**2. Materials and Methods**

Commercial AZ31B rolled plates of magnesium alloy (3%Al, 1%Zn in wt%) with dimensions of 50 mm in length, 20 mm in width, and thickness of 0.8 mm were used as the initial material. The plates faying surfaces were polished until #2000 by emery paper, and finally cleaned while using acetone prior to USW. Final polishing resulted in a decrease of plate thickness, hence the applicable plate thickness was 0.78 mm. Joint welding carried out using a 2500 W, 20 KHz single wedge-reed USW machine MH2026/CLF2500 that was equipped with weld control monitoring system TDM-10AC (TMEIC/SONOMAC JAPAN Inc. Yokohama, Japan). Figure 1 presents a schematic of this machine. Additionally, a real photo of the sonotrode tip, as shown in Figure 1b (up), which has a circular shape with special design of two surfaces, including: "flat region" and "slope region". The flat region of tip head surface (5 mm in diameter) serrated with a knurling pattern that consisted of 14 × 14 rows of indentations (see a magnified view in Figure 1b; down), which can help to better the friction and plastic deformation on the weldment.

**Figure 1.** (**a**) Schematic of single wedge-reed type ultrasonic spot welding (USW) machine used in current research; (**b**) sonotrode tip head (up: side view; down: Magnified normal view of head surface).

An overlap of 20 mm × 20 mm between two plates was considered in order to perform lap shear tensile test on welded samples and, in order to prevent rotation of plates under effect of ultrasonic vibrations, the plates were kept together prior to welding using slight pressing on two points near the tip affecting area. A schematic representation of the welded sample for lap shear tensile testing and/or microstructure analysis is shown in Figure 2a. Additionally, Figure 2b shows a typical macrography of the welded joint intersection including Cu interlayer.

**Figure 2.** (**a**) Schematic of welded sample; (**b**) typical welded joint intersection (RD: Rolling direction).

Lap shear tensile tests on the welded specimens were performed using 0.5 ton-f, DS-3100 (Showa inc., Tokyo, Japan) desk type tensile machine under 1mm/min. cross head speed. The tensile axis was along the length of plates (i.e., perpendicular to vibration direction). For hardness evaluations, an MVK-H1 microhardness testing machine (AKASHI inc., Yokohama, Japan) used according to ASTM E-384-2011. A hardness measurement for welded samples has been conducted inside the flat region.

From the point of view of using or not using interlayer, two types of USW joints were produced, including:


The high-power of 2450W was selected as the maximum capability of USW machine that was allowed by maker under safe conditions. USW parameters (same as impedance and clamping pressure) were optimized to access the maximum mechanical strength for all series of joints. Since that impedance is a variable that changes under different conditions of weldments, like coatings, oxide layers, or machine related design, it should be optimized under the maker's instructions before experiments. Subsequently, pressure optimization has been done under optimized impedance and a desired medium constant energy (e.g., 1500 J energy for joint type A and 1000 J for joint type B). Finally, the main experiments were performed under the optimized obtained pressure of 0.56 MPa for both joint types. The range of applied energy was selected between 800–1600 J, which covers low to high levels of energy and it has been monitored test by test according to lap shear tensile test results. The welding time and tip penetration depth were dependent variables that changed with change of energy. Ultrasonic vibration direction was along the plate width, perpendicular to rolling direction (RD) (see Figure 2a,b). The welded samples were cut from the width intersection (parallel to the vibration direction) for microstructure evaluations, including light microscopy, scanning electron microscopy (SEM) and electron backscatter diffraction analysis (EBSD) by SU5000 (Hitachi high-Technologies Ltd., Hitachi, Japan), electron probe micro-analyzer (EPMA) JXA8200T (JEOL Ltd., Tokyo, Japan), and transmission electron microscopy (TEM) analysis by JEM-2010 (JEOL Ltd., Tokyo, Japan). TEM sample cutting was performed using low speed diamond saw blades and polishing was continued until 0.5 μm (60000 grit) with abrasive polishing paper and finally dimpled with GATAN 656 and PIPS-GATAN 691 (Gatan Inc., Pleasanton, CA, USA). Oil-based diamond slurry was used for the final polishing of light microscopy and SEM-EBSD samples, to prevent oxidative corrosion. For a revealing microstructure with light microscopy, acetic-picral etchant was used, which is a common etchant that is capable of revealing Mg grain boundaries. Mean grain size measurement has been performed with using the line intercept method. Final polishing was continued with ion milling by HITACHI IM-4000 series (Hitachi high-Technologies Ltd., Hitachi, Japan) for EBSD and EPMA analysis.

