Abstract The solidification of metals and alloys and the resulting microstructures, which as a function of thermal and solutal parameters can evolve as planar, cellular and dendritic, are important from a practical point of view, since they strongly influence the properties and quality of the final product. In many practical situations it is impracticable to develop analytical solutions permitting reliable predictions of microstructural growth during unsteady-state solidification conditions. The Phase Field method has become very popular and effective in modeling complex solid/liquid interfaces due to its ability to simulate the interface kinetics and the formation and evolution of different morphologies along the solidification process. In this work, a numerical analysis of the microstructural evolution during the transient solidification of dilute alloys of the Al-Cu-Si system is developed, which uses a phase-field approach for the simulation of ternary alloys. The phase-field, energy and solute concentration equations were numerically solved for the correspondent ternary system, varying the mesh parameters, temperature and alloy composition. The analysis performed were confronted with existing theoretical models and the results obtained are in agreement with the solidification theory.

The prediction of microstructure morphology is fundamental for the manufacture of metallic components, since the expected levels of mechanical properties will be associated with the final aspects of the microstructure. In this work, an Al-7wt.%Si-3wt.%Mg alloy was directionally solidified in unsteady state conditions in order to investigate the influence of the addition of 3wt.%Mg to an Al-7wt.%Si alloy on the solidification evolution. The microstructure of the examined alloy is shown to be characterized by a more complex arrangement of phases, as compared to that of the Al-7wt.%Si alloy, which includes the binary (α-Al+Mg2Si) and refined ternary (α-Al+Si+Mg2Si+Fe-rich IMC) eutectic mixtures. A higher Vickers hardness profile is shown to be associated with a more refined microstructural arrangement. However, for cooling rates lower than 2K/s the microhardness is shown to increase with the increase in the microstructural spacing, which is shown to occur caused by Si macrosegregation and higher content of free Mg.

This study aims to analyze the influence of macrosegregation on microstructure evolution, and of microstructure length scale on the corrosion resistance of a Zn-5.0wt.%Mg alloy casting. The analyzed samples were taken along the length of castings unidirectionally solidified in unsteady state conditions of heat extraction. Microstructure characterization, microhardness, linear polarization and electrochemical impedance spectroscopy (EIS) tests were performed. A unique type of microstructure was observed, characterized essentially by a morphology typified by idiomorphic MgZn2 crystals in a “complex eutectic mixture” [coexistence of stable (Zn+Mg2Zn11) and metastable (Zn+MgZn2) eutectics]. The correlation between thermal and microstructural parameters, permitted experimental growth laws, correlating the evolution of the lamellar eutectic spacing with the cooling rate to be established. Vickers microhardness and electrochemical corrosion tests showed that more refined microstructures associated with higher experimental cooling rates, are related to a better set of higher hardness and corrosion resistance.

The present research aims to characterize 5xxx alloy series, considering chemistries based on the commercial 5052 alloy with three Mg contents (2.4wt.%, 2.6wt.% and 3.2wt.%) through transient directional solidification experiments. Very representative incoming impurities to a given twin-roll casting procedure were reached. As such, the Si, Fe, Cu, Mn and Cr-contents in the tested samples typically trend in between the suitable alloying spectrum. Microstructural analyzes were performed using polarized light microscopy of samples taken from various ingot positions. Growth relationships between the secondary dendritic spacing (λ2) and the growth velocity were determined. The 5052 alloys containing higher Mg content may induce a decrease in λ2 for a certain growth velocity. The hardness values measured across the three directionally solidified castings were directly related to the λ2, which can be considered a fundamental variable affecting mechanical strength. For representative conditions vis-à-vis those employed in industry, it was shown that even relatively small changes in Mg content of the 5052 alloy may have some impact on λ2. These results open new ways to predict the final as-cast microstructure characterizing commercial 5052 alloy products, with a view to controlling not only the dendritic growth but also the Mg content during casting operations.

