Directional Solidification Research Articles

Interface interaction during the preparation of TiAl-(Nb, V) quaternary intermetallic single crystals by directional solidification based on Y2O3 doped BaZrO3/Al2O3 composite ceramic mold

Interface interaction during the preparation of TiAl-(Nb, V) quaternary intermetallic single crystals by directional solidification based on Y2O3 doped BaZrO3/Al2O3 composite ceramic mold

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.

Low-Cost Clean One-Step Production of Solar Silicon from Natural Quartzite

Despite the fact that many groups have tried to achieve solar silicon production using molten salt electrolysis, the motivation for this project is to build on a recent breakthrough in silicon molten salt chemistry. The novel molten salt bath comprised of MgF₂-CaF₂-YF₃-CaO-SiO₂ exhibits low volatility (<0.1 μg/cm2·s), low viscosity (<5 mPa·s), high ionic conductivity (>4 S/cm), SiO₂ solubility (>5 wt%), and yttria-stabilized zirconia (YSZ) solid oxide membrane (SOM) compatibility, which are all at least an order of magnitude better than prior works. Our goal is to produce high-purity silicon from natural 99.7-99.9% pure quartzite (SiO₂) in only one step, utilizing molten salt electrolysis with a state-of-the-art molten salt composition. This process can potentially produce silicon with 4-5N purity at $1.70/kg using <30 kWh/kg, with pure oxygen by-product and zero direct Greenhouse Gas (GHG) emissions. The 4-5N product purity would be sufficient for directional solidification, resulting in $2.20-2.70/kg solar-grade silicon cost, i.e., 90-95% lower than today. Besides considerable silicon price reduction, our novel technology provides a high level of scalability that can control solar silicon price fluctuations, while it is a "green" production.

Enhancing sustainable valorization: Harmless synergistic melting treatment for high-value vitreous products from MSWI fly ash and electrolytic manganese residue

The waste-to-energy and manganese industries face significant ecological challenges due to two major risk sources: municipal solid waste incineration (MSWI) fly ash and electrolytic manganese residue (EMR), especially, the MSWI fly is classified as hazardous waste. High temperature melting is a promising method for harmless disposal of solid wastes. However, it has yet to be industrialized due to the high costs and energy consumption. This study proposes using EMR as an additive to co-melt with MSWI fly ash, aiming to develop a method that minimizes energy consumption while producing high value-added products. To this end, the phase evolution and phase-change cooling characteristics during the co-melting process of MSWI fly ash and EMR were experimentally investigated. XRD and SEM analyses revealed that pure vitreous slag can be obtained when mixtures are heated to 1500 °C for 120 min with ≥40 wt% EMR addition under natural air-cooling conditions. Additionally, to produce vitreous slag by air-cooling and increase MSWI fly ash treatment capacity, the molten mixture with 30 wt% EMR addition was adopted in the directional solidification experiments to establish a predictive model relating the average cooling rate to the glass content. The findings ultimately contribute to the advancement of melting-based industrial applications.

Failure analysis of a first stage turbine blade made of directionally solidified GTD111 superalloy and repaired by welding process

Failure of a turbine blade made of DS GTD111 and repaired by welding process was investigated by fracture surface investigation and metallurgical observation. The methods employed to investigate the fracture surface and the metallurgical structure include optical microscope (OM) observation, scanning electron microscopy (SEM) observation and energy dispersive X-ray spectrometry (EDS) analysis. The fracture surface had penetrated the welding zone and entered the directional solidification zone. The high-temperature oxidation of the fracture surface and the degradation of the grain boundary in the welding zone appear to be the primary causes of numerous secondary cracks. The spallation of the top coating of TBC near the fracture surface also accelerated the formation of the cracks. Based on the metallurgical observation, it was found that the bond coating of TBC did not play any positive role in promoting the initiation and propagation of the cracks.

Effects of the Growth Rate on the High-temperature Tensile Properties and Micro-organization of Directionally Solidified Ti-44Al-9Nb-1Cr-0.2W-0.2Y Alloys

