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Summary of Results:
The combination (joining or welding) of aluminium (Al) and steel is a promising solution in the automotive industry to reduce the weight and fuel consumption of car body structures. Due to the susceptibility to the formation of intermetallic compounds (IMCs) between iron and aluminum, this poses a significant challenge for dissimilar welding of aluminum/steel. In this paper, the formation and growth mechanism of Fe/Al IMCs are reviewed. In this paper, the research progress of various welding methods of aluminum/steel dissimilar welding, including friction stir welding, laser welding and resistance spot welding, is reviewed. The joining mechanisms, interlaminar materials, microstructures, and mechanical properties associated with each method are highlighted. The effects of process parameters, geometry, filler material composition, joint type, sandwich or coating composition, RSW with cover plate, and external energy field on the microstructure and mechanical properties of the joint are also reviewed. In addition, the temperature control range of the aluminum/steel interface under various welding processes is systematically summarized. Some suggestions are put forward for further research directions of aluminum/steel dissimilar welding.
Background:
Over the past few decades, the global manufacturing industry has faced two major challenges: energy shortages and environmental crises. These issues have been recognized as key factors influencing the development of this field. Considering the premise of the development of advanced automobiles, paying attention to environmental protection should be the primary concern. Therefore, an important strategy is to reduce the weight of the car. If a car is 10 percent lighter, its fuel efficiency can be improved by 6 to 8 percent. Low weight not only reduces fuel consumption, but also reduces CO2 emissions and air pollution. Today, in order to reduce CO2 emissions from passenger vehicles, their average fuel economy targets and production have become a major challenge for most countries. Figure 1 shows the different trends in passenger vehicle fuel economy targets for different countries from 2015 to 2025. China's average CO2 emission target for 2020 is 116 g/km, down 30% from 2015.
Figure 1 Trends in passenger vehicle fuel economy targets in different countries from 2015 to 2025
Recently, the automotive industry has begun to explore the use of lightweight materials to achieve these goals. Automotive lightweight materials can be divided into three categories: (i) light alloys (aluminum (Al) alloys, magnesium (Mg) alloys, (Ti) alloys, etc.) ;(ii) advanced high-strength steels (AHSS) ;(iii) composite materials. Lightweight materials are widely used in vehicle structural components, including instrument panels, bumpers, engines, body shells, wheels, suspension systems, brakes, steering systems, batteries, seats, transmissions, and more (Figure 2).
Figure 2 Typical lightweight materials used for different components in automobiles.
Figure 2-1 Timetable of aluminum alloy and magnesium alloy used in automobiles
Figure 2-2 Lightweight materials used in different parts of automobiles
Figure 2-3 Distribution and proportion of the main materials on the white body of the car
Aluminum alloy has become the most widely used lightweight material in automotive structural components. Aluminum alloy has gradually replaced traditional steel materials and has become one of the dominant lightweight materials in the automotive industry. However, they cost a lot, about twice as much as steel. In addition, considering safety and cost factors, it is almost impossible to completely replace steel in structural applications.
As the main structural material in the automotive industry, steel can provide excellent mechanical properties to meet strength requirements at a lower cost. High-strength steel is commonly used in automotive body construction, not only because it is an inexpensive material, but also because it has excellent machinability and superior impact absorption. In general, there is an inherent trade-off between the strength and ductility of various steels [16]. When choosing a material for a body structure, it is important to consider various factors such as raw materials, production, maintenance, and cost. The most effective way to meet the need for lightweight materials is to combine aluminum alloys and steels. As a result, the integration of different metals in multi-material designs is becoming a popular approach that provides desirable and unique material properties.
The main problem when steel is connected to aluminum alloy
The lightweight multi-material strategy has been a promising approach to building low-weight vehicles in recent years. In particular, the combination of aluminum and steel is emerging as a promising way to achieve lightweight, safe, rigid and cost-effective automotive structures. Various joining technologies have been successfully used to join aluminum alloys and steels. To improve joint strength in traditional joining methods such as riveting and bonding, various alternative welding methods such as solid-state welding, fusion welding, or hybrid welding can be used.
