Papers by Author: Gerhard Hirt

Abstract: Friction has a significant influence on almost all metal forming processes. An in situ measurement of the friction stress within the forming process is in general difficult. Therefore, different experimental setups based on the indirect measurement of a friction dependent value are used to determine the friction conditions in laboratory experiments. For example the ring compression test and the conical tube-upsetting test are using the change of the geometrical shape of a specimen to investigate an averaged friction coefficient within the process. The essential advantages of conical tubes are the prevention of sticking friction and a homogeneous displacement and relative velocity along the contact surface depending on the friction conditions and the used cone angle. However, in both methods the development of the friction conditions during the upsetting process and the relative velocity between tool and workpiece are unknown. In this paper an extended setup of the conical tube-upsetting test is presented. The development of the specimen profile is detected by a laser sensor during the process at elevated temperatures. Experiments are conducted for different cone angles and the measured data is compared to FE-simulations. The time-dependent geometric data is used for the calculation of the relative displacement and relative velocity between tool and workpiece at the edge of the contact zone. A comparison with classical nomograms indicates a change of the friction conditions during the upsetting process. Finally, simulations are fitted to the experimental results by using a variable friction coefficient.

Abstract: The design of industrial hot metal forming processes nowadays is mostly carried out using commercial Finite Element (FE) software codes. For precise FE simulations, reliable material properties are a crucial factor. In bulk metal forming, the most important material property is the materials flow stress, which determines the form filling and the necessary forming forces. At elevated temperatures, the flow stress of steels is determined by strain hardening, dynamic recovery and partly by dynamic recrystallization, which is dependent on strain rate and temperature. To simulate hot forming processes, which are often characterized by rapidly changing strain rates and temperatures, the flow stress is typically derived from flow curves, determined at arbitrary constant temperatures and strain rates only via linear interpolation. Hence, the materials instant reaction and relaxation behavior caused by rapid strain rate changes is not captured during simulation. To investigate the relevance of the relaxation behavior for FE simulations, trails with abrupt strain rate change are laid out and the effect on the material flow stress is analyzed in this paper. Additionally, the microstructure evolution due to the strain rate change is investigated. For this purpose, cylinder compression tests of an industrial case hardening steel are conducted at elevated temperatures and different strain rates. To analyze the influence of rapid strain rate changes, changes by one power of ten are performed at a strain of 0.3. As a reference, flow curves of the same material are determined at the initial and final constant strain rate. To investigate the microstructure evolution, compression samples are quenched at different stages, before and after the strain rate change. The results show that the flow curves after the strain rate change tend to approximate the flow curves measured for the final strain rate. However, directly after the strain rate change significant differences between the assumed instant flow stress and the real material behavior can be observed. Furthermore, it can be shown that the state of dynamic recrystallization at the time of the strain rate change influences the material response and relaxation behavior resulting in different slopes of the investigated flow curves after the strain rate change.

Abstract: The prediction of microstructure evolution in addition to the macroscopic material strength, material flow and temperature evolution is becoming increasingly important as more and more complex materials, with properties that are heavily influenced by their microstructure, are being used. This in turn requires refined microstructure models to be parameterized. Compared to flow curve models, the experimental effort for the parameterization of microstructure models increases due to the inclusion of grain size and recrystallization effects. Therefore plenty of experiments are usually performed to fully characterize the material at hand. The increasing versatility of testing machines, like dilatometry with easily variable temperatures, in addition to the growing expenses that go along with increasing the number of experiments for high cost materials, leads to the question whether performing all those experiments is really justified. In this paper the microstructure model StrucSim is parameterized for the nickel-base alloy Inconel 718 and coupled online with a finite-element (FE) simulation to predict the material behaviour during double compression tests. StrucSim combines multiple constitutive equations into a single consistent material model representing also the microstructure. Therefore these constitutive equations are parameterized to their respective metal-physical phenomena to find the initial parameters for StrucSim. Afterwards the set of final parameters is determined by optimizing the initial parameters using the StrucSim algorithm interconnecting the constitutive equations to define a reference model. The reference model is later compared to different final parameter sets parameterized based on reduced experimental data. Beforehand the reference model is coupled with the FE software Simufact.forming to simulate double compression tests and compare them to experiments as a validation of the reference model. Here forces are predicted with a mean deviation (root of the sum of squared relative errors) of 7.6 % and grains sizes with a mean deviation of about 8 μm from the measurements. Afterwards the influence of reducing the available data during parameterization of StrucSim is investigated to evaluate the possibility of reducing the experimental effort. It is shown that when using only 50 % of the data the quality can be maintained with the reduced model. When simulating the double compression tests a comparable deviation regarding the forces and grain sizes is achieved. Reducing the number of experiments by 50 % during materials characterization therefore appears feasible.

