Non-linear model-predictive-control for thermomechanical ring rolling

he authors present a new ring rolling variant that combines a semi-warm forming process of a bearing ring with controlled cooling directly followed by a cold forming process. The aim is to produce near net shape rings with a selected microstructure and high strength without additional consecutive heat treatment. To achieve this, a new and fast control strategy is necessary that not only controls the geometrical forming of the ring, but also considers temperature development and microstructure formation. The proposed control strategy is based on the application of a fast semi-analytical simulation model with a very short response time in combination with a FE-analysis of the thermomechanical ring rolling process. The semianalytical model is used as a predictor and a parallel FEA or experimental results as a corrector for the control model. The aim is to correctly identify transient process parameters needed to achieve defined product properties as a basis for a later implementation in a non-linear modelpredictive-control of thermomechanical ring rolling. The new approach will be described in detail and demonstrated numerically and experimentally

Analysis and modelling of a rotary forming process for cast aluminum alloy A356

Spinning of a common aluminum automotive casting alloy A356 (Al-7Si-0.3 Mg) at elevated temperatures has been investigated experimentally with a novel industrial-scale apparatus. This has permitted the implementation of a fully coupled thermomechanical finite element model aimed at quantifying the processing history (stress, strain, strain-rate and temperature) and predicting the final geometry. The geometric predictions of this model have been compared directly to the geometry of the workpieces obtained experimentally. This study is novel in regards to both the size and shape of the component as well as the constitutive material representation employed. The model predictions are in reasonable agreement with experimental results for small deformations, but errors increase for large deformation conditions. The model has also enabled the characterization of the mechanical state which leads to a common spinning defect. Suggestions for improving the accuracy and robustness of the model to provide a predictive tool for industry are discussed

Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes

Numerical and physical simulation of rapid microstructural evolution of gas atomised Ni superalloy powders

The rapid microstructural evolution of gas atomised Ni superalloy powder compacts over timescales of a few seconds was studied using a Gleeble 3500 thermomechanical simulator, finite element based numerical model and electron microscopy. The study found that the microstructural changes were governed by the characteristic temperatures of the alloy. At a temperature below the γ' solvus, the powders maintained dendritic structures. Above the γ' solvus temperature but in the solid-state, rapid grain spheroidisation and coarsening occurred, although the fine-scale microstructures were largely retained. Once the incipient melting temperature of the alloy was exceeded, microstructural change was rapid, and when the temperature was increased into the solid + liquid state, the powder compact partially melted and then re-solidified with no trace of the original structures, despite the fast timescales. The study reveals the relationship between short, severe thermal excursions and microstructural evolution in powder processed components, and gives guidance on the upper limit of temperature and time for powder-based processes if desirable fine-scale features of powders are to be preserved

Modeling the microstructural evolution during constrained sintering

A numerical model able to simulate solid-state constrained sintering is presented. The model couples an existing kinetic Monte Carlo (kMC) model for free sintering with a finite element model (FEM) for calculating stresses on a microstructural level. The microstructural response to the local stress as well as the FEM calculation of the stress field from the microstructural evolution is discussed. The sintering behavior of a sample constrained by a rigid substrate is simulated. The constrained sintering results in a larger number of pores near the substrate, as well as anisotropic sintering shrinkage, with significantly enhanced strain in the central upper part of the sample surface, and minimal strain at the edges near the substrate. All these features have also previously been observed experimentally.Comment: 9 pages, 7 figure

Computational and experimental thermo-mechanics of metal additive manufacturing : stress, warpage, cracks and properties