The present research work examines the microstructural arrangements formed during the transient solidification of eutectic Sn-0.2wt.%Ni and hypereutectic Sn-0.5wt.%Ni alloys. Also, it examines their respective correlations with solidification thermal parameters: eutectic growth rate (VE) and eutectic cooling rate (ṪE); length scales of matrix and eutectic phases: microstructural spacings and the corresponding tensile properties: ductility and strength. Both alloys were directionally solidified upwards under unsteady-state regime, and characterized by optical and scanning electron microscopy. Concerning the hypereutectic Sn-0.5wt.%Ni, the increase in Ni content is shown to influence both thermal behavior and cellular spacing (λC). The NiSn4 intermetallics is present in the eutectic mixture of both alloys, whilst in the Sn-0.5wt.%Ni alloy the primary phase has been identified by SEM-EDS as the Ni3Sn4 intermetallics. A β-Sn morphological cellular/dendritic transition occurs in the 0.2wt.%Ni eutectic alloy for ṪE> 1.2K/s. Despite that, regular cells in the hypereutectic alloy (0.5wt.%Ni) turns into plate-like cells for ṪE> 1.4K/s. If considered a reference cellular spacing about 20μm (i.e.,λ(c-1/2=0.22), the samples associated with the Sn-0.5wt.%Ni alloy are shown to be associated with higher tensile strengths, but much lower ductility as compared with the corresponding results of the eutectic alloy.

Prediction of microstructure evolution and microsegregation is one of the most important problems in materials science. The dendritic growth and microsegregation provide a challenging simulation goal for computational models of solidification, in addition to being an important technological feature of many casting processes. The phase-field model offers the prospect of being able to perform realistic simulation experiments on dendrite growth in metallic systems. In this paper, the microsegregation and dendritic growth of hypoeutectic Al-Cu alloys under constant cooling rate was simulated using a phase-field model. The main new feature of the present model is based on the fact that the effect of the growth rate is incorporated via an effective partition coefficient that has been experimentally determined for a range of growth rates. It is shown that both models (Phase-field model and Scheil) have significant deviations from the experimental data when the equilibrium partition coefficient is considered in the calculations. Since the predicted results using the models yielded discrepancies from the experimental data, an experimental equation is adopted for calculating the effective partition coefficient from experimental data. The experimental equation is then adopted in the calculations of phase-field model and Scheil's equation, showing a good agreement with the experimental data.

RESUMO Diversas aplicações industriais, tais como nos setores automotivo e aeronáutico, que necessitam de resistência e tenacidade, utilizam ligas do sistema Al-Cu. As ligas do sistema ternário Al-Cu-Ag são empregadas como modelo para o estudo de ligas eutéticas ternárias, além do uso em certo número de aplicações técnicas. Estudos relacionados aos efeitos da adição de Ag em ligas do sistema Al-Cu solidificadas em regime transitório, sob aspectos microestruturais e de microdureza, não são encontrados na literatura. Nesse sentido, o objetivo principal deste trabalho consiste na investigação dos efeitos da adição de 4%Ag na liga Al-4%Cu solidificada unidirecionalmente no sentido vertical ascendente em regime transitório, abordando análises de microestrutura, microdureza e parâmetros de solidificação dados pela velocidade de crescimento (VL) e pela taxa de resfriamento (Ṫ). São apresentadas leis experimentais para as ligas Al-4%Cu e Al-4%Cu-4%Ag que correlacionam os espaçamentos dendríticos primário (λ1) e secundário (λ2) com VL e Ṫ. Foram realizadas análises de Difração de Raios-X (DRX) para a caraterização das fases cristalinas dos compostos presentes na microestrutura das ligas. O difratograma da liga binária mostrou a presença dos intermetálicos θ-Al2Cu, Al-Cu, Al4Cu9 e Al6Fe e, para o caso da liga ternária, adicionalmente foi detectado o intermetálico µ-Ag3Al. Composições químicas de regiões do contorno da dendrita, assim como de seu interior, foram obtidas com o auxílio de um microscópio eletrônico de varredura (MEV) acoplado a um sistema de análise por energia dispersiva (EDS). Diversas posições ao longo do lingote foram analisadas através da técnica de Fluorescência de Raios-X (FRX) com objetivo de se detectar a existência de macrossegregação. Constatou-se que a microdureza não é influenciada pelas duas formas de espaçamentos da matriz rica em Al. Os intermetálicos ρ-Ag2Al e µ-Ag3Al são responsáveis pelo aumento de dureza devido a sua interação com o espaçamento dendrítico terciário e outros intermetálicos.