Abstract: Almost all metal strips with thicknesses of < 2 mm are produced by cold rolling. Thickness variations of cold rolled strips are caused by various factors like fluctuation in strength of the material, the eccentricity of the rolls or thickness variation of the incoming strip. As the demands concerning the thickness variation are ever increasing the Institute of Automatic Control and the Institute of Metal Forming aim at reducing the thickness tolerance of thin, cold-rolled steel and copper strips to 1 μm. As high frequency disturbances are expected, it is assumed that this goal can only be achieved by using a predictive controller in combination with a high precision strip thickness gauge and, for roll adjustment, a piezoelectric actuator in addition to the existing electromechanical actuator. The objective of this work is the constructive implementation and the testing of a thickness gauge based on laser triangulation. The gauge includes guide rollers to prevent strip vibration, a C-frame to allow an inline calibration and mechanical adjustment of the measuring range so that even flexible strip thicknesses can be measured. The designed gauge showed a high repeat accuracy of 0.4 μm for two different metal strips. Furthermore the gauge was used to investigate the dynamics of the thickness change of a steel strip at maximum rolling speed of 5 m/s using a Fourier transformation. This frequency analysis supports the need for a piezoelectric actuator that can also subsequently be dimensioned based on the obtained frequency data.

Abstract: When processing conventional semi-finished metal strips, distinctive changes in the material properties along the strip length are unavoidable. The roller levelling process is sensitive to changes of those strip characteristics. Thus, a process control allowing for an online adaption of the roller levelling machine according to the actual strip characteristics is highly desirable. In order to enable a precise process layout, the calculation by the Finite Element Method (FEM) provides a suited strategy. Furthermore, the coupling of user-subroutines to an FE code offers the possibility to implement and test respective control strategies. This work proposes a control strategy that is based on a force measurement in the first load triangle of a levelling machine. A first FE model including a feedback control is used to calculate the dependence between the force in the first load triangle and the roll intermesh in the last load triangle leading to a flat sheet. The results are transferred to meta models – so called control curves – that give a direct relationship between the measured force and the roll intermesh. Within a second FE setup a feed-forward control based on these control curves is implemented and the proposed control strategy is investigated for varying yield strengths along the strip length. Thus, the time consuming FE simulations that are necessary to obtain the control curves are decoupled from the actual levelling process. According to the obtained results, the introduced approach is able to improve the sheet flatness for thin sheets when a change in the material properties occurs.

Abstract: Twin-roll strip casting represents a promising alternative production route for clad steel strips. The main idea behind the presented research is the introduction of a prefabricated strip into the melt pool of a twin-roll casting process to exploit the heat of the melt to create bonding between the cast strip and the prefabricated strip. Prior investigations proved the general feasibility of this concept for steel-steel combinations and described the bonding of the two layers. This concept is now further investigated with the aim to understand the influence of the process parameters on the bonding. For the experiments an austenitic high manganese steel is cladded with an austenitic stainless steel. Beginning from a starting point determined in numerical simulations, a process window for the introduction of a 0.3 mm thick strip of 1.4301 was identified by process parameter variation during casting experiments. Up to 25 m long clad strips with a thickness ratio between introduced strip and cast strip ranging from 1:6 to 1:10 were produced this way. Micrographic examinations of the clad strips’ cross sections were carried out to describe the influence of the casting parameters on the joining interface. Higher element diffusion was found in strips with bigger thickness ratios, indicating a stronger bonding of the two layers. Afterwards the observations from the micrographic examination were compared to the results of bonding strength which were obtained by a customised shear test. Supporting the findings of the micrographic examinations the average bonding strength rose from around 100 MPa for a ratio of 1:7 to over 300 MPa for the ratio of 1:10. Although the process parameters with the main influence on the bonding strength, the contact time and the thickness ratio, have been identified more research is needed to quantify their influence.

