Multiscale modelling of the influence of convection on dendrite formation and freckle initiation during vacuum arc remelting

Vacuum Arc Remelting (VAR) is employed to produce homogeneous ingots with a controlled, fine, microstructure. It is applied to reactive and segregation prone alloys where convection can influence microstructure and defect formation. In this study, a microscopic solidification model was extended to incorporate both forced and natural convection. The Navier-Stokes equations were solved for liquid and mushy zones using a modified projection method. The energy conservation and solute diffusion equations were solved via a combined stochastic nucleation approach along with a finite difference solution to simulate dendritic growth. This microscopic model was coupled to a 3D transient VAR model which was developed by using a multi-physics modelling software package, PHYSICA. The multiscale model enables simulations covering the range from dendrites (in microns) to the complete process (in meters). These numerical models were used to investigate: (i) the formation of dendritic microstructures under natural and forced convections; (ii) initiation of solute channels (freckles) in directional solidification in terms of interdendritic thermosolutal convection; and (iii) the macroscopic physical dynamics in VAR and their influence on freckle formation. 2D and 3D dendritic microstructure were simulated by taking into account both solutal and thermal diffusion for both constrained and unconstrained growth using the solidification model. For unconstrained equiaxed dendritic growth, forced convection was found to enhance dendritic growth in the upstream region while retarding downstream growth. In terms of dimensionality, dendritic growth in 3D is faster than 2D and convection promotes the coarsening of perpendicular arms and side branching in 3D. For constrained columnar dendritic growth, downward interdendritic convection is stopped by primary dendritic arms in 2D; this was not the case in 3D. Consequently, 3D simulations must be used when studying thermosolutal convection during solidification, since 2D simulations lead to inappropriate results. The microscopic model was also used to study the initiation of freckles for Pb-Sn alloys, predicting solute channel formation during directional solidification at a microstructural level for the first time. These simulations show that the local remelting due to high solute concentrations and continuous upward convection of segregated liquid result in the formation of sustained open solute channels. High initial Sn compositions, low casting speeds and low temperature gradients, all promote the initiation of these solute channels and hence freckles. to study the initiation of freckles for Pb-Sn alloys, predicting solute channel formation during directional solidification at a microstructural level for the first time. These simulations show that the local remelting due to high solute concentrations and continuous upward convection of segregated liquid result in the formation of sustained open solute channels. High initial Sn compositions, low casting speeds and low temperature gradients, all promote the initiation of these solute channels and hence freckles

Fatigue life extension in existing steel bridges. High-Frequency Mechanical Impact treatment and Tungsten Inert Gas remelting in life extension and fatigue crack repair of welded steel structures

This thesis investigates the performance of improved welds with two post-weld treatment methods for application on existing structures. High-Frequency Mechanical Impact (HFMI) treatment and Tungsten Inert Gas (TIG) remelting were used for fatigue life extension of welded structures. Axial fatigue testing was conducted on transversal non-load-carrying attachment treated via the investigated methods. Furthermore, more than 250 test results on different treated welded details were collected, sorted and analysed. HFMI-treatment was found to give a significant fatigue life extension even with the presence of cracks up to 2.25 mm. On the other hand, the efficiency of TIG-remelting was also proven when the crack was completely eliminated after remelting. Even if a small part of the crack remains after remelting, fair fatigue life could be expected. However, it is recommended to use HFMI-treatment or TIG-remelting only when the crack inspection is negative before and after treatment respectively.Complimentary studies showed that the investigated methods induced compressive residual stress, increased the smoothness of the weld toe, increased the local hardness and changed the angular distortion status locally. Moreover, TIG-remelting changed the microstructure in both the fusion zone and the heat-affected zone. HFMI-treatment changed the crack orientation, induced compressive plasticity at the crack tip and caused crack narrowing or even closure. However, these effects were less significant for deeper cracks. Moreover, some practical aspects of the treatment application were investigated. Unlike treating new structures, TIG-electrode should be placed at the weld toe to secure that the maximum fusion depth corresponds to the crack plane. On the other hand, HFMI-indentor should be slanted more toward the base metal than the weld to avoid unintentional crack opening. Moreover, the IIW recommendations for both HFMI-treatment inclination and indentation depth could be extended to cracked structures. The aforementioned investigated parameters (i.e. residual stress, distortions, local hardness and toe\u27s smoothness) were incorporated in fatigue life predictions for both treatment methods. The base metal S-N curve was used to predict the life of specimens treated via TIG-remelting, while Paris law was used to track the crack propagation of HFMI-treated details. The results corresponded well with fatigue test results. Combining TIG-remelting with HFMI-treatment resulted in welds with higher fatigue strength because of the combined effects of crack closure via TIG-remelting and compressive plasticity via HFMI-treatment

