Upward Directional Solidification Research Articles

An experimental investigation of the effects of radiation and thermal parameters on the aluminum 1050 obtained from upward directional solidification

During the reactor operation, aluminum tank and the core components can experience radiation damage, resulting in limitation of its life span. Effects of radiation on the aluminum alloys are not yet fully understood, especially when material is obtained from upward directional solidification under transient heat-flow condition. For this paper, an experimental research has been conducted to investigate the radiation effects on the as-cast aluminum 1050, with focus on the microstructure, microhardness and porosity for different position along casting. Firstly, thermal parameters were determined during solidification experiment and then correlated with cellular spacings, microhardness and porosity content. The experimental results show that thermal parameters such as solidification speed, thermal gradient and cooling rate exerts an influence on the microstructure, microhardness and porosity. Experimental laws for microstructure, microhardness and porosity have been determined, indicating that the increase in thermal parameters has induced a refinement effect on the microstructure, increase in microhardness and a decrease in the porosity content. Further, measurable effects of the gamma radiation on the microstructure, microhardness and porosity formation along the ingot of aluminum 1050, were experimentally determined. It stands out among results that gamma radiation has favored an increase in cellular spacings, microhardness and porosity in ingot, obtained from a vertical upward directional solidification. However, even after the gamma radiation exposure, microstructure, microhardness and porosity found along the casting are dependent of the solidification thermal conditions.

Magnetic field-assisted solidification of W319 Al alloy qualified by high-speed synchrotron tomography

Magnetic fields have been widely used to control solidification processes. Here, high-speed synchrotron X-ray tomography was used to study the effect of magnetic fields on solidification. We investigated vertically upward directional solidification of an Al-Si-Cu based W319 alloy without and with a transverse magnetic field of 0.5 T while the sample was rotating. The results revealed the strong effect of a magnetic field on both the primary α-Al phase and secondary β-Al5FeSi intermetallic compounds (IMCs). Without the magnetic field, coarse primary α-Al dendrites were observed with a large macro-segregation zone. When a magnetic field is imposed, much finer dendrites with smaller primary arm spacing were obtained, while macro-segregation was almost eliminated. Segregated solutes were pushed out of the fine dendrites and piled up slightly above the solid/liquid interface, leading to a gradient distribution of the secondary β-IMCs. This work demonstrates that rotating the sample under a transversal magnetic field is a simple yet effective method to homogenise the temperature and composition distributions, which can be used to control the primary phase and the distribution of iron-rich intermetallics during solidification.

Investigation on machinability in turning of as-cast and T6 heat-treated Al-(3, 7, 12%)Si-0.6%Mg alloys

The machinability in turning of Al-Si-Mg alloys under as-cast and heat-treated conditions has been investigated. Three alloys with 3%, 7% and 12%Si and 0.6%Mg contents (wt%) were prepared. Unsteady solidified ingots were obtained in a vertical upward directional solidification apparatus using an instrumented cylindrical water-cooled mold. Samples extracted from the ingots were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) examinations and by Brinell hardness tests, before and after T6 heat treatments (540 °C solutioning temperature for 10 h, water quenching at 25 °C and artificial aging at 155 °C for 5 h). Specimens in both the as-cast and heat-treated conditions were subjected to machinability tests as a function of cutting force in face turning operation using a CNC lathe machine, fixing parameters as cutting depth, feed rate, cutting speed, and cutting tool. The as-cast microstructure consists of a dendritic α-Al rich phase and an interdendritic eutectic microconstituent (α-Al + Si particles). As the alloy Si content increased, the solidification cooling rate increased, the amount and size of α-Al dendrites decreased while the amount of eutectic mixture increased. After heat treatment, Si particles were found to be more rounded and an increase in fraction of Mg2Si was observed. The hardness profile declines along the length of the as-cast ingots, however the values improved with both alloy Si content and heat treatment. The as-cast alloys showed maximum cutting force at the beginning of the process, decreasing during machining progress, whereas in the heat-treated condition the cutting forces showed values almost constant. The results suggest a more stable machinability behavior during turning of the heat-treated alloys as a consequence of a more homogenous microstructure and hardness.

