Upward Directional Solidification Research Articles

Plate-like cell growth during directional solidification of a Zn–20wt%Sn high-temperature lead-free solder alloy

Abstract Although Zn–Sn alloys have suitable features for high temperature solders, as for example the absence of intermetallic compounds (IMCs) and relatively high melting temperatures, the control of the scale of the microstructure by adequate pre-programming of the solidification thermal parameters remains still a task to be accomplished. The present study focuses on the interrelation among hardness, microstructure features/segregation and solidification thermal parameters. An upward directional transient solidification apparatus was used in order to permit samples along a range of cooling rates to be obtained for such evaluation. The entire Zn–20wt%Sn alloy casting is characterized by a two-phase alternated structure, which resembles the morphology of a lamellar eutectic. Experimental growth laws having −1/2 and −1/4 exponents are proposed relating the interphase spacing to the growth rate and the cooling rate, respectively. The morphology and size of the Zn-rich plate-like cells, as well as the macrosegregation pattern are shown to affect the hardness.

Influence of natural convection during upward directional solidification: A comparison between in situ X-ray radiography and direct simulation of the grain structure

This paper presents a comparison between the in situ and real time observation of a directional solidification experiment carried out at the European Synchrotron Radiation Facility and a direct simulation of grain structure formation in the sample using a two-dimensional cellular automaton–finite element (CA-FE) model. In situ characterization of the columnar-to-equiaxed transition in a refined Al–3.5wt.% Ni alloy is achieved by synchrotron X-ray radiography. Two main characteristics are derived from the radiographs for each nucleated grain, namely the nucleation position in the sample and the orientation of the main trunk relative to the vertical temperature gradient. These data are then used as input for the CA-FE simulations. The CA-FE model takes into account the effects of macroscopic transport of heat, liquid momentum and solute mass on the development of the dendritic grain structure and vice versa. The influence of convection on macroscopic shape of the growth front, the grain structure, the microstructure distribution and macro-segregation is determined by a comparison between the experimental observations and results from the numerical simulations with and without fluid flow. Good qualitative agreement is obtained and limitations that are linked to the two-dimensional approximation and the need for direct tracking of the eutectic grain structure are pointed out.

The effects of microstructure and intermetallic phases of directionally solidified Al–Fe alloys on microhardness

Iron is the dominant impurity in Al of commercial grade purity and is also added deliberately in some Al alloys compositions to improve the high temperature strength. Al–Fe castings show the formation of different metastable intermetallic phases (AlmFe, Al6Fe, etc.), which may affect decisively the mechanical properties. In the present study, upward transient directional solidification experiments were carried out with hypoeutectic Al-1.0 and 1.5wt% Fe alloys. The microstructure evolution and Vickers microhardness (HV) profiles were determined for both alloys, and characterization of Al–Fe intermetallics was carried out. The cell spacing, λc, has been correlated with the local HV microindentation and Hall–Petch type equations are proposed. Comparisons were performed with hardness results available for hypoeutectic Al–Ni alloys. It was found that the microhardness is affected by both the iron content of the alloy and the cellular spacing.

Corrosion and wear resistance of hypoeutectic Zn–Al alloys as a function of structural features

The aim of the present research was to investigate the role of the type of grain macrostructure (columnar, columnar-to-equiaxed transition–CET -, and equiaxed) and microstructure (secondary dendrite arm spacing - λ2) in the wear and electrochemical behavior of hypereutectic Zn–Al alloys (Zn-1wt%Al, Zn-2wt%Al, Zn-3wt%Al and Zn-4wt%Al) directionally solidified in a vertical upward directional solidification device. We also aimed to, correlate the thermal parameters with the electrochemical and wear properties. The results obtained in this alloy system indicate that the corrosion resistance decreases as the concentration of aluminum increases, but that at the highest concentrations (near the eutectic concentration) this resistance depends on the structure. For the same wear conditions, the wear rate of the equiaxed region is lower than the columnar and transition regions. Independently of the type of structure, wear resistance increases as the aluminum concentration increases.For each alloy concentration, the wear resistance increases from the columnar to the equiaxed structure. A clear inverse relation is observed between the wear resistance and λ2, which may explain the increase in wear resistance in the equiaxed region. At low spacings, the wear rate converges to a single value.