Direct test of defect mediated laser induced melting theory for two dimensional solids

We investigate by direct numerical solution of appropriate renormalization flow equations, the validity of a recent dislocation unbinding theory for laser induced freezing/melting in two dimensions. The bare elastic moduli and dislocation fugacities which are inputs to the flow equations are obtained for three different 2-d systems (hard disk, inverse $12^{th}$ power and the Derjaguin-Landau-Verwey-Overbeek potentials) from a restricted Monte Carlo simulation sampling only configurations {\em without} dislocations. We conclude that (a) the flow equations need to be correct at least up to third order in defect fugacity to reproduce meaningful results, (b) there is excellent quantitative agreement between our results and earlier conventional Monte Carlo simulations for the hard disk system and (c) while the qualitative form of the phase diagram is reproduced for systems with soft potentials there is some quantitative discrepancy which we explain.Comment: 11 pages, 14 figures, submitted to Phys. Rev.

Spurious Grain Formation During Directional Solidification in Microgravity

This research is aimed at carrying out a systematic investigation of the nucleation, and growth of spurious “misoriented” grains during directional solidification in the low gravity environment of space. Three Al–7 wt. % Si alloy cylindrical samples (MICAST-6, MICAST-7 and MICAST2-12) were directionally solidified on the Space Station at growth speeds varying from 5 to 50 µms-1 under thermal gradients varying from 14 to 33 K cm-1 in alumina crucibles, under a joint NASA-ESA (European Space Agency) project called, MICAST (Microstructure formation in casting of technical alloys under a diffusive and magnetically controlled convection conditions). The primary purpose of directionally solidifying these three Al-7Si samples in the low gravity environment of space was to eliminate gravity-induced convection in the melt, and grow dendrite array morphology under purely diffusive transport conditions. However, when these directionally solidified samples were extracted from their alumina crucibles, they all showed evidence of surface pores along their length. We believe that these pores formed because in microgravity, there is no imposed force to pull the liquid column down on to the solidifying portion below to continue to feed the volume shrinkage due to liquid to solid phase transformation. There was no additional built-in mechanism, such as a piston and spring, in the MICAST ampoules to keep the melt column pressed onto the solid below. We also believe that even in the absence of gravity, a liquid coulmn which gets detached from the crucible internal waals (forming surface pores), under an imposed positive thermal gradient, would lead to the liquid-solid surface energy driven Marangoni convection. This convection may fragment fragile secondary or tertiary arms of the primary dendrite trees growing in the mushy zone. These broken dendrite solid fragments may lead to the nucleation and growth of spurious grains in the MICAST samples, where the orientation of primary dendrites would be very different from those in their unmelted seed portions. Our purpose is to examine the longitudinal and transverse microstructures of these MICAST samples to study the formation of spurious grains, and investigate if there is any correlation between the location of the observed surface pores and the formation of spurious grains

