Any time trial and error can be eliminated from a manufacturing process that process becomes technologically and financially more efficient. Using software is a promising and proven way to ease the technological development of multi-stage closed-die forging technology. It eliminates trial and error, even in cases that involve complex material flow. QForm Direct was tested for different industrial hot-forging technologies and showed that tooling cost and material waste were significantly reduced.
Designing a single impression forging or the final impression of a multi-impression forging is relatively straightforward. The designer looks at the desired final product and finds a reasonable parting line. He or she then adds draft angles and a machining allowance (if needed).
The more challenging project in closed-die forging die design is finding the optimal preform shape for cases in which the finished forging is too complex as compared to the initial billet shape to be produced efficiently and defect-free in a single blow. There are some empirical guidelines for creating preforms, such as using enlarged drafts and radii and making a smoother profile in crosscuts. Other than these basic guidelines, up until now, finding the optimal preform shape has essentially been based on the educated guesses of experienced die designers.
It is easiest to visualize how a small change in the preform shape can affect die fill in the finish cavity in 2-D (Fig. 1). In this simple cross section, notice how a slight variation in the preform dies can significantly affect the filling of the finish die.
Fig. 1. Unsuccessful preform design results in poor finish die filling.
The preform design is dependent on several key factors, including the material’s deformability, the initial billet shape and the desired final forging shape. The optimal preform shape should achieve complete finish die fill with minimal flash and reduced forming load. It should also avoid laps and flow-through defects. Optimal preform design should minimize die wear by reducing metal sliding over the tool surface during impressions.
An empirical method has been developed to determine the optimum preform die shape (Fig. 2). The method is a potential flow approximation utilizing equipotential surfaces as a preliminary guess. This approach involves finding appropriate surfaces for creating dies for preform shapes followed by verification analysis of the material flow through FEM simulation. It is done by creating equipotential surfaces that are geometrically between the two shapes (the finished forged shape and the billet or preceding preform shape), choosing an appropriate surface and then increasing the volume to accommodate for flash and machining allowances (if needed). Until now this process was time-consuming and difficult to implement practically using general-purpose CAD systems because it required an experienced die designer as well as complex geometric transformations between FE software and CAD programs.
Fig. 2. Optimal preform design results in proper finish die filling.
The QForm Group has automated the complicated process of finding the ideal preform shape with a specially developed CAD program called QForm Direct based on SpaceClaim™ from ANSYS. QForm Direct couples with the QForm-UK platform, which simulates material flow in a die cavity, finds the optimum preform shape and automatically creates the preform die to produce a defect-free forging.
Fig. 3.The billet is placed in finished die (a). The forging blow is simulated (b).
The process of generating a preform shape requires only the selection of a suitable intermediate surface to be incorporated in the preform die cavity design and the identification of the portions of the preform surface that will remain free when the dies reach their final position. Furthermore, preform shape optimization can be achieved in an automatic mode with just one variable parameter, which is the value of the potential that determines the preform shape.
We will now present applications that show how actual problems were solved using the developed method and software implementation for hot closed-die forgings.
Fig. 4. In addition to the laps, there are many red zones shown in QForm’s proprietary “Gartfield” subroutine, indicating potential flow-through defects located on the lower surface of the finished forging.
Aluminum Climbing Gear
This practical example is courtesy of DMM International (Wales, U.K.). The original technology consisted of a piece of climbing gear (impact block) of aluminum alloy 6082 in which the billet is heated to 850°F (455°C) and placed directly in the finish die cavity to be forged in a screw press.
As can be seen in Fig. 3, the die is completely filled but the simulation (Fig. 4) shows several problematic defects. We note several critical laps that are shown by the red dot clusters. These defects are unacceptable in the final product, so it was determined that this forging would require more design iterations (not all shown here). QForm Direct was used to find the optimal preform shape for this forge process (Figs. 5a and 5b).
Fig. 6. With the QForm Direct optimized preform shape, the laps and the flow-through defects do not appear (Gartfield < 0.5).
Steel Arm with Perpendicular Shaft
This example illustrates the practical implementation of the proposed method for forging a non-symmetric part that requires a bent parting line (Fig. 7a) that is even more difficult to produce without defects. The process was carried out using a 1,600-ton mechanical press.
In the initially proposed technological process, a round billet of AISI 1035 steel with a diameter of 1.9 inches and a height of 7.5 inches was heated to 2010°F. It was then upset to 5.1 inches to create a “bulb” shape (Fig. 7b) that was expected to fill the finish die cavity in the most massive zone of the forging.
Fig. 7. Steel arm: the model of the finished forged part (a); placement of the “bulb” preform in finish dies (b); result of the simulation of the finish impression (c).
According to the simulation results, the initial “bulb” preform was found to be inadequate because it did not facilitate sufficient material flow to fill the die in the shaft zone (Fig. 7c). As an alternative, the round billet was bent and placed in finish dies (Figs. 8a and 8b) to achieve better results. Although the shaft area was successfully filled using this method, a significant defect in the form of a massive lap appeared in the shaft-to-arm transition zone, as indicated by the red marks in Fig. 8c. Thus, this technology is also unacceptable.
Fig. 8. The bending of the round billet (a); placing it in finish dies (b); defects prediction after the finish impression (c).
Utilizing QForm Direct, a customized preform and preforming dies were created to address the previous issues with laps in the finish impression. Simulation tests showed that the newly developed preform resulted in no laps in the preliminary impression (Fig. 10). Only minor red marks detecting laps were present in the flash area (Fig. 10b).
Fig. 9. The bent, round billet placed in newly developed preform dies (a); preform shape after preliminary impression (b).
Then this preform was placed in the finish forging impression, and the simulation was carried out. As we see from Fig. 10, the dies are completely filled and there are no defects in the body of the finished forged part.
Fig. 10. General view of the simulation of finish forging impression using the developed preform (a); magnified fragment in the critical transition zone (b).
After this preliminary simulation approved the die design work, actual dies were manufactured and tested in real production (Fig. 11). Once the batch of parts was produced, a thorough inspection was conducted. The inspection revealed that the quality of the product was excellent. The comparison of the cleaned, forged part and the simulation prediction is depicted in Fig. 12.
Fig 11. The finished forged part in a hot state.
The QForm Direct method based on the use of equipotential surfaces for automatically creating the optimal preform is a promising way to ease the technology development of multi-stage closed-die forging technology, which has required significant experience and trial-and-error up until now. It helps even in cases that involve complex material flow. The software was tested for different industrial hot-forging technologies, and it showed that tooling cost and material waste were significantly reduced.
Fig. 12. Comparison of the real finished forged part after cooling and cleaning (a) and its predicted shape by simulation (b).
All graphics courtesy of the author except where noted.
For more information: Co-author Tom Ellinghausen is president of Forge Technology Inc. in Woodstock, Ill. He can be reached at firstname.lastname@example.org or at 815-337-7555. Co-author Nick Biba is director of Micas Simulations Ltd. in Oxford, U.K. He can be reached at Nick@QForm3D.com. For additional information, visit www.forgetechnology.com and www.qform3d.com.