Pinless Friction Stir Welding of AA2024-T3Joint and Its Failure Modes

Trans. Tianjin Univ. 2014, 20: 439-443 DOI 10.1007/s12209-014-2157-2Accepted date: 2013-10-27.*Supported by the Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (No. 131052), the Fundamental Research Funds for the Central Universities (No. 3102014JC02010404), the Research Fund of the State Key Laboratory of Solidifi-cation Processing (No. 108-QP-2014), the Graduate Starting Seed Fund of Northwestern Polytechnical University (No. Z2013019) and the “111” Project (No. B08040).Li Wenya, born in 1976, male, Dr, Prof.Correspondence to Li Wenya, E-mail: liwy@nwpu. edu. cn.Pinless Friction Stir Welding of AA2024-T3Joint and Its Failure Modes *Li Wenya (李文亚)1，Li Jinfeng (李锦锋)1，Zhang Zhihan (张志函)1， Gao Dalu (高大路)1，Wang Weibing (王卫兵)2，Luan Guohong (栾国红)2(1. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China;2. China FSW Center, Beijing 100024, China ) © Tianjin University and Springer-Verlag Berlin Heidelberg 2014Abstract ：The joining of aluminum alloy sheets with thickness less than 2.0 mm is difficult via conventional friction stir welding owing to the defects in the joint, such as root flaw, keyhole and lazy S. In the present research, a newly designed pinless tool with involute grooves on its shoulder surface was applied to weld 1.5 mm thick AA2024-T3. The effects of the rotating speed and welding speed on the microstructure and mechanical properties of the joints were ana-lyzed. The experimental results showed that the root flaw and keyhole were successfully eliminated. The lazy S was also eliminated under the optimized welding parameters. The maximum tensile strength of the joints was 326 MPa, which is about 74.1% that of the base material. Moreover, all the tensile samples fractured from the retreating side.Two fracture modes were observed during the tensile tests, which are related with the lazy S. Keywords ：pinless friction stir welding; AA2024-T3; mechanical property; fracture modeFriction stir welding (FSW ) is a solid state joining technique, which was developed by TWI in 1991 to join the lightweight materials such as aluminum and magne-sium alloys. Owing to the advantages of energy effi-ciency, environment friendliness and versatility, FSW is considered to be the most significant development in metals joining [1-3]. The process is a solid -state, hot -shear joining process, in which a rotating tool with a shoulder and terminating in a threaded pin moves along the butting surfaces of two rigidly clamped plates placed on a back-ing plate. During the FSW process, the root flaw [4] and lazy S [5] seriously decrease the mechanical properties of the joint [4,6,7], especially the fatigue property. Besides, the keyhole remaining at the end of the weld also de-creases the integrality and properties of the joint. Barlas and Ozsarac [8] investigated the effect of welding parame-ters on the joint properties of AlMg3 alloy. They found that the root flaw was easily formed during the FSW process. The main reasons for the occurrence of the root flaw were insufficient stirring and heat input, which de-pends on low tool rotating speed and unsuitable tool tilt angle. In addition, Zhou et al [4] proved that the fatiguelife of the joint decreased from 120.6 MPa for flaw -free welds to 54.7 MPa for flawed welds at 2×106 cycles. Moreover, Wanjara et al [9] found that with increasing weld pitch, the occurrence of a lazy S defect in the weld nugget of FSWed AA6061 became increasingly pro-nounced, though its impact on the bending performance of the weld was negligible.The pinless tool was firstly proposed and applied to the friction stir spot welding (FSSW ) process by Bakavos and Prangnell [10], which has received increasing attention in recent years. Considering that the formation of the root flaw and lazy S defects is associated with the tool pin, the pinless tool is used to eliminate the above mentioned defects. The pinless FSW process, as shown in Fig. 1, is greatly different from the conventional FSW process because the flow of weld metal depends on the grooves on the tool shoulder. The pinless FSW process is also termed as friction flow welding (FFW )[11]. Forcellese et al [12] researched the effect of welding pa-rameters, tool geometry and size on the mechanical prop-erties of the joint in AZ31 sheets using the pin and pinless tools. They found that the ultimate tensileTransactions of Tianjin University Vol.20 No.6 2014—440—strength and ductility values of the joints made by the pin tool were higher than those of the joints made by the