Friction Stir Lap Welding Thin Aluminum Alloy Sheets

High Temperature Materials and Processes 2020; 39: 663–670

Research Article

Tao Wang#, Xue Gong#, Shude Ji*, Gang Xue*, and Zan Lv Friction stir lap welding thin aluminum alloy sheets

https://doi.org/10.1515/htmp-2020-0024 received September 24, 2019; accepted December 3, 2019 1 Introduction

Abstract: In this work, thin aluminum alloy sheets with As a solid-state joining method, friction stir welding thickness of 0.8 mm were friction stir lap welded using (FSW) was invented in 1991 [1–3]. The peak temperature small shoulder plunge depths of 0 and 0.1 mm. The joint during FSW is always lower than the melting point of the formation, microstructure and mechanical properties were base metal (BM)[4,5]. Thus, FSW serves as a promising investigated. Results show that voids appear inside the method, avoiding fusion defects [6–9]. Al alloys are stir zone when the small plunge depth of 0 mm is used widely used in modern industries because of their advan- because the tool shoulder cannot exert a good material- tages of low densities, high strengths, good corrosion collecting effect at such low plunge depth. A plunge depth resistances and fatigue properties [10,11]. FSW has been of 0.1 mm causes tight contact between the shoulder and widely used to join various kinds of Al alloys and the the material and thus results in good material-collecting microstructure and mechanical properties of the welded effect, which is helpful to eliminate the void. Sound joints joints have been studied [12–17]. are attained at a wide range of welding parameters when In actual engineering applications, Al alloys of var- using the shoulder plunge depth of 0.1 mm. No crack is ious thicknesses are required [18]. Researchers studied observed inside the bonding ligament. The joints own higher the joining of the alloys [19–21]. In case of thick sheet ( ) failure loads when the retreating side RS of the joint bares FSW joint, the joint consists of various microstructure the main load during the lap shear tests. The shear failure and mechanical properties [19–23]. Martinez et al. [21] fi load rst increases and then decreases with increasing the studied the microstructure and mechanical properties of rotating and welding speeds, and the maximum failure load thick 7449 Al alloy FSW joint and found that the joint - of6419Nisobtainedat600rpmand150mm/min.Thehard bottom had higher hardness compared to that of the ness of the joint presents a “W” morphology and the BM. The heat gradient along the thickness resulted in minimum hardness is obtained at the heat affected zone. different microstructures in the joint. Upadhyay and The joints present tensile fracture and shear fracture when Reynolds [22] investigated the effect of the backing plates the advancing side and RS bare the main loads, respectively. on the microstructure and mechanical properties of a Keywords: friction stir lap welding, thin sheets, secondary 25.4-mm-thick AA6061 FSW joint and reported that the phase, bonding ligament, lap shear failure load back plates had significant effect on temperature at the joint root. Buchibabu et al. [23] welded a thick Al–Zn–Mg  alloy and investigated the microstructure and mechanical # Xue Gong and Tao Wang contributed equally to this work properties of the joints. They reported that the optimum mechanical properties were achieved with a low rotating  speed of 350 rpm. * Corresponding author: Shude Ji, College of Aerospace For thin Al alloys sheets, low heat input is always Engineering, Shenyang Aerospace University, Shenyang 110136, People’s Republic of China, e-mail: [email protected] needed during welding. However, joining thin sheets * Corresponding author: Gang Xue, Hull Structure Steel and Process has more problems such as thickness reduction and sheet Laboratory, LuoYang Ship Material Research Institute, Luo Yang warping. Therefore, joining thin Al alloy sheets also 471023, People’s Republic of China, e-mail: [email protected] attracted a plenty of attention [24–27]. Ahmed and Saha Tao Wang: Hull Structure Steel and Process Laboratory, LuoYang [24] developed a new fixture for FSW of thin Al alloy ’ Ship Material Research Institute, Luo Yang 471023, People s [ ] - - Republic of China sheets. Huang et al. 25 used two tools to join 0.5 mm Xue Gong, Zan Lv: College of Aerospace Engineering, Shenyang thick 6061 Al alloy and reported that thickness reduction Aerospace University, Shenyang 110136, People’s Republic of China of joints was lower than 2% under rotational velocities

