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Vacuum Brazing of Titanium Alloy and Aluminum Alloy Dissimilar Metals

Titanium alloys are widely used in aerospace, shipbuilding and petrochemical fields due to their low density, high specific strength, excellent corrosion resistance, high temperature strength and good low temperature toughness. However, titanium alloys have low elastic modulus, poor creep resistance, poor welding performance and processing performance, and are expensive, making it difficult for a single titanium alloy to meet the requirements for comprehensive properties of materials under actual working conditions. are often restricted. Aluminum alloy is one of the most commonly used structural materials. It has the advantages of low density, high specific strength, high thermal conductivity, good corrosion resistance and processing performance, and relatively low cost, but its strength is far less than that of titanium alloy. Titanium/aluminum composite components composed of titanium alloy and aluminum alloy have the advantages of the two metals in their respective performance and economy, such as high specific strength, light weight, low cost and good corrosion resistance, etc., which can maximize the use of the two metals. It has a wide range of application prospects in the fields of aero-engine, aircraft structure design, automobile manufacturing, weapon equipment and other fields.

1 Weldability analysis of titanium alloy/aluminum alloy

Due to the difference in physical and chemical properties between titanium alloys and aluminum alloys, there are many difficulties in welding at present. First of all, the melting point of titanium alloy and aluminum alloy is quite different, and the solubility of titanium in aluminum is very small. When fusion welding is used, some low-melting-point elements in the aluminum alloy will be burned and evaporated, and a large number of Ti-Al brittle intermetallic compounds will be formed. (Ti3Al, TiAl3, TiAl, Ti2A15, etc.), which seriously affects the mechanical properties of the joint. Secondly, titanium and aluminum have great differences in physical properties such as online expansion coefficient and thermal conductivity, resulting in different deformation capabilities of the two metals during heating and cooling. There is a large residual stress in the joint after welding, and the weld is easy to Cracks occur. In addition, both aluminum and titanium have strong chemical activity and are prone to form oxide films and reduce wettability. Titanium is also prone to gettering reactions with oxygen, nitrogen and hydrogen elements at high temperatures. In order to avoid the inhalation reaction and prevent the oxidation of the aluminum alloy and the weld, it is necessary to be in a vacuum or to be protected by argon gas during welding. welding method and welding process to reduce the internal stress of the joint and reduce the formation of the interface brittle phase. Due to the relatively low solid solubility between titanium and aluminum, there are few studies on fusion welding, and most of them are vacuum brazing.

In general, under vacuum or inert gas protection, suitable welding process can solve defects such as lack of alloy elements, high temperature oxidation, pores and cracks in Ti/Al dissimilar metal welding, but it cannot completely solve the problem of brittle intermetallic compounds. Therefore, inhibiting the formation of brittle intermetallic compounds is still the main research direction of Ti/Al dissimilar metal welding. In the process of Ti/Al brazing, since the temperature is lower than the melting point of the base metal, the formation of intermetallic compounds can be limited, and the interface reaction can be better controlled.

2 Aluminum-based solder

From the current research, Al-based solder has good wettability to titanium alloy and aluminum alloy under vacuum, protective atmosphere or non-vacuum with auxiliary measures, but the joint strength still needs to be improved, and the intermetallic compound is thicker The problem needs to be further improved by optimizing the solder composition and welding process.

Under vacuum conditions, vacuum brazing experiments were carried out on pure titanium and pure aluminum using Al-30Ag-10Cu, Al-10Cu-8Sn, Al-10Si-1Mg solders. The experimental results show that under the same welding conditions, when using Al-30Ag-10Cu and Al-10Cu-8Sn solders, TiAl3 intermetallic compounds are formed at the titanium side interface, and a small amount of Ag2Al and CuAl2 phases are formed respectively, and the fracture of the joint is Located in the interface compound layer, the tensile strength is only 38 MPa. When Al-10Si-1Mg solder is used, the formed intermetallic compound phase is Ti7Al5Si12, the joint is broken at the location of the solder, and the tensile strength is increased to 70 MPa.

Al-8.4Si-20Cu-10Ge and Al-8.4Si-20Cu-10Ge-0.1Re solders were configured on the basis of Al-Si solder, and the TC4/6061 joint was vacuum brazed under the protection of high-purity argon gas. Welding test. The experimental results show that when 20% Cu and 10% Ge are added to the Al-12Si solder, the liquidus temperature of the solder drops from 592 ℃ to 513 ℃. After adding Cu and Ge, the addition of rare earth elements with a mass fraction of 0.1% can significantly refine the Al-rich phase, transform acicular Al2Cu into bulk (as shown in Figures 1a and 1b), while improving interfacial bonding and increasing TC4/ 6061 joint strength. The average shear strengths of the brazed joints obtained by using Al-8.4Si-20Cu-10Ge and Al-8.4Si-20Cu-10Ge-0.1Re solder at 530℃ were 20 MPa and 51 MPa, respectively.

Fig.1 Microstructure of TC4/6061 brazing interface (530 ℃-60 min)

For Ti/Al brazing, Si element plays an indispensable role in inhibiting Ti-Al intermetallic compounds and improving the mechanical properties of joints, and is the element that has the greatest influence on Ti-Al compounds. The focus is also on the Al-Si based brazing filler metal. On the basis of Al-Si based brazing filler metals, adding other elements can improve some properties of brazing filler metals. For example, adding appropriate amount of Sn and Ga elements can improve the brittleness of intermetallic compounds; Cu and Ge elements can effectively reduce the brittleness of brazing filler metals. The melting point of the material; Zr element can improve the spreadability of the solder; rare earth elements can refine or change the shape of part of the brittle phase, reduce the hardness of the reaction layer, improve the interface bonding, and increase the strength of the joint. In addition, the use of pre-plating aluminum on the surface of titanium alloy can effectively improve the formation and shape of intermetallic compounds, and improve the mechanical properties of joints.

3 Summary

In general, due to the huge difference in physical and chemical properties between dissimilar metals in the connection of titanium alloys and aluminum alloys, a large number of brittle intermetallic compounds are easily formed at the interface by conventional welding methods, which seriously affects the mechanical properties of welded joints. The connection of titanium alloy and aluminum alloy by vacuum brazing has certain advantages in terms of quality and precision of welded joints, but there are still many problems such as high brittleness and low strength of welded joints. The main reason is that the formation of brittle intermetallic compounds is difficult to avoid. Therefore, The main solution is to start from the composition design of the solder to assist the improvement of the brazing process and equipment. For aluminum-based brazing filler metals, the formation of Al-Ti intermetallic compounds is mainly reduced by adding an appropriate amount of Si element, and supplemented by an appropriate amount of Ga, rare earth elements, etc. to improve the brittleness of the intermetallic compounds to improve the mechanical properties of the joint. At the same time, the process method of presetting aluminum layer on the surface of titanium alloy can also be used, and with the assistance of ultrasonic equipment, the oxide film on the surface of the base metal can be effectively removed, and with zinc-based solder, the welding temperature can be further reduced and the generation of intermetallic compounds can be reduced, effectively improve the joint strength.