The present thesis investigates the grain refinement in solidifying Al-7wt%Si hypoeutectic alloy driven by electric currents. The grain size reduction in alloys generated by electric currents during the solidification has been intensively investigated. However, since various effects of electric currents have the potential to generate the finer equiaxed grains, it is still argued which effect plays the key role in the grain refinement process. In addition, the knowledge about the grain refinement mechanism under the application of electric currents remains fragmentary and inconsistent. Hence, the research objectives of the present thesis focus on the role of electric current effects and the grain refinement mechanism under the application of electric currents. Chapter 1 presents an introduction with respect to the subject of grain refinement in alloys driven by electric current during the solidification process in particular, including the research objectives; the research motivation; a brief review about the research history; a short introduction on the electric currents effects and a review relevant to the research status of grain refinement mechanism. Chapter 2 gives a description of research methods. This chapter shows the employed experiment materials, experimental setup, experimental procedure, the analysis methods of solidified samples, and numerical method, respectively. Chapter 3 focuses on the role of electric current effects in the grain refinement process. A series of solidification experiments are performed under various values of effective electric currents for both, electric current pulse and direct current. The corresponding temperature measurements and flow measurements are carried out with the increase of effective electric current intensity. Meanwhile, numerical simulations are conducted to present the details of the flow structure and the distribution of electric current density and electromagnetic force. Finally, the role of electric current effects is discussed to find the key effect in the grain refinement driven by electric currents. Chapter 4 investigates the grain refinement mechanism driven by electric currents. This chapter mainly focuses on the origin of finer equiaxed grain for grain refinement under the application of electric current on account of the importance of the origin for understanding the grain refinement mechanism. A series of solidification experiments are carried out in Al-7wt%Si alloy and in high purity aluminum. The main origin of equiaxed grain for grain refinement is concluded based on the experiment results. Chapter 5 presents three further investigations based on the achieved knowledge in chapter 3 and 4 about the role of electric current effects and the grain refinement mechanism. According to the insight into the key electric current effect for the grain refinement shown in chapter 3, this chapter presents a potential approach to promote the grain refinement. In addition, the solute distribution under the influence of electric current is examined based on the knowledge about the electric current effects. Moreover, the grain refinement mechanism under application of travelling magnetic field is investigated by performing a series of solidification experiments to compare with the experiments about the grain refinement mechanism driven by electric currents shown in chapter 4. Chapter 6 summarizes the main conclusions from the presented work.:Abstract VII Contents IX List of figures XI List of tables XVII 1. Introduction 1 1.1 Research objectives 1 1.2 Research motivation 2 1.3 Research history 5 1.4 Electric currents effects 9 1.4.1 Some fundamentals 10 1.4.2 Role of electric currents effects in grain refinement 12 1.5 Grain refinement mechanism 13 1.5.1 Nucleation theory 13 1.5.2 Equiaxed grain formation without the application of external fields 18 1.5.3 Grain refinement mechanism under the application of electric currents 23 1.5.4 Grain refinement mechanism under the application of magnetic field 29 2. Research methods 31 2.1 Introduction 31 2.2 Experimental materials 31 2.2.1 Solidification 31 2.2.2 Similarity of GaInSn liquid metal and Al-Si melt 32 2.3 Experimental setup 33 2.3.1 Solidification 33 2.3.2 Flow measurements 35 2.3.3 External energy fields 36 2.4 Experimental procedure 38 2.4.1 Solidification 38 2.4.2 Flow measurements 39 2.5 Metallography 39 2.6 Numerical method 41 2.6.1 Numerical model 41 2.6.2 Numerical domain and boundary conditions 42 3. Role of electric currents effects in the grain refinement 45 3.1 Introduction 45 3.2 Experimental parameter 45 3.3 Results 46 3.3.1 Solidified structure 46 3.3.2 Forced melt flow 50 3.3.3 Temperature distribution 58 3.4 Discussion 61 3.5 Conclusions 67 4. Grain refinement mechanism driven by electric currents 69 4.1 Introduction 69 4.2 Experimental parameter 69 4.3 Results 73 4.3.1 Solidified structure of Al-Si alloy 73 4.3.2 Cooling curves of Al-Si alloy 77 4.3.3 Solidified structure of high purity aluminum 78 4.4 Discussion 80 4.5 Conclusions 83 5. Supplemental investigations 85 5.1 A potential approach to improve the grain refinement 85 5.1.1 Introduction 85 5.1.2 Experimental parameter 86 5.1.3 Results and discussion 87 5.2 Macrosegregation formation 90 5.2.1 Introduction 90 5.2.2 Experimental parameter 91 5.2.3 Results and discussion 92 5.3 Grain refinement driven by TMF 97 5.3.1 Introduction 97 5.3.2 Experimental parameter 97 5.3.3 Results and discussion 98 5.4 Conclusions 102 6. Summary 103 Bibliography 105

