Influence of Filler Alloy on Microstructure and Properties of Induction Brazed Al/Cu Joints

2022 This work aimed to clarity the influence of filler alloy on microstructures and properties of induction brazed Al/Cu joints. It was found that the alloying elements in the filler alloy changed the morphology and phase type of interfacial layer in the joint. Mg converted the native Al 2 O 3 film into MgO and stopped the re-oxidation of aluminum. However, excessive Mg caused planar inter-metallic compounds (IMCs) to become wavy, which decrease the ductility of the joint. A suitable amount of Cu and Si removed residual oxide film and resulted in a thin planar IMCs layer, which is beneficial to Al/Cu joint. Al-8Si-4Cu-2Mg-1Ga-0.05Ce filler foil produced an excellent joint consisting of a 2μm Cu 9 Al 4 /CuAl 2 planar layer and free from oxide film. The tensile strength of the joint is higher than that of aluminum. The bend angle is higher 130°. The electrical resistivity of the joint is lower than the theoretical


Introduction
Copper and aluminum have excellent electrical conductivity. Joining of copper to aluminum is often required to transport electric current in many application fields, such as electric vehicles, high-speed railway and power grid. Mechanical connections cannot provide reliable current transmission because the oxide film on aluminum surface can cause the high contact electric resistance. Welding is more suitable for joining Al and Cu than mechanical joining due to its metallurgical reaction. However, inter-metallic compounds (IMCs) will present in the Al/Cu welded joints in the form of Cu x Al y . These IMCs can affect the mechanical properties and electrical properties of Al/Cu joints owing to their brittleness and resistivity characteristics. Many works have focused to control the IMCs by optimizing welding process parameters [1][2][3][4][5][6][7] . Inserting a filler material between Al and Cu is one good method to adjust the phase type, thickness and morphology of Cu x Al y 8 . Meantime, it is helpful to prevent aluminum and copper from re-oxidizing when welding under non-vacuum condition. Some pure metals and alloys have been tried as filler materials to limit the harmful effect of IMCs on the joint properties, such as Ni,Ti,Zn,Zn-Al and Al-Si.
Ni and Ti have high melting point and do not react with Cu and Al, which can prevent the formation of Cu x Al y during welding of copper to aluminum. Yang et al. 9 reported that Ni can prevent the formation of Cu x Al y in the brazed joint, but a new AlNi formed in the joint. AlNi IMC became a source of crack initiation and caused brittle fracture of the joint, thereby decreasing the tensile strength. Further work showed that a few microns thick Al 3 Ni layer in diffusion bonded Al/Cu joint reduced the joint tensile strength 10 . Sahu et al. 11 found that although Ni and Ti interlayer prevented the formation of Cu x Al y during friction stir welding, they became an initiation point of failure and reduced joint ductility.
Zn has low melting point and high diffusion rate. It can react with Cu and Al to form Cu x Al y Zn z and then limit the formation of Cu x Al y . Zhang et al. 12 believed that the formation of Cu x Al y Zn z by Zn interlayer was due to no binary Al x Zn y in Al-Zn binary diagram. Further work showed that both Al x Cu y Zn z and Cu x Al y formed in the friction stir welded Al/Cu joint with Zn interlayer 13 . In friction stir spot welding of Al to Cu [14][15][16] , the Zn interlayer can change Cu 9 Al 4 to Al 4.2 Cu 3.2 Zn 0.7 and decrease the continuous and thickness of CuAl 2 layer, and then improve the tensile shear strength of Al/Cu joint. In ultrasonic spot welding of Al to Cu 17 , the Zn interlayer can lead to the formation of a eutectic structure instead of brittle Cu x Al y, thereby increasing the shear tensile strengths of the joint. In diffusion bonding 18 , the Zn interlayer can stop the formation of Cu x Al y at low diffusion bonding temperature of 200°C and become nucleation point for Cu x Al y at high temperature of 300°C~400°C. Meanwhile, a zinc coating on copper surface can stop the copper re-oxidizing during squeeze casting of Al/Cu bimetal 19 . In friction stir welding of copper to aluminum in air 20 , a Zn interlayer can eliminate the oxide film and reduce the thickness of solidified structure. Although the Zn filler metal can improve the joint strength, the newly formed Cu x Al y Zn z will reduce the ductility of the joint owing to their high hardness, synonymous of low ductility 14 .