Open Access. © 2020 Tao Wang et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 664  Tao Wang et al. higher than 1,500 rpm. For FSW, another joint type was The metallographic samples and tensile specimens lap joint. Friction stir lap welding (FSLW) is formed by were cut by an electrical discharge cutting machine. two or more overlapped sheets. Similarly, lap joint is The metallographic samples underwent a standard polish formed at solid state and therefore owns high properties. procedure and were observed on an optical microscope To the author’s knowledge, no study has focused on (OM; Olympus–GX71, Olympus Corporation) and a scan- lap joint of thin sheets. Therefore, in this work, 2024-T4 ning electron microscope (SEM) equipped with an energy thin Al alloy sheets were lap welded. Different sleeve dispersive X-ray spectrometer after etching using Keller’s plunge depths were used, and the microstructure and reagent. The ISO 25239 was the standard used in this work mechanical properties of the lap joints were studied. for shear testing. The width of the lap shear specimen was 20 mm (Figure 1c). Vickers hardness was measured using an HVS-1000 Vickers hardness tester by a step of 0.5 mm. The testing force of 10 g was applied and the dwell time 2 Experimental was 10 s. Two lines across the joint were tested. The first line was located at the center of the upper sheet and the The 2024-T4 Al alloys of 0.8 mm thick were chosen as the second line at the lower sheet, and its distance from the BM. The dimensions of the BMs were 300 × 50 mm. Before bonding ligament was 0.2 mm. Lap shear tests were per- welding, the sheets were cleaned using 500 # emery formed on an Instron 8801 testing machine at a speed of papers to wipe off the oxide films. FSW-3LM-4012 machine 3 mm/min under room temperature. After the lap shear was used. The dimensions of the tool are shown in Figure 1a. tests, the fracture morphologies were observed using SEM. The diameter of the shoulder was 13.5 mm. The root and tip diameters of the pin were 6 and 5 mm, respectively. The length of the pin was 0.7 mm. Two sheets were lap combined at a width of 70 mm (Figure 1b). Rotating 3 Result and discussion speeds of 400, 600, 800 and 1,000 rpm and welding speed of 50, 100, 150 and 200 mm/min were used. The Figure 2 shows the joint cross section using the plunge titling angle was 2.5°. depth of 0 mm. The shoulder surface slightly contacted

Figure 1: Tool used in experiment (a) and schematic of the welding (b) and lap shear specimen (c). Friction stir lap welding thin aluminum alloy sheets  665

(TMAZ), and SZ. Figure 4 shows the joint cross sections at different welding speeds. The cross sections presented little difference with increase in welding speed. Heat input was reduced at high welding speed, leading to narrower HAZ and TMAZ. At 150 and 200 mm/min, the upper sheets showed a little distortion (Figure 4b and c). Sheet warping is a common problem especially when welding thin sheets [31]. The warping problem was more serious at low heat input. We speculate warping is connected with the forward tool movement. When the heat input is low, the material showed bad plasticity, providing large resistance for tool movement, Figure 2: Cross section of the joint using shoulder plunge depth of easily causing sheet warping. ( ) ( ) ( ) 0mm a and the voids b and c . The results of Figures 2–4 show that sound joints can be obtained when a plunge depth of 0.1 mm is used. The the sheet during welding, resulting in weak friction joint formation of FSW is closely related to the material between the shoulder and the sheet. Rather weak mate- flow behavior during welding. Thus, the schematic of the rial flow behavior was induced. Only very small flash was material flow behavior in this work is shown in Figure 5. observed. Besides, the heat input under this condition Figure 5a shows the material flow behavior using the was not enough to guarantee sufficient material flow plunge depth of 0.1 mm. During welding, the plastic ma- behavior. Thus, voids were formed in the stir zone (SZ; terial of the BM flows under the stirring action of the tool. Figure 2b and c). A small part of the material flows upward due to the Figure 3 shows the joint cross sections welded at plunge of the tool shoulder, forming flash. A large part 0.1 mm plunge depth and at different rotating speeds. of the material flows toward the joint center due to the The cross section presented a typical basin-like mor- collection effect of the shoulder, which is marked using phology similar to other typical FSW joints [28,29]. red arrows in Figure 5a. At the same time, the material No defects were observed, illustrating that sound joints flows along the direction of tool rotation and also flows could be attained at a relatively board range. Relatively downward along the threads during welding [32,33]. rough joint surface was obtained at 400 rpm (Figure 3a). When the material collection effect is strong enough at The upper sheet bent upward at the advancing side (AS); the shoulder, plastic material at the AS of the joint is Thus, resulting in sound surface formation (Figure 3b and enough not to cause void, as shown in Figure 5a. How- c) at increased rotating speed. The alclad layers at the ever, when the plunge depth of 0 mm is used, the mate- BM surfaces were not broken. A bonding ligament was rial collection effect of the shoulder goes down to 0, observed at the SZ. The bonding ligament was of weak which is not enough to compensate the material loss strength [30]. The sizes of the hook and cold lap were caused by the horizontal flow due to tool rotation and very small on the joints. The SZ widths at the bonding the downward flow caused by the thread. Therefore, ligament were, respectively, 5.2, 5.4 and 5.8 mm as shown void appears at the AS of the joint, as shown in Figure 5b. in Figure 3. The cross section was divided intoBM, heat The result in Figure 5 shows that sufficient tool plunge affected zone (HAZ),thermal–mechanically affected zone depth should be guaranteed to avoid the void.