Thermoelectric (TE) ZrNiSn samples with a half-Heusler atomic structure were synthesized by mechanical alloying (MA) and consolidation by either Spark Plasma Sintering (SPS) or hot pressing (HP). X-Ray diffraction patterns of as milled powders and consolidated samples were compared and analyzed for phase purity. Thermal conductivity, electrical conductivity and Seebeck coefficient are measured as a function of temperature in the range 300 K to 800 K and compared with measurements reported for high temperature solid state reaction synthesis of this compound. HP samples, compared to SPS samples, demonstrate increased grain growth due to longer heating times. Reduced grain size achieved by MA and SPS causes increased phonon scattering due to the increased number of grain boundaries, which lowers the thermal conductivity without doping the base system with addition phonon scattering centers. Mechanical characterization of the samples by microindentation and depth sensing indentation for hardness and elastic modulus will be discussed.

This thesis work deals with the volume changes during the solidification of cast irons and peritectic steels. The volume changes in casting metals are related to the expansion and/or contraction of the molten metal during solidification. Often, different types of shrinkage, namely macro- and micro-shrinkage, affect the casting quality. In addition to that, exposure of the metal casting to higher contraction or expansion during the solidification might also be related to internal strain development in samples, which eventually leads to surface crack propagation in some types of steel alloys during continuous casting. In consequence, a deep understanding of the mechanisms and control of the solidification will improve casting quality and production. All of the experiments during the entire work were carried out on laboratory scale samples. Displacement changes during solidification were measured with the help of a Linear Variable Displacement Transformer (LVDT). All of the LVDT experiments were performed on samples inside a sand mould. Simultaneously, the cooling curves of the respective samples during solidification were recorded with a thermocouple. By combining the displacement and cooling curves, the volume changes was evaluated and later used to explain the influence of inoculants, carbon and cooling rates on volume shrinkages of the casting. Hypoeutectic grey cast iron (GCI) and nodular cast iron (NCI) with hypo-, hyper- and eutectic carbon compositions were considered in the experiments from cast iron group. High nickel alloy steel (Sandvik Sanbar 64) was also used from peritectic steel type. These materials were melted inside an induction furnace and treated with different types of inoculants before and during pouring in order to modify the composition. Samples that were taken from the LVDT experiments were investigated using a number of different  methods in order to support the observations from the displacement measurements:  Differential Thermal Analysis (DTA), to evaluate the different phase present; Dilatometry, to see the effect of cooling rates on contraction for the various types of alloys; metallographic studies with optical microscopy; Backscattered electrons (BSE) analysis on SEM S-3700N, to investigate the different types of oxide and sulphide nuclei; and bulk density measurements  by applying Archimedes' principle. Furthermore, the experimental volume expansion during solidification was compared with the theoretically calculated values for GCI and NCI. It was found that the casting shows hardly any shrinkage during early solidification in GCI, but in the eutectic region the casting expands until the end of solidification. The measured and the calculated volume changes are close to one another, but the former shows more expansion. The addition of MBZCAS (Si, Ca, Zr, Ba, Mn and Al) promotes more flake graphite, and ASSC (Si, Ca, Sr and Al) does not increase the number of eutectic cells by much. In addition to that, it lowers the primary austenite fraction, promotes more eutectic growth and decreases undercooled graphite and secondary dendritic arm spacing (SDAS). As a result, the volume expansion changes in the eutectic region. The expansion during the eutectic growth increase with an increase in the inoculant weight percentage. At the same time, the eutectic cells become smaller and increase in number. The effect of the inoculant and the superheat temperature shows a variation in the degree of expansion/contraction and the cooling rates for the experiments. Effective inoculation tends to homogenize the eutectic structure, reducing the undercooled and interdendritic graphite throughout the structure. In NCI experiments, it was found that the samples showed no expansion in the transversal direction due to higher micro-shrinkages in the centre, whereas in the longitudinal direction the samples shows expansion until solidification was complete.   