The increasing requirements for lightweight and safety improvement of modern car bodies have fostered the use of hot-stamped press-hardened ultra-high-strength steel (UHSS) components. Recently, tailored heating has been proven as an effective technology to achieve the tailored properties of UHSS components produced by press hardening. In this work, the thermal absorption of 1.6 mm-thick uncoated sheets of 22MnB5 press hardening steel (PHS) was modified to control the heating rate and further achieve different temperature zones during short-wave infrared heating. The absorption was modified by (i) surface oxidation pre-treatment, (ii) graphite coating with high-reflectivity, and (iii) graphite coating with physical shielding. The surface oxidation pretreatment was identified to play an important role for the absorptivity of 22MnB5 steel. The absorptivity increased with increasing pre-oxidation temperature due to the changes in both surface roughness and the color of the oxide layer. After heating to 900 degrees C, the temperature difference between the graphite coating and the physical shielding or the high-reflectivity coating was 258 degrees C or 167 degrees C, respectively. After press hardening, the specimens revealed a fully martensitic microstructure in the high -absorption zones and a ferritic microstructure with dispersed carbides in the low-absorption zones. The tailored infrared heating also reduced the width of the transition zone between heated and unheated zones to less than 20 mm. This study demonstrates the high effectiveness of this approach for producing press-hardened components with tailored strength.

In additive manufacturing, intrinsic heat treatments take place during deposition that affect the properties of AM-structures. In this work, the influence of thermal cycling on the local microstructure and mechanical properties of nickel-based superalloy in wire-based electron beam directed energy deposition (EB-DED) was investigated. Structures were fabricated using a continuous deposition strategy (CDS) and discontinuous interpass cooling strategy (ICS) revealing changes in thermal profiles, time-temperature history, and microstructure. An altered morphology along the build-up height and interdendritic zones enriched in Nb are formed. Nb and Mo did not show a clear trend of segregation along the build-up height. Lower fractions of the Laves phase and MCs are found for both configurations. Differences between deposition strategies and locations within AM-structures are found for the γ" and δ phase. The higher Nb content in the interdendritic zone promotes the precipitation of γ" and δ phase by shortening the aging times compared to wrought materials. The longer deposition times of ICS favour the precipitation of fine γ" in the interdendritic zone throughout the deposition height. In contrast, the short deposition time of CDS leads to an increase in temperature and a heterogeneous distribution of γ" along the height, i.e. coarsening of the γ" followed by a dissolution along the built-up height. The microstructural changes correlate with the mechanical properties. Structures fabricated with ICS exhibit homogeneous mechanical properties throughout, while the graded microstructure of CDS results in graded mechanical properties and decreasing strength throughout.

Self-piercing riveting (SPR) has established as key technology for cost-efficient mechanical joining of components in the serial production of lightweight car bodies. The SPR process is basically quite stable and reliable; however, irregular process conditions that affect the interlock between the rivet and the joined components cannot be completely avoided. Such conditions may reduce the load-bearing capacity of the joints. Therefore, this experimental study investigates the influence of irregular offsets between the rivet and the die on the quality/integrity and on the load-bearing capacity of SPR lap joints of commercial 1.5 mm-thick EN AW-6016-T4 aluminum alloy sheets. A steel die and semi-tubular high-strength steel rivets of different hardness were used in the experiments. Characteristic cross-section features and the quasi-static shear-tensile strength of the joints were assessed. The rivet-die offset and the hardness of the rivet were both identified to influence the deformation behavior of the rivet and the symmetry of the joint. However, the influence on the shear-tensile strength was negligible, as the horizontal undercut of the rivet and, thus, the interlock between the rivet and the sheets were still sufficient even for notably asymmetric joints. If the undercut is sufficient, the rivet-die offset can be regarded as uncritical with respect to the load-bearing capacity of lap joints.

