Research On The Spring-Back Compensation And Crease Mechanism During Multi-Point Forming Based On Force-Displacement Model

Force-displacement Separated Control Multi-point forming technology(FDSC MPF) is a new flexible forming technology based on conventional multi-point forming. During the forming process, the displacement is controlled effectively by the upper punch elements and the forming force is controlled availably by the lower punch elements, furthermore, there is always valid surface normal constraint. According to the forming sequence, it is divided into the edge zone priority forming and central zone priority forming. The sheet metal is distributed into three regions: the deformation, transition and non-deformation zone because FDSC MPF is characterized by the successive section forming and alternating bending. FDSC MPF has many advantages such as small rebound, unconspicuous indentations and wrinkles, but has the crease.The springback compensation is a practical means to control rebound that affects seriously the machining precision of forming panels. Based on elastic-plastic material model, the springback bending moment is inferred according to the alternating bending and the curve surface equations S=f(R) enables be calculated. The experiment with different thickness, material and the arrangement of punch elements verifies the accuracy and universality of the theory formula. On the basis of the springback compensation principle, the adjustment-figuration radius is reversed by the formula R=f-1(S) to construct the compensation surface of the die and the experiment certifies the validity and high-efficiency of the compensation theory.The single-curvature cylindrical surface and double-curvature saddle surface chosen as the research objects, the angle and height of the crease are the index to represent the crease defect and effect of the forming force, thickness, forming mode, materials, contact friction coefficient, radius of the hemisphere head and rotational friction coefficient in FDSC MPF on the crease is systematically studied. The results show that: alternating bending is the dominant factor to crease; the larger the forming force, thickness, contact friction coefficient and rotational friction coefficient increases, the less obvious the crease. The larger the radius of the hemisphere head, the more obvious the crease. Considered the Bauschinger effect, the smaller strain hardening exponent, the better the surface quality. The crease of the edge zone priority forming is more obvious than that of the central zone priority forming, and the crease is more obvious in the early forming area. There is almost no crease in the conventional multi-point forming. The elastic cushion and dislocation-point enable improve the surface quality and inhibit crease.