With the development of China’s automobile mold manufacturing industry, the domestic automobile mold production has gradually transferred from low-end mold to high-end mold, so the mold manufacturing enterprises have higher and higher requirements for the refinement and high-end mold production from the inside out. Therefore, in this era of both face and strength, whether the stamping design process is reasonable or not is of great significance to designers and manufacturing enterprises. Therefore, as the main parts of automobile body, stamping parts are the key research and development products of various mold manufacturers. In this paper, the mechanism and formability of flanging reverse arc and the control method of flanging reverse arc are described in detail.
Mechanism analysis of flanging reverse arc
The concept of flanging reverse arc
After curve flanging, because of the uneven stress distribution, the flanging surface will be concave or warped after the release of internal stress, which belongs to a kind of springback of flanging.
In the process of curve flanging, sheet metal is mainly divided into three areas: non deformation area (pressing area), fillet and main deformation area (flanging surface). Figure 1 shows a straight line flanging. Fig. 2 when flanging forming, the pressing area is only subjected to downward pressure (stamping direction), so the material does not undergo plastic deformation; the main deformation area is under the action of flanging insert, and the material will be in the stress state of biaxial tension or biaxial compression. During flanging forming, the deformation area cdab reverses downward along AB after flanging, and finally becomes efab. The outer edge CD of deformation area becomes arc EF, and the DC in Figure 3 becomes longer, otherwise it becomes shorter.
Figure 1 straight line flanging
Figure 2 curve extension flanging
Figure 3 Curve compression flanging
Stress distribution of flanging
The maximum circumferential compressive stress occurs at the inner edge of the deformation zone. When the external stress reaches the limit value, wrinkling occurs, resulting in the scrapping of parts.
Figure 6 shows the extended flanging. From the shape point of view, the flanging contour is convex to the outside. When the metal material in the flange deformation area is under the action of tensile stress during deformation, the material will produce elongation deformation, and the thickness will gradually decrease. The maximum circumferential compressive stress occurs on the outer edge of the deformation area. When the value reaches the limit value, the outer edge of the flange deformation area will crack, resulting in parts scrapping.
Circumferential stress distribution of flanging
For compression or tension flanging, the maximum strain value of circumferential stress in the boundary part of metal material during flanging should occur at point G, and the two sides show a decreasing trend. The minimum strain value is 0 at the two ends of a and B. It is assumed that the circumferential stress along the flanging line is cosine distribution. When the metal material is unloaded by the external force, the stress of the sheet will no longer maintain equilibrium, and the internal stress will be released in the form of springback The greater the stress is, the greater the springback value of the material will be, resulting in the uplift and upwarping of the G-point position. The springback trend of the material will be cosine distributed outward with the G-point as the center, resulting in plastic or elastic deformation in the material deformation area, as shown in Figure 7.
Figure 4 straight line flanging
Figure 5 curve compression flanging
Figure 6 curve extension flanging
Figure 7 circumferential stress distribution of flanging
Figure 8 straight line flanging process
Figure 9 curve flanging process
Generation process of flanging reverse arc
When flanging, the blank holder presses the non deformation area, and the corner r of flanging insert contacts with the main deformation area. With the upper die insert moving downward, the main deformation area begins to bend. In Figure 8, the forming process of straight flanging is similar to straight flanging because the flanging stress distribution is uniform. In Figure 9, because the flanging stress distribution is not uniform, and the flanging arc gradually becomes longer or compressed, the curve flanging forming process is arc. After flanging, because the internal stress cannot be eliminated, the warping phenomenon occurs, that is, flanging reverse arc.
Control scheme of flanging reverse arc
Flanging reverse arc is a common defect of stamping parts, which is difficult to solve. At present, several common control schemes are adopted in die and mould factories, such as reducing flanging insert, flanging strong pressing, increasing process gap, adopting pressing material flanging, etc. However, at present, the above methods are not ideal, and the problem can not be completely solved. In addition to the conventional control method, it can also be solved by the scheme of leaning over, which has the disadvantage of increasing the flanging wedge and increasing the development cost. In order to control the flanging reverse arc, the flanging angle is not consistent with the product direction. When the flanging is in place, the flanging surface is strongly pressed to control the reverse arc. Table 1 shows the comparison results of various anti arc control schemes.
Table 1 comparison results of various anti arc control schemes