The theoretical and measured volume changes agreed with each other. The austenite fraction and number of micro-shrinkage pores decreased with increase in carbon content. The nodule count and distribution changes with carbon content. The thermal contraction of NCI is not influenced by the variation in carbon content at lower cooling rates. The structural analysis and solidification simulation results for NCI show that the nodule size and count distribution along the cross-sections at various locations are different due to the variation in cooling rates and carbon concentration. Finer nodule graphite appears in the thinner sections and close to the mold walls. A coarser structure is distributed mostly in the last solidified location. The simulation result indicates that finer nodules are associated with higher cooling rate and a lower degree of microsegregation, whereas the coarser nodules are related to lower cooling rate and a higher degree of microsegregation. As a result, this structural variation influences the micro-shrinkage in different parts. The displacement change measurements show that the peritectic steel expands and/or contracts during the solidification. The primary austenite precipitation during the solidification in the metastable region is accompanied by gradual expansion on the casting sides. Primary δ-ferrite precipitation under stable phase diagram is complemented by a severe contraction during solidification. The microstructural analysis reveals that the only difference between the samples is grain refinement with Ti addition. Moreover, the severe contraction in solidification region might be the source for the crack formation due to strain development, and further theoretical analysis is required in the future to verify this observation. / <p>QC 20170228</p>

Objetivo principal desta dissertação foi verificar os efeitos da temperatura de laminação na formação de martensita induzida por deformação e na sua posterior reversão da martensita para austenita no aço inoxidável austenítico AISI 304L. O estudo foi predominantemente microestrutural e para análise e caracterização foram utilizadas as técnicas de microscopia óptica, microscopia eletrônica de varredura, difração de raios X, medidas de dureza Vickers e medidas de fases ferromagnéticas por ferritoscopia. As amostras foram inicialmente solubilizadas a 1100 ºC por uma hora, visando a dissolução de uma pequena quantidade residual de ferrita encontrada nas amostras na condição como recebida, depois laminadas em diferentes temperaturas, determinando-se curvas de endurecimento por deformação e de formação de martensita induzida por deformação em função do grau de deformação. Em seguida, foram realizados pré-recozimentos a 600 ºC, favorecendo apenas a reversão da martensita para austenita, de maneira que não ocorresse a recristalização. A quantidade e a temperatura de deformação apresentaram forte influência na quantidade de martensita formada, no endurecimento por deformação e na cinética de amolecimento durante o recozimento. Os pré-tratamentos realizados a 600 ºC causaram acentuada reversão da martensita, algum amolecimento e pequeno efeito no tamanho de grão recristalizado durante o posterior recozimento a 600 ºC. / The main objective of the present dissertation was to verify the effects of the rolling temperature on the formation of strain induced martensite and in its subsequent martensite reversion to austenite in a AISI 304L stainless steel. The study was predominantly microstructural and, for the analysis and characterization, several techniques have been used, namely optical microscopy, scanning electron microscopy, X-ray diffraction, Vickers hardness measurements and magnetic phase measurements, using the ferritoscope. The samples were initially solution annealed at 1100 ºC for one hour, aiming at the dissolution of a small quantity of the residual -ferrite found in the samples in the as-received condition; then rolling was performed at different temperatures, evaluating strain hardening and the strain induced martensite as a function of strain. Following, pre-annealing treatments at 600 ºC have been performed, favoring only the martensite to austenite reversion, in a way that no recrystallization would occur. Strain and temperature had a strong influence on the amount of formed martensite, on the strain hardening and on the softening kinetics during annealing. The pre-annealing treatments at 600 ºC caused an accentuated effect on the martensite reversion, some softening and a small effect on the recrystallized grain size during the subsequent annealing at 600 ºC.