ABSTRACT Al-Cu alloys are used in several industrial applications, such as automotive and aerospace manufacturing, which demand mechanical strength and toughness performances. The Al-Cu-Ag ternary alloy system has been investigated to obtain a better understanding on ternary eutectic alloys besides its inherent applications. However, studies concerning the effects on microstructure and hardness due to Ag addition on Al-Cu alloys solidified under transient conditions, are not found in literature. In this sense, the main objective of this work consists in investigating the solidification parameters such as growth (VL) and cooling (Ṫ) rate, microstructure and microhardness regarding the effects of adding 4wt% Ag on a Al-4wt%Cu alloy unidirectionally solidified vertically upwards under unsteady-state solidification conditions. Experimental growth laws correlating the primary (λ1) and secondary (λ2) dendritic spacings with growth and cooling rate are proposed for Al-4wt%Cu and Al-4wt%Cu-4wt%Ag alloys. X-Ray Diffraction (XRD) analyses were performed in order to determine the crystalline phases of the compounds present in the alloys microstructure. The diffractogram of the binary alloy showed the presence of θ-Al2Cu, Al-Cu, Al4Cu9 and Al6Fe intermetallics and, for the case of the ternary alloy, the µ-Ag3Al intermetallic was detected. Chemical compositions at the boundary regions of the dendrite as also inside the dendrite were obtained using scanning electron microscope (SEM) connected to an Energy-Dispersive Spectrometer (EDS) system. Several positions along the ingot were analyzed through the X-Ray Fluorescence (XRF) technique permitting the detection of macrosegregation occurrence. It is shown that the microhardness is not influenced by both spacing patterns of the Al-rich matrix. The ρ-Ag2Al and µ-Ag3Al intermetallics are responsible for the increase in hardness due to the interaction with ternary dendrite arm spacing and other intermetallics.

Nesse trabalho, a transição colunar/equiaxial (TCE) foi investigada experimentalmente na solidificação unidirecional de três ligas multicomponentes à base de alumínio (Al-nSi-3Cu), com “n” igual a 5,5, 7,5 e 9% em massa. Os principais parâmetros analisados foram o gradiente de temperatura (GL), a velocidade de deslocamento da isoterma liquidus (VL), a taxa de resfriamento (TR) e a composição de Si. Um dispositivo de solidificação direcional refrigerado à agua para fluxo de calor transitório foi utilizado. Para todas as ligas examinadas, os resultados mostram que a concentração de soluto inflencia na posição da TCE, a qual ocorre para uma taxa de resfriamento igual a 1.17 0C/s. A análise comparativa entre os resultados desse trabalho com os da literatura, propostos para analizar a TCE em ligas Al-Si, é conduzida e os resultados mostram que o final da região colunar acontece para uma taxa de resfriamento sete vezes maior que a encontrada para ligas Al-Si.

In this work the columnar to equiaxed transition (CET) was experimentally investigated in the unidirectional solidification of three aluminum based multicomponent alloys (Al-nSi-3Cu), with "n" equal to 5.5, 7.5 and 9 wt.%. The main parameters analyzed include the tip temperature gradient (GL), tip growth rate (VL), tip cooling rate (TR) and Si content. A water-cooled solidification experimental setup was developed, and specimens were solidified under unsteady state heat flow conditions. It is shown that for the alloys examined, the solute concentration influences the position of the CET, which occurs for an average cooling rate of about 1.17 0C/s. A comparative analysis between the results of this work and those from literature proposed to analyze the CET during upward vertical solidification of Al-Si alloys is reported and the results show that the end of the columnar region is abbreviated as a result of seven times higher critical cooling rate than that verified for Al-Si alloys.

