Ultrasonic spot welding (USW) is a highly promising welding process for Al/Mg dissimilar alloys due to its advantages of being pollution-free, highly efficient, and having short welding times. Additionally, the process is insensitive to material conductivity and heterogeneity, further adding to its appeal [31]. When welding metals using ultrasonic welding, fast metallurgical bonding can be achieved through the use of the ultrasonic volume effect and interface friction effect. Additionally, this process has the ability to prevent liquid–solid phase transformation, promote metallurgical miscibility, and effectively reduce residual stress and dimensional deviations in the workpiece. This makes ultrasonic welding a widely used and effective method in various industries, such as automotive, aerospace, and electronics manufacturing [19]. The heat input in ultrasonic welding process mainly comes from friction heat, under the action of ultrasonic softening and thermal softening, plastic deformation and metallurgical reaction occur at the interface of the joint, causing permanent bonding [32]. The level of welding energy utilized can impact both the degree of friction and the duration of interface action, resulting in changes to the interface temperature and ultimately affecting the microstructure and mechanical properties of the joint [33]. It is important to consider such variations depending on the specific applications and requirements of the welded components and structures. In the research conducted by Huang et al. [34], it was found that with a constant welding energy input, an increase in ultrasonic amplitude can lead to a corresponding increase in the temperature of the welding interface. Thus, in addition to energy input, amplitude is demonstrated as a crucial factor in the formation of bonds between ultrasonically welded base metals. Welding under larger ultrasonic amplitude can substantially increase the temperature and enhance the metallic bond formation process [35]. During ultrasonic welding, welding pressure is applied on the sonotrode and interacts with the ultrasonic shear stress. This interaction causes surface scratching and plastic deformation, which in turn breaks up the oxide film that is present. Such a process produces the necessary conditions for metal–metal bonding, which is then reinforced by atomic diffusion. In their investigation of high-power USW, Li et al. [36] found that pressure plays a crucial role in determining joint characteristics and dynamic processes. Insufficient pressure can lead to low interface friction, making it difficult to form a complete bonding surface and resulting in large gaps. Conversely, excessive pressure can lead to weld edge cracking, hinder sample vibration, weaken relative friction, and cause a sharp rise in welding temperature. Additionally, excessive pressure can result in excessively thick IMCs, thereby reducing the overall strength of the weld. During ultrasonic welding, the failure of the interface occurs due to the larger strain rate and plastic deformation that occurs at the metal interface, whereas core pulling is caused by elastic deflection and maximum stress concentration at the edge of the welded, as elaborated upon by reference [37].

Recent years have seen a growing recognition of the benefits of adding rare earth elements to magnesium alloys. Such alloys not only exhibit improved formability and corrosion resistance, but their overall mechanical properties are generally found to be better. These favorable characteristics make them particularly attractive for consideration in the manufacture of lightweight multi-material body structures and parts through welding. The welding of Al/Mg dissimilar alloys is a critical aspect of this endeavor and has been discussed in greater detail by reference [38]. In the welding process of Al/Mg dissimilar alloys, the relatively softer magnesium alloy tends to undergo greater degrees of deformation due to the applied welding pressure. Additionally, the core zone of the weld often experiences higher temperatures than the aluminum alloy. According to Macwan et al. [39], welding ZEK100 and Al5754 together resulted in a higher maximum tensile shear load compared to the AZ31/Al5754 welding experiment [40]. They attributed this to the ZEK100′s high ductility at the interface with Al5754, which allowed for better bonding and flowability, even at low welding energy. Another advantage of using ZEK100 is its lack of Al alloy elements, which enabled faster diffusion in ZEK100/Al5754 joints compared to AZ31/Al5754 joints. Interestingly, the peak tensile lap shear load of ZEK100/Al5754 dissimilar joints even exceeded that of AZ31/Al5754 dissimilar joints that included a tin interlayer [41].

Peng et al. [42] found that incorporating a Cu interlayer during the welding of ZEK100/Al6022 dissimilar alloys improved the tensile shear strength of joints by eliminating harmful Mg₁₇Al₁₂ intermetallic compounds. With the more intense plastic deformation at 1500 J welding energy, diffusion at the Mg/Cu and Al/Cu interfaces became more active, leading to the formation of an α-Mg + Mg₂Cu eutectic structure and a maximum mean tensile bond shear strength of approximately 70 MPa [42]. In Peng’s subsequent studies, interlayers such as Ag [43] and Zn [44] were utilized, demonstrating that the use of interlayers can effectively enhance the strength of aluminum/magnesium welded joints and reduce the formation of Al-Mg IMCs. This is a proven method to improve welding strength and reduce welding defects. However, it is important to note that the choice of interlayer material can have varying effects on the strengthening the welding of aluminum/magnesium dissimilar alloys, as the underlying principles differ depending on the chosen interlayer material.

