3.3.1. Light Microscopy Observation

Initial microstructure of base metal is depicted in Figure 8a, where it is related to the plate thickness intersection. Figure 8b depicts a schematic of raw material plate and observation surface for a better understanding. The microstructure of the as-rolled Mg base metal shown in Figure 8a included large elongated grains inside the small equiaxed ones, with a mean grain size of 3.5 μm. The deposited Cu interlayer had a random thickness instead of a uniform layer, as the average thickness of deposited Cu layer was measured to be 2.5 ±1.0 μm.

**Figure 8.** Light microscopy observation of AZ31 base metal with Cu coating, before USW (**a**) microstructure with Cu coating; (**b**) schematic of observed surface; (RD: Rolling direction).

Light microscopy evaluation on USW samples has been completed for two types of samples (A and B), but it was limited to strongest sample among all the samples in each type. The microstructure after ultrasonic welding is presented in Figure 9a,b for two joint types A and B, respectively.

**Figure 9.** Light microscopy microstructure of interface flat region: (**a**) joint type A (1400 J); (**b**) joint type B (1100 J).

Firstly, through light microscopy microstructure for joint type B (Figure 9b) and comparing it with type A (Figure 9a), it was found that the use of copper coating resulted in a joint without discontinuity along the interface flat region. The observed interface showed a smooth flat interface. Some parts of the interface joined in bare contact between two Mg plates and it seems that the thin layer of Cu coating was scratched and removed due to scrubbing under high frequency ultrasonic vibrations and severe friction. As it was mentioned, the deposited Cu layer was a non-uniform coating layer and some parts were very thin when compared to total Cu layer (look at Figure 8a).

Afterwards, a comparison between microstructures before and after ultrasonic welding revealed amounts of grain growth and changing grain structure morphology. Grain size measurement for welded samples base metal was limited to nugget flat region inside a rectangular area of ±200 μm parallel to the interface centerline (except centerline and IMC layer). Morphology after ultrasonic welding for both joint types changed to larger grains with equiaxed structure, as grains were grown in the scale of about three times larger than initial base metal grain size. Figure 10 graphically summarizes the results.

**Figure 10.** Comparison of base metal mean grain size before and after HP-USW for joint type A (1400 J) and joint type B (1100 J).

In joint type A, the mean grain size around the interface measured to be 11.5 μm ± 0.3 in the average (11.2 μm for the top plate and 11.8 μm for the bottom plate). It means that grain growth for this joint was in the scale of 3.3 times compared to initial base metal. For the joint type B, mean grain size around the interface was measured to be about 10.7 μm (10.2 μm for the top plate and 11.2 μm for the bottom plate). In the average, the mean grain size around the interface was about 7% smaller than the non-interlayered joint. This value also shows grain growth after ultrasonic welding that was about three times larger than initial base metal.

Grain growth after USW, has been frequently reported in previous researches in this field. For example, Macwan et al. [26] used USW for welding a rare-earth containing ZEK100 magnesium alloy in different USW energies 500–2000 J and observed that the average grain size in the NZ (nugget zone) increased from 7.6 to 13.6 μm, while grains that were close to the weld interface were relatively coarser. The main effective parameter on grain growth is the enhanced temperature of joint interface due to friction. Accordingly, grain growth due to temperature rising can lead to soften the material, together with the fact that magnesium and its alloys have poor ductility and formability at room temperature, which can improve in elevated temperatures. Poor ductility was mainly related to their hexagonal closest packed (HCP) crystal structure and a lack of enough slip systems in low temperature due to highly anisotropic dislocation slip behavior. According to critical resolved shear stress (CRSS) analysis, only two independent slip systems (related to basal plane) are active in room temperature, while, based on Taylor/Von Mises criterion, at least five independent slip systems are needed for uniformly deformation without failure. As shown in Figure 7, the interface temperature increased to high levels around the melting point of AZ31B. At elevated temperatures, the formability of magnesium improves, owing to the activation of other non-basal slip systems, like secondary (c+a) pyramidal slip systems and easier movement and dislocation climbing [2,32,33]. Moreover, the process time in USW is very short and mentioning the role of high strain rates governing in USW that has an important effect on phenomena to occur in dynamic conditions is needed. Previously, other researchers discussed the role of dynamic recovery (DRV) and dynamic recrystallization (DRX) occurring in USW [9,27,34–37]. High strain rates in USW were reported to be in the scale of 10<sup>3</sup> s−<sup>1</sup> [38]. Under high strain rate deformation, DRX occurs that leads to new equiaxed grain structure, followed by DRV under the effect of elevated temperature that affects on grain size to enlarging. Larger grains decrease the hardness of base metal that lead to softening the material. As vibrations periodically produce, this process can occur several times during the short cycle of the process. Material softening facilitates plastic deformation due to softer structure additionally to the activation of more slip systems in elevated temperature, as previously mentioned. Bakavos et al. described this procedure in more detail [9]. To ensure the effect of grain coarsening on softening the material and tracking the effect of IMC formation on interface mechanical properties, microhardness evaluations were done with a focus on the flat region, which presented in the next section.

## 3.3.2. Microhardness Evaluation

Hardness data after HP-USW obtained from corresponding area of flat region wherein the metallography grain size measurement was performed. Measurements verified the softening of weld base metal after USW, which is attributed to recovery in elevated temperature. The hardness of base metal before USW was 75 HV/10 gf/15 s (average of 10 points), while after the HP-USW of joint type A, it was decreased to 59.9 HV, shows about a 20% reduction in hardness. For joint type B, the average microhardness of base metal that was measured in nearly the same area as joint type A, out of IMC layer, was around 66.7 HV, which shows an 11% reduction in hardness when compared to the as-received material. According to grain size measurements, this evidence is reasonable, as that *Tmax* for joint B was 120 ◦C lower than joint type A, hence the extent of softening is lower. Moreover, according to mean grain size evaluations in the previous section, the grain size of joint type B was 7% smaller than joint type A, which is in agreement with the hardness data achieved.

Hardness evaluation has also been performed on the IMC layer of joint type B, but it cannot assure real data, because the size of indentation effect was larger than the width of the IMC layer. Therefore, the acquired data is an average of the analyzed region, including an IMC layer and base metal. Accordingly, IMC containing region showed a clear increase in the hardness, as its average hardness was 105 HV/10 gf/15 s. This value is 40% more than raw material hardness. The hardness data reported by other researchers on thick IMC layer between Mg and Cu also confirmed an increase in interface hardness. For example, Macwan et.al. reported a sharp increase in hardness at the interface of ultrasonically welded AZ31 to Cu in the scale of 278 HV as related to the Mg2Cu intermetallic compound [15]. By the way, the high hardness of interfacial region is evidence of a eutectic reaction and the formation of IMC products.

#### 3.3.3. Electron Microscopy Analysis

Magnesium etchant that used in metallography trials cannot simultaneously attack pure Cu, and, if some parts of Cu remained unreacted after USW, it is distinctly recognizable with a red tint in light microscopy photos, as that visible in Figure 8a. Meanwhile, interfacial layer in joint type B has a black and white appearance, which affirms the complete reaction of Cu and the formation of IMC products, alongside microhardness findings. Accordingly, for the better interface characterization of joint type B, more precise analysis methods based on electron microscopy are needed.
