Damage Controlled Forming - Caliber Rolling

Based on the increasing demand on light-weighted metal parts with long service time, innovative methods to predict and control damage evolution in metal forming processes need to be developed. With this goal, caliber rolling process is studied on the case-hardening steel 16MnCrS5. Caliber rolling is a hot rolling process to produce semi-finished product and undergoes damage evolution, which can be characterized as nucleation, growth and coalescence of voids. In caliber rolling, several process parameters such as caliber geometry and roll diameter are identified to influence factors the damage evolution and will be investigated in FE simulation. For damage prediction, some existing damage models as well as a further developed damage model will be implemented. Furthermore, rod-shaped metal parts with the same geometry but various damage will be produced by caliber rolling on a universal rolling mill during the research period, which will be subject to further manufacturing processes, e.g. cold extrusion.

For further information, please contact Dorothea Czempas.

Damage Controlled Forming - Flat Rolling

Flat rolling is a forming process for the production of semi-finished sheet metal products. These products are commonly used in the automotive sector for instance to further process them into structural components. The two main goals of flat rolling are the thickness reduction in order to reach the desired geometry and the improvement of the mechanical properties compared to the raw material, usually cast slabs. The cast slab usually contains voids and pores, which are formed during solidification. The pores can be eliminated by mechanical closure and hot pressure bonding of the voids through favorable process conditions. One of the major factors in this regard is considered to be the so-called load path. It describes the sequence of different states of stress and strain throughout the forming history of the part. The main focus of this project is the determination of the spectrum of accessible load paths and their influence on the pore evolution in simulation and experiments on a universal rolling mill and a cold rolling mill.

For further information, please contact Dorothea Czempas.

Thermo-Mechanical Design of Microstructures for Damage Control

As part of the Collaborative Research Center “TRR 188 – Schädigungskontrollierte Umformprozesse” this project aims to determine the influence of hot working microstructures on the damage initialization and evolution in subsequent cold working processes. Therefore, possible microstructural variations for two different steel types with regards to phase composition, morphology and grain size are calculated via the CALPHAD method. Afterwards a deformation dilatometer is used to process specimens reproducing these microstructures. Finally, these specimens are exposed to typical load paths of cold rolling and cold forging processes on a laboratory scale. The resulting damage is characterized and quantified and will later be used to develop optimized process strategies that allow damage control in microstructures during hot working.

For further information, please contact Jannik Gerlach.

Cold Rolling Strategies for Producing Magnetic-Optimized Electrical Steel Sheet in Energy-Efficient Electrical Drives

One way to increase the efficiency of electric drives is to optimize the magnetic properties of the electrical steel used in the magnetic core. In order to quantify the influence of process parameters on these final properties and to create a scientific-theoretical basis for the development of low-loss electrical steel, an interdisciplinary DFG research group, FOR 1897, is working on the integrated process chain modeling. The main task of the IBF is to investigate and simulate the cold rolling process. Experimentally, the IBF will test different rolling strategies on the cold and hot rolling mill. A multi-scale model that includes a macroscopic finite element model and a microscopic crystal plasticity finite element model is created to compute the texture evolution, which makes it possible to determine the influence of different rolling strategies and initial states on the local texture development during cold rolling. By linking the sub-models, it enables model-based process design of low-loss electrical sheets for highly efficient electric drives.

For further information, please contact Jannik Gerlach.

Microstructure Simulation with DIGIMU® and StrucSim

DIGIMU®, developed by the software manufacturer TRANSVALOR S.A., and StrucSim, developed at the IBF, are programs for the simulation of the microstructural development during hot forming. DIGIMU® is based on physical approaches and allows a spatially resolved representation of grain size evolution and average dislocation density. The optimization of the material model parametrization, as well as the targeted application for industrial forming processes are ongoing work in close cooperation with TRANSVALOR. In StrucSim, the microstructure of the material is described by state variables that evolve depending on the process parameters. Thus, microstructural variables such as the mean grain size or recrystallized (RX) fraction can be calculated and the flow stress derived from them. Empirical modelling approaches allow here a low computational time for coupling with fast process models to calculate the evolution of grain size distributions and RX fractions.

For further information, please contact Holger Brüggemann.

Finite-Element Based Process Design for Fabrication of Metal Composites by Roll Bonding

Roll Bonding enables the production of composites with customized combinations of properties. In roll bonding, the bonding partners are permanently joined together by plastic deformation. The bond formation is a complex process influenced by material properties and process parameters. At IBF an Abaqus subroutine has been developed for computing the formation and failure of the bonds. In a DFG transfer project, this subroutine will be further improved to develop efficient process routes for new material combinations. With this subroutine and the Abaqus process model, Roll Bonding can now be mapped. The bond strength is calculated depending on the surface enlargement. The established bond can also loosen again due to unfavorable load condition after roll gap. The influences of parameters such as temperature and height reduction on the bond strength and the bonding status can now be simulated.

For further information, please contact Holger Brüggemann.

