Wen/Xin Tong, Jiaqi Yan, Shang Liu, Xiaofeng Jia, China FAW Co., Ltd
Some parts of the body-in-white are prone to wrinkles and cracks during the stamping process due to the excessive depth of stretching. In this paper, taking the steering pipe string bracket of a certain model firewall as an example, the Finite Element Simulation simulation of the stamping forming process is carried out by Using Autoform software, and the simulation results are analyzed, and finally the secondary drawing can perfectly solve the stamping forming problem of parts with large stretch depth.
With the development of social economy and automobile industry, the status of automobiles in the national economy is becoming more and more important, and it is one of the indicators of evaluating the level of manufacturing industry in a country. The sheet metal parts that make up the car body and chassis can generally be divided into outer covering parts, inner cover parts, structural parts, reinforcement parts and so on. Among them, some structural parts and reinforcements have smaller dimensions, but the space shape is very complex, so automotive sheet metal parts are often difficult to achieve through traditional processes in the design and manufacture of automotive molds. Among them, some parts are difficult to achieve only one stretch in the stamping process, and it is necessary to add a second pull after one pull to obtain qualified parts. Among them, the steering pipe string bracket belongs to this type of part, this paper takes the steering pipe string bracket with a part height of 190mm as an example, and combines the CAE analysis to elaborate the secondary stretching process design of the automotive steering pipe string bracket.
Analysis of the appearance size and workmanship of the part
The three-dimensional diagram of the steering tube string bracket is shown in Figure 1, the part material is DC06, the thickness is 1.4mm, the maximum appearance size is 430mm×270mm, the maximum height is 190mm, the flange edge radius R is 12mm, and there is an irregular oval hole in the vertical wall.
Fig. 1 Three-dimensional diagram of the steering tube string bracket
The part is a typical box-shaped part, the part size is small, but the spatial shape changes dramatically. The focus of the part is that the flange surface is the overlapping surface with the firewall, the dimensional accuracy requires ±0.5mm, to ensure that it has no wrinkle defects, and secondly, because the position and angle of the steering pipe string cannot be changed, the height of the part cannot be reduced.
The traditional process is generally the first blanking, the first order of stretching, the second sequence of trimming, the third sequence of shaping, the fourth sequence of punching. However, due to the small size and high height of this part, only one stretch will often cause cracking at the top of the part and wrinkles on the flange surface. Therefore, to obtain qualified parts, it is necessary to add a second stretch after a pull, and increase the radius of the bottom corner of the flange edge to 15 mm to facilitate the inflow of the piece.
The difference between the secondary drawing and the ordinary drawing is that a press core is added to the upper die to ensure that the part formed by the first drawing will not continue to participate in the second stretch to avoid cracking the top of the part.
The design principles of the secondary drawing process are:
The depth of the pull of a pull should be maximized.
The shape of the pressed surface of the primary stretch and the secondary stretch is consistent, otherwise the sheet will be deformed when the secondary stretch edge is closed.
The position of the primary and secondary stretched stretch ribs on the respective pressed surfaces is the same.
Considering the above factors, the drawing depth of the primary stretch is 120mm, the drawing depth of the secondary stretch is 70mm, and the fourth sequence reshaping reshapes the radius of the bottom corner of the flange edge to 12mm.
The stamping process is planned for OP05 blanking, OP10 primary drawing, OP20 secondary drawing, OP30 trimming, OP40 shaping, OP50 side punching, a total of five sequences to complete.
CAE analysis of parts
The AutoForm incremental solver uses an improved static implicit algorithm that inherits the benefits of the usual static implicit algorithm.
Meshing the imported toolbody model is an important step before simulating the analysis. The meshing is divided into two categories, mold specific meshing and sheet meshing.
Import the tool body model made in UG into AutoForm, and mesh the model through the AutoForm adaptive meshing function. The fault tolerance determines the amount of chordal deviation acceptable when meshing. It should be noted that when meshing, it is necessary to ensure that there are at least 8 mesh elements at the 90° fillet of the model, so the fault tolerance needs to be adjusted appropriately according to the size of the rounded corners of the digital module to ensure simulation accuracy. The smaller the fault tolerance value, the denser the mesh element distribution at the fillets, and the higher the fit accuracy. The fault tolerance of this part is selected at 0.05mm. The maximum side length refers to the maximum side length of the triangular element in the flat area, and the maximum side length of this part is selected at 30mm.
Adjust to a reasonable stamping direction according to the principles of no negative angle in the drawing forming, uniform drawing depth and minimization as much as possible.
Single-action pull-in mode selects single-action pull-in, and secondary pull-in mode selects secondary pull-in mode. The secondary extension adopts an inverted structure. The convex and concave dies and crimping rings in a single stretch are rigid structures, and the tool body is shown in Figure 2. The convex and concave dies, crimping rings and press cores of the secondary stretch also use rigid structures, and the pressed surface is consistent with the pressed surface of the primary stretch, and its tool body is shown in Figure 3.
Figure 2 Tool body for a pull-off
Fig. 3 Tool body of the secondary stretch
The plate material is made of oval shaped material, the thickness is selected 1.4mm, the material is selected DC04, and the physical properties of the material are shown in Table 1.
