Peter Groche

Hydromechanical Deep-Drawing of Aluminium-Alloys at Elevated Temperatures

Abstract This paper presents a technology that combines the different effects used in hydromechanical and warm deep drawing to reduce the drawing force and to increase the transmittable drawing force in the deep drawing process of aluminium sheets by flange heating and by using a counter pressure. Adequate process parameters and an optimised tool design are discussed in thermo-mechanically coupled Finite Element Simulations. The required system parameters, such as temperature and strain rate dependent flow curves, temperature dependent friction coefficients and heat transfer coefficients, were detected in different model experiments for the numerical simulations. Experimental results are presented to highlight the possibilities and limitations of this forming method.

Prediction and inline compensation of springback in roll forming of high and ultra-high strength steels

With the rising interest in lightweight construction, the usage of high and ultra-high strength steels has increased remarkably during the last years. Unfortunately, these steel grades show more springback than mild steels do, which leads in consequence to lower dimensional accuracy. To improve the bended part’s geometry considerable trial-and-error work is necessary since the influence of different bending-parameters (e.g. the bending radius, sheet thickness, yield strength, Young’s modulus, the material’s strain hardening coefficient, …) on the amount of springback is still unknown. The aim of the paper at hand is therefore the investigation of springback for different parameter combinations. Furthermore, an online calibration system for occurring springback during roll forming is presented to compensate springback independent of material or process parameters.

Method for the Optimization of Forming Presses for the Manufacturing of Micro Parts

Abstract In order to enable the efficient and economical production of micro parts with high demands on tolerances and accuracy, it is necessary to establish efficient, productive and dynamically optimized forming presses. This paper introduces a new simulation strategy for the optimization of high precision presses. A newly developed and existing prototype of a linear motor press was used to experimentally verify the simulation results. The experimental investigations, which especially focus on horizontal slide displacements under dynamic loads, showed a good correspondence with the results from the simulations. The presented innovative simulation strategy offers improved possibilities of evaluating and optimizing the dynamic press behavior in the early design process with a reduced effort of experimental optimization.

New Forming Processes for Sheet Metal with Large Plastic Deformation

Due to the high effort involved, bifurcated constructions in mass market products made from sheet metal remained largely unused. Extruded profiles with cross-sections containing bifurcations show the possibility to increase the stiffness and allow modern lightweight design using load optimized structures as well as in box strap, sandwich and stringer constructions or different profiles. The two new forming processes linear flow splitting and linear bend splitting developed at the PtU enable the production of bifurcated profiles in integral style made of sheet metal without joining, lamination of material or heating of the semi-finished product. These forming processes use obtuse angled splitting rolls and supporting rolls to transform the sheet metal at ambient temperature. Whereas the linear flow splitting process increase the surface of the band edge and forms the band into two flanges. At linear bend splitting a bended sheet metal as semi finished product is used. Thereby bifurcations at nearly any place of a sheet metal can be produced. Both processes induce high hydrostatic compressive stresses in the local forming zone during the process which leads to an increased formability of the material and thereby to the realization of large strains. Parts produced are characterized by increased stiffness, high surface hardness and low surface roughness. Experimental investigations have shown an increasing of the band edge surface at maximum splitting depth up to 1800%. By a following forming process new multi-chambered structures and integral stringer construction can be realized with thin walled cross-sections from steel of higher strength.

A Study on Flexible Roll Formed Products Accuracy by Means of FEA and Experimental Tests

Flexible roll formed profiles usually have got highly warped webs if they are manufactured without any supporting tools or process measures. In this paper different methods to improve the accuracy of flexible roll formed parts are described and studied on the basis of a hat‐profile. A new self‐adjusting blank holder system is demonstrated and its potential to reduce warping is experimentally and numerically analyzed. Furthermore the influence of over‐bending the profile’s flange to affect the characteristic strain distribution is studied by means of numerical simulation.

Determination of material properties for hot hydroforming

For the process design of hydroforming in the “hot” temperature range, reliable data are necessary to describe the material behaviour at elevated temperatures under the occurring loads of hot hydroforming processes. State-of-the-art technologies for the investigation of material behaviour, like uniaxial tensile tests or hydraulic bulge tests, do not provide enough similarity with the process of hot hydroforming. This paper describes a new testing technique, capable of realizing high process temperatures and constant strain rates. It represents a further development of the established technology of tube bulge tests. The hardware is described, its functionality is proven and mathematical approaches for the calculation of stress/strain-curves from experimental data are presented.

