The factors affecting superplastic forming are: the structure state of raw materials, grain size, thickness uniformity, forming temperature, forming pressure and deformation speed and other parameters.
- Selection of original blank
The structure state and grain size of the titanium slab for forming are directly related to its superplasticity index. The coarse grains of the Weiss structure phase have very low plasticity and are not suitable for pneumatic forming. Titanium alloy slabs are generally required to have an equiaxed fine grain structure (grain size <10 μm). Therefore, the industrial supply state TC3, TC4 thin plate can basically meet the requirements of use. However, metallographic examination and high temperature tensile test must be carried out on the raw materials to confirm that they meet the requirements before they can be used.
The thickness tolerance of the plates supplied by the industry is large, and the error of the thickness uniformity requirements cannot be greater than ± 0.05mm, and local defects such as thinness, inclusions and segregation are not allowed.
- Molding temperature
According to the high-temperature tensile data of the material, select the appropriate temperature range, and also consider the long-term use temperature of the mold material, easy operation, prevent pollution and other factors to determine a reasonable forming temperature. From the perspective of engineering application, it is more reasonable to choose a lower temperature as much as possible. This is beneficial to prolong the service life of the mold and improve production efficiency, and is conducive to operation.
- Forming pressure and deformation speed
In the pneumatic forming of titanium alloys, the gas pressure is an important parameter to ensure that the parts are attached. According to the shape and size of the part, the thickness of the original slab and the flow stress, a reference value can be obtained.
Deformation speed is a parameter that has a direct relationship with the forming pressure. It can be seen from the formula α = Ke that the flow stress is directly proportional to e. In order to ensure that the flow stress is always in a small range during the forming process, the deformation speed must be controlled in a low range. Generally, it is more appropriate to take the strain rate e <10 ^ (-4) -10 ^ (-1) seconds. In forming, the forming speed is generally controlled by controlling the increase of gas pressure per unit time. For example, in titanium ball bottom forming, the pressure is 0.005-0.01 MPa in the first stage R800mm ball forming, and it is 0.02-0.03 MPa in the second stage R5 arc forming, assuming The deformation of the blank is relatively uniform, and the deformation speed is to be within the range of e <10 ^ (-4) -10 ^ (-1) seconds.
From the perspective of engineering application, the deformation speed should not be too low, as long as the forming speed is as high as possible under the premise of satisfying the forming quality of the parts, it is beneficial to improve production efficiency.