Spurious Grain Formation During Directional Solidification in Microgravity

This research is aimed at carrying out a systematic investigation of the nucleation, and growth of spurious “misoriented” grains during directional solidification in the low gravity environment of space. Three Al–7 wt. % Si alloy cylindrical samples (MICAST-6, MICAST-7 and MICAST2-12) were directionally solidified on the Space Station at growth speeds varying from 5 to 50 µms-1 under thermal gradients varying from 14 to 33 K cm-1 in alumina crucibles, under a joint NASA-ESA (European Space Agency) project called, MICAST (Microstructure formation in casting of technical alloys under a diffusive and magnetically controlled convection conditions). The primary purpose of directionally solidifying these three Al-7Si samples in the low gravity environment of space was to eliminate gravity-induced convection in the melt, and grow dendrite array morphology under purely diffusive transport conditions. However, when these directionally solidified samples were extracted from their alumina crucibles, they all showed evidence of surface pores along their length. We believe that these pores formed because in microgravity, there is no imposed force to pull the liquid column down on to the solidifying portion below to continue to feed the volume shrinkage due to liquid to solid phase transformation. There was no additional built-in mechanism, such as a piston and spring, in the MICAST ampoules to keep the melt column pressed onto the solid below. We also believe that even in the absence of gravity, a liquid coulmn which gets detached from the crucible internal waals (forming surface pores), under an imposed positive thermal gradient, would lead to the liquid-solid surface energy driven Marangoni convection. This convection may fragment fragile secondary or tertiary arms of the primary dendrite trees growing in the mushy zone. These broken dendrite solid fragments may lead to the nucleation and growth of spurious grains in the MICAST samples, where the orientation of primary dendrites would be very different from those in their unmelted seed portions. Our purpose is to examine the longitudinal and transverse microstructures of these MICAST samples to study the formation of spurious grains, and investigate if there is any correlation between the location of the observed surface pores and the formation of spurious grains

Exploring semi-solid alloy deformation with discrete element method simulations and synchrotron radiography

Semi-solid alloys are deformed in a wide range of pressurised casting processes; an improved understanding and modelling capability are required to minimise defect formation and optimise productivity. This thesis combines thin-sample in-situ X-ray radiography of semisolid Al-Cu alloy deformation with 2D coupled lattice Boltzmann method – discrete element method (LBM-DEM) simulations. Mechanisms of strain heterogeneity and localisation are identified during semi-solid deformation in globular Al-Cu alloys with various combinations of initial solid fraction and strain rates. The calibrated set of LBM-DEM simulations is then used to obtain information that is not available in X-ray imaging to extract deeper insights into the semi-solid deformation behaviours observed in the experiments. It is found that the local contraction and dilation of the percolating grain assembly are highly influenced by the initial solid fraction. When deforming a low solid fraction alloy, macroscopic contraction due to grains being pushed together increases the local liquid pressure and expels liquid from the sample surface. In contrast, deforming high solid fraction alloys leads to macroscopic shear-induced dilation by grains pushing each other apart and surface menisci are sucked-in due to the decrease in interstitial liquid pressure. It is also shown that the macroscopic behaviour of semi-solid alloy deformation undergoes a range of rheological transitions with increasing solid fraction. First from a suspension to a percolating solid network and, later, from net dilation to shear cracking. These transitions are investigated with LBM-DEM simulations, and the transition to shear cracking is shown to be related to the local decrease in interstitial liquid pressure caused by shear-induced dilation. The verified coupled LBM-DEM simulation is shown to exhibit a load:deformation response consistent with the critical state framework of soil mechanics, indicating that this approach can be useful for modelling thermomechanics in casting processes.Open Acces

Composition effects on macroscopic solidification segregation of superalloys

This research work investigates primarily the composition effects on solidification macrosegregation, i.e. freckles, in superalloys. First, the freckling mechanism of superalloys is developed using the physical simulation of freckle formation in specially designed model alloys. It is found that freckles originate from dendrite irregularities at the solidification front. The freckle channels flow downward in Nb-containing alloys and upward in T-containing alloy under horizontal solidification conditions. The horizontal solidification correctly simulates the horizontal component in remelt ingot where freckling potential is the highest. Based on the result, key control parameters for freckling are identified.;Thermodynamic simulation approach is employed to study the solidification behavior of superalloys. The theoretical prediction yields relative good agreement with experimental observations of freezing range, solidification sequence, and the occurrence of secondary phases. Solidification diagram of Ni-Cr-Fe-Nb alloy system is developed using the simulation approach to predict the phase relationship during solidification. The simulation results can be used as inputs for freckling criteria.;The sensitivity of freckling criteria relies on the accurate acquisition of the attributes that are critical to the key control parameters. These attributes include interdendritic liquid composition, temperature and fraction of solid. Two techniques are developed in the present study to obtain their correlation. From here, liquid composition and fraction solid at any temperature in the mushy zone can be obtained.;Composition effects on the key control parameters are investigated in detail. Liquid density during solidification depends on the composition and the temperature of the liquid. A significant temperature effect on density change is found. Regression analysis shows that Nb has more pronounced effect than Ti in reducing the fraction solid. The dendrite arm spacing and the solidification front angle are largely dependent on the processing conditions.;Freckling criterion in remelting of superalloys is developed at the end of this study. A composition related is proposed to account for the alloying effect on freckling tendency. The model correctly predicts the freckling pattern in Ti- and Nb-containing model alloys. It also correctly predicts the freckling tendency of industrial alloys