Hybrid components of wood-based materials offer a high potential for automotive lightweight applications. To investigate the bending behavior of hybrid aluminum-wood plates, commercial 1-mm-thick sheets of EN AW-6016-T4 aluminum alloy were adhesive-bonded with 4.2-mm-thick plates of birch wood. Orientations of the wood fibers parallel (longitudinal) as well as perpendicular (transverse) to the rolling direction of the aluminum alloy sheet and three different moisture contents of the wood plate were considered. The hybrid aluminum-wood plates were subjected to three-point bending at room temperature. Simple wood plates without aluminum alloy sheets were also tested. The bending force-bending angle curves monitored during bending, the bending angles at maximum bending force and the surface strains were evaluated. Moreover, a finite element model of the testing setup was created using the LS-Dyna software. The different moisture contents did not significantly influence the bending angle; however, moisture decreased the maximum bending force. Debonding was identified as critical failure mechanism. The FE model that considered the experimentally determined material properties was able to predict the bending behavior for different moisture conditions.

Structural components made of thin tailor-welded blanks (TWB) are essential for the design of modern lightweight car bodies. Investigating the formability of these blanks requires detailed information about the mechanical behavior of the welds. Therefore, this study presents an engineering approach that combines experimental and numerical methods for determining elastoplastic properties and fracture parameters of thin aluminum alloy welds produced by gas metal arc (GMA) welding. This approach considers the entire weld seam including surface features as well as inhomogeneous grain structures in as-welded condition, as these features may considerably influence the actual mechanical behavior. Single-pass welds of aluminum alloys AA-5087 and AA-5554 were deposited on a 1.2 mm-thick sheet of aluminum alloy EN AW-5182 using the Cold Metal Transfer (CMT) welding process. Tensile samples consisting almost exclusively of the weld metal were prepared. The three-dimensional (3D) geometries of these samples were captured using an optical scanner. Initial flow curves describing the plastic deformation behavior of the weld were calculated based on the force-elongation curves obtained from tensile testing of the samples. These flow curves and the meshed 3D geometries of the welds were employed for building numerical models of the tensile testing procedure. The flow curves and the fracture locus were iteratively optimized until the force-elongation curves calculated in the simulations and measured in the tensile tests matched each other. The elastoplastic properties and the fracture parameters can be applied for modeling aluminum welds in forming simulations of tailor-welded blanks consisting of, e.g., aluminum alloys and steels.

Structural components made of thin tailor-welded blanks (TWB) are essential for the design of modern lightweight car bodies. Investigating the formability of these blanks requires detailed information about the mechanical behavior of the weld. Therefore, this study presents an engineering approach that combines experimental and numerical methods for determining elastoplastic properties and fracture parameters of thin aluminum alloy welds produced by gas metal arc (GMA) welding. This approach considers the entire weld seam including surface features as well as inhomogeneous grain structures in as-welded condition, as these features may considerably influence the actual mechanical behavior. Single-pass welds of aluminum alloys AA-5087 and AA-5554 were deposited on a 1.2 mm-thick sheet of aluminum alloy EN AW-5182 using the Cold Metal Transfer (CMT) welding process. Tensile samples consisting almost exclusively of the weld metal were prepared. The three-dimensional (3D) geometries of these samples were captured using an optical scanner. Initial flow curves describing the plastic deformation behavior of the weld were calculated based on the force-elongation curves obtained from tensile testing of the samples. These flow curves and the meshed 3D geometries of the welds were employed for building numerical models of the tensile testing procedure. Flow curves and fracture locus were iteratively optimized until the force-elongation curves calculated in the simulations and measured in the tensile tests matched each other. The elastoplastic properties and the fracture parameters of the aluminum welds can be applied for modeling the weld in deep drawing simulations of tailor-welded blanks consisting of, e.g., different aluminum alloys and steels.

Wood-based materials offer a high potential for the lightweight design of modern car bodies. Therefore, this prestudy investigates the static strength of screw-bonded lap joints of 1 mm-thick sheets of EN AW-6016-T4 aluminum alloy and 4.5 mm-thick plates consisting of three 1.5 mm-thick cross-laminated beech veneers. Plates with cover veneers oriented longitudinal (L) or transverse (T) to the uniaxial loading direction and with veneer stacking orders designated as L/T/L and T/L/T were used. Two self-cutting screws and polyurethane-based adhesive were used for hybrid joining. The stacking order and thus the fiber orientation of the cover veneers influenced the fracture behavior, which became evident in the force-displacement curve monitored during uniaxial shear-tensile testing of the joints. Veneer plates with the L/T/L stacking order withstood higher maximum tensile force but pro-vided lower pull-out resistance of the screws than plates with the T/L/T stacking order. However, regardless of the stacking order of the veneers the adhesive layer between the aluminum alloy sheet and the veneer plate mainly determined the maximum tensile force and thus the static strength of the joints, whereas the screws contributed only little.