Although ultrasonic spot welding is typically used for thin metal parts, its low welding energy input can limit its application in the welding of dissimilar materials. However, as a welding heat source, ultrasonic technology is being increasingly utilized in the hybrid welding process, which combines ultrasonic welding with other welding processes, such as MIG, TIG, or laser welding. This hybrid process has been shown to be effective in overcoming the limitations of ultrasonic welding when used alone, and has thus expanded the range of materials that can be welded using ultrasonic technology.

To improve the strength of the friction-stir-welded joint between AZ31B and 7075 dissimilar alloys, Niu et al. [49] utilized pure zinc foil as the interlayer. The addition of the zinc interlayer caused significant changes to the cross-sectional morphologies of the lap joints. During the welding process, the liquid zinc dispersed in the stir zone (SZ) due to the rotational motion of the pin. The lubrication effect of the zinc liquid was observed to reduce the flow stress of the material and increase its flow rate in the SZ. As a result of the heightened flow rate, the accumulation of material at the advancing side (AS) and retreating side (RS) was reduced [49]. The hook heights at both the AS and RS were reduced under the appropriate process parameters with the addition of the zinc interlayer, while the effective lap width (ELW) and SZ area were enlarged. The zinc interlayer also caused a significant change in the microstructures of the lap joint. The presence of fine Mg-Zn IMCs resulted in a uniform distribution throughout the SZ, instead of the continuous Mg-Al IMCs located at the boundary between the Mg and Al substrates.

The use of ultrasonic-assisted friction stir welding can significantly enhance the welding process by improving its forming ability, reducing the welding load, and preventing the occurrence of holes and tunnel defects within the joint. Not only does this improve welding efficiency, but it also has a positive impact on the grain structure of the weld. Ji et al. [50] conducted ultrasonic-assisted friction stir welding (UaFSW) with added Zn, which effectively improved the quality of Al/Mg dissimilar alloy joints. The mixing region between Mg and Al (Mg/Al MR) of all joints contained Al substrates of varying sizes, forming an effective mechanical interlocking in the joint under each process. During the Zn-added UaFSW, the external ultrasonic assistance helped increase the heat input, further reducing the viscosity and flow stress of the material in the SZ compared to the only-Zn-added FSW process. Additionally, ultrasonic high-frequency vibration was observed to improve the material’s flow behavior. This caused a portion of cold lap and continuous Al substrates to break into small pieces and disperse into the Mg/Al MR due to the large flow rate of the material. The Mg/Al MR in the UaFSW process with ultrasonic assistance was found to be larger than that of the conventional FSW process. Under the maximum ultrasonic power of 1600 W, the diffusion degree of Zn reached its highest value, which resulted in the enlargement of Mg/Al MR area [50]. Furthermore, as the ultrasonic power increased, the cold lap height of the Zn-added UaFSW joint reduced, forming a larger effective sheet thickness (EST).

Friction stir welding has undergone rapid expansion in the industry largely due to its dependability, low cost, superior strength, and ease of use. Moreover, its environmentally friendly and hazard-free products, coupled with its excellent welding qualities and cheap inspection cost, have made it a desirable option for manufacturing, hence its reference as a ‘Green Process’ [51]. By selecting the appropriate FSW welding parameters, it is possible to generate an acceptable Al/Mg joint without forming harmful IMCs layer. To achieve this, researchers have developed various hybrid welding techniques that utilize FSW. These techniques offer promising solutions for managing defects and IMC issues, as well as improving joint effectiveness by refining grain structures, enhancing strength, increasing material flow, and ensuring particle distribution homogeneity. Therefore, these hybrid welding techniques using FSW have significant potential compared to conventional FSW techniques, enabling the fabrication of high-quality joints.

Diffusion bonding is a highly reliable solid-state connection method that eliminates the risks associated with fusion welding defects and heat-affected zones. Furthermore, joints that are produced via diffusion bonding using optimized process parameters often exhibit high strength and dimensional accuracy. Several bonding parameters—such as temperature, pressure, holding time, and surface roughness—significantly impact the joint performance [52]. Additionally, factors like surface preparation, surface cleanliness, and the use of interlayers can also play a critical role in the quality and success rate of diffusion bonding. Reasonable temperature and holding time are critical parameters that can significantly reduce welding defects while also greatly influencing the mechanical properties of diffused joints [53]. Liu et al. [54] and Afghahi et al. [55] independently conducted in-depth investigations on the vacuum diffusion bonding process of Al/Mg alloys and divided the process into four distinct stages [55]. Firstly, under the influence of the concentration gradient, aluminum and magnesium atoms diffuse into each other and react to form a solid Al-Mg solution layer. Secondly, due to the faster diffusion of Al atoms in the magnesium alloy, Al atoms tend to aggregate in the interface layer and the Mg region, leading to the formation of Al₁₂Mg₁₇ intermetallic compounds. Thirdly, as the concentration of Mg at the interface and in the Al region increases, Mg atoms begin to diffuse into the aluminum alloy to form a layer containing the Al₃Mg₂ phase. Finally, as the holding time is increased, Al and Mg atoms continue to diffuse in the interfacial transition region, resulting in the gradual thickening of the intermetallic compounds layer.