Application of high-frequency mechanical impact treatment for fatigue strength improvement of new and existing bridges

This thesis investigates the application of High-Frequency Mechanical Impact (HFMI) treatment for fatigue strength improvement of weldments in existing and new bridges. In the former case, the welds have already been subjected to fatigue loading and accumulated damage before treatment. A fatigue testing program is set up, comprising welded specimens subjected to fatigue loading before HFMI treatment to investigate the efficiency of HFMI treatment on existing structures. Moreover, additional fatigue test results are collected from the literature and analyzed. HFMI treatment is found to be very efficient in extending the fatigue lives of existing structures regardless of the accumulated fatigue damage prior to treatment, given that any surface cracks, if exist, have not grown more than 2.25 mm in depth. For practical applications, HFMI treatment is only recommended if the critical details are verified to be free of any surface cracks. Remelting the surface with a tungsten electrode before HFMI treatment is another solution which has rarely been studied on existing structures. Therefore, several experimental investigations are conducted including fatigue testing, measurement of residual stress, hardness testing and scanning the welds topography to study the effect of combining these two post-weld treatment techniques. The combined treatment is found to be efficient as it induces higher and deeper compressive residual stress and local hardening. These aspects are all considered in numerical simulations conducted to investigate the fatigue behaviour of new and existing weldments treated using this combination. The results verify the superiority of the combined treatment to both individual treatments (TIG & HFMI). Nonetheless, because of the complexity associated with TIG remelting, the combination is only suggested for existing structures containing shallow fatigue cracks which can be fused by a tungsten electrode. One major hindrance to applying HFMI treatment on weldments in steel bridges is the lack of design rules and recommendations such as consideration of stress ratio (mean stress) and overloads.\ua0 Therefore, a correction factor (λHFMI) to account for the mean stress effect is derived. This factor is used to magnify the design stress range for fatigue verification of HFMI-treated welded details existing in road and railway bridges. λHFMI is calibrated using measured traffic data that includes millions of vehicles and hundreds of trains. In addition, the characteristic load combination associated with the serviceability limit state is found to be the most appropriate for verifying the maximum stresses in road bridges. Based on the work conducted in this thesis, a complete methodology is proposed for the design and assessment of HFMI-treated welded details in new and existing steel bridges. Finally, the effect of corrosion on the performance of HFMI-treated weldments is studied by analyzing collected test results. Despite the observed reduction in fatigue endurance of HFMI-treated details due to the removal of top layers improved by residual stresses, the obtained fatigue lives are still longer than the design lives assigned for new welded details even in extreme corrosion conditions. However, corrosion protection and removal of sharp HFMI groove edges via light grinding are still necessary to reduce the susceptibility of weldments to corrosion

Advances in Laser Materials Processing

Laser processing has become more relevant today due to its fast adaptation to the most critical technological tasks, its ability to provide processing in the most rarefied and aggressive mediums (vacuum conditions), its wide field of potential applications, and the green aspects related to the absence of industrial cutting chips and dust. With the development of 3D production, laser processing has received renewed interest associated with its ability to achieve pointed to high-precision powder melting or sintering. New technologies and equipment, which improve and modify optical laser parameters, contribute to better absorption of laser energy by metals or powder surfaces and allow for multiplying laser power that can positively influence the industrial spread of the laser in mass production and advance the existing manufacturing methods. The latest achievements in laser processing have become a relevant topic in the most authoritative scientific journals and conferences in the last half-century. Advances in laser processing have received multiple awards in the most prestigious competitions and exhibitions worldwide and at international scientific events. The Special Issue is devoted to the most recent achievements in the laser processing of various materials, such as cast irons, tool steels, high entropy alloys, hard-to-remelt materials, cement mortars, and post-processing and innovative manufacturing based on a laser

