Directional Solidification Process Research Articles

Removal of Fe impurities from Al alloy scraps by electromagnetic directional solidification combined with Si addition

The removal of Fe from Al alloy scraps is the main challenge for their recycling because the excess of Fe usually forms needle-like phases in the Al alloys, which reduces their mechanical properties. In this study, a combined process of electromagnetic directional solidification and Si addition is proposed to remove the Fe impurities from Al alloy scraps. The influence of the Si content on the removal and distribution of Fe impurities and the removal mechanism of Fe impurities were studied during the electromagnetic directional solidification of Al alloy scraps. In addition, the precipitation pattern of various phases and the removal mechanism of Cu, Mn, Mg, and Zn impurities were studied. The obtained results showed that adding 5–11.4 wt% Si during the electromagnetic directional solidification of Al alloy scrap can promote the precipitation of large bulk α-Fe (Al15(Fe, Mn)3Si2) phase at the bottom and top of the alloy. In other words, Fe and Mn were mainly enriched at the bottom and top of the alloy, which caused the removal of Fe and Mn impurities (Fe and Mn were reduced from 1.1 to 0.32 wt% and from 0.73 to 0.15 wt%, respectively). After the bulk α-Fe phase precipitated, the Al, Chinese script-like α-Fe, Si, θ (Al2Cu), and Q (Al3Cu2Mg9Si7) phases would sequentially precipitate. In the electromagnetic directional solidification, the Mg impurities (from 1.2 to 0.65 wt%) and Zn impurities (from 1.7 to 0.03 wt%) were also removed. This study provides a new green process for the efficient Fe removal from Al alloy scraps.

Effect of hydrogen pressure on pore growth and microstructure of GASAR porous Mg–Ag eutectic alloys

In order to study the effect of hydrogen, as a third non-metallic component, on the macroscopic pore growth and eutectic solidification structure in the directional solidification process, GASAR porous Mg–Ag eutectic alloy ingots were prepared by mould casting technology at different hydrogen pressures. The results show that with an increasing hydrogen pressure, the increased temperature gradient promotes the nucleation and growth of pores; for the microstructure, the increased temperature gradient cause an increasing driving force for solidification structure growth, which leads to the instability of the eutectic cells, and the eutectic structure transitions from lamellar to rod-like.

Numerical simulation of directional solidification process of single crystal blade based on precast

A finite element method was applied to establish a mathematical model of a single crystal blade’s directional solidification process. The model had different angles between the axis of the mold group and the blade. The effects of different radiation angles on temperature variation laws at different blade positions were investigated. The temperature near the center was lower than that near the wall on the upper side of the baffle, while it was higher on the lower side. Moreover, the grain growth of stray particles in blade casting defects was influenced by blade orientation. The grain growth of scattering particles in blade casting defects was faster. The isolated solid-liquid mixing zone was more prone to form at the platform tip near the center when the blade orientation angle was smaller. The 0° orientation angle resulted in the blade’s most severe stray grain phenomenon.

Effect of pulling speed on directional solidification structure of the single crystal blade

In this paper, the influence of withdraw speed on the directional solidification structure of the single crystal blade was studied. The influence of 2.7 mm/min, 5.4 mm/min, and 9.5 mm/min drawing speed on temperature field, temperature gradient, and microstructure were analyzed. The results show that as the casting speed increases, the temperature gradient G decreases, the degree of downward depression of the temperature field also increases, and more stray grains appear at the single crystal blade platform. In addition, it is also found that the increase in the withdrawal speed helps to reduce the secondary dendrite arm spacing and refine the grains. This paper provides a theoretical basis for optimizing the directional solidification process of single crystal blades.

