ Correspon E-mail ad 1 Graduate erim tes 3 friction stir butt welded using a tool (SKD61) composed of 12 mm diameter shoulder and 4.0 mm diameter threaded pin. Chemical compositions in mass% of base metals (BMs) 2024 and 7075 Al alloys are 5.35Cu, 0.67Mn, 2.7Mg, Al balance and 7.99Zn, 2.52Mg, 2.4Cu, 0.1Mn, Al balance, respec- 0921-5107/$ doi:10.1016/j tool is soft and allows the tool to move along the ]. Dissimilar joints of 2024-T3 to7075-T6 Al alloys d for aerospace application to optimize mechanical al properties. h, there are some previous reports about dissimi- of 2024-T3 to 7075-T6 Al alloys [6,7], there is tion about the effect of welding conditions on the ture and mechanical properties of butt welded joints. t study aims to investigate the effects of welding xed location of material on microstructure, hardness , and tensile properties for dissimilar of 2024-T3 to l alloys joints produced by FSW. ding author. dress:
[email protected] (S.A. Khodir). Student of Osaka University, Japan. tively. Mechanical properties at room temperature of the BMs are shown in Table 1. Rotation speed was kept constant at 20 s−1 and welding speed was set at 0.7, 1.2, 1.7 and 3.3 mm/s. Microstructures of various regions of welds were observed in the cross-section of the joint by optical microscopy. The homogeneity of constituents in the stir zone (SZ) was ana- lyzed by SEM-EDS method. Microhardness measurement was carried out 4.3 Ms (50 days) after natural aging. The ten- sile test was carried out at room temperature at a strain rate of 1.2 × 10−3 s−1. 3. Results and discussion 3.1. Macro and microstructures of joints Macroscopic appearances of the cross-section of the joints produced at a rotation speed of 20 s−1 and welding speeds of – see front matter © 2007 Elsevier B.V. All rights reserved. .mseb.2007.09.024 Friction stir welding of dissimilar aluminum allo Saad Ahmed Khodir ∗,1, Toshi Joining and Welding Research Institute, Osaka University, 11-1, M Received 22 May 2007; received in revised form 25 Augu nt study focuses on the microstructure and mechanical properties of dis tir welding. Effects of welding speed and fixed location of base metals o d joints were investigated. SEM-EDS analysis revealed that the stir nge of grain size as well as a heterogeneous distribution of alloying elem produced at welding speed of 1.67 mm/s when 2024 Al alloy was loca vier B.V. All rights reserved. iction stir welding; 2024-T3 Al alloy; 7075-T6 Al alloy; Welding speed; Micro ction stir welding (FSW) is a solid-state joining process 2. Exp Pla A2024 and AA7075 hibayanagi aoka, Ibaraki, Osaka 567-0047, Japan 7; accepted 3 September 2007 ar joints of 2024-T3 Al alloy to 7075-T6 Al alloy produced crostructures, hardness distributions, and tensile properties contains a mixed structure and onion ring pattern with a The maximum tensile strength of 423.0 MPa was achieved n the advancing side. ture; Mechanical properties ental procedures mm thick of 2024-T3 and 7075-T6 Al alloys were S.A. Khodir, T. Shibayanagi / Materials Science and Engineering B 148 (2008) 82–87 83 Table 1 Mechanical properties of base metals Materials Mechanical properties at room temperature Yield stress (MPa) Tensile stress (MPa) Elongation (%) AA2024-T3 327 461 29.5 AA7075-T6 498 593 17.7 0.7 and 3.3 mm/s are shown in Fig. 1. The base metal of 2024 Al alloy was located on the advancing side in Fig. 1(a and b) and on the retreating side in Fig. 1(c and d), respectively. The FSW joint is characterized by four different zones as shown in Fig. 1(a) as follow: stir zone (SZ) around the weld center line, thermomechanically affected zone (TMAZ) on both sides of the SZ, heat affected zone (HAZ) which is surrounding the TMAZ, and non-affected base metal (BM). Onion ring patterns were obviously observed in the SZ irrespective of welding con- ditions and fixed locations of BMs. In