Recently, intermetallic compounds have attracted much attention due to their potential technological applications as high-temperature materials. In particular the intermetallic compounds, associated with the Al-Ni binary system stand out as promising candidates for high-temperature materials for the use in harsh environments. It is expected that a bulk Al-Ni alloy may exceed the strength of many commercial materials. The great challenge in developing these alloys is to manipulate the solidification thermal parameters in order to obtain the desired microstructural features. One of the indicated routes to obtain very refined intermetallic phases dispersed in the microstructure is the spray forming process. The dendritic and eutectic growth dependences on cooling rate are already known for directionally solidified (DS) hypoeutectic Al-Ni alloys. In the case of rapidly solidified (RS) samples, extrapolations of such experimental laws are needed, which can be very helpful to estimate realistic values of high cooling rates imposed during the spray forming process. The present study aims to compare directionally solidified and spray-formed Al-5wt. (%)Ni alloy samples with a view to providing a basis for understanding how to control solidification parameters and the as-cast microstructure. The Al-5.0wt. (%)Ni alloy was shown to have a cellular morphology for the overspray powder size range examined (up to 500 µm). The mean cell spacing decreased from 5.0 to 1.1 µm with the decrease in the powder average diameter. It was found that the experimental cooling rates imposed during the atomization step of the overspray powder solidification varied from 10³ to 2.10(4) K/s. The DSC trace depicted a crystallization peak of an amorphous structure fraction in the smallest Al-5.0wt. (%)Ni alloy powder size range (<32 µm) estimating a 15 µm critical diameter of amorphous powder in the binary Al97.5Ni2.5 (at%) alloy.

Hypoeutectic Al-Ni alloys show a ductile phase α distributed with a β phase Al3Ni fragile where β serves as reinforcement of the structure of the material. The eutectic composition alloys obey the relationship: λ2.v = C, where λ is the eutectic spacing, v is a tip growth rate and C is a constant. The aim of this study is to establish correlations between λ and v for hypoeutectic Al-1%, 3% and 5% Ni alloys. Unsteady-state upward directional solidification experiments were performed, as well as metallography, dissolution of the aluminum matrix and scanning electron microscopy (SEM). The interphase spacing of the three Al-Ni alloys decreased with increasing tip growth rate, with a predominance of a rod-like morphology on intermetallic. It was observed that parameters such as tip growth rate, cooling rate and temperature gradient decreases as the solidification front advances. It was further observed that a single experimental law λ = 1.2 v-0, 5 illustrates the evolution of the interphase spacing for any examined alloy.

O objetivo deste trabalho foi avaliar o comportamento eletroquímico da liga Al-4,5%Cu solidificada em condições de fluxo de calor transitório. Esta evolução foi realizada através de análises de curvas de polarização e testes de espectroscopia de impedância eletroquímica (EIE), em solução de 0,5M de NaCl à 25°C. O perfil de segregação obtido no experimento de solidificação foi caracterizado por segregação positiva e negativa de cobre, respectivamente, na base e no topo da amostra. Igualmente, na prática de fundição convencional, em um mesmo lingote podem ocorrer regiões de segregação de cobre positiva e negativa. O lingote pode apresentar diferentes respostas à corrosão em diferentes partes, como função do arranjo microestrutural. As influências na resistência à corrosão da redistribuição do soluto durante a solidificação, a magnitude dos espaçamentos dendríticos e a distribuição das partículas da fase rica em Al e Al2Cu ao longo do lingote, foram examinadas com coleta de amostras ao longo do lingote. A taxa de corrosão e parâmetros de impedância (obtidos de uma análise de circuito equivalente) são também discutidos.

The purpose of this work was to evaluate the electrochemical behavior of an Al-4.5wt%Cu alloy solidified under unsteady-state heat flow conditions. This evaluation was carried out through the analysis of both potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) tests in a 0.5 M NaCl solution at 25ºC. The experimental segregation profile obtained in the solidification experiment was characterized by positive and negative copper content regions at the bottom and the top of the casting, respectively. Likewise, in conventional foundry practice, in a same casting both positive and negative copper segregation regions may occur. Such casting can exhibit different corrosion responses at different locations. The influences of solute redistribution during solidification, the magnitude of dendritic spacing and hence of the Al-rich phase and of Al2Cu particles distribution along the casting on the corrosion resistance, were examined in samples collected along the casting length. The corrosion rate and impedance parameters (obtained from an equivalent circuit analysis) are also discussed.