Morphology evolution of abnormal tertiary dendrite by diffusion- controlled remelting/resolidification in directionally solidified Sn–Mn peritectic alloy

Although the growth of tertiary dendrite is also important in influencing the properties of alloys, the analysis on it is relatively scarce since the growth of it is always assumed to catch up to the tips of growing primary dendrite. Distinct from this, the abnormal tertiary dendrites which grow towards lower temperature has been observed on thicker secondary branches during upward directional solidification of Sn–40at.%Mn peritectic alloy. The experimental result shows that the dendritic structure is influenced by not only the temperature gradient zone melting (TGZM) but also the Gibbs-Thomson (G-T) effects which can induce diffusion-controlled remelting/resolidification processes. Further examination on this abnormal tertiary dendrite arm shows its gradual decomposition during directional solidification. Analysis indicates that the decomposition of it is closely related with these remelting/resolidification processes. The remelting rate at the back surface of the thicker secondary branch is found to be lower than that at the tip of abnormal tertiary dendrite. However, the remelting process is first finished on the back edge of thicker secondary branch where no abnormal tertiary dendrite exist due to the shorter length there. A diffusion-controlled analytical model incorporating peritectic reaction and both effects is established to analyze the decomposition process, and the Ivantsov's equation-based solutions have been introduced, which agrees reasonably well with the experimental observation.

Dendritic Spacing/Columnar Grain Diameter of Al–2Mg–Zn Alloys Affecting Hardness, Tensile Properties, and Dry Sliding Wear in the As‐Cast/Heat‐Treated Conditions

Inter‐relations of solidification conditions, as‐cast and heat‐treated structures, and mechanical properties are extremely useful in the design of components, especially Al‐based alloys. Herein, the effects of secondary dendrite arm spacing (λ2) and columnar grain diameter (CGD) on hardness, tensile, and dry sliding wear responses of Al−2Mg‐(5 and 8)Zn (wt%) alloys in the as‐cast and heat‐treated conditions, respectively, are focused upon. The alloys are melted and solidified in an instrumented upward directional solidification apparatus under nonsteady‐state heat transfer conditions. Samples are cut from the solidified ingots and subjected to the T6 heat treatment. Samples are characterized metallographically by optical and scanning electron microscopies and mechanically by Brinell hardness, tensile, and dry sliding wear tests. It is found that higher Zn concentration and solidification cooling rates refine λ2 and the CGD. Hardness and tensile properties are improved with the decrease in λ2 in the as‐cast condition of both alloys. After heat treatment, all these properties increase with the decrease in CGD. Wear parameters show that only the alloy having a higher Zn content is associated with better wear resistance. Correlations between the wear rate as a function of λ2 and CGD are established by experimental equations.

Large–scale phase–field lattice Boltzmann study on the effects of natural convection on dendrite morphology formed during directional solidification of a binary alloy

The effects of natural convection on dendrite morphologies formed during solidification of a three–dimensional (3D) bulk crystal have not been clarified yet. Although numerical simulation is key to solve the problem, it is not straightforward from a computational cost point of view to simulate the growth of multiple dendrites and liquid flow simultaneously. In this study, we numerically investigate the effects of natural convection on the dendrite morphologies formed during 3D upward directional solidification of an Al–3 wt%Cu alloy, providing a comparison with two–dimensional (2D) simulations. Large–scale simulations of a model coupling phase–field and lattice Boltzmann methods are performed by parallel computation using multi graphics processing units on a supercomputer. As a result of a series of simulations where gravity is changed, similar results are obtained in the 3D and 2D simulations: the average primary arm spacing increases as the gravity decreases in the negative region, the large upward flow causes unstable dendrite growth, and the downward flow enhances the growth of secondary arms. On the other hand, because the natural convection caused under gravity is less in the 3D than the 2D case, the effect of natural convection in the 3D bulk is smaller than in the 2D. In addition, due to differences in flow patterns in front of dendrite tips, there are some distinctions in dendrite growth between 2D and 3D cases.

