During Directional Solidification Research Articles

Enhancing the removal of Fe and Ti from metallurgical silicon by slagging-directional solidification

Metallurgical preparation of solar-grade silicon is the most popular method, benefiting from the low costs and environmentally friendly. The paper employed a metallurgical method to purify silicon and investigated the removal of Fe and Ti from silicon by directional solidification synergized with slagging refining (slagging-directional solidification, SDS). The result showed that lower solidification velocity and stronger cooling conditions during directional solidification (DS) contributed to removing Fe and Ti impurities from ingots. The concentrations of Fe and Ti at the beginning of the ingot obtained by DS at optimum conditions were 0.04 wt% and 0.002 wt%. Based on this, the influence of SDS on Fe and Ti was investigated, finding the concentrations of Fe and Ti at the beginning of the ingot were 0.021 wt% and 0.001 wt% with removal rates of 98.55 % and 98.86 %. Compared to ingots obtained by DS, the removal rates of Fe and Ti increased by 1.31 % and 1.13 %, respectively. The results showed that SDS has been proven to enhance the removal of Fe and Ti from ingot. Finally, the removal mechanism of SDS was analyzed. The method of directional solidification synergized with slagging refining was a novel method to purify with a low cost and environmentally friendly.

Phase-Field Simulation of the Microstructure Evolution in the Eutectic Alloy NiAl-31Cr-3Mo

The directionally solidified eutectic alloy NiAl-(Cr,Mo) is a promising candidate for structural applications at high temperatures, due to its increased creep resistance compared to its single phase B2ordered NiAl counterpart. This system yields an eutectic trough connecting the invariant reactions of the ternary alloys NiAl-Cr and NiAl-Mo. During directional solidification (DS) along this trough the evolved microstructures of the two-phase eutectic is changing from fibrous to lamellar and back to fibrous morphology while increasing and decreasing the amounts of Mo and Cr, respectively. To investigate these effects in the morphology, the phase-field method has proven to be predestined in the last decades. However, as the modeling of quaternary systems is challenging for the simulation with a grand potential based phase-field model, the focus of this work is on the generation of a material model for one defined compound namely NiAl-31Cr-3Mo. The modeling is validated by investigating the microstructure evolution in two- and three-dimensional simulations of the DS process for two different growth velocities and by investigating their undercooling spacing relationships. The evolving microstructures obtained from three-dimensional large-scale simulations are presented and validated with corresponding micrographs from scanning electron microscopy (SEM) of directionally solidified samples with the same growth velocities. The simulation results show the theoretically expected behaviors and are in qualitative and quantitative accordance with DS experiments. The study of NiAl-31Cr-3Mo serves as the basis for a comprehensive data-driven analysis of microstructure properties and system quantities of the entire quaternary material NiAl-(Cr,Mo). With this, an accelerated design of advanced materials is promoted.

In-situ investigation of the solidification dynamics in an irregular eutectic alloy

We present an in-situ experimental investigation of irregular eutectic growth dynamics observed optically in real time during directional solidification (DS) and rotating directional solidification (RDS) of a model transparent alloy, namely Amino-methyl-propanediol-Succinonitrile (AMPD-SCN). The SCN-rich solid grows with a nonfaceted solid-liquid interface, while AMPD forms thin whisker-like faceted crystals. With RDS, we observed simultaneously four distinct growth dynamics along the solidification front thanks to the rotation induced velocity ramp. At low velocity, AMPD crystals grow individually, with their leading tip ahead of – at a higher temperature than – the SCN-liquid interface which presents a linear undercooling change with V ramp. This regime is defined as a quasi-steady decoupled growth. During RDS, AMPD crystals are found to rotate with the sample in this regime. When they can reach the unsteady coupled-growth regime at higher V, since increasing V favors the formation of AMPD-SCN-liquid trijunctions, they start to branch noncrystallographically. At higher velocity, impurity effects lead to the formation of two-phase fingers. Further increase in the velocity results in the formation of primary SCN dendrites and a fine interdendritic eutectic microstructure. DS experiments confirmed the existence of those regimes at different velocities.

Evaluation of the formation of the silicon-rich area of hypereutectic aluminum-silicon melts treated with alternating electromagnetic directional solidification

In order to replace the high energy consumption and high-polluting Siemens method, aluminum-silicon (Al-Si) solvent refining was used by many researcher to manufacture purified silicon. Aiming at evaluating the formation of the Si-rich areas of hypereutectic aluminum-silicon (Al-Si) melt materials, a series of experiments on the effects of silicon separation from hypereutectic aluminum-silicon under different conditions was carried out. Currently, some researchers used the mechanism to explain silicon formation and separation from hypereutectic Al–Si melts during directional solidification (DS), based on Fick's second law. However, the mechanism cannot explain some experimental phenomena. In the present study, the deficiencies of the mechanism were examined and are presented. To explain the formation of Si-rich areas in detail, combinations of hypereutectic aluminum-silicon melts were analyzed to obtain a better understanding of the properties of hypereutectic aluminum-silicon melts. The experiments were used to confirm that the Si separate from hypereutectic Al-Si melts is also a purification process. Moreover, when the sample dealt with a 0.9 mm/min pulling speed under 3 kHz, the silicon content of the Si-rich areas can reach over 80 wt%. Finally, a novel optimized device was developed to obtain better Si separation from the hypereutectic Al-Si melts.

