Tow hook forging preform technology simulation

Automobile tow hook is an important safety component used to tow the car with the help of external force after the car breaks down. In the process of use, in order to prevent its fracture or wear, there are certain requirements for the strength, hardness and impact toughness of the tow hook.

Automobile tow hook belongs to the long shaft type of bending parts, in order to improve production efficiency, to ensure the use of product performance, the example adopts this production process: roll forging preforming - bending - die forging shaping, and conducts theoretical analysis and finite element simulation of the roll forging preform process.

1. Roll forging process

1.1 Preform drawing

The plan view and three-dimensional schematic diagram of the tow hook forging are shown in Figure 1. Roll forging provide reasonable preform for die forging, and the primary task of its design is the formulation of roll forging preform drawing. The reasonable preform shape before die forging should be close to the calculated. Therefore, the formulation of the roll forging preform drawing should be based on the calculation of the shape and size.

Calculate the preform through the cross-sectional view of the tow hook, as shown in Figure 2. Based on the calculated maximum cross-sectional size, the standard steel model can select φ48 mm round steel billet, then calculate the original preform length.

The final roll forging preform as shown in Fig. 3 can be obtained based on further calculations.

1.2 Determination of roll forging passes and selection of groove system

According to Figure 3, the maximum cross-sectional area and minimum cross-sectional area of the roll forging preform are obtained, and then the number of roll forging passes for tow hooks can be obtained as 2 by further special calculation.

During the roll forging process, the selection of the groove is affected by many factors. Considering the cross-sectional shape requirements of the preform by die forging after roll forging, the elliptical-circular groove system is selected for the tow hook, and use the corresponding method to calculate the intermediate cross-section dimensions, to get the dimensions of these parts as shown in Figure 4.

2. Finite element simulation of roll forging preform

2.1 Finite element modelling

Before setting up the geometric model, firstly based on the calculation of the roll forging preform, and then combined with experience to determine the corresponding dimensions of each section on the arc-shaped roll forging die, thereby setting up the three-dimensional solid model. During the simulation process, the initial temperature of the workpiece is 1180°C, the finite element model of the roll forging is obtained, as shown in Figure 5.

2.2 Simulation results and analysis

The simulation steps of the roll forging process are shown in Figure 6. It can be seen that after the two-pass roll forging process, the radial size of the workpiece changes greatly, especially at the end, it reached a large elongation, and ultimately obtained tow hook preform without flash, burrs, in full compliance with the requirements of the preform making process, indicating the feasibility of roll forging in the tow hook preform making process.

During roll forging, the workpiece gradually deforms in the die cavity. The entire deformation process is completed by each local deformation. Therefore, roll forging requires much less force than the general die forging process. The roll forging torque of the two passes of preform making is shown in Figure 7. The appearance of the wave peak corresponds to the stage of change of the die cavity cross section, which is consistent with the theoretical calculation.

The stress changes at different moments during the roll forging process are shown in Figure 8. The stress distribution during the first pass of deformation is shown in Figure 8 a. The workpiece shown in step 10 is gripped by the die in the middle, and the part in contact with the die has greater stress, while other parts are basically undeformed. As the amount of underpressing increases to the maximum value during deformation, the stress reaches the maximum value.

The stress distribution during the second pass of deformation is shown in Figure 8 b. Comparing the two-pass process, the stress of the second pass is larger. This is because the billet needs to be turned 90°enter the 2nd pass cavity after the 1st pass, forming a narrow and high deformation area.

After rolling forging the workpiece, the final forging obtained by further bending and die forging is shown in Figure 9. It can be seen that the workpiece is well filled, the flash edge is more uniform and no forging defects such as folding, which meets the design requirements. It indicated that the preform design process of roll forging is reasonable and effective. The stroke load curve of the final forging is shown in Figure 10, and the maximum load is 7.88MN, that is, 788t.

3. Conclusion

Through the above numerical and simulation analysis, it can be seen that it is feasible and effective to use the roll forging process to make preform for tow hooks, the special designed roll forging preform are more in line with the final shape of the tow hooks, which can realize precise molding, have the smallest forging flash and a better surface finish, which will also greatly reduce the subsequent machining, effectively reducing costs for forging companies while improving product market competitiveness.

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