Defects of Titanium Alloy Forgings and Their Prevention and Control

During the forging process of titanium alloys, due to improper process specifications and insufficient quality control of raw materials, the forged parts may have various defects. The common defects include the following types:

1. β-ductility
The β brittleness is caused by overheating of the forging. For α and (α + β) titanium alloys, especially (α + β) titanium alloys, if the forging heating temperature is too high and exceeds their β transformation temperature, the low-magnification structure of the forging will have large grains and be in an equiaxed state; the microstructure contains α phases that precipitate along the grain boundaries and within the coarse original β grains, in a strip-like manner. As a result, the plasticity of the forging at room temperature decreases. This phenomenon is called β brittleness.

The overheating defects of titanium alloy forgings cannot be repaired by heat treatment methods. Instead, they must be remedied by reheating the forgings to a temperature below the β transformation point (if the forgings are permitted) and undergoing plastic deformation.

To prevent overheating, when heating the titanium alloy, the furnace temperature should be strictly controlled, and the temperature of the qualified zone in the furnace chamber should be regularly measured. The loading position and quantity of materials should be reasonably arranged, and they should not be excessive. When using resistance heating, baffles should be set on both sides of the furnace chamber to prevent the billets from getting too close to the silicon carbide rods and causing overheating. Measuring the actual β transformation temperature of the alloy for each furnace number is also an effective measure to prevent overheating.

2. Local coarse grains
When forging with a hammer or a press, due to the poor thermal conductivity of titanium alloys, the surface layer of the billet comes into contact with the mold and experiences a significant temperature drop. Additionally, the friction between the surface of the billet and the upper and lower molds has an impact. The middle part of the billet undergoes intense deformation, while the surface deformation is minimal, allowing the original material's structure to remain, thereby forming a new local coarse-grained structure. To avoid local coarse-grained defects in titanium alloys, the following measures can be taken: adopt the pre-stretching process to ensure uniform deformation during final forging; enhance lubrication to improve the friction between the billet and the mold; and fully preheat the mold to reduce the temperature drop of the billet during the forging process.

3. Cracks
The surface cracks in titanium alloy forgings mainly occur when the final forging temperature is lower than the sufficient recrystallization temperature of the titanium alloy. During the die forging process, if the contact time between the billet and the die is too long, due to the poor thermal conductivity of the titanium alloy, the surface of the billet is prone to cool below the allowable final forging temperature, which can also cause surface cracks in the forgings. To control the occurrence of cracks, when forging with a press, a glass lubricant can be used, or when forging with a hammer, the contact time between the billet and the lower die should be shortened as much as possible.

4. Residual casting structure
When forging titanium alloy ingots, if the forging ratio is insufficient or the forging method is improper, casting structures will remain in the forged parts. The solution to this defect is to increase the forging ratio and adopt repeated upsetting and drawing processes.

5. Bright Bars
The so-called bright stripes in titanium alloy forgings are visible bands with abnormal brightness that exist in the low-magnification structure. Due to differences in light angles, these bright stripes can be brighter or darker than the base metal. On the cross-section, they appear as dots or sheets; on the longitudinal section, they are smooth and long strips, with lengths ranging from several millimeters to several meters. The main reasons for the formation of these bright stripes are two: one is the chemical composition segregation of the titanium alloy, and the other is the deformation heat effect during the forging process.

The bright strips have a certain impact on the properties of titanium alloys, especially having a significant effect on plasticity and high-temperature performance. The measures to prevent the appearance of bright strips are to strictly control the segregation of chemical components during smelting; and to correctly select the forging thermal specifications (heating temperature, deformation degree, deformation speed, etc.) to avoid too large temperature differences in different parts of the forging due to the deformation heat effect.

6. α embrittlement layer
The α brittle layer is mainly formed when titanium alloy is subjected to high temperatures, where oxygen and nitrogen diffuse through the loose oxide layer into the metal interior, increasing the oxygen and nitrogen content in the surface metal. This leads to an increase in the number of α phases in the surface structure. When the oxygen and nitrogen content in the surface metal reaches a certain level, the surface structure may be entirely composed of α phases. Thus, a surface layer with more α phases or a completely α-phase composition is formed on the surface of the titanium alloy. This surface layer composed of α phases is commonly referred to as the α brittle layer. If the α brittle layer on the surface of the titanium alloy billet is too thick, it may cause cracking during forging.

The thickness of the α brittle layer is closely related to the type of heating furnace used during forging or heat treatment, the nature of the gas inside the furnace, the heating temperature of the blank or part, and the holding time. As the heating temperature increases and the holding time grows, the thickness increases; as the oxygen and nitrogen content in the furnace gas increases, the thickness also thickens. Therefore, in order to avoid an excessively thick brittle layer, appropriate control must be exercised over the heating temperature, holding time, and gas properties during forging or heat treatment.

Both α, β and (α + β) titanium alloys can form α embrittlement layers. However, α titanium alloys are particularly sensitive to the formation of α embrittlement layers, while β titanium alloys will only form α embrittlement layers when heated above 980℃.

7. Hydrogen embrittlement

There are two types of hydrogen embrittlement: the strain-induced type and the hydrogen compound type. Hydrogen atoms located in the lattice gaps diffuse and accumulate at the stress concentration points after a certain period of time under the action of stress. Due to the interaction between hydrogen atoms and dislocations, the dislocations are immobilized and cannot move freely, resulting in the phenomenon of embrittlement of the matrix. This is called the strain aging type hydrogen embrittlement. Hydrogen dissolved in the solid solution at high temperatures precipitates in the form of hydrogen compounds as the temperature drops, causing the titanium alloy to become brittle. This is called the hydrogen compound type hydrogen embrittlement. Both of these types of hydrogen embrittlement can occur in titanium and titanium alloys.

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

To prevent or reduce hydrogen embrittlement, the furnace should be maintained in an oxidizing atmosphere during forging or heat treatment. For titanium alloy parts with hydrogen content exceeding the specified limit or those of significant importance, vacuum annealing can be carried out to eliminate hydrogen embrittlement.