A new welding method: ultrasonic assisted friction stir spot welding (UAFSSW) was put forward in the present study. UAFSSW was successfully applied to weld dissimilar AZ31 Mg alloy and 6061 Al alloy. Results show that for either conventional FSSW or UAFSSW, sound joints are obtained in the configuration of upper Mg alloy and lower Al alloy. Ultrasonic vibration is beneficial to the upward flow of lower aluminum alloy, the increase of the stir zone (SZ) width and the refinement of the grains in the SZ. All cross sections of the Al–Mg joints exhibit the formation of intermetallic compounds (IMC) in the SZ. The crack of the conventional FSSW joint propagates exactly along the interface between the dissimilar materials and presents an inverted “V” morphology. After reaching the highest point of the hook defect, crack of the UAFSSW joint extends to the keyhole, leaving a portion of Mg alloy on the lower sheet. Conventional FSSW joint and UAFSSW joint show different IMC compositions at the faying interface

Gao, S.S.

Ji, S.D.

Li, Z.W.

Ma, L.

Yue, Y.M.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

SCIENTIFIC AND TECHNICALSECTIONUDC 539.4Investigation of Ultrasonic Assisted Friction Stir Spot Welding of MagnesiumAlloy to Aluminum AlloyS. D. Ji,1Z. W. Li, L. Ma, Y. M. Yue, and S. S. GaoFaculty of Aerospace Engineering, Shenyang Aerospace University, Daoyi Development District,Shenyang, China1 superjsd@163.comA new welding method: ultrasonic assisted friction stir spot welding (UAFSSW) was put forward inthe present study. UAFSSW was successfully applied to weld dissimilar AZ31 Mg alloy and 6061 Alalloy. Results show that for either conventional FSSW or UAFSSW, sound joints are obtained in theconfiguration of upper Mg alloy and lower Al alloy. Ultrasonic vibration is beneficial to the upwardflow of lower aluminum alloy, the increase of the stir zone (SZ) width and the refinement of the grainsin the SZ. All cross sections of the Al–Mg joints exhibit the formation of intermetallic compounds(IMC) in the SZ. The crack of the conventional FSSW joint propagates exactly along the interfacebetween the dissimilar materials and presents an inverted “V” morphology. After reaching thehighest point of the hook defect, crack of the UAFSSW joint extends to the keyhole, leaving a portionof Mg alloy on the lower sheet. Conventional FSSW joint and UAFSSW joint show different IMCcompositions at the faying interface.Keywords: ultrasonic assisted friction stir spot welding, intermetallic compounds, magnesiumalloy, aluminum alloy, fracture position.Introduction. In the last few decades, as the resource shortage and environmentprotection problems become severer, weight-saving structures have become a hotspot inindustries of aerospace and transportations. Therefore, light materials are being extensivelyused now [1–3]. As a new variant of friction stir welding (FSW), friction stir spot welding(FSSW) owns advantage of lower energy consumption, smaller distortion, less weldingdefects and higher joint quality compared to resistance spot welding (RSW) [4]. Being asolid state joining technology, FSSW is more suitable to weld light materials such asaluminum alloys and magnesium alloys [5–8].Since the invention of FSSW, many investigations about FSSW joints of Al alloy orMg alloy have been reported [9–15]. Yin et al. [9] performed FSSW on AZ31 aluminumalloy and reported that better failure loads can be attained under conditions of largerbonded width, outward curved hook and smaller hook height. Bozzi et al. [10] found outthat the FSSW joint strength was largely determined by the stir zone (SZ) width and thehook defect. Besides, some investigations about FSSW dissimilar Al and Mg alloys havebeen published. Rao et al. [13] investigated effects of the tool rotating speed on dissimilarAM60B Mg alloy and 6022-T4 Al alloy FSSW joints and studied the intermetalliccompounds (IMCs) in the SZ. Furthermore, Sato et al. [15] studied the interfacialmicrostructure on the lap shear strength of Al alloy to Mg alloy FSSW joint. Lots ofimportant conclusions can be obtained from these studies. However, before successfulapplication of the Al–Mg FSSW joints, plenty of researches need to be done.