Solid-State Welding of Aluminum to Magnesium Alloys: A Review

Abstract

With the continuous improvement of lightweight requirements, the preparation of Mg/Al composite structures by welding is in urgent demand and has broad prospective applications in the industrial field. However, it is easy to form a large number of brittle intermetallic compounds when welding Mg/Al dissimilar alloys, and it is difficult to obtain high-quality welded joints. The solid-state welding method has the characteristics of low energy input and high efficiency, which can inhibit the formation of brittle intermetallic compounds and help to solve the problem of the poor strength of welded joints using Mg/Al dissimilar alloys in engineering applications. Based on the literature of ultrasonic welding, friction welding, diffusion welding, explosive welding, magnetic pulse welding, and resistance spot welding of Al/Mg in recent years, this paper summarized and prospected the research status of solid-state welding using Mg/Al dissimilar alloys from three aspects: the optimization of welding parameters, the addition of interlayers, and hybrid welding process.

1. Introduction

With the development of the automobile industry, people have put forward higher requirements for means of transportation. Reducing the weight of automobiles without compromising their performance is an inevitable trend [1]. Reducing the weight of a car can improve its fuel economy by 10% to 30%, which leads to a significant decrease in fuel consumption and can help save a lot of money while conserving energy [2]. Aluminum alloy has a wide range of applications in automobile lightweight due to its excellent strength, toughness, and machinability [3]. In addition, magnesium alloy materials offer many advantages, such as low density, high specific strength, high specific stiffness, and excellent fatigue strength, making them ideal for use in structural parts that have low requirements for strength and stiffness, such as automobile engine supports [4]. Magnesium alloy is not easily susceptible to the production of fatigue cracks under the action of alternating loads, has good processing performance, and can be easily machined into different shapes [5,6].

At present, the development of Al/Mg hybrid structures has made some progress, mainly in the areas of extrusion technology, rolling technology, and casting technology. These technologies enable the Al/Mg hybrid structures to have complex geometric shapes, and offer various functions, such as electromagnetic shielding, conduction, heat conduction, and heat transfer [7]. Composite casting is ideal for producing complex structures and parts with specific functionalities, enabling the combination of different materials for synergistic benefits; however, it generally incurs higher costs compared to welding due to material limitations and process complexity, leading to increased equipment and production expenses [8].

Hot roll bonding, also known as hot roll welding, is a high-temperature process used to bond metal sheets by heating them above their melting point [9]. However, it is important to note that hot roll bonding is generally more suitable for thinner sheets. When dealing with thicker metal sheets, multiple rolling processes or preheating may be required to achieve the desired bonding. In contrast, the Cold Roll Bonding (CRB) process involves the cold pressing of two metal sheets together through rolling [10]. The pressure applied during cold rolling promotes atom diffusion, resulting in the formation of a strong bond. However, the application range of cold rolling welding is limited, as it requires specific requirements regarding the material’s shape, thickness, and surface quality. In cases involving large or complex welded joints, cold rolling may not be a feasible option. In contrast, the Accumulative Roll Bonding (ARB) process is primarily suitable for alloy materials with excellent plasticity, such as aluminum alloy and magnesium alloy [11]. ARB requires specialized equipment and meticulous operations to ensure precision and consistency in the rolling and stacking processes. In contrast, the welding process is more straightforward as it does not require any special equipment or complex operations. However, the welding process can still achieve high welding strength, often surpassing or equaling the strength of the base material.