Cu interlayer was deposited through the vacuum vapor deposition method. Deposition was applied just on the overlapping contact area (20 mm × 20 mm) of the AZ31B top plate.

A notch was created at the center part of bottom plate overlap surface for temperature measurements, as shown in Figure 3, and a 0.5 mm K-type thermocouple was inserted inside the notch to near the center of the weld nugget surface.

**Figure 3.** Schematic configuration for temperature measurement.

#### **3. Results and Discussion**

#### *3.1. Lap Shear Tensile Test Results*

Figure 4 shows a comparative graph of maximum lap shear tensile force for two types of samples in different energy levels. All of the tests were optimized to use 0.56 MPa clamping pressure. Each data in the curve is the average of three samples.

**Figure 4.** Comparative lap shear tensile force data vs energy, in high-power ultrasonic spot welding (HP-USW) of AZ31B, with and without interlayer (A: without interlayer; B: using Cu coating as interlayer).

The average maximum force that was achieved with joint type A, was 2485 N, while, in the case of joint type B, it was 2477 N, which is comparable with type A, but the used energy to achieve peak point for interlayered joint of type B was 20% lower, being equal to 1100 J. There were some cases that the maximum force data of joint type B was higher than that for joint type A, or the failure occurred in the base metal (out of weld spot).

The presented curves in Figure 4 imply that after a certain amount of applied USW energy, in both series of samples, with increasing USW energy, the curves reached a maximum and higher levels of used energy resulted in a relatively sharp drop in the maximum lap shear tensile force. For a better understanding of this effect, a macrography of failed samples after lap shear tensile testing is shown in Figure 5, which correlates change in failure behavior with USW energy. Figure 5a–d are related to the joint type B samples in energies of 900, 1000, 1100, and 1200 J respectively. Figure 5e that is related to the strongest sample of joint type A (1400 J) is presented to compare with 5c (strongest sample among joint type B).

**Figure 5.** Failed samples after lap shear tensile testing for joint type B at (**a**) 900 J; (**b**) 1000 J; (**c**) 1100 J; (**d**); 1200 J; and, (**e**) joint type A at 1400 J USW energy.

The full nugget pull-out in samples of joint type B occurred when higher USW energies were applied (≥1100 J), while in lower USW energies (<1100 J), weld button was partly pulled out like a mix of pull out and the interfacial shear fracture. Hence, partial pull-out gradually changed to a complete pull-out of weld button, which is correspondent with the data attained in the related graph in Figure 4. With a comparison between the strongest samples of both joint types (Figure 5c,e), it can be found that the Cu-interlayered sample and the sample without interlayer mostly showed the same failure behavior.

The relationship between different parameters of USW is not simple and the main parameters are energy, power, and clamping force (pressure), and other parameters can be assumed as the dependent parameters like welding time or tip penetration depth that change with changing of energy. There is a famous relationship between power, energy, and time, as that energy equals to the multiplication of power and time. However, according to current research results and many other HP-USW studies, it can be found that this relationship is not completely responsible in HP-USW. For example, in the welding conditions of 2450 W and 800 J energy, the recorded welding time was 0.7 s, while, according to the energy-power relationship, it should be around 0.33 s. The other point is the role of clamping force, which is normally used to stablish a continuous intimate contact between two plates during USW. Logically, it can be considered that with an increasing of clamping force, a preventive force is working opposite to frictional force, which can result in the prohibition of easy movement between two joining parts. However, the application of HP-USW results in the need of increased higher clamping forces to keep the weldments together. Liu et al. [23], who studied the vibration amplitude effects on ultrasonic welding of Cu-Al joints, also noticed that interactions between different USW parameters have great influence on the mechanical performance of the joints, however a moderate clamping force is indispensable in generating intimate contact between the welded samples, and high power levels are required for extensive deformation that is associated with high clamping force.

