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METHODOLOGY FOR A MODEL-BASED CONTROL OF THE BOUNDARY ZONE PROPERTIES DURING MILLING OF TI-6AL-4V
Residual stresses in metal components induced by forming operations significantly influence subsequent manufacturing steps. In particular, two aspects for the final processing of formed thick sheets by cutting and machining operations result: First, the achievable geometric accuracy is limited by distortion due to the modification of residual stresses across the process step sequence. In addition, the residual stresses present in the final component are determined by the process parameter settings of the individual substeps. In this work, the influence of the forming-induced residual stresses on a subsequent cutting operation was quantitatively investigated. For this purpose, the step sequence of forming and wet abrasive cutting for aluminum AA7075 thick sheets was implemented both numerically and experimentally. This allows the residual stress redistribution to be taken into account in future virtual setups of the production chain by specifically adjusting the process parameters.Keywordsresidual stressthick sheet formingmillingFEM
The residual stress state of the sub-surface zone is a significant influencing factor, which determines the strength, lifetime and reliability of a component after machining. A reliable adjustment of this surface characteristic during peripheral milling is currently not possible. It is desired to control the milling process using a model-based approach in order to generate defined residual stresses in titanium components. This paper focuses on the determination of the thermo-mechanical load leading to the residual stress state during peripheral milling of Ti-6Al-4V. A newly developed sensory tool holder for the in-process measurement of the temperatures in the process zone was used. The influence of the process parameters cutting speed, feed per tooth, and radial depth of cut on the residual stress condition is shown. In addition, the varying microgeometry of the tool due to tool wear is taken into account as an observable disturbance variable.
This paper presents the results of a distortion analysis of profiled workpieces after the grinding process and a distortion compensation. Heat treatments and machining processes such as grinding change the residual stress state of steel workpieces in a quite uncontrolled manner. This significantly influences the deformation of the quenched and tempered steel workpieces used in this work. The lack of a detailed representation of profile grinding processes and the stress-related distortions of slim components still poses a great challenge in research. Simulative mapping of the thermo-mechanical effects on profiled components has proved difficult in literature. The influences of different cutting depths and feed speeds of the grinding wheel on the distortion were investigated with developed finite element simulations as well as experimentally. The grinding model, validated by experimental results, expands the understanding of the profile grinding process. This enables a detailed evaluation and an analysis of the occurring effects. As an approximation to linear guide rails, a V-groove was ground to improve the surface quality. Inside the V-groove, the machining parameters and thus the generated three-dimensional heat flux influence the residual stresses. The simulated workpiece distortions match very well to the experiments, especially when varying the defined depths of cut. The results provide a model for distortion prediction. Based on this model, approaches to use a laser-based treatment and deep rolling as distortion compensation strategies can be addressed.
Τhis chapter is aimed at providing current knowledge on the association of surface texture with machining, along with recent advances in surface characterization and evaluation. Various texture parameters, adopted by ISO standards or not, are described and their distinctive power is considered. Arithmetic parameters, statistical and random process functions serve as measures for rendering in-height and inlength surface characteristics and allow multiparameter analysis of the surface, something quite necessary under current high requirements for precision and operation. Theoretical models for roughness parameters and experimental trends with regard to machining conditions are discussed. Isotropy of machined surfaces is also considered and methods for surface typology are finally discussed.
This special issue was initiated by the priority programme 1480 “Modelling, simulation and compensation of thermal effects for complex machining processes” and presents current investigations, new approaches and relevant results with regard to the specific topic. In addition to the contributions from the priority programme, this issue contains selected publications, which complement the substantial overview of novel developments in the area of heat source identification, modelling, simulation, minimisation and compensation of thermal effects in machining operations. © 2015, German Academic Society for Production Engineering (WGP).
The major step of the process chain for regeneration of damaged components is the removal of excess weld material, called re-contouring. This material removal process influences the surface integrity and therefore the functional performance of components. But today the surface integrity, e.g. residual stresses, cannot be predicted to a satisfying degree due to the complex physical effects during the cutting process. This paper investigates the fundamental influence of cutting conditions, tool geometry and weld characteristics on the residual stress formation after 5-axis ball nose end milling of Ti-6Al-4V. It is shown experimentally, that the cutting edge radius is the most influencing factor on residual stresses. Furthermore it is shown, that the thermal effects during cutting have a minor influence on the residual stresses due to the properties of titanium and the ball nose end milling process. Finally a basic physical approach is given to explain the effects by considering only the uncut chip volume, which is generating the final surface.
This paper presents an overview of recent developments in simulating machining and grinding processes along the NC tool path in virtual environments. The evaluations of cutter–part-geometry intersection algorithms are reviewed, and are used to predict cutting forces, torque, power, and the possibility of having chatter and other machining process states along the tool path. The trajectory generation of CNC systems is included in predicting the effective feeds. The NC program is automatically optimized by respecting the physical limits of the machine tool and cutting operation. Samples of industrial turning, milling and grinding applications are presented. The paper concludes with the present and future challenges to achieving a more accurate and efficient virtual machining process simulation and optimization system.
A thermal elastic-viscoplastic finite element model is used to evaluate the residual stresses remaining in a machined component. An improvement in the accuracy of the predicted residual stresses is obtained by: (a) using a modified Johnson–Cook material model that is augmented by a linearly elastic component to describe the material behavior as non-Newtonian fluid; (b) using a remeshing scheme to simulate the material flow in the vicinity of the rounded cutting tool edge without the use of a separation criterion; (c) properly accounting for the unloading path, and (d) considering the thermomechanical coupling effect on deformation. Case studies are performed to study the influence of sequential cuts, cutting conditions, etc., on the residual stresses induced by orthogonal machining.
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