Cracks are a frequent defect in the welding of titanium plates. Titanium weld cracks are cold cracks, mainly caused by hydrogen in the weld. The main sources of hydrogen are moisture and oil in the plate and welding wire, and the ambient humidity is the main reason for the increase of hydrogen in the weld.
Under the action of high temperature during welding, a large amount of hydrogen is dissolved in the molten pool. During the cooling and solidification process of the weld, hydrogen is easily escaped due to the rapid decrease in solubility. If the weld cooling rate is too fast, the hydrogen will not escape and remain in the weld, which will make the hydrogen in the weld supersaturated, so the hydrogen has to diffuse as much as possible, and promote further embrittlement of this area.
If there is a notch effect in this area and the hydrogen concentration is high enough, cracks may occur. Especially in winter construction, the ambient temperature is low, and the water vapor adheres to the AMS 4900 CP Titanium Sheet, which creates conditions for the hydrogenation of the weld. Because the titanium plate is too thin (1.2mm), the steel plate is relatively "warm", the temperature rises slowly, and the corresponding titanium composite layer weld is cooled too fast. During the cooling process, the residual hydrogen in the weld has no time to escape. It exists in the form of supersaturation in the weld, which eventually leads to the appearance of cracks.
Therefore, in the welding process of Grade 23 Ti-6Al-4V ELI Titanium Sheet, the surface of the base metal and welding wire should be carefully cleaned, and the ambient temperature should not be lower than 5 °C. During construction in winter, flame preheats the base steel surface, one is to remove the moisture around the weld; the other is to increase the temperature of the weldment and reduce the cooling rate of the weld.
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