Advanced Materials Research Vol. 1018

Abstract: During machining of metals high temperatures and deformations occur at the surface layers leading to changing component states. Depending on the thermal and mechanical set of stress microstructural changes like residual stresses and grain refinement can be found. Grain refinement is influenced by the amount of deformation. In the present investigations high deformations of the surface layers are realized by using the cutting tool in opposed direction resulting in high negative rake angles. This test setup is used to investigate the material behaviour of Armco-Iron and AISI 4140 at different cutting velocities during machining. Furthermore the flow stresses at different strain rates and temperatures were determined by means of high speed tensile tests. The flow behaviour of the investigated materials is used to explain the results of the machining experiments.

Abstract: The growing demand for micro products of hard and brittle materials requires suitable manufacturing processes, which serve high structure quality and accuracy. Therefore, micro pencil grinding tools are used. While grinding hard and brittle materials the structure quality and accuracy depends on the material removal mechanism. This mechanism is a complex interaction between the process parameters, the material response and the tool geometry. In this paper the influence of the process parameters, particularly the cutting speed and the feed rate, on the material removal mechanism are discussed. Furthermore, a method for the analysis of the structure quality and accuracy and within the material removal mechanism is shown.

Abstract: Galvanic retroreflector moulds suffer from fabrication errors and considerable wear due to the manufacturing process and the subsequent use for injection moulding. To remove degradations like flaws and wear marks from the mould’s optical surfaces without tedious manual labour, a partially automated process chain using a 6-axis robot is being developed combining surface metrology and machining technology. The areal limitation of the structured surfaces excludes conventional rotatory polishing processes. Vibration polishing is applied instead to remove critical surface errors and to enhance surface roughness to a near optical value of Sa ≤ 50 nm. The work presented in this paper gives insights into experiments performed on a robot polishing system and illustrates and the suitability for an application in the aforementioned partially automated process chain.

Abstract: Drilling is the most used machining operation in modern manufacturing. Consequently, a high efficiency is desired which can achieved by proper tool geometries and process conditions. The claim for a high degree of process understanding is met by the attempt to visualize the chip formation on a micro scale, e.g. by the use of the finite element method. While two-dimensional FE-simulations of cutting processes are established 3D is the next step to take for further analysis of the drilling processes. Nevertheless, this puts high challenges to the simulative approach in order to achieve valid results. This paper deals with the modelling and the simulation of drilling operations with respect to tool and workpiece geometry. In order to create an acceptable starting condition it is useful to consider the engagement situation of the tool and the workpiece as an ideal state with a defined uncut chip thickness in front of the cutting edge. This provides shorter calculation times due to earlier steady-state chip formation compared to simulations with workpieces which are modeled with uniform surfaces like planes or cones in the initial state. Therefore, the engagement situation is created by the utilization of a geometric-kinematical modelling technique in which an array of rays and their intersections with a triangular mesh constitute a discrete description of a surface. The obtained set of points is further used for triangulation to generate the workpiece geometry mesh. Finally, finite element simulations of the drilling processes are carried out and are analyzed with respect to drilling torque and feed force.

Abstract: This paper presents a diagram of maximum contact zone temperature T_max versus contact time Δt, based on the analysis of workpiece surface layer properties after cylindrical grinding experiments. Apart from resulting surface layer properties, process quantities (T_max, normal and tangential grinding forces F_n, F_t) are investigated with reference to the resulting workpiece surface layer state as well. Ground workpieces are analyzed by performing Barkhausen noise level measurements together with subsequent metallographic and X-ray diffraction investigations. By mapping characteristic values T_max and the contact time Δt to corresponding surface layer properties, a general analysis of workpiece material response to the thermo-mechanical load during grinding is possible.