Influence of natural convection on microstructure evolution during the initial solidification transient: comparison of phase-field modeling with in situ synchrotron X-ray monitoring data

The influence of natural convection on the evolution of the solid-liquid (s/l) interface during the initial transient of upward directional solidification was studied on Al-4 wt.% Cu alloy by coupling the two dimensional quantitative phase-field model with the Navier-Stokes equations. The simulations were compared with in situ and real-time synchrotron X-ray monitoring data. The origin of natural convection in experiment was the presence of a small unavoidable horizontal temperature gradient. Due to the stringent requirement on the phase-field interface width parameters, the simulated domain could not be chosen as large as the size of the experimental sample. As the calculated fluid flow strength would be weakened by using a smaller domain, a horizontal temperature gradient ten times larger than the estimated experimental value was applied in simulation to recover a fluid flow washing the s/l interface similarly to experiments. Direct comparison to experimental measurements demonstrated that the phase-field simulations with convection qualitatively reproduced the evolution of all the characteristic parameters measured in experiments. Based on these results, the effects of natural convection on the growth dynamics of the s/l interface during directional solidification of alloy were further clarified.

Effect of traveling magnetic field on dendrite growth of Pb-Sn alloy during directional solidification

The influence of melt convection on dendrite growth during the upward-directional solidification of Pb-33%Sn binary alloys was investigated. The melt convection was modulated by traveling magnetic field. When the direction of traveling magnetic field was changed from upward to downward, the primary dendrite spacing gradually increased, and the distribution peak of the primary dendrite spacing shifted to the field of narrower spacing. These result from the different intensities of melt convection, which are controlled by the traveling magnetic field. The effects of the traveling magnetic field on melt convection are similar to those of adjustment in the gravity level, thus, the primary dendrite spacing varies. When the intensity of the traveling magnetic field was 1 mT, and the drawing speed was 50 μm/s, the gravity acceleration reached 0.22 g for the downward-traveling magnetic field and 3.07 g for the upward-traveling magnetic field.

Evolution of eutectic spacing during unidirectional solidification of Al-Ni alloys

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.

Effects of melt flow on the primary dendrite spacing of Pb–Sn binary alloy during directional solidification

The influences of melt flow generated by traveling magnetic fields (TMFs) on dendrite growth during the upward-directional solidification of Pb–33 wt% Sn binary alloy were investigated under the condition of 1× g gravity. When the direction of the TMF was changed from upward to downward, the primary dendrite spacing gradually increased, and the peak of distribution of the primary dendrite spacing shifted to the narrower values. This was caused by the different intensities of melt flow, which was controlled by TMFs. The effects of TMFs on melt flow were similar to those of adjustments in gravity levels; thus, the primary dendrite spacing varied. The effective gravity acceleration, which was used to modulate melt flow, decreased for downward-TMFs and increased for upward-TMFs. As the drawing speed increased, the modulated flow showed more significant influences on primary dendrites spacing.

Al–Fe hypoeutectic alloys directionally solidified under steady-state and unsteady-state conditions

The aim of this work is to evaluate the cellular growth, the nature of Al–Fe intermetallic particles and the eutectic arrangement of Al–Fe hypoeutectic samples solidified at growth rates ranging from 0.05 to 2.5 mm/s. The samples grown at higher solidification velocities were obtained using a water-cooled directional solidification apparatus. A Bridgman-type furnace was used to grow samples in the lower range of solidification velocities and an air-cooled mold was used to generate experimental values in between those obtained by the other two techniques of directional solidification. All casting assemblies were set to support upward directional solidification. Based on the present results, a single experimental power law seems to be enough to fit all experimental values of cell spacing as a function of cooling rate. The wide range of solidification thermal parameters used in the present study was chosen due to the diversity of foundry processes used for the manufacture of Al–Fe alloys components. For instance, low solidification velocities are typical of sand casting processes while high velocities are typical of direct-chill (DC) castings. In order to investigate the nature of the Al–Fe intermetallics, these particles were extracted from the aluminum-rich matrix by using a dissolution technique. Such phases were then investigated by SEM-EDAX microscopy and X-ray diffraction (XRD). It was found that Al 3Fe is the predominant intermetallic phase in the Bridgman-grown samples and Al 6Fe prevails in the samples grown in the water-cooled solidification apparatus.

