- Email: [email protected]
Al joint welded by gas tungsten arc welding-assisted hybrid ultrasonic seam welding
Materials and Design 77 (2015) 65–71
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microstructure and properties of Mg/Al joint welded by gas tungsten arc welding-assisted hybrid ultrasonic seam welding Xiangyu Dai a,b, Hongtao Zhang b,⇑, Jihou Liu b, Jicai Feng a,b a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Laboratory of Special Welding Technology of Shandong Province, Harbin Institute of Technology at Weihai, Weihai 264209, China
a r t i c l e
i n f o
Article history: Received 12 November 2014 Revised 8 March 2015 Accepted 25 March 2015 Available online 28 March 2015 Keywords: Aluminum alloy Magnesium alloy Ultrasonic welding Micro-bond Lap shear strength
a b s t r a c t A novel gas tungsten arc welding (GTAW) assisted hybrid ultrasonic seam welding MgAZ31B and Al6061 alloy sheets with satisfactory joint strength were successfully achieved using a previous GTAW preheating heat source. The preceding GTAW reduced the sheet hardness but enhanced the acoustic softening effect and materials plasticity. Therefore, the direct joining of 1 mm thick MgAZ31B and Al6061 alloy sheets can be obtained without improving the ultrasonic power. The effect of GTAW current on the microstructure and mechanical properties was investigated. The tensile shear strength of the joint increased with GTAW current up to a maximum strength and then decreased dramatically with higher GTAW current. The maximum lap shear strength was 1 kN at a GTAW current of 30 A, approximately 40% of AZ31B Mg alloy base metal. The failure occurred by interface fracture mode, and the fracture patterns exhibited brittle fracture mode with cleavage facet feature. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The successful joining of dissimilar light materials of Al and Mg alloys is advantage for producing lightweight structures, pushing forward the project of energy-saving and emission-reduction in the transportation and aerospace industries [1,2]. However, traditional fusion welding technologies, including resistance spot welding, arc welding and laser welding, are difficult to apply successfully to the joining of Mg and Al dissimilar metals. The formation of brittle Mg–Al intermetallic compounds (IMCs) preferentially acts as the source of microcracks to deteriorate the mechanical property of the joint [3]. In order to achieve a good combination of the properties of Mg and Al alloys [4,5], owing to their low density, high strength/weight ratio and damping capacity, the development of reliable joining process between these metals is crucial. Recently, solid state technologies are attracting increasing interest for dissimilar light metals joining applications. Ultrasonic welding is a solid state joining process that uses high-frequency ultrasonic vibrations under a modest pressure to induce oscillating shears between the faying surfaces to produce metallurgical bonds [6,7]. It involves forming welds by disruption of the interface oxide layer through deformation, and thus in principle avoids liquid phase reactions. The metallurgical defects, such
⇑ Corresponding author. E-mail address: [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.matdes.2015.03.054 0261-3069/Ó 2015 Elsevier Ltd. All rights reserved.
as formation of brittle IMCs, high levels of welding distortions and HAZ damage in fusion weld can be typically avoid [8,9]. Moreover, ultrasonic welding is more efficient than resistance spot welding (RSW) and friction stir welding (FSW), because the energy is predominantly generated at the weld-line and only 0.6–1.5 kJ per weld compared to RSW with 50–100 kJ and FSW with 3–6 kJ per weld [10]. Recently, a number of studies reported on the joining of dissimilar light metals using ultrasonic spot welding [11–15]. A sound ultrasonic spot welding joints could be obtained at usual welding conditions. Studies on joining dissimilar metals via ultrasonic seam welding are few even though the method possesses higher efficiency and weld continuity characteristics. However, conventional ultrasonic seam welding can only be applied to weld metal foils or thin plate (<0.5 mm in thickness) because of the lower welding energy of the power output [16]. For welding of thick metal specimens, higher power welding systems are necessary [17]. In addition, transverse and torsional complex vibration systems with a welding tip vibrating in elliptical or circular locus were also reported to weld thick plate specimens [18]. However, the main drawback of the above methods was that the equipment costly and is difficult to perform. As we all know, the introduction of ultrasound techniques in arc welding with the intention of improving the operational performance and technical characteristics of the welding processes have been studied intensively. There were mainly two methods to introduce ultrasound techniques in arc welding, one used mechanical
66
X. Dai et al. / Materials and Design 77 (2015) 65–71
transducers to obtain ultrasonic energy [19], the other focused on the pulse of current in ultrasonic frequencies to make the arc act not only as a thermal source but also as a mechanism that emits ultrasound to introduce energy in the molten pool [20,21]. The two methods still belonged to the category of the fusion welding field and had achieved significant results in control of weld pool grain size. In dissimilar metals joining, avoiding metal melting was an important method to control the intermetallic compound formation. In this study, gas tungsten arc welding (GTAW) was introduced into ultrasonic seam welding of Mg and Al dissimilar metals for the first time. The preceding GTAW with relatively low heat input can preheat the sheet metal to enhance the ultrasonic weldability. The specimen hardness was reduced, whereas the material plasticity and acoustic softening effect were enhanced. Therefore, at a constant power output, the solid state joining of 1 mm thick Al and Mg alloys sheets was accomplished. The aim of this paper is to perform the direct solid state joining of 1 mm thick Al and Mg alloys without improving the ultrasonic power. In addition, the effect of GTAW current on the microstructure and mechanical properties was investigated in detail.
Table 1 Optimized welding parameters.
2. Experimental
Two different configurations of Al–Mg lap joints were adopted in the experiments. Joint I was for the case where the Mg sheet was placed on top of the Al sheet. Joint II was where the Al sheet was placed on top of the Mg sheet. For Joint I, the Mg sheet was broken under the pressure and ultrasonic vibrations because of the poor plasticity of Mg alloys. For Joint II, continuous and sound welded joints were achieved. The weld appearances of the joints with various GTAW currents are shown in Fig. 2. The conventional ultrasonic seam welding of 1 mm-thick Al and Mg sheets was not obtained because of the low welding energy of the power output. As the GTAW current was from 15 A to 30 A, the welds exhibited good ‘‘knurl pattern’’ surface morphology. Weld defects, such as slipping and tool sticking, were not found. The rotating sonotrode tips have serrated surface to improve gripping of the lapped sheets and prevented slipping between the sonotrode and the sheet. When the GTAW current was increased, the penetration of the sonotrode tips into the Al surface became increasingly deeper, as shown in Fig. 3. The penetration of the sonotrode tips into the Al surface increased from 130 lm at a GTAW current of 20 A to 150 lm at a GTAW current of 25 A and to 400 lm at a GTAW current of 30 A. This result indicated that specimen hardness was reduced and plastic deformation became more extreme with increasing GTAW current. However, poor weld joints were produced at a GTAW current of 35 A. This observation was caused by a large amount of brittle Al–Mg IMCs (Al12Mg17, Al3Mg2) formed and then fragmented caused by ultrasonic vibration.
