Showing papers on "Directional solidification published in 2003"

Anisotropic compressive properties of porous copper produced by unidirectional solidification

Abstract: Porous copper whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification of the melt in a mixture gas of hydrogen and argon. The compressive yield strength of the porous copper with the cylindrical pores orientated parallel to the compression direction decreases linearly with increasing porosity. For the porous copper whose pore axes are perpendicular to the compressive direction, the compressive yield strength slightly decreases in the porosity range up to 30% and then decreases significantly with increasing porosity. The compressive stress–strain curves depend on the compressive direction with respect to the pore direction, which are due to the stress concentration around the pores and the buckling of the copper between the pores. From two different types of stress–strain curve, the energy absorption capacity of the porous copper with the pores parallel to the compressive direction is higher than that perpendicular to the compressive direction at a given porosity.

Microstructure and mechanical properties of Fe–Cr–C eutectic composites

Abstract: In the present investigation, an Fe–Cr–C eutectic alloy was prepared from industry-grade materials and subjected to unidirectional solidification (UDS), through which new types of fibre reinforced composites, eutectic composites, were generated. The composites obtained were examined using X-ray diffraction (XRD), electron microprobe and field emission scanning electron microscopy (FESEM). According to XRD, the composites consist of three phases, i.e. γ, M 7 C 3 and trace amounts of M 23 C 6 . While the compositions of γ and M 7 C 3 were determined by electron microprobe analysis, M 23 C 6 was not detected due to the limited amount present. Fibre morphology was studied as a function of solidification rate ( R ) using FESEM. Fibre diameter, spacing and regularity decreased with increasing solidification rate; however, the volume fraction of fibres remained essentially constant at around 32%. Mechanical testing was carried out on both eutectic composites and as cast alloys. The tensile strength of Fe–Cr–C eutectic composites varied parabolically with solidification rate. When R was 58 mm h −1 , a maximum strength of 2300 MPa was reached, which is about seven times as strong as that of conventional high chromium cast iron of similar composition. Two other alloys with hyper- and hypo-eutectic compositions were included in the present investigation for comparison purposes. It was found that the presence of primary phases significantly diminished fibre regularity and, therefore, the material strength.

Cellular/dendritic transition during unsteady-state unidirectional solidification of Sn–Pb alloys

Abstract: Structural parameters such as grain size, dendritic and cellular spacings, segregated products, porosity and other phases are strongly influenced by the thermal behavior of the metal/mold system during solidification, imposing a close correlation between this and the resulting microstructure. Numerous unidirectional solidification studies have been carried out with the objective of characterizing cellular and dendritic spacings and most of the published work has involved solidification in steady-state heat flow conditions. The objective of this article is to determine the thermal solidification parameters affecting the cellular/dendritic transition as well as to compare theoretical models that predict cellular and primary dendritic spacings with experimental results for unsteady-state solidification. Experiments were carried out in a water cooled unidirectional solidification apparatus and dilute alloys of the Sn–Pb system have been used (Sn 1.5 wt.% Pb, Sn 2.5 wt.% Pb and Sn 5 wt.% Pb).

Stabilization of thermosolutal convective instabilities in Ni-based single-crystal superalloys: Carbide precipitation and rayleigh numbers

Abstract: The influence of carbide precipitation on grain-defect formation during unidirectional solidification of experimental single-crystal Ni-based superalloys has been assessed over a wide range of compositions with large variations in Re, W, and Ta. In all instances, carbon additions of up to 0.15 wt pct were determined to be statistically significant with respect to stabilizing against the formation of grain defects, such as freckle chains, during solidification. Assessment of the segregation behavior of the constituent alloying additions via a Scheil-type analysis enabled estimation of critical Rayleigh numbers denoting the onset of thermosolutal convection. Precipitation of Ta-rich MC carbides near the liquidus temperature of the alloy was found to interact strongly with the mechanisms associated with freckle formation. Segregation analyses and phase-transformation temperature measurements were used to assess the corresponding Rayleigh numbers for the experimental alloys and to modify the Rayleigh criterion to account for carbide precipitation. Mechanisms pertaining to the interaction of carbides with the onset of thermosolutal convection are discussed.

