13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- normal FSSW leaves an exit hole after welding [11]. From the point of view of energy consumption, USW is far more advantageous. For example, welding of aluminum alloys using a USW process consumes only about 0.3 kWh per 1000 joints [4,12], compared to 20 kWh with RSW, and 2 kWh with FSSW [4]. Our previous studies [13] and other investigations [14-17] showed that in the joining of dissimilar Mg-to-Al alloys, the formation of IMCs of Al12Mg17 and Al3Mg2 seems to be unavoidable. Since the mechanical properties of the welded joints are closely related to the formation of the brittle intermetallic layer [18], it is difficult to obtain a strong joint between Mg and Al alloys. In the study of dissimilar Mg-to-steel joint, Santella et al. [19] and Schneider et al. [20] reported that Mg and steel do not react with each other and the joint could be easily broken by hand. To improve the mechanical properties of the Mg-to-Al and Mg-to-HSLA steel joints, Chowdhury et al. [21] (FSSW) and Xu et al. [22] (RSW) have tried to weld Mg-to-Al and Mg-to-HSLA steel joint, respectively, using adhesive placed in-between the faying surface. However, the application of adhesive is a time consuming process. Some researchers have used Zn as an interlayer between Mg and Al alloys [23,24] and Mg and HSLA steel [19,25] for improving the mechanical properties of the dissimilar joints. Others, e.g., Liu et al. [26] and Qi and Liu [27] in the tungsten inert gas (TIG) and hybrid laser-TIG welding of Mg-to-Al alloys, respectively, and Liu et al. [28] in the hybrid laser-TIG welding of Mg-to-steel, have used Sn as an interlayer and also showed the improvement of the mechanical properties. However, it is unclear how Sn interlayer would affect the microstructure of USWed Mg-to-Al and Mg-to-HSLA steel joints, and if the intermetallic layer would form, and whether Sn interlayer would improve the mechanical properties of the joints. This study was, therefore, aimed to identify the effect of the Sn interlayer on the microstructure and lap shear tensile properties of USWed AZ31B-H24-to-Al5754-O and to-HSLA steel. The selection of Sn in the present study was also based on Mg-Sn, Al-Sn and Fe-Sn binary phase diagrams [29-31], which showed that Sn may interact with Mg and generated IMCs, while Sn might be dissolved into Al and Fe to form solid solution of Sn-Al and Sn-Fe. Furthermore, it was selected on the basis of the findings that Sn improved the wettability of Mg, Al and Fe during the welding process [26,28] and also refined the grain size in the Mg alloy [28,32]. 2. Material and Experimental Procedure In the present study, commercial 2 mm thick sheet of AZ31B-H24 Mg alloy (composition in wt.%: 3Al, 1Zn, 0.6Mn, 0.005Ni, 0.005Fe and balance Mg), 1.5 mm thick sheet of Al5754-O Al alloy (3.42Mg, 0.63Mn, 0.23Sc, 0.22Zr, and balance Al), and 0.8 mm thick sheet of high strength low alloy (HSLA) steel (0.06C, 0.227Si, 0.624Mn, 0.006P, 0.004S, 0.013Ni, 0.041Cr, 0.005Mo, 0.044Cu, 0.039Al, 0.003Ti, 0.021Nb and balance Fe) were selected for the USW. The specimens of 80 mm long and 15 mm wide were sheared, with the faying surfaces ground using 120 emery papers, and then washed using acetone followed by the ethanol and dried before welding. During welding a 50 μm thick pure Sn interlayer was placed in-between the work pieces of Mg/Al and Mg/HSLA steel samples. The welding was conducted with a dual wedge-reed Sonobond-MH2016 HP-USW system. The samples were welded at an energy input ranging from 500 to 2500 J at a constant power setting of 2000 W, an impedance setting of 8 on the machine, and a pressure of 0.414 MPa. Four samples were welded in each welding condition. Two of them were used for microstructural examination and microhardness tests, and the other two were used for the lap shear tensile tests. Cross-sectional samples for scanning electron microscopy (SEM) were polished using diamond paste and MasterPrep. A computerized Buehler microhardness testing machine was used for the micro-indentation hardness tests diagonally across the welded joints using a load of 100 g for 15 s except for the thin IMC interlayer, where a load of 10 g was used for 15 s. The mean value of three indentations along the IMC interlayer was taken for a better accuracy with the low
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