MgAZ31B and Al6061 sheets with dimensions of 150 mm 60 mm 1 mm were used. The oxide layers on the faying surface were removed by stainless steel wire brushing and then degreased using acetone before welding. The sheet specimens were clamped between the rotating sonotrode and the anvil in a lap configuration with an overlap distance of 20 mm. The schematic diagram of the GTAW-assisted hybrid ultrasonic seam welding system is shown in Fig. 1. The system mainly consisted of two sections: the ultrasonic seam welding system and the GTAW system. The ultrasonic vibration direction during welding was parallel to the longitudinal direction of the test coupon. The preceding GTAW torch was attached adjacent to a rotating sonotrode and carried out with an angle of 45° adjacent to the joint. It could preheat the sheet metal to enhance the weldability during the ultrasonic welding process. The rotating sonotrode tips had serrated surface to improve gripping of the lapped sheets. The GTAW-assisted hybrid ultrasonic seam welding was carried out at GTAW currents ranging from 15 A to 35 A at a constant of welding speed of 4 mm/s, pressure of 0.44 MPa, vibration frequency of 20 kHz, as listed in Table 1. The metallurgical cross section was observed via scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) after a standard grinding and polishing procedure. To determine the temperature profile during GTAW-assisted hybrid ultrasonic seam welding, weld temperatures were measured using 0.5 mm diameter K-type thermocouples placed carefully at the center of the weld. The Vickers hardness was measured along the transverse cross section of the welded specimen at a load of
Fig. 1. Schematic diagram of GTAW assisted hybrid ultrasonic seam welding system.
B1 B2 B3 B4 B5
GTAW current (A)
Welding speed (mm/s)
Pressure (MPa)
Vibration frequency (kHz)
15 20 25 30 35
4 4 4 4 4
0.44 0.44 0.44 0.44 0.44
20 20 20 20 20
200 gf and a dwell time of 10 s. Five 10 mm-wide specimens perpendicular to welding direction were prepared and subjected to tensile testing machine with a speed of 0.5 mm/min at room temperature. Moreover, the fracture surface was examined via SEM equipped with EDS.
3. Results 3.1. Weld appearance and cross section
3.2. Thermal measurements K-type thermocouples are placed at the center of the weld interface as shown in Fig. 4(a) and the results of peak temperature recorded by the thermocouples are shown in Fig. 3(b). A higher GTAW current resulted in higher temperature at the center of the weld interface. The peak temperature increased gradually from about 245 °C without GTAW hybrid to about 470 °C at a GTAW current of 35 A. This peak temperature was sufficient for mutual diffusion between Mg and Al atoms [22]. Constitutional liquation also occurred. From the Al–Mg binary phase diagram in Fig. 5, two low melting point eutectic reactions were observed: Mg solid solution and Al12Mg17 IMC reaction at 437 °C and Al3Mg2 and Al12Mg17 IMC reaction at 450 °C. Therefore, once the GTAW current reached 35 A, a large amount of Al–Mg IMCs was formed at the center of the weld interface, even though the thermal cycle in ultrasonic welding was very short.
X. Dai et al. / Materials and Design 77 (2015) 65–71
67
Fig. 2. Weld appearances of the joints with increasing GTAW currents of (a) 20 A; (b) 25 A; (c) 30 A.
Fig. 3. Optical images of weld cross sections with increasing GTAW currents of (a) 20 A; (b) 25 A; (c) 30 A.
Fig. 4. (a) Experimental setup of temperature measurement; (b) peak temperature as a function of GTAW current.
Fig. 5. Binary Al–Mg phase diagram with the eutectic reaction temperatures indicated [19].
3.3. Microstructure evolution Fig. 6 shows the SEM backscattered electron micrographs of the weld interface obtained for different GTAW currents. As seen in Fig. 6(a), discrete gaps along the weld interface were observed, which showed lack of bonding at the interface at a GTAW current of 15 A. These gaps appeared when the temperature was not yet risen sufficiently for the materials to deform [23]. This plastic
deformation was not strong enough to form a sound joint, and only part of the join line was effectively bonded. The discrete gaps were less distinct and a dramatic change in materials flow behavior was observed with increasing GTAW current. At a low GTAW current, the plastic deformation was localized at the interface, and the weld join-line remains macroscopically flat. When the GTAW current was increased above 20 A, the plastically deforming region expanded and the weld join-line showed a wave-like or swirl-like appearance because of the heavy plastic deformation [24]. The wave-like or swirl-like structure could lead to a mechanical interlocking and therefore, enhanced the joint strength. The joint strength was a combined effect of waves and micro-bonds along the weld interface [25]. At a higher GTAW of 35 A, a high temperature of 470 °C resulted in extreme reactions between Al and Mg atoms to form a large amount of brittle Al–Mg IMCs, which decreased the joint strength. In addition, a thin dark transition layer (about 1–2 lm thickness) was observed at the interface. To identify further the dark transition layers, EDS line scanning of the weld interface at GTAW current of 20 A and 30 A is shown in Fig. 7. A continuous and smooth transition of atomic content for Al and Mg elements within the 3 lm thickness transition layer is shown in Fig. 7(a). The results indicate that no continuous brittle IMC layers were formed at the weld interface. This result was mainly due to the
68
X. Dai et al. / Materials and Design 77 (2015) 65–71
Fig. 6. SEM micrographs of the joints at GTAW current of (a) 15 A; (b) 20 A; (c) 25 A; (d) 30 A.
Fig. 7. EDS data of weld interface obtained at different GTAW currents: (a) 20 A; (b) 30 A.
lower temperature at the weld interface than in FSW and RSW. However, an extremely narrow platform could be observed at a GTAW current of 30 A in Fig. 5(b), which indicated the existence of brittle IMC layers. When the GTAW current was 35 A, a large amount of brittle Al–Mg IMCs was formed and then fragmented because of ultrasonic vibration. For a separate verification of IMCs, the fracture surface on Mg side of the welded specimen was analyzed via XRD, and the results were shown in Fig. 8. The XRD results showed large peaks of the Al12Mg17 phase and small peaks of the Al3Mg2 and Mg solid solution. This result confirmed that the non-uniform IMC layer contained a large volume of Al12Mg17 phase. 3.4. Vickers micro-hardness Fig. 9 shows the micro-hardness profiles that were determined diagonally along the transverse cross section of welded specimen
Fig. 8. X-ray diffraction of fracture surface on Mg side at GTAW current of 35 A.