Cellular spacings in unsteady-state directionally solidified Sn–Pb alloys

Abstract: Solidification thermal parameters, i.e. transient metal/mold heat transfer coefficient, tip growth rate and cooling rate, and cellular spacings have been measured in dilute Sn–Pb alloys solidified under unsteady-state heat flow conditions. It has been found that the cellular spacings decreased with increasing solute content up to the limit of the cellular range, at about 2 wt.% Pb. A sharp increase in spacing is observed when the growth morphology changes from cellular to cellular/dendritic. The Hunt–Lu’s model applied to cellular growth did not generate the experimental observations for the alloys examined in the present study. It is proposed the insertion of analytical expressions for tip growth rate and cooling rate into the resulting experimental equations permitting to establish empirical formulae relating cellular spacings with unsteady-state solidification parameters like: melt superheat, type of mold and transient metal/mold heat transfer coefficient.

Fixed-grid front-tracking algorithm for solidification problems, part i: method and validation

Abstract: This article presents a method for solving moving-boundary problems associated with phase change. This method is an explicit interface-tracking scheme that involves the reconstruction and advection of the moving interface on a fixed grid. Three distinct steps are undertaken to handle the movement of the interface: advection and reconstruction (tracking); calculation of normal velocities; and the solution of the governing equations for different phases. Details of each step and its implementation are provided. The transient heat diffusion equation in two space dimensions is the governing equation for energy transport. In order to validate the approach, results obtained from the front-tracking scheme are compared with exact analytical solutions for melting problems in Cartesian and cylindrical coordinates. It is shown that the numerical and analytical results are in excellent agreement. Finally, problems involving the multidimensional solidification of a pure aluminum ingot subject to bi-directional heat ex...

Effect of the temperature gradient, growth rate, and the interflake spacing on the microhardness in the directionally solidified Al-Si eutectic alloy

Abstract: An alloy of composition Al-12.6 wt.% Si was prepared using metals of 99.99% purity. Weighed amounts of aluminium and silicon were melted in the vacuum-melting furnace. This irregular eutectic alloys were directionally solidified upward with a constant growth rate V (8.3×10−3 mm/s) and different temperature gradients G (2.0–7.8 K/mm) and also with a constant temperature gradient G (7.8 K/mm) and different growth rates V (8.3–498.7×10 −3mm/s) in the directional solidification furnace. The interflake spacings λ and microhardness HV were measured from both transverse section and longitudinal section of the specimen. The variations of HV with respect to G, V, and λ have been determined by using the linear regression analysis method. It has been shown that HV increases with the increasing values of G and V. On the other hand HV values decreases with the increasing λ values. The Hall-Petch type relationships obtained in this work have been compared with the previous works.

Effect of carbon on microstructure and high-temperature strength of NbMoTiSi in situ composites prepared by arc-melting and directional solidification

Abstract: Improvement of high-temperature strength of Nb–(10–20)Mo–22Ti–18Si alloys by adding carbon and directional solidification technique has been investigated. Vickers hardness HV and compressive strength at 1470 and 1670 K were examined. The as-cast specimen without C composes of Nb solid solution (Nb SS ) and (Nb,Mo,Ti) 5 Si 3 silicide, while (Nb,Ti)C carbide obviously appears when contain more than 2.4 mol% carbon. The amount of carbide and silicide phases increases with increasing carbon content. The directionally solidified samples have the same phases as the as-cast ones and show coarse microstructure slightly oriented to the direction of growth. A small amount of carbon addition lowers both the HV and strength at 1470 and 1670 K, where corresponds with the appearance of the carbide phase. By adding more than 4.8 mol% carbon, however, the HV , 0.2% yield strength ( σ 0.2 ) and maximum strength ( σ max ) tend to increase. The directional solidification samples show higher strength and work hardening rate than the as-cast ones at corresponding C content. The 10 mol% Mo sample is weakened by C addition, but a considerable improvement in strength at elevated temperatures can be predicted when more than 10 mol% C is added to the 20 mol% Mo material. The sliding and debonding of the Nb SS /silicide interfaces is dominant in compressive damage at elevated temperatures.

