In order to make the thermal calibration successful, it is necessary to select the appropriate heating temperature, holding time and forming pressure. The pressure applied to the part during thermal calibration can be as long as it can ensure that the part is pressed into the mold. The large pressure does not have much effect on the shape of the branch, but may result in the shape of the mold and the table. Therefore, the main effect of thermal correction is temperature and time, and temperature is the determining factor. In order to satisfactorily eliminate the rebound, it is only carried out at a certain temperature. The so-called thermal specification is mainly to determine the shape temperature and time. In summary, a suitable thermal specification for the formation of a certain titanium sheet must ensure that the part meets the following basic requirements after being shaped:
1. The parts are well-applied and basically do not need to be manually trimmed. The shape, size and surface quality meet the inspection requirements for titanium plate parts;
2, the mechanical properties of the material is basically stable, the main performance indicators at room temperature and operating temperature meet the requirements;
3. The residual stress inside the part is basically eliminated;
4. The average hydrogen content of the material after hot calibration does not exceed the allowable value of 150p. p. m;
5. The total thickness of the scale and the gas barrier layer shall not exceed half of the allowable negative deviation of the thickness of the sheet;
6. There is no change in the metallographic structure of the material, and the crystal grains have no obvious growth and overheating;
7. Under the premise of satisfying the above requirements, the temperature should be as short as possible and the pressure should be as low as possible to ensure that the parts can be pressed.
Our calibration test was carried out on a domestic RX-1 machine. The materials are TA2 and TC1 with thicknesses of 0.5, 0.8 and 1.0 mm. Use a thermocouple to indicate the temperature of the mold or work area, the temperature can be controlled within ±10 °C; the time is controlled by the time relay, from the time the test piece is placed in the mold, clamped or placed in the working chamber, until the time of removal.
The issue of forming accuracy is now highlighted. Forming accuracy is the most basic requirement for part press forming, and it is the main indicator for measuring the appropriate temperature and time specifications. The method for judging the forming accuracy is to measure the degree of non-sticking after the part is cooled to room temperature, that is, the amount of remaining rebound. Take the arc-shaped wide plate bending piece as an example. It can represent a type of part with an accurate curvature on the aircraft and an accurate shape. 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/mm 2 ; the yield limit α0.2, take 40.5 kg/mm 2 for TA2 and 60.5 kg/mm 2 for TC1.
In summary, under the premise of basically maintaining the forming accuracy, the specification of the thermal calibration of the titanium plate can be initially determined.