UNIVERSITÀ DEGLI STUDI DI PADOVA LEIBNIZ UNIVERSITÄT HANNOVER DIPARTIMENTODIINGEGNERIAINDUSTRIALE CORSODILAUREAMAGISTRALEININGEGNERIADELL’ENERGIAELETTRICA Optimization of Pulsed Magnetic Field Application for Electromagnetic Stirring during the Continuous Casting Author: MattiaGuglielmi[1111022] Supervisors: Pf. Dr.-Ing. EgbertBaake Ing. MicheleForzan Dipl.-Ing. DianaMusaeva AcademicYear2017/18 Tomygrandfather,Bruno. Acknowledgements My first acknowledgement goes to my supervisor, Prof. Michele Forzan, for his pre- cioushelp,guidanceandadviceduringmyMasterstudiesattheUniversityofPaduaand during the development of this Thesis work most of all. I am extremely thankful to Prof. EgbertBaakeandProf. BernardNacke,fromInstitutefürElektrothermischeProzesstech- nik, Univeristät Hannover for the possibility to carry on experimental activities in their institute. They also gave me the opportunity to attend the HES 2017 conference, which representedtomeagreatmotivationandmatterforpersonalgrowth. Aspecialacknowl- edgementgoestoDipl.-Ing. DianaMusaevaandThomasSteinbergfortheirstrongguid- anceandeverydaysupport. IamalsoindebtedtoallthecolleaguesoftheETPforsharing theirtimeandknowledgewithme,notonlyduringmyexperimentalactivities. The greatest thank you goes to my family, who always supported me with patience and understanding, every day and humbly. Sara, Mattia, Claudio and Licia, you all are my second family; thank you for challenging the freezing wind of Hannover for me. Claire, you are my happiness and greatest supporter; my time in Hannover would have not been the same without you. Marco and Fabio, you have been my fellows and guides, but my friends most of all; thank you for sharing the Erasmus adventure together. Thank you Michele for being my matter of motivation and mate during my whole studies at the University; we shared projects, exams and anxiety. Jan, Eike and Emma, you have been thebestroommates. Afinalacknowledgementgoestoallofthepeople,friendsandcolleaguesImetduring my Master studies and the development of my Thesis project especially. Each one of you taughtmesomethingIwillneverforget. Abstract The search for new materials with definite physical properties like homogeneity and high mechanical durability is one of the topics of the highest interest of the modern tech- nologyandindustry. Therefore,therestillremainsawidefieldforscientificinvestigations fordevelopingnewmethodsofmaterialproducingandimprovingexistingtechniques. In the frame of metal production, Continuous Casting is the most cost- and energy- efficient method to mass-produce semi-finished metal products with consistent quality in a variety of sizes and shapes. It transforms molten metal into solid in a continuous processandisthemostefficientwaytosolidifylargevolumesofmetalintosimpleshapes for subsequent processing. Most basic metals are presently mass-produced using such process, including over 750 million tons of steel, 20 million tons of aluminium and 1 milliontonsofcopper,nickelandothermetalsintheworldeachyear. In the majority of cases, properties of the cast metal are predicted with the conditions oftheirmeltingandsolidification,thusdefiningtheseconditionsisakeyforachievingre- quiredqualitiesinthematerial. Magneticfieldsareapowerfultoolforcontinuouscasting to provide expedient flow pattern in the molten material, with prospects for controlling themicrostructureofsolidifyingmetal. Useofthemagneticfieldsiswidelyusedbecause it allows for a completely contactless influence on the melt and a direct control of the flow intensity through changing electrical parameters and by varying the design of elec- tromagnetic (EM) field. Effective stirring of the liquid metal is provided, which brings tointenseheatandmasstransfer,thenhomogenizationoftemperaturefieldandchemical components in the liquid volume. It is easy to notice that efficient stirring of the melt is a key component of the casting process, affecting both the quality of the produced metal anditscost. Thisworkwasintendedtotheinvestigationofmeltflowbehaviourundertheinfluence ofelectromagneticstirring(EMS),whentransientandnotstandardconditionsoccuralong thecastingline: applicationofAMFonthefirstsolidificationareageneratestwotoroidal vortices within the liquid metal, similar to each other in terms of amplitude, velocity and energy. However, when cooling process is too fast or intense, the solidification front shiftsupwarddramaticvariationinthedimensionandvelocityofthetwovorticesoccurs; transition to a unique, large eddy modifies stirring conditions thus intense motion cannot be achieved anymore. Main goal of this thesis was to