Stamping makes the wheels go round

Mish Jaksic, director of engineering at Hayes Wheels explains how his company has used stamping simulation

Fabricated aluminium wheels weigh substantially less than either their steel or cast aluminium counterparts. What is more, because aluminium wheels can be deep drawn, the designer has additional styling freedom – it is possible to eliminate wheel covers and trim with the exception of a small cap around the bolt holes.

By using stamping simulation, US-based Hayes Wheels has successfully produced what it claims is the first deep drawn fabricated aluminium wheels for a 1997 model vehicle. The new wheel uses a 2in. deep draw on a 2in. 3 3in. oval – deeper than ever before on this type of wheel. One of the keys to the success of this application was simulating the stamping operation prior to prototyping to minimise trouble-shooting time on the plant floor.

The first fabricated aluminium wheels were introduced in the late 1970s. These wheels were produced from a full disk without any discontinuities. Besides their weight advantages, fabricated aluminium wheels can be polished to a considerably brighter finish than cast aluminium wheels. And, although fabricated aluminium wheels cost more than steel wheels, they do not require chrome plating, which can make them cost competitive in applications where a bright finish is required.

Despite these advantages, fabricated aluminium wheels have not been widely used in the past for styling reasons. It was not possible to achieve the depth and properties of a cast aluminium wheel because of limitations in the drawing process. Producing a fabricated wheel with the depth of a cast wheel means that materials must be forced to the very edge of their formability limits, creating enormous difficulties in the drawing process. Typically, the greatest problem is tearing. Another significant concern is thinning in high stress areas, which has the potential to cause fatigue problems. While experiments have been conducted on the subject in the past, it had been impossible to produce a successful production part.

The forming process for fabricated aluminium wheels typically starts with a round disc about 17in. in diameter. The first step is to form the disk into the shape of a bowl, which puts the workpiece into tension. The following step packs material into the bottom of the bowl to increase strength at the point where it attaches to the hub. Then, the bowl is reversed so that the workpiece goes into compression. Next, styling features particular to the individual wheel design, such as window pockets, are added. Typically, these features are first pierced and then formed. Then the bolt holes for the wheel nuts are formed and the entire workpiece is flattened to remove any undulations created in the previous forming operations. The final step is piercing the bolt holes.

Because the depth of the draw had never been accomplished before on an aluminium wheel, it was decided to simulate the stamping operation with the PAM-STAMP program from the Troy, Michigan, subsidiary of Engineering Systems International (ESI), with headquarters in Paris, France.

The company started by importing geometry from the CATIA software that was originally used to design the part. PAM-STAMP automatically generated a mesh with 50,000 elements with specially detailed definition in the areas where the greatest elongations are experienced. The model included local restraining systems such as beads. All tools were treated as non-deformable bodies.

The simulation process began with an analysis of the surfaces and their classification between final product surfaces and those necessary for fixing and forming. After a patching operation, the mesh was generated using PAM-STAMP’s adaptive meshing techniques. An especially fine mesh was applied to the bend regions. From the starting position with a flat sheet, the blankholder and the punch were simultaneously pressed, until the prescribed blankholder force was achieved.

Perhaps the most valuable element of stamping simulation was the modelling of the contacts between the blank and tools. The explicit integration technique used in PAM-STAMP enables accurate description of the local flow of the material with small time steps. The contact algorithms used search for contact correspondence between nodes and elements and determine contact forces.

Then the punch was driven until it reached its final position. Finally, all tools were removed and the springback of the sheet was evaluated. The analysis was run on a Silicon Graphics Indigo 2 engineering workstation and took between two to 12 hours depending on the density of the mesh used and the accuracy required. The feasibility of the forming process was judged on the basis of contour plots for the sheet thickness and effective plastic strain as well as the distance of individual elements from the forming limit.

The first iteration showed that the window was torn out earlier than intended. This meant that the draw was not nearly as deep as it was supposed to be. Clearly, the original tooling design would not have been able to produce good parts. The radii of the tooling was modified in order to reduce stresses and thinning.

This process was simplified by the fact that the analysis pinpointed the areas where stress was concentrated during forming. After four iterations of modifying the tooling and re-running the analysis, the simulation results showed that the problems had been resolved.

Now the goal is to improve the simulation process so that they will be able to produce fabricated aluminium wheels on the shop floor with only a single iteration of minor tooling changes. They have already achieved this goal with steel wheels using the same simulation methods on simpler product designs.

One other critical issue which needs to be addressed with simulation is predicting residual stresses which are imparted by the metal forming process. Because wheels are a rotational load-bearing member, fatigue cracks typically start at pierced windows. Unlike conventional finite element analysis, PAM-STAMP software calculates these residual stresses and it is planned to correlate these calculations with fatigue testing later in the development process.

Being able to track residual stresses from one die operation to the next will make it possible to optimise forming operations for fatigue strength of the part. Computer simulation is helping to improve both the design appeal and durability of their wheels.

The proven ability of stamping simulation to resolve manufacturing problems early in the product development cycle will enable the company to achieve the goal of reduced time and cost in the concept-to-market automotive process.

Figure 1: Because the depth of draw had never been accomplished before on an aluminium wheel, the stamping operation was simulated with the PAM-STAMP program from Engineering Systems International (ESI)

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