Mechanical properties and parameters correlation at high speed double pass FSW in AA2050-T84 alloy

H. Plaine¹, C. R. M. Afonso¹, L. Mechanical properties were evaluated by microhardness and tensile tests, the results were used to find the conditions within the optimized parameters window, which are strongly related. Microstructural and microhardness features were observed and correlated with mechanical testes results. The results obtained in this work showed double pass (DS-FSW) friction welding processes at high welding speeds were found to be suitable for the production of similar joints of the AA 2050 alloy -T84, with applications for the aeronautical industry. Were found the optimized conditions for DS-FSW (1,00 mm/rev; 600 rpm; 10 mm/s). Keywords: DS-FSW, Friction Stir Welding, High speed welding, aluminum-cooper-lithium alloys, mechanical properties. AA 2050 aluminum-copper-lithium alloy was designed to have lower density, better specific strength, toughness and elastic modulus, fatigue crack growth resistance properties and exhibits excellent stress corrosion cracking resistance than conventional non-lithium bearing series alloys (2–5).

However, it is known that many metallurgical phenomenons like grain growth, recrystallization, recovery, precipitates nucleation, coarsening and dissolution, are a function of temperature peak reached during the welding process, which in turn is a function of distance from the weld nugget central line. In alloyed 2XXX and 7XXX aluminum series the fusion weld techniques like arc-welding, have particular problems like poor solidification microstructure formation, porosity in the weld zone and weld crack susceptibility, resulting in significant mechanical properties losses (strength, fatigue and fracture resistant joints) compared to their base materials (6–9). Thus, the high performance of these aluminum alloys welds is compromised. Fusion weld techniques, are been replaced by alternative processes: Laser beam welding (LBW) and Friction Stir Welding (FSW) (6,10). Then, the optimal parameters depend on the material to be welded and on the specific criteria adopted to evaluate weld quality.

Figure 1. Operational welding parameters window (12). Fig. show the parameter combination in the “cool” side with insufficient heat input; and also, “hot” side with generation of excessive heat input. Zn0. Mn0. Cu3. Li1-Ag0. Zr0. DS-FSW experimental parameters to 25 kN Condition rpm mm/s Weld pitch (mm/rev) 1 400 4 0,60 2 6 0,90 3 500 8 0,96 4 600 10 1,00 5 700 12 1,03 6 400 7 1,05 7* 8 1,2 8* 500 10 *Condition which probe has been broken Microstructure images were captured by an optical microscope Leica DM IRM integrated to the software Leica Application Suite 3. software, examined under cross-polarized illumination for micrographs. SEM images of fracture mode from tensile tests were obtained using FEI quanta 650 SEM, operated at 15-20 kV. Scanning electron microscopy (SEM) (LEO 435 VPi coupled to Oxford energy dispersive X-ray spectroscopy - EDS); and SEM-EBSD-EDS simultaneous analysis was realized in a Dual Beam (field emission gun - FEG electron beam and Ga source focused ion beam - FIB) ZEISS Dual Beam model AURIGA Compact, which was used as well for FIB “in-situ” polishing of samples surface previous to EBSD maps acquisition at 20 kV.

Second phase area fraction and grain size were determined by image analysis software. Heat input (kJ/mm) versus revolutionary pitch (mm/rev). Error: Reference source not found shows the experimental results of heat input obtained for each rotational pitch. Can be observed an inverse relationship between revolutionary pitch and the corresponding heat input. According to these results, it is suitable to expect a trend of heat input reduction with a rotational pitch (RP) increment, except for the condition 6 that runs away from the rule with higher RP and higher heat input than conditions 1 to 5. Figure 2. Temperature peaks measured and estimated by perpendicular distance to the weld center. D (mm) Side T (°C) Side T (°C) 2. AS 232 RS 276 7 204 243 12 173 206 17 141 168 22 114 131 25 90 108 DSC curves reported by Sidhar et al. for various AA 2050 FSWed samples fabricated for T3 state, compared to samples in the T8 state, showed that an exothermic peak representing T1 precipitation (Al2CuLi) in the range between 240 and 310 °C for T3 state.

This peak is absent to the initial T8 state because has been previously aged. Error: Reference source not foundshows the observed void volumetric defects in AS, in ZM near ZTMA in condition 5, with highest welding speed (12 mm/s and revolutionary pitch 1. mm/rev). Very High travel speeds gives to the work piece less amount of work per unit of length. Consequentially material reach a lower plasticized temperature, i. e. HAZ is a region that experience temperature solicitation, containing elongated grain with similar shape and direction to base material. TMAZ is the adjacent area of nugget zone, a transition from NZ to HAZ. The region experienced both tool mechanical and thermal solicitation in a magnitude to grains become refined and show divergent shape of HAZ elongated shape. Double nugget zone (DNZ) is the region that experiences two thermal cycles and is common region among both NZ of both passes.

Although, there are no significant microstructural differences between first and the second pass, DNZ final microstructure extension through one pass in relation to another still not being well understood. NZ has refined grains due to recrystallization. HAZ receives a thermal cycle the unique responsible for grain growth higher than BM. Figure 6. Crystallographic orientation maps (a) BM (b) NZ (c) HAZ (d) Black and white band contrast for HAZ (e) Crystallographic orientation for EBSD maps. The contrast differences between NZ, TMAZ and HAZ images (Error: Reference source not found) may be associated with grain size differences. The images of Error: Reference source not found (e-f) show in the NZ a similarity between the presence of Cu and Mn (valid also for Fe), however, NZ has also other particles with Mn without Cu and Fe, possibly a lithium formation due to alloy composition.