addition, the size of SZ decreased with rise in welding speed probably due to decreased heat inputs. Although, all joints are free from cracks and tun- nel like defects, kissing bond at the weld root and pores in the SZ were clearly observed at welding speed of 3.3 mm/s espe- cially when the 2024 Al alloy plate was located on the retreating side. Microstructures of the base metals, onion ring structure, and kissing bond defect are shown in Fig. 2. The microstructures of the base me and b), res grains alon constituent grains in 2024 Al alloy are slightly wider and contain higher concentration of constituent particles than that observed in the 7075 Al alloy plates. As shown in Fig. 2(c), onion rings in the SZ of joint welded at a welding speed of 1.2 mm/s consisted of bands with fine and coarse grains. The average grain sizes of the bands are estimated to be 4.1�m and 5.8�m. The microstructure observed at the root of the joint welded at 3.3 mm/s is shown in Fig. 2(d) when the 2024 Al alloy plate was located on the retreating side. It contains residual structures of the two alloy plates with a clear kissing bond observed between them. The structure located on the left hand side belonged to the 7075 Al alloy BM. The other on the right hand side is the elongated grains of the 2024 Al alloy plate. It is said that the kissing bond defect results from the insufficient metal flow of base metals or insufficient penetration depth of the probe at the weld root due to the lack of heat input generated from the friction between the rotating probe and two alloy plates at higher welding speeds especially when the 7075 Al alloy plate located on the advancing side. Close observations of the grain structures of the stir zone are shown in Fig. 3 for joints welded at different welding speeds with locating the 2024 Al alloy plate on the advancing side. Grain size decreases with increasing welding speed. Grain size decreases from 7.9�m at 0.7 mm/s to 3.3�m at 3.3 mm/s. This could be attributed to the lower temperature caused by the lower heat input associated with faster welding speed where the grain size decreases with decreasing heat input [8]. The effect of welding speeds on the grain size in the SZ was almost independent of ed l ratur y pl Fig. 1. Macro where 2024 A tals of 2024 and 7075 Al alloys are shown in Fig. 2(a pectively. The microstructures consist of elongated g the rolling direction with a random distribution of particles recognized as small black particles. The the fix tempe Al allo structures of joints welded at different welding speeds: (a) 0.7, (b) 3.3 mm/s where 2 l alloy was fixed on the retreating side. ocations of Al alloy plates. This suggests that the e in the SZ is almost unaffected by the location of the ates. 024 Al alloy was fixed on the advancing side (c) 0.7, (d) 3.3 mm/s 84 S.A. Khodir, T. Shibayanagi / Materials Science and Engineering B 148 (2008) 82–87 Fig. 2. Optical microstructures BMs and SZ: (a) BM of 2024-T3, (b) BM of 7075-T6, (c) onion ring, and (d) kissing bond defect. Fig. 3. Optical microstructures of SZ of the joints where AA2024-T3 fixed at advancing side: (a) 0.7 mm/s, (b) 1.2 mm/s, (c) 1.7mm/s, and (d) 3.3 mm/s. S.A. Khodir, T. Shibayanagi / Materials Science and Engineering B 148 (2008) 82–87 85 Fig. 4. SEM EDS image o 3.2. EDS a Results the SZ are s 1.7 mm/s a side. The b part of the characteris Fig. 4(b–d Table 2 EDS quantita Alloying elem Cu Zn Mg Mn 2024 Al alloy and EDS images of SZ of the joint welded at 1.70 mm/s of welding speed: (a) SEM i f Mg. nalysis of joints from the SEM-EDS analyses of the advancing side of hown in Fig. 4 for a joint welded at welding speed of nd located the 2024 Al alloy plate on the advancing ands of onion ring are clearly visible in the middle SEM image (BEI mode) as shown in Fig. 4(a). The tics X-ray images of Cu, Zn and Mg, are shown in ), receptively. The brighter bands in Fig. 4(a) were rich in Zn Although b Mg, Zn wa darker ban while the b The resu Mg and M in two BM of Cu, Mg tive analysis of base metals and zones corresponding to Fig. 4(a) for the welding spe ents Content/mass% 2024 Al alloy 7075 Al alloy 1 2 5.95 2.53 5.86 2.6 0.12 8.6 0.24 7.7 2.45 1.2 2.68 1.3 0.65 0.2 0.79 0.1 plate was located in the advancing side. mage of SZ, (b) EDS image of Cu, (c) EDS image of Zn, and (d) while the darker bands were rich in Cu and Mg. oth the 2024 alloy and 7075 alloy contain Cu and s added only to the 7075 Al alloy. Therefore, the ds observed in Fig. 4(a) is related to 2024 Al alloy righter ones are related to 7075 Al alloy. lts from EDS quantitative point analysis for Cu, Zn, n at the positions indicated in Fig. 4(a) as well as s are summarized in Table 2. The mass percentages and Mn in the positions 1, 3, and 5 were almost ed of 1.7 mm/s 3 4 5 6 0 5.80 2.72 5.68 2.42 2 1.06 7.39 0.36 6.83 0 2.82 1.36 2.32 1.28 7 0.47 0.21 0.63 0.09 86 S.A. Khodir, T. Shibayanagi / Materials Science and Engineering B 148 (2008) 82–87 Table 3 Mechanical properties and fracture locations of welded joints in the transverse direction to weld center line Rotation speed (s−1) Welding speed (mm/s) Tensile properties at room temperature AA2024-T3 located in advancing side Yield strength (MPa) Tensile strength (MPa) Elongation (%) Fracture location 20 0.7 274.8 395.0 13.6 HAZ of 2024 1.2 281.5 404.0 14.5 HAZ of 2024 1.7 290 423.0 14.9 HAZ of 2024 3.3 287 398.0 11.4 SZ corresponding to their contents of 2024 Al alloy plate while the concentrations of Zn, Mg and Mn at the positions 2, 4 and 6 were close to 7075 Al alloy plate. From these results, it should be noted that the heterogeneous distribution of alloying elements in the SZ was realized regard- less of welding speed and location of plates. This heterogeneous in microstructures through the SZ might be attributed to the insufficient welding time required for completing diffusion of alloying el the onion r position as SZ. The sm alloy plate to the 7075 3.3. Hardn Microha of joints we in Fig. 5. H of 2024 A Also, softe shifted tow was increas The sof attributed m Fig. 5. Hardn the joints whe their coher scattering o formation chemical c ness) and 7 and SZ reg base metals than that in t of g el ing d ensil tens erent l al h inc tensi mm/ he te mm/ en 2 reasi ion i nd p ile p on valu s, te o 70 and , the ed o ements in the SZ. Also, it should be concluded that ing pattern reflects the difference in chemical com- well as grain size between the lamellar bands in the all grains in darker bands were related to the 2024 Al while the big ones in the brighter bands were related Al alloy plate. ess of joints rdness distributions on the transverses cross-section lded at welding speeds of 0.7 and 3.3 mm/s are shown ardness minimum appeared in the heat affected zone l alloy and its value increased with welding speed. ned zones existing on both sides of the HAZ were ards the welding center line as the welding speed ed. tening of hardness of the HAZ in the joints can be ainly to the coarsening of precipitates where lost amoun alloyin harden 3.4. T The at diff 2024 A strengt imum of 1.7 joint, t of 3.3 Wh an inc reduct bond a of tens located higher Thu plate t defects defects fractur ess profiles at mid thickness transfers to the welding directions of re AA2024-T3 fixed on the advancing side. hardness. 