On Variable BHF Deep Drawing Process

Deep-drawing is a common sheet metal forming method to produce an open hollow parts with punch and dies. Wrinkling and fracture are the main obstacles in sheet metal forming. There are so many factors affecting qualities of deep-drawing parts, such as blank holding force (BHF), the forming force and speed of punch die, die structure, lubricant, material performance, and so on. BHF is an important technical parameter in sheet metal forming, its main function is bringing friction, increasing pulled stress of sheet metal, controlling material flowing, avoiding wrinkling and fracture. So an adequate blank holding force (BHF) is important for a successful deep-drawing process of steel sheets. Conical part, a typical part with curvilinear surface is in large proportion to deep drawing production, therefore it is necessary to study the forming characteristic of conical part, to analyze the influence of different parameters on its forming limit, and to define reasonable blank holding force (BHF). By using numerical simulation and experiment analysis, the influence law of BHF on conical parts deep drawing forming was studied in this paper. The variation of BHF to realize extra low deep-drawing ratio forming so as to improve the part qualities was also discussed and testified by experiments.Finite element model of deep drawn conical parts was established according to the systematic analysis about its forming mechanism in this paper, and the constant BHF and variable BHF were set by considering about the factors affecting sheet metal forming such as friction coefficient, die parameter, speed of punch, and so on. Using the software LS-DYNA3D simulated sheet metal forming process. By observing the simulation results in the post-processor program, the limit drawing height, thickness distribution, stress/strain, FLD and etc were analyzed. The simulation results showed: the limit drawing height was only 42mm and the forming force was big as a constant BHF deep-drawing process was adapted. It also can be observed that the ratio of reduction of thickness at the round corner of punch was about 28%, the biggest pulled stress at the direction of drawing was 600KPa and the pressed stress was relative big. But the obstacles of wrinkling and fracture could be eliminated and the higher forming limit was got if an adequate variable BHF was set. The limit drawing height was to 50mm; the ratio of reduction of thickness at the round corner of punch was smaller than 25% and the distribution of thickness was uniformity; the pulled stress and pressed stress were smaller than those in the drawing process as a constant BHF was added and the forming force can be reduced to 28.2KN. It showed that the flow and distribution of the material could be improved; the more uniform thickness distribution and a higher forming limit height were achieved if the control mode of BHF was changed. And the formability also can be increased if a variable BHF control mode was adapted.Simulation results were also validated by deep drawing experiments of conical parts by using 0.8mm IF steel sheet. The test results indicated that variable BHF was in favors of improving limiting drawing ratio of sheet metal, improving surface quality of parts, reducing the ratio of wrinkling and fracture of parts. The simulated results were basically in accordance with the experimental results although some little deviation existed. Therefore this numerical software can be used to predict the deep drawing with variable BHF control mode.