Showing papers by "Gerhard Hirt published in 2008"

Increasing of the drawing depth using tailor rolled blanks—Numerical and experimental analysis

Abstract: Deep drawing is a common sheet metal forming process. In most cases, sheets with constant thicknesses are formed. At the end of the previous century, new innovative blank technologies have been established for weight saving purposes. The development of the flexible rolling process is an illustrating example for this progression. By changing the roll gap during rolling, longitudinal thickness transitions are produced. The innovative semi-finished product, which is produced in this manner, is called tailor rolled blank (TRB). Its behaviour and characteristics during further processing, especially in forming, are topics of present research. The main emphasis of this paper is placed on the idea that TRB can be used to increase the maximum deep drawing depth compared to blanks having a constant thickness. This can be realised by “weakening” certain areas of the blank in a way that the load in failure at critical areas is reduced. To ensure weight saving in addition to increasing the maximum deep drawing depth, the maximum sheet thickness of the TRB is equal to the constant thickness of the other blanks. The concept is first analysed with the help of numerical simulations and then verified by experimental work.

Production of blanks with thickness transitions in longitudinal and lateral direction through 3D-Strip Profile Rolling

Abstract: 3D-Strip Profile Rolling should enable the production of blanks with a defined thickness profile in latitudinal and longitudinal direction. The production chain of 3D-Strip Profile Rolling will combine Flexible Rolling in a first production step with Strip Profile Rolling in a second step. The control system to adjust the roll gap during 3D-Strip Profile Rolling is currently under development. Nevertheless, some first experiments have shown the general feasibility to produce 3D-profiled blanks. In 3D-Strip Profile Rolling the material will strain harden differently on different locations. This results in a variation of the material properties of the strip. Lateral spread, elastic roll stand deformation and local deformation will be influenced by this variation. To investigate these influences on the complete production process, the complete production chain needs to be modelled in the future with aid of finite element simulations. In this publication a first simulation model is used to study the influence of different grades of strain hardening in a Taylor Rolled Blank on the bulge formation that occurs during the rolling of a rill in this Tailor Rolled Blank.

Adaptive cross-approximation for surface reconstruction using radial basis functions

Abstract: The efficient handling of matrices arising in surface interpolation and approximation with radial basis functions (RBF) is considered. To find a data-sparse approximation of the system matrix, the adaptive cross-approximation (ACA) technique is used. The approximation of the matrix requires O (Nlog2 N ) units of storage and arithmetic operations, where N is the number of interpolation points. Because basis functions are not explicitly used, the implementation is applicable to a wide class of interpolation kernels. Numerical examples involving generated data and measurements of formed sheet-metal parts are presented.

Validation of an improved dislocation density based flow stress model for Al-alloys

Abstract: A microstructure and flow stress model for cell forming metals, 3IVM (3 Internal Variables Model), was developed [1], in which the work hardening and softening effects due to interaction of dislocation densities are taken into account. This model had been applied in through process modelling of sheet production. In the present paper, improvements of 3IVM are introduced. In the new model (3IVM+) the softening effect of recovery by cross slip and by additional dislocation reactions are considered in addition to that of climb. Moreover an improved kinetic equation of state is utilized. For validation, compression tests on a commercial Al-alloy were carried out and the flow curves were determined. The comparison between model predictions and experimental results shows that 3IVM+ can replicate the flow curves much better than the previous model, in particular at low temperatures and in wide temperature range. Since the concept of the model is unchanged, its applicability for through process modelling is not compromised.

Investigations on Thermal Influences for Thixoforging and Thixojoining of Steel Components

Abstract: At the Institute of Metal Forming (IBF) current investigations within the framework of the Collaborative Research Centre 289 are mainly concentrated on forming of semi-solid precursor material of the steel grades X210CrW12 and 100Cr6. One important task is the precise temperature for the composition of solid and liquid phase fraction in the preheated billet. Experimental measurements and numerical simulations show significant heat losses during transport of the billet and after its insertion into the die. These developing temperature gradients influence strongly the resulting temperature field in the formed component. In case of the forming tools the critical increase of the temperature depending on the tool material is shown. As fundamental research in the field of thixojoining the temperature development of the inserts is analysed and demonstrates the feasibility of joining higher and lower melting materials into the semi-solid matrix.

Process Window Determination for Thixoextrusion Processes Using a Steel Alloy

Abstract: A direct semi-solid bar extrusion process is characterised by inserting a feed stock in a container and extruding through a forming die with a punch. Compared to conventional bar extrusion the use of semi-solid material complicates the process due to the requirement of solidification of the material. To achieve the solidification of the semi-solid bar, different basic tool concepts are presented. With a combination of these concepts experiments were carried out using the steel alloy X210CrW12 to detect the influence of the most influencing parameters press velocity, extrusion channel diameter, length and geometry. Numerical simulations enable a better understanding of the process mechanics like temperature development in the billet and forming die as well as the material flow in the deformation zone.

Modelling and optimisation of incremental bulk metal-forming processes for near net shape components

Abstract: In this paper, we present a method to accelerate incremental bulk metal-forming simulations by using different mesh discretisations in two depending meshes. This multimesh method is preventing a loss of local resolution by using a special data transfer. Two example processes show the working of this method and the archieved acceleration.