Cellular Microstructure and Mechanical Properties of a Directionally Solidified Al-1.0wt%Fe Alloy

Upward directional transient solidification experiments have been carried out with an Al-1.0wt%Fe alloy. Tensile tests were carried out with samples collected along the casting length and these results have been correlated with measured cell spacings, since cellular growth has prevailed along the directionally solidified casting. The resulting mechanical properties include ultimate tensile strength, yield tensile strength and elongation. The used casting assembly was designed in such a way that the heat was extracted only through the water-cooled system at bottom of the casting. During non-equilibrium solidification, typical of DC (direct chill) castings, different cooling rates occur from the casting cooled surface up to the top of the casting, causing the formation of metastable intermetallic phases (AlmFe, Al6Fe, etc) in addition to the stable Al3Fe phase. The extensive presence of plate-like Al3Fe phase in the as-cast structure adversely influences the mechanical properties of Al-Fe alloys, since this morphology is more likely to induce microcracks than the fibrous Al6Fe phase. In order to permit an appropriate characterization of these intermetallic phases, they were extracted from the aluminum-rich matrix by using a dissolution technique. These phases were then investigated by optical microscopy and SEM techniques. It was found that the ultimate tensile strength, the yield strength and the elongation increase with decreasing cell spacing and experimental laws correlating cell spacing and these mechanical properties have been established.

The effects of cell spacing and distribution of intermetallic fibers on the mechanical properties of hypoeutectic Al–Fe alloys

The aim of the present investigation was to contribute to provide a basis for understanding how to control solidification parameters, microstructure and mechanical strength of Al–Fe alloys. Upward directional solidification experiments have been carried-out with commercially pure Al and Al–0.5 wt.% Fe, Al–1.0 wt.% Fe and Al–1.5 wt.% Fe alloys. The tensile tests results have been correlated to cell spacing ( λ 1), since cellular growth has prevailed along all obtained Al–Fe castings. The used casting assembly was designed in such way that the heat was extracted only through the water-cooled system at the bottom of the casting. In order to investigate the nature of Al–Fe intermetallic fibers, they were extracted from the aluminum-rich matrix by using a dissolution technique. These fibers were then investigated by SEM-EDAX microscopy. It was found that the ultimate tensile strength, yield tensile strength and elongation increase with decreasing cell spacing. The highest ultimate tensile strength was that obtained for the most refined microstructure, i.e. for the Al–1.5 wt.% Fe alloy sample, where a higher density of eutectic fibers was found distributed in a more homogeneous way along the casting section due to lower cell spacings. In contrast, the elongation was found to decrease with increasing solute content.

Correlação entre propriedades mecânicas e arranjo dendrítico de ligas Sn-Zn utilizadas em solda sem presença de chumbo

O desenvolvimento de solda sem chumbo tem sido uma tarefa árdua para cientistas de materiais pelas preocupações com a saúde e com o meio ambiente devido ao conteúdo de chumbo das soldas convencionais. O objetivo do presente trabalho é investigar a influência dos parâmetros da microestrutura dendrítica nas propriedades mecânicas das ligas sem chumbo: Sn-4 e 12%p Zn. Amostras das ligas em estudo foram obtidas por experimentos de solidificação conduzidos em um aparato que proporciona uma solidificação unidirecional vertical ascendente refrigerado à água (25 °C ±2°C), o qual garante a extração de calor apenas pela base do lingote em regime transitório de extração de calor. Os resultados experimentais permitiram estabelecer correlações entre parâmetros térmicos e espaçamentos dendríticos secundários com a resistência à tração e alongamento. Conclui-se dos resultados experimentais que o alongamento específico mostra-se significantemente melhorado (em torno de 10%) com o refino microestrutural para ambas as ligas. Encontrou-se também que em taxas de resfriamento na faixa entre 0,5 e 10 ºC/s, a liga Sn-4%p Zn é aquela que apresenta a menor variação entre limites de resistência à tração (31 a 32 MPa) e alongamento que a liga Sn-12%p Zn (29 a 33 MPa).

