Reiner Kopp

Forming strategies and Process Modelling for CNC Incremental Sheet Forming

Abstract Incremental Sheet Forming (ISF) is a process capable of producing complex sheet components by the CNC movement of a simple tool in combination with simplified dies. Earlier work revealed two major process limits, namely the limitation on the maximum achievable wall angle, and the occurrence of geometric deviations. The work detailed in this paper focuses on forming strategies to overcome these process limits, including the processing of tailor rolled blanks. Additionally, finite element modelling of the process is presented and discussed with respect to the prediction of the forming limits of ISF.

Mushy/Semi-Solid Metal Forming Technology – Present and Future

Mushy, semi-solid and/or thixo processing of metals (alloys) is becoming popular as a new potential manufacturing technology for parts and components in automobile, electronic and machine industries. Internal structures and mechanical properties of those metals that include solid and liquid fractions are quite different from those of hot or molten metals. Diversified possibilities are known today to process those metals based on die casting, hot metal forming or polymer injection technologies, each of which has its own specific advantages and disadvantages. Up to now thixocasting and thixomolding have been used in industrial applications for light metal alloys. The potentials of those processes are wider by far however. They include the processing of specially designed alloys and composites, the combination of forming and joining processes as well as reduction of production costs and energy consumption.

Some current development trends in metal-forming technology

Abstract Various trends in metal-forming technology which will change future plant construction and production technology are already becoming apparent. The report describes the shortening, flexibilization and integration of a number of processes. Moreover hollow structure technologies become more and more interesting for innovative production. Furthermore FEM-simulation and optimization, also reported on in the paper, are increasingly important tools for the development of new or improved processes and plants. In view of the need to minimize production costs, to increase environmental compatibility and to manufacture products to a defined quality standard, long, complex processes should be shortened as far as is possible or necessary. In the field of strip production, mention should be made of the development of thin slab technology and thin strip casting, in which certain manufacturing steps are eliminated completely. For formed parts, possible methods for shortening the process include forming in the solidus-liquidus range (thixoforming). Another possibility is a combination of forming with heat treatment. Shorter process chains often mean more favourable mechanical properties for the products and hence new applications. Against a background in which forming processes are becoming more flexible in order to enlarge the spectrum of products, it is necessary to use flexible forming units with adaptable links to preceding and succeeding steps as well as universal dies and intelligent controls. For example, robot-manipulated open die forging allows reproducible manufacture of complex forgings with relatively small allowances. A variable rolling gap in the rolling process means that sheet metals can be produced with a defined longitudinal thickness profile matching the load case for the subsequent component. The integration of different production processes also paves the way for new approaches. Existing process limits can be extended and the final properties of components optimized by using partial heating methods during or directly after forming and by coupling forming processes with parting or joining techniques. A promising approach in the field of innovative lightweight construction is the systematic use of hollow structures; new processes for generating cavities and production techniques for processing the new hollow structures are being developed. Apart from suitable tests, physical and numerical simulation can be used to optimize existing or develop new methods of manufacture. Physical simulation may be especially successful in solving questions of material flow. A new material flow simulator is presented. Numerical simulation is used especially for quantitative analysis of local process variables; methods have recently been developed for taking the modification of structure during the forming process into account.

Forming and joining of commercial steel grades in the semi-solid state

Abstract The processing of metals in the semi-solid state (thixoforming) promises to enlarge the spectrum of application for various metals. Compared to casting the quality of the final product can be improved, as less defects related to solidification processes occur. Compared to forging processes, the needed forming loads are lower and more complexly shaped parts can be produced net or near-net in a one step process. On the basis of its outstanding mechanical properties that can be adjusted by adequate heat treatments, steel possesses possibilities for light weight constructions realisable by complex shapes. The semi-solid processing of steels is one focal point at the Institute for Metal Forming (IBF). Thixoforming experiments are carried out to investigate the feasibility of different steel grades. The heating of billets into the forming temperature and the forming operation itself takes place within a protective gas atmosphere to avoid the growth of scale on the billet’s surface. To study the applicability of different tool materials the die halves are built up out of segments. Theoretical approaches to predict the suitability of different steel grades for semi-solid processing especially concerning the width of a process window for thixotropic flow behaviour are presented. Furthermore the results of forming experiments regarding chemical homogeneity and mechanical properties of the thixoformed parts are pointed out. Since first forming experiments using conventional hot working tool steel as die material have shown massive wear and plastic deformation, tailored dies have been developed. The supply of suitable die concepts is one major task to ensure the cost effectiveness of this technology. One major potential of the semi-solid technology in the future may be the combination of the processes forming and joining taking advantage of the high flowability of semi-solid materials. This approach will lead to new possibilities for production of functionally integrated parts leading to new component classes and minimising assembly costs. In this paper results of joining functional steel parts (screw) with a steel component during thixoforming in a one step process are presented.

