Ultrasonic spot welding of Al/Mg/Al tri-layered clad sheets
Introduction
Lightweighting has been regarded as a key strategy in the automotive and aerospace industries to improve fuel efficiency and reduce anthropogenic environment-damaging, climate-changing, human death-causing1 and costly emissions [1], [2], [3], [4], [5], [6]. It has been reported that the fuel efficiency of passenger vehicles can be enhanced by 6–8% for each 10% reduction in weight [7]. Magnesium (Mg) alloy, as the lightest structural metallic material with a density of ∼30% less than aluminum and one fourth of steel, has been increasingly used in the transportation industry to reduce the weight of motor vehicles [1], [2], [8], [9], [10], [11], [12]. However, the concerns about poor corrosion resistance and low room-temperature formability of Mg alloys limit a widespread structural application in transportation industry [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Recently, roll cladding has been identified as a promising technique to improve the corrosion resistance and formability of Mg alloys [24], [25], [26], [27], [28], [29], [30], [31]. In particular, Al-clad Mg alloy sheet can combine the corrosion resistance and formability of an Al alloy with the high strength-to-weight ratio of Mg substrate. Several studies have shown the successful cladding of Al on Mg alloy sheet using hot and cold rolling, which resulted in good surface corrosion resistance and improved formability [24], [25], [32].
The structural application of these Al-clad Mg alloy sheets inevitably involves welding and joining during manufacturing [33], [34], [35], [36], [37]. Due to the challenges of fusion welding in joining Mg alloys, solid-state welding techniques such as friction stir welding (FSW), linear friction welding (LFW), friction stir spot welding (FSSW), and ultrasonic spot welding (USW) are gaining momentum due to their potential of obtaining superior joint properties compared with the fusion welding techniques [33], [34], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]. USW, involving an ultrasonic high-frequency shear vibration to generate localized heat to soften the material at the weld interface, is an emerging and promising technique in especially joining lightweight magnesium and aluminum alloy sheets compared with the conventional resistance spot welding and FSSW due to its low energy consumption and higher efficiency [47], [48], [49], [50], [51], [52], [53], [54].
While several studies on the process optimization and mechanical characterization of rolled Al-clad Mg alloy sheets have been reported [24], [25], [32], to the authors’ knowledge, no report on the USW of such clad sheets has been seen in the literature so far. During USW, intense sliding motion along the weld interface by the vibration of welding tips generates heat due to friction and plastic deformation, resulting in adhesion and forming micro-welds which increase in density and spread over the affected area. It is well understood to have a sufficient friction to achieve good bonding at the interface. However, this friction force at the weld interface may generate shear stresses at the pre-existing Al/Mg clad interface and may decrease its strength. It is unknown if it is feasible to join the Al-clad Mg alloy sheets using USW, and whether the frictional heat at the weld interface would cause inter-diffusion at the Al/Mg clad interface and subsequently generate brittle intermetallics such as Al12Mg17 and Al3Mg2. Also, it is unclear how the generated frictional heat and subsequent softening will affect the sonotrode tip penetration in the clad layer. A deeper penetration may re-expose Mg alloy substrate and therefore increase corrosion susceptibility. Furthermore, it is unclear how failure would occur during the lap shear tensile tests in relation to the welding energy. The purpose of this study was, therefore, to identify the optimum USW parameters and failure mode, so as to achieve the optimal joint strength without damaging protective clad surface layer.
Section snippets
Materials and experimental procedure
The materials used in the present study were 0.99 mm thick Al 1060/Mg alloy/Al 1060 tri-layer clad sheets fabricated by a combined hot and cold rolling process. The composition of Al 1060 and rare-earth element containing Mg alloy were listed in Table 1. The details of clad rolling process have been presented in [24]. Prior to welding, the surfaces of the clad sheets were slightly ground using 120 grit sand paper to remove surface oxides, then cleaned with acetone followed by an ultrasonic
Microstructure
Fig. 2a–f shows typical SEM micrographs of cross-sections of ultrasonic spot welded (USWed) Al/Mg/Al tri-layer clad sheets with increasing welding energy from 25 J to 150 J. At a low energy of 25 J (Fig. 2a), a weld line could be seen as indicated by arrows at the Al/Al weld interface. This indicates that the temperature at the interface has not yet risen sufficiently, giving rise to inadequate bonding, which could be seen later from the fracture surface as well. As the welding energy increased to
Conclusions
- 1.
The Al/Mg/Al tri-layered clad sheets were successfully welded using solid-state USW technique. It was observed that only a very low welding energy of 100 J and a very short time of 0.1 s were required to achieve the optimal welding condition during USW at a low clamping pressure of 0.4 MPa for the tri-layered clad sheets due to the thin and soft nature of Al 1060 clad layer which acted as an interlayer.
- 2.
With increasing welding energy the lap shear failure load initially increased, reached a maximum
Acknowledgements
The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC), Premier’s Research Excellence Award (PREA), NSERC-Discovery Accelerator Supplement (DAS) Award, Automotive Partnership Canada, AUTO21 Network of Centers of Excellence, and Ryerson Research Chair (RRC) program for providing financial support. The authors also thank Ministry of Science and Technology of China (2011DFR50950-05, 2014DFG52810) and Chongqing Science and Technology Commission,
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