© S. D. JI, Z. W. LI, L. MA, Y. M. YUE, S. S. GAO, 2016ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 7Ultrasonic assisted FSW (UAFSW) is a new derivative process of FSW. Theultrasonic vibration can increase the heat input during the welding process and decreasesthe welding force [16]. In the last few years some researches about UAFSW or ultrasonicassisted FSSW (UAFSSW) have been done [16–18]. Rostamiyan et al. [16] investigated themicrostructure and mechanical properties of 6061 aluminum alloy conventional FSSW andUAFSSW joints. Liu et al. [17] and Shi et al. [18] reported the material flow behavior ofUAFSW and discovered that the material flow behavior was evidently enhanced duringUAFSW. However, studies about UAFSSW of magnesium to aluminum alloys have notbeen reported yet. Therefore, in the present study, UAFSSW was applied to weld Mg alloyand Al alloy. Effects of ultrasonic vibration on hook defect, SZ width, microstructure of thejoints, distribution of the IMC and fracture paths were studied in detail.Experiment Process. AZ31B magnesium alloy and 6061-T6 aluminum alloy wereused as the base material (BM) in the present study. Dimensions and configuration of thetwo sheets are indicated in Fig. 1. Prior to welding, all sheets were cleaned with sand paperto clean off the oxidation layer. The rotating tool was composed of a concentric shoulderand a tapered pin. The diameter of the shoulder is 11 mm and the pin has a right-handthreaded. Diameters of the pin root and bottom are 5 to 3 mm, respectively. TheFSW-3LM-4012 machine was used in the experiment. The tool rotated in an anti-clockwisedirection and the tilted angle is 0. Ultrasonic vibration was exerted 5 s before the plunge ofthe pin and it was exerted on the lower plate under the SZ, as shown in Fig. 1. Thevibration frequency was 19 kHz. Pin rotating speed and dwell time were 1000 rpm and 5 s.Other parameters such as plunge speed, shoulder plunge depth, and retracting speed are 5mm/min, 0.3 mm, and 10 mm/min, respectively. After the refraction of the pin, theultrasonic vibration lasted for another 5 s.After welding, the metallographic specimens were cut through the center of the jointusing a wire electrical discharge cutting machine. Then the specimens were burnished,polished and etched with Keller’s reagent. Metallographic analysis was carried out using anoptical microscopy (OLYMPUS-G71). Fracture positions of the specimens were observedusing a stereoscopic microscope (ZSA403). The IMC composition was examined using aSU3500 scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy(EDX) capabilities.Results and Discussion. Cross sections of conventional FSSW and UAFSSW jointsare shown in Fig. 2. It can be seen that in the SZ, 6061 Al alloy flows upwards and mixeswith the AZ31 alloy due to the rotation of the tool. An up-bending interface exists betweenthe AZ31 Mg alloy and the 6061 Al alloy, which is called the hook defect. According toYin et al. [9], the geometry of the hook defect largely influenced the lap shear failure of thejoint and better mechanical properties were attained when the hook height was small. Asshown in Fig. 2a, 6061 Al alloy not only flows upwards into the upper sheet but also flowsoutwards. Hence, hook defect of the conventional FSSW joints curved outwards from theaxis of the rotating pin. In the UAFSSW joints in Fig. 2b, 6061 Al of the lower sheetmainly shows upward flow and the hook defect is flatter.S. D. Ji, Z. W. Li, L. Ma, et al.8 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1Fig. 1. A schematic illusion of the ultrasonic vibration during welding.The SZ width refers to the width from the origin of the hook defect to the keyhole. Asshown in Fig. 2, SZ width is 1.34 mm of conventional FSSW joint while this width of theUAFSSW joint is 1.65 mm. The maximum flow distance of Al alloy on the conventionalFSSW and UAFSSW tool is 1.13 and 1.29 mm, respectively. The results agree with theconclusion of Shi et al. [18]. During the UAFSSW process, the material flow behavior ofthe SZ is enhanced due to the ultrasonic vibration, which means more material adjacent theoriginal SZ will be driven to flow. Therefore, the enhanced material flow behavior isbeneficial to the upward flow of the lower 6061 Al alloy, as shown in Fig. 2b.As shown in Fig. 3, at the end section of the hook defect, mixing degrees between thetwo materials show much