Crack defects and causes in the processing of stamping and drawing parts

Most of the stretch cracks produced by the stamping and drawing parts are at or slightly below the lower end of the workpiece cylinder wall and the outer corner, which is the so-called dangerous section. The main reason is that the strain value at the dangerous section is too large, which exceeds the maximum tensile stress that it can bear, resulting in a significant reduction in wall thickness. The main reasons that affect the tensile cracks of the cylindrical tensile parts are as follows: 1) The mechanical properties of the sheet metal. The smaller the yield ratio of the raw material, the greater the elongation δ, which is more beneficial to the stretching of the stretched part. Because the yield strength s is small, the plastic deformation ability of the raw material is better, the resistance generated during deformation is relatively small, and the tensile stress of the cylinder wall is also reduced; the tensile strength b is large, and the strength of the dangerous section is corresponding Increased, the probability of producing tensile crack defects decreases. The elongation δ of the sheet material is large, and the necking phenomenon is not easy to appear during stamping and stretching, so the dangerous section is greatly thinned and the tensile cracks will be delayed. 2) Stretch coefficient m. The smaller the value m, the greater the degree of deformation of the stretched part each time. Although the stretching times of the stretched part can be reduced, it will cause the wall thickness of the stretched part to become thinner and more likely to be broken. 3) Fillet radius of the die. The radius of the transition fillet of the die is too small. During the stretching process, the bending and straightening deformation of the fillet when the sheet is stretched will produce deformation resistance, causing the friction resistance between the two to rise sharply. The total tensile force also increases correspondingly, and causes the tensile sheet to become too thin, which in turn leads to tensile cracks. 4) The round corner radius of the punch. During the stretching process, if the radius of the rounded corner at the front end of the punch is too small, the bending deformation of the sheet will increase through this part, which will cause the strength of the dangerous section of the sheet to be reduced, so it is more likely to be serious at the dangerous section. Thinning and stretching cracks. 5) Blank holder force and lubrication. The blank holder force of the blank holder should not be too large, otherwise it is difficult for the material to enter the gap between the upper and lower molds during stretching, and the parts are more likely to be broken; during the stretching process, lubrication measures are taken at the transition fillet of the mold. It can reduce the phenomenon of tensile parts cracking. Previous: Advantages of Wire EDM