Objetivo principal desta dissertação foi verificar os efeitos da temperatura de laminação na formação de martensita induzida por deformação e na sua posterior reversão da martensita para austenita no aço inoxidável austenítico AISI 304L. O estudo foi predominantemente microestrutural e para análise e caracterização foram utilizadas as técnicas de microscopia óptica, microscopia eletrônica de varredura, difração de raios X, medidas de dureza Vickers e medidas de fases ferromagnéticas por ferritoscopia. As amostras foram inicialmente solubilizadas a 1100 ºC por uma hora, visando a dissolução de uma pequena quantidade residual de ferrita encontrada nas amostras na condição como recebida, depois laminadas em diferentes temperaturas, determinando-se curvas de endurecimento por deformação e de formação de martensita induzida por deformação em função do grau de deformação. Em seguida, foram realizados pré-recozimentos a 600 ºC, favorecendo apenas a reversão da martensita para austenita, de maneira que não ocorresse a recristalização. A quantidade e a temperatura de deformação apresentaram forte influência na quantidade de martensita formada, no endurecimento por deformação e na cinética de amolecimento durante o recozimento. Os pré-tratamentos realizados a 600 ºC causaram acentuada reversão da martensita, algum amolecimento e pequeno efeito no tamanho de grão recristalizado durante o posterior recozimento a 600 ºC. / The main objective of the present dissertation was to verify the effects of the rolling temperature on the formation of strain induced martensite and in its subsequent martensite reversion to austenite in a AISI 304L stainless steel. The study was predominantly microstructural and, for the analysis and characterization, several techniques have been used, namely optical microscopy, scanning electron microscopy, X-ray diffraction, Vickers hardness measurements and magnetic phase measurements, using the ferritoscope. The samples were initially solution annealed at 1100 ºC for one hour, aiming at the dissolution of a small quantity of the residual -ferrite found in the samples in the as-received condition; then rolling was performed at different temperatures, evaluating strain hardening and the strain induced martensite as a function of strain. Following, pre-annealing treatments at 600 ºC have been performed, favoring only the martensite to austenite reversion, in a way that no recrystallization would occur. Strain and temperature had a strong influence on the amount of formed martensite, on the strain hardening and on the softening kinetics during annealing. The pre-annealing treatments at 600 ºC caused an accentuated effect on the martensite reversion, some softening and a small effect on the recrystallized grain size during the subsequent annealing at 600 ºC.

Lors de l’usinage, les conditions de pression et de température à la surface usinée sont trèsélevées. La microstructure et l’état mécanique du matériau sont impactés, ce qui a desconséquences sur la réactivité chimique de la surface usinée. Dans cette thèse, ons’intéresse au tournage de superfinition de l’acier inoxydable martensitique X4CrNiMo16-5-1traité thermiquement (dénommé APX4 optimisé). Il présente la particularité d’êtremultiphasé à la température ambiante (martensite, ferrite et austénite). L’objectif est dequantifier les relations entre les conditions de coupe, les propriétés d’usage et ladégradation par corrosion des surfaces usinées. Des essais d’usinage en coupe orthogonale(type QST) ont été réalisés afin de comparer directement les résultats expérimentaux auxprédictions numériques 2D des surfaces usinées.Les changements de phase du matériau ont été étudiés par des essais de calorimétriejusqu’à 1200°C. Les cycles thermiques réalisés ont mis en évidence différentestransformations métallurgiques. L’étude bibliographique complémentaire permet deconclure quant aux possibilités de transformation de phase en usinage, qui sont quasiinexistantes du fait des cinétiques de chauffage extrêmement élevées en tournage.Les échantillons usinés ont été caractérisés avec une étude microstructurale parmicrographie optique et par MEB-EBSD. Ces deux techniques mettent en évidence lesdéformations importantes en extrême surface indiquées par l’étirement des îlots de ferriteparallèlement à la surface usinée. Ces résultats microstructuraux ont été mis en relationavec les résultats de microdureté Vickers. Une relation de la dureté superficielle a étéexprimée en fonction des conditions de coupe. La technique EBSD a permis de montrer pourles échantillons usinés dans les conditions les plus sévères la déformation de la ferrite avecapparition de sous-joints de grain, un affinement microstructural de la martensite et uneaccentuation de l’affinement de la matrice martensitique à proximité de la ferrite déformée.Une simulation numérique de prédiction des déformations à l’échelle macroscopiquea été réalisée. Ces résultats numériques ont été comparés aux résultats de déformation etde microdureté issus d’une campagne d’essais. Des essais numériques complémentairesappliqués à l’échelle de la microstructure (matériau considéré biphasé) permettent ded’appréhender la déformation de la phase ferritique et son influence locale sur ladéformation de la