In recent years, modern materials such as high strength steels and aluminum are being increasingly applied in the production of automotive stamped parts. Since stamped parts made from these materials are more affected by springback than those made from conventional deep-drawn steel, springback and springback compensation have become challenging issues in the automotive industry. To meet these challenges and achieve accurate springback simulation, accurate process description is necessary.
Springback is analyzed using an A-pillar reinforcement part. Most of the part geometry is formed during the first operation known as deep drawing. After trimming the flange and piercing the holes, the part is almost in its final shape. In the last operation, the small flange at the bottom of the part is flanged up. Figure 1 shows the sheet after each of the process steps. From left to right are shown: the initial blank, the sheet after deep drawing, the sheet after trimming and piercing, and the final part after flanging.
Figure 1: Sheet in subsequent process steps: blank, drawing, trimming and piercing, flanging.
The process described in Figure 1 can be simulated applying two different setups, A and B, which are described in detail below. Figure 2 shows the springback simulation results based on these different setups. After a free springback analysis is performed, the values shown represent displacements in normal direction. The difference in the springback values for the two simulations begs the question as to why there is a difference.
Figure 2: Springback results based on simulation setups A and B.
An accurate simulation of the entire forming process includes drawing and secondary operations as well as springback. The initial position of the tools is presented in Figure 3. The first operation, drawing operation, a single action drawing with a double curved binder is carried out. A segmented drawbead with varying geometry controls the material flow. The drawing operation is identical for both A and B simulations; however the secondary operations are simulated differently.
Figure 3: Definition of the drawing operation.
The four images in Figure 4 represent drawing, trimming, flanging and springback. After the drawing setup is carried out as explained above, all trimming and piercing contours are defined on the part. During the simulation, the elements inside the piercing contour and outside the trimming contour are simply deleted. This is a very common approach in the stamping simulation. In order to simulate the flanging operation and avoid unwanted bending of the part, the sheet is clamped. The flat top area is held down with a flat tool. At the end of the process, the part is released from all tools and constraints and a free springback is then applied.
Figure 4: Forming process setup A.
The simulation setup shown in Figure 5 is similar to the process setup in a press shop. The five images represent drawing, segmented trimming T30, segmented trimming T40, flanging and springback. This precise representation of the complete process is called a full cycle simulation. After the drawing setup is carried out as explained above, the trimming and piercing process is split up into two operations. In reality, the trimming process is segmented since it must be possible for the trimmed flange to be released into the scrap. If the complete flange were trimmed at once, it would be impossible to remove the scrap material during automated manufacturing. In trimming and piercing, a great amount of force is required to separate the sheet material. Thus, the sheet must be strongly clamped to avoid unwanted movement and forming. The second and the third image in Figure 5 below represent how the pad and post follow the part geometry. During the flanging operation, the part is also completely clamped to avoid unwanted movement and forming. The last step in the simulation is to release all the tools and constraints and finally apply a free springback.
Figure 5: Forming process setup B.
Springback results presented in Figure 2 show differences based on their simulation setups. The main difference between setups A and B is the tool kinematics of the secondary operations. A potential cause for different results could be a certain amount of plastic deformation during tool closure in the trimming operations T30 and T40. To determine whether plastic deformation occurs during tool closure, the plastic strain rate is analyzed. Figure 6 shows the plastic strain rate at closing of pad and post in operation T30. The plastic strain rate is not only analyzed at the middle layer but also at the top and bottom layers of the sheet. The middle image, which represents the middle layer, hardly shows any plastic strain rate whereas the top and bottom layers show a certain amount of plastic strain rate at the radii. This deformation through thickness indicates some bending deformation at these radii. Generally, elastic-plastic bending deformations result in distinguishable geometry deviation due to springback. The springback caused by the slight bending deformation due to closing the pad and post only occurs in process setup B. In process setup A, closing of tools at secondary operations is not simulated. As a result, this springback effect does not appear in the simulation result.
Figure 6: Plastic strain rate at closing of pad and post in operation T30.
In summary, in order to obtain reliable springback results, the correct process conditions must be considered, i.e. the process setup must be studied carefully and described correctly in the simulation. Accurate springback results are dependent on an appropriately selected process description as it can significantly influence the results.
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