X. Dai et al. / Materials and Design 77 (2015) 65–71
69
Therefore, failure occurred easily through the interface. When the GTAW current was increased, the lap shear strength increased in the beginning because of the high temperatures, which accelerated the diffusion and deformation between the Mg and Al sheets. However, the lap shear strength decreased sharply at a high GTAW current of 35 A because of a large amount of brittle Al–Mg IMCs formation. 3.6. Failure behavior
Fig. 9. Micro-hardness profile in welded joints at different GTAW currents.
made at GTAW currents of 20, 25, and 30 A. We concluded that the micro-hardness variation along the transverse cross section was significant. The hardness in the vicinity of the weld near the Al and Mg side could increase from 65–75 HV to 85–110 HV and from 60–70 HV to 75–120 HV, respectively, because of strain hardening in the vicinity of the weld. Furthermore, the highest value of hardness was produced at the interface zone, approximately 230–270 HV, because of the hard and brittle Al–Mg IMCs formation. Moreover, the micro-hardness increased with increasing the GTAW current. This result was due to a large amount of brittle Al–Mg IMCs formed with increasing temperature. 3.5. Lap shear test Fig. 10 shows the lap shear test results obtained from dissimilar Al–Mg welds as a function of GTAW current. The tensile shear strength of the joint increased with GTAW current until the maximum strength was reached and then decreased dramatically with higher GTAW current. The maximum lap shear strength was 1 kN at a GTAW current of 30 A, with approximately 40% of AZ31B Mg alloy base metal. The joining of 1 mm thick Al and Mg sheets via conventional ultrasonic seam welding was not achieved because of the low welding energy of ultrasonic power. The low welding energy of ultrasonic power resulted in a low micro-bond density.
Typical macroscopic images of the failed lap shear tensile samples are shown in Fig. 11(a). All the failure of joints occurred by interface fracture mode rather than the pull-out mode, even though the maximum lap shear strength could reach 1 kN (approximately 40% of AZ31B Mg alloy base metal). SEM micrographs fracture surface of the joints made at GTAW currents of 20 and 30 A on the Mg side are shown in Fig. 11(b) and (c), respectively. The fracture patterns exhibited brittle fracture mode with a cleavage facet feature. At a low GTAW current of 20 A, microbonded bright spots were observed at the periphery of the sonotrode tip footprint. The bright spots were parallel to the direction of ultrasonic vibration. The micro-bonds increased in density and spread across the faying surface and coalesced with each other when the GTAW current reached 30 A. To ascertain this phenomenon, EDS box analysis was performed on the fracture surface of the Mg side, as shown in Table 2. The EDS results showed that 42% Al atom was present at a GTAW current of 30 A, which was higher than 31% Al atom at a GTAW current of 20 A. This result was attributed to the extreme diffusion of Al atom and plastic deformation at higher temperatures (or higher GTAW currents). 3.7. Weld defects Based on the above analysis, a good ultrasonic weld should have dense interfacial bonds without discrete gaps and extreme deformation. At lower values of GTAW current, a substantial density of gaps or unbonded areas at the weld interface is shown in Fig. 12(a). This result was due to lower temperature that enables sufficient plastic flow the interface. Only part of interface was effectively bonded. However, at higher GTAW currents, the ‘‘tool sticking’’ issue occurred frequently because the friction between the two sheets was larger than the friction between the sonotrode and top sheet or between the anvil and lower sheet. The motion at the interface stopped and the work pieces would either slide over the anvil, or the sonotrode loose traction on the top part. Therefore, the sheets could be welded to the sonotrode or the anvil as shown in Fig. 12(b). 4. Discussion As described above, direct welding of 1 mm thick MgAZ31B/ Al6061 dissimilar alloys via GTAW-assisted hybrid ultrasonic seam welding can be performed if the proper value of GTAW current was selected. The conventional ultrasonic seam welding can only be used to join metal foils or thin plate specimens (generally <0.5 mm in thickness) because of the limitations in the welding energy of the ultrasonic power. The ultrasonic power (P) is dependent on the sheet hardness (H) and sheet thickness (d) and can be expressed by the following equation [26]:
P ¼ kH
Fig. 10. Lap tensile shear strength.
1:5 1:5
d
ð1Þ
where k is the proportion constant, H is sheet hardness, and d is the sheet thickness. The ultrasonic power increases exponentially with increasing sheet hardness and thickness. However, the high ultrasonic power welding systems results in high cost and inconvenient
70
X. Dai et al. / Materials and Design 77 (2015) 65–71
Fig. 11. Typical fractured welded joints: (a) macroscopic images of failed lap shear tensile samples; SEM images of the fractured surface on Mg side at GTAW of (b) 20 A and (c) 30 A, and EDS box analysis in the magnified fracture surface area.
Table 2 Chemical compositions based on the EDS results (at.%). Spot
Mg
Al
B2 B4
69 58
31 42
manipulation of the ultrasonic equipment. In the present work, gas tungsten arc welding (GTAW) is initially introduced into ultrasonic seam welding. The preceding GTAW can preheat the sheet metal to reduce the specimen hardness and enhance the specimen plasticity
and acoustic softening effect. The transfer of the welding energy is more efficient. Therefore, the direct joining of 1 mm thick MgAZ31B and Al6061 alloy sheets can be obtained without improving the ultrasonic power. To analyze the mechanism of GTAW-assisted hybrid ultrasonic seam welding, a physical model is established in Fig. 13. The formation process of the joint can be considered to occur in sequence as follows: (1) Micro-bond formation: When static pressure and tangential force are applied using the rotating sonotrode (in Fig. 13(a)), the faying surface come into close contact and
Fig. 12. Weld defects: (a) unbonded areas; (b) tool sticking.
X. Dai et al. / Materials and Design 77 (2015) 65–71
71
Fig. 13. Weld formation model: (a) real contact; (b) mirco-bond formation; (c) GTAW current of 20 A; (d) GTAW current of 30 A.
