Forging technology has undergone continuous development during the past few years. The improved possibilities offered by material-flow simulation enable the forging of increasingly complex parts. The use of component finite-element-method (FEM) simulation within forges has enabled greater coordination of part design and the forging process, thereby helping to uncover and further exploit lightweighting potential.
Lightweighting in the Combustion Engine
The connecting rod in the heavy-duty truck we studied is made of the microalloyed steel 23MnVS3, which has a tensile strength of 850 MPa. Now, however, there are new microalloyed steels on the market with a tensile strength of 1,160 MPa, thereby allowing a reduction in the shaft cross section and a weight reduction of 51 grams without compromising the safety factor. Modern bainitic steels with even higher tensile strength promise a weight-saving potential amounting to an additional 20 grams.
The vehicle camshaft is produced as a solid cast-iron shaft. After analysis, a lightweighting project was proposed by Tekfor. The submitted suggestion shown in Figure 1 uses tubular material shaped into a functional contour by means of internal high-pressure forming with multidirectional tool movements. The strength and wear-resistance of the cams certainly still require more detailed analysis, however.
For the crankshaft, various suggestions for optimized steel materials were made. High-strength, microalloyed or bainitic steels that, like the current material, do not require any additional heat treatment following forging or steels with a very high degree of purity due to a reduced sulfur content should allow smaller dimensioning thanks to a longer service life.
At the same time, solutions based on design/forging concepts were also proposed. Hatebur suggests building the crankshaft from individual parts. This would allow pockets and boreholes to be forged easily into the individual parts. Schuler expands this idea by proposing the joining of the individual parts by means of a shrink-fit. Trumpf takes it a step further, suggesting the use of hollow journals that are then joined by laser welding with the individual forged parts.
Lightweighting in the Transmission and Powertrain
The rotor shaft in the power-split transmission is designed as a two-part solution (Figure 2). The hollow shaft is joined by press-fit into the electric sheet stack carrier. Torque transmission inevitably requires a very thick-walled solution. The lightweighting solution here aims at guiding the bearing bending torque across a much larger diameter, namely the seat of the electric sheet stack, thereby achieving an overall mass reduction.
Further down the powertrain, there is a slide joint in the half shaft. The outer side of the joint is round and turned on the outside. The lightweighting proposal aims at forging an outside contour that follows the inner side. In so doing, the wall thickness remains sufficient for induction hardening on the inner side.
Another source of lightweighting potential that has not yet been quantified is achieved by forging the part from the steel 50CrMnB5-3 (H50) 1.7136. This generates a higher core strength directly through cooling from the forging heat than that of the induction-hardened carbon steel used in the part. This could improve the load-bearing capacity of the surface and enable the joint to have a smaller design.
The connecting flange that connects the output of the differential transmission with the half shaft, which is fastened to the flange of the joint housing mentioned above, could likewise lead to a mass saving of about 10%. Here, San Grato suggests a deeper cavity, which can be produced cost-efficiently by means of forging.
In the analyzed vehicle, the connection between the inner and outer side of the constant-velocity drive shaft is achieved by means of a solid shaft. In the weight-optimized proposal, the half shaft is produced as a hollow design by means of tube swaging.
Lightweighting in the Electric Rear Axle Drive
The first lightweighting proposal in this application area aims at assembling six instead of four differential bevel gears. In this way, torque transmission is distributed across double the number of gear flanks, and the entire system can be designed significantly smaller (Figure 3).
The input gear is fastened on the outside of the differential housing. Here, the lightweighting proposal from Hirschvogel addresses material savings beneath the tooth root in the areas where less torque is transmitted to the teeth. Furthermore, by piercing during the forging process, it is possible to produce a contoured borehole that saves weight between the mounting holes. Here, the case-hardening steel 16MnCrV7-7 (H2) 1.8195 with cost-efficient alloying elements for increasing hardenability could further increase the load-bearing capacity of the gears through higher tooth strength. For gear components, Daido recommends its DCDG steel, which demonstrates a 40% higher pitting strength and a 20% higher tooth-root fatigue strength, thereby allowing smaller and lighter dimensioning.
TimkenSteel provides quantifiable data in its lightweighting proposal that can be applied to several power-transmitting components. One component, previously made from a case-hardening steel of the ME grade, can withstand loads on the flank that are 300 MPa higher when using ultraclean steels. Depending on the load state of the components, mass savings of 10-30% are possible.
In the analyzed vehicle, the input gear is joined to the differential transmission using several threaded fasteners. Trumpf proposes edge-to-edge laser welding, which would lead to a material saving of 1 kg.
The carrier that connects the differential to the chassis frame is made of cast iron and weighs 6.56 kg. Bharat Forge, Hammerwerk Fridingen, Hirschvogel and Lasco suggest weight-optimized versions, which could achieve a weight saving of 10-20%. Hirschvogel and Leiber propose switching to forged aluminum, which should lead to a mass saving of 30%.