Microstructures are the strategic link between materials processing and materials behavior. A dendritic structure is the most frequently observed pattern of solidified alloys. The microstructural scales of dendrites, such as primary and secondary arm spacings, control the segregation profiles and the formation of secondary phases within interdendritic regions, determine the properties of cast structures. In this work, the influence of thermosolutal convection on dendrite arm spacings is experimentally examined in the downward vertical unsteady-state directional solidification of Sn-Pb hypoeutectic alloys. The experimental observations are compared not only with the main predictive theoretical models for dendritic spacings but also with experimental results obtained for Sn-Pb alloys solidified vertically upwards. Primary dendritic arm spacings have been affected by the direction of growth, decreasing in conditions of downward vertical solidification when compared with those grown vertically upwards. Further, the unsteady-state lambda1 predictive models did not generate the experimental observations.

The amount, distribution, size and chemical composition of non-metallic inclusions have a direct influence on steel properties. By controlling size and chemical composition of these inclusions, it is possible to get a product with good quality. The identification of the nature and the control of inclusion formation are very important for steel cleanness. The behavior of these inclusions is predictable, in some extent, by the determination of the chemical composition of non-metallic phases that form such inclusions. With the objective of studying the chemical composition, the size and the distribution of such inclusions, samples of aluminum and silicon killed low carbon steels were collected in a national steel industry in the secondary refining and continuous casting stages. These samples were analyzed in the scanning electron microscope (SEM) coupled to an energy dispersive analysis system (EDS). From the results, it was possible to evaluate the nature of inclusions and to analyze the effectiveness of the refining process in the reduction of the number and area fraction of the inclusions. It was also possible to verify that the inclusions that remained after treatment, are less damage both to the steel properties as to the continuous casting process (clogging of the submerged valve).

A crescente demanda por aços de alta qualidade com elevada limpeza interna tem levado a uma contínua melhoria nas práticas de produção do aço, em especial na etapa de refino secundário. O interesse especial é no controle das inclusões não-metálicas, devido ao seu efeito prejudicial nas etapas subseqüentes (lingotamento e conformação plástica) e devido às sua grande influência nas propriedades do produto final. Através do controle do tamanho, da quantidade e da composição química das inclusões, é possível obter um produto final de alta qualidade. O controle da formação das inclusões não-metálicas e a identificação de suas fases constituintes são de suma importância para a obtenção de aços limpos. Nesse trabalho, amostras de aço de baixo teor de carbono, desoxidado ao silício, coletadas em uma indústria siderúrgica, são analisadas e é demonstrado que, pela comparação do teor total de cálcio e de oxigênio no aço, é possível prever os tipos de inclusões presentes no aço.

The increasing demand in recent years for high-quality steel products has led to the continuous improvement of steelmaking practices. There is a special interest in the control of non-metallic inclusions due to their harmful effect on the subsequent stages and their great influence on the properties of the final product. Through the control of the amount, size and chemical composition of the inclusions it is possible to obtain a final product of good quality. The control of the formation of non-metallic inclusions and the identification of their constituent phases are of extreme importance for the obtaining of clean steels. In this work, samples collected in a steelmaking process are analyzed and it is demonstrated that by comparing the total calcium and oxygen contents in the steel, it is possible to have an overview of the types of oxide inclusions present.

The aim of this work is to develop a mathematical model to predict the depth of laser treated zone in the LTH process. The Fourier equation of heat conduction is solved by using the Finite Difference Method in cylindrical coordinates in order to study the temperature distribution produced in a workpiece and hence to obtain the depth to which hardening occurs. The theoretical simulations are compared with results produced experimentally by a CO2 laser operating in continuous wave, showing good agreement.