2016 International Conference on Material Science and Civil Engineering (MSCE 2016) ISBN: 978-1-60595-378-6 Numerical Simulation on the Hot Stamping Process of an Automobile Protective Beam Han-wu LIU a, Xin-yong GONG, Xue-yun SHA and Hui-ming SUN Sch. of Mechanical and Electrical Engineering, North China Institute of Science and Technology, Sanhe, Hebei, 065201, China a hanwu-liu@sohu.com As an important method of plastic forming, sheet metal forming was widely applied to automobile manufactory. High strength sheet steels were used in automobile stamping parts recently, but traditional cold stamping method was hard to produce body parts with complex structure and shape as the steel sheet strength is greater than 1000 MPA, and then the hot stamping forming process arose at the historic moment. In this paper, the hot stamping process of automotive protective beam was studied. The numerical simulation and analysis were conducted for the forming technological property, temperature variation of sheet metal and possibility of spring back and wrinkling by using finite element analysis software DYNAFORM. Possible defects could be predicted, which provided a new theoretical method and technical support to reduce the production cost of similar products and shorten the development cycle. Keywords: Temperature Field; Protective Beam; Hot Stamping; Spring Back; Wrinkling. 1. Introduction Based on transformation strengthening, dual phase steel (DP) composed of ferrite and martensite which showed low yield ratio, high initial hardening rate, good strength and ductility matching characteristics, had developed into an advanced high strength steel (AHSS) [1]. Project research of ultra light steel auto body showed that the amount of DP would reach 80% in future auto body. The hot forming of DP had its own process characteristics [2]. As high deformation resistance of super steel, forming load was very high in the process of cold forming for large parts, so the cost of punch machine was improved. However, hot stamping method had the advantages of good plasticity, high forming limit, easily forming, less forming load, and so on [3]. Hot stamping process of automobile protective beam was relatively simple, but it was difficult to judge the state of stress and strain accurately, and control deformation [4]. Many defects might appear such as insufficient stamping height, wrinkle, crack, shape skew and so on. Hot stamping process of DP800 automobile protective beam was studied and its forming limit, distribution of principal stress and strain, variable material thickness, forming dimension 520
precision and possible breakage were simulated and analyzed to accelerate the development and application of DP in automobile industry [5.6]. Table 1. Mechanical Properties of DP800/500. Material Elongation Yield Stress Tensile Strength Elastic Modulus / % σs / MPa σb/ MPa E 2. Mathematical Modeling and Stamping Process Simulation of the Beam The material of automobile protective beam was DP800/500 and its mechanical properties were shown in Tab.1 [7]. Mathematical model established by Pro/E software (Figure 1) was introduced into DYNAFORM to mesh and set material, and the final mesh was shown in Figure 2. Figure 1. Mathematical Model. Figure 2. Mesh Graph. Simulation results of hot stamping were shown in Figure 3, in which convex and concave die were hidden for clarity. In the primary stage of stamping, an appropriate blank holder force was loaded to the metal sheet by press plate firstly. Bending force was exerted on the sheet by convex die, meanwhile the reverse force was generated in support point of concave die corner, which were both motive power of bending deformation. Bending radius of the metal sheet decreased with downward movement of the convex die gradually, and the position of support point mentioned above was also changed, resulting in shortening the bending arm. As the bending radius reached a fixed value, plastic deformation of the sheet occurred at the radius corner of the convex die. The bending radius would be reduced at the end of motion displacement. Straight edge of U shape part had been deformed in the direction of concave die until the sheet contact completely with convex and concave die. 521
Figure 3. Stamping process simulation of the beam. 3. Numerical Simulation of Spring back in Hot Stamping Process In terms of drawing and bending, due to different direction of tangential stress for inner and outer layer material, direction of final elastic recovery was distinct, which intensifying dimension deviation of workpiece, so the research on spring back was very critical. In this paper, spring back value was marked as angle variation between bottom and side edge at the end of hot stamping forming. As wrinkle and thickness thinning appearing in the forming process, judgement result was average from four different positions. Measurement angles in Figure 4 were 93.953, 85.474, 85.550, 85.984. 6 combinations of stamping speed and initial sheet temperature were set up, and corresponding spring back values Figure 4. Positions of Angle Measurement. were listed in Table.2 Punctuation marks are used at the end of equations as if they appeared directly in the text. 522