matrice martensitique. Ces résultats sont cohérents avec les résultatsexpérimentaux.Les phénomènes de corrosion par piqûres ont été étudiés par le biais d’essais depolarisation à l’aide de la microcellule électrochimique et d’essais spécifiques pulsés. Cesderniers essais ont permis l’analyse des piqûres générées (en diamètre et en densité). Larésistance à la corrosion localisée des surfaces d’acier inoxydable martensitique reste bonneaprès usinage, même améliorée (densité de piqûres plusieurs fois plus faible que l’état deréférence). Ce comportement est corroboré à l’état de compression de la surface.La microdureté de surface, qui est contrôlée majoritairement par l’avance, conditionne lepotentiel de piqûre. L’augmentation du diamètre de piqûre a été reliée à l’affinementmicrostructural (observé à partir d’un certain seuil de microdureté superficielle). / During machining, pressure and temperature conditions at the machined surface are veryintensive. Machined material microstructure and mechanical state are changed, whichimpacts the chemical reactivity of the machined surface. In this PhD study, we focus onsuperfinish turning of heat treated martensitic stainless steel X4CrNiMo16-5-1 (namedoptimized APX4). It has to be mentioned that this material has a multiphased microstructureat ambient temperature (martensite, ferrite and austenite). The aim is then to quantifyrelationships between cutting conditions, properties and the deterioration (corrosion) ofmachined surfaces. Machining trials in orthogonal cutting configuration (QST) have beenrealized in order to compare experimental results directly to 2D numerical forecast ofmachined surfaces.Material phase transformations have been studied by calorimetry tests until 1200°C. Testshave evidenced different metallurgical transformations. The complementary bibliographystudy allows to conclude that phase transformation possibilities during machining are veryquasi nonexistent due to extremely high heating kinetics in turning.Machined samples have been characterized by a microstructural study by opticalmicrography and SEM-EBSD. Both techniques highlight high strains in extreme machinedsurface as indicated by the stretching of ferrite islands in parallel to the machined surface.Microstructural results have been linked to the Vickers microhardness results. Surfacehardness has been expressed as a function of the cutting conditions.EBSD measurements have shown for machined samples in the most severe conditionsstraining of the ferrite with low angle grain boundaries, microstructural refining ofmartensite and heightening of martensitic matrix close to the strained ferrite.A numerical simulation predicting strains at the macroscopic scale has been carried out.These numerical results have been compared to strain and microhardness results arise fromone trials campaign. Further numerical simulations applied at the microstructure level(considered as biphased material) allow understanding of ferritic phase strain and its localinfluence on martensitic matrix strain. These results are consistent with experimentalresults.Pitting corrosion phenomenon has been studied by polarization testing using theelectrochemical microcell and specific pulsed testing. Last used method has been conductedto analyze the generated pits (diameter and density). Localized corrosion resistance ofmachined martensitic stainless steel surfaces remains good, even improved (few times lowerpitting density than the reference state). This behaviour is corroborated to the compressivestate of the surface.Surface microhardness, which is mainly controlled by the feed rate, conditions the criticalpitting potential. Increased pitting diameter has been linked to microstructural refining(observed above a surface microhardness level).

科学研究費補助金 研究種目:基盤研究(B)(2) 課題番号:11555183 研究代表者:金武 直幸 研究期間:1999-2001年度

[ES] La fabricación aditiva, de sus siglas en inglés AM (Additive Manufacturing) es un proceso que construye objetos sólidos tridimensionales mediante la superposicióon de materiales basados en un modelo de diseño asistido por ordenador. La AM está llamada a convertirse en la próxima revolución industrial, transformando el panorama del desarrollo y la producción. La AM ofrece numerosas ventajas, como posibilidades de diseño complejas y flexibles, la eliminación de procesos intermedios como el mecanizado, la independencia de los costes de producción del tamaño de los lotes, la reducción de los residuos de material, las estructuras ligeras, las reparaciones personalizadas de las máquinas y la capacidad de desarrollar nuevos materiales, entre otras ventajas. En las tecnologías de fabricación aditiva que emplean un rayo láser como fuente de energía, la materia prima inicial (en forma de polvo o cable) es fundida por la fuente de calor láser de forma controlada, capa a capa, hasta crear un componente con dimensiones finales o casi finales. Estas tecnologías implican someter el material impreso a un proceso térmico único, en el que el material se funde en un área muy específica y luego se enfría rápidamente a velocidades extremadamente altas de hasta 10^6 K/s. Por lo tanto, las microestructuras que surgen de los procesos de fabricación en AM difieren significativamente de las que se consiguen en los procesos