Effects of interphase boundary anisotropy on the three-phase growth dynamics in the β(In) – In2Bi – γ(Sn) ternary-eutectic system

We present an experimental investigation on the effects of the interphase energy anisotropy on the formation of three-phase growth microstructures during directional solidification (DS) of the β(In)–In2Bi–γ(Sn) ternary-eutectic system. Standard DS and rotating directional solidification (RDS) experiments were performed using thin alloy samples with real-time observation. We identified two main types of eutectic grains (EGs): (i) quasi-isotropic EGs within which the solidification dynamics do not exhibit any substantial anisotropy effect, and (ii) anisotropic EGs, within which RDS microstructures exhibit an alternation of locked and unlocked microstructures. EBSD analyses revealed (i) a strong tendency to an alignment of the In2Bi and γ(Sn) crystals (both hexagonal) with respect to the thin-sample walls, and (ii) the existence of special crystal orientation relationships (ORs) between the three solid phases in both quasi-isotropic and anisotropic EGs. We initiate a discussion on the dominating locking effect of the In2Bi–β(In) interphase boundary during quasi steady-state solidification, and the existence of strong crystal selection mechanisms during early nucleation and growth stages.

Numerical simulation of dendrite growth in Ni-based superalloy casting during directional solidification process

Abstract An understanding of dendrite growth is required in order to improve the properties of castings. For this reason, cellular automaton−finite difference (CA−FD) method was used to investigate the dendrite growth during directional solidification (DS) process. The solute diffusion model combined with macro temperature field model was established for predicting the dendrite growth behavior. Model validation was performed by the DS experiment, and the cooling curves and grain structures obtained by the experiment presented a reasonable agreement with the simulation results. The competitive growth of dendrites was also simulated by the proposed model, and the competitive behavior of dendrites with different misalignment angles was also discussed in detail. Subsequently, 3D dendrites growth was also investigated by experiment and simulation, and both were in good accordance. The influence on dendrites growth of initial nucleus was investigated by three simulation cases, and the results showed that the initial nuclei just had an effect on the initial growth stage of columnar dendrites, but had little influence on the final dendritic morphology and the primary dendrite arm spacing.

Spurious Grain Formation at Cross-Sectional Expansion During Directional Solidification: Influence of Thermosolutal Convection

Formation of spurious grains during directional solidification (DS) of Al-7 wt.% Si and Al-19 wt.% Cu alloys through an abrupt increase in cross-sectional area has been examined by experiments and by numerical simulations. Stray grains were observed in the Al-19 wt.% Cu samples and almost none in the Al-7 wt.% Si. The locations of the stray grains correlate well where numerical solutions indicate the solute-rich melt to be flowing up the thermal gradient faster than the isotherm velocity. It is proposed that the spurious grain formation occurred by fragmentation of slender tertiary dendrite arms was enhanced by thermosolutal convection. In Al-7 wt.% Si, the dendrite fragments sink in the surrounding melt and get trapped in the dendritic array growing around them, and therefore they do not grow further. In the Al-19 wt.% Cu alloy, on the other hand, the dendrite fragments float in the surrounding melt and some find conducive thermal conditions for further growth and become stray grains.

Numerical study of melt flow under the influence of heater-generating magnetic field during directional solidification of silicon ingots

The alternating current (AC) in the resistance heater for generating heating power can induce a magnetic field in the silicon melt during directional solidification (DS) of silicon ingots. We numerically study the influence of such a heater-generating magnetic field on the silicon melt flow and temperature distribution in an industrial DS process. 3D simulations are carried out to calculate the Lorentz force distribution as well as the melt flow and heat transfer in the entire DS furnace. The pattern and intensity of silicon melt flow as well as the temperature distribution are compared for cases with and without Lorentz force. The results show that the Lorentz force induced by the heater-generating magnetic field is mainly distributed near the top and side surfaces of the silicon melt. The melt flow and temperature distribution, especially those in the upper part of the silicon region, can be influenced significantly by the magnetic field.