On the other hand, it should be noted that Mg/Al dissimilar alloys may experience incomplete solution treatment during the diffusion bonding process, which can result in suboptimal mechanical properties. Therefore, comprehensive solution treatment and aging treatment may be necessary after the diffusion bonding process to ensure good mechanical performance [56]. It is worth mentioning that employing a suitable interlayer material is considered to be an effective approach in achieving sound bonding without compromising the mechanical properties. The interlayer composition can be tailored to meet the specific requirements of the joint, such as phase constituent and mechanical properties, which offer significant flexibility during the bonding process [57]. Zhang et al. [58] investigated diffusion bonding aluminum and magnesium using a Ni interlayer for the first time. The results showed that dissimilar metals of Mg/Al could be successfully joined by diffusion bonding with a Ni interlayer and that the Mg–Al intermetallic compounds were impeded. Later, they compared the thin Al film and Ni foil interlayer [59] and found that the Ni foil interlayer eliminated the formation of Mg–Al intermetallic compounds, while the addition of an thin Al film to the interlayer improved the properties of the Mg–Ni intermetallic compounds, and the shear strength of the joints was improved by the addition of the Ni foil and thin Al film interlayer. Javad et al. [60] used a cold-rolled copper interlayer for Al/Mg diffusion bonding. The cold-rolled process increased the strain of the copper interlayer, which in turn increased the energy of the surface atoms between the copper layer and the internal atoms, thus promoting diffusion among atoms. The findings suggested that the copper interlayer, with suitable cold rolling, can effectively enhance the bond strength between aluminum and magnesium. Guo et al. [61] employed three different thicknesses (30 μm, 50 μm, and 100 μm) of pure Zn interlayers in the Al/Mg diffusion bonding process. They compared the bond strength of the Mg/Zn/Al joints with and without the Zn interlayer and found that the shear strengths of the Mg/Zn/Al joints were significantly higher when the Zn interlayer was utilized, while the strength varied with the thickness of the Zn interlayer. Specifically, the Mg/Zn₅₀/Al joint exhibited the highest shear strength, measuring 38.56 MPa, while the Mg/Zn₃₀/Al and Mg/Zn₁₀₀/Al joints demonstrated lower shear strengths, possibly due to the production of Mg-Al-Zn ternary compounds and the low strength of pure Zn, respectively [61].

Diffusion bonding is an ideal solution for joining dissimilar materials with considerably different melting points due to its solid-state welding process. Furthermore, vacuum conditions can be employed to prevent oxidation and welding-pore formation when joining Mg alloys with Al alloys. In addition, interlayers play a pivotal role in preventing the formation of unwanted Mg–Al intermetallic compounds while also optimizing the microstructures of the joints. Notably, the employment of hybrid diffusion bonding techniques can effectively enhance the strength of Mg/Al welded joints by optimizing atomic diffusion. Overall, diffusion bonding with appropriate interlayers and hybrid methods is an effective approach to producing high-quality joints between Mg and Al alloys.

Explosive welding is a relatively advanced method for producing material composites and is commonly used to join both similar and dissimilar metals that cannot be welded using traditional welding techniques [62]. One of the key advantages of explosive welding is its ability to produce a thin transition zone with a minimal amount of intermetallic compound formation. Due to its small welding heat input, it can effectively eliminate the issue of brittle and hard compounds commonly produced when welding aluminum and magnesium, leading to a deteriorated joint performance [63]. During explosive welding, energy is transferred in the form of waves, which in turn form a distinct waveform interface at the dissimilar alloy welding joint. Zhang et al. [64] investigated the microstructure evolution of AA6061/AZ31B explosive welding and found that the formation of adiabatic shear bands (ASBs) in the magnesium alloy plate can be explained by the stress wave theory. ASBs are more likely to form in metals with a HCP crystal structure, such as Mg, than in those with an FCC crystal structure, such as Al, during explosive welding. Additionally, fine equiaxed grains were observed in the ASBs. The content changes of magnesium and aluminum show that they exist in a specific proportion in the melting zone, and metallurgical bonding occurs at the interface of the composite plate. The melting zone consists of a mixture of Al₃Mg₂ and Al₁₂Mg₁₇ intermetallic compounds [65]. Common interface joint defects, such as cracks, adiabatic shear bands, and intermetallic compounds can be mitigated through heat treatment, the use of an intermediate layer, gas-protected explosive welding [66], and other techniques.