Zn-Al alloy is a commonly filler metal for brazing Al and Cu. The main research focused on the effect of Zn-Al composition on the IMCs in interface layer and brazing seam. For Zn-(15~28)Al filler material 21 , Al x Cu y Zn z was formed in interface layer and CuAl 2 was formed in brazing seam. For Zn-(2~25)Al filler material 22 , the amount of bulk Cu x Al y *e-mail: wxuegang@163.com in brazing seam was decreased with the increase of Al in Zn-Al filler material. The thickness of interfacial layer and the morphology of Cu x Al y in brazing seam were also changed with the variation of Al in Zn-Al filler material 23 . For Zn-22Al-(0.5~2)Si filler material, Si can prevent the diffusion of atoms and change the growth rate and morphology of CuAl 2 , which improve the corrosion resistance of the joint 24 . Moreover, Al x Cu y Zn z was formed in the interface layer when Zn-22Al-1.5Si filler material was used to ultrasonic assisted braze Al to Cu 25 . For Zn-22Al-xTi filler material, the doping of Ti in Zn-22Al filler material can limit the formation of IMCs and change the morphology of Cu 9 Al 4 26 . Similarly, the addition of Ce in Zn-22Al-0.05Ce filler material can decrease the growth rate of IMCs and then increase the joint shear strength 27 . That is to say, adding Si, Ti and Ce to Zn-Al filler metal is beneficial to control IMCs and improve joint performance.
Al-Si alloy is another filler metal for brazing or transient liquid phase bonding of Al to Cu. Weigl et al. 28 found that Al-12Si filler materials decreased the local formation of IMCs and then significantly enhanced the ductility of Al/ Cu joint. Li et al. 29 considered that it was difficult for Si to form IMCs with Cu or Al due to its non-metallic properties, which can control the growth of IMCs. The joint brazed with Al-12Si filler metal had the highest tensile strength and the joint brazed with Al-5Si filler metal had the highest electrical conductivity 30 . In transient liquid phase bonding with Al-11Si-4Cu-2Mg and Al-4.5Si-1Cu-1Mg interlayer 31 , Mg had no influence on the formation of IMCs and Si decrease the thickness of IMCs by suppressing the growth of CuAl 2 , which lead to a high shear strength and conductivity joint.
As mentioned above, many kinds of pure metal and alloy have been tried as filler materials to control Cu x Al y and stop oxidation during welding of Al to Cu. Alloy is more suitable as filler materials than pure metals to attain high quality joint by comprehensive utilization of alloying elements. However, few works have achieved the best match of strength, ductility and conductivity of Al/Cu joint. In this work, four Al-Si-Cu-Mg-Ga-Ce filler foils were developed to induction braze copper to aluminum. The influence of alloying elements on microstructure and proprieties of the joint was investigated, and the relationship between IMC and joint performance was also discussed.

Materials and Methods
Commercially pure copper plates in size of 5mm×50mm×50mm and commercial pure aluminum plates in size of 5mm×50mm×60mm were utilized for parent materials in this work. The faying surface to be brazed was 5mm×50mm. Four different alloys were used as filler materials in the form of foil and their composition were given in Table 1. All filler foils had the same content of Ga and Ce and different content of Si, Cu and Mg.
The filler foils were made using a rapid solidification method as shown in Figure 1. Firstly, these six components were mixed according to the designated composition and arc smelted into a bulk master alloy. Then, the bulk master alloy was put into a quartz tube and induction heated to fully molten state. Finally, the molten master alloy was pushed to the rotating copper roller by high pressure argon gas. A foil was prepared by rapid cooling from molten state to room temperature solid state. Figure 2 showed the schematic diagram of induction brazing for butt Al/Cu joints. The induction brazing process was similar to our previous work 32 . The aluminum plate and the copper plate were fixed using mechanical clamps. The filler foil was inserted between the faying surfaces. An axial loading was employed to fix the filler foil and provide brazing pressure. A thermocouple was located at the Al side near the faying surface to measure the induction brazing temperature. An induction coil was installed around the outside of copper plate to supply heat energy. A distance between the induction coil and the welding zone was 11mm to ensure the uniform temperature in the faying area. The welding area was covered with a flow of 30L/min argon gas to stop the re-oxidation of faying area. The induction brazing temperature was 590°C~600°C, which was 60°C higher than the melting point of filler foil. The induction  brazing time was 2s. The induction brazing pressure was firstly 4MPa before the filler foil melted and then 9MPa after the filler foil melted. The filler foil and the Al/Cu joints were characterized with scanning electron microscopy (SEM) of Hitach SU70, energy-dispersive X-ray spectroscopy (EDS) of Horiba Ex250 and X-ray diffraction (XRD) of PANalytical B.V XPert. The tensile strength and bending ductility of the Al/Cu joints were tested using a universal test machine. The tensile specimen and the bending specimen were machined in the dimensions, 110mm×12mm×5mm