difference. On conventional FSSW joint, mixing degree between6061 Al and AZ31 Mg is not so adequate and the intersection where IMC always appearcan be clearly recognized. Better mixing is formed on UAFSSW joint. As shown in Fig. 3b,plenty of complex lamellar-like shear bands formed between 6061 Al alloy and AZ31 Mgalloy. Therefore, the conclusion can be attained that ultrasonic vibration can enhancematerial mixing when welding dissimilar alloys.In FSW, the microstructure of the SZ plays a predominant part on joints quality. Ingeneral, uniform and fine grains of the SZ always results in better strength and higherhardness. On the contrary, non-uniform grains of the SZ leads to poor joint strength. Thegrains of the SZ on the conventional FSSW joint and UAFSSW joint are shown in Fig. 4. Itcan be seen that compared with grains shown in Fig. 4a, uniform and refiner grains areobserved on the UAFSSW joint. During the UAFSSW process, the fluidity of material isevidently enhanced due to the ultrasonic vibration. Higher flow velocity is beneficial forthe strain and strain rate of the material in SZ, which is beneficial to the dynamicInvestigation of Ultrasonic Assisted Friction Stir Spot Welding ...ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 9Fig. 2. Cross sections of the FSSW joints: (a) conventional FSSW joint; (b) UAFSSW joint; (c)magnified view of the region marked (a); (d) magnified view of the region marked (b).Fig. 3. Mixing of the dissimilar materials on two different joints: (a) conventional FSSW joint; (b)UAFSSW joint.recrystallization. Therefore, finer grains are attained in the SZ of the UAFSSW joint. Asshown in Fig. 4, the average size of the grains decreases from 8 to 4 m when theUAFSSW is used.Fracture positions of the conventional FSSW joints and UAFSSW joints are shown inFig. 5. It can be seen that both joint fracture at the hook defect firstly. When it comes to theregion adjacent to the keyhole, crack propagation path shows difference. As shown inFig. 5a, in conventional FSSW joint, crack propagates exactly along the interface betweenthe two materials and presents an inverted “V” shape. On the UAFSSW joint, afterinitiating at the tip of the hook, the crack propagates along the hook and then upwards. Atthe region adjacent to the keyhole, crack shows a flat fracture path towards the keyholeinstead of exactly along the interface. A portion of Mg alloy remains at the lower Al alloy,as shown in Fig. 5e and 5f.Figures 6 and 7 show the SEM images and the EDX analysis results of the region E,F, and G marked in Fig. 5. As shown in Fig. 6a, the main component of the IMC is47.329% Mg and 51.246% Al at the Mg side. While at the Al side, the IMCs are mainlycomposed of 32.037% Mg and 64.637% Al. Therefore, it is believed that on the lapinterface of conventional FSSW joints, the Al3Mg2 is the main component of the IMC. Asshown in Fig. 7, region B locates at the interface between the two dissimilar alloys. Themain components of the IMC are 53.464% Mg and 43.484% Al. It can be seen thepercentage of the Mg is much bigger than that on the conventional FSSW joint. Possiblereason for this is that when the ultrasonic vibration was used, material flow behavior at thelap interface was evidently enhanced. The mixing degree between the two materialsbecomes better on the UAFSSW joint, leading to an average elemental composition. It isbelieved on the UAFSSW joints, Al12Mg17 is the main component of the IMC.10 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1Fig. 4. Grains in the SZ on two different joints: (a) conventional FSSW joint; (b) UAFSSW joint.Fig. 5. Fracture positions of the joints: (a) general view of the conventional FSSW joint; (b) magnifiedview of the region A; (c) magnified view of the region B; (d) general view of the UAFSSW joint;(e) magnified view of the region C; (f) magnified view of the region D.S. D. Ji, Z. W. Li, L. Ma, et al.Conclusions. In the present study, UAFSSW was used to weld dissimilar Mg alloyand Al alloy, effects of ultrasonic vibration on the material flow, mixing degree, SZ grainsand fracture modes of the conventional FSSW and UAFSSW joints were studied. Thefollowing conclusions can be drawn:1. Ultrasonic vibration is beneficial for the upward flow of the lower sheet material.The upward flow distance and width