Structure lightweight design uses an optimized shape or topology to benefit from higher stiffnesses and structural advantages.
Incremental Sheet-Bulk Metal Forming
Incremental sheet-bulk metal forming offers multiple lightweight design possibilities. Complex-shaped parts and functional components with tailored properties can be produced. The production of high-strength steel parts from sheet metal is one of the key benefits of this process. Bulk forming processes are applied to sheet metal to produce near net-shaped components, by the plastic change of shape with two- and three-axial stress and strain conditions . This means that a flat sheet is thickened in the out of plane direction by a forming operation and/or functional elements, like gears, attachment points or thickness changes that are integrated in the circumference of the sheet, as shown in Fig. 10. In Fig. 10, F is the tool force; Fclamp is the clamping force. The edge of the part can be thickened, and functions like gear teeth can be integrated.
An adaption of the sheet-bulk metal forming process is the incremental sheet-bulk metal forming process (iSBMF). The possibility to produce non-rotationally symmetric parts is characteristic for this process. The iSBMF process enables the efficient manufacturing of load adapted and functional components . Possible applications for this process are seat adjuster or starter gears, which currently are produced conventionally, e.g., by milling, but are possible parts where iSBMF can be applied to reduce the parts weight without decrease in the parts performance, especially when using high-strength steels, as shown in Fig. 11.
The iSBMF process can be applied to produce parts to meet specific dimensions and design requirements for a specific use case, like in Fig. 11b. For this purpose, Sieczkarek et al.  developed a novel five-axis forming press to meet the special needs for such a process, as shown in Fig. 12.
Future research in this area will deal with the investigation of the forming behavior of hybrid parts and components. Lightweight applications for the iSBMF process are the manufacturing of gears for the drive train and seat adjuster gears.
Joining by Forming
Joining by forming is a process concept, which joins two materials by plastic deformation of at least one joining partner. This process is especially interesting in lightweight design since it supersedes screws, rivets or other parts in a joining operation by a form, force or material fit. Joining by plastic deformation offers the potential of improved accuracy, reliability and environmental safety and the opportunity to design new products by joining dissimilar materials .
Magnetic Pulse Welding
Dissimilar materials, such as hybrid driveshafts, can meet the mechanical requirements with the benefit of reduced weight. The joining of dissimilar metallic tubes to a hybrid component is challenging for conventional fusion welding processes. Therefore, magnetic pulse welding (MPW) can be utilized to join such components since it is based on the high-velocity collision between the joining partners without an additional heat source . The MPW process and the flyer acceleration due to an electric discharge are shown in Fig. 13, in which C is the capacitor; Ri is the resistor. The process allows a material-bonded joint, and the process principles are as follows: A capacitor bank provides electrical energy that can be discharged within several microseconds and induces a secondary current through the magnetic field in the electroconductive flyer. The flyer is accelerated in radial direction by the Lorentz forces that act between the coil and the flyer. The flyer collides with the fixed inner part at a high velocity and creates a high pressure at the joining partners interface. As a result, a so-called jet is formed which consists of surface particles of both joining partners , accompanied by a characteristic light emission to monitor and control the MPW process .
Utilizing the MPW process, Lueg-Althoff et al.  presented successful experimental research to join dissimilar metallic tubes (aluminum flyers to steel parents, aluminum flyers to copper parents) with reduced wall thicknesses. A possible application is the production of driveshafts that consist of different materials . With this approach, a bi-metallic stub shaft was produced to an aluminum–steel drive shaft, as shown in Fig. 14.
Joining by Die-less Hydroforming
To produce joints without the need of an external heat source, joining by die-less hydroforming (DHF) can be used. Müller et al.  presented a joining process to produce overlap joints by means of hydraulic expansion. The principle lies in the difference of the elastic recovery of the two joining partners due to a deformation and therefore a remaining radial contact pressure in their contact area. This offers great potential for joining parts in lightweight applications. Rotational symmetric as well as non-rotational symmetric  profiles can be joined. Interference-fit joints and form-fit joints can be produced. Interference-fit joints rely on the friction between the overlap of the joining partners. Form-fit joints generate an undercut between the inner and the outer joining partners, as shown in Fig. 15. The form-fit joint has great advantages by the produced undercut for connections under torque load. The bearable load already increases significantly with minimal overlap of some tenths of a millimeter .
For the manufacturing of lightweight structures, DHF is a great alternative to thermal-based joining processes.
Next to the production of driveshafts, another application is the production of lightweight framework structures. Marré et al.  investigated the connection of several tubular parts by a node to create an assembly inspired by the lightweight frame structure for the BMW C1E motorcycle, as shown in Fig. 16.
Profiles/Tubes with Variable Cross Section
The adjustment of tube diameters and bending curvatures can be used to produce load-adjusted structures with variable tube topology and the possibility to form high-strength materials, which is a current trend in lightweight design . Due to the increased tendency of springback, high-strength materials are challenging to bend .
Incremental Tube Forming
To overcome and reduce problems related to springback of high-strength materials, the incremental tube forming (ITF) process was invented . ITF combines the technology of incremental tube spinning and continuous tube bending, where both processes take place simultaneously. The process offers the possibility to produce tubes with variable diameter and freely definable bending curvatures, as shown in Fig. 17, in which RL is the bending radius.
The initial tube is fed through rotating spinning rolls to set the wanted tube diameter and the magnitude for the stress superposition. At the same time, the bending tool superposes an additional bending moment to give the tube the desired shape, as shown in Fig. 17a. Thus, tubes can be bent from high-strength steels DP 800 and DP 1000, respectively, as shown in Fig. 17b. The stress superposition causes a significant reduction in the bending moment, compared to conventional tube bending processes . The possibility to almost freely form high-strength materials can be used to produce parts like anti-roll bars for automotive applications.
