What is the reason why titanium alloy and aluminum are difficult to weld?
Titanium alloy is a lightweight, high-strength, corrosion-resistant material. Due to its excellent properties, it is widely used in aerospace, medical equipment, chemical industry and other fields. However, the welding performance of titanium alloys is relatively poor, mainly for the following reasons:
Oxide layer formation: A solid oxide layer is easily formed on the surface of titanium alloy. This oxide layer not only increases the difficulty during the welding process, but also reduces the welding strength. Before welding, some special pretreatment methods, such as pickling or mechanical polishing, are usually required to remove the oxide layer and thereby improve the welding performance.
Low thermal conductivity: The thermal conductivity of titanium alloy is relatively low, resulting in a large temperature gradient near the weld, which can easily cause welding deformation and cracks. In order to reduce the temperature gradient, it is often necessary to adopt measures such as preheating and controlling the welding speed, which increases the complexity of the welding process.
Hydrogen sensitivity: Titanium alloy is sensitive to hydrogen and easily absorbs hydrogen during the welding process, resulting in hydrogen embrittlement. Hydrogen embrittlement may cause brittle fracture of welded joints, so some measures need to be taken, such as controlling the hydrogen content in the welding environment, to reduce the risk of hydrogen embrittlement.
Selective dissolution: Titanium alloy is prone to selective dissolution with some metal elements at high temperatures, forming a brittle phase, which affects the performance of welded joints. Therefore, special attention needs to be paid to avoiding this selective dissolution when selecting welding materials and welding processes.
High melting point: The relatively high melting point of titanium alloys requires high temperatures for the welding process, thereby increasing energy consumption and welding equipment requirements.
In order to solve these problems, welding titanium alloys usually requires the use of specialized welding processes, including inert gas shielded welding, electron beam welding, laser welding and other advanced welding technologies. In addition, selecting appropriate welding materials, controlling welding parameters, and adopting pretreatment methods are also important means to improve the welding quality of titanium alloys.
To sum up, the reasons why titanium alloy and aluminum are difficult to weld are as follows:
1. Aluminum and titanium react easily with oxygen
⑴ Aluminum reacts with oxygen to form a dense and refractory Al2O3 (oxide film) with a melting point as high as 2050°C, which hinders the combination of the two base materials and makes the weld prone to inclusions.
⑵Titanium begins to oxidize at 600°C. The higher the temperature, the more serious the oxidation will be, forming TiO2 (titanium dioxide), forming an intermediate brittle layer in the weld, reducing the plasticity and toughness.
2. Aluminum and titanium react differently at different temperatures
⑴. At 1460°C, aluminum and titanium form a TiAl (titanium aluminide) compound containing 36.03% aluminum mass fraction, which increases the brittleness of the metal.
⑵ Aluminum and titanium form a TiAl3 (titanium trialuminide) compound containing 60% to 64% aluminum mass fraction at 1340°C.
⑶ After aluminum and titanium are melted, when the mass fraction of titanium is 0.15%, a solid solution of titanium is formed in aluminum.
3. The mutual solubility of aluminum and titanium is very small
⑴At 665°C, the solubility of titanium in aluminum is 0.26%~0.28%. As the temperature decreases, the solubility decreases.
⑵When the temperature drops to 20°C, the solubility of titanium in aluminum drops to 0.07%, making it difficult to combine the two base materials.
The solubility of aluminum in titanium is more limited, which makes the formation of welds between the two base materials very difficult.
4. Aluminum and titanium have strong water absorption at high temperatures.
⑴Liquid aluminum can dissolve a large amount of hydrogen, but it is almost insoluble in the solid state. As the weld solidifies, the hydrogen does not have time to escape and form pores.
⑵Hydrogen has a high solubility in titanium. At low temperatures, hydrogen accumulates in the pores, reducing the plasticity and toughness of the weld and easily causing brittle cracks.
5. Aluminum forms brittle compounds with titanium and other impurities
⑴. The oxide formed by aluminum and oxygen increases the brittleness of the metal and makes welding difficult.
⑵Titanium and nitrogen form titanium nitride, which reduces the plasticity of the metal.
⑶Titanium and carbon form carbides. When the mass fraction of carbon is greater than 0.28%, the weldability of both base metals will significantly deteriorate.
6. Aluminum and titanium react differently at different temperatures
⑴The thermal conductivity of aluminum and titanium is very different. Aluminum (206.9W·m-2·K-1) is approximately 16 times larger than titanium (13.8W·m-2·K-1).
⑵The linear expansion coefficients of aluminum and titanium are very different, and aluminum is about 3 times larger than titanium. Prone to cracking under pressure.
7. The alloy elements in aluminum and titanium burn and evaporate
⑴When aluminum or aluminum alloy melts, elements with melting points lower than it, such as magnesium, zinc, etc., begin to burn or evaporate.
⑵ When the melting point of titanium or titanium alloy is reached (1677°C), alloy elements such as aluminum burn and evaporate more, resulting in uneven chemical composition of the weld and reduced strength.
