Why is it difficult to weld titanium and aluminum?

Release time：2024-09-24 10:04:17 丨 Number of visits：

Titanium and aluminum have the characteristics of low density, high specific strength, and good corrosion resistance, and are widely used in aerospace, transportation, vehicle manufacturing, chemical industry, and other fields.

The complex working conditions in modern engineering pose higher challenges to the service performance of workpieces, promoting the development and application of composite structures. Composite components composed of titanium alloy and aluminum alloy can maximize the performance characteristics of both materials.

Due to significant differences in thermal and mechanical properties between titanium and aluminum, they are prone to issues such as porosity and cracking during the welding process. One of the important reasons for the deterioration of the joint performance of Ti/Al heterogeneous materials is the formation of intermetallic compounds due to metallurgical reactions.

So what are the reasons for the difficulty in welding titanium and aluminum?

Aluminum and titanium are highly reactive with oxygen

1. Aluminum and oxygen react to form dense and refractory Al2O3 (oxide film), with a melting point of up to 2050 ℃, which hinders the bonding of the two base metals and makes the weld prone to inclusions.

2. Titanium begins to oxidize at 600 ℃, and the higher the temperature, the more severe the oxidation, generating TiO2 (titanium dioxide), which forms an intermediate brittle layer in the weld seam, reducing plasticity and toughness.

Aluminum and titanium undergo different reactions at different temperatures

At 1460 ℃, aluminum and titanium form TiAl (titanium aluminate) type compounds with an aluminum mass fraction of 36.03%, which increases the brittleness of the metal.

At 1340 ℃, aluminum and titanium form TiAl3 (titanium aluminate) compounds with an aluminum mass fraction of 60% to 64%.

After melting aluminum and titanium, a solid solution of titanium in aluminum is formed when the mass fraction of titanium is 0.15%.

The mutual solubility between aluminum and titanium is very low

At 665 ℃, the solubility of titanium in aluminum ranges from 0.26% to 0.28%, and decreases with decreasing temperature.

When the temperature drops to 20 ℃, the solubility of titanium in aluminum decreases to 0.07%, making it difficult for the two base metals to bond.

3. The solubility of aluminum in titanium is even more limited, which poses great difficulties for the formation of welds between the two base metals. Aluminum and titanium have great high-temperature gas absorption properties

4. Liquid aluminum can dissolve a large amount of hydrogen, but it is almost insoluble in solid state. When the weld solidifies, hydrogen does not have time to escape and form pores.

5. Hydrogen has a high solubility in titanium, and at low temperatures, hydrogen accumulates into pores, reducing the plasticity and toughness of the weld and making it prone to brittle fracture.

Aluminum forms brittle compounds with titanium and other impurities

The oxide formed by aluminum and oxygen increases the brittleness of the metal, making welding difficult to carry out.

2. Titanium and nitrogen form titanium nitride, which reduces the plasticity of the metal.

3. Titanium and carbon form carbides, and when the mass fraction of carbon is greater than 0.28%, the weldability of the two base metals significantly deteriorates.

Aluminum and titanium undergo different reactions at different temperatures

The thermal conductivity of aluminum and titanium is significantly different, with aluminum (206.9W · m-2 · K-1) being about 16 times higher than titanium (13.8W · m-2 · K-1).

The coefficient of linear expansion of aluminum and titanium is significantly different, with aluminum being about three times larger than titanium. Cracks are prone to occur under stress.

Alloy elements in aluminum and titanium are burned and evaporated

When aluminum or aluminum alloy melts, elements with lower melting points such as magnesium, zinc, etc. begin to burn or evaporate.

When the melting point of titanium or titanium alloy is reached (1677 ℃), aluminum and other alloying elements are burned and evaporated more, resulting in uneven chemical composition of the weld and a decrease in strength.