Preliminary in situ and real-time study of directional solidification of metallic alloys by x-ray imaging techniques

Abstract: During directional solidification of a binary alloy, the solid–liquid interface exhibits a variety of patterns that are due to the Mullins–Sekerka instability and governed by the growth conditions. It is well known that properties of the grown material are largely controlled by the microstructures left in the solid during processing. Thus, a precise mastering of the solidification is essential to tailor products in a reproducible fashion to a specified quality. One major difficulty for this study is the real-time and in situ observation of the interface, especially for metallic alloys. A possibility is to use an intense and coherent third generation x-ray beam. By combining different x-ray imaging techniques (absorption/phase contrast radiography and diffraction topography), we have studied the directional melting and solidification of aluminium-based alloys. The preliminary results show the great potential of these techniques for the study of the coupling between stress effects and microstructure formation in solidification processing.

Three-dimensional simulation of freckle formation during binary alloy solidification: effect of mesh spacing

Abstract: Three-dimensional numerical simulations are performed of freckling during directional solidification of a binary metal alloy. The purpose of the study is to evaluate the sensitivity of the predictions to the grid resolution. Detailed results are provided for the liquid concentration, solid fraction, and liquid velocity distributions at different times. It is shown that, while it is possible to simulate freckle formation with a relatively coarse grid, many local details are predicted inaccurately. The results for the finest grid indicate that complete grid independence is difficult to achieve given present-day computing resources.

Cellular-to-dendritic transition during the directional solidification of binary alloys

Abstract: The transition from a cellular to dendritic microstructure during the directional solidification of alloys is examined through experiments in a transparent system of succinonitrile (SCN)-salol. In a cellular array, a strong coupling of solute fields exists between the neighboring cells, which leads not only to multiple solutions of primary spacing, but also includes multiple solutions of amplitude, tip radius, and shape of the cell. It is found that these multiple solutions of different microstructural features in a cellular array, obtained under fixed growth conditions and compositions, play a key role in the cell-dendrite transition (CDT). The CDT is controlled not only by the input parameters of alloy composition (C 0), growth rate (V), and thermal gradient (G), but also by microstructure parameters such as the local primary spacing. It is shown that the CDT is not sharp, but occurs over a range of growth conditions characterized by the minimum and maximum values of V/G. Within this transition range, a critical spacing is observed above which a cell transforms to a dendrite. This critical spacing is given by the geometric mean of the thermal, diffusion, and capillary lengths and is inversely proportional to composition in weight percent.

Seaweed to dendrite transition in directional solidification.

Abstract: We simulate directional solidification using a phase-field model solved with adaptive mesh refinement. For small surface tension anisotropy directed at 45 � relative to the pulling direction we observe a crossover from a seaweed to a dendritic morphology as the thermal gradient is lowered, consistent with recent experimental findings. We show that the morphology of crystal structures can be unambiguously characterized through the local interface velocity distribution. We derive semiempirically an estimate for the crossover from seaweed to dendrite as a function of thermal gradient and pulling speed.

Effect of Al2O3 particles on mechanical properties of directionally solidified intermetallic Ti–46Al–2W–0.5Si alloy

Abstract: The effect of Al2O3 particles on microhardness and room-temperature compression properties of directionally solidified (DS) intermetallic Ti–46Al–2W–0.5Si (at.%) alloy was studied. The ingots with various volume fractions of Al2O3 particles and mean α2–α2 interlamellar spacings were prepared by directional solidification at constant growth rates ranging from 2.78×10−6 to 1.18×10−4 ms−1 in alumina moulds. The ingots with constant volume fraction of Al2O3 particles and various mean interlamellar spacings were prepared by directional solidification at a growth rate of 1.18×10−4 ms−1 and subsequent solution annealing followed by cooling at constant rates v varying between 0.078 and 1.889 K s−1. The mean α2–α2 interlamellar spacing λ for both DS and heat-treated (HT) ingots decreased with increasing cooling rate according to the relationship λ∝ ν −0.46. In DS ingots, microhardness, ultimate compression strength, yield strength and plastic deformation to fracture increased with increasing cooling rate. In HT ingots, microhardness and yield strength increased and ultimate compression strength and plastic deformation to fracture decreased with increasing cooling rate. The yield stress increased with decreasing interlamellar spacing and increasing volume fraction of Al2O3 particles. A linear relationship between the Vickers microhardness and yield stress was found for both DS and HT ingots. A simple model including the effect of interlamellar spacing and increasing volume fraction of Al2O3 particles was proposed for the prediction of the yield stress.

