Ageing Behavior of Friction Stir Welding AA7075-T6 Aluminum Alloy T. AZIMZADEGAN*, GH.KHALAJ*, M.M. KAYKHA**, A.R.HEIDARI*** *Department of Materials Science and Engineering, Saveh branch, Islamic Azad University, Saveh, Iran,tohid_s69@yahoo.com ** Faculty of Engineering, Department of Mechanical Engineering University of Zabol, Zabol, IRAN, mm_kaykha@yahoo.com *** Member of young researcher club, I.A.U, Zabol Branch, arg.heidari@gmail.com Abstract: - The microstructures of FSWs in age-hardening 7075-T6 aluminum alloy have been characterized by scanning electron microscopy. Different planes of section, in addition to the more conventional transverse section, have been used to examine the different microstructural zones present in FSWs. Tensile testing has been employed to examine the natural and artificial ageing response of the FSWs, several months after welding. The ageing response of the alloys has been related to the thermal cycle experienced during the welding operation. Key-Words: Friction stir welding, Aging, Tensile test, AA 7075. 1 Introduction Friction stir welding (FSW) is a relatively new route to conduct welding operations in solid-sate conditions [1]. During the process, a rotating pin is first inserted in the material and after a dwell time, it moves along the joint line. Friction between the rotating tool and the workpiece generates heat which reduces the flow stress of material around the rotating pin and tool shoulder, and then weld joint is produced by material flow from the advancing side to the retreating side of the weldment. Several researches have been conducted to characterize the effect of postwelding treatment on mechanical and microstructures properties in aluminum alloys. For instance, Barcellona et al. [2] have studied the effect of post-welding heat treatments on the joint strength of 2024-T4 and 7075 T6. They have shown that Post-welding heat treatments can improve the material mechanical characteristics and overall can increase the joints resistance. Mahoney et al. [3] have investigated the changes in tensile properties produced by FSW 7075 T651 Al and to determine the effect, if any, of postweld low-temperature aging. Oertelt et al. [4] have reported that the effect of thermal cycling on friction stir welds AA 2195 led to a decrease in dislocation density and precipitation of the second phase within and along the grain boundaries. Whilst early work concentrated on establishing process parameters necessary to produce high quality friction stirs welds in AA 7075-T6 alloy, there is a need to understand further the microstructural developments and ageing characteristics of such joints. 2 MATERIALS AND METHODS The composition the employed aluminum alloy is given in Table 1. FSW was performed on a 5 mm thick plate with 100 mm in length, along the welding line, and 50 mm in width. The welding tool is made of H13 tool steel with a hardness of 52 HRC having shoulder height of 10 mm and diameter of 20 mm. The shoulder face was designed as a concave cone. The welding tool was tilted 3_ about vertical axis, and a cylindrical welding pin was employed in the experiments. The diameter of the pin was 6 mm, and the pin height was 4.9 mm. The range of rotational speeds utilized for the welding was varied between 1000 and 10 rpm, whereas the longitudinal speeds were varied from to 80 mm/min (0.67 to 1.3 mm/s). Table 1. The chemical composition of the employed alloy. Element Al Zn Mg Cu Cr Wt.% 90.07 5.6 2.5 1.6 0.23 Temperature measurements were also made on the welded joint during the process by means of thermocouples placed at distances of 5 and 25 mm from the centerline of weld. Thermocouples were attached to a data acquisition system at the rate of 10 Hz, and data collection was accomplished with the system attached to a personal computer. The ISBN: 978-1-61804-014-5 183
