Factors Affecting Springback in U-Shaped High-Strength Steel Components

1. Experimental Materials

1.1 Material Performance Parameters

The experiment studied DP590 high-strength steel, whose chemical composition is shown in Table 1, and performance parameters are shown in Table 2.

Table 1 Chemical Composition of the Material (mass fraction)

Table 2 Material Performance Parameters

1.2 Stress-Strain Curve

The tensile test was carried out according to the GB/T 228.1-2010 test standard, and the true stress-strain curve obtained from the test data is shown in Figure 1.

2. Finite Element Model Establishment

2.1 Model Establishment

Based on the NUMISHEET2011 international standard case and the size of commonly used beam parts in actual production, the size of the plate used for the U-shaped part is determined to be 100mm × 360mm × 1.2mm.

The assembled two-dimensional structure is shown in Figure 2, and the geometric dimensions of the mold parts are shown in Table 3. The model was created in UG NX software according to the drawing, and the explosive structure was assembled as shown in Figure 3.

It was then imported into Dynaform for meshing and basic process parameter setting, as shown in Figure 4.

• 1. Punch
• 2. Flanging Ring
• 3. Sheet Metal
• 4. Die

Table 3 Geometric Dimensions of Mold Parts

2.2 Reasons for Springback and Evaluation Standards

During the stamping process, the sheet metal will undergo large amounts of elastic deformation.

In the final stage of stamping, the unloading of the load will cause the elastic energy generated earlier to be released, causing the internal stress to recombine, thereby changing the shape of the part. This results in the springback defect of the sheet metal.

Now, the middle section of the U-shaped part is selected to measure the springback situation, as shown in Figure 5.

The size of the springback is characterized by the complementary angle μ of the angle θ between the surfaces AB and CD. The larger the angle μ, the larger the springback amplitude.

3. Investigation of Factors Affecting Springback Angle

The factors that affect the springback of stamping parts are mainly mold structure, sheet metal shape and mechanical properties, and stamping process parameters.

In view of the structural and stamping process characteristics of the U-shaped part, four main factors including flanging force, friction coefficient, sheet metal thickness, and punch corner radius were selected to explore the influence of different factors on the variation of the springback angle.

3.1 Influence of Flanging Force

Under the conditions of using DP590 high-strength steel with a thickness of 1.2 mm and a friction coefficient of 0.125, and a gap (G1) of 1.2 mm between the punch and the die, the influence of flanging force on springback was studied by performing forming and springback simulations with flanging forces ranging from 10 kN to 120 kN.

The influence of the flanging force on the springback angle obtained through simulation is shown in Table 4 and Figure 6.

Table 4 Relationship between Flanging Force and Springback.

As shown in Figure 6, the springback angle of the part initially increases and then decreases. From Table 4, it can be seen that when the flanging force varies from 10 kN to 120 kN, the maximum change in springback angle is 5.216°, indicating that the control of flanging force has a more obvious effect on springback angle.

The reason for the above phenomenon is: when the flanging force is small, the part is mainly formed by bending. The sheet metal produces bending stress under the action of the punch, and the bending stress is released after the punch unloads, which leads to an increase in springback.

When the flanging force increases to a certain extent, the plastic deformation of the material increases, causing the amount of elastic deformation released by the sheet metal to decrease, and therefore the springback also decreases.

3.2 Influence of Friction Coefficient

Under the conditions of using a flanging force of 40 kN and a gap of 1.2 mm between the punch and the die, the influence of friction coefficient on springback was studied by performing forming and springback simulations with friction coefficients ranging from 0.025 to 0.275.

The influence of the friction coefficient on the springback angle obtained through simulation is shown in Table 5 and Figure 7.

Table 5 Relationship between Friction Coefficient and Springback.

Table 5 shows that when the friction coefficient varies from 0.025 to 0.275, the maximum change in springback angle is 4.676°, indicating that the control of friction coefficient has a more obvious effect on springback angle.

As shown in Figure 7, the springback angle of the sheet metal increases first and then decreases with the increase of the friction coefficient.

The reason for the above phenomenon is: initially, because the friction coefficient is small, the frictional resistance of the material is small, and the forming process of the sheet metal is mainly bending, resulting in a large amount of springback.

As the friction coefficient gradually increases, the frictional resistance of the material also increases. When the frictional resistance is large enough, the edge of the sheet metal is difficult to move, and the forming process of the part is mainly plastic deformation.

After the punch is unloaded, the sheet metal will release less elastic potential energy, and therefore the springback of the sheet metal will decrease.

3.3 Influence of Sheet Metal Thickness

Under the conditions of using a flanging force of 40 kN, a gap of 1.2 mm between the punch and the die, a friction coefficient of 0.125, and a punch radius of R5 mm, the influence of sheet metal thickness on springback was studied by performing forming and springback simulations with sheet metal thicknesses ranging from 1.0 mm to 1.8 mm.

The influence of sheet metal thickness on the springback angle obtained through simulation is shown in Table 6 and Figure 8.

Table 6 Relationship between Sheet Metal Thickness and Springback.

As shown in Table 6, when the sheet metal thickness varies from 1.0 mm to 1.8 mm, the maximum change in springback angle is up to 8.511°, which is the largest among the four selected factors, indicating that the control of sheet metal thickness has a relatively obvious effect on the springback angle.

As shown in Figure 8, the springback angle of the sheet metal decreases with the increase of sheet metal thickness.

This is mainly because, under the condition of constant punch radius, the larger the sheet metal thickness, the smaller the ratio of bending radius to sheet thickness, resulting in less bending deformation of the sheet metal and a lower proportion of elastic deformation in the total deformation.

Therefore, when the punch is unloaded, the springback of the sheet metal decreases.

3.4 Influence of Punch Corner Radius

Under the conditions of using a flanging force of 40 kN and a friction coefficient of 0.125, the influence of punch corner radius on springback was studied by performing forming and springback simulations with punch corner radii ranging from R3 mm to R7 mm.

The influence of punch corner radius on the springback angle obtained through simulation is shown in Table 7 and Figure 9.

Table 7 Relationship between Punch Corner Radius and Springback.

As per the observations made from Table 7 and Figure 9, increasing the radius of the punch fillet from 3mm to 7mm results in an increase of springback angle from 6.388° to 8.553°.

This behavior is attributed to the rise in contact area between the punch fillet and sheet metal, which subsequently increases the bending deformation, leading to a greater springback effect.

Nevertheless, from Table 7, it is notable that the change in maximum springback value from 3mm to 7mm in the punch fillet radius is only 2.165°, which is the smallest effect among the four selected factors.

This implies a minimal influence of the punch fillet radius over the springback angle.