Tesi en modalitat de compendi de publicacions, amb una secció retallada per drets de l'editor.The objectives of this thesis are (i) to understand the thermal, metallurgical and mechanical behavior during AM, (ii) to shed light on the generation of residual stresses and the stress-induced deformations and cracks, and (iii) to further propose several effective strategies to control such defects. To improve the efficiency and reliability of the research investigation, an enhanced thermomechanical finite element framework for AM is developed and validated by numerous in-situ temperature and displacement measurement experiments. Furthermore, the calibrated model is employed to perform a large number of thermal and mechanical analyses in AM processes. For this purpose, Ti6Al4V titanium alloy is selected as the printing material for this investigation due to its wide application in aeronautics and astronautics. First of all, the effect of the complex thermal histories experienced on the metallurgical evolution and the formation of layer bands in multi-layer multi-pass Ti6Al4V blocks fabricated by laser directed energy deposition (DED) are explored. Based on the analysis of the predicted thermal histories and the experimental microstructure observation, the quantitative processing-thermal-microstructure-microhardness relationship is established. Next, the mechanical behavior of AM-components in terms of residual stresses, part warpages and cracks are analyzed in detail. Here, the influence of the scanning strategy on the heat transfer process and the evolution of the thermally induced mechanical variables in laser-based AM are studied to reduce residual stresses and deformations of rectangular DED-parts. Next, the thermal deformation of several different thin-walled structures printed by laser powder bed fusion (LPBF) are experimentally and numerically investigated in order to control the stress-induced warpages and to increase the geometrical precision of AM lightweight components. The generation of residual stresses and the key factors for their development are elucidated. A novel strategy to optimize the design of the substrate structures is proposed to mitigate the residual stresses induced by AM process. Moreover, a systematical evaluation on the effectiveness of different strategies to control the residual stresses in AM is carried out. Lastly, the formation mechanism of cracks is explored by analyzing the mechanical behavior of two T-shape parts deposited on different substrates without and with grooves. An innovative strategy to optimize the substrate geometry lowering its mechanical stiffness is proposed to prevent cracks during LPBF. Finally, a proposal to achieve high-quality Ti6Al4V AM-builds with lower residual stresses and homogeneous microstructures is detailed based on the better understanding on the process-structure-property interactions, and the formation and control of residual stresses in AM processes. This thesis presents further insight into the interactive thermal-metallurgical-mechanical behavior in metal AM and provides a comprehensive framework to guide AM designers to optimize process configuration when fabricating complex metal components.Los objetivos de esta tesis son (i) comprender el comportamiento térmico, metalúrgico y mecánico durante la AM, (ii) arrojar luz sobre la generación de tensiones residuales y las deformaciones y grietas inducidas por la tensión, y (iii) proponer además varias estrategias efectivas para controlar tales defectos. Para mejorar la eficiencia y la confiabilidad de la investigación, se desarrolla y valida un marco de elementos finitos termomecánicos mejorado para AM mediante numerosos experimentos de medición de desplazamiento y temperatura in situ. Además, el modelo calibrado se emplea para realizar una gran cantidad de análisis térmicos y mecánicos en procesos AM. Para este propósito, se selecciona la aleación de titanio Ti6Al4V como material de impresión para esta investigación debido a su amplia aplicación en aeronáutica y astronáutica. En primer lugar, se explora el efecto de las complejas historias térmicas experimentadas en la evolución metalúrgica y la formación de bandas de capas en bloques de Ti6Al4V multicapa y multipaso fabricados mediante deposición de energía dirigida por láser (DED). Con base en el análisis de las historias térmicas pronosticadas y la observación de la microestructura experimental, se establece la relación cuantitativa procesamiento-térmico-microestructura-microdureza. A continuación, se analiza en detalle el comportamiento mecánico de los componentes AM en términos de tensiones residuales, deformaciones parciales y grietas. Aquí, se estudia la influencia de la estrategia de escaneo en el proceso de transferencia de calor y la evolución de las variables mecánicas inducidas térmicamente en AM basada en láser para reducir las tensiones residuales y las deformaciones de las piezas DED rectangulares. A continuación, se investiga experimental y numéricamente la deformación térmica de varias estructuras diferentes de paredes delgadas impresas por fusión de lecho de polvo láser (LPBF) para controlar las deformaciones inducidas por el estrés y aumentar la precisión geométrica de los componentes ligeros AM. Se dilucidan la generación de tensiones residuales y los factores clave para su desarrollo. Se propone una estrategia novedosa para optimizar el diseño de las estructuras del sustrato para mitigar las tensiones residuales inducidas por el proceso AM. Además, se lleva a cabo una evaluación sistemática de la eficacia de diferentes estrategias para controlar las tensiones residuales en AM. Por último, se explora el mecanismo de formación de grietas analizando el comportamiento mecánico de dos piezas en forma de T depositadas sobre diferentes sustratos sin y con ranuras. Se propone una estrategia innovadora para optimizar la geometría del sustrato reduciendo su rigidez mecánica para evitar grietas durante LPBF. Finalmente, se detalla una propuesta para lograr construcciones AM de Ti6Al4V de alta calidad con tensiones residuales más bajas y microestructuras homogéneas basada en una mejor comprensión de las interacciones proceso-estructura-propiedad, y la formación y control de tensiones residuales en los procesos AM. Esta tesis presenta una visión más profunda del comportamiento termo-metalúrgico-mecánico interactivo en la fabricación aditiva de metal y proporciona un marco integral para guiar a los diseñadores de fabricación aditiva para optimizar la configuración del proceso al fabricar componentes metálicos complejos.Postprint (published version