and 110mm×10mm×5mm, respectively. Both tests were carried out at room temperature with a loading rate of 5 mm/min. The joint tensile strength as well as the joint bending ductility was obtained from the average value of three measures. The electrical resistance of the Al/Cu joint was tested using micro-ohmmeter of Tinsley 5898 with four-point method. The electrical resistivity of the joint was then calculated based on the size of the tested sample and the electrical resistance. Figure 3 is the appearance of Al-8Si-4Cu-2Mg-1Ga-0.05Ce filer foil. The filler foil is homogenous and continuous without voids. However, there is some porosity in the foil SEM image. This may be due to the fact that the surface of the copper roller is not completely flat. When the molten alloy is cooled on the surface of the copper roller to form the thin filler foil, the uneven surface of the copper roller will generate porosity on the surface of the filler foil. The porosity of the foil does not affect the brazing of copper to aluminum as the foil elements can diffuse quickly to fill the porosity during heating and then form a thin homogenous liquid layer. Other three foils have the same appearance. The melting temperature of these foil are between 530°C and 540°C, and their thickness are between 60~80μm. Figure 4 shows the results of XRD and EDS of Al-8Si-4Cu-2Mg-1Ga-0.05Ce foil. The main phases in the foil are   Al, Al 9 Si and CuAl. Mg and Ga are observed in the EDS, but no obvious Mg-or Ga-containing phases can be found in the XRD possibly owing to their low content. No Ce can be measured by EDS and XRD due to its quite low content. Other three foils have the similar results of XRD and EDS as this foil. Figure 5 shows the microstructures of Al/Cu joints induction brazed using four filler foils. A thin interfacial layer is found in all induction brazed joints. There is no eutectic phase in the joints which is different from the braze joint made by Zn-Al filler metals [21][22][23][24][25][26][27] . The thickness of the interfacial layer is about 2~3μm for all joints, which is far less than the thickness of the filler foil with 60~80μm. This indicates that the filler foil has achieved sufficient diffusion during induction brazing. In other word, this six-element filler alloy has the potential to induction braze copper and aluminum in short bonding time. The interfacial layer has four types of morphology, which are discontinuous, delamination, planar interface and wavy interface. The reason should be the different matching of alloying elements in the filler foil. The characteristics of these interfacial layers are studied in detail below.

Joint microstructure
The interfacial microstructure of the induction brazed Al/Cu joint by Al-8Si-2.5Cu-1Ga-0.05Ce foil is identified using EDS and XRD, as shown in Figure 6 and Figure 7, respectively. The continuous layer on copper side is Cu 9 Al 4 . The discontinuous mixed layer on the aluminum side is Al 2 O 3 and CuAl 2 . The distribution of O in the interfacial microstructure shows that Al 2 O 3 locates between Cu 9 Al 4 and CuAl 2 . Xu et al. 33 founded that the native Al 2 O 3 was broken and located at the interface of Cu/CuAl 2 during ultrasonic bonding of Al and Cu. Sheng et al. 34 reported that the newly Al 2 O 3 formed at the interface of CuAl 2 /Al during the heat treatment of cold rolled Al/Cu joint. Different from these two joints, Al 2 O 3 is between CuAl 2 and Cu 9 Al 4 in the induction brazed joint. This should be a combination of original Al 2 O 3 and newly formed Al 2 O 3 . Although the surface of aluminum is machined before induction brazing in this work, a thin Al 2 O 3 film will form immediately in air. Xu et al. 35 reported that the thickness of the native Al 2 O 3 film is about 5nm. The thickness of Al 2 O 3 is about 1μm in this joint. This indicates that some new Al 2 O 3 form during induction brazing. In other words, Al-8Si-2.5Cu-1Ga-0.05Ce filler foil can disrupt Al 2 O 3 but cannot remove Al 2 O 3 during induction brazing in non-vacuum condition. The distribution density of Si on copper side is higher than that on aluminum side and there is no Si segregation in the joint. This indicates that Si has diffused sufficiently from the filler foil to base metals, and the diffusion rate of Si to the copper side is faster than that of the aluminum side. Figure 8 and Figure 9 show the EDS and XRD results of the interfacial microstructure of Al/Cu joint induction brazed by Al-6Si-2.5Cu-2Mg-1Ga-0.05Ce foil. A three-layer composite structure of Cu 9 Al 4 /MgO/CuAl 2 is formed between copper and aluminum in the joint. Cu 9 Al 4 layer is continuous, and CuAl 2 and MgO are discontinuous. The IMCs layer is     delaminated by MgO. In the work by Cheng et al. 36 , the native Al 2 O 3 film can be converted into MgO and MgAl 2 O 4 by Mg during brazing of 5083 aluminum alloy. In this work, the original Al 2 O 3 film is fully transformed into MgO when the filler foil contains 2% Mg by weight. This indicates that it is beneficial to add Mg in the filler foil to remove the native Al 2 O 3 film. Moreover, the thickness of MgO is greater than 5nm. Some MgO are formed during induction brazing in air because Mg is more active than Al. That is to say, Mg in the filler foil can stop the re-oxidation of aluminum.