Microstructural evolution during upward and downward transient directional solidification of hypomonotectic and monotectic Al–Bi alloys

Upward directional unsteady-state solidification experiments were performed with both a hypomonotectic Al–2.0 wt% Bi alloy and a monotectic Al–3.2 wt% Bi alloy. Besides, the monotectic composition (3.2 wt% Bi) was directionally solidified under downward transient heat flow conditions, which enables the effects of melt convection on the final microstructure to be evaluated since the collective downward movement of Bi-rich particles is favored in such case. This is due to the density differences between the two coexisting liquid phases. The thermal parameters such as cooling rate, growth rate and thermal gradient were experimentally determined by data collected from cooling curves recorded along the casting length. The monotectic features observed in the Al–3.2 wt% Bi alloy castings, i.e. the interphase spacing and Bi-rich particles diameter were correlated with the growth rate and thermal gradient. The cell spacing was experimentally determined for the Al–2.0 wt% Bi alloy as a function of both the cooling rate and tip growth rate. These experimental data were compared with the main predictive cellular growth models from the literature. A comparison between upward and downward unsteady-state solidification results for the interphase spacing and Bi-rich particles diameter has also been conducted.

Gravity-driven inverse segregation during transient upward directional solidification of Sn–Pb hypoeutectic alloys

Upward unidirectional solidification experiments were performed with Sn–4.0 wt.%Pb and Sn–12.5 wt.%Pb alloys. Transitory conditions of heat flow extraction were sustained during these solidification experiments, which mean that all solidification variables (cooling rate, tip growth rate, thermal gradient, and metal/mold heat transfer coefficient) have varied freely along solidification. In this work, the macrosegregation phenomenon is experimentally and theoretically evaluated considering both solidification shrinkage and gravity induced flows. The numerical model is based on a one-dimensional solution consisting of an implicit/explicit time integration scheme to couple thermal and solutal fields, which is supported by a finite volume numerical modeling technique to be solved. The applied solidification system permitted columnar growth to prevail along the castings. Since the Pb-rich melt tends to flow downward (in favor of the gravity vector), the bottom of the castings exhibited positive Pb content, mainly induced by gravity. The numerically simulated and experimental temperature and concentration profiles were compared and a very good agreement has been observed.

Cellular growth during transient directional solidification of hypoeutectic Al–Fe alloys

Investigations have been made of the solidification structure of three hypoeutectic Al–Fe alloys, which were directionally solidified under unsteady-state heat flow conditions. The experiments have been carried out by using a casting assembly, which was designed in such way that the heat was extracted only through the water-cooled system at the casting bottom, promoting vertical upward directional solidification. A combined theoretical/experimental approach was used in order to quantitatively determine the solidification parameters: tip growth rate and cooling rate of Al–0.5 wt%Fe, Al–1.0 wt%Fe and Al–1.5 wt%Fe alloys castings. The experimental results include transient metal/mold heat transfer coefficients, hg, determined from comparisons between the experimental thermal profiles in castings and the simulations provided by a finite difference heat flow program. Cellular microstructures prevailed along the entire castings for any alloy experimentally examined. The cell spacing variation with cooling rate and tip growth rate has been characterized by −0.55 and −1.1 experimental power laws, respectively. The experimental cell spacings were compared with the theoretical predictions furnished by cellular growth models.

PARÂMETROS TÉRMICOS, MACROESTRUTURA E MICROESTRUTURA NA SOLIDIFICAÇÃO DIRECIONAL DA LIGA Al-20%Sn

Al-Sn alloys are widely used in tribological applications. Aluminium and tin form immiscible alloys and the type of the microstructural arrangement causes a good wear resistance. In this study, thermal and microstructural analysis were carried out along an Al-20wt%Sn alloy ingot produced by vertical upward directional transient solidification. The experimental thermal readings were fitted to numerical predictions in order to determine the global heat transfer coefficient (h g ) and were also used to calculate the evolution of thermal variables (tip growth rate and cooling rate) during solidification. The growth rate and the cooling rate play a key role in the microstructural formation. The dendritic microstructure has been characterized by secondary dendritic arm spacings (λ 2 ) which were experimentally determined and correlated to solidification thermal variables. The behavior presented by the Al-20wt%Sn alloy during solidification was similar to that of other aluminum alloys, i.e., the dendritic network became coarser with decreasing cooling rates, indicating that the immiscibility between aluminum and tin has not a significant effect on the relationship between the secondary dendritic arm spacing and cooling rate. An experimental dendritic growth law has been