Welding dissimilar materials, like aluminum and magnesium, is challenging due to their varying properties, such as melting points, thermal conductivities, specific heats, and thermal expansion coefficients. During solidification, cracking may occur at the welded interface due to the induced stresses from the mismatch in thermal expansion coefficients or the presence of brittle phases, like intermetallic phases [12]. According to Figure 1, one of the primary obstacles in the use of Al/Mg dissimilar metals is the formation of intermetallic compounds (IMCs) at the weld interface. These compounds, namely Al₁₂Mg₁₇ and Al₃Mg₂, are generated during the eutectic reactions. Specifically, the reaction of L → Mg + Mg₁₇Al₁₂ takes place at a temperature of 437 °C, while the reaction of L → Al + Mg₂Al₃ occurs at 450 °C [13]. The existence of such compounds has a detrimental effect on the welding strength [14]. During solid-state welding, the growth of IMCs occurs through diffusion, in which the diffusion rate of Mg in Al is higher than that of Al in Mg. In this process, the initial IMC island tends to be γ-Mg₁₇Al₁₂, which has a higher tendency to nucleate on the surface of Al. In turn, on the γ-Mg₁₇Al₁₂/Al side interface, the second layer of IMCs, β-Mg₂Al₃, is formed [15]. The Mg₁₇Al₁₂ phase has a much higher formation energy than that of the Mg₂Al₃ phase (−0.024 eV/atom). As a result, the Mg₂Al₃ phase grows faster and displays a significant hardness of 663.26 HV. This specific microstructure pattern is a critical factor that significantly affects the overall strength and toughness of the joint [16].

Solid-state welding, recognized as a promising technology due to its high efficiency, short processing time, and low energy input, is capable of producing high-quality welds for metals that are similar or dissimilar in nature [17]. Ultrasonic consolidation is the advanced process of the sequential solid-state joining of metal foils or sheets by ultrasonic welding [18]. The research on ultrasonic welding parameters mainly focuses on welding energy, oscillation amplitude, as well as welding pressure. Other process parameters during ultrasonic welding include ultrasonic frequency and welding time. Ultrasonic frequency is generated by a transducer designed to operate at a specific frequency, while welding time usually fluctuates and is influenced by other parameters [19]. As a solid-state joining method, friction stir welding (FSW) is employed in the welding of various materials, like aluminum, magnesium, and steel [20]. The intricate nature of the FSW process, influenced by a multitude of parameters, such as tool geometry, process parameters, plunging depth, tool tilt angle, tool offset, etc., renders the investigation of its thermal and mechanical behavior a complex undertaking [21]. Diffusion bonding refers to the solid-state joining method with high temperature pressing without visible deformation and relative movement. The quality and properties of the joint can be enhanced through the appropriate selection of process parameters, such as temperature, pressure, and holding time during diffusion bonding [22]. Explosive welding is a solid-state method used to join different layered metal materials. This process creates a metallurgical bond through high-speed oblique collision between two metal layers. It effectively prevents extensive melting at the bonding interface by limiting heat transfer and minimizes the formation of intermetallic compounds [23]. Magnetic pulse welding (MPW) is a solid-state joining process that does not require a shielding atmosphere or input heat like traditional welding methods [24]. MPW utilizes a high-intensity magnetic field to accelerate one metal workpiece towards another, and the collision creates a high-speed deformation, leading to metallurgical bonding between the two workpieces. This method is commonly used for joining dissimilar metals or materials with different melting points [25]. Resistance spot welding (RSW) is a high-speed and adaptable process that is well-suited for automation, making it ideal for mass production. The quality of RSW is influenced by various welding parameters, with the most crucial ones being welding current, welding time, and electrode force [26].

Solid-state welding shows no apparent fusion or heat affected zone (HAZ) during the welding of dissimilar metals. The weld defects, such as the formation of brittle IMCs, high levels of welding distortions and HAZ damage in fusion welding could be typically avoided [27]. However, no matter which method is adopted, it is impossible to avoid the metallurgical reaction between the magnesium alloy and aluminum alloy to produce brittle Mg-Al IMCs, resulting in an increase in the tendency of brittle fractures and a significant deterioration in the quality of the joints [28]. Therefore, the interlayers are widely used in Mg/Al solid-state welding, which can prevent the metallurgical reaction between aluminum and magnesium liquid phases during the welding process [29]. The hybrid welding methods can also optimize the Mg/Al welding metallurgical bonding interface, reduce the occurrence of pores and cracks, regulate the diffusion and distribution of elements on both sides of Mg/Al alloys, affect the formation of intermetallic compounds, and improve the strength of welded joints [30]. Solid-state welding technology has a profound potential impact, as it combines advanced techniques and excellent results, which can improve the quality of welded joints. In future applications, it can bring more stable, efficient, and reliable solutions to various industries.