tradicionales. Además, los materiales que se emplean principalmente en la AM no se han diseñado explícitamente para estas tecnologías. Las características específicas de los procesos de AM pueden utilizarse para lograr microestructuras y propiedades distintas en aceros que han sido adaptados para aprovechar las rápidas velocidades de enfriamiento y la historia térmica del proceso, entre otros factores. Por el momento, el número de calidades de acero comerciales disponibles en el mercado de la AM es limitado. Diversas industrias demandan nuevos grados de acero con menor densidad para disminuir el peso sin comprometer las propiedades mecánicas. Los aceros con alto contenido en manganeso se consideran materiales muy prometedores para aplicaciones estructurales debido a su excepcional combinación de resistencia y ductilidad, con una baja densidad. Sin embargo, a pesar de sus excepcionales propiedades, los aceros con alto contenido en manganeso se enfrentan a diversas limitaciones o retos durante las técnicas de procesado convencionales. Afortunadamente, la solidificación rápida puede resolver estos problemas. En este sentido, las tecnologías de AM basadas en láser proporcionan velocidades de enfriamiento rápidas, así como flexibilidad en términos de diseño geométrico. Los nuevos retos de estas tecnologías implicarán la microsegregación y el agrietamiento en caliente o hot cracking en inglés, que se producen durante la solidificación. Esta tesis está dedicada a explotar el método CALPHAD para realizar cálculos termodinámicos con el fin de diseñar varios aceros con alto contenido en manganeso que puedan prevenir eficazmente los problemas de solidificación rápida en AM. Las composiciones de acero diseñadas se produjeron en forma de polvo para AM mediante atomización con gas. Se analizaron los polvos para determinar su microestructura en relación con la química y la velocidad de enfriamiento. Ajustando adecuadamente los parámetros de impresión, estos polvos de acero con alto contenido en manganeso se imprimieron con éxito en AM, dando lugar a densidades relativas superiores al 99.9%. Se analizó la microestructura de estas muestras totalmente densas y se comparó con sus respectivos polvos, con el fin de identificar cualquier diferencia resultante de las variaciones en la velocidad de enfriamiento y los ciclos térmicos. Por último, tras definir el mejor conjunto de condiciones de impresión para cada composición de polvo, se produjeron varias muestras para evaluar las propiedades mecánicas. / [CA] La fabricació additiva, de les seues sigles en anglés AM (Additive Manufacturing) és un procés que construïx objectes sòlids tridimensionals mitjançant la superposició de materials basats en un model de disseny assistit per ordinador. L'AM està cridada a convertir-se en la pròxima revolució industrial, transformant el panorama del desenvolupament i la producció. L'AM oferix nombrosos avantatges, com a possibilitats de disseny complexes i flexibles, l'eliminació de processos intermedis com el mecanitzat, la independència dels costos de producció de la grandària dels lots, la reducció dels residus de material, les estructures lleugeres, les reparacions personalitzades de les màquines i la capacitat de desenvolupar nous materials, entre altres avantatges. En les tecnologies de fabricació additiva que empren un raig làser com a font d'energia, la matèria primera inicial (en forma de pols o filferro) és fosa per la font de calor làser de manera controlada, capa a capa, fins a crear un component amb dimensions finals o quasi finals. Estes tecnologies impliquen sotmetre el material imprés a un procés tèrmic únic, en el qual el material es funde en una àrea molt específica i després es refreda ràpidament a velocitats extremadament altes de fins a 10^6 K/s. Per tant, les microestructures que sorgixen dels processos de fabricació en AM diferixen significativament de les que s'aconseguixen en els processos tradicionals. A més, els materials que s'empren principalment en l'AM no s'han dissenyat explícitament per a estes tecnologies. Les característiques específiques dels processos d'AM poden utilitzar-se per a aconseguir microestructures i propietats diferents en acers que han sigut adaptats per a aprofitar les ràpides velocitats de refredament i la història tèrmica del procés, entre altres factors. De moment, el nombre de qualitats d'acer comercials disponibles en el mercat de l'AM és limitat. Diverses indústries demanden nous graus d'acer amb menor densitat per a disminuir el pes sense comprometre les propietats mecàniques. Els acers amb alt contingut en manganés es consideren materials molt prometedors per a aplicacions estructurals a causa de la seua excepcional combinació de resistència i ductilitat, amb una baixa densitat. No obstant això, malgrat les seues excepcionals propietats, els acers amb alt contingut en manganés s'enfronten a diverses limitacions o reptes durant les tècniques de processament convencionals. Afortunadament, la solidificació ràpida pot resoldre estos problemes. En este sentit, les tecnologies d'AM basades en làser proporcionen velocitats de refredament ràpides, així com flexibilitat en termes de disseny geomètric. Els nous reptes d'estes tecnologies implicaran la microsegregació i