Effect of rotary friction welding on mechanical properties of 6060 Al alloy

1. INTRODUCTION

Aluminum (Al) alloys are widely used as structural materials for their good mechanical and metallurgical properties, excellent corrosion resistance and significant low density (i.e., good for light weight applications). Furthermore, the mechanical properties of Al alloys can be improved with work hardening or heat treatment. However, the high heat input can cause a decline in the mechanical strength of these alloys when they are joined with fusion welding processes (Kou, 2020Kou, S. (2020). Welding Metallurgy. 3rd edition, Wiley and Sons, Hoboken, NJ.). In addition, Al alloys are sensitive to hot cracking when fusion welding is used (Soysal, 2020Soysal, T. (2020). A criterion to find crack-resistant aluminum alloys to avoid solidification cracking. Sci. Technol. Weld. Joi. 26 (2), 99-105. https://doi.org/10.1080/13621718.2020.1843260.; Soysal, 2021Soysal, T. (2021). A new solidification cracking test: stationary weld-pool deformation test. Sci. Technol. Weld. Join. 26 (8), 622-630. https://doi.org/10.1080/13621718.2021.1985369.) due to the solidification shrinkage (liquid density > solid density) and the thermal contractions. Thus, usually a proper filler alloy is needed to change the chemical content of the weld to avoid the cracking issue (Coniglio and Cross, 2009Coniglio, N., Cross, C.E. (2009). Mechanisms for solidification crack initiation and growth in aluminum welding. Metall. Mater. Trans. A 40, 2718-2728. https://doi.org/10.1007/s11661-009-9964-4.; Soysal and Kou, 2019Soysal, T., Kou, S. (2019). Effect of filler metals on solidification cracking susceptibility of Al alloys 2024 and 6061. J. Mater. Process. Technol. 266, 421-428. https://doi.org/10.1016/j.jmatprotec.2018.11.022.). A solid-state welding process can be an alternative solution for these problems because the joining occurs under the melting temperatures of the materials, and it can offer a lower heat input. For example, the rotary friction welding (RFW), as a solid-state welding process, can be used for various materials, bring smaller heat affected zones compared to the fusion welding processes, and save materials and energy (Dey et al., 2009Dey, H.C., Ashfaq, M., Bhaduri, A.K., Prasad Rao, K. (2009). Joining of titanium to 304L stainless steel by friction welding. J. Mater. Process. Technol. 209 (18-19), 5862-5870. https://doi.org/10.1016/j.jmatprotec.2009.06.018.; Teker and Karakurt, 2020Teker, T., Karakurt, E.M. (2020). Examination of mechanical properties of high chromium white cast iron/AISI1030 steel welded by friction welding with nickel interlayer. Sci. Technol. Weld. Joi. 25 (2), 150-156. https://doi.org/10.1080/13621718.2019.1648720.).

RFW is performed rotating one of the two workpiece relative to the other stationary one and applying a compressive axial friction pressure. The friction between the two workpiece produces heat and causes the interfacial portions of the material to plasticize. The plasticized material is displaced by the axially applied pressure from the interface, and this knocks the interface oxide layers and other contaminants to allow joining. As a result of this deformation process, a flash collar is formed, and the axial length of both workpiece is shortened. When the desired shortening (burn-off distance) is managed, the rotation is stopped, and a forging pressure is used for a period of time to assist consolidation of the joint (Teker, 2013Teker, T. (2013). Evaluation of the metallurgical and mechanical properties of friction-welded joints of dissimilar metal combinations AISI2205/Cu. Int. J. Adv. Manuf. Technol. 66 (1-4), 303-310. https://doi.org/10.1007/s00170-012-4325-7.).

