13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- indentation load of 10 g. To evaluate the mechanical strength of the welded joints and establish the optimal welding conditions, tensile shear tests of the welds were conducted to measure the lap-shear failure load using a fully computerized United testing machine at a constant crosshead speed of 1 mm/min at room temperature in air. In the tensile lap shear testing, restraining shims or spacers were used to minimize the rotation of the joints and maintain the shear loading as long as possible. X-ray diffraction (XRD) was carried out on both matching fracture surfaces of Mg-Al and Mg-HSLA steel sides after tensile shear tests, using CuKα radiation at 45 kV and 40 mA. The diffraction angle (2θ) at which the X-rays hit the samples varied from 20° to 100° with a step size of 0.05° and 2 s in each step. 3. Results and Discussion 3.1 Microstructural evaluation Microstructural characterization was conducted across the weld line of the samples. Fig. 1(a) and (b) show microstructures at the center of weld nugget of USWed Mg/Al and Mg/HSLA steel joints without a Sn interlayer, respectively. Sound joints were obtained since no large defects were present, such as crack or tunnel type of defects. It is seen from Fig. 1(a) that there was a heterogeneously distributed IMC layer between the Mg and Al alloy sheets. In our previous study [6] of USW of Mg/Al alloys without Sn interlayer, the non-uniform IMC layer had a solidified microstructure containing the brittle phase through the eutectic reaction, liquid→Al12Mg17+Mg. In the USWed Mg/HSLA steel joint, as there was no interaction between Mg and Fe, the interface of AZ31B-H24 and HSLA steel was clear without transitional zone, as shown in Fig. 1(b). Due to a large difference of hardness, the sections of Mg alloy and steel were not in the same level in the process of metallographic sample preparation, indicating by white arrows where some hydroxides produced, which will be confirmed by EDS analysis later. Fig. 1(c) and (d) show the welded joints of Mg/Al and Mg/HSLA steel with a Sn interlayer, which could be clearly seen. However, this interlayer was no longer pure Sn interlayer after USW. It became a layer of Sn-Mg2Sn eutectic structure, which will be identified in the following sections. Figure 1. Microstructure of the dissimilar USWed joints made with a welding energy of 1000 J, (a) Mg/Al and (b) Mg/HSLA steel without a Sn interlayer, and (c) Mg/Al and (d) Mg/HSLA steel with a Sn interlayer. 3.2 Energy-dispersive X-ray spectroscopy (EDS) analysis Fig. 2(a) and (b) show the SEM image at the center of the nugget zone (NZ) of USWed Mg/Al with a Sn interlayer, and its EDS line scan, respectively. The chemical composition (in at.%) at points A and B was 64.4% Mg - 36.4% Al - 1.2% Sn and 63.5% Mg - 21.8% Al - 14.7% Sn, respectively, which suggests that the dark area (A) had less Sn than the white area (B). Fig. 2(c) and (d) show the SEM image at the center of the NZ of USWed Mg/HSLA steel with a Sn interlayer, and its EDS line scan, respectively. The chemical compositions (in at.%) at points C (Fig. 2(c)) was 70.3% Mg-29.7% Sn, suggesting that only Mg and Sn elements were present in the interlayer. The Al5754-O Al12Mg17 AZ31B-H24 (a) (b) AZ31B-H24 HSLA steel HSLA steel AZ31B-H24 Al5754-O AZ31B-H24 Eutectic Sn-Mg2Sn layer Eutectic Sn-Mg2Sn layer (c) (d)
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