Papers by Keyword: Forming

Abstract: Due to current ecological and economic developments there is a growing demand for functional components with complex and closely tolerated geometrical features. Conventional sheet and bulk metal forming operations leads to products which are often limited in their geometrical and functional variety. A promising approach is the process-class sheet-bulk metal forming (SBMF). SBMF is characterised by the application of bulk and sheet forming operations on sheet metals [1]. This combination leads to locally and temporally varying load conditions regarding stress as well as strain states. In order to get high quality parts, controlling the material flow is of major importance. Modified Surfaces, so-called tailored surfaces represent an innovative approach to control the material flow. The objective of the current study is the experimental investigation of the effectiveness of locally adapted tribological conditions using workpiece-and tool-sided tailored surfaces within SBMF processes. The study has shown that the local adaption of workpiece and tool surface increased the heights of functional elements. Thus, using locally adapted tribological conditions leads to an improvement of the quality of the produced gearing components. In a further step the influence of surface modifications on the surface properties of the manufactured components are analysed. Additionally, investigations regarding the wear behaviour of tool-sided surface adaptions lead to the assumption, that the effectiveness of tailored surfaces is reduced during the operating time of the tools.

Abstract: Cu-Cr-Zr-Ti alloys are widely used for fabrication of thrust chamber in liquid rocket engines, because of their high thermal conductivity and adequate strength. The alloy should be used in peak aged condition to achieve the best combination of strength and conductivity. However, realization of final component involves different stages of forming, intermediate heat treatment and brazing operations, which limit the usage of the alloy in peak aged condition. The formability of material depends on grain size to a large extent. Larger grained metals have better formability, but the roughened surface that results from stretching metal with coarse grain structure results in surface defects like orange peel. Copper alloys on cold working sometimes exhibit a surface pebbling effect termed as ‘orange peel’ or alligator skin’.The plates of this copper alloy, finish hot rolled at 925 to 975°C and annealed at 980±10°C for 10 ±2 min followed by air cooling are subjected to different stages of forming to get the final profile of thrust chamber. During the course of forming of plate for divergent thrust chamber, orange peel defect is observed in areas of higher deformation. Detailed characterization of these formed shells in terms of microstructure, grain size and mechanical property has been carried out to find out the exact cause of this defect. Study revealed that the plate has coarse grain structure, which has resulted in orange peel defect during forming. To overcome this effect, the plates were realized with modifications in finish hot rolling and annealing temperatures, which has shown improvement in grain refinement and mechanical properties. It has suppressed the orange peel defect during forming.

Abstract: Nickel based super alloy MoNiCr was developed as the material with high corrosion and creep resistance against aggressive molten fluoride salt environment. This medium is used by Fluoride salt cooled High temperature Reactor (FHR) and Molten Salt Reactor (MSR) systems. The elements Mo, Cr and Fe are most important alloying elements accompanied by very low amount of Al + Ti. Necessity to prepare hot formed semi products such as sheets, bars, wires and tubes was the motivation for this new experimental program execution searching for suitable hot forming conditions. In the case of MoNiCr components forming, recrystallization is a crucial process allowing successful forming. Special attention was paid to this process after finding out, that this process is running very slowly or even not running at all, if inappropriate conditions are met. Proper temperature and strain rate range were determined and furthermore the influence of cold deformation on recrystallization progress was determined. It was found that cold deformation before hot forming as well as after hot forming can be successfully implemented into technological chain. Almost fully recrystallized microstructure can be achieved using synergy of both cold and hot deformation.

Abstract: The properties of fiber reinforced materials are depending on the fiber direction. During draping processes - which are necessary to form complex structures - the fiber direction and therefore the resulting properties of the final part are changing.To ensure that the fibers in the final complex structure are placed exactly in the direction needed, a new approach is investigated.The idea is to define the orientation of the reinforcement fibers based on the distribution of forces in a complex structure under certain loading determined by a structural simulation. Best lightweight behavior is achievable in the final complex structure. A three-dimensional mesoscopic model of the directed fibers is created using FEM-software. In reverse draping simulations the three-dimensional fibers are formed from complex shape to a two-dimensional flat sheet.In manufacturing the two-dimensional patches can be created using the tailored fiber placement process. With this process it is possible to place the fibers orientated to the required paths. The patches are formed to the necessary three-dimensional shape by a real draping process. The relative sliding behavior of crossing fibers can be achieved by varying the stitch during the TFP process.Using that approach it is possible to create lightweight structures in which fibers are orientated directly along the load paths of the three-dimensional application.