A design of non-uniform thickness mould for controlling temperature gradient and S/L interface shape in directionally solidified superalloy blade

The heat transfer behaviors of turbine blade have been simulated using finite element method in three dimensions. The temperature gradient distribution and the solid/liquid (S/L) interface shape evolution were investigated during directional solidification (DS). Numerical results were compared with experimental results and good agreement was found. The experimental results demonstrate that solidification defects, such as freckles and stray grains, were observed on the platform of the blades. Numerical results indicate that this phenomenon should be attributed to a lower axial temperature gradient and a concave S/L interface shape. In order to overcome the shortcomings, two types of non-uniform thickness mould are designed to obtain higher temperature gradient ahead of S/L interface and flat S/L interface, contributing to reducing or eliminating the solidification defects in the blades during direction solidification.

Phase-field simulation of the microstructure evolution in the eutectic NiAl-34Cr system

The directionally solidified eutectic alloy NiAl-34Cr possesses promising properties for structural applications at high temperatures, such as increased creep resistance compared to the stoichiometric NiAl, while keeping the excellent oxidation behavior of the binary intermetallic compound. As microstructure and material properties are usually closely linked together, a deeper understanding of the microstructure evolution is crucial to design materials with defined properties. To simulate the eutectic reaction during directional solidification (DS), a thermodynamically consistent phase-field model based on the Grand potential approach is applied. Two- and three-dimensional simulations of the DS process for three different growth velocities are performed. The evolving microstructures obtained from large-scale simulations are presented and compared with micrographs from scanning electron microscopy (SEM) as well as with the analytic Jackson-Hunt approach in 2D and 3D. The simulation results are in qualitative and quantitative accordance with both, DS experiments and analytical solutions.

Investigation of iron contamination of seed crystals and its impact on lifetime distribution in Quasimono silicon ingots

The use of seed plates during directional solidification (DS) of Quasimono silicon ingots causes additional yield losses compared to standard multi-crystalline ingots due to an increased area of low minority carrier lifetime (“red-zone”) in the bottom region. This effect is attributed in literature mainly to iron impurities which are contaminating the seed crystal during heat up and afterwards the as-grown ingot during solidification. However, the contamination mechanisms itself are still not completely understood.Therefore, in this work the seed contamination mechanisms by iron and their effect on the lifetime distribution in the bottom region of Quasimono silicon ingots were investigated. For this purpose special crystal growth experiments in a laboratory-scale DS furnace were carried out by using diffusion barriers at the crucible/seed and seed/melt interfaces in order to separate the different contamination paths. The results show that the iron diffusion path from the crucible into the seed plates plays an important role. But in addition to this it will be demonstrated that an even more important iron contamination path is by gas phase transport from furnace parts via the furnace atmosphere to the seed crystals.

Evaluation of solder/substrate thermal conductance and wetting angle of Sn–0.7 wt%Cu–(0–0.1 wt%Ni) solder alloys

Wettability is defined as the tendency of a liquid metal to spread on a solid surface. The solder/substrate thermal conductance represented by a heat transfer coefficient, hi, states the heat ability to flow across the solder/substrate interface during solidification, being affected by the wettability. This letter aims to investigate the wetting behavior of Sn–0.7wt%Cu–(Ni) lead-free solders by measuring the contact angles (θ) on a copper substrate as a function of the alloy Ni content. Furthermore, time-dependent hi profiles are determined from thermal readings during directional solidification (DS) of Sn–0.7wt%Cu–(xNi) alloys. It is shown that a smaller initial contact angle (θi=32.7) characterizes the Sn–0.7wt%Cu–0.05wt%Ni associated with the highest hi value of 11,500t−0.03W/m2K (t: time).

Effect of Solidification Rate on Grain Structure Evolution During Directional Solidification of a Ni-based Superalloy

The effect of solidification rate on grain structure evolution during directional solidification (DS) of a Ni-based superalloy was explored. It was found that a high solidification rate led to sharper texture and smaller grain size in the DS samples. One of the most important findings in this work was that such result was not in accordance with the general concept, and the sharper texture was accompanied by the larger grain size. To explain the contradiction, the modeling samples with five grains were produced and the effect of solidification rate on the evolution of grain texture was illustrated based on the modeling samples.

A closely-complete peritectic transformation during directional solidification of a Ti-45Al-8.5Nb alloy

A closely-complete peritectic transformation (PT) was found during directional solidification (DS) of a Ti-45Al-8.5Nb alloy under a low temperature gradient. The closely-complete PT can align the peritectic α phase and control the lamellar orientation. The conditions to obtain a complete PT in primary β TiAl alloys were theoretically evaluated in the dendrite regime.

