Showing papers by "Gerhard Hirt published in 2016"

Closed-loop control of product properties in metal forming

Abstract: Metal forming processes operate in conditions of uncertainty due to parameter variation and imperfect understanding. This uncertainty leads to a degradation of product properties from customer specifications, which can be reduced by the use of closed-loop control. A framework of analysis is presented for understanding closed-loop control in metal forming, allowing an assessment of current and future developments in actuators, sensors and models. This leads to a survey of current and emerging applications across a broad spectrum of metal forming processes, and a discussion of likely developments.

Synergetic effect of laser patterning and micro coining for controlled lubricant propagation

Abstract: In this study, the anisotropic spreading behavior of Poly-(alpha)-olefin oil (kinematic viscosity of 7.8 cSt at 100 °C) on stainless steel samples (AISI 403) having periodic, channel-like structures produced by hot micro-coining (periodicity of 400 μm and depth of 40 μm) as well as multi-scale structures (coining and laser patterning) was investigated. These results were compared to the behavior of periodic channels fabricated by direct laser interference patterning (periodicity of 5 μm and depth of 1 μm). The spreading behavior of a droplet (3 μl) was studied for a polished reference as well as for all modified surfaces and recorded by a digital light microscope. From this study, it can be concluded that the polished reference leads to an isotropic spreading behavior resulting from the stochastic surface roughness without any preferential orientation whereas all structured samples induce an anisotropic spreading behavior but with different degrees of anisotropy. The observed behavior can be well correlated with pinning induced by the grooves thus hindering the droplet propagation perpendicular to the grooves and the generation of capillary forces which favor the droplet movement along the grooves. It could be proved that the structural depth is a very desicive parameter with regard to the resulting spreading behavior. The multi-scale surface combining large structural depths and the steeper pattern geometry of the micro-coined surface with much smaller grooves of the laser-structure shows the largest anisotropic spreading behavior due to a stronger pinning and increased capillary forces.

Effect of the Interdependence of Cold Rolling Strategies and Subsequent Punching on Magnetic Properties of NO Steel Sheets

Abstract: Nowadays, optimization of non-oriented (NO) electrical steels toward lower iron-loss, improved, and isotropic magnetizability is critical to the improvement of rotating electrical machines. The whole production process chain adjusts the microstructure evolution, e.g., grain size and crystallographic texture, determining the magnetic properties. In particular, the interdependence of raw material properties and the resulting mechanical stress distribution during final assembly, e.g., punching, leading to magnetic property deterioration is crucial for the optimization of NO steel properties of rotating machines. This paper studies the effect of different cold rolling strategies, annealing treatments, and sheet metal blanking (punching) regarding microstructure evolution, magnetic properties, and deterioration.

Investigation of a composite ring rolling process by FEM and experiment

Abstract: Roll bonding is a well-known process to produce composite sheet metals. Transferring this principle to ring rolling would allow to produce seamless radial composite rings, which combine the advantages of different material properties. This process is studied, both experimentally and using FEM-simulation. For the FEM an explicit model is applied including the kinematic control algorithms of the radial-axial ring rolling machine. In the used model the occurrence of unexpected asymmetrical joints can be reproduced, which are observed during the rolling experiments. By varying the influencing parameters on the asymmetric joint in the FEM a plausible reason was found.

A New FE-Model for the Investigation of Bond Formation and Failure in Roll Bonding Processes

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.

Effects of Process Parameter Variation on the Bonding Strength in Clad Steel Strips by Twin-Roll Strip Casting

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.

Development of a Laser Triangulation Gauge for High Precision Strip Thickness Control

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.

Inverse modelling approach in 3-point bending for elasto- plastic material parameter identification of thin spring steel

Abstract: Under process conditions such as bending of flat wire made from high strength spring steel, the occurring strains are many times higher than the maximum strains determined from uniaxial tensile tests. To determine the elasto-plastic material behaviour of high strength spring steel (X10CrNi18-8), an inverse modelling approach using a simple testing method is presented. A 3-point bending test with the resulting force-displacement measurements is used for the inverse analysis. The inverse approach is used for determining the Young's modulus and hardening parameters of the Ludwik-Hollomon's law for bending of high strength spring steel. FE simulations with the optimised material data meet the experimentally measured punch forces during bending. The optimised material data considerably enhances the springback prediction.

Realistic modelling of the tool kinematics of radial-axial ring rolling machines in finite element simulation

Abstract: For simulating metal forming processes by means of Finite Element programs it is required to define all tool motions beforehand. This is one of the major difficulties of the conventional Finite Elements Analysis (FEA) for simulating ring rolling processes, since in reality the motions are controlled by closed-loop control systems according to current sensor values. A solution is given by integrating control algorithms into the Finite Element model. In a previous publication the authors have presented a method in which the algorithms of an industrial control system of ring rolling machines are coupled with the Finite Element model. Although this approach enables modelling with realistic kinematic conditions, it has the major drawback that the algorithms of the used control are not disclosed to the users. Hence, it will not be possible to modify the controller for new processes and process optimization. In this paper, therefore, a set of reasonable and simple control algorithms is introduced, which can be u...

Numerical Investigation of a Process Control for the Roller Levelling Process Based on a Force Measurement

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.

Extended Conical Tube-Upsetting Test to Investigate the Evolution of Friction Conditions

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.

Investigation on Hardening and Softening Behavior of Steel after Rapid Strain Rate Changes

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.

Influence of die geometry and material selection on the behavior of protective die covers in closed-die forging

Abstract: Due to the fact that tooling costs make up to 30% of total costs of the final forged part, the tool life is always one main research topic in closed-die forging [1]. To improve the wear resistance of forging dies, many methods like nitriding and deposition of ceramic layers have been used. However, all these methods will lose its effect after a certain time, then tool repair or exchange is needed, which requires additional time and costs. A new method, which applies an inexpensive and changeable sheet metal on the forging die to protect it from abrasive wear, was firstly proposed in [2]. According to the first investigation, the die cover is effective for decreasing thermal and mechanical loads, but there are still several challenges to overcome in this concept, like wrinkling and thinning of the die cover. Therefore, an experimental study using different geometries and die cover materials is presented within this work. The results indicate the existence of feasible application cases of this concept, since conditions are found under which a die cover made of 22MnB5 still keeps its original shape even after 7 forging cycles.

On the impact of rolling direction and tool orientation angle in Rotary Peen Forming

Abstract: Shot Peen Forming processes are suitable to produce surface curvatures that are commonly required for aircraft fuselage as well as structural components. The so called Rotary Peen Forming is an alternative process for manufacturing sheet metals with slight curvature. The forming tool consists of impactors which are connected flexibly to a rotating hub and thus moving on a circular trajectory. An industrial robot guides the Rotary Peen Forming tools. As a result, the machine design is more compact compared to traditional Shot Peen Forming.In the present work, the impact of both, the tool orientation angle and the rolling direction, on the curvature of aluminum AA5083 samples is examined. By means of a point laser measurement, the set-up enables a distance control to adjust a determined indentation depth.It can be shown, that the highest curvature is achieved when the tool is orientated parallel and when the rolling direction of the sheet metal is transversal to the curvature plane.