Abstract: Roll bonding is a process to join two or more different materials permanently in a rolling process. A typical industrial application is the manufacturing of aluminum sheets for heat exchangers in cars where the solder is joined onto a base layer by roll bonding. From a modelling point of view the challenge is to describe the bond formation and failure of the different material layers within a FE-process model. Most methods established today either tie the different layers together or treat them as completely separate. The problem for both assumptions is that they are not applicable to describe the influence of tangential stresses that can cause layer shifting and occur in addition to the normal stresses within the roll gap. To overcome these restrictions in this paper a 2D FE-model is presented that integrates an adapted contact formulation being able to join two bodies that are completely separated at the start of the simulation. The contact formulation is contained in a user subroutine that models bond formation by adhesion in dependence of material flow and load. Additionally if the deformation conditions are detrimental already established bonds can fail. This FE-model is then used to investigate the process boundaries of the first passes of a typical rolling schedule in terms of achievable height reductions. The results show that passes with unfavorable height reduction introduce tensile and shear stresses that can lead to incomplete bonding or can even destroy the bond entirely. It is expected that, with adequate calibration, the developed FE-model can be used to identify conditions that are profitable for bond formation in roll bonding prior to production and hence can lead to shorter rolling schedules with higher robustness.

Abstract: In the past, different methods have been investigated regarding the production of curved workpieces by open-die forging as a significant demand for open-die forged parts with excellent mechanical properties exists. Current ways to produce curved workpieces by open-die forging have the disadvantages of complex preforms and inflexibility. One alternative approach to realize the production of curved workpieces by open-die forging is to actively control the material flow in open-die forging by superimposed manipulator movements during a forging stroke. Since the currently formed material between the forging dies is in a plastic state during the deformation, the required bending forces can be reduced significantly. This concept is investigated in a first study for the cold-forging of an aluminum alloy by numerical simulation and experiments on laboratory scale. For the process, the bite ratio, height reduction and the intended bending angle were identified as most important process parameters. Both numerical and experimental investigation proved the feasibility of the process principle since the bending forces, moments and the bending work can be reduced significantly.

Abstract: To reduce the failure of dies by abrasive wear, mechanical fatigue and thermal fatigue in closed-die forging usually measures like nitriding, deposition of ceramic layers by Physical Vapour Deposition (PVD), Chemical Vapour Deposition (CVD) or deposition welding are used. However, after some time wear appears and the dies have to be replaced. A new concept implements sheet metal die covers, which are placed on the die engraving during forging and will absorb abrasive wear, thermal and mechanical load. The inexpensive cover will be replaced quickly by a new one after it is worn-out. This concept is regarded in a first numerical and experimental study by comparing a covered die (C) and an uncovered die (U) for the production of the same part. A one-time use of the die cover showed a reduction of the peak temperature by 140 K and of the temperature amplitude by 37 %. The temperature reduction and the increase of inner radii of the engraving to fit the 1mm thick die cover doubled the expected die life time. The experiment showed that the soft deep drawing steel DC04 is not suitable in the current case for a die cover and a higher strength sheet metal should be applied.

Abstract: In cold extrusion of aluminum alloys adhesive wear can be prevented by an excessive lubrication of the process. While this causes additional process steps also environmental risks have to be addressed. Hence, dry metal forming, i.e. avoiding lubrication by means of coatings and topography modifications is highly desirable. In this paper first results concerning the behavior of tailored surfaces under dry metal forming conditions for pure aluminum are presented. Different surface treatments (laser polishing and Mo₂BC coating) of the tool steel AISI H11 are tested in a compression-torsion-tribometer under conditions adapted from cold extrusion. Normal stresses six times higher than the initial yield stress of the tested workpiece material pure aluminum (AA1050-O) are applied. Furthermore, a strategy for the characterization of aluminum adhesions to the tool is introduced. The influences of different topographies and the presence of a coating on the loss of material due to adhesive wear are investigated.