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What are the common forging defects in titanium alloys?

Update:2021-08-20
Abstract:

At present, the common forging defects in titanium allo […]

At present, the common forging defects in titanium alloys mainly include overheating and unevenness, voids, cracks, etc. These defects are generally easy to find in the microstructure inspection or ultrasonic inspection of titanium alloy products, mainly in the forging process of titanium alloy products. It is formed by improper control of parameters, so it is necessary to select the appropriate deformation rate (forging equipment), heating forging temperature, pass deformation and cooling rate after forging according to different characteristics of titanium alloy materials during the forging process. Titanium alloy has excellent comprehensive properties such as low density, high specific strength, high temperature resistance, corrosion resistance and non-magnetic properties, making it one of the most promising metal structural materials in the contemporary aerospace field.

1. Void defects
Studies have shown that the plastic deformation process of metal materials is accompanied by changes in the structure of the structure, mainly including grain growth, equiaxed grain elongation, grain rotation and sliding, dislocation proliferation, dynamic recovery and recrystallization, and void nucleation and growth. Big wait. Grain boundary slip is the main mechanism of plastic deformation. Grain boundary slip will cause local stress concentration and hinder the further occurrence of grain boundary slip. When the stress concentration cannot be eliminated by dislocation movement, the void will nucleate and then grow. Big. The cavity preferentially nucleates at the triangular grain boundary. As the amount of deformation increases, the cavity begins to grow, and the cavity does not grow in an isometric state, but in an elliptical manner. The cavity easily diffuses to the grain boundary shared by the parallel tensile stress, thereby forming a directed vacancy stream in the tensile stress direction, and continuously gathering toward the center of the cavity, so that the cavity can grow parallel to the tensile direction. A large number of documents mentioned that the alloy is prone to "pitting" and voids during the forging process. Through the analysis of the formation mechanism of TA7 titanium alloy "pitting" and void defects, we have summarized a set of prevention of void defects in TA7 titanium alloy forgings. The effective method is to strictly control the deformation per fire time ≤50%, and strictly control the deformation rate. It is best to use hydraulic or hydraulic forging, and try to avoid hammer forging, which has achieved good results in production.

2. Forging thermal effect
In the forging deformation of titanium alloy, under normal circumstances, the central part is a severely deformed area, so the center is the area with the highest temperature rise. The temperature rise of the central part is the main basis for formulating the forging process. When using a forging hammer with a faster forging speed to forge titanium alloys, the central thermal effect during the forging process must be considered, and the billet cannot be continuously hammered. For titanium alloy forging, it is recommended to use a press or fast forging machine under conditions. This type of forging equipment has a low impact speed, and the instantaneous strain rate of the blank during the forging process is low, the deformation heat generated is not very obvious, and there is enough time for deformation Thermal diffusion will not cause the instantaneous temperature of the heart to increase significantly.

3. Uneven organization
In terms of forging process, firstly, proper high temperature homogenization treatment is adopted when the ingot is forged. The microscopic intragranular dendrite segregation in the columnar structure region of the ingot is improved and eliminated by homogenization annealing or deformed recrystallization; secondly, in alloys During the die forging process of the billet and the finished product, an appropriate post-forging cooling method is adopted to control it to suppress the appearance of coarse α blocks in the microstructure. After the above-mentioned TC17 titanium alloy forging is sub-β die forging, the use of air cooling is the inducement for the appearance of coarse α blocks. After forging, the cooling rate is slow, the degree of subcooling is small, and the nucleation rate is low, so the α phase has enough time to grow to form coarse α piece.

After forging, water-cooling or oil-cooling fixes all or part of the deformed structure of forging produced crystal defects (dislocations, sub-crystals) and increased dislocation density to room temperature, and adds a large number of crystalline cores for recrystallization in the subsequent heat treatment process. During heat treatment, the precipitation mechanism of the β phase changes from the induced nucleation mechanism under the air-cooling condition to the independent nucleation method, resulting in fine, chaotic and intertwined strips of primary α and secondary α. This structure can significantly improve the alloy's comprehensiveness. performance.
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