Klaus Siegert

Recent developments in hydroforming technology

Abstract This paper shows an overview about possibilities of hydroforming sheet metal as well as hydroforming tubes and extrusions. Coming from the deep drawing process with rigid dies, specially designed dies for presses with multipoint cushion systems required for hydroforming sheet metal are discussed. Further special press systems for presses with high ram forces are shown.

Superimposing Ultrasonic Waves on the Dies in Tube and Wire Drawing

The forces during wire and tube drawing can be reduced by ultrasonically oscillating dies. It is a major problem of conventional wire and tube drawing to introduce high forces into the forming area. Compared to conventional wire and tube drawing, the forming process limits can be extended by superimposing ultrasonic waves due to decreasing drawing forces. Different techniques can be used to excite the die. One possibility is the variation of the vibration mode. In tube and wire drawing, the dies are usually excited longitudinally. If the vibration direction is parallel to the drawing direction, the main influence will be on the friction between workpiece and die. The Institute for Metal Forming Technology of the University of Stuttgart, Germany started a project to investigate the effect of ultrasonic waves on the tribology and on the formability of the work-piece. The objective of this investigation is to separate the ultrasonic effect on the surface from the volume effects. This paper shows that the reduction of the sliding friction between a longitudinal oscillating die and the workpiece can be explained by the so-called Sliding Friction Vector Effect (SFVE). A statistical evaluation of roughness-measurements makes it possible to show the effect of the ultrasonic vibration on the friction and gives an insight into the operation of the SFVE. The results are compared with wire and tube drawing experiments of copper and Ti-alloys. New tube- and wire-drawing experiments with longitudinally vibrating dies support the theoretical approach. The surface quality of the manufactured workpieces can be improved and the productivity increased.

Pneumatic Bulging of Magnesium AZ 31 Sheet Metals at Elevated Temperatures

Abstract This paper deals with forming magnesium sheet metal components like auto body parts by pneumatic bulging. Furthermore, this paper deals with the evaluation of true stress-true strain curves as well as with forming limit curves determined by pneumatic bulging at temperatures up to 350°C. It can be shown that magnesium AZ31 is quite good formable at temperatures in the range of 250°C to 350°C. Outgoing from these fundamental investigations, an auto body sheet metal component was formed by pneumatic bulging at a temperature of about 300°C. Furthermore, the strains over the component were measured with an automated grid analysis. This analysis shows a nearly equal distribution of the strains over the component.

Wire drawing with ultrasonically oscillating dies

Abstract The drawing forces during wire drawing can be reduced by applying ultrasonically oscillating dies. A major problem of conventional wire drawing is to introduce high forces into the forming area. Compared to conventional wire drawing, the forming process limits can be extended due to the decreasing drawing forces. Different techniques can be used to excite the die. A possibility is the variation of the mode of oscillation. In tube and wire drawing, the dies are usually excited longitudinally. However, in certain cases it would be advantageous to excite the dies in a radial mode. Furthermore, there is the possibility of varying the vibration direction. If the vibration direction is parallel to the drawing direction, the influence on the friction force will be the greatest. If the vibration direction is perpendicular to the drawing direction, the influence on the forming properties of the workpiece will be the greatest. Since longitudinally oscillating dies at their natural frequency generate a standing wave, there is additionally the possibility to locate the die into the nodal- or into the antinodal points. With ultrasonically oscillating dies, the static forming forces can be reduced. The superimposed dynamic stress reduces the measured static forces because of the oscillating movement of the die. One reason for the force decrease is the reduction of friction between die and workpiece. Another reason is the absorption of sonic energy by the lattice dislocations and the change of the forming properties of the workpiece. Mounting pins were developed to induce the dynamic oscillation and to realize high static loads upto 10 kN. In these investigations, the die oscillates longitudinally at the resonant frequency of approximately 22 kHz. There is an oscillating movement of the die with a velocity of maximal 0,5 m/s and an amplitude of maximal 10μm. The experimental equipment was installed in a hydraulic drawing machine. The experiments were performed with highly alloyed austenitic stainless steel wires. Diameter reductions from 3,7 mm to 3,5 mm, from 3,7 mm to 3,0 mm, and from 3,7 mm to 2,5mm were investigated. The drawing velocity was varied in the range between 1 m/min and 10 m/min. Two longitudinally perpendicular excited dies were used. Type A was in the antinodal point of the standing wave and type B was in the nodal point of the standing wave. The experimental results show that applying die-type A, a drawing force reduction of nearly 12 % can be achieved. However, the side-effect of this die-type was the activation of another vibrational mode in the workpiece. Because of resonant effects, the drawing force decreases periodically up to 40 %. Therefore, the wire-diameter has changed at the corresponding locations. Using die-type B, it is possible to achieve drawing force reductions upto 20 %. Compared with the die-type A, the drawing force curve was smoother. Corresponding to the drawing force curve, the wire surface was smooth.