the local surface oxide layer between contacting asperities began to break down. At these clean contacts, metallic adhesion can take place. The initial adhesion spots are much smaller than the micro-bonds and then grew to form micro-bonds under the action of surrounding material plastic deformation (in Fig. 13(b)). The preceding GTAW can preheat the sheet specimens to decrease the hardness of the sheet specimen. Therefore, the area of real contact increased with increasing GTAW current. (2) Weld formation: With the formation of the micro-bonds and sufficient plastic deformation, inter-diffusion can take place in the vicinity of the micro-bonds areas. The number and length of micro-bonds generated per unit unwelded contact area are proportional to the GTAW current because the preceding GTAW can preheat the sheet specimens to improve material plasticity. At a low GTAW current of 20 A, the fractured surface as shown in Fig. 11 indicates that the microbonded bright spots have low density and parallel to the vibration direction. When the GTAW current increases to 30 A, the micro-bonds increases in density and spread across the faying surface to coalesce with each other. Therefore, the tensile shear strength of the joint increased with GTAW current until a maximum strength of 1 kN was reached. 5. Conclusions Direct joining of 1 mm-thick MgAZ31B and Al6061 alloy sheets via GTAW-assisted hybrid ultrasonic seam welding was successfully achieved without improving the ultrasonic power. The results could be summarized as follows: (1) The preceding GTAW was introduced into ultrasonic seam welding to preheat the sheet metal to enhance weldability. The specimen hardness reduced, the material plasticity and acoustic softening effect were enhanced. (2) The higher GTAW current resulted in a higher temperature at the center of the weld interface. At a low GTAW current, plastic deformation was localized at the interface and the weld join-line remained macroscopically flat. When the GTAW current was increased above 20 A, the weld join-line revealed a wave-like or swirl-like appearance because of the heavy plastic deformation. At a higher GTAW of 35 A, the extreme reactions between Al and Mg atoms to form a large amount of brittle Al–Mg IMCs, which deteriorated the joint strength. (3) The maximum lap shear strength was 1 kN at GTAW current of 30 A and approximately 40% of AZ31B Mg alloy base metal. All instances of joint failure occurred by the interface fracture mode rather than the pull-out mode, and the fracture patterns exhibited brittle fracture mode with a cleavage facet feature.
References [1] Liming Liu, Daxin Ren, Fei Liu, A review of dissimilar welding techniques for magnesium alloys to aluminum alloys, Materials 7 (2014) 3735–3757. [2] H.T. Zhang, X.Y. Dai, J.C. Feng, Joining of aluminum and magnesium via preroll-assisted A-TIG welding with Zn interlayer, Mater. Lett. 122 (2014) 49–51. [3] H.T. Zhang, J.Q. Song, Microstructural evolution of aluminum/magnesium lap joints welded using MIG process with zinc foil as an interlayer, Mater. Lett. 65 (2011) 3292–3294. [4] E. Schubert, M. Klassen, I. Zerner, C. Walz, G. Sepold, Light-weight structures produced by laser beam joining for future applications in automobile and aerospace industry, J. Mater. Process. Technol. 115 (2001) 2–8. [5] H.T. Zhang, X.Y. Dai, J.C. Feng, Interfacial microstructure and mechanical properties of Al/Mg butt joins made by MIG welding process with Zn–Cd alloy as interlayer, J. Wuhan Univ. Technol. 29 (2014) 1258–1264. [6] C. Doumanidis, Y. Gao, Mechanical modeling of ultrasonic welding, Weld. J. (2004) 140–146. [7] J.E. Gould, Joining aluminum sheet in the automotive industry – a 30 year history, Weld. J. (2012) 23–34. [8] Shinichi Matsuoka, Hisashi Imai, Direct welding of different metals used ultrasonic vibration, J. Mater. Process. Technol. 209 (2009) 954–960. [9] S. Elangovan, S. Semeer, K. Prakasan, Temperature and stress distribution in ultrasonic metal welding – an FEA-based study, J. Mater. Process. Technol. 209 (2009) 1143–1150. [10] D. Bakavos, P.B. Prangnell, Mechanisms of joint and microstructure formation in high power ultrasonic spot welding 6111 aluminium automotive sheet, Mater. Sci. Eng., A 527 (2010) 6320–6334. [11] V.K. Patel, S.D. Bhole, D.L. Chen, Influence of ultrasonic spot welding on microstructure in a magnesium alloy, Scr. Mater. 65 (2011) 911–914. [12] A. Panteli, Y.C. Chen, D. Strong, X.Y. Zhang, P.B. Prangnell, Optimization of aluminium to magnesium ultrasonic spot welding, JOM 64 (2012) 414–420. [13] V.K. Patel, S.D. Bhole, D.L. Chen, Formation of zinc interlayer texture during dissimilar ultrasonic spot welding of magnesium and high strength low alloy steel, Mater. Des. 45 (2013) 236–240. [14] V.K. Patel, S.D. Bhole, D.L. Chen, Fatigue life estimation of ultrasonic spot welded Mg alloy joints, Mater. Des. 62 (2014) 124–132. [15] M. Shakil, N.H. Tariq, M. Ahmad, M.A. Choudhary, J.I. Akhter, Effect of ultrasonic welding parameters on microstructure and mechanical properties of dissimilar joints, Mater. Des. 55 (2014) 263–273. [16] J. Tsujino, T. Ueoka, T. Kashino, F. Sugahara, Transverse and torsional complex vibration systems for ultrasonic seam welding of metal plates, Ultrasonics 38 (2000) 67–71. [17] C.Y. Zhang, D.L. Chen, A.A. Luo, Joining 5754 automotive aluminum alloy 2mm-thick sheets using ultrasonic spot welding, Weld. J. (2014) 131–138. [18] J. Tsujino, T. Ueoka, Welding characteristics of ultrasonic seam welding system using a complex vibration circular disk welding tip, Jpn. J. Appl. Phys. 39 (2000) 2990–2994. [19] V.C. Tiago, E.N.B. Carlos, Ultrasound in arc welding: a review, Ultrasonics 56 (2015) 201–209. [20] L.B. He, L.M. Li, H.W. Hao, M.S. Wu, R.L. Zhou, Grain refinement and high performance of titanium alloy joint using arc-ultrasonic gas tungsten arc welding, Sci. Technol. Weld. Join. 11 (2006) 72–74. [21] X.Z. Chen, Z. Shen, J. Chen, Y.C. Lei, Q.Y. Huang, Effect of GTA-ultrasonic excitated-voltage on microstructure and impact toughness of CLAMs weldments, J. Jiangsu Univ. (Nat. Sci. Ed.) 32 (2011) 724–727. [22] V.K. Patel, S.D. Bhole, D.L. Chen, Microstructure and mechanical properties of dissimilar welded Mg–Al joints by ultrasonic spot welding technique, Sci. Technol. Weld. Join. 17 (2012) 202–206. [23] J.W. Yang, B. Cao, X.C. He, H.S. Luo, Microstructure evolution and mechanical properties of Cu–Al joints by ultrasonic welding, Sci. Technol. Weld. Join. 19 (2014) 500–504. [24] A. Panteli, J.D. Robson, I. Brough, P.B. Prangnell, The effect of high strain rate deformation on intermetallic reaction during ultrasonic welding aluminium to magnesium, Mater. Sci. Eng., A – Struct. 556 (2012) 31–42. [25] R. Jahn, R. Cooper, D. Wilkosz, The effect of anvil geometry and welding energy on microstructures in ultrasonic spot welds of AA6111-T4, Metall. Mater. Trans. A 38 (2007) 570–583. [26] J.L. Pan, Welding Handbook, China Machine Press, Beijing, 1995.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microstructure and properties of Mg/Al joint welded by gas tungsten arc welding-assisted hybrid ultrasonic seam welding Xiangyu Dai a,b, Hongtao Zhang b,⇑, Jihou Liu b, Jicai Feng a,b a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Laboratory of Special Welding Technology of Shandong Province, Harbin Institute of Technology at Weihai, Weihai 264209, China
a r t i c l e
i n f o