In this experiment, Ti-44Al-9Nb-1Cr-0.2W-0.2Y alloy was prepared by the directional solidification method. The effect of different growth rates on tensile properties and microstructure orientation at high temperature was studied. Three kinds of alloys with different growth rates of 10 μm/s, 15 μm/s and 20 μm/s were prepared. The results show that the tensile properties of the alloy at 800 ℃ decrease with increasing growth rate, and recrystallization occurred at the position of intracrystal fracture in the microstructure. The size of columnar crystals decreases with the increase of the growth rate, increasing the number of grains and decreasing the orientation difference between the growth and axial directions as well as the preferred orientation of lamellar, and the anisotropy of the material, which leads to the obvious decrease of the tensile strength and plasticity. Combined with electron backscattering diffraction test results, the lamellar orientation of the effective parts of the three specimens after high temperature stretching was studied. It was found that the axial preferred orientation of the alloy specimens decreased obviously with the increase of the growth rate, and the orientation became disorderly and the uniformity of lamellar thickness decreased gradually with the increase of the growth rate. In addition, it was found that a new single-phase γ phase is formed in the microstructure after high temperature stretching, and the distribution range increased with the increase of the growth rate, which seriously degraded the axial preferred orientation of the alloy. It can be concluded that the directionally solidified alloy with a growth rate of 10 μm/s has better microstructure orientation and high temperature tensile properties.

Microstructure evolution and mechanical properties of directionally solidified Al0.5CoCr1.3FeNi1.3(Ti, Si, B, C)0.3 multiphase complex concentrated alloy

Microstructure evolution and mechanical properties of Al0.5CoCr1.3FeNi1.3(Ti, Si, B, C)0.3 multiphase complex concentrated alloy (CCA) prepared by directional solidification combined with quenching during solidification (QDS) technique were studied. The directionally solidified (DS) samples were prepared at constant growth rates V ranging from 2.78 × 10−6 to 5.56 × 10−5 m/s, the constant temperature gradient in liquid at the solid-liquid interface of GL = 16.5 × 103 °C/m and then quenched at a cooling rate of about 50 °C/s. The solidification of the studied CCA begins with the growth of columnar FCC(A1) dendrites and is finalised by the formation of a multiphase microstructure in the interdendritic region. The DS samples subjected to isothermal annealing at 900 °C for 25 h show a strong brittle-ductile transition (BDT) behaviour characterised by brittle-ductile transition temperature (BDTT) between 650 and 700 °C at a strain rate of 1 × 10−4 s−1. The tensile deformation curves show only a strain hardening stage at test temperatures up to 650 °C. At temperatures ranging from 750 to 900 °C, the alloy shows anomalous strain hardening behaviour characterised by one strain hardening stage and three well-distinguished strain softening stages.

Structure and diffuseness model of solid-liquid interface for binary alloys

Using the morphological instability at the solid–liquid interface as a basis by the maximum entropy production rate principle (MEPR), a model is presented on the morphological structure and diffuseness of the interface during directional solidification of binary alloys. It is shown that, the independent diffuseness theory of Cahn and the Jackson roughness criterion can be unified at a limiting condition under this new MEPR solidification model. The model under the principle of MEPR is applied to describe the evolution of atomistically smooth and rough interfaces through the evaluation of the size of the solid–liquid interface and the of number atomic layers. The model is tested with data for binary alloys of aluminium, lead, and tin at varying solute concentrations. The results showed strong agreement with available data from experimental measurements.

The Stray Grains from Fragments in the Rejoined Platforms of Ni-Based Single-Crystal Superalloy

Nickel-based single crystal superalloy is the most important material for blade preparation. However, some solidification defects inevitably occur during the process of preparing single-crystal blades through directional solidification. In this study, in order to study the origin of misorientation defects during solidification, a model with rejoined platforms was designed according to the geometry of single-crystal guide vanes. Electron Back-Scattering Diffraction (EBSD) was used to quantify the orientation deviation of the dendrites and identify the solidification defects in the rejoined platforms. The results showed that stray grain defects appeared in the platforms and their misorientation changed gradually, not abruptly. Combined with the simulation results, it was proposed that the stray grains formed as the result of the dendrites fragment, which was induced by solute enrichment in the mushy zone during solidification. Meanwhile, it was accompanied by a obvious dendritic deformation, which was caused by solidification shrinkage stress. This suggested that the fragmentation was induced by multiple factors, among which, the concave interface shape provided favorable conditions for solute enrichment, and the dynamic variability in the local thermal gradient and fluctuations of the solidification rate might play catalytic roles.