Numerical modelling of cold crucible induction melting (CCIM) process and fabrication of high value added components of titanium and its alloys

This dissertation concerns the development of a numerical modelling of cold crucible induction melting (CCIM) and the fabrication of high value added components of titanium and its alloys. Titanium and its alloys have emerged as a very attractive metal for numerous applications: medical prostheses, aerospace industry, automotive industry, power generation, sport equipment, and marine engineering. The reason lie in their attractive properties, such as excellent biocompatibility, high specific strength, excellent corrosion resistance, excellent high temperature creep resistance, and good fracture toughness. However, the application of titanium is often limited by its relatively high cost. This high cost of titanium makes casting very attractive route. However, is it difficult to cast these alloys by conventional casting techniques because of the titanium reactivity at high temperatures, which reacts with the crucible and mould components. The CCIM process is currently the most effective means of melting these alloys. The CCIM is an innovative process to melt high melting point reactive materials such as titanium alloys. The melting and casting of the material is performed in vacuum or in a protective atmosphere in order to prevent any contamination of the charge. Moreover, a water cooled segmented crucible is used instead of a ceramic crucible to avoid any kind of reaction among the charge and the crucible. The magnetic field generated by an external coil penetrates through the slits of the crucible and generates induced currents in the charge, which are responsible of melting it due to Joule heating. The drawbacks of this process are the poor efficiency due to great percentage of heat that is removed by the cooling system and the small superheat of the melt, which can cause solidification problems. In this dissertation, we have selected the CCIM process to melt and cast titanium alloys. The aim of this dissertation consists on increasing the scientific knowledge about the CCIM process by means of both a numerical and an experimental approach. The main part of the dissertation focuses on the development of a numerical modelling of CCIM to optimize of the main parameters of the process. The task of optimizing melt superheat faces the challenge of finding optimal combination of crucible height to diameter ratio, number of inductor turns, crucible design, current strength, and frequency. Variation of any of the after mentioned factors influences the shape of melt meniscus and, as a result, flow pattern and energy balance. The second part deals with the set-up of an installation of CCIM and the fabrications of titanium components. As a result of the present work some goals have been achieved, being the most important: a) Development of numerical modelling of CCIM, b) setting up of a CCIM installation, and c) casting of titanium parts.Tesi honek “Cold crucible induction melting (CCIM)” prozesuaren simulazio numerikoaren gainean eta prozesu honen bidez titaniozko balio erantsi altuko osagaiak ekoizteko modua tratatzen du. Titanioak eta bere aleazioek interes handia sortu dute aplikazio industrial askotan: mediku protesiak, aeronautika, automozioa, energia generazioa, kirol ekipamendua eta itsas ingeniaritza. Arrazoia bere ezaugarri erakargarrietan errotzen da: biokonpatibilitate bikaina, erresistentzia espezifiko altua, korrosioaren aurkako erresistentzia ezin hobea, tenperatura alturako isurpenaren aurkako erresistentzia paregabea eta hausturaren aurkako erresistentzia. Hala ere, bere kostu altuak bere aplikazioak murrizten ditu. Galdaketa-prozesuek kostu txikiagoko produktuetara daramate. Hala ere, zaila da aleazio hauek galdaketa prozesu konbentzionalekin urtzea, tenperatura handitan erreaktibotasun handia daukate eta. CCIM prozesua aleazio hauek galdatzeko prozesu eraginkorren arten dago gaur egun. CCIM prozesua material erreaktiboak urtzeko prozesu berritzailea da. Bai urtze bai galdaketa hutsean edo atmosfera babesle baten egiten da materialaren erreakzioa saihesteko. Gainera, ohiko zeramikozko arragoen ordez segmentudun kobrezko arragoa erabiltzen da. Kanpoko harilak