microstructural studies were made utilizing optical and scanning electron microscopy. After welding, the samples were sectioned normal to the welding direction and then prepared by grinding disks and polished and finally etched with Keller_s reagent: 150 ml H2O, 3 ml HNO3, 6 ml HCl, and 6 ml HF [5]. To evaluate the mechanical properties of the welded part, longitudinal and transverse tensile tests were also performed. Tensile tests for the material within the nugget zone as well as the hardness testing were carried out 3 weeks after the welding experiments and T6 treatment. However, tensile tests for heat-affected zone (HAZ) were performed 6 months after welding operation to ensure a stable state of natural aging in the above region. 3 Discussion Tensile tests have been carried out to evaluate the effect of rotational and longitudinal speeds on the mechanical properties of the nugget zone and HAZ. The stress-strain curves of base material and the weld zone are compared in Figure 1. In addition, the results of the tensile tests for friction stir-welded 7075-T6 Al alloy are shown in Table 2. The results show a reduction in yield and ultimate tensile strengths in the weld nugget irrespective of the welding speeds. This may be attributed to the elimination of the strengthening precipitates in the weld nugget and the thermo- mechanically affected zones [6]. strengthening particles is presented as compared to higher heat input. Table 2: Tensile properties of friction stir welded 7075-T6 Al alloy. Base metal Longitude direction Transverse direction Longitude speed (mm/min) ---- 60 Rotational speed (rpm) ---- 1200 10 1200 10 Yield Strength (MPa) 480 290 356 290 283 223 271 215 Ultimate strength (MPa) 567 484 518 476 5 385 430 376 Elongation (%) 8.2 6.5 9.5 7.5 7 3 5.3 2.8 Fig. 1. The stress-strain curves for base and FSWed 7075- T6 Al alloy with different rotational speeds at longitude speed of mm min-1. The effect of higher and lower ratios of the rotational speed to longitude speed (high and low heat input, respectively) on the HAZ microstructure is shown in Figure 2. For lower heat input the strengthening precipitates in the HAZ is roughly coarsened and lower density of very fine Figure 2: SEM micrographs of the HAZ at different rotational and longitude speeds, (a) rpm and mm min -1, (b) 1000 rpm and 80 mm min -1. 3.1 Artificial ageing A post-welding heat treatment was performed for a set of joints in order to reproduce the T6 treatment. The treated welded specimens were analyzed in order to highlight the new density of the insoluble particle values. In particular an almost ISBN: 978-1-61804-014-5 184
uniform recovery of the hardness was obtained along the transverse joint section, indicating a strong improvement of the joint mechanical characteristics. In Figure 3 the values of density of the insoluble particles are shown. Figure 3: AA7075-T6 welded joint density of the insoluble particles. A strong increase is obtained in the density values along the joint section at different distances from the joint symmetry axis; the differences with the parent material were due to some irregularities in the development of the post-welding heat treatment, such as the temperature control in the utilized oven and/or the temperature of the quench medium Post welding heat treatments were tested with the aim to increase the joint resistance, through precipitation hardening. 3.2 Effect of natural ageing on the strength of HAZ Process parameter variations, such as changes in rotational or longitude speed, will produce different thermal profiles (similar to Figure 4). The effect of such variations on the precipitate distribution can be investigated with the presented models. Figure 5 indicates the stress-strain curves of the HAZ in parallel to weld line for different welding parameters. In the HAZ the precipitates are overaged by the lower temperatures experience in this region during welding, leading to more reduction in the strength of this region. As the material naturally ages with time (several months), the formation of strengthening precipitates increases the strength of the weld zones, although the original strength level is never fully regained. As seen in Figure 5, the ultimate tensile strength increased with increasing rotational speed due to a higher heat input in the HAZ, that resulting in dissolve the formed coarse precipitates in this region during welding. Figure 4: Thermal profiles for different welding speeds, (a) 1000 rpm and 80 mm min -1, (b) 1000 rpm and mm min -1. Figure 5: The stress-strain curves of the HAZ in parallel to weld line for different welding parameters. The model used to capture the HAZ microstructure evolution during welding of 7XXX series aluminum alloys have been adopted from Bjorneklett et al. [7]. This model use a combination of chemical thermodynamics and diffusion theory to describe the dissolution, reprecipitation, and natural aging kinetics occurring sequentially in time as a result of welding. According to this model equilibrium solves temperature ( T eq ), as a function of composition (C), is given as [7]: ISBN: 978-1-61804-014-5 185