Investigation of Skin-Pass Rolling With a Focus on Surface

An important characteristic of rolled aluminium strips for use in the automotive outer skin is the surface quality. The topography of the surface and in particular the number of roughness peaks as well as the volume of closed lubrication pockets influence the success of the subsequent process steps deep drawing and painting.
The work carried out so far has investigated the relationship between the process kinematics of skin-pass rolling and the transfer mechanisms. For this purpose, the kinematics of a process model of flat rolling was transferred to a mesomodel to describe the surface imprinting. With regard to the imprinting of the surface, a good correspondence between simulation and experiment could be shown.
In the medium term, the numerical model is intended to enable a knowledge-based design of the skin-pass process for aluminium alloys, taking into account global and local influences.

For further information, please contact Michael Riedel.

Void Closure in Open-Die Forging

Large ingots for open-die forging are commonly produced in ingot-casting processes. Despite improvements in the casting qualities, casting defects such as voids, gas porosities and pores cannot be completely avoided. One of the goals of open-die forging is therefore, besides realization of the final geometry as well as a homogeneous deformed microstructure, the closure and healing of the voids, whereby the success depends on the process control. Delivery specifications for forging companies are still based on experienced-based safety factors, which guarantee a safe closure and healing of the voids. In order to design shorter and more efficient process chains, the IBF has therefore used finite element simulations (FEM) and forging tests to develop a reliable criterion for describing void closure during open-die forging. Both the changing load directions during open die forging and the occurring shear are taken into account.

For further information, please contact Moritz Gouverneur.

Investigation of Influencing Factors on Ring Rolling Processes

Today, the goal of minimizing the material input and the machining ratio through near-net-shape ring rolling is of considerable economic and ecological importance, since not only the costs, but also the energy input are decisively determined by the amount of material used. In some cases, non-reproducible defects are found in the rolled rings under obviously identical conditions. These defects can include cracks in the component, but also a coarse and/or unevenly distributed microstructure. In this research project, therefore, influencing factors on material damage in ring rolling processes were investigated. In a first step, comprehensive data recording was carried out in the companies of the project's advisory board. Subsequently, the data determined in this way has been applied to FEM simulations of upsetting, piercing and ring rolling processes in order to reproduce the influences of the fluctuating parameters on the material damage using damage models. In particular, the speed of ring-growth and the mandrel roll geometry turned out to be decisive values for the material damage during ring rolling. Based on the results of the research project, it is now possible to better adapt the selection of the system, the tool and the process kinematics to the product to be achieved and accordingly to save material and energy costs in the form of smaller oversizes and less reworking time and scrap.

For further information, please contact Laurenz Kluge.

Local Heat Treatment of Strain-Hardened Steels

Current lightweight design strives for high strength steels, which at the same time offer sufficient formability. To tailor the properties of low alloy steel accordingly, strain hardening combined with a subsequent local heat treatment presents a promising alternative. This approach can be used to locally increase the formability of semi-finished parts as well as to adapt the property distribution of the sheet metal at best to the function of the final part. In the joint research project of the Institute of Metal Forming and the Fraunhofer Institute for Laser Technology, a crash box serves as an example part. Local softening strategies, which increase the energy absorption capacity, are developed, at first, by means of FE simulations. Dynamic impact tests of real crash boxes confirm that the deformation path can be reduced by 28 % compared to a globally heat-treated crash box and thus weight can be saved.

For further information, please contact Lisa-Marie Reitmaier.

FE Simulation of Multi-Stage Bending Processes

The stamping and bending technology is used for the production of complex bending parts, for example for the electric industry. The process design is mainly based on expert knowledge and experimental testing. Aim of the cooperation project with Phoenix Feinbau GmbH & Co. KG is the development of precise FE models to describe multi-stage bending processes and the springback behavior of the produced parts. One key aspect is the identification of material data of high-strength spring steels by means of an inverse modeling approach under bending conditions. Experimental investigations are further carried out to implement the FE boundary conditions correctly. The validated FE models are then applied to examine and evaluate different influencing factors on the final parts in stamping and bending processes.

For further information, please contact Thomas Bremen.

Phase Transformation in Nickel-Base Alloys

Nickel-base alloys exhibit good corrosion and high-temperature properties. Due to their high creep resistance, they are ideally suited for usage under extreme conditions e.g. aircraft engines. Therefor the precipitation microstructure and the kinetics of phase transformations are the most decisive factors for the good creep resistance at high temperatures. Within the framework of a ZIM project, a software tool is currently being developed at the IBF in cooperation with the company GTT* for the calculation of phase transformation kinetics and TTT diagrams of nickel-base alloys. This tool will be used to predict phase fractions as a function of alloy composition and temperature profile, thus enabling a simulation-based optimization of the processing of these alloys. Further, this software-tool will be used for various other metallic alloys and slags in the future.

*Gesellschaft für Technische Thermochemie und -physik mbH

For further information, please contact Holger Brüggemann.