Crack initiation and propagation dominated by strain localization in quasi-single crystal and poly-crystalline of a Ni-based complex concentrated alloy

Cracking is the precursor of the final fracture for most engineering materials, often affected by various microstructural characteristics including grain orientation, boundary, precipitation, etc. In this study, the fracture behaviour of a typical Ni-based complex concentrated alloy (CCA) was systematically investigated. These CCAs were fabricated using fine-grain casting and directional-solidification processes, which consists of quasi-single crystal (QSC) and poly crystalline structures respectively. Quasi-static tensile test results combined with morphology characterizations and crystal plasticity analyses suggest that microcracks tend to nucleate at the matrix/carbide interfaces in QSC and then propagate along the maximum slip direction at the crack tips. The crack path showed a distinctive zig-zag shape, beneficial for consuming the crack propagation energy. In consideration of carbide morphology, script-typed carbides were found to be very effective in hindering dislocation movements, resulting in admirable work hardening ability. In polycrystalline, however, microcracks are commonly found near the carbides that precipitate at grain boundaries, then developed quickly to connect with each other, leading to premature intergranular fracture. Grain boundary carbides were detrimental to mechanical performance. Considering the large amounts of investigations on second phase and precipitation, these new findings about carbide-related crack behaviours could be extended to other second phase hardening materials, providing new routes to design high-performance alloys by controlling the precipitation morphology and position.

Influence of the cooling rate on the solidification path and microstructure of a AlCoCrFeNi2.1 alloy

The AlCoCrFeNi2.1 alloy is a promising eutectic high entropy alloy which may act as an in-situ composite, showing a trade-off between mechanical properties and ductility. Controlling the microstructure formation is crucial for achieving the best balance since it is closely related to the cooling rates during solidification. Hence, the present study determined the relationship between different cooling rates and the formed microstructures. As such, the AlCoCrFeNi2.1 alloy was subjected to a directional solidification process so that samples solidified at different cooling rates could be generated. The examined samples corresponded to a range from 1 to 6 °C/s, which resulted in microstructures composed of primary dendrites and uncoupled eutectic, with varying secondary dendritic spacings from 18 to 54 μm and eutectic spacings from 4.5 to 10 μm. The use of CALPHAD, XRD, SEM-EDS, and EBSD phase mapping enabled to determine that the primary FCC dendrites formed in all samples enveloped by the FCC + B2 eutectic. Moreover, the solidification cooling rate affected both dendritic (primary phase) and eutectic length-scales. Finally, the solidification sequence path was able to be changed, and the primary FCC dendrites were also able to form, due to the non-equilibrium nature of the solidification process, which imposed particular segregation and growth conditions.

Effect of the Solidification Mode on the Pore Growth of Gasar Porous Mg‐Ag Alloy

AbstractGasar porous Mg–3 wt% Ag alloy is prepared by metal–gas eutectic directional solidification process. The pore structure is related to the solidification mode of the alloy. The pores grow stably under the solidification conditions of planar, cellular, and columnar dendrites but not under the condition of equiaxed dendrites. The numerical simulation shows that the solidification velocity and temperature gradient decrease rapidly with an increase in the sample height, but their ratio remains unchanged. During the solidification process, the solidification mode of the alloy changes from cellular crystals to columnar dendrites, and finally to equiaxed dendrites. The calculated results of the solidification mode transition are consistent with the microstructure morphologies of the Mg–Ag alloy at different heights.

Effect of the withdrawal rate on the microstructure and creep behavior of the directionally solidified nickel-based superalloy

The effect of the withdrawal rate on the microstructure and creep behavior of a directionally solidified nickel-based superalloy DZ125L is investigated in this study. A directionally solidified nickel-based superalloy is directionally solidified using the high rate directional solidification (HRS) process at various withdrawal rates. The results show that the closely packed dendrite arms and well distributed γ′ phase can significantly enhance the creep rupture life of the alloys, while the carbides and γ/γ′ eutectics affect the strength failure and fracture of materials. A series creep tests are performed on the nickel-based directionally solidified superalloy after different withdrawal rates. With an increasing withdrawal rate, the creep life of the alloy first increases and then decreases. The K–R creep damage model is established, which can reflect the quantitative relation between the withdrawal rate and creep life. The model simulation results are in good agreement with the tests.