However, due to the significant differences in the thermophysical properties of the two metals, the dissimilar welding of aluminum alloys to steel remains a very important challenge. Table 1 lists the physical properties of Al and Fe. There are also significant differences between aluminum and steel, such as melting point and coefficient of thermal expansion, resulting in poor metallurgical compatibility. In addition, Al is highly active and prone to the production of dense Al2O3 oxide films. Al2O3 has a high melting point (2050°C), which may reduce the strength of the joint.
Table 1 Comparison of the physical properties between Al and Fe
Figure 3 shows the Fe—Al equilibrium phase diagram. During the solidification process, various intermetallic compounds are formed, namely Fe3Al, FeAl2, FeAl, FeAl3 and Fe2Al5. Aluminum-rich IMCs form less plastic and ductile joints than iron-rich IMCs. Therefore, the excessive formation and growth of IMCs at the interface of different materials must be avoided and inhibited.
Fig.3. Fe-Al binary phase diagram
The focus of this review is on the use of various welding methods to attach aluminum alloys to steel. The low solid solubility of aluminum alloys and steels during welding results in poor metallurgical compatibility of welds, while large differences in thermophysical properties pose even greater challenges. Thermal cracks, porosity, and other defects are common during welding. In addition, during the welding process of aluminum alloy and steel, the formation of hard and brittle Fe-Al IMC will significantly reduce the mechanical properties of the welded joint. Therefore, controlling the formation and growth of IMCs and reducing welding defects have become an important research area for aluminum-steel welding.
Previous review articles on the dissimilar joining of aluminum alloys to steel have provided only brief and inadequate analyses. In addition, in the dissimilar welding of aluminum alloy and steel, there is a lack of attention to interface temperature control, which affects the growth of IMC layer. In this paper, the mechanism of friction stir welding (FSW), laser welding (LW) and resistance spot welding (RSW) for the welding of aluminum alloy and steel dissimilar materials is reviewed.
This article summarizes the progress of FSW, LW, and RSW for joining aluminum alloys and steels (Figure 4). The following section provides a comprehensive review of the advantages, processes, mechanisms, and corresponding joint microstructures and strengths obtained by the above welding methods.
Fig.4 Overview of ASW, LW, RSW and other welding methods for aluminum/steel.
Formation of intermetallic compounds
In the Fe-Al phase diagram (Figure 3), two types of IMCs can be seen: Fe-rich (Fe3Al and FeAl) and Al-rich (Fe2Al5, FeAl2 and FeAl3). The type of IMC formation depends on the local temperature and material composition during the welding process. Phases with lower Gibbs free energy (ΔG) are more likely to form during solidification in welding. The Gibbs free energy sequence of the various Fe single-bond Al IMCs is as follows:
Table 2 lists the lattice structure and melting point of Fe single-bond Al IMCs. At a eutectic temperature of 1150 °C, the reaction can be expressed as follows:
Table 2 Lattice structure and melting point of different Fe—Al IMCs
In addition, due to the mutual diffusion reaction of Fe atoms and Al atoms, the FeAl phase can be formed. For example, Pan et al. observed the FeAl phase at the Fe2Al5 grain boundary of an RSW welded joint of Al/steel.
For dissimilar welding of aluminum alloy and steel, inhibiting the growth of IMC is a key technical issue, if its growth and growth are not controlled, too much IMC will affect the mechanical properties of the joint. Reducing the thickness of the Fe-Al-IMC layer can improve the overall performance of the joint. However, some studies have shown that the imc layer of a thin layer can effectively inhibit interfacial failure by ensuring strong binding strength. Therefore, understanding the formation mechanism of IMC is essential to improve the performance of joints.
