Germany’s Lightweight Forging Initiative continues with its Phase II, in which a light commercial vehicle is dismantled and its appropriate component parts are examined for weight-saving by forging, material selection and design parameters.
For a medium-sized passenger car, forging allows a weight reduction of 42 kg in powertrain and chassis parts (see the Lightweight Forging Initiative article in the April 2016 issue of FORGE), but how much can be saved in a light commercial vehicle (LCV) that weighs 2,394 kg? The Lightweight Forging Initiative discussed this issue, and its investigations showed that a potential weight savings of 99 kg can be identified by the use of forging, alternative steel materials or lightweight design concepts.
Phase I: Passenger Cars
The Lightweight Forging Initiative was formed in 2013 by 15 forging and nine steelmaking companies under the auspices of the German Forging Association (IMU) and the Steel Institute VDEh. During the first phase, a medium-sized passenger car was analyzed, and a lightweight design potential of 42 kg was identified for components in the powertrain and chassis. Based on the tremendous interest that the results received from customers and driven by the intensive cooperation within the two participating industries, a decision was made to launch Phase II in 2015 to focus on the lightweight design potential of forgings in an LCV. Phase II of the Lightweight Forging Initiative brings 17 forging companies, 10 steelmakers and one engineering-service supplier together.
Phase II: Light Commercial Vehicle
During the second phase of the initiative, an LCV was analyzed with the aim of making it lighter by using forged components. In contrast to cars, the weight of LCVs still continues to increase from one generation to the next. However, goals for decreasing CO2 emissions in cars likewise apply to LCVs. It should also be noted that the total cost of ownership is more critical in commercial vehicles than in cars, so weight reduction leading to decreased fuel consumption has a bigger impact on purchasing decisions. Finally, lower vehicle weight means a higher payload can be transported, which is a factor in the purchasing decision of a commercial-vehicle owner.
In 2013, 1.44 million LCVs (gross vehicle weight up to 3.5 tons) were sold in the European Union. Here, legislation requires a reduction in CO2 emissions of 13% to stand at a CO2 value of 182 g/km by the year 2020. The vehicle chosen for this lightweight design potential analysis is very representative of this class. The vehicle has a 2.1-liter four-cylinder diesel engine with 120 kW of power, a manual six-speed transmission and rear-wheel drive, thus representing the most widely sold configuration. The total mass balance for powertrain and chassis, body, interior and electronics, as well as the spectrum of manufacturing processes applied are shown in Figure 2.
We used the same procedure as we did in Phase I to generate ideas for lightweighting potential. Having found an appropriate vehicle for our Phase II studies, we purchased a second-hand one (12 months old, 23,000 km odometer reading). The vehicle was then dismantled. The members of our group analyzed all 2,536 parts and brainstormed some lightweight design ideas over the course of two workshops. The ideas were classified according to weight-reduction potential, possible impact on manufacturing cost and, finally, according to the level of implementation difficulty.
Three Idea Groups for Lightweight Design Potential
In total, 535 ideas for lightweight design potential were generated for parts made from rolled long material (forgings, bolts, nuts, tubes or springs). With the classification data attributed to each idea, an overview for a meaningful prioritization of lightweight design suggestions was easily generated. The ideas were clustered into three groups in a portfolio chart (Figure 3). On the horizontal axis, the ideas have been placed with cost impact versus realization potential (with a weighting coefficient of 2:1). On the vertical axis, the amount of possible weight savings is shown.
The first group of ideas (Group A in Figure 3) is “Quick Wins.” These ideas should be pursued at a priority pace. They offer a decrease in weight with little or no cost increase and pose only little or no difficulty in implementation. The Lightweight Forging Initiative, however, wants to emphasize that its findings are not at all to be understood as criticism of the vehicle’s manufacturer or its designers. Rather, they are meant as suggestions in order to apply modern forging and materials techniques and technologies (especially modern steel solutions) to support the megatrend of lightweight design.
Group B of Figure 3 encompasses ideas that offer weight-reduction potential at increased cost and require greater implementation efforts. It should be noted that these efforts need to be compared thoroughly with other weight-reduction options in a vehicle that are currently dominating the headlines (CFRP, sheet-metal steels, plastics). Forging is a proven technology and can offer a better weight-reduction cost per kilogram than many other manufacturing methods – if given its appropriate attention (which is one of the primary goals of the Lightweight Forging Initiative).
Group C in Figure 3 is the class of “Tough Nuts.” Ideas in this group require more cost and effort for a lightweight design action.
