The lattice type, melting point, thermal conductivity, linear expansion coefficient, and chemical composition of copper and titanium plates are very different, which brings great difficulties to the welding of the two, mainly reflected in the following aspects:
Welding Difficulty Analysis
Welds are prone to forming pores
First, copper and titanium have strong hydrogen absorption ability at high temperatures, and hydrogen has a high solubility in liquid copper and titanium. Second, high-temperature metallurgical reactions will produce gas in the molten pool. Third, oxygen and nitrogen gases around the welding area are easily absorbed in the molten pool. During the crystallization process of the molten pool, the gas cannot completely escape from the surface of the molten pool, so it remains in the weld to form pores.
Welded joints have a high tendency to crack
When copper and titanium are welded, eutectics and hydrides will be formed on the two parent metal sides, which are very easy to crack under the action of welding stress. Specifically:
1. Copper and bismuth form a (Cu + Bi) eutectic with a eutectic point of 270°C.
2. Copper and lead form a eutectic (Cu + Pb) with a eutectic point of 326°C (the original text "Cu + Al" is incorrectly expressed here, and it is inferred that it should be "Cu + Pb" in combination with the following text).
3. Copper and ferrous sulfide form a eutectic (Cu + Cu₂O) with a eutectic point of 1067°C.
4. Flake hydride TiH₂ is formed on the metal side of the titanium parent material, causing hydrogen embrittlement.
5. The linear expansion coefficients of copper and titanium differ by more than 1 times, and greater stress will be generated during welding.
Low mechanical properties of welded joints
1. The oxide film can weaken the intergranular bonding between copper and titanium. For example, when the oxygen content in the weld reaches 0.38%, the bending angle of the joint is reduced from 180° to 120°.
2. A large amount of eutectics and hydrides will significantly reduce the plasticity and toughness of welded joints.
3. The mutual solubility of copper and titanium is very small. They are easy to form intermetallic compounds at high temperatures, such as Ti₂Cu, TiCu, Ti₃Cu₄, Ti₂Cu₃, TiCu₂, and TiCu₄. These compounds increase brittleness, reduce plasticity, and significantly reduce the corrosion resistance of weld metal.
Welding process
Excellent welding joints can be obtained by vacuum diffusion welding, argon arc welding, plasma arc welding, brazing,,g, and electron beam welding of copper and titanium or titanium alloy.
Vacuum diffusion welding
The characteristics of vacuum diffusion welding are that the joints are not oxidized, the welds are beautiful in appearance, and the product quality is good. The main operation process is as follows:
1. Pretreatment of copper base metal: Taking T2 copper as an example, it is cleaned with trichloroethylene before welding to remove oil and other debris. Then etch in 10% sulfuric acid solution for 1min, then wash with distilled water, and then anneal at 820-830℃ for 10min.
2. Pretreatment of titanium base metal: Taking TA2 titanium as an example, after cleaning with trichloroethylene, it is etched by vibration for 4 minutes in a 2% by volume HF and 50% by volume HNO₃ aqueous solution to remove the oxide film, and then cleaned with water and alcohol.
3. Assembly and welding: The two cleaned base metals are assembled according to the process requirements, and then placed in a vacuum furnace for welding. The welding parameters are: welding temperature is 810℃±10℃, pressure is 5-10MPa, time is 10min, and vacuum degree is 1.3332×10⁸-1.3332×10⁹MPa. An intermediate diffusion layer can be added between the two base metals. Usually, niobium metal is used as the diffusion layer material, or no intermediate diffusion layer is added. The joint surface should be carefully cleaned after welding.
Argon arc welding
When welding copper and titanium with argon arc welding, the use of cerium tungsten electrode can improve the welding quality and benefit human health. For example, when welding copper alloy (QCr0.5) and titanium alloy (TC2), niobium can be used as a transition layer material, and the purity of argon gas is 99.8% to obtain high-quality joints.