Finally, the images of Error: Reference source not found (c-e) compares the volumetric fraction and particle size of different weld regions (MB, HAZ and NZ). Figure 7. Energy-dispersive X-ray spectroscopy (EDS) of different elements and in different regions for condition 4 (a) Mn-HAZ (b) Fe-HAZ (c) Cu-HAZ (d) Cu-BM e) Cu-NZ (f) Mn-NZ. When calculating these quantities, it is possible to observe, in relation to the MB, the increase in the particle size of Cu, higher in the HAZ than in the NZ, and the increase of the volumetric fraction in the HAZ and a reduction in the NZ. This high deterioration can be explained by DNZ experienced twice the thermo-mechanical solicitation; giving highlight to the two times received peak temperatures (18). Figure 8. Micro-hardness profile of the condition 4. Near top surface of the first pass (3 mm below), center (mid-thickness) and top surface of the second pass (3 mm above) (fastest condition).

Disposing tensile specimens machined from longitudinal direction, was possible to evaluate the effect of the welding conditions on the tensile properties. From the results can be deduced that weld pitches higher than 1,00 mm/rev lies down into a “cold side” in the right side of the Optimal Parameter Window (see Error: Reference source not found), as the heat input is not enough to completely plasticize the base material that allows a correct weld consolidation. The maximum percentage relative to the BM values reached for Ys, Us and elongation was 67%, 78% and 38% and the absolute values are quite close for conditions 1-4 (Table 4). Table 4. Tensile properties values and percentage relative to BM. Condition Ys (MPa) Us (MPa) Elongation (%) Weld pitch (mm/rev) 1 289 ± 9 (62%) 407 ± 1 (76%) 4,9 ± 0,1 (37%) 0,60 2 310 ± 4 (67%) 411 ± 1 (77%) 5,1 ± 0,4 (38%) 0,90 3 293 ± 3 (63%) 417 ± 1 (78%) 3. An inverse relationship between heat input and weld pitch was observed.

The initially worked weld pitch range was between 0. mm/s, to avoid flash formation and lack of heat for effective mixing of material. The highest process peak temperature estimated was on the reatriting side (RS) was 276 °C The lower temperatures for DS-FSW are not sufficient to achieve complete dissolution of the T1 precipitates in the NZ and in the HAZ partial dissolution and less coarsening. The refining effect is secondary when compared to the dissolution of precipitates and intermetallic particles. Were found the optimized conditions DS-FSW: condition 4 (1,00 mm / rev; 600 rpm; 10 mm / s). ACKNOWLEDGMENTS REFERENCES 1. Lavernia, E. J. Grant N. Rioja, R. J. Liu J. The Evolution of Al-Li Base Products for Aerospace and Space Applications. Metall Mater Trans A, 43(9):3325–37. B. Lukin, V. I. Kolobnev, N. I. Zhang, X. Problems and issues in laser beam welding of aluminum-lithium alloys.

Journal of Manufacturing Processes, 16(2):166–175. Recuperado de: http://dx. doi. El, Mehtedi M. Simoncini, M. Double side friction stir welding of AA6082 sheets: Microstructure and nanoindentation characterization. Materials Science and Engineering: A, 590:209-217. doi. Welding aspects of aluminum-lithium alloys [Internet]. Aluminum-Lithium Alloys: Processing, Properties, and Applications, p. Recuperado de: http://dx. doi. org/10. Waters, T. Varner, C. Journal of Materials Processing Technology Optimizing weld quality of a friction stir welded aluminum alloy. J Mater Process Tech, 222:188–96. doi. Alexis, J. Andrieu E, Delfosse J, Lafont MC, Blanc C. Characterisation and understanding of the corrosion behaviour of the nugget in a 2050 aluminium alloy Friction Stir Welding joint. Corrosion Science, 73:130-142. Recuperado de: http://dx. Deschamps, A. Microstructure mapping of a friction stir welded AA2050 Al–Li–Cu in the T8 state.  Philosophical Magazine, 94(13):1451-1462. Pouget, G. Reynolds, A.

Mishra, R. S. Reynolds, A. P, Baumann, J. A. W. Jata, K. V. Semiatin, S. L. R. Cox, C. D. Ballun, M. C. Sivapragash, M. A. Comparative Study of the Mechanical Properties of Single and Double Sided Friction Stir Welded Aluminium Joints. Procedia Eng, 38:3951–61. Recuperado de: http://dx. C. Li, H. P. Friction stir weld of 2060 Al–Cu–Li alloy: Microstructure and mechanical properties. J Alloys Compd, 649:19–27. Local fatigue crack propagation behavior of a two-pass friction stir welded aluminum alloy.  Mechanical Engineering Journal, 1(6):SMM0057-SMM0057. Hassan, K. A. A. In 2nd Int. Symp Friction Stir Weldong. Jata, K. V. Semiatin, S. Roy, G. G. Debroy T. Numerical Simulation of Three-Dimensional Heat Transfer and Plastic Flow During Friction Stir Welding. Metallurgical and materials transactions A, 37(4):1247-1259. matdes. Niwas, R. Kumar, P. Bhambhu, R.

Effect of Tool Pin Profile on Mechanical Properties of Single and Double Sided Friction Stir Welded Aluminium Alloy AA19000, 5(6):1338–41. H. Shah, L. H. Ishak, M. Mechanical and microstructural characterization of single and double pass Aluminum AA6061 friction stir weld joints. Sabari, S. S. Malarvizhi, S. Balasubramanian, V. Influences of tool traverse speed on tensile properties of air cooled and water cooled friction stir welded AA2519-T87 aluminium alloy joints.