Titanium alloy forging defects and their prevention

When forging titanium alloy, there may be various defects in forging parts due to improper process specifications and lax quality control of raw materials. Common defects are as follows:

1. Beta brittleness
Beta brittleness is caused by overheat of forgings. α and (α+β) titanium alloys, especially (α+β) titanium alloys, if the forging heating temperature is too high, exceeding the β transition temperature, resulting in low microstructure and large grains of forging, is equiaaxial; In the microstructure, the α phase precipitates along the grain boundary of the coarse original β grain and in the grain. The result is a reduction in the plasticity of forgings at room temperature, a phenomenon called beta brittleness.

Overheating defects in titanium alloy forgings cannot be repaired by heat treatment methods, but must be repaired by reheating to a plastic deformation below the β transition temperature (if the forgings allow).

In order to prevent overheating, when titanium alloy is heated, the furnace temperature should be strictly controlled, the temperature of the qualified area of the furnace should be regularly determined, and the loading level and loading amount should not be much. When resistance heating is used, baffles should be set on both sides of the furnace to avoid overheating caused by excessive proximity of the billet to the silicon carbide rod. Measuring the actual β transition temperature of each furnace number alloy is also an effective measure to prevent overheating.

2. Local coarse crystal
During forging on the hammer or press, due to the poor thermal conductivity of titanium alloy, the temperature of the billet surface and the mold contact process is reduced a lot, coupled with the influence of the friction between the billet surface and the upper and lower die of the mold, the middle part of the billet is strongly deformed, and the surface deformation degree is small, so that the organization of the raw material is retained, and a new local coarse crystal is formed.

In order to avoid the local coarse crystal defect of titanium alloy, the following measures can be taken: adopt the pre-forging sequence to make the deformation uniform during the final forging; Strengthen lubrication, improve the friction between billet and mold; Fully preheat the die to reduce the temperature drop of the blank during forging.

3. Crack
The surface crack of titanium alloy is mainly produced when the final forging temperature is lower than the full recrystallization temperature of titanium alloy. In the process of die forging, the contact time between the billet and the die is too long, because of the poor thermal conductivity of titanium alloy, it is easy to cause the surface of the billet to cool below the allowed final forging temperature, and also cause surface cracks of the forging. In order to control the occurrence of cracks, glass lubricant can be used when forging on the press, or forging on the hammer to shorten the contact time between the blank and the lower die as much as possible.

4. Residual casting tissue
When forging titanium alloy ingot, if the forging ratio is not large enough or the forging method is improper, the forging will remain the casting structure. The method to solve this defect is to increase the forging ratio and use repeated upsetting.

5. Glitter
The so-called bright bars in titanium alloy forgings are visible strips with unusual brightness that exist in low-power tissues. Due to the difference in light Angle, the bright strip can be brighter than the base metal, but also darker than the base metal. In cross section, it is point-like or flaky; In the longitudinal section, it is a smooth strip, whose length varies from more than ten millimeters to several meters. There are two main reasons for the production of bright bars: one is the chemical composition of titanium alloy segregation, and the other is the deformation thermal effect of the forging process.

The glitter has a certain effect on the properties of titanium alloys, especially on the plasticity and high temperature properties. The measures to prevent the appearance of bright strips are to strictly control the segregation of chemical components in smelting; Correct selection of forging thermal specifications (heating temperature, deformation degree, deformation speed, etc.) to avoid forging temperature due to deformation thermal effect and the difference is too large.

6. α embrittlement layer
The α embrittlement layer is mainly caused by the diffusion of oxygen and nitrogen to the interior of the metal through the loose oxide skin of titanium alloy at high temperature, which increases the content of oxygen and nitrogen in the surface metal, thus increasing the amount of α phase in the surface tissue. When the oxygen and nitrogen content of the surface metal reaches a certain value, the surface tissue may be completely composed of α phase. In this way, the surface of the titanium alloy forms a surface layer with more α or completely α phase. The surface layer composed of this α phase is often called the α embrittlement layer. The α embrittlement layer on the surface of titanium alloy billet is too thick, which may cause billet cracking during forging.

The thickness of the α embrittlement layer is closely related to the type of heating furnace used in forging or heat treatment, the nature of the gas in the furnace, the heating temperature of the blank or part and the holding time. The thickness increases with the increase of heating temperature and holding time. It thickens with the increase of oxygen and nitrogen content in furnace gas. Therefore, in order to avoid this embrittlement layer is too thick, the heating temperature, holding time and furnace gas properties of forging or heat treatment must be properly controlled.

α, β and (α+β) titanium alloys may form α embrittlement layers. However, α titanium alloy is particularly sensitive to the formation of α embrittlement layer, while β titanium alloy will form α embrittlement layer when heated to 980℃ or above.

7. Hydrogen embrittlement
There are two types of hydrogen embrittlement: strain time type and hydride type. Under the action of stress, the hydrogen atoms in the lattice gap diffuse and accumulate to the stress concentration gap after a certain time. Due to the interaction between hydrogen atom and dislocation, the dislocation is pinned and cannot move freely, which makes the matrix brittle. The hydrogen dissolved into the solid solution at high temperature precipitates out in the form of hydride with the temperature dropping, and the phenomenon that makes the titanium alloy brittle is called hydride hydrogen embrittlement. Both types of hydrogen embrittlement can occur in titanium and titanium alloys.

Hydrogen embrittlement problem is caused by excessive hydrogen content in titanium alloy. Therefore, the hydrogen content in industrial titanium alloys must be controlled within 0.015%.

In order to prevent or reduce hydrogen embrittlement, the furnace should be slightly oxidizing when forging or heat treatment, and vacuum annealing can be carried out to eliminate hydrogen embrittlement for titanium alloy parts with hydrogen content exceeding the regulations and important parts.