Energy is one of the more attentional parameters in USW. Generally, USW energy can influence final strength in different ways comprising an increase in sonotrode tip penetration depth (or decreasing the joint thickness as a result), expanding the weld nugget surface area, and raising the temperature or heat delivered to the weldment. In lower energies, the produced frictional heat is not enough to bring the interface temperature up to levels that are necessary for completely occurring interfacial phenomena, like diffusion, eutectic reaction, etc. In other words, in lower energy levels, if IMC's formed at the interface during USW, they cannot diffuse and distribute correctly in the base metal and may play a role of imperfection at the interface that attenuates joint strength, because of less energy and inadequate heat input.

On the other side, when the applied energy is more than that needed to achieve a perfect joint (such as energies > 1100 J in joint type B), it can lead to an increase in the maximum interface temperature and extra melting possibly occurs at the interface layer, which encounters the highest rate of temperature rise. Thermochemistry data available for Mg-Cu intermetallic system implies that the formation of both IMC products in this system (Mg2Cu and MgCu2) is exothermic due to the negative enthalpy of formation [24]. It means that generated heat in these exothermic reactions may assist in temperature rising, however in small amounts. Consequently, if significant amounts of liquid phase are present at the interface, the rapid cooling after USW may result in brittle solidified reaction products, especially near the nugget edge walls. Several researchers reported the occurrence of melting during the use of interlayer, especially at the nugget edge walls. In recent research by Peng et al. [25] for dissimilar USW of Mg-ZEK100 to Al-6022 while using a 50 μm Cu interlayer, they reported locally melting at the nugget edges that were attributed to the high stress concentration that caused increase in temperature and lead to molten eutectic reaction products of Mg + Mg2Cu, which were squeezed in nugget edges that resulted in interfacial fracture. The same evidence was observed with Macwan for HP-USW of Mg to Cu, and in high levels of USW energy (1500 J) the diffusion eutectic layer was squeezed out of the nugget edge [15,26]. Some melting at the nugget edge walls were rather visible, as it can be found in a higher energy sample of Figure 5d. The pulled-out weld button has a rather smaller area when compared to 5c, which is evidence of the high energy effect.

In the other hand, higher energy effects on joint thickness and it results in the thinning of the joint, thereupon introducing more stress concentration, leading to crack initiation in the weld nugget. Other researchers have frequently reported this behavior. For example, Peng et al. [27], during HP-USW of Al-6022, achieved the maximum mechanical strength at the intermediate energies of 1200–1400 J and the final strength was decreased by increasing the energy to higher levels, hence they attributed this to crack growth and stress concentration that were caused by deeper tip penetration. The measured average joint thickness for A and B joint types showed that total weldment initial thickness of 1.56 mm before USW (2 mm × 0.78 mm), was decreased to about 0.81 mm for joint A (1400 J energy) after HP-USW, while, in the case of type B, which used the Cu interlayer and made in lower energy of 1100 J, the final joint thickness was 0.95 mm. This can be found from Figure 6a,b in a critical region at the end of interface flat part that continues with the slope region.

**Figure 6.** Comparison between macrographs of joint type A and B in transition region from flat part to slope part of the interface; (**a**) Joint type A (1400 J); (**b**) joint type B (1100 J).

In Figure 6a, which is related to joint type A, the arrow markers are notifying some cracks near the slope region that are introduced due to high energy and deep penetration of the sonotrode tip, resulting in extra thinning of joint thickness, especially in the top plate. Moreover, the region in Figure 6a that is confined within dashed lines shows a transitional region between the flat and slope part, with extreme plastic deformation and flow, which led to discontinuity in nugget edges under the effect of higher energy levels. Opposite to Figure 6b, when using the Cu coating interlayer, the joint thickness improved under lower energy USW, followed with an appearance of low melting point IMC products in a transitional region from flat to the slope part of interface (marked within a dashed

line area). In comparison with Figure 6a, cracks cannot be seen in similar area. However, in this case, severe difference in hardness of the IMC product and the Mg base metal can cause brittleness in weld nugget edge (end of flat region). IMC formation and its effect will be discussed in more detail in next sections, which refer to temperature measurements and hardness evaluations, etc.