Experimental analysis of the columnar-to-equiaxed transition in directionally solidified Al–Ni and Al–Sn alloys

Experiments were carried out to investigate the columnar-to-equiaxed transition (CET) during the upward vertical directional solidification of hypoeutectic Al–Ni and Al–Sn alloys. For the growth conditions examined both the thermal and solutal profiles inside the mushy zone and in the overlying melt ahead of the dendrite tips are expected to be stable, since the solute enrichment increases the melt density, and no solute movement induced by convection will affect the CET. From the experimental thermal profiles, tip cooling rates, tip growth rates and thermal gradients have been determined. It was found that the CET occurs for a critical value of cooling rate, with the columnar growth prevailing throughout the casting for cooling rates higher than such critical value, which for a particular binary alloy system is independent of the solute concentration in the hypoeutectic range. For Al–Sn and Al–Ni alloys the critical cooling rates were found to be 0.30 K/s and 0.16 K/s, respectively.

Influence of Forced Convection on Columnar Microstructure during Directional Solidification of Al - Ni Alloys

This paper presents a summary of cellular and dendritic morphologies resulting from the upward directional solidification of Al – Ni alloys in a cylindrical crucible. We analysed the coupling of solid-liquid interface morphology with natural and forced convection. The influence of natural convection was first analyzed as a function of growth parameters (solute concentration, growth rate and thermal gradient). In a second step, the influence of axial vibrations on solidification microstructure was investigated by varying vibration parameters (amplitude and frequency). Experimental results were compared to preliminary numerical simulations and a good agreement is found for natural convection. In this study, the critical role of the mushy zone in the interaction between fluid flow and solidification microstructure is pointed out.

Secondary dendrite arm spacing and solute redistribution effects on the corrosion resistance of Al–10 wt% Sn and Al–20 wt% Zn alloys

In general, aluminum alloys provide the most significant part of all shaped casting manufactured. An optimum range of properties can be obtained as a function of different cooling rate processes, such as sand, plaster, investment, permanent molds and die castings. It is well known that the dendritic network affects not only the mechanical properties but also the corrosion resistance. However, the literature is scarce on reports concerning the influences of dendrite arm spacing on corrosion resistance and mechanical behavior. The aim of this study is to investigate the influence of as-cast microstructure features, i.e., dendrite arm spacing and solute redistribution on the corrosion resistance of samples of aluminum alloys. In order to investigate the electrochemical behavior of solute and solvent of different aluminum systems, samples with the same order of magnitude of dendritic spacings were analyzed to permit comparison between Al–10wt% Sn and Al–20wt% Zn alloys. A casting water-cooled assembly promoting upward directional solidification was used in order to obtain controlled casting samples of these alloys. In order to characterize the dendritic structure, longitudinal sections from the directionally solidified specimens were analyzed by using optical and electronic microscopy techniques. The corrosion resistance was analyzed by both the electrochemical impedance spectroscopy technique and Tafel extrapolation method conducted in a 3% NaCl solution at room temperature. Although both systems present an Al-rich dendritic matrix, different responses to corrosive action as a function of dendritic spacing have been detected.

Cellular growth during transient directional solidification of Pb–Sb alloys

Cellular spacings and solidification thermal variables were determined for three dilute Pb–Sb alloys, which were solidified under unsteady-state heat flow conditions. The experiments have been carried out by using a casting assembly, which was designed in such way that the heat was extracted only through the water-cooled system at the casting bottom, promoting vertical upward directional solidification. An experimental approach, based on thermal readings collected at six different positions from the casting surface, was used in order to quantitatively determine the solidification thermal variables: tip growth rate and cooling rate of Pb–0.3 wt%Sb, Pb–0.85 wt%Sb and Pb–1.9 wt%Sb alloys castings. After grinding, polishing and etching procedures, both longitudinal and transverse sections of the castings were characterized by a typical cellular microstructure which prevailed along the entire casting. The cell spacing variation with cooling rate and tip growth rate was characterized by −0.55 and −1.1 experimental power laws, respectively. The experimental cell spacings were compared with the theoretical predictions furnished by cellular growth models.