The lattice type, melting point, thermal conductivity, linear expansion coefficient, and chemical composition of copper and titanium plates are very different, so it is very difficult to weld.
1. Welds are prone to pores
(1) Copper and titanium have strong hydrogen absorption capabilities at high temperatures, and hydrogen is more soluble in liquid copper and titanium.
(2) Gas is generated in the high-temperature metallurgical reaction molten pool.
(3) The oxygen and nitrogen gas around the welding area are immersed in the molten pool. During the crystallization process of the molten pool, all the gas cannot escape from the surface of the molten pool and remains in the weld to form pores.
2. Welded joints are prone to cracks
When copper and titanium are welded, eutectic and hydride can be formed on the metal sides of the two base materials, which can easily cause cracks under the action of welding stress.
(1) Copper and bismuth form a (Cu+Bi) eutectic with a eutectic point of 270°C.
(2) Copper and aluminum form a (Cu+Pb) eutectic with a eutectic point of 326°C.
(3) Copper and ferrous sulfide form a eutectic (Cu+Cu2O) with a eutectic point of 1067°C.
(4) Flake-like hydride TiH2 is formed on the metal side of the titanium base material, causing hydrogen embrittlement.
(5) The linear expansion coefficients of copper and titanium are more than 1 times different, which will produce greater stress during welding.
3. The mechanical properties of welded joints are low
(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 joint bending angle decreases from 180° to 120°.
(2) A large amount of eutectic and hydrides significantly reduce the plasticity and toughness of welded joints.
(3) The mutual solubility of copper and titanium is very small, and intermetallic compounds are easily formed at high temperatures. Such as Ti2Cu, TiCu, Ti3Cu4, Ti2Cu3, TiCu2, and TiCu4, which increase the brittleness, reduce the plasticity, and significantly reduce the corrosion resistance of the weld metal.
Excellent welding joints can be obtained by vacuum diffusion welding, argon arc welding, plasma arc welding, brazing, and electron beam welding between copper and titanium or titanium alloys.
For example: vacuum diffusion welding is used. The characteristics of vacuum diffusion welding are that the joints do not oxidize, the weld seams have a beautiful appearance, and the product quality is good. The main operation process is: before welding, the copper base metal (such as T2) is cleaned with trichloroethylene to remove oil stains and other impurities. Then it is etched in 10% sulfuric acid solution for 1 minute, washed with distilled water, and then annealed. The annealing temperature is 820~830°C and the annealing time is 10 minutes.
After cleaning the titanium base metal (TA2) with trichloroethylene, use a vibration method to etch it for 4 minutes in a 2% volume fraction of HF and a 50% volume fraction of HNO3 aqueous solution to remove the oxide film, and then clean it with water and alcohol.
(4) Assemble the two cleaned base metals according to process requirements, and then put them into 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 1.3332×10-8~1.3332×10-9MPa. An intermediate diffusion layer can be added between the two base metals. Usually, the diffusion layer material is niobium metal, or no intermediate diffusion layer is required. Carefully clean the joint surface after welding.
If argon arc welding is used to weld copper and titanium, choosing cerium tungsten electrodes 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 the transition layer material, and the argon purity is 99.8% to obtain high-quality joints.