Diffusion time and chemical composition during aluminum/steel welding are the two main factors that affect the formation of IMC. A schematic diagram of the formation mechanism of Fe-Al IMCs is shown in Figure 5 below. This can be broadly divided into four phases: (i) Al and Fe atoms diffuse towards the interface, and the rate of mutual diffusion is slower (Fig. 5(a));(ii) This diffusion rate becomes faster at the interface, and the Fe atoms reach a saturation concentration in the Al matrix. At this point, the Fe4Al13 IMC begins nucleation. Since the diffusion rate of Fe2Al5 is higher than that of other Fe-Al IMCs, discontinuous Fe2Al5 IMCs are formed (Fig. 5(b));(iii) As the interfacial welding energy increases, the discontinuous Fe2Al5 grains exhibit rapid growth and eventually fuse to form a continuous structure. Due to the vacancies in the Fe2Al5 crystal structure, the Fe2Al6 grains grow vertically along the Al/steel interface, forming coarse columnar crystals (Fig. 5(c));(iv) After the final stage of the welding process, the needle-like Fe4Al13 IMCs appear near the Fe2Al5 layer after curing (Fig. 5(d)).
Fig. 5 Mechanism of formation of Fe-Al IMCs at different stages: (a) stage 1, (b) stage 2, (c) stage 3, (d) stage 4.
In recent years, the mechanical properties, microstructure evolution, morphology and IMC thickness of aluminum alloy and steel welding process have been studied. In terms of IMC hardness, Fe2Al5 IMC exhibits a higher hardness as compared to FeAl3. In addition, for Fe2Al5, as the main Fe-Al IMC product, the Fe2Al5 IMC used for RSW grows in the direction of [001], perpendicular to the interface. There are two types of IMC morphologies formed at the weld interface (Figure 6). The first IMC is a tongue-like IMC near the steel side, while the second is a needle-like IMC near the Al side.
Figure 6.IMC topography at the weld interface: (a) backscattered electron (BSE) image and (b) electron backscatter diffraction (EBSD) image
However, excessive IMC layer growth can deteriorate the quality and performance of the joint. Different types of IMC vary in hardness and toughness. Typically, during the welding process of aluminum/steel, various Fe-Al intermetallic phases such as Fe3Al, FeAl2, Fe2Al5, FeAl, and FeAl3 are formed. Aluminum-rich IMCs are the most common phase, although some iron-rich IMCs (FeAl and Fe3Al) also deserve great attention.
Insufficient or excessive IMC formation can be detrimental to welded joints. Therefore, when welding different materials, the thickness of the IMC layer needs to be controlled within an acceptable range. When the predicted thickness exceeds the critical value, thickness control can be achieved through various methods, such as the optimization of process parameters and the addition of intermediate transition layers.
Fig.7 ΔG of Fe-Al IMCs at different temperatures
Fig.8. Different FSWs for different combination types: (a) butt weld, (b) lap weld.
Fig.9 Illustration of the different stages of the aluminum-steel FFSW process: (a) FFSW initial stage, (b) FFSW welding stage, (c) FFSW holding stage and (d) FFSW retraction stage.
Fig. 10.Illustration of the aluminum-steel RFW process at different stages [76]:(a) the initial stage, (b) the welding stage and (c) the upsetting stage.
Fig.11 IMC distribution of the aluminum/steel interface in different welding combinations: (a) lap combined welding and (b) butt combined welding.
Laser welding
The fusion welding process, known as laser welding (LW), is shown in Figure 15. It has the significant advantages of high energy density, fast welding speed and low heat input, which is conducive to the joining of dissimilar materials.
Fig.12 Schematic diagram of the laser welding process
Compared with the solid-state welding method, the LW process of aluminum and steel has the advantages of low heat input, small thermal deformation, thin thickness of IMC layer, and good joint quality. However, challenges remain when using laser welding due to welding defects and reflectivity.