Investigation of Thermo-Electro-Magnetic force on equiaxed grain motion during upward directional solidification

It has been shown that the coupling between the magnetic field and the temperature gradient generates a Thermo-Electro-Magnetic (TEM) Force that dramatically modifies the trajectory of the equiaxed grains during solidification of Al – 10 wt%Cu metallic alloys. We analyze the motion of equiaxed grains both experimentally by Synchrotron X-ray radiography, and theoretically by numerical modeling. The time evolution of trajectory characteristics, like the deviation angle of the equiaxed grain and the grain velocities, were determined as a function of the temperature gradient, the cooling rate and the direction of the magnetic field. By means of numerical modeling, we performed calculations of the Thermo-Electro-Magnetic (TEM) force and Thermo-Electro-Magnetic-Convection (TEMC) in the liquid acting on a solid particle which mimics an equiaxed envelope. Numerical calculations of the viscous drag force were performed firstly on a dendrite envelope, considering the wall confinement. This allowed us to calculate the terminal grain velocity. It was found that the measured grain velocities, as well as their deflection angle measurements were in good agreement with the numerical predictions.

Cellular/dendritic transition, dendritic growth and microhardness in directionally solidified monophasic Sn-2%Sb alloy

Abstract Horizontal directional solidification experiments were carried out with a monophasic Sn-2%Sb (mass fraction) alloy to analyze the influence of solidification thermal parameters on the morphology and length scale of the microstructure. Continuous temperature measurements were made during solidification at different positions along the length of the casting and these temperature data were used to determine solidification thermal parameters, including the growth rate ( V L ) and the cooling rate ( T R ). High cooling rate cells and dendrites are shown to characterize the microstructure in different regions of the casting, with a reverse dendrite-to-cell transition occurring for T R >5.0 K/s. Cellular ( l c ) and primary dendrite arm spacings ( l 1 ) are determined along the length of the directionally-solidified casting. Experimental growth laws relating l c and l 1 to V L and T R are proposed, and a comparative analysis with results from a vertical upward directional solidification experiment is carried out. The influence of morphology and length scale of the microstructure on microhardness is also analyzed.

Microstructure Evolution and Macro Segregation of Directionally Solidified Hypoeutectic and Hypereutectic Lead-Tin Alloys

Unidirectional solidification is preferred to multidirectional solidification for growing crystals in a particular direction. An experimental set-up consisting of Bridgman type of upward directional solidification was employed for the present investigation. The main aim of the present investigation was to assess the effect of unidirectional upward solidification on the segregation of off-eutectic Pb-Sn alloys at different translational speeds of the experimental set-up. Solidification experiments were conducted on hypoeutectic and hypereutectic Lead-Tin alloys. Different combinations of growth rate V and composition Co. were used to investigate their effect on longitudinal macro segregation. Macro segregation along the length of the samples was observed in hypoeutectic Pb-Sn alloys whereas no such macro segregation was observed in hypereutectic alloys. The intensity of longitudinal macro segregation was found to increase with the increase in initial tin content of the alloy, increase in distance from the chill end and decrease in the solidification rate.

