Defects and Prevention of Titanium Alloy Forgings
Defects and Prevention of Titanium Alloy Forgings
1 β brittleness
β Brittleness is caused by overheating of forgings. α And（ α+β） Titanium alloys, especially（ α+β） Titanium alloy, if the forging heating temperature is too high and exceeds its limit β The transformation temperature causes the low magnification structure of the forging to have large grains, which are equiaxed; In microstructure α Along the thick and primitive edges β The grain boundaries and interiors of the grains precipitate in a strip like manner. The result is a decrease in plasticity of forgings at room temperature, a phenomenon known as β Brittleness.
The overheating defects of titanium alloy forgings cannot be repaired by heat treatment, but must be reheated to β Plastic deformation below the transition temperature (if allowed by the forging) is necessary for repair.
In order to prevent overheating, when heating titanium alloys, the furnace temperature should be strictly controlled, the temperature of the qualified zone of the furnace should be regularly measured, and the loading position and amount should not be too large. When using resistance heating, baffles should be installed on both sides of the furnace to avoid overheating caused by the billet being too close to the silicon carbide rod. Check the actual condition of each furnace number alloy β Changing temperature is also an effective measure to prevent overheating.
2. Localized coarsening
When forging on a hammer or press, due to the poor thermal conductivity of titanium alloy, the temperature decreases significantly during the contact process between the surface of the billet and the mold. In addition, the friction between the surface of the billet and the upper and lower molds of the mold causes strong deformation in the middle part of the billet, and the degree of surface deformation is small, which preserves the structure of the raw material and forms new local coarse grains.
To avoid local coarse grain defects in titanium alloys, the following measures can be taken: adopting a pre forging process to ensure uniform deformation during final forging; Strengthen lubrication and improve friction between billets and molds; Fully preheat the mold to reduce the temperature drop of the billet during the forging process.
3. Cracks
The surface cracks of titanium alloy forging are mainly generated when the final forging temperature is lower than the sufficient recrystallization temperature of titanium alloy. During the forging process, if the contact time between the billet and the mold is too long, due to the poor thermal conductivity of titanium alloy, it is easy to cause the surface of the billet to cool below the allowable final forging temperature, which can also cause surface cracks in the forging. To control the occurrence of cracks, glass lubricant can be used when forging on a press, or the contact time between the billet and the lower die can be shortened as much as possible when forging on a hammer.
4. Residual casting structure
When forging titanium alloy ingots, if the forging ratio is not large enough or the forging method is improper, the forging will retain the casting structure. The solution to this defect is to increase the forging ratio and use repeated upsetting and drawing.
5. Bright bar
The so-called bright strips in titanium alloy forgings are visible bands with abnormal brightness that exist in the low magnification structure. Due to differences in lighting angles, the bright bars can be brighter or darker than the base metal. On a cross-section, it appears as a point or sheet shape; On the vertical section, it is a smooth strip with lengths ranging from over ten millimeters to several meters. There are two main reasons for the formation of bright stripes: one is the chemical composition segregation of titanium alloys, and the other is the deformation heat effect during the forging process.