Table 2. Springback Angle for different Stamping Speed and Initial Temperature. 1 2 3 4 5 6 Velocity (mm/s) 5000 1000 500 3000 3000 3000 Temperature( C ) 800 800 800 400 600 1000 Position 1 93.953 85.139 85.955 83.834 84.429 86.377 Position 2 85.474 86.138 94.222 83.653 83.836 85.544 Position 3 85.550 85.281 85.334 82.878 84.832 85.588 Position 4 85.884 85.132 84.846 83.768 84.342 86.433 Average 94.236 94.578 94.218 96.467 95.640 94.015 Spring back 1.236 1.578 1.218 3.467 2.640 1.015 In the presence of errors, the impact of stamping speed on spring back was very small, which could be ignored. However, in the allowable range, the higher initial temperature of hot stamping, the less obvious was spring back. The hot stamping could reduce amplitude of spring back compared with the cold stamping 4. Analysis of Wrinkling and Thinning in the Hot Stamping Process The stamping process (stamping speed-5000 mm/s, initial sheet temperature-800 ) was simulated. Wrinkling of the stamping part could be observed in the forming limit diagram (Figure 5), in which region of red, yellow, green, rose pink, deep pink, light blue representing crack, cracking tendency, safety, wrinkle, severe wrinkle, wrinkle tendency, respectively. Wrinkle phenomenon was concentrated in four side wall of drawing corner in this process of stamping, which been subjected to severe extrusion and friction. But the wrinkle was in the range allowed and the track did not exist in this stamping. Figure 5. Forming Limit Diagram. Figure 6. Thinning. Diagram of sheet thickness variation was shown in Fig.6, in which the thinning area mainly occurring in corner position of the workpiece. Generally, stamping process required thickness thinning should be less than 15%. The maximum of thinning for this stamping was 1.829 mm and the thinning rate was 8.6%, so the above requirement was satisfied. 523
5. Temperature Field Analysis of the Hot Stamping Process For this forming material, the thermal conductivity, radiation coefficient, conduction coefficient was 40.0 W/mK, 7.6 SBC, 20.0W/m 2 K. Initial temperature of the die (forming sheet) was 50 (500 ) and the temperature field of the stamping part was shown in Fig.7. Temperature of the sheet was continuously decreased. At the beginning of stamping, Heat transfer existed between the die and forming sheet as their temperature difference. Compared with bottom, both sides of the U shape sheet contracted earlier and longer time with the die, and more heat was transferred away, so its temperature drop was the lowest. As no cooling system equipped in the hot forming die, workpiece temperature could be reduced quickly for only one sheet processing, but for continuous processing, in which heat was transferred only by convection and radiation, mold temperature would increase consistently to affect forming quality, thus processing stability could not be guaranteed. So cooling system was required to transmit the mold heat timely, ensure the stability of the forming part. 6. Conclusions Figure 7. Temperature Field of the Hot Stamping Process. In this paper, pre-processor, solver, post-processor was set as DYNAFORM, LS-DYNA Solver, ETA/Post-Processor (LS-PREPOST 4.0) respectively. The hot stamping forming process of AHSS automobile protective beam was simulated by using adaptive remeshing and penalty function contact algorithm. 524
Phenomena of spring back, wrinkling, thinning, and temperature field of the forming part were analyzed. The following conclusions are gained: 1) By dynamic simulation of hot stamping for protective beam, thickness thinning of the metal sheet and slight wrinkle in the sidewall were observed, which was basically consistent with the forming process. 2) As stamping speed of the convex die and initial temperature of the metal sheet were set as variables only, stamping speed had little effect on spring back of sheet metal forming, however, spring back value decreased with the initial temperature within the allowable range. 3) Though analysis of temperature field, temperature drop of stamping sheet was continuous, especially for some region, which met the requirement that temperature drop should larger than 27 /s. For continuous stamping process, cooling system should be designed in the mold as the above requirement could not be satisfied. 4) It is feasible to produce DP800 automobile protective beam by hot stamping process, which can meet the requirement of practical application. But radius and clearance of the mold, holding time were not considered in spring back analysis, which needs further study. References 1. Ekkehard D. Hot Stamping-A New Hot Forming Technology. ThyssenKrupp Tech Forum. 2005 (7). 2. Merklein M., Lechler J. Investigation of the Thermo-Mechanical Properties of Hot Stamping Steel. Journal of Materials Processing Technology. 2006, 177. 3. Geiger M., Merklein M., Lechler J., etc. Basic Investigations on Hot Sheet Metal Forming of Quenchenable High Strength Steels. 2nd International Conference on New Forming Technology. 2007. 4. Turetta A., Bruschi S., Ghiotti A. Investigation of 22MnB5 Formability in Hot Stamping Operations. Journal of Materials Processing Technology. 2006, 177. 5. Klhn. Hot Forming Makes Higher Strength. Modern Metal Processing, 2006(11). 6. Karbasian H, Klimmek C, Brosius A, etc. Numerical Process Design of Hot Stamping Processes Based on Optimized Thermo-Mechanical Characteristic. CHS2, 2008. 7. Akerstrom P., Wikman B., Oldenburg M. Material Parameter Estimation for Boron Steel from Simultaneous Cooling and Compression Experiments. Modeling and Simulation in Materials Science and Engineering. 2005 (13). 525