The Role of Si and Cu Alloying Elements on the Dendritic Growth and Microhardness in Horizontally Solidified Binary and Multicomponent Aluminum-Based Alloys

Horizontal directional solidification (HDS) experiments were carried out with Al-3wtpctCu, Al-3wtpctSi, and Al- 3wtpctCu-5.5wtpctSi alloys in order to analyze the interrelation between the secondary dendrite arm spacing (λ 2) and microhardness (HV). A water-cooled horizontal directional solidification device was applied. Microstructural characterization has been carried out using traditional techniques of metallography, optical, and SEM microscopy. The ThermoCalc software was used to generate the phase equilibrium diagrams as a function of Cu and Si for the analyzed alloys. The effects of Si and Cu elements on the λ 2 and HV evolution of the hypoeutectic binary Al-Cu and Al-Si alloys have been analyzed as well as the addition of Si in the formation of ternary Al-Cu-Si alloy. The secondary dendrite arm spacing was correlated with local solidification thermal parameters such as growth rate (V L), cooling rate (T R), and local solidification time (t SL). This has allowed to observe that power experimental functions given by λ 2 = Constant (V L)−2/3, λ 2 = Constant (T R)−1/3 and λ 2 = Constant (t SL)1/3 may represent growth laws of λ 2 with corresponding thermal parameters for investigated alloys. Hall–Petch equations have also been used to characterize the dependence of HV with λ 2. A comparative analysis is performed between λ 2 experimental values obtained in this study for Al-3wtpctCu-5.5wtpctSi alloy and the only theoretical model from the literature that has been proposed to predict the λ 2 growth in multicomponent alloys. Comparisons with literature results for upward directional solidification were also performed.

Evolution of Columnar-Dendritic Microstructure during Upward Directional Solidification of Eutectic and Off-Eutectic Pb-Sn Alloys

Upward directional solidification experiments were carried out on various Pb-Sn alloys using water cooled copper chills. Different combinations of growth rate V and composition Co were used to investigate their effect on microstructure evolution during solidification. The effect of solidification velocity and composition on the structure formation is analyzed. It is observed that the hypoeutectic Pb-Sn alloys exhibit columnar dendritic structure with α phase rich region. In case of hypereutectic alloys the columnar dendritic structure is with β phase rich region. In both the cases dendritic structure was found to surround by the eutectic structure, a typical result of non equilibrium cooling. The eutectic composition exhibits complete eutectic structure aligned in the direction of solidification.

Analysis of the Microstructural Evolution during the Transient Upward, Downward and Horizontal Directional Solidification of the Al-1.2wt%Pb Monotectic Alloy

Monotectic alloys are of great industrial importance because of their favorable tribological behavior. Many studies in order to better understand the morphologies obtained by monotectic reaction have been developed. To better understand the morphologies obtained by monotectic Al-1.2wt%Pb alloy reaction, especially in relation to induced convective flow, this paper aims to compare the microstructural evolution of the alloy obteind by directional solidification in transient heat-flow conditions in upward, downwand and horizontal solidification devices. It was observed a morphology transition from particles to fibers on upward and downward cases and a morphology of only particles on the horizontal case. The classical relationship used for eutectic growth, λ2v = C, which was considered applicable to monotectic reactions, didn’t seemed to be valid in the interphase spacing evolution for the downward device, however power functions (λ = C.va) were found in all cases.

Influence of upward and horizontal growth direction on microstructure and microhardness of an unsteady-state directionally solidified Al-Cu-Si alloy

In order to analyze the effect of the growth direction on dendrite arm spacing (λ1) and microhardness (HV) during horizontal directional solidification (HDS), experiments were carried out with the Al-3wt.%Cu-5.5wt.%Si alloy and the results compared with others from the literature elaborated for upward directional solidification (UDS). For this purpose, a water-cooled directional solidification experimental device was developed, and the alloy investigated was solidified under unsteady-state heat flow conditions. Thermal parameters such as growth rate (VL) and cooling rate (TR) were determined experimentally and correlations among VL, TR, λ1 and HV has been performed. It is observed that experimental power laws characterize λ1 with a function of VL and TRgiven by: λ1=constant(VL)-1.1 and λ1=constant(TR)-0.55. The horizontal solidification direction has not affected the power growth law of λ1 found for the upward solidification. However, higher values of λ1 have been observed when the solidification is developed in the horizontal direction. The interrelation of HV as function of VL, TR and λ1 has been represented by power and Hall-Petch laws. A comparison with the Al-3wt.%Cu alloy from literature was also performed and the results show the Si element affecting significativaly the HV values.