Table 1 Physical properties of materials
In the drawing forming of auto parts, the stretch ribs or stretch sills are generally used. The stretching rib increases the feed resistance at each position on the pressed surface, controls the flow direction of the material, adjusts the amount of material inflow, and greatly improves the drawing conditions of the part. According to the design principles of the stretch rib, taking into account the calculation time of the CAE analysis, the equivalent stretch rib is used instead of the solid rib. The equivalent stretch ribs of primary and secondary stretches are shown in Figures 4 and 5.
Fig. 4 Equivalent stretch rib of a single stretch
Fig. 5 Equivalent stretching rib of secondary stretching
According to experience, the lubrication value due to the different lubrication conditions, generally between 0.16 ~ 0.19, this article selects 0.15, the physical description of the value is, the mold research and coordination, the light is in place, no lubricant is used, and the sheet is coated with anti-rust oil.
The membrane unit operation speed is faster, but the accuracy is low, the shell unit is generally used in the sheet thickness is greater than 2.5mm, and the subsequent process is secondary stretching, flanging and biting edge. Therefore, both the primary and secondary pull-ins are shell elements. The brimming force of the primary stretch is set to 100000N, and the brinking force of the secondary stretch is set to 300000N.
The mutual adaptation of the sheet metal element and the mold specific unit is also a factor affecting the analysis results, in general, the mold specific grid should be denser than the plate grid. The specific values of the radius penetration value and the maximum element angle are selected according to the three-level standard of sheet material refinement in Table 2, the radius penetration value is 0.16mm, and the maximum element angle is 22.5°.
Table 2 Tertiary criteria for sheet material refinement
Analysis of CAE results
For steering tube string brackets, cracking and flange edge wrinkles are the two most important indicators to determine whether they meet the requirements.
Analysis of part formability cloud diagram and forming limit diagram
As shown in Figure 6, the formability cloud chart shows the formability and defect trend of the part through different colors, which is the most important means for us to qualitatively analyze the quality of stamping. The formability cloud map shows the failure risk area of the part, but cannot give quantitative results, and it needs to be evaluated together with the results such as the thinness cloud map and the wrinkle cloud map. The determination principle of formability cloud diagram is FLD diagram.
Figure 6 Formability cloud diagram
The forming limit diagram can comprehensively reflect the deformation of the parts in the complex stress strain situation, such as cracking and wrinkles, and the forming limit diagram can quickly find the defects of the material in the forming process, and take corresponding measures to change its forming conditions, avoid the occurrence of defects, and improve the forming performance of the parts.
As shown in Figure 7 is the forming limit diagram, which is a coordinate diagram with the secondary strain as the X axis and the main strain as the Y axis. The basic curve is the formation limit curve of the material, which directly reflects a characteristic curve of the material properties, and the curve is biased down a certain distance to obtain the area where there is a risk of cracking. From the figure, it can be seen that most of the points are in the green area, some points are in the blue area, and a small number of points are in the purple area, and they are some distance from the forming limit curve. Therefore, it can be concluded that most of the areas of the part are stretched sufficiently, but there are certain areas of insufficient stretching, and even the possibility of wrinkles, which needs to be determined through further analysis.
Fig. 7 Forming limit diagram
Analysis of part thinning cloud charts
The formability cloud map provides a rough idea of the shape of the part, but you also need to analyze the thinness cloud map to determine whether the part is at risk of cracking. The Thinner Cloud chart represents the thinning and thickening of the sheet and has the same function as the Thickness Cloud Chart, with values of:
Where Thickness is the current thickness of the sheet and Original thickness is the initial thickness of the sheet. The thinning rate is negative to indicate that the sheet is thinner, which is a positive indication of the thickness of the sheet.
By querying the relevant information, it is understood that DC06 sheets have the risk of cracking when the thinning rate reaches 25%. As shown in Figure 8, it can be seen from the figure that the maximum thinning of the part appears at the top, and the thinning rate reaches 21.78%, far from reaching 25%, so it is judged that there is no risk of cracking in this part.
Figure 8 Thinning rate cloud chart
Analysis of part wrinkle clouds
Since the assessment of the wrinkles of the formability cloud map is very rough, we need a more accurate evaluation criterion for the wrinkles , the wrinkle cloud map, as shown in Figure 9. The basic definition of a wrinkling cloud map is:
where r is an anisotropic index, ε1, ε2 are the primary strain and the secondary strain.
It is clear from Fig. 9 that parts of the part are at risk of creping, and that the largest part of the crepe is located in the corner near the flange edge, with a value of 3.12%, which can be completely eliminated by adding a teardrop-like suction rib. The important flanged surface is basically not wrinkled.
Fig. 9 Wrinkled cloud diagram
conclusion
This paper uses AutoForm software as a tool to study the secondary drawing stamping process of parts with large drawing depth by taking the steering pipe string bracket as an example, and finally through the analysis of the simulation results, it is concluded that the secondary drawing can perfectly solve the problem of unforming of parts with large drawing depth.
However, due to the complexity of the production situation in reality, so far, finite element simulation analysis can not completely replace the actual test, but with the further deepening of computer and finite element applications, numerical simulation analysis will be able to guide the actual production.
——Article from: Forging and Stamping, No. 24, 2021