( ) T eq C 32000 R ln ( C 2590) 273 Also, the metastable solves temperature ( T defined as [7]: ( C) ' eq (1) ) is 32000 Ω 32000 T eq 273 32000 ln( 2590) R C (2) where 2γv Ω = m (3) where r denotes the contribution of the interface curvature to the reaction enthalpy, where particle/matrix interfacial energy, is the is the molar volume of the precipitates, and is the initial (mean) particle radius. Referring to Figure 6, dissolution of the hardening phase (i.e. η' in AA7075) is the main factor contributing to strength loss during welding in the HAZ. The dimensionless strength parameter,, is expressed as [7]: 2 0 S1 S min 1 0 0 r = = S max S min r0 α (4) where S1 denotes a specific mechanical property such as Vickers hardness (HV) and strength. The maximum and minimum values ( and, respectively) represent the hardness or strength in the age-hardened and fully reverted conditions, respectively. Figure 6: Thermal profiles showing temperature range of microstructure evolution in the HAZ during welding of AA7075-T6. As seen in Figure 2b, growth of the nonhardening phase (η) occurs during the cooling leg of the thermal cycle, particularly for temperatures between and in the HAZ, leading to solute depletion within the aluminum matrix. This, in turn, reduces the precipitation potential and contributes to the development of a permanent soft zone within the partly reverted region of the weld HAZ after prolonged roomtemperature aging which agrees well with the equation [8]. It follows from Figure 5 that the strength recovery due to natural aging occurs more or less evenly across the entire soft zone of the welds. The HAZ strength level depends on the interplay between two competing processes that occur during and after welding, respectively, i.e., dissolution and reprecipitation. For the higher heat input (peak temperature higher than, rotational speed equal to 10 rpm), the precipitates of and dissolve during welding as well as the mean solute concentration in the matrix increase. Therefore, strength of the HAZ after natural aging increase that this is in good agreement with experimental observation as illustrated in Figure 5. It is noted that there is a considerable different ( 50 MPa) between strength of the samples with different welding speeds. 4 Conclusion Post-welding heat treatments can improve the material mechanical characteristics and overall can increase the joints resistance. It should be observed that aluminum alloys are often very difficult material to be thermally treated; in this way accurate monitoring and control activities have to be applied during heat treatments in order to avoid the insurgence of defects. Undesirable further precipitates may appear as a consequence of the heat treatment; in this way proper checks have to be developed depending on the material. References: [1] W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Templesmith, and C.J. Dawes Friction Stir Butt Welding Great Britain Patent 9125978.8, 1991. [2] A. Barcellona, G. Buffa, L. Fratini and D. Palmeri On microstructural phenomena occurring in friction stir welding of aluminium ISBN: 978-1-61804-014-5 186
alloys Journal of Materials Processing Technology 177 (2006) pp. 3 343. [3] M.W. Mahoney, C.G. Rhodes, J.G. Flintoff, R.A. Spurling, and W.H. Bingel " Properties of Friction-Stir-Welded 7075 T651 Aluminum" METALLURGICAL AND MATERIALS TRANSACTIONS A, VOLUME 29A, (1998) pp.1955-1964. [4] G. Oertelt, S. S. Babu, S. A. David and E. A. Kenik "Effect of Thermal Cycling on Friction Stir Welds of 2195 Aluminum Alloy" Welding Research Supplement, (2001) pp. 71-79. [5] M e t a l s H a n d b o o k, 8 t h e d., V o l. 8, Metallography, Structures and Phase Diagrams, ASM, Metals Park, OH, 1973, pp. 124. [6] H.J. Liu, H. Fujii, M. Maeda, and K. Nogi Tensile Properties and Fracture Locations of Friction-Stir-Welded Joints of 2017-T351 Aluminum Alloy J. Mater. Proc. Tech. 142 (2003), pp. 692 696. [7] B.I. BjØrneklett, Ø. Grong, O.R. Myhr, A.O. Kluken, A process model for the heat-affected zone microstructure evolution in Al-Zn-Mg weldments, Metal. Mater. Trans 30A (1999) pp. 2667-2677. [8] K.A.A. Hassan, P.B. Prangnell, A.F. Norman, D.A. Price, S.W. Williams, Effect of welding parameters on nugget zone microstructure and properties in high strength aluminium alloy friction stir welds, Sci. Tech. Weld. Join, 8 (2003) pp. 257-268. ISBN: 978-1-61804-014-5 187