l'esquerdament en calent, o hot cracking en anglés, que es produïxen durant la solidificació. Esta tesi està dedicada a explotar el mètode CALPHAD per a realitzar càlculs termodinàmics amb la finalitat de dissenyar diversos acers amb alt contingut en manganés que puguen previndre eficaçment els problemes de solidificació ràpida en AM. Les composicions d'acer dissenyades es van produir en forma de pols per a AM mitjançant atomització amb gas. Es van analitzar les pólvores per a determinar la seua microestructura en relació amb la química i la velocitat de refredament. Ajustant adequadament els paràmetres d'impressió, estes pólvores d'acer amb alt contingut en manganés es van imprimir amb èxit en AM, donant lloc a densitats relatives superiors al 99.9%. Es va analitzar la microestructura d'estes mostres totalment denses i es va comparar amb les seues respectives pólvores, amb la finalitat d'identificar qualsevol diferència resultant de les variacions en la velocitat de refredament i els cicles tèrmics. Finalment, desprès de definir el millor conjunt de condicions d'impressió per a cada composició de pols, es van produir diverses mostres per a avaluar les propietats mecàniques. / [EN] Additive manufacturing (AM) is a process that builds three-dimensional solid objects by layering materials based on a computer-aided design model. AM is set to become the next industrial revolution, transforming the landscape of development and production. AM provides numerous benefits, including complex and flexible design possibilities, the elimination of intermediate processes like machining, production cost independence from batch size, reduced material waste, lightweight structures, customized machine repairs, and the ability to develop new materials, among other advantages. In additive manufacturing technologies that employ a laser beam as an energy source, the initial raw material (in the form of powder or wire) is melted by the laser heat source in a controlled manner, layer by layer, until a component with final or nearly final dimensions is created. These technologies involve subjecting the printed material to a unique thermal process, where the material is melted in a very specific area and then rapidly cooled at extremely high rates of up to 10^6 K/s. Hence, the microstructures that arise from the manufacturing processes in AM differ significantly from those achieved in traditional processes. Moreover, the materials predominantly employed in AM have not been explicitly designed for these technologies. The specific characteristics of AM processes can be utilized to achieve distinct microstructures and properties in steels that have been tailored to take advantage of the rapid cooling rates and thermal history of the process, among other factors. For the moment, the number of commercial steel grades available in the AM market is limited. Various industries are demanding new steel grades with lower density to decrease weight without compromising mechanical properties. High manganese steels are regarded as highly promising materials for structural applications due to their exceptional combination of strength and ductility, with low density. Nevertheless, despite the exceptional properties of high manganese steels, they encounter various limitations or challenges during conventional processing techniques. Fortunately, rapid solidification may solve these issues. In this sense, laser-based AM technologies provide rapid cooling rates, as well as flexibility in terms of geometric design. The new challenges of these technologies will involve micro-segregation and hot cracking occurring during solidification. This thesis is dedicated to exploiting the CALPHAD method to perform thermodynamic calculations in order to design various high manganese steels that can effectively prevent fast solidification issues in AM. The steel compositions designed were produced in the form of powder for AM using gas atomization. Powders were analyzed to determine their microstructure in relation to the chemistry and cooling rate. By adjusting properly, the printing parameters, these high manganese steel powders were successfully printed in AM, resulting in relative densities exceeding 99.9%. The microstructure of these fully dense samples was analyzed and compared to their respective powders, in order to identify any difference resulting from variations in cooling rate and thermal cycling. Lastly, after defining the best set of printing conditions for each powder composition, various samples were produced to evaluate the mechanical properties, to determine the correlation between the composition, microstructure and properties of these steels. In addition, lattice structures that are close to final part geometries were constructed to quantify the energy absorbed during compression by one of these high manganese steels. The results were then compared to those of 316L, revealing that the high manganese steel absorbs roughly twice as much the specific energy in compression. This finding demonstrates the potential of these novel AM steels for use in industrial applications. / Sánchez Poncela, M. (2024). Design of High Mn Fe-Mn-Al-C Low Density Steels for Additive Manufacturing [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/205174