X-Ray Videomicroscopy Studies of Eutectic Al-Si Solidification in Al-Si-Cu

Al-Si eutectic growth has been studied in-situ for the first time using X-ray video microscopy during directional solidification (DS) in unmodified and Sr-modified Al-Si-Cu alloys. In the unmodified alloys, Si is found to grow predominantly with needle-like tip morphologies, leading a highly irregular progressing eutectic interface with subsequent nucleation and growth of Al from the Si surfaces. In the Sr-modified alloys, the eutectic reaction is strongly suppressed, occurring with low nucleation frequency at undercoolings in the range 10 K to 18 K. In order to transport Cu rejected at the eutectic front back into the melt, the modified eutectic colonies attain meso-scale interface perturbations that eventually evolve into equiaxed composite-structure cells. The eutectic front also attains short-range microscale interface perturbations consistent with the characteristics of a fibrous Si growth. Evidence was found in support of Si nucleation occurring on potent particles suspended in the melt. Yet, both with Sr-modified and unmodified alloys, Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form at the Si-particle interfaces.

Microstructure development during directional solidification of Ti–45Al–8Nb alloy

Microstructure development and phase transition were investigated by quenching during directional solidification (DS) of Ti–45Al–8Nb (at.%) alloy. The solidification route and process can be controlled by adjustment of the solidification condition. A coexisted region of L + β + α was found at a low growth rate. The high temperature α phase, produced by peritectic transformation or β → α solid-state transformation or direct solidification from interdendritic liquid, was determined by the solidification conditions and can be identified by checking the β-segregation pattern in dendritic region. Lamellar orientation control was achieved when the interdendritic α grew on the top of peritectically formed α. High temperature α can be aligned continuously although α partly grew from the region of primary β interdendrite. Some elongated B2 particles were generated due to β-stablizers were centralized in interdendritic liquid during DS process. The volume fraction of the B2 particle decreased with decreasing growth rate ( V).

Mechanism of competitive grain growth in directional solidification of a nickel-base superalloy

The structure evolution of bicrystal (BC) samples during directional solidification (DS) was explored in an attempt to understand the mechanism of competitive grain growth. It was found that in the case of diverging dendrites the favorably oriented grain overgrows the misaligned grain. However, in the case of converging dendrites the result differs from the prediction of the generally accepted model for competitive grain growth. First, the unfavorably oriented dendrites are able to overgrow the favorably oriented dendrites. Second, the misaligned grain overgrows the favorably oriented grain by blocking the dendrites of the favorably oriented grain at the grain boundary. Based on the experimental results, the process by which a favored 〈0 0 1〉 texture is developed during DS process of a nickel-base superalloy is illustrated.

Influence of solidification conditions on the castability of nickel-base superalloy IN792

The effect of withdrawal rate on hot tearing or solidification cracking during directional solidification (DS) was explored. High withdrawal rates were found to reduce the hot tearing tendency. There is a general refinement of the microstructure when the withdrawal rate is increased. First, the dendrite arm spacing appears smaller. Second, the columnar grain size is reduced. Third, the remaining liquid is finely dispersed and increasingly discontinuous. Finally, a more uniform distribution of strain is obtained because of the presence of more grain boundaries, and grain boundary cohesion becomes stronger due to the elimination of continuous liquid films. The reduced hot tearing susceptibility is attributed to the uniform distribution of strain and stronger grain boundary cohesion.

Effect of Zr and B on castability of Ni-based superalloy IN792

The effect of Zr and B on hot tearing susceptibility of the Ni-based superalloy IN792 during directional solidification (DS) was studied. The Zr and B concentrations in the experimental alloys ranged from 0 to ∼550 ppm. The results indicate that Zr or B does not influence the castability when added individually. However, when both Zr and B are present in the alloy, high hot tearing susceptibility was found, the effect being particularly strong if Zr concentration was high. The castability results cannot be explained by simple solidification characteristics such as total freezing range (obtained from differential scanning calorimetry (DSC)) or by the amount of eutectic liquid (derived from the fraction of interdendritic γ/γ′ obtained from quantitative metallography). However, the present results can be interpreted in terms of formation of continuous films of liquid at grain boundaries (GBs) during the final stages of solidification rather than enclosed pockets. Such thin films of liquid may reduce GB cohesion and promote hot tearing.

Microstructure of Directionally Solidified Ti-52Al-2W-0.5Si Intermetallic

The phase transformations controlling the microstructural evolution during directional solidification (DS) using the Bridgeman process and subsequent cooling are described for a Ti-52Al-2W-0.5Si intermetallic. The DS structure consists mainly of columnar dendrites with various shapes. α-Ti with a dendritic morphology is the primary solidified phase, followed by an incomplete peritectic reaction liquid (L)+α → γ-TiAl. The remaining liquid solidified as y and silicides. During cooling after solidification a dendrites transform into ordered α 2 and y with various morphologies, including near fully lamellar, script α 2 in a y matrix and blocky α 2 in a y matrix. Very fine tungsten-rich precipitates are also observed. The DS alloy is quite brittle. The microstructural evolution and the reason leading to the brittleness of the alloy are discussed.