Numerous studies have been conducted on the application of solid-state welding methods for joining aluminum–magnesium dissimilar alloys. However, a comprehensive review of various solid-state welding methods for aluminum–magnesium alloys is still lacking. This paper aims to provide a detailed review of this topic, offering insights into future trends and conclusions, with the objective of establishing a solid foundation for further research.

2. Solid-State Welding Methods

2.1. Ultrasonic Spot Welding

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, as shown in Figure 2, 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. As demonstrated in Figure 3, 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.

2.2. Friction Stir Welding

Friction stir welding is a solid-state welding process used to connect dissimilar metals between Al and Mg alloys by rotating a pin located at the bottom of the tool into the workpiece and moving along the joint [45]. The process entails stirring and mixing the metals while avoiding liquefaction, thus producing a strong and high-quality weld joint. If welding process parameters, like material position bias, rotational speed, welding speed, and heat input, are either too high or too low, the insufficient mixing of aluminum and magnesium can occur within the joint core area, leading to irregular voids or holes [29]. It is crucial to maintain proper process parameters to ensure thorough mixing and the formation of a strong joint. In friction stir welding, it is commonly accepted that approximately 80% of the heat is generated through friction between the shoulder of the tool and the workpiece, while the remaining 20% of the heat is generated from the friction between the mixing pin and the workpiece [46]. This distribution of heat is attributed to the differing contact geometries between the tool components and the workpiece during the welding process and has been validated through theoretical modeling and empirical observations. Sekon et al. [47] compared the effects of tool pin profile, rotation speed, and welding speed. They categorized the tool pin profile into cylindrical, tapered threaded, and square shapes. It was found that the tool pin profile is the most influential parameter and that the square pin profile demonstrated a significant reduction in cracks and groove defects in the FSW process due to its better stirring ability. Firouzdor et al. [48] welded AA6061 and AZ31, and the joint strength was better when the magnesium alloy was on the front side, especially when the tool was offset to magnesium. They discussed how moving pins into magnesium can help them better bond with magnesium, promote its flow, and catch up with Al. It seems that AZ31, when the tool deviates from AZ31, can penetrate into the mixing zone and interlock with Al, thereby improving the strength of the joint.

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, as depicted in Figure 4 [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 as shown in Figure 5 [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.

2.3. Diffusion Bonding

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, as shown in Figure 6 [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, as shown in Figure 7, 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.

2.4. Explosive Welding

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 (Figure 8) [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 (Figure 9) [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.

2.5. Magnetic Pulse Welding

Magnetic pulse welding is a high-speed solid-phase welding technology driven by electromagnetic force based on pulse power technology and the electromagnetic induction principle [70]. It offers numerous advantages, including short welding time, high welding efficiency, and low heat input, which can significantly reduce the tendency of intermetallic compounds generated at the interface of welded joints. As a result, it has significant advantages in the connection of dissimilar metals [71]. The corrugated structure interface, which involves a combination of mechanical interlocking and metallurgical bonding, is crucial for obtaining magnetic-pulse-welded joints of aluminum/magnesium dissimilar alloys with outstanding mechanical properties [72]. With an increase in discharge energy, the initial kinetic energy of the flying plate grows, generating longer wavelengths and larger amplitudes at the interface after impact, thereby increasing the binding area and carrying capacity (Figure 10). The high-speed collision between the fly plate and substrate creates severe plastic deformations at the joint interface, leading to grain refinement. Grain refinement is primarily due to dynamic recrystallization caused by severe plastic deformation [73].

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 (Figure 11). 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.

