Interfacial structure and strength of Al-25Si-4Mg-1Cu joint brazed with Zn interlayer

A high-quality joint of the parent metal Al-25Si-4Mg-1Cu alloy (AlSi alloy) was brazed to itself using Zn layer as a filler material. The typical microstructure of AlSi/Zn/AlSi joint was Al-Zn solid solution, primary Si phase and α -Al phase. When brazing temperature was increased from 400 to 405 ◦ C, Zn began to melt and reacted with Al diffused from base material, then Si phase formed. While the increase in brazing temperature to 415 ◦ C caused the gradual buring of Zn from Al-Zn solid solution, and α -Al remained. Furthermore, the shear strength of the joint increased because of α -Al phase and

application was still limited, because of its difficult machinability. 5,6 Therefore, AlSi alloys need to be jointed to form a sealed integrated circuit for signal transmission, and the study of alloy joining properties is inevitable. [7][8][9][10] And, the brazed joint between AlSi alloy and itself (AlSi-AlSi joint) is necessary.
Recently, some researchers have attempted to join aluminum alloys with alloy steels by braze-welding. The research showed that the wettability and penetration of the galva-nized sheet were good, without leakage of the material at the root. 11 Liu et al. 12 studied the AlSi alloy by friction stir welding, and formed a fine α-Al and eutectic Si eutectic structure in the joint, because of the rapid cooling during the solidification. In addition, the fine structure was beneficial to improving shear strength of the joint. Wang et al. [13][14][15] investigated the hypereutectic AlSi alloy with transient liquid phase bonding under ultrasonic assistance with Zn-based solder. The research revealed that as the process developed, much more Si particles gradually migrated to the center of soldering seam and the shear strength of the joint increased to 94.2 MPa. Xia et al. 16,17 realized good brazed joint of Cu/Al with AlSi filler metal. The research showed that Mg 2 Si and Al 2 Cu formed, because of the reaction between base metal and filler alloy, and the mechanical properties of the joint was improved. However, the research about the brazing of Al-25Si-4Mg-1Cu alloy to itself was few reported.
Therefore, this paper aimed to study the mechanism of microstructure and mechanical properties of AlSi alloy brazed joints. Zn was used as interlayer because the eutectic temperature of Al-Zn is only 382 • C, which was far lower than the melting point of the base material. In addition, the mutual solubility of Al-Zn was very large, which was conducive to the generation of liquid phase during the diffusion process. This article mainly studied the effect of brazing temperature and holding time on microstructure and mechanical properties of the joints. Furthermore, it revealed the mechanism of microstructure and illustrated the effect of microstructure on the mechanical properties of the joints.

EXPERIMENT PROCEDURE
In this test, AlSi alloy base material used in this experiment adopts a rectangular parallelepiped plate with a specification of 20 × 10 × 2 mm 3 . Before brazing, the base material was polished by 400#, 800#, 1500#, and 2000# sandpaper, cleaned in acetone solution for 10 min, and put in alcohol to prevent the oxidation of the Al matrix. The solder selected for this experiment was Zn foil. The Zn foil with a thickness of 300 um was rolled to 30 um using a tablet press, washed with alcohol and 5% hydrochloric acid alcohol solution to remove surface impurities and oxide film, and put into alcohol to prevent the surface from contamination. The samples had a three-layered structure with Zn interlayer sandwiched between AlSi alloys, and the samples were fixed with a self-made device, as schematically shown in Figure 1A. The width of the overlaping was 1 mm. Fix it with a fixture and put it into a vacuum brazing furnace for brazing. In order to ensure the sample stable during brazing process, this fixture exerted a continuous pressure on the sample. In addition, the pressure was only 10 3 Pa playing a fixed role and it was too small to effect the thickness of the joint. Then the samples were put in the furnace and the vacuum degree of the furnace kept at 1.2 × 10 −4 ∼ 2.0 × 10 −4 Pa. The process parameters were determined based on the Al-Zn binary phase diagram, and the relatively precise process parameter window was selected by preliminary exploration experiments. The specific parameters were shown in Figure 1B. The cross-sections of the joints were cut by automatic precision cutting machine (Shengyang Kejing Automation Equipment Co., Ltd.). The microstructures of the brazed joints were analyzed by emission scanning electron microscope (SEM, Helio Nanolab600i, USA) equipped with an energy dispersive spectrometer (EDS, INCA Energy 300, UK). An X-ray diffraction analysis (XRD, Bruker D8A A25, Germany) was used to detect the phases in the brazing seams. During XRD measurement, the angle increment was 0.02 • with 1000 W. The shear strength of the joint was tested by Electronic Universal Tester (Instron-1186) with the rate of 0.5 mm/min. Every five joints was a group and the average value was the shear strength of the joint under the same condition.