A two-layer interfacial structure is formed in the Al/Cu joint by Al-8Si-4Cu-2Mg-1Ga-0.05Ce foil, as shown in Figure 5c. The XRD results show that the two-layer structure is Cu 9 Al 4 /CuAl 2 , as shown in Figure 10. Different from the interfacial structure by Al-6Si-2.5Cu-2Mg-1Ga-0.05Ce, there is no MgO in this joint. Both Cu 9 Al 4 layer and CuAl 2 layer have homogenous structure and planar interfaces. That is to say, more Cu and Si in the filler foil can fully extrude the newly formed MgO and result in a thin IMCs layer.
The XRD and EDS results of the joint induction brazed by Al-8Si-4Cu-2.5Mg-1Ga-0.05Ce filler foil are shown in Figure 11 and Figure 12. Similar to the joint by Al-8Si-4Cu-2Mg-1Ga-0.05Ce filler foil, thin Cu 9 Al 4 layer and CuAl 2 layer are formed between copper and aluminum. However, the interface between CuAl 2 layer and aluminum is wave, as shown in Figure 5d. This indicates that excess Mg in the filler foil can change the interface of IMCs from planar to non-planar. Lumley et al. 37 reported that more addition of Mg in aluminum alloys can result in expansion of the sintered aluminum alloy. Wei et al. 31 reported that excessive addition of Mg in Al-Si filler foil can lead to a rough interface during transient liquid phase bonding of copper and aluminum. This indicates that Mg in the filler foil can not only remove the Al 2 O 3 film, but also affect the interface morphology. Figure 13 shows the tensile and bending properties of Al/Cu joints made with four different filler foils. All joints are necked on the aluminum side during the tensile test, indicating that the tensile strength of all joints is higher than that of aluminum. The bending angle of all joints is different. The joint made by Al-8Si-4Cu-2Mg-1Ga-0.05Ce filler foil (#3) does not fail when it is bend to 130°, indicating an excellent ductility. The others three joints fail when bending angle is lower than 90°, showing a low ductility. Based on the microstructure and mechanical properties, a    thin laminar IMCs layer can result in a high quality Al/Cu joint in strength and ductility. However, both Al 2 O 3 (or MgO) and the non-planar interface of IMCs layer can decrease the ductility of the joint. Figure 14 shows the bending fracture of the joints made using Al-6Si-2.5Cu-2Mg-1Ga-0.05Ce filler foil (#2) and Al-8Si-4Cu-2.5Mg-1Ga-0.05Ce filler foil (#4). There are some tearing ridges in the fracture surface of the two joints as shown in Figure 14b and Figure 14c. This indicates that both ductile fracture and brittle fracture occur simultaneously when bending tests are performed in the two joints. A large number of inter-granular fractures are also found in both fracture surfaces as shown in Figure 14a and Figure 14d. That is to say, both joints have low ductility because inter-granular fracture is a characteristic of brittle fracture. Moreover, cracks are observed in the fracture surface of the joint made by Al-6Si-2.5Cu-2Mg-1Ga-0.05Ce foil (#2) and dimples are found in the fracture surface of the joint made by Al-8Si-4Cu-2.5Mg-1Ga-0.05Ce foil (#4), as shown in Figure 14b and Figure 14d, respectively. This is the reason that the ductility of the former joint (bending angle=25°) is lower than that of the latter (bending angle=60°). Figure 15 shows the electrical resistivity of Al/Cu joints made by different filler foils, the resistivity of copper and aluminum are also given. The order of electrical resistivity of all the joints induction brazed with different filler foils is Cu<#3<#2<#1<#4<Al. Based on the microstructure and electrical properties, a planar continuous IMCs layer can lead to the best conductivity, while a wavy continuous IMCs results in the lowest conductivity. Abbasi et al. 38 proposed theoretical resistivity to evaluate the conductivity of Al/Cu joint. The theoretical resistivity was decided by the volume fraction and resistivity of copper and aluminum under ignoring the existence of interfacial IMCs. In this study, the resistivity of Al/Cu joint made using Al-8Si-4Cu-2Mg-1Ga-0.05Ce foil (#3) is lower than the theoretical resistivity, showing an excellent conductivity.