Another application for the ITF process is the manufacturing of titanium tubes to run at elevated temperatures . Titanium tubes offer a high strength-to-weight ratio and corrosion resistance which makes them ideal for lightweight applications, especially in corrosive environments. Unfortunately, titanium suffers from its low formability in combination with high strength, so it is challenging to form in a defined bending operation. The adaption of the ITF process with an additional inductor can be used to heat up the titanium tube for higher ductility. In combination with the reduced bending forces by stress superposition, the device offers the potential to bend titanium tubes to complex shapes with good dimensional stability .
Incremental Profile Forming
The production of profiles with variable cross sections, the incremental profile forming (IPF) process, was developed. The process allows the production of tubes and profiles with variable cross-sectional designs along the center line of a tube or profile . The process combines high process flexibility with high workpiece complexity. Grzancic et al.  developed a new machine concept and a prototype which allows the processing of thin-walled tubes and profiles with diameters up to 80 mm, as shown in Fig. 18. The highly flexible forming process can be considered as incremental, since it preceeds in several steps.
Figure 18a shows the general process principle that was first described by Staupendahl et al. , in which zp is the profile feed; rst,1 and rst,2 are the styluses feed; α is the rotation. A varying number of styluses are arranged circularly around the profile axis which can move independently from one another into the profile. The initial cross section can be diverse, e.g., quadric, circular, rectangular, etc. The forming of the tube takes place while the profile is fed in axial direction and the styluses penetrate the workpiece in radial directions . To increase the flexibility, either the workpiece or the styluses can rotate—both concepts have the same effect on the forming operation. The process can apply a symmetric or an asymmetric shape into the workpiece, depending on the tooling setup, as shown in Fig. 18b. Through the flexibility of this process, multiple combinations of tools, tool geometries, process setups and profile shapes including an enormous amount of final workpiece shapes can be realized, as shown in Fig. 18c. With special tool kinematics, profile forming from the inside is also possible. Such profiles can be utilized in lightweight applications to meet specific design requirements or increase stiffnesses in multiple directions by a change of the profiles topology.
Damage-Controlled Forming Processes
Damage in metal-formed parts is not a failure, but a product property that decreases the performance of components and needs to be considered in the dimension of products . Due to the appearance and evolution of voids, damage is evolving in a part during a specific forming operation and reduces its load bearing capacity . It is important to understand this when dealing with ductile damage, failure and product properties, especially when dealing with lightweighting in metal-formed parts. As a result of the known damage state inside the part, it is possible to properly rate a construction for its specific use case. The Collaborative Research Centre TR188 “Damage Controlled Forming Processes” has the aim to introduce a paradigm shift in forming industry. Instead of only considering the manufacturability, forming-induced product properties, e.g., damage, should be considered in the process and product design cycle—a reorientation from formability to usability is initiated in the TR188 research center.
Damage in Cold Forging
Product properties are dependent on the damage evolution introduced by the forming operation. Reducing and knowing the amount of damage help to further increase the lightweighting potential. Tekkaya et al.  experimentally investigated the damage evolution and fatigue strength of cold-forged parts and of parts produced by air bending. The increase in the extrusion ratio leads to a lower damage and increased fatigue strength, whereas in an air bending operation the damage can be influenced by process variations such as stress superposition. Cold-forged parts with different axial stress values during the extrusion process exhibit different damage evolutions, as shown in Fig. 19a. Modeling parameters are as follows: cone angle 2α = 90°; friction coefficient m = 0.08 (Coulomb); elastic die (55NiCrMoV6); elastic–plastic workpiece (16MnCr5); initial rod diameter d0.
The process results in lower damage for higher strains (Fig. 19b), whereas the specimen exhibiting the lower strain has a higher axial tensile stress (Fig. 19a). As a result, the fatigue strength of the specimen who were strained to a lower value (ε = 0.5) shows the lower fatigue strength (Fig. 19b). Thus, a larger extrusion strain leads to less damage and increased fatigue strength .
Damage in Bending
Air bending experiments with and without an elastomer cushion were performed, as shown in Fig. 20a, b. Parameters for simulation are as follows: friction coefficient m = 0.3; die and tool are assumed as rigid bodies; sheet metal is modeled with elastic–plastic material and isotropic hardening (DP1180); elastomer cushion is modeled with the Mooney–Rivlin model. Bending with the superimposed compressive stresses of the elastomer cushion decreases the stress triaxiality η = σh/σV (hydrostatic stress divided by von Mises stress) by 18%, as shown in Fig. 20c, d. Triaxiality is typically defined between − 1 and 1. A negative triaxiality results in void decrease and shrinkage, while a positive triaxiality results in void growth and therefore increased damage values. As a consequence, the damage introduced by the elastomer cushion should be lower which could be proved by micrographs of the bending specimens after the bending operations. As a result, the fatigue life of the specimens formed into the elastomer cushion is higher by approximately 10%.
A new process was developed by Meya et al. . The novel sheet metal bending process was developed to adjust the load path to control the damage evolution by a superposition of radial stresses during bending—namely radial stress superposed (RSS) bending, as shown in Fig. 21 . Through the changed stress state of the new process, a reduction in triaxiality can be achieved which inhibits the initiation and growth of voids. By the control of the damage evolution during the bending process, a better product performance, e.g., in terms of impact absorbance, can be achieved.