Application of fuzzy control for a hydraulic forging machine

Abstract Most metal-forming processes are non-linear, non-stationary and time-variant. Nonetheless, linear controllers or LTI-control systems are used for many of these processes. Because of their simplicity of application, fuzzy controllers have recently been implemented for many industrial processes in Europe, in Japan and in the USA. Fuzzy control may be characterized as an approach using controllers which can be designed without a mathematical model of the process and which are inherently non-linear. Fuzzy control is based on a priori expert knowledge which can be described in the form of a language. This paper discusses one application of fuzzy control, an adaptive fuzzy controller for a hydraulic forging machine which minimizes geometrical and material defects of the workpiece by controlling the servo valve and the pump power. The paper describes the use of the fuzzy controller with two processes, upsetting and thixoforging. The design procedure for the controller and experimental results are discussed.

The Technical and Commercial Potential of an Incremental Ring Rolling Process

A novel incremental ring rolling process is proposed, in which a narrow mandrel is moved both radially and axially. The process has potential to allow flexible near-net-shape forming of both hot and cold rings, and has been assessed by experiments on commercial cold ring rolling machines, a physical simulation using wax rings, and two finite element models. The results suggest that the process is technically feasible although the cycle time increases with the degree of flexibility and the stability of deformation depends on careful design of the tool path.

Integration of physically based models into FEM and application in simulation of metal forming processes

To obtain higher accuracy in FEM simulations the incorporation of microstructure evolution models becomes more and more important. From the point of view of metal physics it is well known that effects like recrystallization and deformation texture have a big influence on the material properties, especially the mechanical ones. The present article will give an overview about parts of the research activities in the Collaborative Research Centre (SFB 370) of the Deutsche Forschungsgemeinschaft (DFG). Three different types of microstructure models have been developed at the IMM and were coupled at the IBF to an implicit FEM code. The so-called flow-stress model is based on dislocation density evolution to describe the flow curve of metals, mainly at high temperatures. The Taylor-type model is able to describe deformation texture during metal forming. The third model is a modified cellular automaton to predict grain size and microstructure evolution during static recrystallization. The simulation of a rolling trial of the Al-alloy AA3104 including the named three models has been made and the results will be validated with experimental findings.

Simple compression test and simulation of an Sn–15% Pb alloy in the semi-solid state

Abstract A rheological investigation has been conducted using a simple compression test and a numerical simulation on a semi-solid Sn–15% Pb alloy with a solid volume fraction of 0.55 and 0.60. The stress–strain curve for the semi-solid state was divided into three zones; the first is the compaction zone, the second is the structure breakdown zone and the last is the thixotropic zone. Three sets of power law factors, one for each zone, were calculated. The factor m was larger than 1.0 in the first compaction zone, between zero and 1.0 in the second zone and less than zero in the third zone. The calculated load–stroke curves with the factors were compared with experimental results. The friction mechanism of the semi-solid forming process is different from that of conventional solid forming processes and therefore is discussed in more detail. The calculated curve with three sets of factors represents the experimental results comparatively well.

Different concepts of thixoforging and experiments for rheological data

Abstract The Institute for Metal Forming (IBF) works on the field of thixoforming in the framework of a Collaborative Research Centre. One forming process to be examined is thixoforging. This process is similar to closed die forging. During thixoforging the available initial material is formed completely to the component. As possible drip off losses may occur during the inductive heating to the semi-solid state and the following billet-transport to the die, an exact volume of the initial material cannot always be ensured. Different tool- and forming-concepts may solve this problem of varying initial material volume. One possibility is to work with excess-material, on the one hand in order to strip off an existing oxide skin before the forming process and on the other hand to direct the excess-material into defined areas in or outside of the component. Another different forming process is the so-called thixo transverse impact extrusion. During the process the material is squeezed sideways from a gating section into a closed die. Therefore the component volume is exactly adjusted and the excess-material remains in the gating section. Simulating the forming process saves time and costs. The simulation of metal forming in the semi-solid state can be difficult, as rheological models and suitable data are required. These data are being obtained by different experiments, such as the classical concentric cylinder and capillary rheometers. But these are only conditionally inserted in the temperature range of semi-solid aluminium. In order to obtain suitable data for this material as well, different experiments have to be used. Upsetting tests, known from the forming technology to investigate flow stresses, can only be used as long as the definiteness of the stress-conditions is guaranteed. Therefore the IBF examines different experiments in order to determine suitable data.

A New Rolling Process for Strips with a Defined Cross Section

Abstract A new rolling process is being developed for producing cold rolled strips with a defined cross section. These strips feature differences in thickness up to 50% perpendicular to the rolling direction. A special roll system is used to achieve a material flow exclusively in the direction of the strip width. As a consequence the strip width is increased significantly. In this way a wide variety of different cross sections can be produced. The range of application for these kind of strips cover novel load-adapted and load-optimized structures with different thicknesses, e.g. special profiles and special section tubes.