Control of dimensional accuracy of hollow turbine blades during investment casting

Because of their variable airfoil cross sections and internal hollow cavities with complex serpentine shapes, hollow turbine blades are prone to sectional deformation during investment casting, resulting in dimensional errors. To control the deformation of turbine blades, a novel one-step reverse compensation method based on a variable shrinkage factor is proposed in this study. First, the directional solidification process was simulated to predict the deformation distribution of the turbine blades. The deformation of the airfoil and platform increases further after constraints removal, with the trailing edge of up to 0.4192 mm. Subsequently, the shrinkage rate of each finite element node of the turbine blades was calculated to guide the mold design. Finally, three cross sections of the blade were validated using a coordinate measuring machine. The experimental results show that the optimized deformation using the proposed method is within the design requirement of 0.15 mm, and the maximum deformation of the blade has lowered by 68.2 %, which indicates that the dimensional accuracy of the hollow turbine blades is effectively controlled.

Effect of Withdrawal Rate on Solidification Microstructures of DD9 Single Crystal Turbine Blade.

Single crystal superalloys are widely used in the manufacturing of turbine blades for aero-engines due to their superior performance at high temperatures. The directional solidification process is a key technology for producing single crystal turbine blades with excellent properties. In the directional solidification process, withdrawal rate is one of the critical parameters for microstructure formation and will ultimately determine the blade's properties. In this paper, the as-cast microstructures in the typical sections of a DD9 single crystal (SX) superalloy turbine blade were investigated with 3 mm/min and 5 mm/min withdrawal rates during the directional solidification process. With increased withdrawal rate, the dendrite morphologies tended to become more refined, and the secondary dendritic arms tended to be highly developed. The dendrite in the blade aerofoil section was more refined than that in the tenon section, given the same withdrawal rate. Additionally, with increasing withdrawal rates, the size and dispersity of the γ' precipitates in the inter-dendritic (ID) regions and dendritic core (DC) tended to decrease; furthermore, the size distributions of the γ' precipitates followed a normal distribution law. Compared with the ID regions, an almost 62% reduction in the average γ' sizes was measured in the DC. Meanwhile, given the same withdrawal rate, at the blade's leading edge closest to the heater, the γ' sizes in the aerofoil section (AS) were more refined than those in the tenon section (TS). As compared with the decreasing cross-sectional areas, the increased withdrawal rates clearly brought down the γ' sizes. The sizes of the γ-γ' eutectics decreased with increasing withdrawal rates, with the γ-γ' eutectics showing both lamellar and rosette shapes.

Modeling and experimental study on deformation prediction of thin-walled turbine blades during investment casting process

Due to variable cross-sections and a thin-walled structure, gas turbine blades have stringent dimensional, and geometrical tolerance requirements. Single-crystal hollow blades are manufactured using the following investment casting processes: ceramic core preparation, wax injection, ceramic coating, wax removal, metal casting, and finishing. The main causes of the final casting deformation are wax pattern deformation, core deflection, and metal solidification warpage. This paper proposes a numerical simulation method to predict the deformation of the wax pattern, core deflection, and the directional solidification (DS) process of large single-crystal blades. Additionally, it investigates the displacement field and the influence of casting process parameters on dimensional accuracy. Three groups of DS process parameters were selected for experiments, and the deformation prediction was in agreement with the experimental results. The selected blade section deformation is the smallest when the pouring temperature is 1530°C and the withdrawal rate is 5 mm/min. The proposed finite element model is efficient to predict the deformation in all the investment casting processes, providing geometric guidance for the control of the dimensional accuracy of the turbine blade.

Numerical and experimental investigation of the removal of Fe impurity during different DS process under axial magnetic field

The effects of axial magnetic field on the different directional solidification (DS) process were studied by using a transient numerical model of heat and mass transfer and corresponding experiments. The shape of melt-crystal (m-c) interface, melt flow morphology, thermal stress and Fe impurity concentration in the crystal for different DS process have been studied. The simulation results indicate that axial magnetic field provided a slight convex m-c interface shape, better flow pattern, lower thermal stress as well as lower Fe impurity concentration throughout the entire DS process. To further verify the simulation results, multi-crystalline silicon (mc-Si) ingots were prepared and testing were carried out. The experimental results are in good agreement with the simulation results. The experimental results show that a slight convex m-c interface shape was obtained, and the Fe impurity concentration in the middle area of the ingot was lower and higher minority carrier lifetime could be achieved by using the axial magnetic field.

