Forging processes (hot, warm and cold) are used to produce many important automotive components. When automotive companies award contracts, price is often the deciding criterion. Innovations are either not inquired about or part and system development is already so far advanced that it is too late to incorporate lightweight design proposals. The Lightweight Forging Initiative was formed to emphasize the contributions that forging makes to the automotive megatrend of lightweight design.
There are several developments in the steel industry, as well as with forging companies, with respect to wire and bar materials. These all have the aim of supporting customers in their lightweight design efforts. However, each individual development provides only an isolated solution, perhaps with only negligible transferability to other areas of application. This is because solutions are developed in each case for the specific requirements in a vehicle, and even then, the results may not be published.
The Potential for Lightweight Forgings in Automotive Design
The majority of projects we have seen deal with the car body or with lightweight design solutions based on working with sheet metal. However, lightweight design potential in the powertrain and chassis is rarely considered. Lightweight design potential achieved through material selection or through forging operations has not yet been thoroughly analyzed. The German steel manufacturing and forging industries have thus begun the task of demonstrating design, material and production engineering solutions, the success of which may be measured in lightweight design, cost and implementation potential.
Fifteen forging companies and nine steel manufacturers formed The Lightweight Forging Initiative (LFI) under the auspices of the German Forging Association and the VDEh steel institute. Without any public funding, the companies are financing the first step of the initiative to be taken – a lightweight design potential study. This was carried out by the automotive engineering research institute Forschungsgesellschaft Kraftfahrwesen mbH Aachen (fka).
LFI’s Reference Vehicle
A diesel all-wheel-drive vehicle with low mileage was purchased for reference purposes. It was systematically disassembled, and the parts were documented in a database. Experts from the participating companies gathered to generate lightweight design proposals.
The results of our lightweight design study concluded that, from a forging perspective, many parts from the powertrain (fuel injection, engine, transmission, transfer gearbox, propeller shaft and drive shafts) and the chassis offer weight-reduction potential. Some opportunity is also seen in the car body, mainly for fasteners. The lightweight design ideas focus on 838 kg of component weight, or about 48% of the vehicle. Notably, some components contributing greatly to its weight cannot be produced by forging. Therefore, parts such as the engine block, cylinder head and others were not analyzed.
In total, a lightweight design potential of 42 kg (92.5 pounds) was identified (Figure 2). The lightweight ideas submitted identified an average weight-reduction potential of 10% for the components analyzed. We noticed that some ideas will lead to both a reduction in weight and costs. Other ideas demonstrated lightweight design potential but with somewhat higher costs and increased development efforts.
During our study, 399 ideas were generated from reference vehicle parts. These were categorized as ideas relating to material, design or concept. The ideas generated are not fully developed solutions, and some may work and some may not. However, they demonstrate the ingenuity of the steelmaking and forging industries and will surely fuel future simultaneous engineering discussions.
Weight Reduction by Material Selection
There are many new developments in the area of steel materials for forgings. An example is bainitic grades that can be processed as cost-efficiently (i.e., without additional heat treatment) as dispersion-hardening steels but achieve mechanical values comparable to quenched-and-tempered steels. Lightweight design potential may be leveraged economically by using these steels. One example is the trailer coupling that may be designed with less weight using a stronger steel grade. The use of higher-strength fasteners can also help in weight reduction due to their large number in vehicles.
The use of lightweight metals may lead to lightweight design solutions. A case in point is a chassis bearing on the rear axle. Here, we switched from steel to high-strength aluminum with a larger contact surface. Furthermore, the part has a hollow design with internal undercut. Both may be implemented cost-effectively and easily by means of cold forging (Figure 3).
Weight Reduction by Design
We performed some geometrical optimizations on a crankshaft. In the area of the pin bearings, recesses may be forged into the part. Imbalance considerations show that this can lead to material savings on the counterweights as well.
Geometries that are not rotationally symmetric may be produced easily by forging. We examined one flywheel with pockets at three sites on its circumference. A solution involving machining alone would not allow the lightweight design potential of this part to be exploited (or would only do so at higher costs), since milling these pockets would be expensive.
Additional lightweight design potential may be tapped by exploiting the forming possibilities offered by forging. This is shown in Figure 4.
Other geometrical possibilities allow more effective dimensioning, which leads to parts that are smaller and can bear greater load. Here, differential pinions and speed gears are continually optimized for weight reduction, removing material where possible without sacrificing strength or performance. Gearwheels are attracting particular attention due to the high numbers used in transmissions. Additional optimization can be achieved through a non-rotationally symmetric design of the gear rim connection with reduced wall thicknesses.
An example of weight-reduction potential in a chassis part is the wheel hub. Depending on the wheel bearing generation, the functional integration of antifriction bearings directly on the wheel hub has already led to weight savings, which was the case in the reference vehicle analyzed. A lightweight design proposal we considered represented a large weight reduction (Figure 5). Due to its bold design, however, it also falls under the category of suggestions involving significantly higher implementation efforts.
Finally, the hexagon shape on nuts and bolts is a classic, almost iconic design element. It may be deviated from by exploiting the design possibilities of cold forging. Although only a few grams per part are saved, the high number of these fasteners means that the lightweight design advantage multiplies to a corresponding degree in the vehicle.
Conceptual Lightweight Design Potential
Lightweight design generated by means of concept changes is highly effective because of its more-revolutionary and disruptive character and not just evolving mindset. However, this can also magnify the implementation obstacles. We considered a lightweight design proposal whose implementation hurdles still need to be tested. The lightweight design proposal foresees that torque transfer is achieved via Hirth gears, which may be produced ready-for-assembly by means of forging both on the output shaft and on the tripod. The proposal, therefore, not only leads to a reduction in weight of 25.1% but to the omission of the welding process and to a reduction of effort in vehicle assembly (Figure 6).
Summary and Outlook
The results outlined summarize the innovations of the German steel and forging industries. Assessing material, design and forging innovations demonstrates that a significant weight reduction of 42 kg (92.5 pounds) appears possible on the vehicle we analyzed. With respect to the potential of lightweight design, the areas of powertrain and chassis are just as important as the car body. Steel materials and forging technology may be used to achieve lightweight design, with the cost of lightweight design below that incurred by using many new types of materials or manufacturing technologies.
To utilize these lightweight designs, it is necessary to consider material and forging potential in the early phases of part development. Here, there are tried-and-tested simultaneous engineering processes. However, these need to be used for more components than is currently the case. The purchasing process should begin during early phases of development, when lightweight design proposals of the supplier can still flow from material or production engineering into part design.
The Lightweight Forging Initiative has determined that not only is there potential to be tapped, but also that there is a need for research. For example, the correlation between the cleanliness of the steel and the fatigue strength needs to be better quantified. Activities of The Lightweight Forging Initiative have included communication of the results within the industry. The second phase of this initiative will be covered in an article to appear in the next issue of FORGE. Please refer to www.LIGHTWEIGHTforging.com for further information.
Co-author Dr.-Ing. Hans-Willi Raedt is vice president advanced engineering, Hirschvogel Automotive Group. He may be contacted through the Hirschvogel website. Co-author Dipl.-Ing. Frank Wilke is vice president technical customer service, Deutsche Edelstahlwerke. He may be reached at Frank.Wilke@DEW-Stahl.com. Co-author Dipl.-Ing. Dipl.-Wirt. Ing. Christian-Simon Ernst is project manager, Institut für Kraftfahrzeuge (ika), RWTH Aachen University. He can be reached at firstname.lastname@example.org.