Titanium alloy is "stable and reliable" at normal temperature, but becomes extremely "lively" at high welding temperatures (over 800°C), which is the root of all problems:
1. Oxidation pollution: Titanium at high temperatures will "absorb oxygen" crazily and form a brittle and hard oxide layer (TiO₂). It is like a layer of glass slag wrapped on the weld, which may break with a light touch. Experimental data shows that when the oxygen content of the weld exceeds 0.15%, the impact toughness of titanium alloy will plummet by more than 50%.
2. Risk of hydrogen embrittlement: Moisture in the air and oil on the surface of the welding wire will release hydrogen during welding. When hydrogen penetrates into titanium alloys, it will form needle-shaped hydrides (TiH₂), making the material brittle, which may cause "unpredictable fracture", especially in low-temperature environments.
3. Stress cracks: Titanium alloy has a small thermal expansion coefficient, but local high temperatures during welding will cause severe thermal expansion and contraction, producing huge stress in the weld and heat-affected zone. If the welding speed is too fast and the cooling is uneven, the stress will "tear" the weld apart and form cracks.
How to solve？
1. Tungsten arc welding (TIG welding)
Suitable for thin-walled parts below 3mm (such as medical implants, precision instrument parts), the advantages are stable arc, easy control of heat input, and precise reduction of oxidation. The key is to do a good job of "triple protection": welding gun nozzle protection (argon gas flow rate 15-25L/min), back argon filling protection (flow rate 5-10L/min), and drag hood trailing protection (covering areas with temperatures above 400°C) to ensure "air isolation" throughout the welding process.
2. Vacuum electron beam welding:
In a vacuum environment (vacuum degree ≥1×10⁻³Pa), high-energy electron beams are used to bombard titanium alloys, and the melted area is pure and free of impurities. It is especially suitable for thick plates (10-100mm) and complex structural parts (such as rocket fuel tanks). The weld depth-to-width ratio can reach 10:1, the heat-affected zone is extremely small, and the mechanical properties of the titanium alloy can be retained to the maximum extent.
3. Laser welding:
Laser energy density is high, the welding speed is 3-5 times faster than TIG welding, and it is suitable for mass production (such as automotive titanium alloy chassis parts). However, attention should be paid to "heat input control" - too high a power will cause the weld to burn through, and too low a power will easily cause failure to fuse. Usually, an inert gas shield is required to prevent side oxidation.
4.Diffusion welding:
When titanium alloy needs to be welded with steel, copper and other materials, diffusion welding is the best choice. Under high temperature (800-950°C) and high pressure (10-50MPa), atoms on the surface of the two metals diffuse into each other to form a strong joint and avoid the generation of brittle and hard intermetallic compounds. "Titanium alloy-stainless steel" bone nails in the medical field are typical applications.