The cooling rate effects on the Nb-B inoculant potential of refining grains and dendritic scale in 6201 alloy

Nb-B refiners are thought to be more stable than Ti-based refiners, becoming promising inoculants for Al alloys. However, the combined effect of adding NbB on dendritic scale and grain size (GS) with a wide range of solidification cooling rates remains unknown. It is well known that the shape of the α-Al phase is determined by a balance between inoculating components content/type and cooling rate. The inoculation of Al alloys with Ti-based inoculants, for example, significantly lowers GS while having no significant effect on secondary dendritic arm spacing (SDAS). Increased cooling rate, on the other hand, results in grain refinement, but only to a limited extent. Because of a combination of appropriate mechanical properties and suitable electrical conductivity, the 6201 alloy is widely employed in the transmission and distribution of electricity. Despite being a thermomechanically treated alloy, it goes through a melting and solidification process first, which entail research on this level of processing. ln order to assess the impact of adding NbB to the 6201 alloy’s refinement level (either macro or microscopic), directional solidification (DS) and centrifugal casting (CC) processes were utilized to obtain samples for examination. For the DS samples, cooling rates were determined, whereas for the CC samples, they were approximated for two alloys: 6201 (reference) and 6201-0.5wt.%Nb-0.1wt.%B alloys. Comparing the refined and non-refined alloy results, the GS of the DS 6201-NbB alloy was much smaller (about 13 times) across a range of cooling rates from 0.5 K/s to 20 K/s, whereas only 3 times smaller GS values were detected for CC samples at higher cooling rates. For particular cooling rates of 2.5 K/s and 240 K/s, the GS/SDAS ratios of values related to slow and fast cooling were 6.9 and 6.2, demonstrating that the cooling rate sensitivity of both GS and SDAS was similar in the NbB-inoculated alloy.

Creep behavior of silica bar core during directional solidification process of single crystal superalloy

Creep behavior of silica bar core during directional solidification process of single crystal superalloy

Numerical Investigation on the Effect of Thermal Gate Moving Rate on Directional Solidification Process

The temperature field distribution during the growth of crystalline silicon by the directional solidification (DS) method is an important factor affecting the growth rate, the shape of the melt-crystal (m-c) interface, and thermal stress. To solve the problem of m-c interface convexity at the early stage of crystal growth caused by supercooling at the bottom center of silicon ingot during DS. In this paper, a two dimensional (2D) global transient numerical model based on a large-size ALD-G7 (G7) crystalline silicon ingot furnace is established and experimentally verified. Based on the model, the influence of different bottom thermal gate moving process curves on the convexity of the m-c interface at the early stage was studied, with emphasis on the changes in temperature field, m-c interface, and thermal stress at the early stage of crystal growth. We have designed three cases, case 1 uses the original moving process curve of bottom thermal gate, case 2 and case 3 adjust the process curve to 0.95 and 0.9 of the original ratio, respectively. The numerical simulation results show that compared with case 1, the average heat dissipation at the bottom of the silicon ingot in case 2 and case 3 is reduced by 2.7 kW·m−2 and 4 kW·m−2, and the maximum thermal stress is also reduced by 3 MPa and 4 MPa, separately. The maximum convexity of the interface is reduced by 25 mm and 20 mm, and the convexity is reduced by 55% and 44% on average, respectively.

Analysis of grain structures and impurity distribution in mc-silicon grown by directional solidification: Computational and experimental approach

The grain structures and grain boundary characteristics such as grain orientation and formation of Σ 3 grain boundaries have been analysed in the wafers sliced from central and peripheral bricks of the ingot grown by G1 (15 Kg) directional solidification process. The orientation of grains and grain boundary characteristics in the wafers sliced from different parts of the ingot have been investigated using EBSD analysis. Large number of grains with different orientation are observed at the bottom of the ingot and the grain size increases from bottom to top as the growth proceeds. The density of grain boundary decreases with respect to the growth direction of the ingot. Simulation analysis has been done to investigate the overall growth process and the distribution of two major impurities such as carbon and oxygen impurities in the ingot. Simulation results are compared with experimental results and the relation between formation of grain boundaries and impurity concentration within the ingot has been analysed.

