| Due to its advantages of no oxidation, no decarburization, low distortion, saving energy and full automation of heat treatment process, vacuum heat treatment technique has become the hotspot in the development of international heat treatment technique. Vacuum hot bulge forming is a special forming process technique. According to material high temperature softening and stress relaxation principle, it utilizes the difference in the thermal expansion coefficient between workpiece and die to form the workpiece through thermal expansion force of the die under high temperature. Differs from the other metal forming techniques, deformation of the workpiece takes place elastic-plastic deformation at low stress levels and the amount of plastic deformation is directly related to heating temperature and holding time in the vacuum hot bulge forming processes. Due to the complexity in vacuum heat treatment and hot bulge forming process, the traditional researches in this field focus on the experiments and experience.It will increase research and developed period and product cost. With the development of computer software and hardware and the perfecting of finite element method, finite element analysis becomes a very powerful technique to study vacuum heat treatment and hot bulge forming processes. The simulation is able to collect information of various fields during the vacuum heat treatment and hot bulge forming processes, which provides scientific foundation for diterming the optimal process parameters and ensuring the quality of the workpiece.The present research chooses GH4169 super alloy and BT20 titanium alloy as the research objects, utilizes the nonlinear finite element software MSC. Marc to simulate the vacuum heat treatment and hot bulge forming process, analyzes the temperature field, deformation field and stress field, predicts the vacuum thermal hysteresis time, and explores the methods to confirm the technological parameters for vacuum hot bulge forming of titanium alloy cylindrical workpiece, which provides the theoretical guidance for actual manufacture. The main research content and findings are as follows:1. A 2-D nonlinear finite element model is developed for the heat transfer process in the vacuum heat treatment furnace. The influence of nonlinear factors, such as radiative heat transfer and material thermal physical parameters changing with temperature etc., were considered. The temperature field of heat transfer process for vacuum heat treatment furnace was calculated and analyzed using FE software MSC.Marc. Intelligent PID control of virtual vacuum heat treatment furnace temperature was carried out. The temperature of 750℃and 1150℃is chosen in the simulative computation of the uniformity of furnace temperature and corresponding experiments are conducted to testify the results. The simulaton results of temperature are in good agreement with experimental results, which lay a theory fundation for the virtual manufacture of the vacuum heat treatment processes.2. Heat transfer FE model of vacuum heat treatment furnace is utilized in the numerical simulation of the temperature field in vacuum heating process of the large-scale complex workpiece of GH4169 alloy and various sizes of BT20 titanium alloy specimen. The thermal hysteresis time of GH4169 superalloy workpiece and BT20 titanium alloy specimen during vacuum heating process was predicted. The temperature of GH4169 alloy workpiece and BT20 alloy specimen during vacuum heat treatment process were measured. The experimental results of temperature profiles agree well with the simulated results. Effect of thermal hysteresis time on annering stucture of BT20 alloy was analyzed. The simulated results show that the coefficient of thermal hysteresis time lies between 0.8 to 1 min/mm in vacuum heating process for BT20 titanium alloy with effective thickness thinner than 20 mm. It is concluded that a reasonable thermal hysteresis time should be chosen to ensure the quality and reliability of the workpiece.3. A thermo-mechanical coupling finite element model is established for the vacuum hot bulge forming process of BT20 titanium alloy cylindrical workpiece. FE software MSC.Marc is utilized in the simulative computation of the temperature field, deformation field and stress field in the vacuum hot bulge forming process of BT20 titanium alloy cylindrical workpiece, and it is also used to analyze the impact of the factors such as heating temperature, workpiece thickness, clearance between workpiece and die on vacuum hot bulge forming. A proposal for the technological parameters and design size of die in the vacuum hot bulge forming process of various types of BT20 titanium alloy cylindrical workpiece is put forward, which can be put into practice for real industrial production.4. A model optimization research is conducted on the thermo-mechanical coupling model for the vacuum hot bulge forming process of BT20 titanium alloy. On the premise of keeping the computation precision, auto-increment technique and two step simulation are used to raise effectively the computing efficiency. Numerical simulation is conducted for vacuum hot bulge forming'process of various types of BT20 titanium alloy conical workpiece. And the effect of heating temperature and wall thick of workpiece on vacuum hot bulge forming is analyzed. The corresponding experiments were carried out. The simulated results are in good agreement with the experimental results. The simulation results show that the best temperature range for the vacuum hot bulge forming of BT20 titanium alloy cylindrical workpiece is between 750 and 850℃. Titanium alloy cylindrical workpiece with wall thick thinner than or equal to 3mm can be bending as well as sizing formed by means of vacuum hot bulge forming, but those with wall thick thicker than 3mm can only be sizing formed rather than bending formed by means of vacuum hot bulge forming.
|
PDF Full Text Request