The typical microstructure of AlSi alloy brazed joint
The typical microstructure of AlSi-AlSi joint brazed with Zn interlayer was shown in Figure 2. The full view of the joint was shown in Figure 2A and the enlarged view of zone A was shown in Figure 2B. From Figure 2A,B, it can be seen that the joint formed well without defects, such as pores and cracks. According to EDS (see Table 1) and XRD analysis (in Figure 2C), it can be inferred that the microstructure of the joint was mainly composed of Al-Zn solid solution, Si particles and α-Al phases. During brazing, Zn element diffused into AlSi alloy and formed Al-Zn solid solution. [18][19][20] Furthermore, Si element was stable under the brazing temperature, so Si element existed as Si particle phase. 21 In order to analysis the diffusion process of elements in the joint, the main elements distribution of the joint was investigated. Figure 3 shows the microstructure and the element distribution of the joint brazed at 415 • C for 12 min. It can be clearly observed that Al element diffused into the brazing seam. Zn element was mainly distributed in the center of the brazing seam. In addition, the primary Si particles appears at the center of the brazing seam. It was worth noting that Cu element and Mg element diffused into the brazing seam during the joint formation process, and no intermetallic compounds formed. It may be because the brazing temperature is not enough for Cu element and Mg element to react with other elements. Then Al-Si solid solution, Si particles and α-Al formed in the joint. This feature was also confirmed in Figure  center of the brazing seam. Therefore, it can be inferred that during brazing, Al element and Si element diffused into brazing seam and Zn was able to dissolve into Al infinitely.

Effect of brazing temperature on the microstructure and mechanical properties of the joint
To further reveal the diffusion process of elements, the effect of brazing parameters on the microstructure and mechanical property of the joints were analyzed. Figure 5 is the microstructures of the joints with brazing temperature changing from 400 to 415 • C and holding time kept at 12 min. Comprehensive observation of the four macro pictures of the joints (see Figure 5A-D). It can be seen that the width of the brazing seam narrowed slightly as the temperature increasing. The main reason may be that the Al-Zn miscibility is very large and the fluidity of the Al-Zn eutectic solution is very good. Under high vacuum, the boiling point decreased, then the Zn was lost or even burned with the temperature increasing. 20 So, the width of the brazing seam became smaller with the brazing temperature increasing. Figure 5A shows the microstructure of the joint brazed at 400 • C and the enlarged view of area A. It can be observed that micro-voids existed in the joint. The enlarged view of zone A showed that a small amount of Zn remained in the brazing seam. It may be because the brazing temperature was not enough to cause the Zn interlayer to fully react. Figure 5B was the microstructure of the joint brazed at 405 • C. It can be observed that the joint was good without defects, such as micro-voids. Besides, the enlarged image of zone B showed that Zn interlayer has fully reacted, and the microstructure was mainly composed of Al-Zn solid solution and Si particles. In Figure 5C, the joint brazing at 410 • C was also good, while Al-Zn solid solution decreased. It may be because when the brazing temperature increased to 410 • C, Zn began to evaporate and burn. Then, Al-Zn solid solution reduced and α-Al structure appeared in the brazing seam. In Figure 5D, the brazing temperature was 415 • C. The composition of the brazing seam was similar to that of the joint brazed at 410 • C, mainly composed of Al-Zn solid solution, a-Al and Si particles. Based on the above analysis, it can be inferred that the brazing temperature had a great influence on the microstructure of the joint. When the brazing temperature below 400 • C, Zn remained in the brazing joint and micro-voids existed in the joint. With the brazing temperature increasing, the Al-Zn solid solution significantly reduced, and the α-Al and Si particles started to appear in the joint.
Furthermore, the effect of brazing temperature on shear strength of the joint was shown in Figure 6. The fracture microstructure of the joints brazed for 12 min at 400 and 415 • C were shown in Figure 6A, and B, respectively. The fracture diagram was shown in Figure 6C. According to EDS analysis (Table 2), from Figure 6A, it can be seen that the fracture was combined brittleness and toughness, because the fracture was composed of plane and dimple. In addition, the mechanical properties of the Zn element were poor, which resulted in low shear strength of the joint. When the brazing temperature increased to 415 • C, the fracture was composed of Al-Zn solid solution, α-Al and more Si particles, while Zn disappeared (see Figure 6B and Table 2). Due to the difference in the atomic size of the solute and solvent of α-Al and Al-Zn solid solution, the formation of solid solution resulted in lattice distortion in a local range and formed an elastic stress field, so the shear strength of the joint was greatly increased to 65.6 MPa (see Figure 6D). 22 In addition, based on EDS analysis of the two fractures, it can be inferred that the fractures were both engendered in the brazing seam (see Figure 6C). Therefore, the analysis of the microstructure of fracture was consist with that of joint.   as the holding time extending from 8 to 14 min, the width of brazing seam narrowed. In addition, with brazing time extending, Al-Zn solid solution decreased gradually and α-Al phase appeared and gradually increased. Figure 7A is the SEM image of the joint brazed for 8 min. It can be seen that the microstructure of the joint was good. Then, the high magnification diagram of zone A showed the microstructure of the joint was mainly composed of Al-Zn solid solution and Si particles. In addition, Si particles treated as reinforcing phase in the joint, so the shear strength of the joint reached 40 MPa. Figure 7B is the SEM image of the joint brazed for 10 min. It can be seen that the amount of Al-Zn solid solution reduced and the number of Si particles increased, while α-Al phases existed. It was worth noting that the number of Si particles in the brazing seam increased, which had an important impact on mechanical properties of the joint (see the high magnification diagram of zone B). Figure 7C is the SEM image of the joint brazed for 12 min. It can be seen that the amount of Al-Zn solid solution further decreased, while the number of α-Al phases increased. It may be because with holding time extending, Zn gradually burned and Al remained. Moreover, Si particles in the brazing seam was almost no change. Moreover, it may be because Zn burned seriously with long brazing time, micro-voids appeared uniformly distributed in the brazing seam. And the micro-voids formed after Zn burning improved the mechanical properties of the joint, because they were able to absorb energy of residual stress. Therefore, the shear strength of the joint reached 65.6 MPa. In Figure 7D, it shows the SEM image of the joint brazed for 14 min. It can be seen that Al-Zn solid solution nearly disappeared, and the micro-voids in the brazing seam increased, which deteriorated the mechanical properties of the joint. According to the above analysis, it can be inferred that with brazing time extending, Zn excessively burned, and α-Al remained in the joint. The toughness of α-Al was good, so the shear strength of the joint increased. In addition, because of Zn burning, many micro-voids formed and uniformly distributed in the joint. When the number of micro-voids was suitable, the shear strength of the joint was improved. Furthermore, for the effect of holding time on mechanical properties of the joint, shear strength tests and microstructure observation of fracture were performed. Figure 8 shows the fracture microstructure of the joints brazed at 415 • C for different holding times. It can be observed that the joints were all combined brittleness fracture and toughness fracture. Figure 8A shows the fracture microstructure of the joint brazed for 8 min. It can be seen that the joint was mainly composed of a large amount of Al-Zn solid solution and a small amount of primary Si phases. With the brazing time extending to 10 min, the fracture was composed of Al-Zn solid solution and Si particles, in addition, a small amount of α-Al appeared in the fracture (see Figure 8B). With the brazing time further extending to 12 min, it can be found that the fracture was also basically composed of α-Al, Si particles, and Al-Zn solid solution. It was worth noted the amount of Al-Zn solid solution decreased, while that of α-Al and Si particles increased (see Figure 8C). Furthermore, α-Al and Si particles were beneficial to improve the shear strength of the joint and the shear strength reached 65.6 MPa. Figure 8D shows the joint brazed for 14 min. It was found that the fracture was composed of α-Al and Si particles. In addition, Al-Zn solid solution disappeared, the shear strength of the joint greatly reduced.