of SZ on the UAFSSW joint are much bigger thanthose on the conventional FSSW joint. The mixing degree of the dissimilar materials ismuch better when ultrasonic vibration is used.2. The grain size of the SZ on the UAFSSW joint is smaller than that on theconventional FSSW joint because of the ultrasonic vibration.3. On the conventional FSSW joint, crack exactly propagates along the fayinginterface of dissimilar materials. On the UAFSSW joint at the region adjacent to thekeyhole, crack shows a flat fracture path towards the keyhole instead of exactly along theinterface, leaving a portion of Mg alloy at the lower Al alloy.4. The main component of the IMC at the lap interface on the conventional FSSWjoint is Al3Mg2 and the main component of the IMC at the lap interface on the UAFSSWjoint is Al12Mg17.Acknowledgments. This work is supported by the National Natural Science Foundationof China (No. 51204111), the Education Department Foundation of Liaoning Province(Nos. LJQ2012015 and L2012047), the Natural Science Foundation of Liaoning Province(No. 2013024004) and the Project of Science and Technology Department of LiaoningProvince (No. 2013222007).1. Y. H. Yin, A. Ikuta, and T. H. North, “Microstructural features and mechanicalproperties of AM60 and AZ31 friction stir spot welds,” Mater. Des., 31, 4764–4776(2010).ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 11Fig. 6. EDX results on the conventional FSSW joint: (a) region E; (b) region F.Fig. 7. EDX results of region G on the UAFSSW joint.Investigation of Ultrasonic Assisted Friction Stir Spot Welding ...2. H. Badarinarayan, Y. Shi, X. Li, and K. Okamoto, “Effect of tool geometry on hookformation and static strength of friction stir spot welded aluminum 5754-O sheets,”Int. J. Mach. Tool. Manuf., 49, 814–823 (2009).3. R. Z. Xu, D. R. Ni, Q. Yang, et al., “Influencing mechanism of Zn interlayer additionon hook defects of friction stir spot welded Mg–Al–Zn alloy joints,” Mater. Des., 69,163–169 (2015).4. Z. H. Zhang, X. Q. Yang, J. L. Zhang, et al., “Effect of welding parameters onmicrostructure and mechanical properties of friction stir spot welded 5052 aluminumalloy,” Mater. 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Baudin, et al., “Influence of FSSW parameters onfracture mechanisms of 5182 aluminum welds,” J. Mater. Process. Technol., 210,1429–1435 (2010).11. S. O. Yoon, M. S. Kang, Y. J. Kwon, et al., “Influences of tool plunge speed and toolplunge depth on friction spot joining of AA5454-O aluminum alloy plates withdifferent thicknesses,” Trans. Nonferrous Met. Soc. China, 22, s629–s633 (2012).12. V. X. Tran, J. Pan, and T. Pan, “Fatigue behavior of aluminum 5754-O and 6111-T4spot friction welds in lap-shear specimens,” Int. J. Fatigue, 30, 2175–2190 (2008).13. H. M. Rao, W. Yuan, and H. Badarinarayan, “Effect of process parameters onmechanical properties of friction stir spot welded magnesium to aluminum alloys,”Mater. Des., 66, 235–245 (2015).14. P. C. Lin, Z. M. Su, R. Y. He, and Z. L. Lin, “Failure modes and fatigue lifeestimations of spot friction welds in cross-tension specimens of aluminum 6061-T6sheets,” Int. J. Fatigue, 38, 25–35 (2012).15. Y. S. Sato, A. Shiota, H. Kokawa, et al., “Effect of interfacial microstructure on lapshear strength of friction stir spot weld of aluminum alloy to magnesium alloy,” Sci.Technol. Weld. Join., 15, No. 4, 319–324 (2010).16. Y. Rostamiyan, A. Seidanloo, H. Sohrabpoor, and R. Teimouri, “Experimental studieson ultrasonically assisted friction stir spot welding of AA6061,” Arch. Civ. Mech.Eng., 15, 335–346 (2015).17. X. C. Liu and C. S. Wu, “Material flow in ultrasonic vibration enhanced friction stirwelding,” J. Mater. Process. Technol., 225, 32–44 (2015).18. L. Shi, C. S. Wu, and X. C. Liu, “Modeling the effects of ultrasonic vibration onfriction stir welding,” J. Mater. Process. Technol., 222, 91–102 (2015).Received 03. 08. 201512 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1S. D. Ji, Z. W. Li, L. Ma, et al.

Investigation of Ultrasonic Assisted Friction Stir Spot Welding of Magnesium Alloy to Aluminum Alloy

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Investigation of Ultrasonic Assisted Friction Stir Spot Welding of Magnesium Alloy to Aluminum Alloy

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)