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Why is it difficult to weld titanium and aluminum?

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 physical and mechanical properties between titanium and aluminum, they are prone to various problems such as porosity and cracks during the welding process. One of the important reasons for the deterioration of the joint performance of Ti/Al dissimilar materials is the formation of intermetallic compounds through metallurgical reactions.

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

Aluminum and titanium easily interact with oxygen

1. Aluminum and oxygen react to form a 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 seam prone to inclusions.

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

Aluminum and titanium produce different reactions at different temperatures

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

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

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

The solubility between aluminum and titanium is very low

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

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 combine.

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 high temperature air absorption properties

1. Liquid aluminum can dissolve a large amount of hydrogen, which is almost insoluble in the solid state. When the weld solidifies, hydrogen cannot escape in time to form pores.

2. 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 cracking.

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.

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

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

Aluminum and titanium produce different reactions at different temperatures

1. There is a significant difference in thermal conductivity between aluminum and titanium, with aluminum (206.9W · m-2 · K-1) being about 16 times larger than titanium (13.8W · m-2 · K-1).

2. There is a significant difference in the coefficient of linear expansion between aluminum and titanium, with aluminum being about three times larger than titanium. Cracks are prone to occur under stress.

Burning and evaporation of alloying elements in aluminum and titanium

When aluminum or aluminum alloys melt, elements with lower melting points such as magnesium and zinc begin to burn or evaporate.

When reaching the melting point of titanium or titanium alloy (1677 ℃), aluminum and other alloy elements burn and evaporate more, resulting in uneven chemical composition of the weld and reduced strength.

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