experimentally study the effect of low-frequency pulsed magnetic field (PMF) on Galinstan eutectic alloy, to optimize the stirring effect even during the transition of vortices and increase the rate of equiaxed structure within the solidifying metal. Application of AMF was initially investigated with with the use of numerical model: experimental activity was carried on in parallel to validate simulation results and tounderstand conditions when the transition happened; relative position between melt and the inductor was changed to simulate shifting of the solidification front. Once shape and velocity of initial vortices dramatically changed, their characteristic rotation frequency was calculated and PMF with the same frequency was applied, to achieve resonance between mechanical (eddies velocity) and electrical (inductor’sfeedingcurrent)parameters. Motionofmeltflowwasquantifiedwithvelocity of vortices and it was measured thanks to Ultrasound Doppler Velocity (UDV) meter; averagestandarddeviationandfastFouriertransform(fft)ofpulsedcomponentofvelocity werefinallycalculated. ThisthesisconsistsofAbstract,4Chapters,includingConclusionsandReferences. Chapter 1 gives a description of the Continuous Casting process, including limits and advantages for quality improvement of final products; stirring methods and finally objectivesofthisinvestigationaredescribed. Chapter 2 includes detailed theoretical background for dealing with the stirring pro- cess,thuselectromagnetic,thermalfieldsandhydrodynamicequations. In Chapter 3 numerical and experimental activities with the application of AMF are described,plusthefirstresultsfromthesimulation. Chapter 4 deals with optimization of the stirring effect, thus starts from previous results to properly apply PMF; results from PMF application are described at the end of theChapter. Sommario La ricerca di materiali sempre nuovi e con proprietà fisiche specifiche, quali buona omogeneità ed elevata resistenza meccanica, rappresenta ora un campo di forte interesse sia in ambito sperimentale che industriale. Per questo motivo, la ricerca scientifica sta cercando di sviluppare metodi sempre nuovi per la produzione dei diversi materiali, o di migliorareletecnologieesistenti. Il processo di Colata Continua (in inglese Continuous Casting) rappresenta il metodo più economico ed efficiente per la produzione di metalli semi-lavorati, caratterizzati da buona qualità, di diverse dimensioni e forme. Esso trasforma il metallo durante un pro- cessocontinuodisolidificazioneecostituisceperciòilmetodopiùefficacepersolidificare grandiquantitatividimetalloinformesemplici,finalizzateadunasuccessivalavorazione. Ogni anno nel mondo, grandi quantità di metallo di uso comune vengono prodotte sulla base di tale processo; si possono contare 750 milioni di tonnellate di acciaio, 20 di allu- minioed1milionedirame,nickel,oltreanumerosialtrimateriali. La conoscenza delle condizioni di fusione e solidificazione consente, nella maggior parte deicasi, dipredirre conbuona accuartezzale proprietàdel prodottofinito. Ottenere determinate qualità e proprietà dello stesso significa quindi conoscere a fondo le con- dizioni di lavorazione del materiale. Uno degli strumenti più influenti nel processo di colata consiste nell’impiego di campi magnetici, con lo scopo di generare mescolamenti di comportamento opportuno nel metallo fuso, quindi controllare la sua struttura micro- cristallina in fase di solidificazione. I campi magnetici consentono anche l’interazione col metallo senza alcun contatto ed un controllo diretto dell’intensità del mescolamento tramite opportuna variazione dei parametri elettrici. Grazie alle proprietà conduttive del metallo stesso, l’azione del mapo magnetico produce un effetto di stirring che omoge- nizza temperatura e proprietà chimiche nel liquido. Uno stirring efficace ed efficiente costituiscequindilachiaveperprodurremetallidibuonaqualitàedabassocosto. Obiettivo di questo lavoro di tesi è quello di analizzare il comportamento del met- allo fuso influenzato dal processo di stirring elettromagnetico durante la colata continua, in condizioni operative non standard. l’applicazione di un campo magnetico alternato (AMF) in fase di primo raffreddamento dà origine nel metallo liquido a due vortici di formatoroidalesimilitraloro,interminidiampiezza,velocitàedenergia. Concondizioni “non standard” ci si riferisce ad un processo di raffreddamento troppo veloce o intenso, tale per cui il fronte di solidificazione tende a muoversi verso l’alto, determinando una variazionedidimensioneevelocitàdeiduevorticiprodotti;siverificaquindiilpassaggio