In order to make the heat calibration successful, the appropriate heating temperature, holding time and forming pressure must be selected. As long as the pressure applied to the parts during hot calibration is enough to ensure that the parts are pressed into the mold, the greater pressure will not have a great effect on the dendrite, but may cause the mold and the table to form. Therefore, it is the temperature and time that affect the effect of thermal correction, and temperature is the determining factor. In order to eliminate the springback satisfactorily, only at a certain temperature. The so-called thermal specification is mainly to determine the calibration temperature and time. In summary, a suitable thermal specification for the formation of a certain titanium plate must ensure that the parts meet the following basic requirements after calibration:
- 1. The parts are well pasted and basically do not require manual trimming. The shape, size and surface quality meet the inspection requirements of titanium plate parts;
- 2. The mechanical properties of the material are basically stable, and the main performance indicators at room temperature and service temperature meet the regulations;
- 3. The residual stress inside the parts is basically eliminated;
- 4. The average hydrogen content of the material after thermal calibration does not exceed the allowable value of 150p. p. m;
- 5. The total thickness of the oxide scale and the gas-permeable layer shall not exceed half of the allowable negative deviation of the sheet thickness;
- 6. There is no change in the metallographic structure of the material, no obvious growth of grains and overheating;
- 7. Under the premise of meeting the above requirements, the temperature should be as low as possible and the time should be as short as possible, and the pressure should be suitable to ensure that the parts can be pressed.
Our shape calibration test is carried out on domestic RX-1 machine tool. The materials are TA2 and TC1, and the thickness is 0.5, 0.8 and 1.0 mm. Use a thermocouple to indicate the temperature of the mold or working area. The temperature can be controlled within ± 10 ℃; the time is controlled by the time relay, starting from the time the test piece is placed in the mold, the mold is closed, or placed in the working cavity, until it is taken out.
Now focuses on the problem of forming accuracy. Forming accuracy is the most basic requirement for parts to be formed by pressing, and it is the main indicator to measure whether the temperature and time specifications are appropriate. The method to judge the forming accuracy is to measure the degree of non-molding after the part is cooled to room temperature, that is, the amount of remaining springback. Let’s take the arc-shaped wide plate bending piece as an example. It can represent a class of parts with small curvature on the plane, and the shape requires accurate. The relative bending radius of such parts is large (such as the relative bending radius R / t = 200 before rebound). If the elastic modulus E = 10500 kg / mm2; yield limit α0.2, 40.5 kg / mm2 for TA2 and 60.5 kg / mm2 for TC1.
In summary, on the premise of basically maintaining the forming accuracy, the specifications of the thermal correction of the titanium plate can be preliminarily determined.