4. Conclu In the fr alloy plate of kissing plate was were forme grain sizes AA7075-T6 located in Advancing side Yield strength (MPa) Tensile strength (MPa) Elongation (%) Fracture location 270.0 392.0 12.1 HAZ of 2024 264.0 394.0 12.5 HAZ of 2024 280.0 381.0 9.0 SZ 283 340.0 7.5 SZ ency with the matrix due to the thermal history. The f hardness values in the SZ can be attributed to the of onion ring pattern that consists of bands having omposition similar to those 2024 alloy (lower hard- 075 alloy (higher hardness). Hardness in the TMAZ ions were slightly decreased in comparison with the . The higher hardness observed in the SZ and TMAZ the HAZ can be attributed to the increase in the dissolved precipitates that increased the contents of ements (in-solution) available for the precipitation uring natural aging [9]. e properties of joints ile properties and fracture locations of joints welded welding speed are summarized in Table 3. When loy was located on the advancing side, the tensile reased with welding speed up to 1.7 mm/s. The max- le strength 423 MPa was achieved at welding speed s. Due to the occurrence of kissing bond at the root of nsile properties of joint decreased at a welding speed s rather than that at 1.7 mm/s. 024 Al alloy plate was located on the retreating side, ng in welding speed more than 1.2 mm/s led to the n tensile properties due to the formation of kissing ores in the SZ as shown in Fig. 1(d). The enhancement roperties of the joints observed when 2024-T3 was the advancing side was mainly due to the slightly es of minimum hardness in the HAZ [10]. nsile properties of FSW butt joints of 2024 Al alloy 75 Al alloy plate depended mainly on the welding hardness of the joint. When joints were free from ir tensile properties were controlled by hardness and ccurred in the HAZ on the side showed minimum sions iction stir welding of 2024 Al alloy plate to 7075 Al , the rise in welding speed tended to the formation bond and pores especially when the 2024 Al alloy located on the retreating side. Onion ring patterns d and characterized by bands of different equiaxed and heterogeneous distribution of alloying elements S.A. Khodir, T. Shibayanagi / Materials Science and Engineering B 148 (2008) 82–87 87 in the SZ regardless of welding speed and fixed locations of base metals. Hardness minima were observed in the HAZ both sides and their values increased with welding speed. Quite wider scatter- ing of hardness was observed in the SZ corresponding to the onion ring pattern. Defect-free joints were fractured in the HAZ on 2024 Al alloy side, viz. hardness minimum area, while the defect-containing joints fractured in the SZ. Maximum tensile strength of the joints of 423 MPa was achieved at a welding speed of 1.7 mm/s when 2024 Al alloy plate was located on the advancing side. Acknowledgements This work was supported by a grant-in-aid for scientific research B (project No. 17360353) and a grant-in-aid for cooper- ative research project of nationwide joint-use Research Institutes on Development Base of Joining Technology for new Metallic Glasses and Inorganic Materials from The Ministry of Educa- tion, Science, Sports and Culture, Japan. References [1] C.J. Dawes, W.M. Thomas, Weld. J. 75 (1996) 41–45. [2] O.V. Flores, C. Kennedy, L.E. Murr, D. Brown, S. Pappu, M.N. Brook, J.C. McClure, Scripta Materialia 38 (1998) 703–708. [3] R.S. Mishra, Z.Y. Ma, Mater. Sci. Eng. R 50 (2005) 1–78. [4] C.I. Chang, C.J. Lee, J.C. Huang, Scripta Materialia 51 (2004) 509– 514. [5] H.R. Shercliff, J.R. Michael, A. Taylor, T.L. Dickerson, Mech. Ind. 6 (2005) 25–35. [6] L. Cederqvist, A.P. Reynolds, Weld. J. 80 (2001) 281–287. [7] P. Cavaliere, R. Nobile, F.W. Panella, A. Squillace, Int. J. Mach. Tools Manuf. 46 (2006) 588–594. [8] S.A. Khodir, T. Shibayanagi, M. Naka, Mater. Trans. 47 (2006) 185– 193. [9] M.J. Jones, P. Heurtier, C. Desrayaud, F. Montheillet, D. Allehaux, J.H. Driver, Scripta Materialia 52 (2005) 693–697. [10] S.A. Khodir, T. Shibayanagi, Mater. Trans. 48 (2007) 1928–1937. Friction stir welding of dissimilar AA2024 and AA7075 aluminum alloys Introduction Experimental procedures Results and discussion Macro and microstructures of joints EDS analysis of joints Hardness of joints Tensile properties of joints Conclusions Acknowledgements References