Study of Microstructure and Solute Partitioning in a Cast Ti-48Al-2Cr-2Nb Alloy by Quenching during Directional Solidification Technique

Abstract: One major hindrance to effective implementation of cast gamma TiAl-based intermetallic alloys in aircraft engines lies in the variability of their mechanical properties resulting from chemical and microstructural heterogeneities. In the present work, the buildup of microsegregation in a cast Ti-48Al-2Cr-2Nb alloy is investigated through experiments of quenching during directional solidification (QDS). The solidification process, as well as the partitioning of alloying elements, between the solid and liquid phases, is investigated. Considering experimental conditions, the α-hcp phase is found to be the primary solidifying phase. A low dendrite tip temperature of 1475 °C was estimated from thermal recordings. These observations could be explained considering the value of the thermal gradient (around 4 °C/mm). Quantitative values of partition coefficients are proposed for Al, Cr, and Nb. In addition to Al, Cr is found to segregate in interdendritic regions, whereas Nb tends to be retained in the Ti-rich inner dendrites. Considering experimental cumulative solute distributions, the buildup of microsegregation can be satisfactorily represented on the basis of Gulliver-Scheil assumptions. Due to high-temperature quenching, the QDS experiments are also found to be appropriate to the study of high-temperature phase transformations and microstructural development of TiAl-based alloys. The results of QDS experiments are discussed with regard to the range of microstructural and chemical heterogeneities determined within Ti-48Al-2Cr-2Nb investment castings. Finally, regarding solid-state phase transformations subsequent to solidification, the study attempts to explain the formation of B2 phase particles stabilized by the ternary additions.

Effect of growth rates and temperature gradients on the spacing and undercooling in the broken-lamellar eutectic growth (Sn-Zn eutectic system)

Abstract: The Sn-Zn system has a eutectic structure of a broken lamellar type. Dependence of the broken-lamellar spacing λ and the undercooling ΔT on V and G were investigated, and the relationship between them was examined. A Sn-Zn (99.99%) high-purity eutectic alloy was melted in a graphite crucible under vacuum atmosphere. This eutectic alloy was directionally solidified upward with a constant growth rate V (8.30 µm/s) and different temperature gradients G (1.86–6.52 K/mm), and also with a constant temperature gradient (6.52 K/mm) and different growth rates (8.30–165.13 µm/s) in a Bridgman-type directional solidification furnace. The lamellar spacings λ were measured from both transverse and longitudinal sections of the specimen. The λ values from the transverse section were used for calculations and comparisons with the previous works. The undercooling values ΔT were obtained using growth rate and system parameters K 1 and K 2. It was found that the values of λ decreased while V and G increased. The relationships between lamellar spacing λ and solidification parameters V and G were obtained by linear regression analysis method. The λ2 V, ΔTλ, ΔTV −5, and λ3 G values were determined using λ, ΔT, V, and G values. The experimentally obtained values for the broken-lamellar growth (Sn-Zn eutectic system) were in good agreement with the theoretical and other experimental values.

Reusable crucible for silicon ingot growth

Abstract: A silicon nitride crucible is coated with a crucible release coating for use in directional solidification of multicrystalline silicon ingots. The crucible preferably includes reaction bonded silicon nitride crucible. After removing the silicon ingot, the release coating is easily removed and the crucible can be repeatedly recoated and reused.

Analysis of yield rate in single crystal casting process using an engineering simulation model : Special issue on solidification science and processing for advanced materials

Abstract: A 2-D engineering model for grain selection has been developed taking the columnar dendrite growth theory into consideration. After evaluating this model via a unidirectional solidification experiment, the single-crystal casting process was simulated. Since the time required for calculation is rather short, a statistical analysis has been performed for the first time. The yield rate of well-oriented single crystal is increased by increasing the initial number of grains on the chill plate. However, the yield rate does not exceed approximately 90%. A detailed investigation of the formation mechanism of misorientation has revealed two possible processes (Type A and Type B) that may occur during single crystal casting process.