Mechanics of the solid-state bonding under severe thermomechanical processes

Friction stir welding (FSW) has found increased applications in automotive and aerospace industries due to its advantages of solid-state bonding, no fusion and melting, and versatility in various working conditions and material combinations. The extent and quality of the solid-state bonding between workpieces in FSW is the ultimate outcome of their industrial applications. However, the relationship among processing parameters, material properties, and bonding extent and fidelity remains largely empirical, primarily because of the lack of the mechanistic understanding of (1) tool-workpiece frictional behavior, and (2) bonding formation and evolution. In this dissertation, to study the underlying mechanism of tool-workpiece frictional behavior and bonding evolution at workpiece-workpiece interface during solid-state bonding process, firstly, a numerical model that take advantage of Coupled Eulerian Lagrangian (CEL) method is implemented to investigate the stick-slip behavior at tool-workpiece interface. An analytical model is also developed to correlate the stick-slip fraction to processing parameters such as the tool spin rate, and further to derive dimensionless functions for torque and heat generation rate predictions. These analyses provide the critical strain rate and temperature fields that are needed for the bonding analysis. Then, we note that the interfacial solid state bonding process under applied thermomechanical loading histories is a reverse process of the high temperature creep fracture of polycrystalline materials by grain boundary cavities, in this regard, a general modeling framework of bonding fraction evolution was derived, which directly depends on the stress, strain rate, and temperature fields near the interface. Finally, Based on the stick-slip contact analysis and the understanding of solid-state bonding mechanism, an approximate yet analytical solution has been developed to derive the bonding fraction field from the given processing, geometric, and material constitutive parameters, and the predicted ultimate bonding extent with respect to these parameters becomes a figure of merit for the study of processing window for industrial applications and design of the FSW process

Thermomechanical Modeling of Microstructure Evolution Caused by Strain-Induced Crystallization

The present contribution deals with the thermomechanical modeling of the strain-induced crystallization in unfilled polymers. This phenomenon significantly influences mechanical and thermal properties of polymers and has to be taken into consideration when planning manufacturing processes as well as applications of the final product. In order to simultaneously capture both kinds of effects, the model proposed starts by introducing a triple decomposition of the deformation gradient and furthermore uses thermodynamic framework for material modeling based on the Coleman--Noll procedure and minimum principle of the dissipation potential, which requires suitable assumptions for the Helmholtz free energy and the dissipation potential. The chosen setup yields evolution equations which are able to simulate the formation and the degradation of crystalline regions accompanied by the temperature change during a cyclic tensile test. The boundary value problem corresponding to the described process includes the balance of linear momentum and balance of energy and serves as a basis for the numerical implementation within an FEM code. The~paper closes with the numerical examples showing the microstructure evolution and temperature distribution for different material samples