Research on Aluminum-based Brazing Filler for LF21 Aluminum Alloy Vacuum Brazing

LF21 is an aluminum alloy with high plasticity, good workability, and good corrosion resistance and weldability. It is especially suitable for the production of deep-rolled parts, welded parts, and parts that work in corrosive media under light loads. Therefore, it is widely used. In the field of aerospace. The development of a low melting point, strength is in line with the requirements, and a certain anti-corrosion properties of the material is the key to solving the vacuum welding of LF21 aluminum alloy. The general vacuum welding aluminum alloy is mainly made of Al-Si system material, which is mainly based on eutectic composition (containing Si7%12%). The eutectic alloy has good wettability and fluidity, and its corrosion resistance is also very good. The adulterated material has certain plasticity, and is the most widely used one, but its melting point is relatively high.

This article mainly based on Al-Si eutectic, add Mg, Cu, Ni and other elements to reduce the melting point of the traitor, improve the wettability of the traitor, and study the effect of the change in its content on the performance of the fraying joint, and finally use The scanning electron microscopy and energy spectrum analysis methods analyze the microstructure of the new adult material and its vacuum welded joints.

2 Test materials and methods 2.1 Test parent materials 2.2 Test solders 2.3 Test method The 464 DTA differential thermal analyzer was used to conduct a melting point test, and the appropriate melting point of the melting point was selected to conduct the test on the spread performance of the adult material according to GB 11364-89 (see) Determine the test specifications, and perform the welding of the shear strength properties of the frazzled joints. The dimensions of the joints shall be in accordance with GB11363-89. After the welding is completed, the scanning, electron microscope shall be used to observe, analyze and contrast the different materials of the traitor and the joints.

Spreading Test Specimen, Brazed Welded Joint Lapping Shear Test Result and Analysis Table 2 Spreading Performance Test Results Solder Spreading Area/mm2 Test Value Average 1186, Table 3 Measurement Data of Solder Joint Shear Strength 3.1 Solder Melting Point The test results showed that the melting point of the material was too high and neither of them was suitable. The results show that the addition of Cu, the alloying element, can significantly reduce the liquidus temperature of the aluminum-based material. However, when the content of Cu is lower than 10%, the melting point of the material is not greatly reduced, and the Cu content is reduced by about 20%. More obvious.

3.2 Spreadability Test Results The spreadability test results are shown in Table 2.

It can be seen from Table 2 that the fluidity of No. 2 and No. 3 traits is significantly higher than that of No. 1 traits, indicating that the addition of Cu and Si elements improves the fluidity of the traitors and increases as the content of Cu and Si increases. The fluidity of the material also increases.

3.3 Determination of shear strength of solder joints The data of the measurement of shear strength of the spilt joints are shown in Table 3.

It can be seen from Table 3 that the excessive addition of Cu to the general assembly increases the brittleness of the traitor, which is detrimental to the performance of the joint. The addition of a certain amount of Ni can replace the content of Cu to a certain extent, so as to reduce the plasticity of Cu traits. The destruction. And all the cracks of the adulterated welding specimens that were stretched at room temperature occurred on the rapist, indicating that the fratricide was the weakest area of ​​the joint.

3.4 Microstructure analysis 3.4.1 The microstructure analysis of the solder can be seen from the following. 1 The material of the spilt is mainly a network-like matrix, on which fine eutectic phases are uniformly distributed between black and white, according to Al-Si-Mg III. The metaphase diagram shows that the cullet mainly consists of a (Al) and Si phase eutectic, which also contains a small amount of Mg2Si phase. The Mg2Si phase is a solid solution strengthening phase in the aluminum alloy and dissolves in the (Al) phase. Among them, it contributes to the performance of the joints. The structures of the 2 and 3 traits are similar and are all composed of dendritic tissue. It is known from the state diagram of the Al-Cu binary alloy that the mass fraction of Cu exceeds the limit solid solubility of Cu in Al and there is a difference in the traitor. The multi-zero (Al2Cu) phase, combined with the Al-Si-Cu ternary phase diagram, shows that the adulterant consists mainly of the a (Al) phase and the lamellar Si phase and 0 (Al2Cu), while the lamellar Si phase and The brittle 0 (Al2Cu) phase is detrimental to the performance of the joint.

3.4.2 Analysis of the microstructure of the brazed joints The microscopic maps of the joints of the brazed joints of 1, 2 and 3 can be seen. 1 The spread of the adulterous material of the adulterous joint is more adequate, and the offenders are mainly composed of a single gray and white. In the black phase, during the process of welding, first, the Al in the base material dissolves into the spilt, and the Al content in the raping layer that is in contact with the base metal increases, and when it cools and solidifies, it begins to crystallize from the surface of the base material. The interfacial region crystallizes out of a grayish white a (Al) solid solution, and the center of the fratricide is mainly composed of a (Al) phase and a black flaky Si phase eutectic; The interface of the virginity interface is mainly a gray (a) phase, but there is a wide range of irregular structures in the center of the fratricide, mainly composed of a large number of black Si phase segregation and brittle 0 (Al2Cu) phases, and these The performance of the solder joint is disadvantageous. From the joint organization of the 2 traits, it can be seen that the grain grows and the culprit extends toward the base material. According to the analysis, due to the diffusion of element Si, Cu, etc. from the material into the grain boundary, the diffusion is faster than the diffusion into the grain boundary. At the same time, there are more low-melting substances such as segregation of impurities on the grain boundary of the parent material, and Si that has diffused into the grain boundary. The eutectic reaction of Cu and other elements may occur, which further reduces the melting point and causes liquefaction of the grain boundary. This may lead to insufficient local material and void defects. Such a joint organization is bound to have an adverse effect on its performance. Therefore, we can conclude that the mechanical performance of the joints that are different from one another is better than the performance of joints that are between two and three thousand.

3 Conclusions The Al-Si-Mg material is suitable for the vacuum welding of LF21 aluminum alloy and can obtain excellent joints; Cu element can greatly reduce the melting point of the material, and at the same time, it also improves the fluidity of the material. Has played a significant role, but has an adverse effect on the performance of the joint; the addition of Ni element can replace part of the Cu element content, reduce the damage of Cu on the plasticity of the adulterous material, and is conducive to the improvement of joint performance; Si element can also be reduced to some extent The melting point of the fornicating material increases the fluidity of the fornicating material. However, if the Si content is too high, the segregation of Si at the offending seam will occur, which will affect the mechanical properties of the joint.