In the aluminum/steel LW process, two different welding modes are observed: laser deep penetration welding mode and laser beam conduction welding mode. When the ratio of width to depth >1, the heat conduction welding mode occurs. On the other hand, when the ratio of width to depth is significantly less than 1, keyholes or deep penetration welds are formed.
The researchers have conducted special research on the joint structure, connection mechanism, joint failure mode, new technology and IMC formation law of aluminum/steel dissimilar materials. Lap joints are typically used for laser keyhole welding. In addition, the most common problems in this welding process are spatter, elemental loss, and solidification cracking. To address these issues, proper packing selection is essential to improve joint performance. The standard procedure for aluminum/steel LW is as follows (see Figure 13):
1. In the first stage, the wire melts under the action of laser irradiation into droplets, which are deposited on the steel surface (Fig. 13(a));
2. In the second stage, the melting process begins at the edge of the Al matrix metal, resulting in the liquefaction of the Zn-Al alloy, which is subsequently combined with the liquid Al-Metal species. As a result, a molten pool is formed (Fig. 13(b));
3. In the third stage, there is mutual diffusion between Fe and Al (Fig. 13(c));
4. A supersaturated solid solution is formed in the fourth stage, which is transferred to the IMC by a diffusion reaction (Fig. 13(d)).
Fig.13 Schematic diagram of aluminum-steel LW process: (a) the first stage, (b) the second stage, (c) the third stage, and (d) the fourth stage.
Laser brazing is a more efficient welding method compared to the traditional LW method. This technology allows for precise control of the heat input and effectively inhibits the growth of IMCs. Therefore, based on its welding characteristics, the laser offset is used to regulate the energy of the aluminum/steel weld. Figure 14 shows a schematic diagram of the aluminum/steel laser bias welding mechanism. When the laser beam is applied to the surface of the steel, the steel melts from the surface to the interface, forming metallic vapors and small holes (Figure 14(a)). The keyhole continuously absorbs laser energy, resulting in an increase in temperature and melting of the aluminum-based metal. The melt pool contains a higher quality molten steel compared to Al, resulting in the diffusion of the Al and Fe elements followed by the formation of metallurgical bonding. The diffusion of chemical elements is shown in Figure 14(b).
Fig. 14 Schematic diagram of the laser bias welding mechanism of aluminum/steel: (a) laser irradiation and (b) elemental diffusion.
Laser power and welding speed have a significant impact on the structure and properties of aluminum/steel LWed joints. Therefore, it is very critical to choose reasonable aluminum/steel LW process parameters. It has been reported that laser power affects diffusion behavior and interface microstructure. When Al and Al-Si coated steels are laser lap welded at lower laser powers, the IMC formed at the Al/steel interface is Fe2Al7Si (Fig. 15(a)). As the laser power increases, the interfacial temperature increases gradually. When the interface temperature exceeds the melting point of Fe2Al7Si, only needle-like FeAl3 IMCs are formed at the interface (Fig. 15(b)). With further diffusion of Fe atoms, the Fe2Al5 phase begins to form (Figure 15(c)). When the laser power is high, Fe2Al5 and FexZnx IMCs are formed at the interface (Fig. 15(d)).
Fig. 15 Diffusion behavior and microstructure evolution of the steel/aluminum interface at different laser powers: (a) 1500 W, (b) 1700 W, (c) 1900 W, and (d) 2100 W.
The effect of rotational lasers on the microstructure evolution and performance of welded joints has been studied. The results show that the rotational laser brazing of aluminum and steel can obtain higher joint strength than traditional laser brazing, which is mainly due to the lower energy density of the rotary laser brazing joint, resulting in a thinner IMC layer thickness and improved joint quality.