The load-bearing capacity of hybrid riv-bonded aluminum-magnesium lap joints under shear-tensile loading was studied with particular focus on their static strength and fatigue performance. Sheets of 1.5 mm-thick EN AW6016-T4 aluminum alloy were joined with sheets of 2 mm-thick AZ91 magnesium alloy using two highstrength steel rivets and epoxy-based adhesive. Local deformation-induced fracture of the comparatively inductile magnesium alloy sheet at the rivet holes during riveting at room temperature was intentionally tolerated. The lap joints were heat-treated to peak-age the aluminum alloy (condition T4 -> T6) and to cure the adhesive between the sheets. Characteristic cross-section features and hardness maps were measured for assessing the quality of the joints and thus for proving the general capability of the riv-bonding process. The fracture behavior of the inductile magnesium alloy sheet determined the static strength as well as the fatigue performance at load ratios of 0.1 and 0.5. Both, load amplitude and mean load, influenced the site of fatigue crack initiation and the path of crack propagation. Local fracture of the inductile magnesium alloy sheet at the rivet hole is tolerable, if riv-bonded lap joints are just exposed to cyclic loading with low amplitudes.

This work investigates the influence of the sheet edge condition on the fracture behavior of riv-bonded aluminum-magnesium lap joints under monotonic-static and cyclic-dynamic shear-tensile loads. Therefore, sheets of 1.5 mm-thick EN AW-6016-T4 aluminum alloy were joined with sheets of 2.0 mm-thick AZ91 magnesium alloy using two C5.3×6.0-H4 rivets and epoxy-based adhesive. The side edges of the sheets were either shear-cut or milled after cutting. Before testing, the joints were heat-treated at about 180-200 °C for 20 min in order to cure the adhesive and to peak-age the aluminum alloy. The cyclic load maximum was about 40 % of the monotonic load maximum. The cyclic load minimum was 10 % of the cyclic load maximum, i.e., the load ratio was R = 0.1. The edge condition of the sheets did not have any significant influence in monotonic-static testing; however, in cyclic-dynamic testing the number of cycles to fracture was about four-times higher for samples with milled side edges than for samples with shear-cut side edges. Hence, the potential load capacity of riv-bonded aluminum-magnesium joints cannot be exploited under cyclic loading, if the magnesium sheet has edges with poor quality.

The load-bearing capacities of (i) self-piercing-riveted, (ii) adhesive-bonded and (iii) hybrid riv-bonded lap joints of commercial 1.5-mm-thick EN AW-6016-T4 sheets were compared under both quasi-static and cyclic shear-tensile loads. The joints were heat-treated to cure the adhesive and to peak-age the aluminum alloy. The joint quality/integrity was assessed based on characteristic cross-sectional features and hardness maps. Riveted joints showed notably lower static strength and fatigue performance than riv-bonded joints. Hence, the adhesive layer provided the main contribution to both the static and the cyclic load-bearing capacities of riv-bonded joints, whereas rivets contributed only little. However, if bonding was insufficient the potential capacity of the joints could not be exploited. Under quasi-static loading fracture occurred at the joint; therefore, joints of high quality/integrity were important. Even under cyclic loading at high load amplitudes fracture occurred at the joint, but at comparatively low load amplitudes fracture rather occurred at the sheets next to the joint. Hence, the joint quality/integrity mainly determines the static fracture and the low-cycle fatigue fracture, whereas the sheet properties mainly determine the high-cycle fatigue fracture.

In order to exploit the advantages offered by multi-material design, this work studies the feasibility of joining aluminum alloys with high-strength steels (HSS) against the favorable joining direction by using self-piercing riveting (SPR) combined with adhesive bonding, so-called riv-bonding. Therefore, riv-bonding of four joint configurations including different aluminum alloy sheets (AW-6014-PX, AW-6451-T4), HSS sheets (HC420LA, HC450X) and rivet types (C5.3×8.0-H4, U5.5×5.0-H6) was experimentally investigated. Moreover, riv-bonding of two joint configurations was exemplarily modeled using the Simufact Forming finite element (FE) software. The viscoelastic properties of the liquid adhesive layer between the sheets were substituted with “equivalent” elastoplastic properties to model the adhesive as solid with strain rate-dependent flow behavior. Good agreement of joint cross sections and force-displacement curves between experiments and simulations confirms that the presented numerical model of riv-bonding is suitable for predicting both the joinability of aluminum alloys with HSS and the final quality of hybrid joints.