Numerical simulation approach to investigate the effect of gas tube design on the impurities distribution and thermal properties of multi-crystalline silicon ingot grown by directional solidification process

Multi-crystalline Silicon (mc-Si) grown by directional solidification (DS) system is considered as a crucial photovoltaic material due to its relatively low cost and high yield. The numerical modelling is very useful for understanding the impurities segregation during mc-Si growth process. In the present paper, numerical simulation is carried out to investigate the impact of Argon gas tube design as shower head. In this Argon gas tube modification act on the non-metallic impurities, melt-crystal interface, temperature distribution and stress levels in the silicon ingot during the different stages of solidification process. The proposed design resulted in remarkable reduction in the back transfer of CO from heater component of the DS furnace. The generated von misses stress is also discussed as the function of different stage of solidification. The results concluded that the stress level at the initial stage of solidification is lower but it is large at above 75 % solidification rate. Also, we found that the modified gas tube design gives better temperature distribution with melt-crystal interface (m-c) shape. The modified argon tube design will be helpful for enhancing the quality of silicon ingot by controlling the chemical reaction between silicon melt and impurity transport.

The role of incoming flow on crystallization of undercooled liquids with a two-phase layer

Motivated by important applications of crystallization phenomena, we consider a directional solidification process for a binary melt with a two-phase (mushy) layer in the presence of weak melt flow. We consider the steady-state solidification scenario, so that the two-phase layer filled with solid and liquid material keeps its thickness. In addition, we consider that the melt flows onto the two-phase layer slowly in the opposite direction to directional crystallization and solidifies there. A complete analytical solution to non-linear two-phase layer equations is constructed in a parametric form, where the solid phase fraction represents a decision variable. The temperature and solute concentration distributions, mushy layer permeability and average interdendritic spacing as well as solidification velocity and mushy layer thickness are analytically determined. We show that incoming melt flow plays a decisive role on mushy layer parameters and internal structures. The solid phase fraction within the two-phase layer and its thickness essentially grow while the mushy layer permeability and average interdendritic spacing decrease with increasing intensity of incoming melt flow.

Application of phase field model coupled with convective effects in binary alloy directional solidification and roll casting processes

Based on the Kim-Kim-Suzuki (KKS) phase field model coupled with the thermodynamic parameters, the transformation process from columnar dendrites to equiaxed crystals during directional solidification of aluminium alloy was simulated, and the effects of phase field parameters on the growth morphology and dendrite segregation were discussed. Furthermore, considering the effect of the microcosmic flow field, the convection influence gradient term is introduced into KKS formula near the solid-liquid interface, and the phase field model considering flow field was applied to the inherent convective environment of the actual roll casting process, also the multiple dendrites growth behavior of magnesium alloy under the action of microscopic convection was further explored. When coupling calculation of microscopic velocity field and pressure field, the staggered grid method was used to deal with the complex interface. The combined solution of Marker in Cell (MAC) algorithm and phase field discrete calculation was realized. In order to further describe the influence of convection on the solidification process, the roll casting experiments are used to verify the impact growth of multiple dendrites under convection. The results show that the dendrites undergo solute remelting and the dendrites melt into equiaxed crystals, showing the phenomenon of Columnar to Equiaxed Transition (CET).

A method of removal of nickel impurity from electrolytic magnesium

The Ni impurity existing in electrolytic Mg cannot be removed using the common settling method since there is a deep eutectic reaction between Ni and Mg. Thermodynamic calculations in this work indicate that the equilibrium solute distribution coefficients of Ni between solid and liquid Mg above the eutectic temperature are far less than 1. The directional solidification method was firstly employed in this work to remove the Ni impurity from the electrolytic Mg. The results show that the Ni impurity in the electrolytic Mg can be almost completely removed (<5 ppm) after the directional solidification process, which meets the requirement of high-purity Mg for Ni content tolerance and is therefore promising for the production of high-purity Mg.