Germany's Lightweight Forging Initiative for Hybrid Vehicles and Heavy-Duty Trucks

When one thinks of forging, “lightweight” is not a word that immediately springs to mind. When speaking of “lightweight forging,” however, what initially appears to be a paradox is, on closer inspection, a cost-efficient approach for achieving lightweighting advances in automotive applications suited to high-volume production. By exploiting the potential offered by forging, it is possible to reduce the mass of a medium-sized vehicle by 42 kg and that of a light commercial vehicle by 99 kg.^[1,2]

Phases I and II of the Lightweight Forging Initiative (LFI) were covered by FORGE in its April 2016 and June 2016 editions, respectively. During the current Phase III, the Lightweight Forging Initiative – now with an international partner structure^[3] – is demonstrating the lightweighting potential in a split-axle hybrid vehicle. Transmission components of a heavy-duty truck were also analyzed (Figures 1 and 2).

Vehicles and Procedure of the Lightweight Forging Initiative

Determining lightweighting potential is achieved using an actual vehicle, in this case a full hybrid SUV with an electrically supported gasoline engine at the front and an electric motor on the rear axle. The heavy-duty truck segment is being analyzed based on a transmission, propeller shaft and the rear axle.

To start the process, the vehicles and systems were disassembled and documented in a database at automotive research company Forschungsgesellschaft Kraftfahrwesen GmbH. Experts from the participating companies then held workshops to develop lightweighting ideas (Figure 3).

Overview of Lightweighting Potential

As part of the study, 816 kg of components in the hybrid vehicle were considered. More than 3,600 components from the powertrain, chassis and electronic systems of the hybrid vehicle were analyzed, and a total of 732 lightweighting ideas were developed during the workshops.

The ideas were then considered and classified according to the criteria of lightweighting potential, production effort and the efforts for production and market implementation. This allows the relevant ideas to be assessed with respect to a lightweighting benefit-cost ratio, as shown in Figure 4.

Overall, the study identified a lightweighting potential for the hybrid vehicle of 93 kg, or 11% of the component mass analyzed. The ideas are then categorized as quick wins, moderate potential or tough projects, depending on the potential weight savings and implementation effort required.

In the second focus of Phase III, the commercial vehicle powertrain – a total of 460 components with an overall weight of 909 kg – was similarly analyzed. Of this total, 290 kg stems from the transmission and 619 kg from the propeller shaft and rear axle. As part of the expert workshop, more than 250 lightweighting ideas based on design, material and production possibilities were developed. The resulting lightweighting potential adds up to a total of 124 kg, or about 14% of the component mass analyzed.

The ideas can be categorized as lightweighting achieved either by material, design or production concepts. However, these distinctions are not clear-cut, and they are sometimes mutually dependent (Figure 5).

The following will provide concrete examples for more detailed insight into the lightweighting potential that has been developed.

Lightweighting with Material Innovations

Modern, high-strength steels can make a significant, cost-efficient contribution to reducing the weight of individual vehicle components and, thus, of the total vehicle weight. High-strength steels allow higher loads, performance and durability, even when it comes to parts subject to dynamic loads such as crankshafts, connecting rods, gearwheels and bearings. The steel manufacturers and forging companies of LFI^[3] proposed the use of about 20 different steels that enable lighter and leaner part design.

These steels encompass a broad composition, metallographic and property spectrum. Some of the suggested precipitation-hardening, ferritic-pearlitic and quench-and-temper high-strength steels, as well as those provided with a bainitic structure after cooling from the forging temperature, are shown in Figure 6. On the one hand, achieving the desired mechanical properties requires a meticulous, state-of-the-art steel production process. On the other hand, targeted further processing that takes both the part and the material into account is required. This may take the form of hot, warm or cold forging, for instance.

High-strength dispersion-hardening steels such as 38MnVS6 and 46MnVS6, which achieve their mechanical properties through controlled cooling from the forging heat and without additional heat treatment, were suggested by some steel companies in our group for components such as connecting rods, crankshafts and wheel carriers. In some cases, the strengths of such dispersion-hardening steels can exceed those of conventional tempered steels.

High-strength bainitic steels sometimes achieve even higher strengths through controlled cooling from forging heat and, simultaneously, improved toughness properties. Figure 6 shows several bainites. In Figure 7, as well as in the subsequent Part IV article to appear in the August issue of FORGE, several bainites are suggested. The percentage states how much heavier the series part is compared to the lightweighting proposal.

The different steel producers we worked with all had suggestions of which steels they recommended for an assortment of applications to achieve various desired properties. In some cases, the weight savings achieved by using high-strength steels are not only possible in the case of forgings. The use of certain forged steels that can withstand higher loads represents an alternative to cast parts.

The analyses carried out within the framework of LFI demonstrate that considerable weight savings could be achieved in the case of tubes, such as those used in shock absorbers. Figure 7
illustrates and summarizes the lightweighting potential of high-strength steels in automotive applications, including driveshaft differentials, steering knuckles, connecting rods and damper tubes for shock absorbers.

Lightweighting in the Transmission: Material as the Key Factor

Transmissions for converting torques and speeds are also used in hybrid powertrains on both axles. Besides numerous geometrical/forging technology proposals relating to the transmission parts used, the choice of material naturally also offers a great source of lightweighting potential.

If gearwheels can withstand higher loads on the flank and tooth root and if shafts can endure greater torsional and bending loads, then the entire system design can be rendered smaller and lighter. Where necessary, however, the press fit of gearwheels on shafts also needs to be considered.

To estimate the lightweighting potential achieved with material optimizations, the Institute of Product Engineering at the Karlsruhe Institute of Technology was commissioned to set up a model of the transmission using a spreadsheet. Data on load and load-bearing capabilities form the input values for this model, which estimates the system weight via the transmission topology.

By varying the load-bearing values, it is possible to estimate the effectiveness of material optimizations on the lightweighting potential of the transmission. The image shows the transmission on the rear axle. The various material input values are listed successively. With the 20% increase, that seems possible through the use of ultraclean carburizing steels.^[5]

Correspondingly, the table calculates possible weight savings and reduced assembly spaces. In this way, lightweighting reserves are uncovered by increasing the load-bearing capacities, ideally in combination with one another. Hence, it is definitely worth looking at improved steels in order to achieve advances in transmission lightweighting.

Summary: Lightweighting Achieved with Material Concepts

Steel is the most important advanced material for the automotive industry. New insights continue to be generated that enable both the performance and the cost-efficiency of steel materials (e.g., through controlled cooling from the forging heat) to be increased even further.

The examples outlined here demonstrate this. It is becoming increasingly important to involve all partners of the process chain – from the steelworks and the forging company up to the manufacturer of the finished component – to achieve optimizations during a joint development process.

Co-author Dr.-Ing. Hans-Willi Raedt is vice president Advanced Engineering at Hirschvogel Automotive Group, Denklingen, Germany. He can be reached at Hans-Willi.Raedt@hirschvogel.com. Co-author Dr.-Ing. Thomas Wurm is head of technical customer support and application development, Georgsmarienhütte GmbH, Georgsmarienhütte, Germany. He can be reached at thomas.wurm@gmh-gruppe.de. Co-author Alexander Busse, M. Sc., is consultant at fka GmbH, Aachen, Germany. He can be reached at busse@ika.rwth-aachen.de. For additional information visit www.massiverleichtbau.de.