Welding conditions and welding parameters of RFW have still been investigated for aluminum alloys to obtain sound welds. Li et al. (2018)Li, X., Li, J., Jin, F., Xiong, J., Zhang, F. (2018). Effect of rotation speed on friction behavior of rotary friction welding of AA6061-T6 aluminum alloy. Weld. World 62, 923-930. https://doi.org/10.1007/s40194-018-0601-y. has observed the friction behavior of RFW while welding 6061-T6 Al alloy by changing the rotation speed from 500 to 2100 rpm and indicated that an unbounded area appears in the weld after exceeding the rotation speed of 1500 rpm. Etesami et al. (2015)Etesami, S.A., Enayati, M.H., Karimzadeh, F., Rasta, V. (2015). Investigating the properties of friction welded 2014 aluminum joints prepared with different rotational speeds. Trans. Indian Inst. Met. 68 (3), 479-489. https://doi.org/10.1007/s12666-014-0475-7. has also studied the friction welded 2014 Al samples with various rotation speeds, and they showed that increasing the rotation speed decreased the weld defects and increased the hardness and the strength of the material. Kimura et al. (2006)Kimura, M., Choji, M., Kusaka, M., Seo, K., Fuji, A. (2006). Effect of friction welding conditions on mechanical properties of A50502 aluminum alloy friction welded joint. Sci. Technol. Weld. Joi. 11 (2), 209-215. https://doi.org/10.1179/174329306X89242. used friction welding process and studied the influence of the welding conditions on the mechanical properties of 5052 Al alloy to increase the joint efficiency by keeping the rotation speed the same, and changing the friction pressure and friction time. The effect of rotation speeds have also been studied for joining dissimilar metals by friction welding, and the speed range of 1400 to 1600 rpm or 1700 rpm were reported as proper speeds to have sound welds (Sun et al., 2018Sun, Y., He, D., Xue, F., Lai, R. (2018). Effect of tool rotation speeds on the microstructure and mechanical properties of a dissimilar friction-stir-welded CuCrZr/CuNiCrSi butt joint. Metals 8 (7), 526-541. https://doi.org/10.3390/met8070526.; Teker et al., 2018Teker, T., Yılmaz, S.O., Karakurt, E.M. (2018). Effect of different rotational speed on mechanical and metallurgical properties of friction welded dissimilar steels. Mater. Test. 60 (2), 135-141. https://doi.org/10.3139/120.111135.). Furthermore, the effect of rotation speeds on the microstructure (deformed zones) of the joint of alumina/6061 Al alloy was investigated, and it was reported that increasing the rotation speed increases the size of fully plasticized deformed zone (Ahmad Fauzi et al., 2010Ahmad Fauzi, M.N., Uday, M.B., Zuhailawati, H., Ismail, A.B. (2010). Microstructure and mechanical properties of alumina-60601 aluminum alloy joined by friction welding. Mater. Desing 31 (2), 670-676. https://doi.org/10.1016/j.matdes.2009.08.019.).

Al alloys welded with friction welding are expected to show good mechanical properties (Kimura et al., 2005Kimura, M., Choji, M., Kusaka, M., Seo, K., Fuji, A. (2005). Effect of friction welding conditions and aging treatment on mechanical properties of A7075-T6 aluminium alloy friction joints. Sci. Technol. Weld. Joi. 10 (4), 406-412. https://doi.org/10.1179/174329305X44125.; Threadgill et al., 2009Threadgill, P.L., Leonard, A.J., Shercliff, H.R., Withers, P.J. (2009). Friction stir welding of aluminium alloys. Int. Mater. Rev. 54 (2), 49-93. https://doi.org/10.1179/174328009X411136.; Dursun and Soutis, 2014Dursun, T., Soutis, C. (2014). Recent developments in advanced aircraft aluminum alloys. Mater. Desing 56, 862-871. https://doi.org/10.1016/j.matdes.2013.12.002.; Fu et al., 2015Fu, B., Qin, G., Li, F., Meng, X., Zhang, J., Wu, C. (2015). Friction stir welding process of dissimilar metals of 6061-T6 aluminum alloy to AZ31B magnesium alloy. J. Mater. Process. Technol. 218, 38-47. https://doi.org/10.1016/j.jmatprotec.2014.11.039.) but the optimum welding parameters should be used to obtain sound welds with superior mechanical properties. The mechanical properties of 6061 Al alloy are improved with the heat treatment of T5 which is artificially aging process applied (at a temperature between 150 and 300 ºC) on the material cooled from the elevated extrusion temperatures (without solution treatment) (ASM Handbook, 1981ASM Handbook (1981). Heat treating of aluminum alloys. 9th Ed., Vol. 4, American Society for Metals, Metals Park, OH, pp. 841-879.). In this study, 6060 Al alloy was selected and welded by RFW, and mechanical tests have been conducted on the welds made with various rotation speeds to study the mechanical properties of the alloy.