Regarding the heat input factor, Jin et al. investigated that the type of IMC remained constant at different laser powers, as shown in Figure 16. At a laser power of 1500W, Fe2Al5-xZnx and FeZn10 were identified as reactive layers. Studies have shown that key factors such as laser offset distance, laser power, and welding speed can improve the strength of aluminum/steel butt joints. The results show that with the change of laser power, welding speed and defocusing distance, the joint strength increases first and then decreases, as shown in Figure 16.
图16 1500–1900 W激光功率范围内激光铝/钢接头的界面微观结构：: (a) 1500 W, (b) 1700 W and (c) 1900 W。
Laser-arc composite welding (LAHW) technology has been widely used in many welding fields due to its good welding ability and deep penetration ability. However, similar to a single deep penetration LW, the keyway is in a fluctuating state, and the keyway periodically collapses, leading to the formation of air bubbles and eventually pore defects in the welded joint. At the same time, an external magnetic field was also used to suppress the porosity in a single laser weld.
Currently, researchers are showing great interest in magnetically assisted laser beam welding. Fig. 17 is a schematic diagram of aluminum alloy laser-MIG composite welding under the action of external magnetic field.
Fig.17. Schematic diagram of laser-metal-inert gas (MIG) composite welding assisted by an external magnetic field
Fig.18 Schematic diagram of the RSW process
Fig.19. Appearance and mechanical properties of aluminum/steel RSWed joints
Fig.20 Macroscopic morphology of interlocking electrodes
Fig.21 Fracture mechanism of oxide film on aluminum/steel RSWed joint[161]:(a) RSWed interface of alumina alloy and steel and (b) RSWed interface of unaluminized alloy and steel.
Fig.22 The temperature change of the interface center and the growth process of the Fe2Al5 layer thickness under different electrode combinations
conclusion
Due to the increasing demand for lightweighting in automobiles, dissimilar welding of aluminum alloys to steel is a key issue. Due to the presence of IMC at the interface, this poses a significant challenge for dissimilar welding of aluminum/steel. On the basis of summarizing and reviewing the dissimilar welding of aluminum alloy and steel, the following conclusions and future development trends are summarized:
- The formation of IMCs at the aluminum/steel weld interface is unavoidable. Optimization of process parameters, use of optimized FSW tools, selection of filler material and joint type, addition of intermediate layers, RSW with cover plate and application of external energy field can effectively control the thickness of the IMC layer and improve the mechanical properties of aluminum/steel welded joints.
- Controlling the temperature of the aluminum/steel interface is an innovative measure to inhibit the overgrowth of the IMC layer. For aluminum-to-steel FSW, the interface temperature should be controlled at 250–500°C. For LW of aluminum and steel, the interface temperature should be controlled at 1052–1270 °C when using zinc-aluminum filler and 860–1032 °C when using Al-Si filling. For RSW of aluminum and steel, the intermediate stage of the RSW process, which is the fastest growing stage of Fe2Al5 IMC, should be controlled.
- It was found that the use of intermediate transition layers such as HEAs, Cu, V, and Ni/Cu could inhibit the formation of Fe-Al IMCs and improve the quality of LWed connectors. The use of Zn, HEA, AlSi12, Als-Mg, AlCu28, Al and Al2O3, and Ni+Al2O3 also limited the formation of Fe-Al IMCs in the RSW process of different materials.
- The development of composite welding methods includes the combination of multiple joining techniques to reduce the weaknesses of individual technologies and improve the performance of composite welded joints. New composite welding techniques, such as resistive element welding, resistance riveting, friction stir riveting, and laser riveting, require further study of their process characteristics. The design of rivets, the selection of electrodes, process parameters and temperature distribution are worth exploring through experiments and finite element methods.
- The growth of the IMC layer of the aluminum/steel joint is related to the interfacial temperature. Reducing the thickness of the IMC layer is beneficial to improve the mechanical properties of the joint. However, there is a lack of systematic research on the precise temperature control of the aluminum/steel interface. Development of a finite element method for predicting temperature distributions, which will be valuable for optimizing process parameters.
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