2.6. Resistance Spot Welding

Resistance spot welding is one of the primary welding methods used in automobile body welding because of its high production efficiency, simple operation, lack of filler material requirement, and easy realization of automation [76]. During the resistance spot welding process, material properties and welding process parameters (primarily including welding current, welding time, and electrode pressure) will impact the temperature distribution characteristics of the welding position. This then affects the size and overall welding quality of the solder joints [77]. The contact pressure is influenced by the workpiece temperature, workpiece deformation, and electrode pressure. The size of the molten core increases with the increase in welding time. However, if the molten core’s size becomes too large, it can produce sputtering in the actual welding process. This can result in the loss of welding materials, decreased molten-core-forming ability, a reduction in size, along with cracks, shrinkage holes, and other defects [78]. During the process of the resistance spot welding of magnesium alloy and aluminum alloy, porosity is a common welding defect that can arise due to various reasons. Firstly, porosity formation is related to the steep reduction in hydrogen solubility during the liquid-phase solidification process. For instance, the solubility of 1 atm hydrogen in 100 g of pure aluminum during the solidification process can decrease from 500 mm³ in the liquid phase to 50 mm³ in the solid phase. Secondly, it is caused by the formation of bubbles that evolve due to the evaporation of evaporative elements, such as Mg and Zn, in welding materials during welding. Thirdly, due to uneven heating and cooling speeds, the liquid phase near the middle interface solidifies last, forming a porosity in this region due to the action of interfacial shrinkage stress.

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 (as shown in Figure 12). 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.

3. Solid-State Welding with Interlayer

Even in solid state welding, during which temperatures generally remain below the eutectic lines, diffusion reactions between atoms of aluminum and magnesium can occur, causing the formation of both intermetallic compounds as continuous layers [86]. Recent trends in Al-Mg dissimilar solid-state welding have focused on the use of interlayer or filler metals in order to attenuate the negative effects of Al-Mg-based IMCs, as evidenced by

Table 1. List of recent research progress on Al-Mg welding with interlayer metal in chronological order.