On the Development of Novel Light Weight Profiles for Automotive Industries by Roll Forming of Tailor Rolled Blanks

Due to continuously increasing demands on safety, comfort and ecological performance, different lightweight construction concepts have been searched for and applied by the automotive industry lately. This paper focuses on the development of novel light weight profiles by roll forming of tailor rolled blanks (TRBs). It covers analytics, experiments and FE analysis. At the beginning, state of the art tools were used for a fundamental process layout. The results show, that their application with respect to the varying sheet thicknesses within the blanks is generally acceptable. However, it may be concluded that TRBs as semi-finished products for roll forming operations call for a well adapted pre-cut of the blank. In a second step, roll forming of TRBs to a symmetrical U-profile with constant roll gap was investigated. For this purpose, experiments on a conventional roll forming line and corresponding FE simulations were conducted. Both show very similar results for the final bending angle of the part. Nevertheless, variability in bending angle of up to 14.3° is witnessed between the areas of different sheet thickness on one part. Thus, one may conclude, that a sheet thickness dependent adaptation of the roll gap is necessary for closer tolerances. A vertical and horizontal adjustability of the rolls seems appropriate to meet this purpose. With respect to these findings, two different tool kit concepts were developed and investigated by means of FE analysis. Both aim at a real-time adjustment of the roll gap to the actual sheet thickness within the stand. On the one hand, a force-driven self-positioning of the rolls was simulated. On the other hand the positioning of the rolls was preset in accordance to the feed of the sheet and its thickness distribution. Both concepts are discussed by their effect on the final bending angle of the roll formed U-profile.

Potential of mechanical surface treatment for mould and die production

Abstract The use of mechanical surface treatment methods can extraordinarily increase the productivity in mould and die making processes. The present paper shows how deep rolling and machine hammer peening can smoothen machined surfaces in order to substitute manual polishing processes. Initial roughness of Ra > 3 μm can be smoothed to Ra < 1 μm, independent from the treated material. For a further improvement of the surface quality, a closer look is given to the influence of geometric effects in hammer peening. Both procedures also increase the surface hardness by work hardening. The influence on the attainable work hardening depth is examined in detail. When combined with thermal hardening operations, hardness and smoothness are still influenced positively, although this combination may be constrained by crack nucleation beneath.

Fundamentals of EMPT-Welding

A well-suited solid state welding process for treatment of tubular structures is the electromagnetic pulse welding technique (EMPT): A pulsed magnetic pressure loads the structure to be welded within a few microseconds and accelerates one of the both contact partners (the so called “flyer”) onto a stationary one. When the flyer strikes the stationary contact partner, contact normal stresses far above 1000 MPa act on the interfacial zone between flyer and stationary part. As a result of these high interfacial loads, a layer of several micrometers thickness next to the interface is severely plastically strained. Hence, the oxide layers covering both contact partners are cracked. These chipped oxide particles are blown out of the joining area by a so called “jet”. This jet is caused by the air between the two joining partners being compressed and accelerated due to the movement of the flyer. The result of both phenomena –the oxide chipping by severe plastic deformation of the interfacial zone and the particle blow out caused by the jet– is a pure metallic interfacial zone, loaded by contact normal stresses. The conjunction of the highly reactive metallic surface and the contact normal stresses establishes a metallic bonding, whose strength equals at least the strength of the weaker contact partner. This report presents the results of a collaborative research project between the Institute for Production Engineering and Forming Machines (PtU) and PSTproducts GmbH. Experimental welding analysis is accompanied by numerical work for the study of the underlying mechanisms of solid state welding with respect to interfacial plastic deformation and contact loads. Additional metallographic work gives insight into the microscopical structure of the interfacial joint zone.

Process Stability in the Tube Hydroforming Process

Abstract Material properties have a significant influence on the process stability in tube hydroforming, particularly in series production. Mainly tubular material with longitudinally oriented welding lines is used in tube hydroforming. A new test method was developed to examine the important properties of the semi-finished product, such as flow pressure or maximum circumferential elongation during the hydroforming process. Knowing these parameters, the process control can be adjusted according to the differences of the varying semi-finished product charges. Another option to improve the process stability is a new process control strategy, which uses the volume flow instead of the conventionally used pressure as control variable. A volume control enables the production of sound parts made of different materials with varying wall thicknesses without changing the process control settings.