The role of alloying elements in filler foil
According to Al-Cu binary phase diagram, Al and Cu can form eutectic liquid phase at 548°C. Theoretically, Al and Cu can be joined by the eutectic phase when it is cooled to room temperature. In the work by Han et al. 39,40 , an IMCs layer and a eutectic phase(α-Al+CuAl 2 ) were formed when copper and aluminum were hold at 550°C~580°C for 10~25min. The joint was brittle due to inhomogeneous bonding by different morphologies of IMCs, such as island, dendritic and layer. In this work, Al-Si-Cu-Mg-Ga-Ce alloy was used as filler material to induction aluminum to copper and produced a high quality Al/Cu joint. The influence of alloying elements on controlling IMCs and removing oxide film are discussed based on the results above.
Al is the main element in the filler foil. From the perspective of welding metallurgy, the main element of the filler metal should be the base metal element. So, Al and Cu can be used as main component in the filler metal. However, the melting temperature of Cu (1083°C) is higher than that of Al (660°C). An elevated brazing temperature is needed to melt the filler metal when Cu is used as main element in the filler metal. The higher the welding temperature, the easier it is to form IMCs. Therefore, using Al as main element is more conducive to controlling the formation of IMCs.
Cu atom has high diffusion ability due to its small atomic radius of 1.28Å 33 . It can diffuse into Al 2 O 3 film with a few nanometers thick 41 . The native surface Al 2 O 3 film on aluminum is about a few nanometers. Therefore, Cu can diffuse through the oxide film and then react with aluminum base metal. According to Al-Cu binary phase diagram, Cu can form an eutectic liquid(Al-33Cu) with Al at 548°C. The surface Al 2 O 3 film can be disrupted by the eutectic liquid. Therefore, Cu in the filler foil helps to disrupt the oxide film. Theoretically, 33mass% of the Cu in the filler foil can form the most eutectic phase, which is good for disrupting the oxide film. However, more Cu in the filler foil will form more Cu x Al y 42 . So the Cu content in the filler foil should be controlled within a certain range. The maximum content of copper is 5.8mass% in aluminum solid solution. In this work, 2.5mass%~4mass% Cu has the ability to disrupt the oxide film and control IMCs.
Si can form a eutectic phase (Al-12Si) with Al at 577°C according to Al-Si binary phase diagram. The eutectic phase can also help to disrupt the surface oxide film. Moreover, Si atom has good compatibility with Al atom and Cu atom, which can enhance the liquid fluidity. Turriff DM 43 found that the surface oxide film could not prevent the diffusion of Si when there is liquid between faying surfaces. Therefore, Si has the ability to disrupt the oxide film. Mazar Atabaki et al. 44 found that Si in filler metal was beneficial to the partial disruption of oxide film. Further work showed that Si in filler materials could inhibit the formation of CuAl 2

31
. Therefore, the addition of Si in the filler foil is beneficial to disrupt the oxide film and control Cu x Al y . In the work 28,45,46 , the filler material was often added with 12mass% Si, which can form the most eutectic phase at a lower welding temperature, so as to control IMCs. In this study, Al-Si-Cu-Mg-Ga-Ce multi-element alloy was used as filler alloy. The Si content in the filler alloy was relatively high, ranging from 6mass%~8mass%. The high content of silicon in the filler alloy can improve the wettability of the liquid filler alloy to copper and aluminum solid substrate. According to the Al-Cu-Si ternary phase diagram, 6mass% Si can form a ternary eutectic liquid phase at 524°C. The liquid filler alloy can fully fill the welding area at welding temperature (590°C~600°C) and then increase the diffusion path between copper and aluminum. Since Si has good compatibility with copper and aluminum. Si can diffuse into both copper and aluminum base metals and avoid the formation of Si-containing phases in the joint. Therefore, Si in the filler alloy was beneficial to avoid the presence of Si-containing phases, and only Cu x Al y phases appeared in the joint, as shown in Figure 16a. In addition, Al and Cu was induction brazed by the inter-diffusion of copper and aluminum. Some Cu x Al y phases were formed when copper and aluminum diffused to a certain concentration in the joint area. The characteristics of the Cu x Al y phase determined the welding of Cu and Al. Si diffused towards the copper and aluminum on both sides, but more towards the copper side during induction brazing, as shown in Figure 16b~d. Si decreased the inter-diffusion between copper and aluminum, especially the diffusion of copper to the aluminum side, thereby suppressing the formation of the Cu x Al y phases.