No.	Interlayer	Welding Method	Configuration	Material (Al-Mg)	Author	Year
1	Pure Sn interlayer	USW	Lap joint	AA5754(top, t)-AZ31(bottom, b)	V.K. Patel et al. [41]	2012
2	Pure Ni foil interlayer	Diffusion bonding	Lap joint	Pure aluminum and pure magnesium	Jian Zhang et al. [58]	2012
3	Pure Ni/Au-coated Ni interlayer	RSW	Lap joint	AA5754(t)-AZ31B(b)	P. Penner et al. [81]	2013
4	Al/Mn coating	USW + cold spray	Lap joint	AA6011-T4(b)-AZ31-H24(t)	A. Panteli et al. [90]	2013
5	Silver film interlayer	Diffusion bonding	Butt joint	pure Al 1060 and pure magnesium	Yiyu Wang et al. [91]	2013
6	Pure Al foil and pure Ni foil interlayer	Diffusion bonding	Lap joint	Pure Al and pure magnesium	Jian Zhang et al. [59]	2013
7	Pure Zn/Zn-coated steel interlayer	RSW	Lap joint	AA5754(t)-AZ31B(b)	P. Penner et al. [82]	2014
8	Pure Ni foil interlayer	Diffusion bonding	Lap joint	Pure Al 1060(t) and pure magnesium(b)	Zhang J et al. [92]	2014
9	Pure Zn interlayer	RSW	Lap joint	AA5052-H12(t)-AZ31B(b)	Y. Zhang et al. [93]	2015
10	Pure Ni foil interlayer	RSW	Lap joint	AA5754-O(t)-AZ31B-H24(b)	M. Sun et al. [80]	2015
11	Pure Zn interlayer	Arc-assisted ultrasonic seam welding	Lap joint	Al6061(t)-AZ31(b)	X. Dai et al. [94]	2015
12	Sn-coated steel interlayer	RSW	Lap joint	AA5052(t)-AZ31B(b)	M. Sun et al. [79]	2016
13	Sn/Zn composite interlayer	Arc-assisted ultrasonic seam welding	Lap joint	Al6061(t)-AZ31B(b)	Xiangyu Dai et al. [95]	2016
14	Zn-coated All2024	FSW	Lap joint	Al2024(b)-AZ31(t)	R. Z. Xu et al. [96]	2016
15	Pure Zn foil interlayer	FSW	Lap joint	Al2024-AZ31	Rongzheng XU et al. [97]	2017
16	Pure Zn interlayer	FSW	Butt joint	Al6061-T6(RS)-AZ31(AS)	A. Abdollahzadeh et al. [98]	2018
17	Pure Ni interlayer	Diffusion bonding	Butt joint	Pure magnesium and pure aluminum	Fuxing Yin et al. [99]	2018
18	Pure Zn interlayer	USW	Lap joint	Al6082-T6(t)-AZ31B(b)	Xiaoyan Gu et al. [100]	2019
19	Pure Zr foil interlayer	FSW	Lap joint	Al6061(t)-AZ31(b)	Yang Zheng et al. [101]	2019
20	Pure Zn interlayer	UaFSW	Lap joint	Al7075-T6(b)-AZ31B(t)	Shude Ji et al. [102]	2019
21	Pure Zn foil interlayer	FSW	Butt joint	Al7075-T6(AS)-AZ31B(RS)	Jinglin Liu et al. [103]	2019
22	Pure copper foil interlayer	USW	Lap joint	Al6022-T43(b)-ZEK100-O(t)	H. Peng et al. [42]	2019
23	Pure zinc foil interlayer	FSW	Lap joint	Al7075-T6(b)-AZ31B(t)	Shiyu Niu et al. [49]	2019
24	Cold rolled copper interlayer	Diffusion bonding	Butt joint	Pure aluminum and pure magnesium	Javad Varmazyar et al. [60]	2019
25	Pure Zn foil interlayer	UaFSW	Lap joint	Al7075-T6(b)-AZ31B(t)	Shude Ji et al. [50]	2019
26	Pure Zn interlayer	Diffusion bonding	Lap joint	Al5083(t)- ZK60(b)	Yangyang Guo et al. [61]	2019
27	Pure Sn foil interlayer	FSW	Lap joint	Al5052H32(t)-AZ31(b)	Bo Zheng et al. [104]	2020
28	pure Ag foil interlayer	USW	Lap joint	Al6022-T43(b)-ZEK100-O(t)	H. Peng et al. [43]	2020
29	Pure Ag foil interlayer	Diffusion bonding	Lap joint	Al 5083(t)-AZ31(b)	Hatef Shakeri et al. [105]	2020
30	Pure Ni foil interlayer	FSW	Lap joint	AA6061-T6(b)-AZ31B(t)	Sachin Kumar et al. [106]	2021
31	Pure tin foil interlayer	FSW	Lap joint	Al7075-T6(t)-AZ31-H24(b)	Omid Karimi-Dermani et al. [107]	2021
32	Cd interlayer	FSW	Butt joint	Al7075(AS)-AZ31(RS)	Satya Kumar Dewangan et al. [108]	2021
33	Zinc-coated AZ80	Diffusion bonding	Lap joint	Al7075(t)-AZ80(b)	R.J. Golden Renjith Nimal et al. [109]	2021
34	Sn foil interlayer	FSW	Lap joint	AA6061(t)-AZ31(b)	Bandi A et al. [110]	2021
35	Pure Ni foil interlayer	FSW	Butt joint	Al6061-T6(RS)-AZ31B(AS)	Shao-kang Dong et al. [111]	2022
36	Sn and Sn–9Zn solders	USW	Lap joint	Al6061(b)-AZ31(t)	Yingzong Liu et al. [112]	2022
37	Pure Zr foil interlayer	FSW	Lap joint	Al6061(t)-AZ31(b)	Yang Zheng et al. [113]	2023
38	Pure Zn foil interlayer	FSW	Butt joint	Al6061-T6(AS)-AZ61(RS)	Mukesh Kumar et al. [114]	2023
39	Vertical and horizontal zinc interlayer	FSW	Butt joint	Al7075(AS)-AZ31(RS)	Dewangan, S. K et al. [115]	2023
40	AgCuZn (Silver-based) interlayer	Diffusion bonding	Lap joint	Al3003(b)-AZ31(t)	Yang T et al. [116]	2023
41	Zr foil interlayer	FSW	Lap joint	Al6061(t)-AZ31(b)	Yang Zheng et al. [117]	2023
42	pure Zn foil interlayer	USW	Lap joint	AA6022-T43(t)-ZEK100-O(b)	H. Peng et al. [44]	2023

