Investigation On Thermal Contact Behavior And Prediction Of Mechanical Properties And Process Optimization In Hot-Stamping

In order to improve the safety of vehicles and reduce emissions to the environment pollution, hot stamping is increasingly used in automobile structure part. Heat transfer related issues in hot stamping are the key factors of affecting the mechanical properties, In this paper, based on the pilot hot stamping line developed by WISCO-HUST joint laboratory, the method of calculating the thermal contact resistance is studied. The effect of non-uniform cooling on parts quality is analyzed. And the scheme of the uniform cooling problem is solved. According to phase transformation dynamics model, the method of the mechanical properties prediction after stamping is expressed. Then the key process parameters are optimized in tailored properties process. Finally, based on the study above, the study on the method of improving cold stamping process to hot stamping process is studied by using finite element method. The study results are as follows:Thermal contact resistance between sheet metal and die is the key fact of influencing parameters in thermal and physical properties of the heat transfer in hot stamping process. Based on the cooling curve of blank and flat die obtained by experiment, the inverse heat conduct problem is solved by using sequential function method. The surface heat flux and temperature between blank and die are calculated. And the thermal contact resistance is calculated based on equation. The results show that the higher of pressure between blank and die, the faster the heat flux crossing the contact surface. The thermal contact resistance falls with the temperature decrease. And there are the singularities near the start temperature of martensite phase transformation. The range of singularity decreases with the pressure increasing. Under the same pressure, the thermal contact resistance decreases with time and gets stable value. The stable value decreases with the temperature increasing. According to the law of energy conservation, martensite phase latent of WH1300HF steel is calculated by using surface heat flux. And the parameters of Koistien-Marburger martensite phase transformation model are calibrated.Based on the typical parts a car front bumper as an example, this paper expounds the main reason of non-uniform cooling parts is the uneven contact pressure and clearance between parts and sheet metal. Contrast analysis of the parts under the condition of uniform and non-uniform cooling parts temperature field distribution and the boundary heat flux density distribution, the method of even cooling is put forward by using modified mold block to improve contact condition between die and blank.Based on the corrected phase transformation kinetics model, parts cooling path is used to calculate the phase fractions. The method to predict the final mechanical properties of parts is discussed. The key parameters of phase transformation model are corrected by flat die experiment. The key parameters are obtained by using optimizing method for tailored properties process.In order to investigate the influence of cooling path on friction behavior in hot stamping, the friction coefficient is measured by using hot strip drawing ribo-simulator. The experimental results show that when the strip was cooled at a Martensite start temperature. This dependence of friction coefficient on the die temperature was related to the microstructure difference. For the strips cooled at higher die temperatures, the microstructure consisted of more ferrite and pearlite and less martensite and the hardness of the strip was reduced as a result. As the surface was of the corrugated nature consisting of micro-valleys and peaks, the decline in hardness could allow the micro-peak regions to deform more easily and thus increased the resistance of mechanical deformation at the friction interface. The dynamic friction coefficient increased, accordingly. In addition, when the hardness of the strip was reduced to a level below the hardness of its friction counterpart, adhesive wear occurred during sliding, which tended to weld the contacting surfaces together. The relative sliding would then shear off the adhering points and generate dissociating abrasive particles in the process, which in turn would lead to the increase in the friction coefficient.Duo to the die design defects, the B pillar is difficult to forming. Based on the above research, B pillar can be formed by using optimized blank. Spring back angle between bottom and side wall of B pillar increases compared with the numerical model, which is different to that formed by cold forming.