Figure 3: Cross section of the joints using shoulder plunge depth of Figure 4: Cross section of the joints using shoulder plunge depth of 0.1 mm at (a) 400 rpm, (b) 600 rpm and (c) 800 rpm. 0.1 mm at: (a) 100 mm/min, (b) 150 mm/min and (c) 200 mm/min. 666  Tao Wang et al.

(a) Tool rotating because of both the stirring and the heat input during Small flash Small flash welding. Due to complete dynamic recrystallization, ﬁne

rotating and small grains were observed in SZ. Similarly, the size of Material flow along tool Material collection Material collection the secondary phase particles was small (Figure 6d). by shoulder by shoulder

Dow D ownwa Figure 7 shows the element distributions at the SZ.

r d ﬀ -

rd Some secondary phases with di erent sizes were ob

f flow l ow served in the SZ (Figure 7a). Al 2024 alloy possesses a precipitation strengthening property. The main secondary (b) Tool rotating phase is Al2Cu. The element distribution is shown in Small flash Material loss Small flash Figure 7b. Rather high Cu content was observed at the otating Weak material Material f low along to ol r Weak material phases, which corresponded well with the distribution of

collection by shoulder collection by shoulder Do Al and Cu elements in Figure 7c and b. Do

wnward w n

war The microstructure of the bonding ligament is shown d

f fl lo o in Figure 8. No crack was observed inside the bonding

w w ligament (Figure 8a). As introduced above, the pin used

Figure 5: Schematic of the material flow using 0.1 mm (a) and 0 mm (b)in this work had a length of 0.7 mm and the shoulder shoulder plunge depths. plunge depth was 0.1 mm. Therefore, the pin tip touched the lap interface during welding. The alclad layers at both the upper and lower sheets were stirred. Figure 8b ff - Figure 6 shows the microstructures at di erent re and c show the element distribution of the bonding gions of the joint. Figure 6a shows the microstructure ligament. Much high Al alloy content was observed be- of the BM. Some big grains with irregular sizes were cause the bonding ligament is composed of pure Al. observed. Inside the grains and at the grain boundaries, Little Cu element was observed at the bonding ligament some secondary phases were observed. The secondary (Figure 8c). - phases appeared black under OM. Under SEM, the second Figure 9 shows the microstructure of the SZs welded ary phases appeared white. The microstructure of HAZ is at different welding speeds. As shown in Figure 6, the SZ shown in Figure 6b. The grains were a little larger than underwent complete dynamic recrystallization, so fine those of the BM. Quantity of the secondary phase was and exquisite grains were observed. The size of the grains smaller than that of the BM, as shown in the OM and was sensitive to heat input. Higher heat input was pro- SEM images. This was because the HAZ only underwent duced at 50 mm/min and, therefore, the grains were large heat input but not mechanical stirring during welding. (Figure 9a). With increase in welding speed, the heat Grains with irregular sizes were observed at TMAZ input decreased, i.e., the grains had short time to grow. (Figure 6c). The highly deformed grains in TMAZ were