. A variety of interlayer metals have been incorporated into the process, including nickel, iron, tin, Zr, Cd, silver, copper, zinc, and manganese. The fundamental idea behind this approach is straightforward: the metallic interlayer/filler is positioned between the base metals during welding and serves as a substitute for Al-Mg reactions by forming reactions with aluminum, magnesium, or both base metals through the course of welding. This mechanism serves to limit the extent of Al-Mg reactions, thereby mitigating Al-Mg-based IMC phases [87]. These two examples illustrate that the suppression of Al-Mg-based IMCs can be achieved through the interlayer’s role as a “physical barrier” if its melting temperature is high enough to remain solid and thus preclude mixing between Al and Mg, or as a “chemical barrier” if the interlayer’s presence favors the formation of less detrimental alternative IMCs. The chemical barrier aspect will be discussed in greater depth in the following subsection [88]. Despite using the same interlayer material, the thickness of the interlayer still plays a significant role in determining the metallurgical reaction and intermetallic compound composition of Al-Mg welded joints. To guide the selection of interlayers, Shah [89] and colleagues suggest a three-step guideline: firstly, the enthalpy of formation (ΔH) and Gibbs energy of the compound (ΔG) should be lower than those of the Al-Mg system; secondly, the relative brittleness of the expected compound should be lower than the hardness values of Al-Mg-based IMCs; and lastly, a trial stage is necessary to assess the effectiveness of the interlayer.

It has been found that when using low-melting-point materials, such as tin, zinc, and copper, although the addition of a third element has a certain effect on reducing the production of Al-Mg IMCs, due to the low melting point of the interlayer metal, it will also melt during the welding process. The molten interlayer material makes the magnesium and aluminum phases on both sides mix in the liquid state, and it cannot completely avoid the direct reaction between magnesium and aluminum. Although the high-melting-point interlayer material can remain intact in the welding process, due to the huge difference in performance and structure of Al/Mg, the high-melting-point interlayer is often difficult to meet the requirement of achieving high metallurgical bonding strength with both sides of Al/Mg. The role of rare earth elements is now as an important additive element of Al-Mg alloy, which is widely used in the field of Al/Mg casting and welding. The intermediate layer containing rare earth elements can optimize the composition of alloy phase in the weld during Al/Mg welding, refine the grain size in the weld, reduce the hardness of the welded joint, and improve the strength of the joint.

4. Hybrid Solid-State Welding Methods

Hybrid welding is an effective technique that reduces welding defects and improves welding quality by integrating the advantages of different welding technologies with varying heat input and power density [118]. In the Al/Mg welding processes, various techniques, such as laser, arc, pulsed current, magnetic field [119], and ultrasonic, are used to improve the performance of a single joint.

Laser-arc hybrid welding is a research hotspot in Al/Mg fusion welding [120,121,122,123], and single lasers or arcs are also widely used as auxiliary processes [124,125,126,127]. Hybrid laser-friction stir welding combines the benefits of laser heating and friction stir welding to achieve efficient and high-quality welding [124]. The laser beam heats the metal, causing partial melting, while the stirring pin rotates and applies pressure to mix the molten metal [125]. This mixing process ensures a uniform and continuous state of the metal in the weld, resulting in strong and connected joints. Dai et al. [96] used gas tungsten arc welding (GTAW) preheating as heat source to assist hybrid ultrasonic seam welding on MgAZ31B and Al6061 alloy sheets and achieved satisfactory joint strength. In this hybrid welding process, GTAW technology enables the melting and welding of metals at high temperatures, while the ultrasonic seam welding technology eliminates surface oxide layers and contaminants through the application of vibration and pressure. This process improves weld quality and strength, reduces porosity and defects, and enhances the surface quality of the joint. In Dai’s subsequent research, Zn [126] and Sn [127] were used as the interlayers for Al/Mg arc-assisted ultrasonic seam welding to improve the mechanical properties of Mg–Al dissimilar joints by suppressing the formation of brittle Mg–Al IMCs.

The ultrasonic field not only promotes mixing between Al and Mg materials, but also inhibits the growth of intermetallic compounds and improves grain recrystallization, thus significantly improving joint strength in Al/Mg dissimilar welding [128,129,130,131,132,133].