Mg . When there is no Mg in the filler foil, the Al 2 O 3 will stay in the final joint. When Mg content is 2mass%, the surface Al 2 O 3 film is decomposed to small MgO particles. However, 2.5mass% Mg in the filler foil changes the interface morphology of CuAl 2 from planar to non-planar. The reason may be that excess Mg disrupts the equilibrium state of liquid near aluminum side and then leads to some rough nucleation for CuAl 2 . Overall, Mg can decompose the oxide film and change the interface morphology of IMCs.
Gallium can form a eutectic liquid phase (Al-99Ga) with aluminum at 26.6°C. The eutectic phase can remove surface oxide film and enhance the bonding of aluminum. Bhadeshia 48 and Mahmoudi Ghaznavi et al. 49 have reported that gallium can be used as filler metal to flux-free braze pure aluminum and transient liquid phase diffusion bonding of aluminum alloy. However, gallium will segregate in the aluminum grain boundaries, leading to brittleness. Therefore, there are some restrictions when using gallium as filler metal, such as low welding temperature, short welding time and controllable dosage. In this study, gallium is added into the aluminum alloy filler foil to disrupt the oxide film during induction brazing. Meantime, the content of gallium in filler foil is 1mass% to avoid the detrimental effect of liquid gallium on the aluminum by minimizing its dosage. In addition, solid state foil is easier to handle than liquid gallium for brazing.
Rear earth elements can segregate at grain boundary and then change the formation mechanism of Al 2 O 3 at high temperature, such as Y, Ce, La and Hf. Ji F 50 found that 0.03~0.05mass% Ce in Zn-Al-Ce filler metal can decrease the grain size of filler metal and improve its wettability on aluminum base metal. In this study, 0.05mass%Ce is used to improve the wettability of filler metal and then enhance induction brazing.
Different from the existing Zn-Al and Al-Si filler metal, a multi-element alloy of Al-Si-Cu-Mg-Ga-Ce is developed as filler foil to disrupt oxide film and control IMCs in this study. Chang 51 calculated liquidus projections of Al-Cu-Mg-Si in the Al corner, as shown in Figure 17. There are three quaternary eutectic point, which are Al-27.2Cu-2.7Mg-5.3Si at I 1 , Al-32.9Cu-7Mg-0.6Si at I 2 , Al-1.2Cu-34Mg-0.1Si at I 3 , as shown in Equation 3, 4 and 5, respectively. It can be found that some intermetallic compounds are formed when the liquid Al-Cu-Si-Mg is cooled to a solid state, such as CuAl 2 , Al 5 Cu 2 Mg 2 Si 6 , Mg 2 Si, Al 2 CuMg and (Al,Cu) 49 Mg 32 . These intermetallic compounds are harmful to the ductility. In this work, the induction brazing temperature is 590°C~600°C, which is higher than the quaternary eutectic temperature of 448°C~509°C. Therefore, these intermetallic compounds can be avoided at the brazing temperature. In other words, the multi-element alloy filler foil can control IMCs by matching the elements at high brazing temperature.

Mechanism of Al/Cu induction brazing
Brazing is a common welding method to join copper and aluminum. Usually, a flux is applied to remove the surface oxide film before brazing, such as AlF 3 -CsF. Then, a filler  metal is used to form the joint without brazing pressure, such as Zn-Al, Al-Si. The brazed joint is composed of brazing seam and interface layer. The brazing seam is often a eutectic phase related with the filler metal, and the interface layer is IMCs. Both interfacial IMCs and brazing seam can affect the joint properties. For example, the brazing seam can decrease the joint ductility. Moreover, the residual corrosive flux will adversely affect the joint properties. Therefore, many researches have been devoted to design the filler metal and fluxes to achieve a high quality joint. In this study, a novel filler metal is developed to induction braze aluminum and copper under brazing pressure. Unlike conventional brazing, induction brazing joint consists of a thin IMCs layer, which has integral properties in term of strength, ductility and conductivity. The mechanism of Al/Cu induction brazing is shown in Figure 18. Induction brazing process includes three stages of heating, holding and cooling. A brazing pressure, 4MPa before the filler foil melted and 9MPa after the filler foil melted, is applied during induction brazing, as shown in Figure 18a.