Lightweighting in the Chassis
One suggestion concerns the stabilizer (Figure 5), which is a bent tube with a constant wall thickness. Benteler suggests using a tube with variable wall thickness as a starting material. In this way, the highly loaded arc areas demonstrate a larger wall thickness, while those parts subject to lesser loads have a thinner wall thickness. With this load-oriented design, over 2 kg of weight could be saved in the stabilizer. voestalpine proposes using a high-strength spring steel for this part to render it more lightweight.
The strut bearing in the analyzed vehicle is a complex assembled part comprising several joined steel sheets. Here, switching to an aluminum forged part could lead to a weight saving of approximately 200 grams.
The direct connection of the chassis to the driver – the steering system – also offers lightweighting potential. Yamanaka Engineering suggests forging the steering rack over a mandrel using a tube as raw material. The forging group of Nissan agrees with this approach but without the supporting mandrel. Hollow forging processes for such components are already in use. JFE suggests using a high-strength steel in the gears in order to achieve smaller and thus lighter overall dimensioning.
Steering knuckles (Figure 5) and wheel carriers made of cast iron can be replaced by forged aluminum with barely any geometrical changes because very similar strength values are achieved. Depending on requirements, small geometric adjustments to the part may be necessary to achieve the same stiffness values. From a forging standpoint, a geometrical optimization of the components would be beneficial for increasing quality.
In the case of the wheel hub, several suggestions based on the rotationally symmetric part as found in the vehicle aim at removing material from the round outer side. Cotarko suggests piercing openings in the flange, an operation that can also be carried out on the forging press. Lightweighting proposals based on design concepts were also put forward. Replacing the connecting pot of the brake disk with the star-shaped arms of the wheel hub not only offers assembly space savings across the width but also a significant lightweighting potential.
Finally, the rear strut can also serve as a lightweighting example. Switching from a welded sheet-metal design to a forged-aluminum solution generates greater flexibility when it comes to stiffening elements. So, in spite of the lower Young’s modulus, weight savings can be achieved while attaining increased longitudinal stiffness.
Lightweighting in a Heavy-Duty Commercial Vehicle
During the third phase of the Lightweight Forging Initiative, the heavy-duty truck segment was also analyzed to demonstrate the lightweighting potential offered by forging. Based on the transmission, a propeller shaft and a rear axle, numerous mass-saving possibilities are revealed. Here, the rear axle is a welded design comprising a central cast part, a brake carrier and a hollow axle stub.
The brake carrier in this assembly is a very planar forged part (Figure 6). The lightweighting proposal shown in the image aims at concentrating material on the load paths only. Openings and indentations can be introduced during forging without any great effort so that a considerable reduction in mass is achieved.
The connecting flange of the propeller shaft is largely designed as a rotationally symmetric part. From a forging standpoint, it is easy to remove material areas subject to less load, thereby generating a lighter part.
It is still possible to achieve clear weight savings in the transmission area even when retaining the rotational symmetry, as the countershaft shows. Here, Seissenschmidt proposes switching from a solid to a hollow shaft. Starting with tubular material, a hollow form can be produced by means of swaging. It was also suggested that mass savings could be gained by means of more-pronounced contours on the gearwheels in the transmission.
Kamax sees lightweighting potential in the heads of fasteners by using an internal hex design, which also offers advantages during assembly. Significant weight savings can also be achieved by using high-strength material with a strength class of 15.9U, considering all boundary conditions (e.g., hydrogen resistance). Lightweighting potential achieved with high-strength fasteners is also seen by steel manufacturer Nippon Steel through the use of a steel with outstanding resistance to hydrogen embrittlement. ArcelorMittal also claims a lightweighting potential when using the steel FreeForm, which can be cold- or hot-forged.
Even though forging is the oldest metal production technology, the industry continuously works on creative advanced developments – ones that can be used for achieving optimizations in the lightweighting of parts. This applies both at the industrial level, as shown by the numerous examples outlined in this article, as well as in academic contexts.
Through the interplay of improvements achieved by means of material and production technology concepts and the involvement of all process-chain partners, it is possible to generate significant lightweighting advances. The companies of the steel and forging industries can accompany their customers in mastering these challenges.
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 firstname.lastname@example.org. Co-author Alexander Busse, M. Sc., is Consultant at fka GmbH, Aachen, Germany. He can be reached at email@example.com. For additional information visit www.massiverleichtbau.de.
- Damm, E.B., Glaws, P.C., Findley, K.O.: The Effects of non-metallic Inclusions on mechanical Properties and Performance of Steel, AISTech2016, 16-19 May 2016, Pittsburgh, USA
- TimkenSteel: https://www.youtube.com/watch?v=DjdAZUIi6bk