It is not news that automotive industry is trying to reduce the overall weight of its final products – vehicles. One commonly accepted way to achieve this goal is to look into material weight reduction. In other words, build parts as strong as normal but with lighter materials.
Aluminum is very often quoted as a great alternative to steel parts. However, there are two issues to overcome: the higher raw material cost and the complexity of forging. While the higher cost of raw material might come down with greater demand, there are ways to reduce the overall cost of forging an aluminum part.
This article considers the case of a connecting rod, comparing a “traditional” way of forging the part to an optimized process. While we would like to make it as simple as possible by squeezing an aluminum billet between two forging dies and get the part out in one blow, we will see that it might not be the best technical option or the most economical way. In addition, the quality of the part might not reach the required strength. This article provides concrete optimized designs to achieve the part quality needed and reduce the overall cost.
Single-Step vs. Multiple-Step Forging Processes
The main question to answer while developing a forging sequence is the material spread. While working with high-strength materials such as steel grades, the main purpose of the spreading is to fill the die cavity and avoid folding the material while keeping a good grain flow or flow lines.
When working with lighter and lower-strength materials, the spreading operation becomes essential since we need to induce enough deformation to reach the required strength. In addition, we want to reduce cost by reducing the material consumption for each part. A single-step forging process does not allow for either the control over the spreading or the initial billet size since the cavity must be filled with the final dimensions in one blow only. Consequently, a multi-blow forging operation seems more adapted. But can we keep the cost down?
We are going to review the technical aspect first to make sure we can forge the part properly, and then we will consider the economic aspects of these two approaches. We will use the same equipment for all tests: a mechanical press set to 20 RPM.
The single-step forging operation is simple. The length and diameter of the billet are calculated based on the volume of the cavity to be filled and the overall design of the dies. The billet must roughly cover the entire length of the die’s cavity and the billet diameter selected among the diameters available by the material suppliers. The billet selected in this example is 494 mm long (19.4 inches) and 56 mm (2.2 inches) in diameter.
The billet is positioned to balance the material within the die cavities as shown in figure 1. The operation is one blow, with the top die moving downward until it reaches the final height needed to make the product in its as-forged dimensions (Fig. 2). The part is then heat treated, deburred and drilled to obtain the final part.
While the simulation shows that there is enough material to fill the cavity, we see clearly that a fold is created within the large diameter of the part (see Fig. 3). In this condition, the part cannot be forged in one blow since the final dimensions and geometry of the final product cannot be changed. Therefore, a pre-forming operation is required to flow the material in the die and spread it accordingly to avoid folding the material within the final part.
In addition, for reference in later analysis, the force required to forge the part this way is 983 metric tons. We will examine this metric to estimate the equipment required to forge this part. Similarly, we can notice in figure 4 the strain induced within the part (cross section) during this single-step operation.
There are several ways to spread the material in pre-forming operations. An open-die forging process can be used with the operator stretching the material through a multi-blow process with a hammer. However, this process is non-reproducible since the skills of the operator figure prominently and, hence, inconsistency in product uniformity is introduced. Other operations with a multi-cavity die can also be used to flow the material properly, but in this particular case – and because our goal is to re-duce the amount of raw material – we decided to use a cross-wedge rolling pre-forming process.
A cross-wedge rolling operation rotates the billet through two cylinders (flat dies can also be used) with grooves shaping the pre-form while rotating. This operation stretches the billet; extending it to the length of the die. After the cross-wedge rolling operation, we perform a flattening operation and the final forging operation.
The multiple-step forging shows a properly flowed final forged part without defect. As opposed to the single-step forging operation, the material does not fold during the forging operations due to the correct distribution of the material during the pre-forming stage.
In the single-step forging, we used a billet 494 mm long and 56 mm in diameter. In the multiple-step forging, we used a billet 331 mm (13 inches) long and 56 mm (2.2 inches) in diameter. The maximum force needed to forge the part is 983 tons for a single-step forging and 590 tons in the finishing operation of the multiple-step forging. By simply adding pre-forming operations, we were able to reduce the raw material for each part by 33%. In addition, we reduced the required force needed to forge the part by 40%.
To push the analysis further, we decided to optimize the multiple-step forging process and see how much raw material reduction we could achieve.
Multiple-Step Forging Process Optimization
The objective of this optimization is to reduce the amount of raw material needed to forge these connecting rods and reduce the forging tonnage required in the three operations overall. Throughout table 1, the flattening operation still requires about 1,000 tons, which is about the same tonnage as the single-step forging requires.