Microstructural development and mechanical properties of a near-eutectic directionally solidified Sn–Bi solder alloy

Sn–Bi solders may be applied for electronic applications where low-temperature soldering is required, i.e., sensitive components, step soldering and soldering LEDs. In spite of their potential to cover such applications, the mechanical response of soldered joints of Sn–Bi alloys in some cases does not meet the strength requirements due to inappropriate resulting microstructures. Hence, careful examination and control of as-soldered microstructures become necessary with a view to pre-programming reliable final properties. The present study aims to investigate the effects of solidification thermal parameters (growth rate — VL and cooling rate — ṪL) on the microstructure of the Sn–52wt.%Bi solder solidified under unsteady-state conditions. Samples were obtained by upward directional solidification (DS), followed by characterization through metallography and scanning electron microscopy (SEM). The microstructures are shown to be formed by Sn-rich dendrites decorated with Bi precipitates surrounded by a complex regular eutectic mixture, with alternated Bi-rich and Sn-rich phases. Experimental correlations of primary (λ1), secondary (λ2), tertiary (λ3) dendritic and eutectic spacings (λcoarse and λfine) with cooling rate and growth rate are established. Two ranges of lamellar eutectic sizes were determined, described by two experimental equations λ=1.1 VL−1/2 and λ=0.67 VL−1/2. The onset of tertiary branches within the dendritic array along the Sn–52wt.%Bi alloy DS casting is shown to occur for cooling rates lower than 1.5°C/s.

Electrochemical and Mechanical Behavior of Lead-Silver and Lead-Bismuth Casting Alloys for Lead-Acid Battery Components

The present study focuses on the interrelation of microstructure, mechanical properties, and corrosion resistance of Pb-Ag and Pb-Bi casting alloys, which can be used in the manufacture of lead-acid battery components, as potential alternatives to alloys currently used. A water-cooled solidification system is used, in which vertical upward directional solidification is promoted permitting a wide range of microstructures to be investigated. Correlations between microstructural arrays, tensile strengths, and corrosion resistances of Pb-1 wt pct Ag, Pb-2.5 wt pct Ag, Pb-1 wt pct Bi, and Pb-2.5 wt pct Bi alloys are envisaged. It is shown that a compromise between corrosion resistance (represented by the corrosion current density) and mechanical properties (represented by the ultimate tensile strength) can be obtained. Comparisons between specific strengths and mechanical/corrosion ratios are also made. It is also shown that, for microstructures solidified under cooling rates higher than 10 K/s, the Pb-Ag alloys exhibit higher specific strength and mechanical/corrosion ratio. In contrast, for casting processes in which the cooling rates are lower than 5 K/s, the dilute Pb-Bi alloy (i.e., 1 wt pct Bi) is shown to have more appropriate requirements for lead-acid battery components. Comparisons between specific strengths, mechanical/corrosion ratio, and relative weight and cost with Pb-Sn and Pb-Sb alloys are also made.

Measurements of microhardness during transient horizontal directional solidification of Al-Rich Al-Cu alloys: Effect of thermal parameters, primary dendrite arm spacing and Al2Cu intermetallic phase

In this work, the effect of the growth rate (VL) and cooling rate (TR), primary dendritic arm spacing (λ1) and Al2Cu intermetallic phase on the microhardness was investigated during transient horizontal directional solidification of Al-3wt%Cu and Al-8wt%Cu alloys. Microstructural characterization of the investigated alloys was performed using traditional techniques of metallography, optical and SEM microscopy and X-Ray diffraction. The microhardness evolution as a function of the thermal and microstructural parameters (VL, TR, and λ1) was evaluated using power and Hall-Petch type experimental laws, which were compared with other laws in the literature. In order to examine the effect of the Al2Cu intermetallic phase, microhardness measurements were performed in interdendritic regions. Finally, a comparative analysis was performed between the experimental data of this work and theoretical models from the literature that have been proposed to predict primary dendrite arm spacing, which have been tested in numerous works considering upward directional solidification.