Article history: Received 12 November 2014 Revised 8 March 2015 Accepted 25 March 2015 Available online 28 March 2015 Keywords: Aluminum alloy Magnesium alloy Ultrasonic welding Micro-bond Lap shear strength
a b s t r a c t A novel gas tungsten arc welding (GTAW) assisted hybrid ultrasonic seam welding MgAZ31B and Al6061 alloy sheets with satisfactory joint strength were successfully achieved using a previous GTAW preheating heat source. The preceding GTAW reduced the sheet hardness but enhanced the acoustic softening effect and materials plasticity. Therefore, the direct joining of 1 mm thick MgAZ31B and Al6061 alloy sheets can be obtained without improving the ultrasonic power. The effect of GTAW current on the microstructure and mechanical properties was investigated. The tensile shear strength of the joint increased with GTAW current up to a maximum strength and then decreased dramatically with higher GTAW current. The maximum lap shear strength was 1 kN at a GTAW current of 30 A, approximately 40% of AZ31B Mg alloy base metal. The failure occurred by interface fracture mode, and the fracture patterns exhibited brittle fracture mode with cleavage facet feature. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The successful joining of dissimilar light materials of Al and Mg alloys is advantage for producing lightweight structures, pushing forward the project of energy-saving and emission-reduction in the transportation and aerospace industries [1,2]. However, traditional fusion welding technologies, including resistance spot welding, arc welding and laser welding, are difficult to apply successfully to the joining of Mg and Al dissimilar metals. The formation of brittle Mg–Al intermetallic compounds (IMCs) preferentially acts as the source of microcracks to deteriorate the mechanical property of the joint [3]. In order to achieve a good combination of the properties of Mg and Al alloys [4,5], owing to their low density, high strength/weight ratio and damping capacity, the development of reliable joining process between these metals is crucial. Recently, solid state technologies are attracting increasing interest for dissimilar light metals joining applications. Ultrasonic welding is a solid state joining process that uses high-frequency ultrasonic vibrations under a modest pressure to induce oscillating shears between the faying surfaces to produce metallurgical bonds [6,7]. It involves forming welds by disruption of the interface oxide layer through deformation, and thus in principle avoids liquid phase reactions. The metallurgical defects, such
⇑ Corresponding author. E-mail address: [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.matdes.2015.03.054 0261-3069/Ó 2015 Elsevier Ltd. All rights reserved.
as formation of brittle IMCs, high levels of welding distortions and HAZ damage in fusion weld can be typically avoid [8,9]. Moreover, ultrasonic welding is more efficient than resistance spot welding (RSW) and friction stir welding (FSW), because the energy is predominantly generated at the weld-line and only 0.6–1.5 kJ per weld compared to RSW with 50–100 kJ and FSW with 3–6 kJ per weld [10]. Recently, a number of studies reported on the joining of dissimilar light metals using ultrasonic spot welding [11–15]. A sound ultrasonic spot welding joints could be obtained at usual welding conditions. Studies on joining dissimilar metals via ultrasonic seam welding are few even though the method possesses higher efficiency and weld continuity characteristics. However, conventional ultrasonic seam welding can only be applied to weld metal foils or thin plate (<0.5 mm in thickness) because of the lower welding energy of the power output [16]. For welding of thick metal specimens, higher power welding systems are necessary [17]. In addition, transverse and torsional complex vibration systems with a welding tip vibrating in elliptical or circular locus were also reported to weld thick plate specimens [18]. However, the main drawback of the above methods was that the equipment costly and is difficult to perform. As we all know, the introduction of ultrasound techniques in arc welding with the intention of improving the operational performance and technical characteristics of the welding processes have been studied intensively. There were mainly two methods to introduce ultrasound techniques in arc welding, one used mechanical
66
X. Dai et al. / Materials and Design 77 (2015) 65–71
transducers to obtain ultrasonic energy [19], the other focused on the pulse of current in ultrasonic frequencies to make the arc act not only as a thermal source but also as a mechanism that emits ultrasound to introduce energy in the molten pool [20,21]. The two methods still belonged to the category of the fusion welding field and had achieved significant results in control of weld pool grain size. In dissimilar metals joining, avoiding metal melting was an important method to control the intermetallic compound formation. In this study, gas tungsten arc welding (GTAW) was introduced into ultrasonic seam welding of Mg and Al dissimilar metals for the first time. The preceding GTAW with relatively low heat input can preheat the sheet metal to enhance the ultrasonic weldability. The specimen hardness was reduced, whereas the material plasticity and acoustic softening effect were enhanced. Therefore, at a constant power output, the solid state joining of 1 mm thick Al and Mg alloys sheets was accomplished. The aim of this paper is to perform the direct solid state joining of 1 mm thick Al and Mg alloys without improving the ultrasonic power. In addition, the effect of GTAW current on the microstructure and mechanical properties was investigated in detail.
Table 1 Optimized welding parameters.
2. Experimental
Two different configurations of Al–Mg lap joints were adopted in the experiments. Joint I was for the case where the Mg sheet was placed on top of the Al sheet. Joint II was where the Al sheet was placed on top of the Mg sheet. For Joint I, the Mg sheet was broken under the pressure and ultrasonic vibrations because of the poor plasticity of Mg alloys. For Joint II, continuous and sound welded joints were achieved. The weld appearances of the joints with various GTAW currents are shown in Fig. 2. The conventional ultrasonic seam welding of 1 mm-thick Al and Mg sheets was not obtained because of the low welding energy of the power output. As the GTAW current was from 15 A to 30 A, the welds exhibited good ‘‘knurl pattern’’ surface morphology. Weld defects, such as slipping and tool sticking, were not found. The rotating sonotrode tips have serrated surface to improve gripping of the lapped sheets and prevented slipping between the sonotrode and the sheet. When the GTAW current was increased, the penetration of the sonotrode tips into the Al surface became increasingly deeper, as shown in Fig. 3. The penetration of the sonotrode tips into the Al surface increased from 130 lm at a GTAW current of 20 A to 150 lm at a GTAW current of 25 A and to 400 lm at a GTAW current of 30 A. This result indicated that specimen hardness was reduced and plastic deformation became more extreme with increasing GTAW current. However, poor weld joints were produced at a GTAW current of 35 A. This observation was caused by a large amount of brittle Al–Mg IMCs (Al12Mg17, Al3Mg2) formed and then fragmented caused by ultrasonic vibration.