Microstructure mechanism of the joint brazed by Zn interlayer
According to the above analysis, the formation process of microstructure of the joint can be revealed. And the schematic diagram of microstructure mechanism of the joint was shown in Figure 9. As shown in Figure 9, the evolution process of the joint microstructure was divided into four stages, including the melting stage of Zn interlayer, the reaction stage of Al-Zn solid solution, precipitation stage of α-Al and Si particles and homogenization stage of the joint microstructure. Analysis by synthesis from Figure 9A-F, it can be inferred that as brazing temperature increasing to 415 • C, Zn interlayer began melting and Al-Zn solid solution formed. While Si particles from AlSi alloy remained in the joint, as shown in Figure 9A,B. With brazing time increasing, Zn interlayer melted completely and much Al-Zn solid solution formed, as shown in Figure 9C. Then with brazing time further increasing, Zn burned from Al-Zn solid solution and α-Al and particles remained in the joint (see Figure 9D). Finally, as brazing process going, a lot of Zn burned from Al-Zn solid solution, while much α-Al and Si particles remained in the joint, as shown in Figure 9E.

CONCLUSIONS
In this paper, the high-quality joint of AlSi-AlSi joint brazed by Zn interlayer was achieved. Then the effects of brazing temperature and holding time on the microstructure and mechanical property of the joints were investigated. In addition, the microstructure mechanism of the joint was analyzed, and the following conclusions were drawn: 1. When the brazing temperature was 415 • C and holding time was 12 min, the shear strength of the joint reached 65.6 MPa. The typical microstructure of the joint was mainly composed of Al-Zn solid solution, α-Al and Si particles. 2. With brazing temperature increasing, Zn gradually burned and the amount of Al-Zn solid solution reduced, while α-Al remained in the joint. Furthermore, the toughness of α-Al was good, so the shear strength of the joint increased.