Figure 7: Secondary phases: (a) SEM image, (b) element distribu- Figure 6: Microstructures at (a) BM, (b) HAZ, (c) TMAZ and (d) SZ. tion, (c) Al and (d) Cu. Friction stir lap welding thin aluminum alloy sheets  667

a typical “W” morphology at the upper SZ. The BM of 2024-T4 Al alloy had a hardness of approximately 140.3 HV. The hardness showed an obvious decrease at HAZ and TMAZ. The minimum hardness of approximately 117.6 HV was obtained at HAZ using 800 rpm, which was attributed to large grain size and less amount of secondary phases. The hardness showed an increase (133.1–135.6 HV) at SZ because of the small grains and the precipitated secondary phases. Al 2024-T4 alloy was one precipitation-strengthened Al alloy. The hardness of SZ was affected by both grain sizes and secondary phases. Gen- erally speaking, more second phases with even distribution result in high strength, such as the BM. On the contrary, less second phases with uneven distribution result in low strength, such as the HAZ and TMAZ. The Figure 8: Morphology of bonding ligament (a), element distribution of Al (b) and Cu (c). heat input at 800 rpm was higher than that at 400 rpm. Thus, the grains were larger, and more secondary phases were dissolved at 800 rpm, resulting in lower hardness. Therefore, the size of the grains became smaller (Figure 9b The hardness of the lower SZ is shown in Figure 10b. The and c). Inside the SZ, some black lines were observed lower SZs had obvious less widths. It was seen that the (Figure 9a and b). Figure 9d shows its SEM image. The hardness was similar at the upper and lower SZs and line scan result in Figure 9d shows that the black lines higher rotating speed resulted in lower hardness. were rich in Cu. Figure 9e and f show the element dis- Figure 11 shows the lap shear failure loads of the tributions of this area. The results show that the white joints at different rotating speeds. Figure 11a shows the spots were the secondary phases that precipitated inside failure loads of the joints when the AS of the joint bore the SZ. the main load. The failure load first increased at 600 rpm Figure 10 shows the hardness curves of the joints at and then decreased at 800 rpm. The maximum failure different rotating speeds. The hardness curve presented load of 2,330 N was obtained at the rotating speed of

Figure 9: Microstructure of the SZ using (a) 50 mm/min, (b) 100 mm/min and (c) 150 mm/min, (d) SEM image, (e) element distribution and (f) Cu. 668  Tao Wang et al.

Figure 10: Hardness of the joints using diﬀerent rotating speeds: (a) the upper SZ and (b) the lower SZ.

Figure 11: Lap shear properties: (a) joint AS bears the forces and (b) joint RS bears the forces.

600 rpm. The minimum failure load of 1,855 N was surface of the joint (Figure 12a). Shear fracture mode obtained at 400 rpm. The load-displacement curve was obtained when the joint RS bore the lap shear force. showed that the joint made at 600 rpm possessed bigger Crack initiated at the cold lap and then propagated along displacement. Figure 11b shows the failure loads of the the bonding ligament, finally reaching the hook at the joints when the retreating side (RS) of the joint bore the AS. The upper and lower sheets were separated from main load. The failure loads were much higher than those each other (Figure 12b). Figure 11 shows that the joints when the AS of the joints bore the load. This was attrib- have higher failure loads when the joint RS bore the lap uted to different fracture loads, which was discussed shear force. This attributed to larger bonded width of the in the following section. The maximum failure load of bonding ligament. For the tensile fracture mode, the 6,419 N was obtained at 600 rpm. Similarly, the joint load-bearing distance was the thickness of the upper using 600 rpm had bigger displacement, as shown in SZ, which has a value smaller than 0.8 mm because the Figure 11b. shoulder had a plunge depth of 0.1 mm. For shear frac- Figure 12 shows joint fracture positions and morphol- ture mode, the bonded width of the bonding ligament ogies. Two fracture modes were obtained. Tensile fracture was larger than the diameter of the pin tip (5mm). mode was obtained when joint AS bore the lap shear Figure 12c shows the fracture surface of the tensile frac- force. Crack first initiated at the hook at AS and then ture. Some ridges were observed, whose magnified views propagated through the SZ, finally reaching the upper are shown in Figure 12d and e. Numerous dimples with Friction stir lap welding thin aluminum alloy sheets  669

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