MgAZ31B and Al6061 sheets with dimensions of 150 mm 60 mm 1 mm were used. The oxide layers on the faying surface were removed by stainless steel wire brushing and then degreased using acetone before welding. The sheet specimens were clamped between the rotating sonotrode and the anvil in a lap configuration with an overlap distance of 20 mm. The schematic diagram of the GTAW-assisted hybrid ultrasonic seam welding system is shown in Fig. 1. The system mainly consisted of two sections: the ultrasonic seam welding system and the GTAW system. The ultrasonic vibration direction during welding was parallel to the longitudinal direction of the test coupon. The preceding GTAW torch was attached adjacent to a rotating sonotrode and carried out with an angle of 45° adjacent to the joint. It could preheat the sheet metal to enhance the weldability during the ultrasonic welding process. The rotating sonotrode tips had serrated surface to improve gripping of the lapped sheets. The GTAW-assisted hybrid ultrasonic seam welding was carried out at GTAW currents ranging from 15 A to 35 A at a constant of welding speed of 4 mm/s, pressure of 0.44 MPa, vibration frequency of 20 kHz, as listed in Table 1. The metallurgical cross section was observed via scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) after a standard grinding and polishing procedure. To determine the temperature profile during GTAW-assisted hybrid ultrasonic seam welding, weld temperatures were measured using 0.5 mm diameter K-type thermocouples placed carefully at the center of the weld. The Vickers hardness was measured along the transverse cross section of the welded specimen at a load of
Fig. 1. Schematic diagram of GTAW assisted hybrid ultrasonic seam welding system.
B1 B2 B3 B4 B5
GTAW current (A)
Welding speed (mm/s)
Pressure (MPa)
Vibration frequency (kHz)
15 20 25 30 35
4 4 4 4 4
0.44 0.44 0.44 0.44 0.44
20 20 20 20 20
200 gf and a dwell time of 10 s. Five 10 mm-wide specimens perpendicular to welding direction were prepared and subjected to tensile testing machine with a speed of 0.5 mm/min at room temperature. Moreover, the fracture surface was examined via SEM equipped with EDS.
3. Results 3.1. Weld appearance and cross section
3.2. Thermal measurements K-type thermocouples are placed at the center of the weld interface as shown in Fig. 4(a) and the results of peak temperature recorded by the thermocouples are shown in Fig. 3(b). A higher GTAW current resulted in higher temperature at the center of the weld interface. The peak temperature increased gradually from about 245 °C without GTAW hybrid to about 470 °C at a GTAW current of 35 A. This peak temperature was sufficient for mutual diffusion between Mg and Al atoms [22]. Constitutional liquation also occurred. From the Al–Mg binary phase diagram in Fig. 5, two low melting point eutectic reactions were observed: Mg solid solution and Al12Mg17 IMC reaction at 437 °C and Al3Mg2 and Al12Mg17 IMC reaction at 450 °C. Therefore, once the GTAW current reached 35 A, a large amount of Al–Mg IMCs was formed at the center of the weld interface, even though the thermal cycle in ultrasonic welding was very short.
X. Dai et al. / Materials and Design 77 (2015) 65–71
67
Fig. 2. Weld appearances of the joints with increasing GTAW currents of (a) 20 A; (b) 25 A; (c) 30 A.
Fig. 3. Optical images of weld cross sections with increasing GTAW currents of (a) 20 A; (b) 25 A; (c) 30 A.
Fig. 4. (a) Experimental setup of temperature measurement; (b) peak temperature as a function of GTAW current.
Fig. 5. Binary Al–Mg phase diagram with the eutectic reaction temperatures indicated [19].
3.3. Microstructure evolution Fig. 6 shows the SEM backscattered electron micrographs of the weld interface obtained for different GTAW currents. As seen in Fig. 6(a), discrete gaps along the weld interface were observed, which showed lack of bonding at the interface at a GTAW current of 15 A. These gaps appeared when the temperature was not yet risen sufficiently for the materials to deform [23]. This plastic
deformation was not strong enough to form a sound joint, and only part of the join line was effectively bonded. The discrete gaps were less distinct and a dramatic change in materials flow behavior was observed with increasing GTAW current. At a low GTAW current, the plastic deformation was localized at the interface, and the weld join-line remains macroscopically flat. When the GTAW current was increased above 20 A, the plastically deforming region expanded and the weld join-line showed a wave-like or swirl-like appearance because of the heavy plastic deformation [24]. The wave-like or swirl-like structure could lead to a mechanical interlocking and therefore, enhanced the joint strength. The joint strength was a combined effect of waves and micro-bonds along the weld interface [25]. At a higher GTAW of 35 A, a high temperature of 470 °C resulted in extreme reactions between Al and Mg atoms to form a large amount of brittle Al–Mg IMCs, which decreased the joint strength. In addition, a thin dark transition layer (about 1–2 lm thickness) was observed at the interface. To identify further the dark transition layers, EDS line scanning of the weld interface at GTAW current of 20 A and 30 A is shown in Fig. 7. A continuous and smooth transition of atomic content for Al and Mg elements within the 3 lm thickness transition layer is shown in Fig. 7(a). The results indicate that no continuous brittle IMC layers were formed at the weld interface. This result was mainly due to the
68
X. Dai et al. / Materials and Design 77 (2015) 65–71
Fig. 6. SEM micrographs of the joints at GTAW current of (a) 15 A; (b) 20 A; (c) 25 A; (d) 30 A.
Fig. 7. EDS data of weld interface obtained at different GTAW currents: (a) 20 A; (b) 30 A.
lower temperature at the weld interface than in FSW and RSW. However, an extremely narrow platform could be observed at a GTAW current of 30 A in Fig. 5(b), which indicated the existence of brittle IMC layers. When the GTAW current was 35 A, a large amount of brittle Al–Mg IMCs was formed and then fragmented because of ultrasonic vibration. For a separate verification of IMCs, the fracture surface on Mg side of the welded specimen was analyzed via XRD, and the results were shown in Fig. 8. The XRD results showed large peaks of the Al12Mg17 phase and small peaks of the Al3Mg2 and Mg solid solution. This result confirmed that the non-uniform IMC layer contained a large volume of Al12Mg17 phase. 3.4. Vickers micro-hardness Fig. 9 shows the micro-hardness profiles that were determined diagonally along the transverse cross section of welded specimen
Fig. 8. X-ray diffraction of fracture surface on Mg side at GTAW current of 35 A.