2. MATERIALS AND METHODS

Pairs of 6060-T5 Al cylindrical rods with the size of 12 mm diameter and 75 mm length were joined with the RFW using three different rotation speeds: 1500, 1600 and 1700 rpm. The welded rods were grouped as S1, S2 and S3 regarding the rotation speeds. The process parameters used to weld these rods are shown on Table 1 with respect to the groups. The chemistry of the 6060-T5 Al alloy is given on Table 2. Mechanical properties of the 6060-T5 Al materials are shown in Table 3 (provided by Metalreyonu.com), Metalreyonu (2021)Metalreyonu (2021). 6060 Al Mechanical Properties. available at: https://www.metalreyonu.com.tr/icerik/6060. .

The welded rods were cut perpendicular to the joint line to study the weld zones. First, the samples were rough grinded and polished (Al₂O₃particles of 0.25 µm used for final polishing step), and then etched using the Keller’s etching solution (2.5 mL HNO₃, 1.5 mL HCl, 1 mL HF and 95 mL distilled water) for 3-5 s to be able clearly observe the joint. After that, the samples were observed using optical microscopy (OM: LEICA DM750) and scanning electron microscopy (SEM: ZEISS EVO LS10). They were analyzed with energy dispersive x-ray spectroscopy (EDS) using point analysis and elemental mapping. Vickers micro-hardness measurements were taken under 100 g loading for 5 s by the ONESS Q10 test device. Tensile test specimens were machined in a way that the joint line located at the middle of the specimens. The tensile test specimens were prepared with respect to ASTM E8-E8M (2013)ASTM E8/E8M (2013). Standard Test Methods for Tension Testing of Metallic Materials. ASTM International, West Conshohocken, Pensilvania, EE.UU. standards. Figure 1 shows the front view of the round tensile specimen with the dimensions. The tensile tests were performed with the drawing rate of 0.5 mm·min^-1 using the UTEST UTM-8050 test device with the load capacity of 50 kN. After the tensile tests, the fracture surfaces were examined under SEM.

3. RESULTS AND DISCUSSION

3.1. Macrograph of the welds

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The surface and cross-section macrographs of the welds made with different rotational speeds are shown in Fig. 2 (a-b). As it can be seen from the figure, more flash produced in the sample S3 compared to the other two samples due to the use of a higher rotational speed. The bonding between the two welded 6060 Al rods seemed better in the sample S3 because the shape of the flash was rounder than the other two. These can be attributed to more heat generated during the welding process of the sample S3. The heat caused the material around the interface to be viscous and move out when the forging pressure was applied at the terminal stage of the RFW (Etesami et al., 2015Etesami, S.A., Enayati, M.H., Karimzadeh, F., Rasta, V. (2015). Investigating the properties of friction welded 2014 aluminum joints prepared with different rotational speeds. Trans. Indian Inst. Met. 68 (3), 479-489. https://doi.org/10.1007/s12666-014-0475-7.). On the other hand, the heat input in the sample S1 was the lowest among all. This caused the formation of the flash zone with the conical shape because the forging pressure on the rods resulted in a visible plastic deformation and hence indicated a weak bonding between them. The sample S2 indicated a transition between these two samples. None of these samples had cracks at the joints, unlike the reported cracks in the fusion welding (Soysal and Kou, 2019Soysal, T., Kou, S. (2019). Effect of filler metals on solidification cracking susceptibility of Al alloys 2024 and 6061. J. Mater. Process. Technol. 266, 421-428. https://doi.org/10.1016/j.jmatprotec.2018.11.022.). After welding, the amounts of shortening (burn-off) in the samples were detected as S1= 3, S2= 5 and S3= 7 mm.

Figure 2.  Macrograph of the weld joints.

3.2. Mechanical tests

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The micro-hardness measurements undertaken in the transverse cross-sections of the samples with error bars are given on Fig. 3. All the samples had a similar trend. The hardness increases from the base metal to the joint line, as Fig. 3 shows. Around the joint line, although it was difficult to separate the data points from each other in the figure, it was observed that the sample S3 had slightly higher hardness values. On the other hand, the hardness values of the samples S1 and S2 seemed greater than that of the sample S3 in the heat affected zone. The high hardness values of these two samples could be related to the plastic deformation and the heat input generated during welding. Perhaps, when the heat input was low, the plastic deformation generated with forging action could cause the hardening of the samples in the heat affected zones. Although at some points there were some fluctuations in the hardness values as it ca be observed in the graph, the hardness measurements overall show that there was no visible softening from the weld interface to the base metal.