Abstract: During forming, defects can occur and have to be taken into account because they can significantly affect the mechanical performance of the part. This experimental study shows the type and number of defects is a function of the punch geometry, the process parameters, the orientation of the fabric with respect to the punch and the inter-ply friction. Inter-ply friction has a huge effect on the quality of the preform when inter-ply sliding occurs. This inter-ply Friction leads to several overhanging yarn shocks that generate high tangential forces, which inhibit the relative sliding of plies.

Abstract: The results of in-plane shear tests performed on 5-hardness satin woven carbon/PPS thermoplastic prepregs are described. The experimental analyses are based on bias-extension tests performed in an environmental chamber. The results are given for different temperatures on both side of the melting point. This range of temperature is those of the part during a thermoforming process. In another hand it is shown that second-gradient energy terms allow for an effective prediction of the onset of internal shear boundary layers which are transition zones between two different shear deformation modes. The existence of these boundary layers cannot be described by a simple first-gradient model.

Abstract: A methodology to calculate surface strains from a rectangular grid placed on a forming blank is introduced. This method consists of treating the grid points as nodes of a finite element (FE) model and assigning elements to the grid. The strains are then computed following FE analysis. If higher order elements are used, also more information within the element can be obtained which allows a coarser grid without loss of accuracy. This is the major advantage of the approach presented herein.

Abstract: The bending deformation of thermoplastic prepregs is one of the key deformation modes in the thermoforming due to its crucial role in the wrinkling occurrence. The influence of temperature is of main importance because the viscous effect of resin is temperature dependent and prepregs thermoforming is usually performed closed to resin’s melting point. The currently available bending test devices are not adapted for thermoplastic prepregs since these devices can only be operated at room temperature. To solve this problem, a new cantilever test with an optical measuring performed in an environmental chamber is proposed. The bending properties of PPS-carbon satin prepregs are measured at a series of high temperatures. It’s shown that the bending stiffness of the fore-mentioned pepregs is strongly affected by the temperature and shows a non-linear bending behaviour. The measured bending properties are used to simulate a thermoforming process. The influence of bending properties on the simulation results, especially to the wrinkling is presented as well.

Abstract: The ability of a draping simulation to accurately predict the outcome of a forming process mainly depends on the accuracy of the input parameters. For pre-impregnated composites, material must be characterised in the same conditions as forming occurs, i.e. in temperature regulated environment. Given the issues encountered while testing specimens enclosed in a thermal chamber and mounted on a tensile testing machine, new test methods have to be developed. A new approach using a Dynamic Mechanical Analysis system is presented for the investigation of tensile properties perpendicular to fibre direction of unidirectional pre-impregnated composites. Analyses are focused on a unidirectional carbon fibre thermoplastic tape reinforced polyamide 6 in its molten state. Quasi-static tests are performed at forming temperature for different loading rates with specimens of different geometries in order to assess the reproducibility of the test method.

Abstract: In recent years, laser forming of round plates into bowl or dome shapes by use of circular, radial and circular-radial patterns have been investigated. Usually formed circular plates using circular or linear patterns are distorted as asymmetric saddle shapes. In this study, a new flower pattern has been proposed to form round plates by laser. To make this pattern, the laser beam scans several petal paths on a circular blank. Laser forming of round plates by the proposed pattern have been studied by three dimensional finite element method. The results have been compared for the flower pattern and other conventional circular and radial pattern. In addition experiments have been conducted to verify the numerical results. The results show that the deformed parts by the petal path are more symmetrical in comparison with circular and radial patterns. It was found that in laser forming of dome-shaped parts, scanning by petal paths prevent distortion and increase the geometrical symmetry of deformed parts by laser. It was shown that laser radiation on petal paths improves the deformation process of laser forming of circular steel sheets.