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Research on vacuum brazing technology of dissimilar metals

1 Applications of titanium and aluminum

Titanium and its alloys have a series of advantages such as high melting point, small linear expansion coefficient and elastic modulus, high strength, non-magnetic, good medium-temperature strength and low-temperature toughness, and excellent corrosion resistance. Aluminum and its alloys have many advantages such as low density, high specific strength, good corrosion resistance, good thermal stability, easy forming, good recyclability and low price. Therefore, both are widely used in aerospace, national defense, petroleum industry and other fields. However, aluminum alloy has lower strength and heat resistance, while titanium alloy has higher density and higher price. Therefore, as the requirements for “reducing mass, improving thrust-to-weight ratio, and increasing payload” in the field of aeroengine, automobile and aircraft structural design are getting higher and higher, the need to weld aluminum alloy and titanium alloy to form a composite structure has become increasingly urgent. Therefore, the aluminum-titanium dissimilar metal composite structure has broad application prospects and use value in the future, and welding technology will play an irreplaceable key role in the optimization and design process of the composite structure.

2 Weldability analysis of aluminum-titanium dissimilar metals

First of all, in terms of physical properties, aluminum and titanium are quite different. For example, the melting point of titanium is more than 1,000 degrees Celsius higher than that of aluminum, its thermal conductivity is about 1/20 of aluminum, and its linear expansion coefficient is only 1/3 of aluminum. Therefore, when the two are welded, especially during the fusion welding process, a large welding stress will be generated, and welding deformation and cracks will easily occur under the action of stress, thereby reducing the performance of the joint and affecting its reliability.

Secondly, in terms of chemical properties, both aluminum and titanium are very chemically active and can easily form a dense oxide film on the surface, thereby reducing their surface wettability and ultimately increasing the difficulty of connecting the two. In addition, the aluminum-titanium binary alloy phase diagram (Figure 1) shows that the solid solubility between them is low, and a variety of intermetallic compounds can be formed at different temperatures, thereby increasing the brittleness of the joint and reducing the mechanical strength of the welded joint. performance.

It can be seen that selecting appropriate welding methods and processes is crucial to achieving reliable welding between aluminum and titanium dissimilar metals; and the study of the connection technology between the two has important engineering application value and scientific research significance.

3 Research status of aluminum-titanium dissimilar metal brazing

Vacuum brazing uses a metal material with a lower melting point than the base metal as the filler metal. The weldment and the filler metal are heated to a temperature higher than the melting point of the filler metal and lower than the melting point of the base metal. The liquid filler metal is used to wet the base metal, fill the joint gap and A method of mutual diffusion with the base material to achieve connection between the base materials. Therefore, this method is one of the more suitable technologies for welding aluminum-titanium dissimilar metals with large differences in physical and chemical properties.

In the late 1980s, Japanese scholar Takemoto et al. prepared Al-30Ag-10Cu, Al-10Si-1Mg, Al-10Cu-8Sn and Al-Cu-Mn by adding Cu, Ag, Si and other elements to pure aluminum. -Cr and other alloy solders, and the above-mentioned solder materials have been used to successfully realize the brazing connection of aluminum-titanium dissimilar metals. This method of lowering the melting point of solder by adding alloying elements lays the foundation for research on solder preparation in aluminum-titanium dissimilar metal brazing technology.

4 Vacuum brazing

Since the powdered solder is easily oxidized, the oxide film on its surface will hinder the mutual reactions between the solders and between the solder and the base metal. Therefore, the solder is first made into the form of box sheets, and then the aluminum alloy and titanium alloy are processed. Brazing. The selected brazing material is AlSi₁₀Mg, and the brazing temperature is controlled within the range of 580 to 640°C. The study found that compared with powder solder, foil solder can effectively reduce the adverse impact of oxide film on the welding process. On the side close to the aluminum alloy, the composition of the interface reaction layer is Ti₉(Al,Si)₂₃; while on the side close to the titanium alloy, the composition of the interface reaction layer is Ti₇Al₅Si₁₂:. Since the melting points of aluminum-based filler metal and aluminum alloy base metal are not much different, it is difficult to control the welding process. Subsequently, by reducing the content of Si element and increasing the content of Mg element in the above-mentioned solder material, Al-Si-10Mg foil solder material was prepared. The welding experiments of cp-Ti and 1050AI dissimilar metals showed that the adjustment of alloy element composition reduced The melting point of the solder is increased, so it can reduce the brazing temperature to a certain extent. However, due to the formation of discontinuous Ti₅Al₁₂Si₃ and continuous Ti₇Al₅Si₁₂ intermetallic compounds at the interface, the adverse effects of the intermetallic compounds generated by the reaction between the solder and the base metal on the joint performance cannot be avoided. In addition, it should be pointed out that the formation mechanism of different intermetallic compounds in joints still needs to be further studied, which is also one of the key areas of current dissimilar metal welding research.

In recent years, research on vacuum brazing of aluminum-titanium dissimilar metals has been carried out. In view of the important role of Si element in reducing the melting point of aluminum-based solder, research on solder preparation technology mainly focuses on Al-Si alloy solder system. By adding a small amount of Mg element to Al-11.5Si alloy, aluminum-titanium dissimilar metals were brazed in the form of foil brazing filler metal. Research has found that this process can reduce the melting point of the solder and promote the diffusion reaction between the solder and the base metal. However, the intermetallic compounds generated by the reaction between the solder and the base metal at the interface are still the main reason for affecting joint performance.

In order to further reduce the impact of intermetallic compounds on joint performance, based on Al-Si alloy, Al-Si-Sn-Ga multi-component alloy solder was prepared by adding different contents of Sn and Ga elements to the solder. The effects of the two elements on intermetallic compounds at the interface were studied. Experimental results show that the above two elements can effectively inhibit the formation of brittle phases, thereby improving joint performance. At the same time, the effects of the addition of Sn and Ga elements on the melting point and wettability of Al-Si solder were studied. The brazing experiments of LF21 aluminum alloy and TCA titanium alloy showed that adding the two elements can reduce the melting point of the solder. , It has a good effect on improving the wetting and spreading properties of solder and the mechanical properties of joints. It can be seen that the reasonable matching of Sn and Ga elements and the optimization and design of the components in the solder play an important role in improving the joint structure and improving joint performance. In addition to Sn and Ga elements, through vacuum brazing experiments of 6061 aluminum alloy and TC4 titanium alloy, it was found that although a variety of compound structures are also generated at the interface that harm the joint performance, the evolution process of the microstructure at the interface and the joint performance The analysis found that the addition of Cu and Ge elements can effectively reduce the melting point of Al-Si solder and improve the wetting and spreading properties of the solder on the titanium alloy surface.

5 Conclusions on research on vacuum brazing of dissimilar metals

Based on the above analysis, it can be seen that on the basis of Al-Si solder, by adding a variety of alloy elements, preparing a multi-component alloy solder with good comprehensive properties is one of the effective ways to achieve aluminum-titanium dissimilar metal brazing. However, for multi-component alloy solder, The reaction mechanism between alloy filler metal and base metal still needs to be studied in depth.