Figure 3.  Micro-hardness measurements of the samples.

Figure 4 shows the tensile test curves and the bar chart (inset) represents the ultimate tensile strength (UTS) of the samples. The tensile test curves of the samples S1 and S2 failed after exceeding the linear elastic region. However, the sample S3 had a typical ductile tensile test curve of 6xxx series of aluminum alloys (Jha et al., 2015Jha, S.K., Balakumar, D., Paluchamy, R. (2015). Experimental analysis of mechanical properties on AA 6060 and 6061 aluminum alloys. Int. J. Eng. Res. Appl. 5 (4), 47-53.). As can be seen from the bar chart, the UTS values of the samples S1, S2 and S3 were respectively found as 190.5, 197.7 and 217.7 MPa. Both the UTS and the tensile test curves of the samples showed that the mechanical properties of the sample S3 were much better than those obtained for the other two samples. When a comparison was made among the UTS of all the welds and the UTS of the base metal (215 MPa from Table 3), it could be seen that the UTS values of the samples S1 and S2 were under the UTS of the base metal, but the UTS of the sample S3 was slightly higher. The tested tensile test specimens are shown in Fig. 5. While the samples S1 and S2 showed brittle fracture characteristics (no obvious necking), the sample S3 showed ductile fracture characteristics by having a conical fracture geometry. Although the samples S1 and S2 fractured at the middle (most probably at the weld interface), the sample S3 fractured a bit far away from the middle. Figure 6, SEM fracture surface images of the sample S3. As it can be seen from the figure, the surface morphology of the fracture had dimples, which are typical of ductile fractures.

Figure 4.  Tensile test results of the samples. The inset bar plot provides the value of the ultimate tensile strength (UTS) measured for each sample.

Figure 5.  Macrograph of the samples after tensile testing.

Figure 6.  SEM fracture surface images of the sample S3.

3.3. Characterization of microstructure

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The OM images of S1 and S2 samples are displayed in Fig. 7 (a-c) and Fig. 8 (a-c). SEM images of the sample S3 are given on Fig. 9 (a-c). Figures respectively show the left side, the center, and the right side of the weld zone. Unconnected zones (Li et al., 2018Li, X., Li, J., Jin, F., Xiong, J., Zhang, F. (2018). Effect of rotation speed on friction behavior of rotary friction welding of AA6061-T6 aluminum alloy. Weld. World 62, 923-930. https://doi.org/10.1007/s40194-018-0601-y.) cracks or other weld defects were not seen in these images. The joint line or weld interface has been indicated on Fig. 9. The typical zones for the rotary friction welding (Ahmad Fauzi et al., 2010Ahmad Fauzi, M.N., Uday, M.B., Zuhailawati, H., Ismail, A.B. (2010). Microstructure and mechanical properties of alumina-60601 aluminum alloy joined by friction welding. Mater. Desing 31 (2), 670-676. https://doi.org/10.1016/j.matdes.2009.08.019.; Teker, 2013Teker, T. (2013). Evaluation of the metallurgical and mechanical properties of friction-welded joints of dissimilar metal combinations AISI2205/Cu. Int. J. Adv. Manuf. Technol. 66 (1-4), 303-310. https://doi.org/10.1007/s00170-012-4325-7.) were identified and showed in the figure. The fully plasticized deformed zone (FPDZ), the deformed zone (DZ) and the partially deformed zone (PDZ) were formed as a result of friction and forging pressures and the heat generated during welding. From the comparison of Figs. 8 to 9, it can be suggested that the increase in the rotation speed resulted in the widening of the zones: FPDZ, DZ and PDZ.

Figure 7.  OM images of the sample S1 (FPDZ: Fully plasticized deformed zone, DZ: Deformed zone and PDZ: Partially deformed zone)

Figure 8.  OM images of the sample S2 (FPDZ: Fully plasticized deformed zone, DZ: Deformed zone and PDZ: Partially deformed zone).