In heating stage from room temperature to the melting point of filler foil, solid-state induction happens among filler foil, copper and aluminum. The surface Al 2 O 3 film starts to detach from aluminum base metal due to the chemical reaction by filler foil, as shown in Figure 18b. Because the solid-state diffusion rate is low, the filler foil has no significant effect on the removal of surface oxide film. When the filler foil is heated to melt, the liquid filler foil will wet both surfaces of copper and aluminum. Three reactions will happen between copper and aluminum. At copper/liquid filler foil, Al atom in filler foil will diffuse into copper base metal due to the large diffusion coefficient in liquid. Copper solid solution will form firstly and then Cu 9 Al 4 when more and more Al diffuses into Cu solid solution. On the aluminum side, liquid filler foil will permeate the crevice between Al 2 O 3 film and aluminum base metal. Meanwhile, Al 2 O 3 will be changed to MgO by the chemical reaction with Mg in liquid filler foil. At liquid/aluminum interface, Cu will diffuse into aluminum base metal and forms aluminum solid solution and then CuAl 2 . Moreover, some liquid will be expelled to the edge by the increased pressure. The movement of liquid will disrupt the oxide film. In this stage, small Cu 9 Al 4 and CuAl 2 will segregate on the copper and aluminum, respectively, as shown in Figure 18c.
In holding stage, both Cu 9 Al 4 and CuAl 2 will grow up with inter-diffusion between copper and aluminum. Cu 9 Al 4 will firstly become continuous on copper side. Some CuAl 2 and MgO particles will be expelled to the edge by the flow of liquid. Meantime, a eutectic liquid phase will be formed at the temperature above 548.2°C. The eutectic liquid will also accelerate the diffusion of copper and aluminum and enhance the growth of IMCs. These two IMCs will become thick and other new IMCs will form if there are too much liquid resulted from long time holding. Therefore, the thickness of IMCs can be controlled by adjusting the thickness of filler foil and the holding time. In addition, the aluminum base metal near faying surface will be extruded partly by the brazing pressure for its low high temperature strength as shown in Figure 18d. The extrusion of aluminum can stop the faying surface from oxidation.
In cooling stage, the liquid on the edge will solidify. Both Cu 9 Al 4 and CuAl 2 will grow up and become continuous thin layer as shown in Figure 18e. Under high cooling condition with argon flux, no solid-state phase transformation occurs as both Cu and Al have low diffusion rate in solid Cu 9 Al 4 and CuAl 2 layer. So, no other IMCs are formed in this stage.
Different from brazing, the filler metal of induction brazing can fully diffuse into base metals in short induction brazing time. There is no braze seam in the joint, only a thin interface layer.

Relationship between joint properties and interfacial IMCs layer
From the viewpoint of welding metallurgy, an ideal Al/Cu dissimilar joint should be a solid solution and free of IMCs. However, it is difficult to prevent the formation of IMCs during welding due to the high affinity of copper and aluminum. The thickness and morphology of IMCs have great impact on the joint properties. In the work 52 , the strength, ductility and conductivity of friction welded Al/Cu joints were reduced when the thickness of interfacial Cu x Al y was increased. Many investigations have shown that the thickness of IMCs should be less than 2~5μm 53 . In this paper, the thickness of interfacial IMCs is 2~3μm, and all joints have high tensile strength equivalent to that of aluminum base metal. However, all joints have different morphologies, and their ductility and conductivity are different. There are controversial opinions on the impact of IMCs morphologies on joint performance. Some research found that the continuous IMCs layer can enhance the metallurgical bonding and then increase the joint strength and ductility 1,2,54 . However, other research found that discontinuous IMCs can increase the joint strength and ductility 3,31 . On the other hand, some investigations found that a planar IMCs layer can increase the conductivity, the strength and ductility of the joints 4,5 . However, other investigations thought that the wavy IMCs was good for metallurgical bonding 6,7 .
Based on the welding of aluminum to copper, there are three phase structures in the joint, such as IMCs, oxides and solid solution. The morphology of IMCs can be described using contact radius (r) and spacing (λ). The planar IMCs can be obtained when r→∞ and the continuous IMCs can be obtained when λ=0.
In multiphase alloy, second phase particles will prevent the movement of dislocation and then increase the strength. The strengthening effect can be expressed by Equation 6.
where τ is shear stress driving the movement of dislocation, G is shear elastic modulus of second phase, b is burgers vector of dislocation, λ is spacing between second phase particles.
