*3.2. Interface Temperature Measurements*

It is important be informed regarding rising temperature level during USW. The main factor affecting maximum process temperature (*Tmax*) is friction between the contacting surfaces. The friction is a result of shear force that is induced by transversely ultrasonic vibrations. As shown in Figure 3, temperature measurements have been conducted through the insertion of an accurate 0.5 mm diameter K-type thermocouple inside a notch at the centerline of joint in bottom plate. Temperature profiles that were obtained for different types of weldment combinations, as shown in Figure 7a and variation of *Tmax* against distance from weld nugget center presented in the graphs of Figure 7b. Measurements were just performed on the strongest joint for each joint type. It means HP-USW under 1400 J for joint type A and 1100 J for joint type B.

**Figure 7.** Interface temperature measurements for joint type A (1400 J) and joint type B (1100 J): (**a**) Comparative temperature profile for HP-USW of joint types A and B; (**b**) Maximum temperature of interface in different distances from weld center.

With looking at the temperature profiles of Figure 7a, it can be found that temperature firstly was increased from ambient temperature to a high level around 400 ◦C for joint B and 520 ◦C for joint A, sharply, just in a short duration of 0.2–0.3 s. Then, continuing of ultrasonic vibrations, led to completion of the bonding process, and temperature was increased to near the peak of *Tmax* were observed. There is a clear difference between joint A and B at the maximum point, except the difference in *Tmax* level. With using thin interlayer of Cu for joint type B, the peak temperature cycle in the curve flattened and *Tmax* has longer persistence when compared to the joint type A. This can be predicated to the role of Cu that serves as a heat sink during the USW process. Without using the Cu interlayer (as in joint type A), the interface temperature increased and reached to *Tmax* as a sharp peak with a duration of only 30 ms at the maximum point, and, while using the Cu interlayer besides lower energy, the peak shape somewhat flattened and was maintained near the *Tmax* for a duration of 450 ms. After the *Tmax* welding process is completed and ultrasonic vibrations stopped, weldment temperature decreased to near 300 ◦C very fast and then cooled down to ambient temperature, which continued for long time and nearly the same rate between two joint types.

Melting temperature (*Tm*) for pure Mg is 650 ◦C, while *Tm* of AZ31B is 630 ◦C [28]. Knowing this, the *Tmax* for joint type A (Power: 2450 W; Energy: 1400 J) was recorded at 638 ◦C, which is around the melting point of the AZ31B Mg alloy. However, referring to the microstructure observations that will be shown in Section 3.3 it is difficult to summarize that melting took place and it seems that the process

still occurs in the solid state. One reason might be cited to the short time of maximum temperature persistence with an appearance of a sharp peak in temperature profile. Additionally, looking at the temperature distribution in graphs of Figure 7b for joint type A represents *Tmax* as accessible near the weld center and it quickly drops more than 120 ◦C in the half way of flat region. In Figure 7b, the melting point (*Tm*) of AZ31B pointed out with a dashed line as a useful guide. The calculation of the surface area of joint type A where the temperature remains near the melting point of AZ31B proves that just 25% of weld nugget area can reach the maximum temperature (i.e., 4.6 mm2 from nugget surface area exposed to *Tmax*, against 19.6 mm2 total nugget surface area in flat region). While, 75% of weld nugget area in joint type B fairly reach to maximum temperature and do not show a quick drop. Haddadi et al. [29] also evaluated the temperature drop from the nugget center to the weld edge and found that the peak temperature dropped by 50–130 ◦C at the weld edge for HP-USW of 0.93 mm thickness Al-6111.

In the other side, for joint type B (Power: 2450 W; Energy: 1100 J), the recorded *Tmax* was 518 ◦C, which is apparent in Figure 7a, and the temperature distribution for this joint is presented in Figure 7b. In order to obtain a better understanding, two dashed lines that are related to critical temperatures in Mg-Cu binary system, as drawn in Figure 7b to show the situation of interfacial region. It can be found that most of interface flat region lies in the temperature values over Mg2Cu IMC formation eutectic temperature (*Teu*), which is 487 ◦C, but it cannot exceed *Teu* of MgCu2 IMC product, which is 552 ◦C. Therefore, it is predictable to achieve a kind of interfacial area that contains Mg2Cu as the main composition.

It is inferable that the Cu interlayer affects temperature distribution through improving the heat conductivity of interface. Copper has good electrical and heat conductivity and its thermal conductivity is four times larger than AZ31B Mg alloy (385 W/m-K for Cu; 96 W/m-K for Mg). Referring to this, the Cu effects on the interface to have a better heat distribution during the short time of process. Accordingly, with the matching graphs of Figure 7a,b for joint type B, it is clear that all parts of interface flat region have a chance of staying near maximum temperature for longer duration.

It is noteworthy to say that the Cu coating thin layer not only affected the thermal characteristics, but also the vibration behavior of interface. When using a Cu interlayer, USW impedance (a characteristic of vibration behavior) needs to set in lower levels to achieve proper resonance at the contact area of the two plates. In a research by Haddadi and Fadi [30], for dissimilar ultrasonic welding between steel and Al, they found that using a soft zinc coating on the steel surface altered the vibration behavior of the plate and the long sharp amplitude spike in the amplitude-time curve obviously decreased. They concluded that the surface condition of the steel sheet, which was confirmed by measurement of the net weld energy for each combination, affected the power delivery (when a fixed weld time was set). Furthermore, in that research, a eutectic reaction between Zn and Al led to a low melting point IMC reaction product that is molten under temperature rise that is caused by friction. The presence of this molten phase affected the amplitude of vibrations [30,31]. In the case of Mg-Cu, as it was explained, there is a eutectic point lower than process temperature that has the potential of making a molten phase and affecting the interface vibrational behavior. The effect of interlayer on impedance is also experimentally monitored through lap shear tensile force data before optimizing the energy effect on mechanical strength. At a constant USW energy and pressure, a change in impedance affected the process time and tip penetration depth and, accordingly, the optimized parameters that were used in experiments related to the effect of energy on maximum lap shear force.