Investigation of segregation and density profiles in the mushy zone of CMSX-4 superalloys solidified during downward and upward directional solidification processes

Single-crystal CMSX-4 superalloy bars were casted by using downward (DWDS) and upward (Bridgman) directional solidification processes. The interdendritic segregation along the mushy zone of the quenched samples was measured and the corresponding concentration profiles are comparatively presented. The profiles are translated into liquid density profiles using a numerical model. It was deduced from this observation that, along the mushy zone of DWDS solidified samples, the content of elemental Al increased with increasing distance from the tip of the dendrites, but Re and W gradually decreased. The segregation profiles of these alloying elements in the Bridgman solidified samples demonstrated the same tendency. The results of density calculations suggest that the interdendritic melt becomes lighter with increasing depth in the mushy zone of the Bridgman solidified samples. This indicates that density inversion occurred. The density profiles in the DWDS solidified samples showed an analogous tendency. However, due to the opposite direction of growth, the density inversion did not appear in the DWDSs′ samples. This suggests that the DWDS process can essentially avoid the occurrence of freckles, caused by density inversion, which are serious macro-defects in single-crystal superalloy turbine blade.

Mechanical performance and microstructure array of as-cast lead–silver and lead–bismuth alloys

The aim of this study is to establish correlations between mechanical properties of Pb–Ag and Pb–Bi alloys and parametric features of their as-cast microstructures, as well as to develop a comparative analysis with the corresponding properties of Pb–Sn alloys considering applications of these alloys in the manufacture of Pb-acid battery components. A wide range of microstructures are obtained using an upward water-cooled directional solidification system. Ultimate (UTS) and yield tensile strengths (YS) and elongation are experimentally determined as a function of cellular and dendritic spacings, and Hall-Petch type experimental equations are proposed relating UTS to these microstructure parameters. Despite the higher specific strengths of Pb–Ag alloys, as compared with those of Pb–Bi and Pb–Sn alloys, their corresponding relative costs are the highest of all Pb-based alloys examined. It is found that the Pb–Bi and Pb–Sn alloys examined have similar specific strengths and relative costs.

In Situ Investigation of Dendrite Deformation During Upward Solidification of Al-7wt.%Si

The in situ investigation of dendrite deformation during solidification processing is an ongoing challenge because of the technical difficulties in carrying out experimental observations and direct measurements in metallic systems that are opaque to visible light and have a high melting temperature. Over the last 20 years, x-ray imaging has been established as a method of choice to overcome this experimental barrier by taking advantage of the increased capabilities of third-generation synchrotron x-ray sources, providing information on growth process and semisolid deformation that is not available otherwise. In this article, we present results showing that the unique combination of synchrotron x-ray radiography and synchrotron white beam x-ray topography can help to reveal the deformations that dendritic microstructures undergo during the upward directional solidification of Al-7wt.%Si alloys. Particular focus will be placed on the bending phenomena of dendrites because of gravity, which may precede fragmentation in the case of well-developed secondary arms.

Experimental Studies on Microstructure Evolution and Macro Segregation during Upward Directional Solidification of Lead-Tin Alloys

Directional solidification experiments were carried out on hypoeutectic and hypereutectic Pb-Sn alloys. Different combinations of growth rate V and composition Co. were used to investigate their effect on longitudinal macro segregation. Macro segregation along the length of the samples was observed in hypoeutectic Pb-Sn alloys whereas no such macro segregation was observed in hypereutectic alloys. The intensity of longitudinal macro segregation was found to increase with the increase in initial tin content of the alloy, increase in distance from the chill end and decrease in the solidification rate.