X. Dai et al. / Materials and Design 77 (2015) 65–71
69
Therefore, failure occurred easily through the interface. When the GTAW current was increased, the lap shear strength increased in the beginning because of the high temperatures, which accelerated the diffusion and deformation between the Mg and Al sheets. However, the lap shear strength decreased sharply at a high GTAW current of 35 A because of a large amount of brittle Al–Mg IMCs formation. 3.6. Failure behavior
Fig. 9. Micro-hardness profile in welded joints at different GTAW currents.
made at GTAW currents of 20, 25, and 30 A. We concluded that the micro-hardness variation along the transverse cross section was significant. The hardness in the vicinity of the weld near the Al and Mg side could increase from 65–75 HV to 85–110 HV and from 60–70 HV to 75–120 HV, respectively, because of strain hardening in the vicinity of the weld. Furthermore, the highest value of hardness was produced at the interface zone, approximately 230–270 HV, because of the hard and brittle Al–Mg IMCs formation. Moreover, the micro-hardness increased with increasing the GTAW current. This result was due to a large amount of brittle Al–Mg IMCs formed with increasing temperature. 3.5. Lap shear test Fig. 10 shows the lap shear test results obtained from dissimilar Al–Mg welds as a function of GTAW current. The tensile shear strength of the joint increased with GTAW current until the maximum strength was reached and then decreased dramatically with higher GTAW current. The maximum lap shear strength was 1 kN at a GTAW current of 30 A, with approximately 40% of AZ31B Mg alloy base metal. The joining of 1 mm thick Al and Mg sheets via conventional ultrasonic seam welding was not achieved because of the low welding energy of ultrasonic power. The low welding energy of ultrasonic power resulted in a low micro-bond density.
Typical macroscopic images of the failed lap shear tensile samples are shown in Fig. 11(a). All the failure of joints occurred by interface fracture mode rather than the pull-out mode, even though the maximum lap shear strength could reach 1 kN (approximately 40% of AZ31B Mg alloy base metal). SEM micrographs fracture surface of the joints made at GTAW currents of 20 and 30 A on the Mg side are shown in Fig. 11(b) and (c), respectively. The fracture patterns exhibited brittle fracture mode with a cleavage facet feature. At a low GTAW current of 20 A, microbonded bright spots were observed at the periphery of the sonotrode tip footprint. The bright spots were parallel to the direction of ultrasonic vibration. The micro-bonds increased in density and spread across the faying surface and coalesced with each other when the GTAW current reached 30 A. To ascertain this phenomenon, EDS box analysis was performed on the fracture surface of the Mg side, as shown in Table 2. The EDS results showed that 42% Al atom was present at a GTAW current of 30 A, which was higher than 31% Al atom at a GTAW current of 20 A. This result was attributed to the extreme diffusion of Al atom and plastic deformation at higher temperatures (or higher GTAW currents). 3.7. Weld defects Based on the above analysis, a good ultrasonic weld should have dense interfacial bonds without discrete gaps and extreme deformation. At lower values of GTAW current, a substantial density of gaps or unbonded areas at the weld interface is shown in Fig. 12(a). This result was due to lower temperature that enables sufficient plastic flow the interface. Only part of interface was effectively bonded. However, at higher GTAW currents, the ‘‘tool sticking’’ issue occurred frequently because the friction between the two sheets was larger than the friction between the sonotrode and top sheet or between the anvil and lower sheet. The motion at the interface stopped and the work pieces would either slide over the anvil, or the sonotrode loose traction on the top part. Therefore, the sheets could be welded to the sonotrode or the anvil as shown in Fig. 12(b). 4. Discussion As described above, direct welding of 1 mm thick MgAZ31B/ Al6061 dissimilar alloys via GTAW-assisted hybrid ultrasonic seam welding can be performed if the proper value of GTAW current was selected. The conventional ultrasonic seam welding can only be used to join metal foils or thin plate specimens (generally <0.5 mm in thickness) because of the limitations in the welding energy of the ultrasonic power. The ultrasonic power (P) is dependent on the sheet hardness (H) and sheet thickness (d) and can be expressed by the following equation [26]:
P ¼ kH
Fig. 10. Lap tensile shear strength.
1:5 1:5
d
ð1Þ
where k is the proportion constant, H is sheet hardness, and d is the sheet thickness. The ultrasonic power increases exponentially with increasing sheet hardness and thickness. However, the high ultrasonic power welding systems results in high cost and inconvenient
70
X. Dai et al. / Materials and Design 77 (2015) 65–71
Fig. 11. Typical fractured welded joints: (a) macroscopic images of failed lap shear tensile samples; SEM images of the fractured surface on Mg side at GTAW of (b) 20 A and (c) 30 A, and EDS box analysis in the magnified fracture surface area.
Table 2 Chemical compositions based on the EDS results (at.%). Spot
Mg
Al
B2 B4
69 58
31 42
manipulation of the ultrasonic equipment. In the present work, gas tungsten arc welding (GTAW) is initially introduced into ultrasonic seam welding. The preceding GTAW can preheat the sheet metal to reduce the specimen hardness and enhance the specimen plasticity
and acoustic softening effect. The transfer of the welding energy is more efficient. Therefore, the direct joining of 1 mm thick MgAZ31B and Al6061 alloy sheets can be obtained without improving the ultrasonic power. To analyze the mechanism of GTAW-assisted hybrid ultrasonic seam welding, a physical model is established in Fig. 13. The formation process of the joint can be considered to occur in sequence as follows: (1) Micro-bond formation: When static pressure and tangential force are applied using the rotating sonotrode (in Fig. 13(a)), the faying surface come into close contact and
Fig. 12. Weld defects: (a) unbonded areas; (b) tool sticking.
X. Dai et al. / Materials and Design 77 (2015) 65–71
71
Fig. 13. Weld formation model: (a) real contact; (b) mirco-bond formation; (c) GTAW current of 20 A; (d) GTAW current of 30 A.