Melt thermal-rate treatment for uniform solute distribution and improved mechanical properties of an Al-Zn-Mg-Cu alloy prepared by direct-chill casting

Melt thermal-rate treatment (MTRT), a promising technique for improving microstructure uniformity, has been rarely applied in direct-chill (DC) casting. In this study, we investigated the effects of MTRT on the microstructure and mechanical properties of an Al-Zn-Mg-Cu alloy billet prepared using DC casting. Results indicate that MTRT improved the uniformity of solute distribution in the Al matrix, thereby enhancing mechanical properties such as hardness, tensile strength, and ductility. In particular, yield strength, ultimate tensile strength, and elongation of the near-surface region of the billet increased from 250 to 274 MPa, from 319 to 393 MPa, and from 2% to 8%, respectively. It has been reported that twinned dendrite grains (TDGs) form on the surface of billets because of directional solidification or DC casting at high cooling rates. However, this study reveals that the formation of TDGs was induced in the central region of the billet by MTRT. Furthermore, the effects of TDGs on the mechanical properties are evaluated and compared with those of regular grains of MTRT billets. Overall, these findings suggest that MTRT is an efficient technique used to improve the mechanical properties of Al alloys fabricated using DC casting.

Eutectic growth kinetics as an indicator for glass-forming ability

A metallic glass is known to be often formed more easily by cooling of an alloy melt whose composition is near a deep eutectic. However, the eutectic solidification kinetics, which is closely related to the glass-forming ability (GFA), is rarely reported. In this paper, directional solidification of such a glass-forming eutectic alloy (Zr48Cu36Al8Ag8) has been conducted to investigate the eutectic growth kinetics in terms of the evolution of eutectic structure as a function of the growth rate (V). It was found that, at lower growth rates, the alloy solidified in a dual-phase lamellar structure characterized by the lamellar spacing (λ), with Vλ2 being a constant. As V increased up to a critical value of 400 μm/s, λ decreased and eventually diminished (λ→0), where the eutectic transitioned into a glass. This critical growth rate (i.e., Vce) can be interpreted as a result of the competition between the glass formation and the eutectic growth, in which the glass formation sets a kinetic upper limit for the eutectic growth. Moreover, Vλ2 is found to be kinetically equivalent to Vce, which can be used as a GFA indicator and is readily measurable at low growth rates.

Jet Engine Gas Turbine Blades: Beyond the Super Alloys

The High Pressure Turbine (HPT) blades of a turbo-engine, whether used in land-based power- generating equipment or in aircraft propulsion systems, are among the most severely loaded engineering components; they operate at very high temperatures, they are subjected to very high centrifugal stresses, they are susceptible to creep and fatigue damage and they are also exposed to adverse conditions such as corrosion, oxidation, sulphidation, foreign object damage, etc. Nickel-based super alloys were used to make turbine blades in the 1940s and the Turbine Inlet Temperature (TIT) has been steadily increased over the years due to material innovations such as improved alloy chemistry and protective coatings, processing innovations such as directional solidification and single crystal blade fabrication and design innovations such as complex internal cooling channels that enable the blades to operate at temperatures beyond their melting point. With the improvements attainable with the super alloys reaching the limit, efforts are underway to reduce the weight of the blades by making them out of lower density materials such as intermetallic compounds of titanium and aluminum. With the reduction in the centrifugal forces, the weight of the supporting structures such as the bearings can also be reduced, resulting in gains in fuel consumption and reduction in the generation of exhaust gases. Intermetallic compounds such as titanium aluminides and nickel aluminides, composites using such intermetallics as matrices, and ceramic matrix composites with silicon carbide fibers in a silicon carbide matrix, are being introduced in a phased manner; some of these materials have already shown their potential in the Low Pressure Turbine (LPT) stages and are advancing through the higher temperature parts of the turbo-engine with the ultimate goal of replacing the heavier super alloys. These new materials are difficult to fabricate and innovative methods based on centrifugal casting, forging and additive manufacturing from powders are being developed. This paper describes some of these important innovative developments.

Modeling of Creep Deformation Behavior of DZ411 and Finite Element Simulation of Turbine Blade

Creep tests were conducted on DZ411 material at 930 °C and 850 °C, and creep curves were recorded and employed in normalization creep model building. A yield function suitable for directional solidification nickel-based materials was proposed in an ascending-order approach. Combined with the normalized creep model and the proposed function, a creep subroutine was compiled to simulate the creep deformation behavior of a turbine blade. The typical boundary conditions of the blade were determined and used for finite element analysis. According to the analysis results, the assessment positions for the actual application of a turbine blade were determined and checked for endurance intensity. The phenomenon of deviation angle between crystal axis and blade height direction in actual casting was further analyzed. Multiple angles and directional deviation angles were simulated for 10,000 h creep deformation. Considering the difficulties and challenges of the complex geometric structures of blades, it is necessary to conduct creep tests of DZ411 material and a simulation analysis of a real blade. Based on the above analysis and discussion, the present work sheds light on finite element analysis and has great potential for structural analyses in the engineering applications of complex high-temperature structures.