Viscoplastic self-consistent (VPSC) modeling was used for investigating the deformation behavior of commercial EN AW-7075-T651 aluminum alloy at room temperature under quasi-static tension and compression (i) parallel, (ii) diagonal and (iii) transverse to the rolling direction. Textures of the as-received plate and of the samples after tensile and compression testing were determined using electron backscatter diffraction (EBSD). Euler angles and area fractions of the grains were used as input for calculating direction-dependent flow curves and pole figures of the deformed material. The coefficients of the integrated Voce strain hardening law were adjusted in order to fit the calculated flow curves to flow curves obtained from tensile and compression testing. Pole figures calculated with the VPSC modeling method were validated with pole figures obtained from EBSD analysis of deformed samples. VPSC modeling was successfully applied for predicting the general deformation behavior of EN AW-7075-T651 under both tension and compression. However, texture evolution during tensile testing was negligible, whereas notable texture evolution during compression testing occurred beyond a critical strain value.

An innovative tube bending process supported by hydraulic pressure was patented in 1995. This process was designed for producing butt-welding fittings which particularly fulfil the dimensional specifications of the ASME B16.9, MSS SP-75, and MSS SP-43 standards. Since the patent has meanwhile expired, the bending process is now available for common use. This work presents a novel finite element (FE) model build using the software package LS-DYNA, which enables determining the process window for bending defect-free butt-welding fittings. This model was exemplarily applied for investigating the bending process of a 90° stainless steel elbow based on a straight tube of nominal pipe size (NPS) 6. Comparing the calculated geometry of the elbow with the actual geometry of an industrially produced elbow revealed very good dimensional agreement. This confirms that the presented model can be utilized for determining suitable process conditions, which allow for producing defect-free components.

Increasing the share of battery electric mobility in the transport mix, while at the same time integrating more renewable volatile energy sources in the grid, brings along a variety of challenges such as possible power quality, voltage and grid stability issues. Deploying decentralized energy storage devices in electric vehicle (EV) fast charging stations as buffer storage is one way to mitigate these problems and help store renewable energy in the grid. This paper describes a system integrating a high performance flywheel energy storage system (FESS) in a fully automated fast charging station. A holistic approach is pursued, determining the FESS's energetic target properties by analyzing user behavior in a variety of use cases in conjunction with in-depth grid simulations. Based on these data, a FESS design is proposed and approaches to reach the desired energetic target properties are discussed. Critical components are presented with a focus on CFRP rotor design, as it is crucial for achieving the required energy content at lowest specific cost.

The industrial production of thick walled hydro formed steel parts is a process difficult to control. In particular the prevention of cracks in the production of these parts is very important. It is of utmost importance to have a virtual tool to predict forming results. Standard methods for the simulation of hydro formed parts base upon processes using a shell element formulation and implement a forming limit curve (FLC) for crack prediction. But the forming limit curve is limited to the case of linear strain paths. The initial FLC is no longer valid in the case of nonlinear strain paths. Because of the geometric specifications of the investigated parts - thick walls, compact dimensions, high strains - and the known limitations of the forming limit curve - which don't accord to the hydro forming process - these standard simulation methods are not applicable for the present investigations. A new approach to simulate thick walled hydro formed parts is the use of a volume element formulation in combination with a more complex failure criterion, which gives information about the risk of ductile normal fracture and ductile shear fractures with nonlinear strain paths. The onset of necking must be predicted directly by the volume elements. The aim of this work is to implement the failure criteria in a hydroforming simulation and to compare the results of the simulation with real cracked test parts. The commercial FEM code PamStamp 2G is used as a solver and a comprehensive fracture model is applied. This fracture model distinguishes between two mechanisms responsible for ductile fracture. One is the void growth and coalescence (ductile normal fracture) and the other one is the shear failure model (ductile shear fracture).