Heat treatment is a type of hot processing technology involving heating, insulation, cooling, and other methods to adjust the microstructure. It is commonly used in explosive composite plates to mitigate explosive stress, improve processing performance, and produce high-quality explosive products [67]. The main effect of heat treatment on the interface is two-fold: first, it eliminates machining strengthening, and second, it changes the content of intermetallic compounds. Recrystallization annealing can cause adiabatic shear bands on the magnesium side to disappear, while also producing fine and uniform grains [68]. Chen et al. [69] conducted a study on the effect of multi-pass rolling and post-rolling heat treatment on the microstructure at the interface and the mechanical properties of explosive-welded Mg/Al composite plates. They discovered that the mechanical properties of the composite plates first increase and then decrease with an increase in annealing temperature and holding time [69]. Refined grains improve the strength of the composite plates through grain boundary strengthening. However, when the annealing temperature is further increased, the thickness of the diffusion layer increases significantly and leads to a reduction in the mechanical properties of the composite plates.

Explosive welding is suitable for both similar and dissimilar materials, particularly for dissimilar materials with great differences in physical and chemical properties. Heat treatment after welding can effectively refine the grain structure. Although it cannot change the interface composition after welding, it can effectively improve the strength of the welded joint. Interlayers are rarely used in Al/Mg explosive welding, due to the drastic deformation of the welding interface, as well as the safety and simplicity of using explosive welding. Generally, explosive-welded clad plates are relatively thick and subsequent rolling is sometimes employed to produce thin laminates. Heat treatment can efficiently eliminate weld defects and stress concentration created by rolling.

Al/Mg magnetic pulse welding belongs to high-energy dissimilar metal welding. Under large deformation and high strain rate, the interface between dissimilar metals will produce residual stress and dislocation appreciation, rendering the joint unstable [74]. Heat processing and heat treatment after butt welding can significantly improve the quality of dissimilar metal welds. A good interface can improve mechanical properties. Li et al. [75] annealed welded joints at different temperatures. The microstructure and mechanical properties of welded Al/Mg joints were not significantly affected at temperatures of up to 200 °C. However, at 200 °C, the Al₁₂Mg₁₇ intermetallic compound layer formed, and at 300 °C, both Al₁₂Mg₁₇ and Al₃Mg₂ intermetallic compound layers formed.

Despite its numerous advantages, the spread of magnetic pulse welding is limited due to the higher initial investment in electromagnetic technology. There are only a few studies on Al-Mg magnetic pulse welding. Most of them focus on optimizing welding parameters and subsequent heat treatment and do not involve the addition of intermediate layers or hybrid welding processes. However, these studies on Al-Mg magnetic pulse welding demonstrate that in the joint, there are no significant amounts of Al-Mg intermetallic compounds or obvious intermetallic compound layers, indicating that magnetic pulse welding can produce effective Al-Mg welding joint strength.

Sun et al. [79] successfully prevented the formation of the Al-Mg intermetallic compound and achieved strong Al/Mg resistance spot welded joints by inserting a Sn-coated steel interlayer between the two base metals before welding. The failure during mechanical loading occurred inside the Al fusion zone away from the Al/steel interface. The use of a Sn-coated steel interlayer not only improves the joint’s appearance but also greatly enhances the strength of aluminum–magnesium welding. Additionally, high-melting interlayers, such as Ni [80], Au-coated Ni [81], and Zn-coated steel [82], have also been used in the study of resistance spot welding of Al/Mg dissimilar alloys. They remain solid during welding and inhibit the reaction between Al and Mg.

Al/Mg resistance spot welding is a complex process with significant differences in the physical and chemical properties of Al/Mg, which results in low welding strength. As a result of these challenges, few research papers have focused on Al/Mg resistance spot welding. However, with the development of welding technology, there are new processes being developed to improve resistance spot welding’s welding performance and expand its potential application in Al/Mg resistance spot welding. Li et al. [83] developed a novel electrode, named the Newton Ring (NTR) electrode. The NTR electrode’s unique surface morphology improves the weld quality by changing the current density distribution and creating a ring-shaped nucleation process for the nugget. This, in turn, leads to more stable nugget shape and size, with the nugget causing no significant deviation during continuous welding tests. Lin et al. [84] used magnetically assisted resistance spot welding (MA-RSW) for joining Al/Mg alloys. Compared to traditional resistance spot welding, the MA-RSW joints exhibited better strength, improved toughness, and greater energy absorption capacity. Shah et al. [85], on the other hand, integrated high-frequency ultrasonic vibration into the resistance spot welding process, creating a new joining technique known as ultrasonic resistance spot welding (URW). Comparing URW to traditional resistance spot welding, up to a 300% increase in strength and over a 150% increase in displacement to failure can be achieved.