Abstract: Because of the extensive use of thin sheet metals to reduce the weight of vehicles, wrinkling is becoming a more common and one of the most undesirable failures in the sheet forming process. Generally, experiments for studying wrinkling phenomena can be divided into two methods: actual forming of typical parts such as annular cup test on the one hand and the tests of specially designed sample geometries like the Yoshida Buckling test on the other. Recent experiments indicate that the plastic strains at the onset of wrinkling in the Buckling Test with Yoshida samples are too small to reflect reality of deep and stretch drawing conditions. Therefore, in this paper, in order to enhance the accuracy of the prediction of wrinkling, a new modified Yoshida specimen is provided for numerical simulation. The fundaments of the different buckling phenomena are going to be explained considering the energy theory in metal forming processes. Meanwhile, the influences of the changeable sample geometries in order to cause different stress distribution within loaded area of specimen have been investigated.

Abstract: Within today’s sheet processing lines, roller levellers are included in the production chain to eliminate initial curvature and reduce internal stresses of the sheet material. Despite the desire to achieve fully automated industrial processes, roller levellers still have to be set manually by an operator based on his experience and empirical data. Therefore, this paper evaluates an enhanced numerical approach to predict the vertical roll position, the so called roll intermesh, in the last load triangle. To gain the respective machine setting, a closed-loop control based on an actual curvature measurement is implemented in the finite element (FE) programme Abaqus utilising a user-subroutine. Thus, the presented FE model allows the adjustment of the roller leveller leading to a flat strip in a single simulation run within the accuracy of the FE prediction. Additionally, the FE model provides the chance to develop and test closed-loop controls for roller levelling. Complementing the results gained from the FE model, experiments have been conducted on a down-sized roller leveller with aluminium sheets (AA5005). First results obtained with the presented numerical model proved that the roll intermesh of the last load triangle was determined successfully and the use of an actual curvature measurement within the FE model provides enhanced accuracy.

Abstract: Due to the global energy policy according to the aims of economic efficiency and environmental compatibility the production of lightweight constructions is nowadays a need in many fields of mechanical engineering, especially in automotive and aerospace sectors. The substitution of conventional deep drawing steels with modern sheet metals, i.e. aluminium or high strength steel, provides a positive contribution towards this trend. Prerequisite for an efficient application of these resources is a comprehensive material characterization including the analysis of the formability as well as the failure behaviour. Regarding the accurate prediction of forming behaviour, a robust failure criterion is necessary in order to perform the numerical simulation designs by using finite element method (FEM). The leading criterion used for sheet metal forming is the forming limit diagram (FLD). The FLD gives a graphical description of material formability by plotting the major strain values related to the minor strain values to failure in a curve called forming limit curve (FLC). With the improvement of the digital image correlation (DIC) techniques, which provide very accurate strain measurement, new methods relied on the strain history of the specimen during the test are proposed to determine the FLC. Within this paper, the time dependent method based on the coefficient of correlation is investigated. In particular an improvement dependent on experimental parameter is presented in order to increase its sensibility and reliability for different materials.

Abstract: The composite extrusion process, which is investigated at the Institute of Forming Technology and Lightweight Construction (IUL), allows the combination of different materials within an aluminum profile. In contrast to the useage of reinforcing elements, this paper focuses on the embedding of functional elements, such as isolated electric conductors. Results of the experimental and numerical investigations are presented within this paper.

Abstract: In the manufacturing process of body in white components made from sheet metal it is state of the art to accompany the process by means of finite element analysis. A main criterion for determining a feasible tool design and process parameters is the prediction of material failure, which can be categorized in instability and ductile fracture. The ductile fracture failure mode is more likely to occur, as more advanced high strength steels and aluminium alloys are used for body in white components. Therefore various approaches have been presented to model ductile fracture over the past years. However, there is no guideline to determine which models are suitable for predicting ductile fracture. The same applies when it comes to choosing experiments and calibration of model parameters. A suitable model calibration is vital, as the fracture prediction depends on the determined model parameters. Usually an isotropic material behaviour is assumed for calibration of fracture models. However, sheet metals can show an anisotropic material behaviour due to the rolling process. Therefore it is arguable if an isotropic material model can be applied when fracture models are calibrated.