Taking a deep drawing part of railway freight car as an example, combined with the application of Pro / E and jstamp / NV software, this paper expounds the parameter setting idea and verification process of unfolding calculation, process splitting, gap correction, unit selection, etc. The combination of traditional stamping process analysis method and finite element method can solve practical production problems.

When analyzing the drawing process and setting the die parameters of stamping parts, the traditional method is to look up the table to calculate the total drawing coefficient, and then deduce the drawing times and the drawing shape of each process step by step. But often because the shape of the part example in the manual is inconsistent with the actual working condition, or the new material can not find the experience number, so in the process of making the process plan, a lot of approximate treatment will be done, resulting in the deviation of process analysis, causing the problem to move back, making the mold debugging cycle long and the cost high. With the wide application of sheet metal forming analysis software in engineering, in practical work, more and more traditional methods are used as the original input. After finite element calculation, the original input is corrected, and finally the appropriate process plan and die design input are obtained. In this paper, taking an approximate part of railway freight car as an example, the whole process is described to realize the combination of traditional technology and finite element method. With the help of finite element analysis, the drawing shape of each process is optimized. By observing the clearance between punch and die of post-processing section, the problem caused by the clearance between punch and die of traditional 1.1t is avoided.

Creation of part information and material model

The model used in this paper is shown in Figure 1. It is a typical revolving part with certain taper and flanging flange. Compared with the deep drawing part without flange, some parts are not converted into inner wall and the forming conditions are harsh. See Table 1 for the part information.

Figure 1 sectional view of parts

Table 1 part information

In sheet metal forming analysis, material parameters are very important, but too complex parameters are not easy to obtain. Jstamp / NV software can easily fit the real stress-strain curve (Fig. 2) according to the data in Table 1 through the material fitting function, which can be used for analysis and calculation, and it can also meet the accuracy requirements through test verification.

Figure 2 material fitting curve

Estimate initial blank expansion

In this paper, the equal mass method is used to calculate the blank expansion (Fig. 3), and its calculation principle is the same as that of table checking calculation, which can ensure the calculation accuracy, and is applicable to various shapes, which is not limited by shape, convenient and quick, and has strong practicability. However, no matter which method is only the initial input, the thickness of each part after actual forming is uneven, and necessary correction is also made according to the analysis results.

Figure 3 principle of expansion calculation

Trimming quantity determination: considering the height deviation after forming due to the anisotropy and positioning error of the material, the trimming allowance of the flange cylinder is taken as 4mm. After modeling the final part by CAD software, the trimming amount of 4mm is added in the height direction of the part, and the quality of the process model is m=0.732kg.

New virtual blank: the parts of the rotating body are all round in expansion. Because the circle can determine the shape as long as the thickness and diameter are two elements, the new round blank with the same thickness as the part in Figure 1 is created, and the diameter φ is given to a preliminary value, such as 180mm.

Expansion calculation: adjust diameter φ to make the blank mass close to the mass of parts m=0.732kg. According to the above calculation, rough estimation of the blank diameter is 200mm, and it is recorded as dz=200mm.

Determine the first drawing die surface

The technological process is preliminarily determined as cutting (φ 200) → n drawing → flanging trimming. After the process decomposition is adopted, the forming calculation model of the part is transformed into the three drawing shape in Figure 4. As long as the shape can be formed and then flanged, the manufacturing of the parts can be realized.

Find the existing mold design manual in hand. Because of the existence of side wall slope, the reference model with identical shape can not be found. The drawing coefficient is decomposed by analogy. Find the ultimate drawing coefficient of 06cr19ni10 as shown in Table 2. According to the table, it can be seen that the total drawing coefficient is 0.39 less than the primary limit drawing coefficient of stainless steel 0.5. Therefore, the part needs to be realized by multiple drawing. According to table 3, it is calculated that when k1=0.52, the first drawing die size is calculated, dp1=k1 × dz=0.52 × 200= φ 104, and the diameter of the first deep drawing die is φ 104mm. When K2 = 0.82, the dimension of the secondary drawing die is calculated: DP2 = K2 × DP1 = 0.82 × 104= φ 85.3, and the diameter of the secondary drawing die is φ 85.3mm. When K2 = 0.82, the dimension of the third drawing die is calculated: DP3 = K2 × DP2 = 0.82 × 85.3= φ 69.9, and the diameter of the third drawing die is φ 69.9mm.

From the calculation, the drawing coefficient can be adjusted properly after the finite element calculation. The key dimensions of the opening of the die are calculated by the above derivation. However, the deep drawing depth, convex and concave die fillet of each process are obtained by analysis and calculation. Follow the following principles:

Figure 4 drawing of drawing die surface

(1) because the first drawing performance of stainless steel is good, the drawing coefficient is as close to the limit of the material as possible.

(2) after the initial drawing, the flange of the part’s circumference is thickened obviously and the rigidity is large. The force required to pull it into the concave die again later needs to be transferred from the weakest position of the bottom fillet, which is easy to cause the weak position to pull the crack. Therefore, when drawing for the first time, the material brought into the hole of the concave mold shall be as much as possible.

(3) large fillet should be used for initial drawing, and local stress should be avoided as much as possible.

Initial die surface calculation

After comprehensive analysis, the initial die surface size is shown in Figure 4, and from left to right, it is in turn: one drawing, two drawing and three times drawing.

Optimization of finite element analysis

The pro/e parametric 3D modeling software is used to model the convex and concave model surfaces and blanks of each process, and then it is introduced into jstamp/nv to analyze and calculate each process. When the calculation is not ideal, continue to adjust the corresponding process parameters, and finally determine the following process flow chart (as shown in Fig. 5).

Mold design and key points

Setting of die clearance

Figure 5 calculation flow chart of each process

Table 3 maximum thickness of inner sheet material of die

Figure 6 thickness value of key points of secondary drawing

Figure 7 calculation of forming force

The maximum thickness of sheet material entering the die after forming of each process is counted in Table 3. In the design of drawing die, the clearance between convex and concave is 1.1t (t is the plate thickness), in this case, 1.1t=3.3mm. After calculation, the maximum local thickness of the sheet metal has reached 3.58mm during the third drawing, as shown in Figure 6. If we do not pass the simulation analysis, we will set the clearance of convex and concave die according to 3.3mm. During the process of die test, the convex and concave die will be stuck and the workpiece can not be pressed in place. At this time, we may doubt that the pressure of the equipment is not enough, and we will draw our attention to another direction. The finite element simulation analysis solves this problem in advance.

Calculation of blank holder force and selection of equipment tonnage

The deformation force curve is analyzed by deep drawing. Make the blank holder force more reasonable, for example: in a deep drawing, after analysis and calculation, the blank holder force at this time only needs 2 tons, so that the die design is reasonable. In the past, in the mold design, the large blank holder was first set up according to the space. Then, during the debugging process, if the problem was solved, the blank holder was reduced, the debugging time was long, and the mould cost was high.

Figure 8 photo of real object after direct flanging

Figure 9 photos of flanging after heat treatment

At the same time, the deformation force curve is also an important basis for the reasonable selection of equipment tonnage (Fig. 7). In the “setting of die clearance” part of this paper, the initial die clearance is set to 1.1t. When observing the forming force curve, the forming force soars to more than 2000 tons, and it has not yet reached the bottom. It is found that the gap setting is unreasonable only after observing the profile.

After repeated tension and compression, the work hardening phenomenon of the material is obvious, the plasticity is reduced, and the cracking phenomenon occurs in the flanging process (as shown in Figure 8). After heat treatment, annealing and recrystallization, flanging is carried out, and good results are achieved (as shown in Figure 9).