**1. Introduction**

Mg alloys are increasing in demand due to their high specific strength (strength to density ratio) and they have attractive applications in automotive industries, aerospace, medical implants, electronic appliances, and sport equipment [1,2]. Different methods are available for the joining of magnesium alloys. The primary methods include tungsten arc welding (TIG), laser beam welding (LBW), resistance spot welding (RSW), electromagnetic welding (EMW), electron beam welding (EBW), diffusion bonding, soldering, brazing, adhesive bonding, and mechanical fastening. Fusion based welding methods, like TIG, LBW, RSW, etc., have major problems that are mostly related to low boiling temperature of magnesium alloys that leads to porosities and hot cracking in the weld line and heat affected zone (HAZ). The other main limitations include high energy consumption and the need of consumables [3,4]. Therefore, the development of solid-state processes, like friction stir welding (FSW), friction stir spot welding (FSSW), and ultrasonic spot welding (USW) helped to avoid problems that are related to melting and provide opportunities for dissimilar material welding [5–7]. However, FSW and FSSW have the limitations of making "keyhole" and the need to fixture and a long process time [5]. USW is considered to be a solid-state welding process, with low energy consumption and time saving

process and no need for consumables or atmosphere control. It is using relatively high frequency ultrasonic vibrations (20–40 KHz) that induce the workpieces to be held together with simultaneous pressure, which leads to frictional shear forces parallel to the interface. Under the effect of frictional shear forces, surface oxide layers break down and two joining parts approach bare contact and the temperature increases due to friction, and then local welding occurs between the asperities of two faying surfaces. Finally, the spreading of local micro-welds and coalescence results in the creation of a metallurgical bond in the weld spot [5,7–11].

High-power ultrasonic spot welding (HP-USW) is an effective way of achieving sound strong joints among different USW methods. This method uses higher power and lower energy and time when compared to other spot-welding processes [12]. The use of high energy levels (or equally longer welding times) may cause extra-thinning of the joint and induce cracks in the weld nugget that limits access to maximum joint strength [9]. Furthermore, the use of interlayer in USW mostly related to weldability improvement, especially in dissimilar joining. Typically, use of metallic foil interlayer is more convenient and cost effective than other methods, like sputtering, plating, etc. [8]. The first trials of current research also performed using Cu foil in different thicknesses and USW energies. However, those experiments resulted in relatively strong joints, but the use of Cu coating was more effective in achieving better weldability and mechanical properties.

According to authors knowledge and a review on previous research studies, the use of Cu as an interlayer in the AZ31B Mg alloy "similar USW" has not been recorded. Previous efforts mostly were related to processes other than USW, the same as TLP (transient liquid phase), eutectic, or diffusion bonding. HP-USW of AZ31 Mg alloy classifies similar joining, but, when Cu used as an interlayer in contact with Mg, it forms a dissimilar welding combination at the interface. Hence, it is beneficial to review the Mg-Cu bonding processes and diffusion behavior, as well as AZ31 that is interlayered with Cu through other processes to find out the possible interactions in this system.

Diffusion kinetics and the identification of interfacial layers between Mg and Cu (pure metals) were studied by Nonaka et al. [13] and they reported the formation of Mg2Cu as major interfacial layer near Mg and a thin layer of MgCu2 adjacent to Cu (410–475 ◦C). Additionally, they suggested that the diffusion of Cu in Mg is faster than the inverse. However, in a nearly same recent study by Dai et al. [14], there was agreement about formation of two intermetallic layers between Mg and Cu, but they reported a different interdiffusion priority based on measured diffusion coefficients and activation energy data. These are basic studies regarding the Mg-Cu reaction diffusion couple, but, in the field of ultrasonic welding, Macwan and Chen [15] applied HP-USW for joining AZ31 to Cu (2000 W, 1000–2500 J) and observed the formation of a diffusion layer around the interface containing Mg + Mg2Cu that spread just towards the Mg side. They proposed a mechanism that is mainly based on the preferable diffusion of Cu in Mg to form Mg2Cu intermetallic compound besides liquid phase formation through the eutectic reaction. This behavior was mostly attributed to the differences in the physical properties between Cu and Mg, such as melting point, thermal conductivity, heat capacity, and atomic radius.

One of trials to use the Cu interlayer was related to the field of eutectic bonding, where Elthalabawy et al. [16,17] studied joining AZ31 to 316L steel using a Cu insert (at 530 ◦C), and found that the eutectic reaction between Mg and Cu after 1 min. resulted to formation of 4 distinct interfacial layers in Mg part. Middle interfacial layers included Mg2Cu and a layer of ternary Mg + Cu + Al composition wherein Al content depleted from AZ31 base metal to participate in the Al-containing product.

The participation of AZ31 alloying elements in interfacial phenomena makes it more complicated, which can lead to non- stoichiometric compositions. Anyway, the formation of intermetallic compound (IMC) reaction products in dissimilar joining is prevalent and, in most cases, it has negative effect on mechanical properties of the joint, because they can embrittle the interface, especially when a continuous layer formed. Panteli et al. [18–20], during HP-USW of AZ31 to Al-6111, observed a 5 μm thickness continuous IMC layer along the interface, which resulted in low tensile shear strength with interfacial fracture. Additionally, they found that, under dynamic high strain rates of USW, the rate

of IMC product formation was twice when compared to normal static conditions [18–20]. Patel et al. [21,22] also used HP-USW to join AZ31-Al 5754 and AZ31-HSLA steel combinations while applying a 50 μm thickness Sn interlayer and, finally, the Sn interlayer improved the weld strength in lower energies. The improvement was attributed to the formation of a composite-like Sn + Mg2Sn eutectic structure instead of forming a brittle IMC product without an interlayer.

The objective of the current research is to study the effects of using a Cu coating interlayer in HP-USW of AZ31B Mg alloy on the weldability, mechanical strength, and process optimization from the point of view of energy, time, and interface characterization.
