13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- chemical composition (in at.%) at point D was 62.3% Mg-37.7% O, which suggested the presence of galvanic corrosion product by forming magnesium hydroxide of Mg(OH)2 during the metallographic sample preparation [33,34]. The occurrence of galvanic corrosion was attributed to the large difference between Mg and Fe positioned in the galvanic series. In both Fig. 2(a) and (c), the IMC layer displayed a composite-like eutectic structure at the center of the weld nugget, where the Sn-containing fine white particles were distributed homogeneously or as a network in the interlayer. EDS line scan revealed that the intensity of Al was lower than that of Mg in the NZ of the USWed Mg/Al joint (Fig. 2(b)), and little or no Fe present in the NZ of USWed Mg/HSLA steel (Fig. 2(d)). This was due to the higher solubility of Sn in Mg than Sn in Al and Sn in Fe. Therefore, these results (Fig. 2(d)) in conjunction with the Mg-Sn phase diagram [29] suggested the presence of Mg2Sn phase, where the eutectic structure consisting of β-Sn (or Mg-Sn solid solution) and Mg2Sn, which would occur at a temperature of as low as 203°C [29]. Figure 2. (a) SEM micrograph at the center of NZ of USWed Mg/Al joint and (b) EDS line scan across the interlayer in (a), (c) SEM micrograph at the center of NZ of USWed Mg/HSLA steel joint and (d) EDS line scan across the interlayer in (c) made at a welding energy of 1000 J. In the USW, the simultaneous application of localized high-frequency vibratory energy and moderate clamping force leads to a fast relative motion and rubbing/friction heat at the interfaces [7,8] between Al-Sn (Mg/Al joint) or Fe-Sn (Mg/HSLA steel joint) and Mg-Sn (in both types of Eutectic Sn-Mg2Sn layer C D (b) (a) (d) B Al5754-O AZ31B-H24 Eutectic Sn-Mg2Sn layer A HSLA steel AZ31B-H24 (c)
RkJQdWJsaXNoZXIy MjM0NDE=