For continuous IMCs in Al/Cu joint, λ=0. So, τ→∞. That is to say, the continuous IMCs can increase greatly the joint strength. As shown in Figure 5 and Figure 13, the bending strength of the joints with continuous interfacial layers (#3 and #4) are higher than that of the joints with discontinuous interfacial layers (#1 and #2). Comparing Figure 5c and Figure 5d, Figure 13c and Figure 13d, it was found that the two joints (#3 and #4) had the same thickness of IMCs layer and the same phase type of IMCs. However, the ductility of the two joints was different with the different interfacial morphologies of CuAl 2 . The bending angle of the joint (#4) with non-planar interface CuAl 2 was lower than that of the joint (#3) with planar interface CuAl 2 , indicating the non-planar interface of IMCs layer with the same thickness and phase type decreases the ductility of the joint. It is well known that the hardness of CuAl 2 is much higher than that of copper and aluminum. The hard CuAl 2 becomes a brittle point in the CuAl 2 /Al structure. The more CuAl 2 , the lower the joint ductility. The contact area between non-planar interface CuAl 2 and aluminum was more than that between planar interface CuAl 2 and aluminum. The ductility of the joint was then decreased due to more CuAl 2 /Al contact points resulted from non-planar interface CuAl 2 . For discontinuous IMCs in Al/Cu joint, λ≠0. So, the strengthening effect depends on IMCs, oxides and solid solution, as expressed by Equation 7.
where G IMC , G oxide and G sl are shear elastic modulus of IMCs, oxide and solid solution, respectively. λ IMC , λ oxide and λ sl are spacing between IMCs, oxides and solid solution, respectively.
Since the different contents and sizes of IMCs, oxides and solid solution can lead to different values of λ, discontinuous IMCs maybe increase the joint strength by matching the oxides and solid solutions. On the other hand, the resistance of Al/Cu joint is a sum of a resistance of copper (R Cu ), a resistance of aluminum (R Al ), a resistance of second phase (R sp ) and a contact resistance (R c ), as expressed by Equation 8. The metal to metal contact resistance (R c ) can be expressed by Equation 9  where ρ is resistivity of metal, L is contact length between metals. For continuous IMCs in Al/Cu joint, R sp is the resistance of IMCs.
Then, the contact morphology between IMCs and base metals determines the contact resistance R c . As the resistivity of IMCs is higher than that of base metals, more IMCs in the interface will reduce the contact length (L) between copper and aluminum. Compared with wavy IMCs, the planar IMCs can increase the contact length between copper and aluminum and then decrease R c . The total joint resistance R is then reduced by the planar continuous IMCs. Figure 15 also showed that the resistance of the joint with wavy continuous IMCs layer is lower than that of the joint with planar continuous IMCs layer. For discontinuous IMCs in Al/Cu joint, R sp is a sum of resistance of IMCs (R IMC ), oxides (R oxide ) and solid solution (R sl ). It can be expressed by Equation 10. According to Ohm's law, the resistance of second phases is related to their resistivity and volume. The resistivity order of these three second phases are ρ oxide >ρ IMC >ρ sl . Compared with single IMCs, oxide and solid solution may be increase or decrease the R sp , which depends on their volume. Therefore, discontinuous IMCs can increase the joint conductivity by matching the oxides and solid solutions.
As discussed above, the planar continuous IMCs can increase the strength, ductility and conductivity of the Al/Cu joint. Moreover, the discontinuous IMCs can also have a beneficial effect on joint performance by controlling the content of various second phases in the joint.

Conclusion
In this article, four types of Al-(6~8)Si-(2.5~4)Cu-(0~2.5) Mg-1Ga-0.05Ce were designed to induction braze copper and aluminum in air. The influence of Mg, Si and Cu in the filler foil on induction brazing behavior was investigated and the main conclusions are drawn below.
1. The Al-Si-Cu-Mg-Ga-Ce foils can fully diffuse into base metals in short induction brazing time. There is only a thin interfacial layer in the joint and free from eutectic phase. 2. Mg in the filler foils can convert the native Al 2 O 3 film into MgO. It is helpful to stop the re-oxidation of aluminum base metal in non-vacuum induction brazing. However, excessive Mg will change the interface morphology of IMCs from planar to nonplanar. Cu and Si in the filler foil are beneficial to clear the oxide film, and a suitable amount of Cu and Si can fully remove residual oxide film and result in a thin IMCs layer.

A thin planar IMCs layer can result in a high quality
Al/Cu joint in strength, ductility and conductivity. However, residual oxide and the non-planar interface of IMCs layer can decrease the ductility of the joint. Al-8Si-4Cu-2Mg-1Ga-0.05Ce filler foil can produce an excellent induction brazed joint in argon flux instead of vacuum. The joint consists of a 2μm Cu 9 Al 4 /CuAl 2 planar layer and free from oxide film. The strength, ductility and conductivity of the joint are optimally matched. The tensile strength of the joint is higher than that of aluminum base metal. The bend angle is higher 130°, showing high ductility. The electrical resistivity of the joint is lower than the theoretical value, showing excellent electrical conductivity.