We decided to work with a smaller billet diameter available from our supplier and adjust the billet length to fill the dies correctly. In order to quicken the simulation, we worked with theoretical pre-forms instead of running the cross-wedge rolling simulation. Of course, all these pre-forms are easily rolled through a cross-wedge rolling process. The flattening and finishing forging dies remain the same across the iterations.
Our FORGE® (not affiliated with FORGE magazine) software platform enables us to run automatic optimization to find the best iteration. For this article, we decided to focus on only four iterations for clarity. Iteration 3 is the best scenario since it reduces the raw material, keeps the cavity full and yields enough flash to eject the part properly. We were able to reduce the raw material weight by 13% from the reference multiple-step forging and 41% from the single-step forging. In addition, we reduced the overall tonnage by 46% from the reference multiple-step forging (flattening) and by 43% from the single-step forging.
Therefore, the multiple-step forging process enables companies to save money on raw material (smaller billets) and on equipment (lower tonnage; smaller equipment). We are now going to review and compare the economic impact of each option.
To evaluate the economic impact, we have to consider two main aspects: We buy an extruded aluminum billet, and we sell the scrap. To evaluate the scrap ratio, we use the following utilization ratio formula, where Y is the utilization ratio, Wf is the weight of the final forged part and Wi is the weight of the initial billet.
Y = Wf/Wi
If we then multiply this result by 100%, we get the utilization ratio as a percentage. The scrap ratio S is then expressed as:
S = 1-Y
In this example, we used the price for aluminum 6061 based on the London Metal Exchange (LME) and the Midwest Prime (MWP) for the North American market. LME and MWP fix the price of the extruded billet based on the prime material of the alloy, while only a percentage of it is considered for the price of the scrap material. The price of the extruded billet consists of the raw material price from LME and MWP, plus the extrusion and cutting operations and transportation. The price of the scrap material includes a percentage of the prime material and transportation. For this analysis, the price of an extruded billet is $2.067/pound. The price of scrap is $1.24/pound.
Figure 9 shows the evolution of this connecting rod from billet to finished part. Figure 10 tabulates the key points to consider for the economic impact evaluation. The billet cost as explained above and raw material loss per billet are important indicators to estimate the cost, loss and gain between our forging options. We also consider these figures for an automotive production of 20,000 parts/week, and 1 mil-lion parts/year.
Looking at figure 10, we see that the optimized option (iteration 3) reduces the billet cost by 44% compared to the original single-step forging option and 17% with the original multiple-step forging option. We quantify the direct impact of the optimization process, which appears worthy of doing. If we look at the potential annual gain with an average of 1 million parts (mindful that we forge two parts from each billet), the production savings is about $1.3 million compared with the single-step forging process.
These figures show the massive benefit of adopting a multiple-step forging process rather than a single-step forging process. It also shows the benefit of running through an optimization process. However, it is important to keep in mind that the multiple-step forging, as described in this article, requires the purchase of cross-wedge rolling equipment. Based on the benefits showed in this study for one product, we believe the investment will be quickly offset by the gains.
Connecting rods make a great case study because they are a high-volume product. The multiple-step forging might not be for all forged products, but large volumes are very sensitive to raw material cost, and a small fraction of material savings results in a large annual number. The simulation is essential to achieve the design optimization and evaluate multiple options available to designers.
Simulation is no longer a luxury. It is a necessity for companies to compete in a tight market. While single-step forging might still be an acceptable option for other products, we have shown it is most likely not the most economical. Investing in new equipment is a risk, but an engineering department equipped with simulation can easily demonstrate its benefits.
Using the FORGE® platform, it is clear (in this example) that multiple-step forging is the only solution to forge the parts without defects. In addition, the cost savings is significant enough to justify the in-vestment in new pre-forming equipment while keeping the tonnage low to forge the part on existing equipment.
As for the cost of simulation, there are many benefits since the simulation can be used on every part and operation the company has. The simulation helps back up tool designers’ decisions and fosters creativity since mistakes are made on the computer only. Finally, the cost of simulation software is only a small fraction of the gain demonstrated in this example.
Author Nicolas Poulain is Director of Sales and Technology for Transvalor Americas in Chicago, Ill. He can be reached at 312-219-6029, ext. 1001, or at email@example.com. For additional information, visit www.transvalorusa.com.
Nicolas Poulain has collaborated on this article with Lyès Hacini, an independent consultant and former Director of the Centre of Expertise and Innovation on Aluminum at AluQuébec and Associate Professor at the École de Technologie Supérieure in Montreal, Canada. Lyès is a mechanical engineer specializing in the design and modeling of industrial processes. He worked at Arvida Research and Development Centre (Rio Tinto) as a research scientist before working as a Forging Process Design and Development Engineer for Raufoss Automotive Components Canada.