X-radiography front tracking gradient furnace for directional solidification of bulk Al-alloys

A unique gradient furnace for directional solidification experiments with bulk Al-alloy samples developed at German Aerospace Center is presented. It allows for in situ process control in solidifying samples by using x-radiography, and further insight into the solidification process is gained in combination with x-ray computational tomography on the solidified samples. Tracking of interfaces during directional solidification of bulk samples via in situ x-radiography (TIREX) enables the investigation of the melting process and observation of the movement of the entire mushy zone through the sample, tracing the solid–liquid interface during directional solidification and correlating the observations with the microstructure of the samples. Monitoring the temperature profile inside the sample by in situ observation of the length of the mushy zone is particularly important because the temperature gradient G and the rate of interfacial growth v determine the microstructure of solidification. The x-radiography setup offers temporal and spatial resolutions of 0.5 s and 70 μm, respectively, with a field of view of 10 × 50 mm2. Constant solidification velocities of up to 0.15 mm s−1 at a temperature gradient of up to 8 K mm−1 can be achieved in a temperature range of 537–1373 K. A flat solid–liquid interface inside a rod-like sample with 5 mm diameter is achieved by surrounding the sample by thermal isolating graphite foam. Performance tests with hypoeutectic Al–10 wt. % Cu alloy samples show the functionality of the furnace facility.

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.

Effect of solidification direction on microstructure and mechanical property of single crystal superalloy CMSX-4

The microstructure and mechanical property of single crystal superalloy CMSX-4 fabricated by downward direction solidification (DWS) and upward direction solidification (UWS) were systematically investigated by optical microscope (OM), scanning electron microscope (SEM), electron probe microanalyzer (EPMA), nano-indentation instrument and finite element simulation method. Meanwhile, the effect of remelting heat treatment on microstructure evolution and mechanical property of the superalloy was also explored. Compared to the traditional UWS process, the DWS process was superior in inhibiting the formation of freckle defects and alleviating the segregation of difficult-diffusion elements. Moreover, the expected application of thin shell casting in the DWS process was also more beneficial to the control of freckles and the refinement of microstructure. The change of solidification direction would alter the solute distribution in the mushy zone. Different convection patterns were then formed under the action of gravity, thereby affecting the microstructure of the casting. In the following process of remelting heat treatment, the generation and elimination of incipient melting structures occurred in both UWS and DWS samples. However, more incipient melting structures in the DWS samples made longer elimination time required. After remelting heat treatment, the mechanical property of the DWS sample was comparable with that of the UWS one, since the element segregation in the final microstructure was reduced, and the size, morphology and uniformity of γ' phase were also adjusted.

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.

Evaluasi Efisensi Riser Untuk Bentuk Riser Samping yang Berbeda Menggunakan Metode Simulasi Casting

As the solidification reaches the hot spot area, no molten metal remains and shrinkage is formed. To anticipate the shrinkage, a riser is added to the casting system. An optimal riser design would produce free shrinkage components. One of the factors that affect riser efficiency is the riser shape. This study aims to find the most efficient side riser shape by using simulation software. The riser shape of tubes, tubes with a half sphere on top, hemispheres, conical tubes, tubes with an oval cross-section, and cubes are used in this experiment, with the volume of all risers kept constant. The most efficient shape of the riser is the tube. The tube riser produces a larger modulus. The tube riser generated directional solidification. The same pattern can be seen in the niyama criteria and solidification temperature, where the tube riser has a more continuous pattern compared to other riser shapes.&#x0D;

Programming Crystallographic Orientation in Additive-Manufactured Beta-Type Titanium Alloy.

Additively manufactured metallic materials typically exhibit preferential <001> or <110> crystallographic orientations along the build direction. Nowadays, the challenge is to program crystallographic orientation along arbitrary 3D direction in additive-manufactured materials. In this work, it is established a technique of multitrack coupled directional solidification (MTCDS) to program the <001> crystallographic orientation along an arbitrary 3D direction in biomedical beta-type Ti-Nb-Zr-Ta alloys via laser powder bed fusion (LPBF). MTCDS can be achieved via directional solidification of coupled multi-track melt pools with a specific temperature gradient direction. This results in continuous epitaxial growth of the β-Ti phase and consequently sets the <001> crystallographic orientation along an arbitrary 3D direction. This way, relatively low elastic modulus values of approximately 60 ± 1.2GPa are customized along an arbitrary 3D direction. It is expected that MTCDS can be generalized to a wide range of applications for programming specific crystallographic orientations and, respectively, tailoring desired properties of different metallic materials.