Ultrasonic-assisted friction stir welding is a welding method that combines ultrasonic vibrations and friction stir welding technology [128]. The vibrational effect of the ultrasonic waves can induce effective micro-vibrations in the metal within the welding area, facilitating plastic flow and improving the overall quality and formability of the welded joints [129]. Additionally, the vibrational effect of the ultrasonic waves promotes the release of gas bubbles and the expulsion of liquid metal during the welding process, thereby reducing porosity in the weld joint and enhancing its density [130]. Ultrasonic resistance welding (URW) is an ultrasonically assisted resistance spot welding process, in which ultrasonic vibration enhances nugget formation and minimizes porosity defects [131]. The ultrasonically assisted process facilitates the fracture and breakdown of surface oxides and contaminants, reducing contact resistance and the heat generation rate [132]. The principle of ultrasonic resistance welding involves placing the metal part to be welded between two electrodes, generating local heating on the contact surface through the flow of current, and applying directional pressure through ultrasonic vibration [133]. Ultrasonic vibrations effectively reduce the oxide layer and impurities on the contact surface, improving the quality and strength of the weld. Additionally, the ultrasonic vibration promotes the plastic deformation of the material, resulting in a more solid weld.

The use of pulsed current eliminates cracks, tunnels, and other defects in traditional Al/Mg friction welding, effectively reducing the thickness and quantity of intermetallic compounds in the band structure zone and Al-Mg interface, and improving joint efficiency [134]. Pulse-current-assisted friction stir welding is a specialized welding technique that combines pulse current heating with friction stir welding. During the welding process, the welding joint is heated by applying pulses of current [135]. The pulse current heating technique provides additional heat intermittently, resulting in accelerated softening of the welding materials and the promotion of weld formation, to improve welding quality, reduce welding deformation, enhance welding strength, and increase welding efficiency [136]. However, this process requires the precise control and adjustment of pulse current parameters to achieve optimal welding outcomes.

Processes such as ultrasonic, arc, cold rolling, hot rolling, and solid phase welding technology are studied widely to connect foil or filler metal as an interlayer between two dissimilar material plates in various processes, such as friction stir welding, resistance spot welding, diffusion welding, and ultrasonic welding. Moreover, hybrid welding processes improve components and systems that are in direct contact with the base material, such as stirring tools and electrodes, to better distribute and regulate the input of different energy in the process, optimizing welding parameters. Although using an intermediate layer or filler metal can effectively improve the quality of Al/Mg welding and is simpler and cheaper, hybrid welding processes significantly improve the strength of the joint in the welding of Al/Mg dissimilar alloys using an interlayer.

5. Summary and Outlook

In recent years, welding technology has seen rapid development, with the latest solid welding methods inheriting and innovating traditional methods, while also developing and perfecting new approaches. Despite this progress, the welding process remains complex and is influenced by many factors. Recent studies have resulted in the following findings:

(1)

Optimizing welding parameters such as current, time, force, tool profile, travel speed, etc., is crucial for enhancing the overall performance of aluminum/magnesium dissimilar alloy welding and has demonstrated successful welding outcomes. Furthermore, refining the welding process itself can also lead to a substantial enhancement in the quality of welds.

(2)

In solid phase aluminum/magnesium welding using an intermediate layer, it has been found that metals with a high melting point can effectively resist contact between liquid aluminum and magnesium, thereby hindering the formation of aluminum–magnesium intermetallic compounds. Moreover, by using an intermediate layer metal that undergoes an aluminum–magnesium metallurgical reaction, alloying can be achieved, leading to a change in the composition of the compound phase in the welded joint, making it a promising area for further development.

(3)

Hybrid welding methods make effective use of a range of welding approaches, while avoiding the drawbacks of a single method. Of particular importance is the application of ultrasonic waves in composite welding, which opens up new possibilities for dissimilar alloys. Furthermore, the development of composite welding techniques is leading to the creation of new process equipment and technologies for welding dissimilar alloys, making it a highly promising area for future development.

(4)

The welding of aluminum/magnesium and other dissimilar metals continues to pose significant challenges in the field of welding engineering. While progress has been made through advancements in welding parameters, intermediate layer use, and composite welding, further efforts are needed to develop lower-cost and more efficient techniques for dissimilar alloy welding.