Why is it difficult to weld titanium and aluminum?

Titanium and aluminum are widely used in aerospace, transportation, vehicle manufacturing, chemical industry and other fields due to their low density, high specific strength and good corrosion resistance.The complex working conditions in modern engineering pose a higher challenge to the service performance of workpieces, and promote the development and application of composite structures. The composite component composed of titanium alloy and aluminum alloy can maximize the performance characteristics of the two materials.Due to the significant differences in thermophysical properties and mechanical properties between titanium and aluminum, many problems such as porosity and cracks are easy to occur in the welding process. The formation of intermetallic compounds due to metallurgical reaction is one of the important reasons for the deterioration of Ti/Al dissimilar material joints.What are the reasons for the difficulty in welding titanium and aluminum?Aluminum and titanium react easily with oxygen1. The aluminum and oxygen react to form dense and refractory Al2O3 (oxide film), whose melting point is up to 2050 ℃, which hinders the combination of two base metals, and the weld is easy to produce inclusions.2. Titanium begins to oxidize at 600 ℃. The higher the temperature is, the more serious the oxidation is. TiO2 (titanium dioxide) is generated, forming an intermediate brittle layer in the weld, reducing the plasticity and toughness.Aluminum and titanium react differently at different temperatures1. At 1460 ℃, aluminum and titanium form a TiAl (titanium aluminide) type compound containing 36.03% aluminum, which increases the brittleness of the metal.2. At 1340 ℃, aluminum and titanium form TiAl3 (titanium trialuminate) compound with aluminum content of 60%~64%.3. After the aluminum and titanium are melted, when the mass fraction of titanium is 0.15%, the solid solution of titanium in aluminum is formed.The mutual solubility of aluminum and titanium is very small1. At 665 ℃, the solubility of titanium in aluminum is 0.26%~0.28%. With the decrease of temperature, the solubility becomes smaller.2. When the temperature drops to 20 ℃, the solubility of titanium in aluminum decreases to 0.07%, making it difficult to combine the two base metals.3. The solubility of aluminum in titanium is more limited, which brings great difficulties to the formation of weld between two base metals.Aluminum and titanium have great air absorption at high temperature1. Liquid aluminum can dissolve a large amount of hydrogen, which is almost insoluble when it is solid, and hydrogen cannot escape to form pores when the weld is solidified.2. Hydrogen has a great solubility in titanium. At low temperature, hydrogen polymerizes into pores, which reduces the plasticity and toughness of the weld and is prone to brittle cracking.Aluminum forms brittle compounds with titanium and other impurities1. Oxides formed by aluminum and oxygen increase the brittleness of metal, making welding difficult.2. Titanium and nitrogen form titanium nitride, which reduces the metal plasticity.3. Titanium and carbon form carbides. When the mass fraction of carbon is greater than 0.28%, the weldability of the two base metals becomes significantly worse.Aluminum and titanium react differently at different temperatures1. The thermal conductivity of aluminum and titanium is very different. Aluminum (206.9W · m-2 · K-1) is about 16 times larger than titanium (13.8W · m-2 · K-1).2. The linear expansion coefficients of aluminum and titanium differ greatly, and aluminum is about 3 times larger than titanium. Cracks are easily produced under stress.Evaporation of alloy elements in aluminum and titanium1. When aluminum or aluminum alloy is melted, elements lower than its melting point, such as magnesium and zinc, begin to burn or evaporate.2. When the melting point of titanium or titanium alloy is reached (1677 ℃), aluminum and other alloy elements are burned and evaporated more, resulting in uneven chemical composition of the weld and reduced strength.