sortutako kanpo magnetikoa arragoaren arteketatik barneratzen da eta indukziozko korronteak sortzen ditu kargan, karga bera urtuz Joule beroketagatik. Prozesu honen arazoak efizientzia eskasa (hozte sistemak xurgatzen duen beroagatik) eta solidotzearazoak eragin ditzakeen gainberotze txikia dira. Tesi honetan, CCIM prozesua aukeratu dugu titaniozko aleazioak galdaketa prozesuaren bidez fabrikatzeko. Tesi honen helburua CCIM prozesuaren gaineko ezaguera zientifikoa handitzean datza bai ikuspegi teorikoa bai ikuspegi esperimentala erabiliz. Tesiko alderdi nagusia prozesuaren parametro nagusiak optimizatzeko CCIM prozesuaren zenbakizko modelizazioaren garapenaran gainean tratatzen du. Gainberotze tenperatura optimizatzeko zeregina arragoaren altura diametro ratioa, harilaren espira kopurua, arragoaren diseinua, korrontea eta frekuentziaren balio optimoa aurkitzean datza. Aipatutako faktoreen edozein aldaketek meniskoaren egoeran eragiten du, eta ondorioz, jariakinaren patroian eta energi balantzean. Tesiaren bigarren atalak CCIM instalazio bat abiarazteaz eta titaniozko osagaiak fabrikatzeaz dihardu. Lan honen emaitz garrantzitsuenak hurrengokoak dira: a) CCIM prozesuaren modelo numerikoaren garapena. b) CCIM prozesuaren instalazio baten abiaraztea. c) Titaniozko piezen galdaketa.Esta tesis trata sobre el desarrollo de un modelo numérico del “cold crucible induction melting (CCIM)” y la fabricación de componentes de alto valor añadido de titanio y sus aleaciones mediante este proceso. El titanio y sus aleaciones se han convertido en un metal muy atractivo para numerosas aplicaciones: prótesis médicas, industria aeroespacial, industria de automoción, generación de energía, deporte e ingeniería marina. La razón radica en sus propiedades atractivas, tales como excelente biocompatibilidad, alta resistencia específica, excelente resistencia a la corrosión, excelente resistencia a la fluencia a alta temperatura y buena resistencia a la fractura. Sin embargo, la aplicación de titanio es a menudo limitada por su coste relativamente alto. Los procesos de fundición conducen a productos de menores costes. Sin embargo, es difícil fundir estas aleaciones por técnicas de moldeo convencionales, debido a la reactividad de titanio a altas temperaturas, que reacciona con el crisol y molde. El proceso CCIM es actualmente el medio más eficaz de fusión de estas aleaciones. El CCIM es un proceso innovador en la que la fusión y colada del material se realiza bajo vacío o dentro de una atmósfera protectora y donde se utiliza un crisol refrigerado segmentado de cobre en vez de los habituales crisoles cerámicos para evitar cualquier tipo de reacción entre la carga y el crisol. El campo magnético generado por una bobina externa penetra a través de las ranuras del crisol y genera corrientes inducidas en la carga, las cuales son las responsables de la fusión debido al calentamiento Joule. Los inconvenientes de este proceso son la baja eficiencia debido al gran porcentaje de calor que se elimina por el sistema de refrigeración y el pequeño sobrecalentamiento del metal fundido, que puede causar problemas de solidificación. En esta tesis, hemos seleccionado el proceso CCIM para fundir y colar las aleaciones de titanio. El objetivo de esta tesis consiste en aumentar el conocimiento científico sobre el proceso CCIM tanto de un modo numérico como un modo experimental. La parte principal de la tesis se centra en el desarrollo de un modelo numérico de CCIM para optimizar de los principales parámetros del proceso. La tarea de optimizar sobrecalentamiento se enfrenta al reto de encontrar la combinación óptima de la altura del crisol a diámetro, número de espiras del inductor, diseño del crisol, intensidad de corriente y frecuencia. La variación de cualquiera de los factores mencionados influye en la forma del menisco del metal líquido y, como resultado, el patrón del fluido y el balance de energía. La segunda parte trata de la puesta en marcha de una instalación de CCIM y la fabricación de componentes de alto valor añadido de titanio. Como resultado de este trabajo se han logrado algunos objetivos, siendo los más importantes: a) Desarrollo de modelos numéricos del CCIM. b) Puesta a punto de una instalación CCIM. c) Fundición de piezas de titanio