Total Possible Savings
For the whole LCV, total weight savings of 99 kg were identified on the basis of weight-saving potential that may be achieved through forging, alternative materials or design concepts. The steel-based lightweight design potential reaches 65 kg. The lightweight potential by using nonferrous metals could contribute another 34 kg since this vehicle exhibits a higher ratio of iron-based solutions (e.g., cast iron parts) than the passenger car analyzed in Phase I.
Implementing all the best lightweighting proposals would mean that the weight of the powertrain and chassis in this vehicle could be reduced by 11.7%.
Removing Unneeded Material
Many lightweight design ideas have been detailed in CAD models, comparing the new idea with the original component. This allows for a comparatively precise calculation of weight reduction. Some lightweight ideas were simulated in FEA programs to confirm their validity. Similar to Phase I, weight reduction was primarily generated by removing unneeded material, thus utilizing the shaping possibilities of forging technology to a greater extent.
Secondly, new high-performance steels have enabled lightweight designs. For selected components, the use of aluminum alloys to replace cast iron or sheet-steel components enabled notable weight reductions. The economic validity of these ideas, as in all cases, needs to be assessed thoroughly. Finally, some conceptual ideas might enhance lightweighting potential in the LCV. The broad spectrum of these ideas can be accessed in the full report of the Lightweight Forging Initiative available at www.lightweightforging.com.
Stronger Steels for Lighter Transmissions
The entire field of automotive technology needs lightweight design. Therefore, the Lightweight Forging Initiative has considered it worthwhile to explore the relationship between the increased cost of higher-performance steels and their possible effect on the weight reduction of transmissions. In order to do so, a transmission design study was commissioned at the Institute of Product Engineering (IPEK) at Karlsruhe Institute of Technology (KIT).
The data for the LCV’s manual transmission took into account tooth-flank load, tooth-root load, shrink-fit torque-transfer capacity and shaft fatigue of the medium-alloyed carburizing steel used. Based on fixed input values (engine power, torque and vehicle speed) and on the transmission topology, it is now possible to vary pitting resistance and tooth-root fatigue strength. Depending on the increase in these strength properties, the model can predict savings in system weight and size (Figure 5). The decreased size of the gear wheels and shafts is directly taken into account. An additional program step calculates the secondary weight effects of reduced-size transmission housings.
In order to compare steel-based weight-reduction cost against possible weight loss now, strength parameters for a higher-alloyed steel were fed into the transmission model, which resulted in a predicted weight savings of 2.45 kg. If the manual transmission was made of high-alloy steel, it would be necessary to switch about 21 kg of shafts and gear wheels to the same steel. However, this steel is more expensive.
If we assume our model is accurate and the input weight of the forged components drops by the predicted amount (2.45 kg), the total cost increase for the lightweight design is only 2 euros. Thus, a weight saving of 2.45 kg may be achieved at an increased cost of less than 1 euro per kg of saved weight. Saving weight by using higher-performance steels in transmission applications is a cost-effective lightweighting measure. This not only applies to the transmission itself but to all systems where gears mesh (differentials, transfer boxes, etc.). Additionally, the transmission model predicts that further weight reduction can be expected with even higher strength values.
Economical Weight-Reduction Solutions
The Lightweight Forging Initiative has demonstrated on two different vehicles that modern forging technology and forging materials, especially high-strength steels, can significantly contribute to economical weight-reduction solutions in the automotive industry. In its second phase, the importance and effectiveness of high-quality steel in transmission applications has been more intensely examined.
A federally funded cooperative (10 research institutions and 60 industrial partners), the Lightweight Forging Initiative project that started in May 2015 with an anticipated duration of three years will yield even more lightweight design potential in the future. One key insight will always remain true: Only by good communication can the optimum combination of component design, materials and manufacturing technology be achieved in order to ensure the development of lightweight solutions in mass production at a competitive lightweight design cost.
The Lightweight Forging Initiative has, during Phase I, demonstrated a lightweight design potential of 42 kg in the powertrain and chassis of a midsize passenger car. This successful undertaking was continued in Phase II in the LCV segment. A vehicle was dismantled, and all the components were documented. Material, forging and conceptual lightweight design ideas were generated in hands-on workshops. Additionally, the cost of weight reduction by using stronger transmission steels was quantified in a transmission design study.
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 Dr.-Ing. Dipl.-Ing. Dipl.-Wirt. Ing. Christian-Simon Ernst is senior engineer, fka Forschungsgesellschaft Kraftfahrwesen mbH Aachen. He may be reached at firstname.lastname@example.org.
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