How it can optimize formability, reduce springback
August 1, 2009
Warm forming, or forming material heated in the 200-degree-C to 500-degree-C range, offers dramatic improvement in forming properties over room temperature forming for many aluminum or magnesium alloys—without exotic heat sources or tooling.
In all applications, being able to form complex features and details in a single step is an advantage. In addition, for automotive applications, improved forming characteristics open the possibility for more use of aluminum and magnesium to reduce vehicle weight.
In the 300- to 500-degree-C range, aluminum and magnesium become superplastic, and elongations exceeding 100 percent are common for alloys that would be capable of only 20 percent elongation at room temperature. Classic superplastic forming processes are slow and require special alloys. The goal in warm forming is to achieve reasonable production rates with familiar alloys. The change in material properties at these temperatures reduces or eliminates springback (see Figure 1). Furthermore, it reduces flow stress, lowering force and pressure requirements, and increases elongation, allowing you to form shapes that are more complex and with finer details and tighter radii than can be formed at room temperature.
This article's focus is on applying warm forming to sheet forming operations, specifically stamping and sheet hydroforming with liquid or gas. Whether the sheet is stamped with a punch and die or hydroformed with a single tool, the material and system considerations are similar. When you evaluate the suitability of warm forming for an application, focus on the material and its properties.
The first step is to consider the material you are forming—how increasing temperature changes the metal's properties and how beneficial the change will be. If material selection is flexible, you may consider alternatives that are better-suited to warm forming; for example, switching from a 3000 series aluminum to a 5000 series aluminum.
Obtaining accurate and relevant data about properties can be a challenge. Properties may be available in the literature from published research, and some may be available from material suppliers. It is best to have the alloy tested the same way you will form it. Tensile data is a starting point, but sheet bulge tests are more useful because they impose biaxial strain like those induced by forming operations.
In addition to the lower flow stresses and greater elongation, another factor gains importance at elevated temperature—strain rate dependence. The strain rate dependence can be as significant as the strain hardening and must be included in the properties.
To evaluate the materials and determine the optimal temperature, a useful tool is finite element simulation of the process (see Figure 2). Finite element simulation is a way to reduce the risk associated with adopting a new process. Also, it determines the forces and pressures required so the system can be sized properly. In most warm forming operations, the process is under isothermal conditions. This eliminates the need for more complex thermal and mechanical modeling that would be required in a nonisothermal situation.
Aluminum's increased sensitivity to strain rate at warm-forming temperatures requires flexible control of the punch speed. This is easily accomplished with modern servo-controlled hydraulic presses. The punch velocity can be varied during the cycle as needed to maintain the strain rate in the desired range. Results from the simulation can guide the development of the velocity profile.
Once you have selected the material grade and temperature range, the next step is to select the optimal heating approach for the tooling, blank, and pressure medium, if used. The goal is a uniform temperature condition. There are two primary approaches to heating tooling. The first is to embed electrical heating elements in the tooling, such as cartridge or band heaters; the other is to circulate heated fluid through the tooling. Ceramic plates insulate the tooling from the frame to minimize heat loss and power requirements. The blank may be heated by contact with the tooling or preheated. Preheating can reduce cycle time. When preheating is necessary, induction heaters, infrared heaters, or ovens can be used.
When embedded heaters are used, the tooling generally is divided into multiple zones for control to ensure temperature uniformity. Each component has a different heat loss rate. In a small-tool setup, the punch, die, and binder each has its own control (see Figure 3). If the tools are large and complex, each tool may have zones.
With the circulated fluid approach, a single fluid source temperature control is used, and the temperature distribution is managed by adjusting the fluid flow distribution (see Figure 4). The use of liquid is limited to temperatures of 300 degrees C or lower. Liquids are heated in a closed circulation system to limit loss and oxidation.
These considerations apply to tooling for either sheet stamping or sheet hydroforming. In warm sheet hydroforming, gas or liquid is the pressure media. Each has advantages, so the decision requires looking at the complete process.
If gas is used as a pressure media, it may be preheated. Gas pressurization has proven adequate and reduces the cleaning effort. In both gas and liquid methods, some form of lubrication may be required. The gas is vented to the atmosphere.Gas pressurization for warm sheet hydroforming is convenient. One limitation is that the practical upper pressure is around 5,000 PSI, but this is more than sufficient for most applications because of the reduced flow stress with increased temperature. The low-heat capacity makes it easy to preheat when needed. Also, it is clean and dry.
Gas pressure can be controlled directly using a servo valve, followed by a preheater, or it can be controlled indirectly with a servo-controlled intensifier followed by a preheater (see Figure 5).
With an intensifier, the control can be based on pressure or volume. The optimal choice of gas type depends on possible chemical reactions. Straight pressurized air is a possibility.
When circulating liquid has been selected to heat the tooling, it can be used as the pressure media and possibly for preheating. Liquid pressurization for warm sheet hydroforming has performance advantages over gas.
High pressures are available for forming thick-walled parts. Much lower compressibility means that pressure or volume control is likely to be more responsive. For lower pressures (less than 5,000 PSI), a pump and servo-valve approach with preheater can be used. High pressures and volume control can be achieved using a servo-controlled intensifier. The liquid pressurization is also an excellent means of preheating because of its relatively high heat content.
Warm forming affords great improvement in material formability, but a key question is the cost of implementation. The first consideration is the additional equipment required—heaters and temperature controls. Another consideration is the power required to operate the heating equipment. Heating and cooling may increase the cycle time, but only minimally. A third consideration is that the reduction in forces and pressures allows the use of smaller presses and reduced mechanical energy consumption.