Figure 9.  SEM images of the sample S3: a) the weld zone; b) left side of weld zone; c) right side of the weld zone (FPDZ: Fully plasticized deformed zone, DZ: Deformed zone and PDZ: Partially deformed zone).

EDS point analyses were done on the sample S3 at three different locations, as shown in Fig. 10. The local composition measurement at the middle corresponds to the weld interface. While the measurement on the left side was taken from the partially deformed zone, the one on the right side was taken from the deformed zone. The amount of Al, Si and Mg which are the main alloying elements of this alloy did not fluctuate much in all the three measurements. This indicated that the joining process did not cause any major composition gradients in the weld zone, unlike the fusion welding.

Figure 10.  EDS analysis of the sample S3.

Elemental EDS mapping of the weld zone of the sample S3 is given on Fig. 11. Overall elemental mapping and the elemental mapping for each element of the weld zone are demonstrated in the figure. The alloying elements in the aluminum matrix were homogenously distributed all around in the weld zone. Unlike the welds made with fusion welding, it seemed that there was no clear macro-segregation in the weld zone. Therefore, the sample 3 carries the desired characteristics of the microscopic examination.

Figure 11.  Elemental mapping of the weld zone of the sample S3.

This study clearly indicated that the rotational speed of 1700 rpm was the ideal speed (among all the conditions investigated) to weld 6060 Al alloy with the rotary friction welding. Since this speed was at the upper border of the recommended speed range of the friction welding (Sun et al., 2018Sun, Y., He, D., Xue, F., Lai, R. (2018). Effect of tool rotation speeds on the microstructure and mechanical properties of a dissimilar friction-stir-welded CuCrZr/CuNiCrSi butt joint. Metals 8 (7), 526-541. https://doi.org/10.3390/met8070526.; Teker et al., 2018Teker, T., Yılmaz, S.O., Karakurt, E.M. (2018). Effect of different rotational speed on mechanical and metallurgical properties of friction welded dissimilar steels. Mater. Test. 60 (2), 135-141. https://doi.org/10.3139/120.111135.), higher speeds were not studied not to have unbounded areas in the weld zone (Li et al., 2018Li, X., Li, J., Jin, F., Xiong, J., Zhang, F. (2018). Effect of rotation speed on friction behavior of rotary friction welding of AA6061-T6 aluminum alloy. Weld. World 62, 923-930. https://doi.org/10.1007/s40194-018-0601-y.). The mechanical tests used in this study showed that the rotary friction welding of 6060 Al alloy with the rotation speed less than 1700 rpm resulted in brittle and hard joints. On the other hand, the mechanical properties of the sample made with the rotation speed of 1700 rpm were found desirable. Furthermore, the UTS of the sample S3 can be comparable to the UTS of the base metal. Thus, the friction welding with the 1700 rpm rotation speed actually did not cause a significant reduction in the strength of the joined portion of the material. None of the welding defects observed in the fusion welding has been seen in the joint, as shown with the microscopy images. Thus, the rotation speed of 1700 rpm can be a good choice to friction weld the 6xxx series of aluminum alloys which are prone to cracking when they are welded by fusion welding without dissimilar filler metals (Soysal and Kou, 2019Soysal, T., Kou, S. (2019). Effect of filler metals on solidification cracking susceptibility of Al alloys 2024 and 6061. J. Mater. Process. Technol. 266, 421-428. https://doi.org/10.1016/j.jmatprotec.2018.11.022.). Since dissimilar filler metals reduce the mechanical properties of the base metal (Othman et al., 2010Othman, N.K., Bakar, S.R.S., Jalar, A., Syarif, J., Ahmad, M.Y. (2010). The effect of filler metals on mechanical properties of 6 mm AA 6061-T6 welded joints. Adv. Mat. Res. 154-155, 873-876. https://doi.org/10.4028/www.scientific.net/amr.154-155.873.), using friction welding with the optimum parameters can offer quality welds and good mechanical properties.

4. CONCLUSION

The aluminum alloy 6060-T5 was welded by rotary friction welding with three different rotation speeds: 1500, 1600 and 1700 rpm. The mechanical properties of the welded samples were studied with hardness and tensile tests. The following conclusions were drawn.