Closed-Loop Control System for Blank Holder Forces in Deep Drawing

Abstract When drawing non-axissymmetric sheet metal parts it is necessary to control the flow of material between the lower and upper binder in such a manner that prevents the occurrence of both tears and wrinkles in the drawn part. One possibility for the control of the material flow is through the deliberate adjustment of the normal forces. If one can measure the flow-in of the material in the die as a function of punch penetration with a special sensor, and if this information can be used to produce an average empirical desired flow-in curve, then with the implementation of modern hydraulic drawing facilities in the press table and/or the implementation of special dies with hydraulically supported segmented binders it is possible to build a closed-loop control system which allows for the automatic response of the drawing facilities or the hydraulic systems of the drawing die in light of changing friction conditions.

Influencing the Friction in Metal Forming Processes by Superimposing Ultrasonic Waves

Abstract The possibility of influencing the friction in metal forming processes by superimposing ultrasonic waves will be discussed in general. It can be shown by experimental investigations that it is possible to reduce the friction forces and to improve the surface quality of formed parts. A new friction law including the influence of superimposed ultrasonic waves and the surface and lubrication properties will be described.

Closed loop control of the friction force. Deep drawing process

Abstract When drawing non-axisymmetric sheet metal parts it is necessary to control the flow of material between the lower and upper binder so that the drawn part is free of tears and free of wrinkles. The material flow can be determined by the friction force between the sheet metal and the lower and upper binder. It is possible to determine a friction force curve over the stroke which realizes the desired material flow. The friction force in turn is directly influenced by the blank holder force (BHF). With a closed loop control which uses the friction force as control parameter and the BHF as feedback parameter it is possible to realize the predetermined friction force curve over the stroke, even if the tribological conditions between the sheet metal and the upper and the lower binder will change. This can be carried out by automatically changing the BHF in such a way that the desired friction force and in turn the desired material flow is achieved. To built up a closed loop control, a special die with hydraulically supported segmented binders can be used. This system is able to vary the BHF during the forming process, so that a BHF feedback is possible in the closed loop. To measure the actual friction force as control parameter, a special sensor must be integrated in the die. Such a sensor can be realized with piezo electric load cells.

Bulge testing under constant and variable strain rates of superplastic aluminium alloys

The present paper deals with superplastic forming of aluminium alloy AA5083 sheet metals tested at specific strain rates, temperatures and counter pressures by means of bulge testing using circular and elliptical dies and by the cone-cup testing method. Further, differences from batch to batch can lead to a different strain rates at the maximum m value. It is shown by experimental investigations that pulsating strain rates can lead to higher m values and to increased thickness strains.

Process control in blanking

Abstract This paper summarizes the results of testing the performance of in-die sensors for controlling the stamping process. Piezo based as well as strain gage based in-die sensors were mounted at different locations within a progressive die. Furthermore, these signals were compared with load curves of press frame mounted sensors and the acoustic emission acquired from a triaxial acceleration sensor. The signals acquired from the different in-die sensors at the different mounting locations showed all the different phases of the process. It was shown that changing process variables such as punch-die clearance, material properties, stroke rate and punch-die alignment results in different load-stroke curves. Based on these results it could be concluded that in-die sensors provide the most detailed information about the process.

Anisotropy and formability of AA5182-0 aluminium alloy sheets

Abstract This paper presents a new yield criterion for orthotropic sheet metals and its implementation in a theoretical model of the forming limit diagrams. The equivalent stress equation shows that the shape of the yield surface is defined by eight material parameters. The minimisation of an error-function has been used for the numerical identification of these coefficients. The parameters are established in such a way that the constitutive equation associated to the yield surface reproduces the plastic behaviour of the actual material. The uniaxial yield stresses (σ 0 , σ 45 , σ 90 ), biaxial yield stress (σ b ), uniaxial anisotropy coefficients (r 0 , r 45 , r 90 ) have been used in identification. The new yield criterion has been implemented in the Marciniak-Kuczynski theory in order to predict the limit strains. The theoretical forming limit curves have been compared with the experimental ones. The friction free tests, the hydraulic bulge test (for the positive minor strains) and the tensile test for plane strain and for uniaxial tensile test (for the negative minor strains) are used. The predicted yield surface and forming limit diagrams for AA5182-0 aluminium alloy sheets are in good agreement with the experimental ones.