SEM Interface Observation

Figure 11a,b present the SEM observation on joint type B. Two white lines were graphically added inside the photos to create better perception regarding the interfacial region. The total thickness of Cu deposited layer before USW was only about 2.5 μm, but the affected interfacial region is much wider than the initial thickness of the Cu coating interlayer and it has an average width of around 9 μm, i.e., near four times larger than the initial thickness of the Cu coating. This is evidence of diffusion during the eutectic reaction.

This region can be seen in more detail at higher magnification, as in Figure 11b. The main reaction products are visible as discrete white particles, instead of a continuous layer. Apart from the composition of these particles, the dispersion of such hard IMC products inside a soft matrix of AZ31B base metal performs a composite like structure at the interface that can participate in the strengthening of the welded joint. Previously, in Section 3.1 it was reported that failure occurred in the pull out mode from the nugget edge walls. It means that crack initiation was not introduced from the flat region interface and it is a sign of strong sound joint.

The occurrence observed in Figure 11b specifies that reaction products were found in farther distances, like diffusion in base metal. This can be stimulated under the effect of enhanced temperature and ultrasonic vibrations. As it was noticed before, the interface temperature rises to over *Teu* of Mg2Cu, which predicts the presence of a liquid phase that is able to diffuse through material imperfections, like grain boundaries. The EBSD results in next section will show this matter in more detail.

**Figure 11.** Interface SEM observation of joint type B (1100 J), (**a**) low magnification; (**b**) high magnification.

Interdiffusion of Mg and Cu in steady state conditions were studied before and discussed briefly in the introduction, but the lack of consensus in diffusion data makes diffusion behavior difficult to interpret. For example, activation energy data for the diffusion of Cu in Mg or vice versa are not compatible in different studies. Nonaka et al. [13] studied the reaction diffusion in Mg-Cu diffusion couple between 410–475 ◦C and observed that Kirkendall markers shifted towards the Cu-rich side, so they suggested that the diffusion of Cu is faster than Mg and they measured the activation energy of Mg2Cu as 156 kJ/mole. Additionally, the growth rate of Mg2Cu was much larger than MgCu2. Other research by Dai et al. [14] in nearly the same conditions (400–460 ◦C) similarly found that, at all examined temperatures, two intermetallic layers formed between Mg and Cu, including the Mg2Cu layer close to Mg side and MgCu2 in contact with Cu side. The Mg2Cu layer (with almost no solubility) was much thicker than MgCu2. They also measured the activation energies for both IMCs that resulted in a lower value for Mg2Cu (139 kJ/mole) when compared to MgCu2 (147 kJ/mole). As it is clear, their data were a little different from reference [13]. The other important information that was obtained in reference [14] was related to activation energies for diffusion of Mg in Cu (139 kJ/mole) and Cu in Mg (164 kJ/mole), which imply different interdiffusion behavior when compared to reference [13].

However it is not the objective of current study to derive the diffusion data and, as was mentioned before, the USW process is more complicated than the original diffusion couples in steady state static conditions, due to high strain rates and vibrations in short time (less than 1 s for USW when compared to 24–72 h in reaction diffusion), but some findings, such as formation priority and layer thickness of Mg2Cu, are in relative concurrency.

The formation of these two reaction products is based on a simple Mg-Cu system, while, in the presence of major alloying elements of AZ31B, the synthesis of other by-products also is expectable. For example, the Al content in AZ31B Mg alloy can contribute to form non-stoichiometric binary or ternary IMC products, together with Cu.