pinless tool when the shoulder diameter was 8 mm, while a strongly beneficial effect on the strength and ductility was obtained by increasing the shoulder diameter from 8 mm to 19 mm. Moreover, they investigated the mechani-cal properties of dissimilar FSWed joints and the plastic flow behavior of the joints during the pinless FSW proc-ess [13,14]. However, these researches are just a beginning. To date the research on the pinless FSW of aluminum alloy sheets has not been reported. In this study, there-fore, 1.5 mm thick AA2024-T3 was pinless friction stir welded. The effects of welding parameters on the mech-anical properties of the joints were investigated in detail.Fig. 1 Schematic diagram of pinless friction stir weldingprocess1 Experimental proceduresAA2024-T3 sheets of size 110 mm × 100 mm × 1. 5 mm were pinless friction stir welded. The welding speed of 20—60 mm/min and the rotating speed of 600—950 r/min were used to generate different welds with different process parameters. The pinless tool made of H13 steel was characterized by a plain shoulder of 10 mm in di-ameter with involute grooves on its shoulder surface, asshown in Fig. 2. The depth of the grooves is 0. 3 mm.(a ) Full view (b ) Shoulder surfaceFig. 2 Schematic diagram of pinless toolThe microstructure was examined by optical micro-scope (OM ). The microhardness profiles were measured throughout the weld zone of the upper surface by a hard-ness testing machine and the testing load is 0. 5 kg hold-ing for 10 s. The room temperature tensile test was per-formed at a cross -head speed of 1 mm/min using a com-puter -controlled testing machine. The tensile propertiesof each joint were evaluated using three tensile samples cut from the same joint. After the tensile test, the fracture features of the samples were analyzed by scanning elec-tron microscope (SEM ).2 Results and discussion2.1 Macro - and micro -morphologyFig. 3 shows the macro -morphology of the top and bottom surfaces of the pinless FSWed joint at the rotating speed of 600 r/min and welding speed of 20 mm/min. It can be seen from Fig. 3 that the sheet was successfully butt FSWed by the pinless tool. The top surface of the weld is smooth, and there seems no root flaw on the bot-tom surface of the weld. Fig. 4 (a ) shows the cross -sectional morphology of the weld. According to the size and morphology of the grains, the weld is divided into four regions: base material (BM ), nugget zone (NZ ), thermo -mechanically affected zone (TMAZ ) and heat affected zone (HAZ ). The high magnifications of these four regions are shown in Fig. 4 (b )—(d ). The FSW process transforms the slightly elongated and larger grains in the BM to dynamically re -crystallized and equi-axed finer grains in the NZ. The TMAZ lies outside the NZ and is distinguishable from the NZ. The grains in the TMAZ are significantly rotated and refined. The plastic deformation of grains in the HAZ is due to the thermal effect during the welding process and the grains are elon-gated. The boundary between the TMAZ and NZ in the AS is clearer than that in the RS due to different flow behaviors of the weld metal during the pinless FSW process. The area of the NZ is larger than that of the con-ventional FSW joint. Besides, the root flaw is fully elimi-nated by the pinless FSW process.(a )Top surface(b ) Bottom surfaceFig. 3 Macro -morphology of pinless FSWed jointLi Wenya et al: Pinless Friction Stir Welding of AA2024-T3 Joint and Its Failure Modes—441—(a ) Full view of pinless FSWed joint (AS: advancing side; RS: retreating side; NZ: nugget zone; BM: base material; TMAZ: thermo-mechanicallyaffected zone; HAZ: heat affected zone )(b ) NZ (c ) BM (d ) Mixed zone in ASFig. 4 Cross -sectional microstructures of the pinless FSWed joint2.2 Lazy S The lazy S defect has seriously decreased the me-chanical properties of the joints, especially the fatigue life. To detect and visualize the possible lazy S defect, the FSWed joints were immersed in high concentration NaOH solution for 2 h. Fig. 5 (a ) and (b ) show the top and bottom surfaces of the eroded joints, respectively. Fig. 5 (c )—(e ) show the cross -sectional morphology of the joints made at different welding parameters. It can be found that the lazy S defect is eliminated at the welding parameters of 600 r/min and 20 mm/min, 750 r/min and 60 mm/min as well as 950 r/min and 40 mm/min, thus the lazy S defect could be successfully avoided at suitablewelding parameters by pinless FSW.(a ) Top surface at 750 r/min (b )Bottom surface at 750 r/min(c ) Cross-sections at 600 r/min (d ) Cross-sections at 750 r/min (e ) Cross-sections at 950 r/minFig. 