the local surface oxide layer between contacting asperities began to break down. At these clean contacts, metallic adhesion can take place. The initial adhesion spots are much smaller than the micro-bonds and then grew to form micro-bonds under the action of surrounding material plastic deformation (in Fig. 13(b)). The preceding GTAW can preheat the sheet specimens to decrease the hardness of the sheet specimen. Therefore, the area of real contact increased with increasing GTAW current. (2) Weld formation: With the formation of the micro-bonds and sufficient plastic deformation, inter-diffusion can take place in the vicinity of the micro-bonds areas. The number and length of micro-bonds generated per unit unwelded contact area are proportional to the GTAW current because the preceding GTAW can preheat the sheet specimens to improve material plasticity. At a low GTAW current of 20 A, the fractured surface as shown in Fig. 11 indicates that the microbonded bright spots have low density and parallel to the vibration direction. When the GTAW current increases to 30 A, the micro-bonds increases in density and spread across the faying surface to coalesce with each other. Therefore, the tensile shear strength of the joint increased with GTAW current until a maximum strength of 1 kN was reached. 5. Conclusions Direct joining of 1 mm-thick MgAZ31B and Al6061 alloy sheets via GTAW-assisted hybrid ultrasonic seam welding was successfully achieved without improving the ultrasonic power. The results could be summarized as follows: (1) The preceding GTAW was introduced into ultrasonic seam welding to preheat the sheet metal to enhance weldability. The specimen hardness reduced, the material plasticity and acoustic softening effect were enhanced. (2) The higher GTAW current resulted in a higher temperature at the center of the weld interface. At a low GTAW current, plastic deformation was localized at the interface and the weld join-line remained macroscopically flat. When the GTAW current was increased above 20 A, the weld join-line revealed a wave-like or swirl-like appearance because of the heavy plastic deformation. At a higher GTAW of 35 A, the extreme reactions between Al and Mg atoms to form a large amount of brittle Al–Mg IMCs, which deteriorated the joint strength. (3) The maximum lap shear strength was 1 kN at GTAW current of 30 A and approximately 40% of AZ31B Mg alloy base metal. All instances of joint failure occurred by the interface fracture mode rather than the pull-out mode, and the fracture patterns exhibited brittle fracture mode with a cleavage facet feature.
References [1] Liming Liu, Daxin Ren, Fei Liu, A review of dissimilar welding techniques for magnesium alloys to aluminum alloys, Materials 7 (2014) 3735–3757. [2] H.T. Zhang, X.Y. Dai, J.C. Feng, Joining of aluminum and magnesium via preroll-assisted A-TIG welding with Zn interlayer, Mater. Lett. 122 (2014) 49–51. [3] H.T. Zhang, J.Q. Song, Microstructural evolution of aluminum/magnesium lap joints welded using MIG process with zinc foil as an interlayer, Mater. Lett. 65 (2011) 3292–3294. [4] E. Schubert, M. Klassen, I. Zerner, C. Walz, G. Sepold, Light-weight structures produced by laser beam joining for future applications in automobile and aerospace industry, J. Mater. Process. Technol. 115 (2001) 2–8. [5] H.T. Zhang, X.Y. Dai, J.C. Feng, Interfacial microstructure and mechanical properties of Al/Mg butt joins made by MIG welding process with Zn–Cd alloy as interlayer, J. Wuhan Univ. Technol. 29 (2014) 1258–1264. [6] C. Doumanidis, Y. Gao, Mechanical modeling of ultrasonic welding, Weld. J. (2004) 140–146. [7] J.E. Gould, Joining aluminum sheet in the automotive industry – a 30 year history, Weld. J. (2012) 23–34. [8] Shinichi Matsuoka, Hisashi Imai, Direct welding of different metals used ultrasonic vibration, J. Mater. Process. Technol. 209 (2009) 954–960. [9] S. Elangovan, S. Semeer, K. Prakasan, Temperature and stress distribution in ultrasonic metal welding – an FEA-based study, J. Mater. Process. Technol. 209 (2009) 1143–1150. [10] D. Bakavos, P.B. Prangnell, Mechanisms of joint and microstructure formation in high power ultrasonic spot welding 6111 aluminium automotive sheet, Mater. Sci. Eng., A 527 (2010) 6320–6334. [11] V.K. Patel, S.D. Bhole, D.L. Chen, Influence of ultrasonic spot welding on microstructure in a magnesium alloy, Scr. Mater. 65 (2011) 911–914. [12] A. Panteli, Y.C. Chen, D. Strong, X.Y. Zhang, P.B. Prangnell, Optimization of aluminium to magnesium ultrasonic spot welding, JOM 64 (2012) 414–420. [13] V.K. Patel, S.D. Bhole, D.L. Chen, Formation of zinc interlayer texture during dissimilar ultrasonic spot welding of magnesium and high strength low alloy steel, Mater. Des. 45 (2013) 236–240. [14] V.K. Patel, S.D. Bhole, D.L. Chen, Fatigue life estimation of ultrasonic spot welded Mg alloy joints, Mater. Des. 62 (2014) 124–132. [15] M. Shakil, N.H. Tariq, M. Ahmad, M.A. Choudhary, J.I. Akhter, Effect of ultrasonic welding parameters on microstructure and mechanical properties of dissimilar joints, Mater. Des. 55 (2014) 263–273. [16] J. Tsujino, T. Ueoka, T. Kashino, F. Sugahara, Transverse and torsional complex vibration systems for ultrasonic seam welding of metal plates, Ultrasonics 38 (2000) 67–71. [17] C.Y. Zhang, D.L. Chen, A.A. Luo, Joining 5754 automotive aluminum alloy 2mm-thick sheets using ultrasonic spot welding, Weld. J. (2014) 131–138. [18] J. Tsujino, T. Ueoka, Welding characteristics of ultrasonic seam welding system using a complex vibration circular disk welding tip, Jpn. J. Appl. Phys. 39 (2000) 2990–2994. [19] V.C. Tiago, E.N.B. Carlos, Ultrasound in arc welding: a review, Ultrasonics 56 (2015) 201–209. [20] L.B. He, L.M. Li, H.W. Hao, M.S. Wu, R.L. Zhou, Grain refinement and high performance of titanium alloy joint using arc-ultrasonic gas tungsten arc welding, Sci. Technol. Weld. Join. 11 (2006) 72–74. [21] X.Z. Chen, Z. Shen, J. Chen, Y.C. Lei, Q.Y. Huang, Effect of GTA-ultrasonic excitated-voltage on microstructure and impact toughness of CLAMs weldments, J. Jiangsu Univ. (Nat. Sci. Ed.) 32 (2011) 724–727. [22] V.K. Patel, S.D. Bhole, D.L. Chen, Microstructure and mechanical properties of dissimilar welded Mg–Al joints by ultrasonic spot welding technique, Sci. Technol. Weld. Join. 17 (2012) 202–206. [23] J.W. Yang, B. Cao, X.C. He, H.S. Luo, Microstructure evolution and mechanical properties of Cu–Al joints by ultrasonic welding, Sci. Technol. Weld. Join. 19 (2014) 500–504. [24] A. Panteli, J.D. Robson, I. Brough, P.B. Prangnell, The effect of high strain rate deformation on intermetallic reaction during ultrasonic welding aluminium to magnesium, Mater. Sci. Eng., A – Struct. 556 (2012) 31–42. [25] R. Jahn, R. Cooper, D. Wilkosz, The effect of anvil geometry and welding energy on microstructures in ultrasonic spot welds of AA6111-T4, Metall. Mater. Trans. A 38 (2007) 570–583. [26] J.L. Pan, Welding Handbook, China Machine Press, Beijing, 1995.