5 Macro -morphology of eroded joints obtained at different welding parameters2.3 T ensile testsFig. 6 shows the tensile strength of the joints ob-tained at various rotating and welding speeds. It is found that the tensile strength decreases with increasing therotating speed. The possible reason for this is that the heat input increases when the rotating speed increases, which is adverse to the mechanical property of the joint as higher temperature significantly impacts the joint mi-Transactions of Tianjin University Vol.20 No.6 2014—442—crostructure, such as the volume fraction of S phase pre-cipitates and Guinier -Preston -Bagaryatskii zone. The ten-sile strength changes irregularly with the increase of the welding speed. The maximum value of the tensile strength is found to be 326 MPa at 600 r/min and 60 mm/min, which is 74.1% that of the BM. Furthermore, it is found that the lazy S defect seriously reduces the ten-sile strength of the joint. The experimental results also showFig. 6 Tensile strength of the joints made at different welding parametersthat the joints with or without the lazy S defect all frac-ture in the RS. In summary, the lazy S defect is formed in the RS and seriously decreases the tensile strength of the joint. The tensile strength may increase when the lazy S defect is eliminated.2.4 Fracture morphologyFig. 7 shows the cross -sectional morphology of the fracture joints, where two typical modes with and without the lazy S defect are indicated in Fig. 7 (a ) and (b ), re-spectively. The fracture path of the joint is presented by the dotted line. It can be found that the joint with the lazy S defect fractured as an irregular S shape from the posi-tion of the lazy S defect, while the joint without the de-fect fractured from the position at a tilt angle of about 45°. The results show that the retreating side of the pinless FSWed joint is the weak connection part becauseall the joints fractured at this side.(a ) With lazy S defect (b ) Without lazy S defectFig. 7 Representative cross -sections of fracture jointsAn examination of the tensile fracture surfaces does provide useful information pertaining to the role and/or contribution of the intrinsic microstructure features to the strength and ductility of pinless FSW AA2024-T3 joints. Representative fracture surface features of the joints with and without the lazy S defect are shown in Fig. 8. Fig. 8 (a ) indicates the fracture surface of the failure joint with the lazy S defect produced at 750 r/min and 40 mm/min, where the typical regions of B and C are highly magni-fied and shown in Fig. 8 (b ) and (c ), respectively. Fig. 8 (b ) presents a little river pattern covered with a few shal-low dimples and tearing edges. A distinct layered ledges(a ) With lazy S at 750 r/min and 40 mm/min (b )Magnified Region B in (a ) (c )Magnified Region C in (a )(d ) Without lazy S at 750 r/min and 60 mm/min (e ) Magnified Region E in (d )Fig. 8 Representative fracture surface features of failure jointsLi Wenya et al: Pinless Friction Stir Welding of AA2024-T3 Joint and Its Failure Modes—443—and groove -like features are also observed in Fig. 8 (b ) and (c ). These facts indicate that the failure has mixed fracture characteristics of quasi -cleavage and dimples. Moreover, it can be observed from Fig. 8 (d ) and (e ) that the fracture surface of the joint without the lazy S defect produced at 750 r/min and 60 mm/min is covered with dimples of varying sizes and shapes. Tearing ridges are also observed on the fracture surface. This indicates that the joint without the lazy S defect has more excellent ductility than that with the lazy S. 3 ConclusionsIn the present work, a series of AA2024-T3 welds were made by pinless FSW at various welding parame-ters. The microstructure and tensile property of the joints were compared and discussed, which provides the fol-lowing highlights.(1) The root flaw and keyhole are fully eliminated during the pinless FSW process. The lazy S defect is eliminated at the welding parameters of 600 r/min and 20 mm/min, 750 r/min and 60 mm/min as well as 950 r/min and 40 mm/min.(2) The tensile strength decreases with increasing the rotating speed, but the tensile strength changes irregu-larly with the increase of the welding speed. Besides, the lazy S defect seriously decreases the tensile strength of the joint.(3) The failure of the joint with the lazy S defect has mixed fracture characteristics of quasi -cleavage and dimples. 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