Electromagnetic Cold Crucible Research Articles

Melting spectral emissivity measurement of metal during the melting and solidification process

In the field of energy-intensive industrial processes, investigating the spectral emissivity of molten metals is crucial for improving energy utilization and efficiency. However, previous research efforts have not fully explored the measurement of melting emissivity. This study presents an effective method for measuring the spectral emissivity of molten metals. A new experimental apparatus was developed to measure the melting spectral emissivity within a range of 2–8 μm and temperatures ranging from 873 K to 2373 K. The sample was rapidly melted using a specially designed electromagnetic cold crucible, while its spectral radiance was detected using a Fourier transform infrared (FTIR) spectrometer. To enhance energy utilization rate, numerical simulation of the cold crucible improved induction heating performance. Additionally, calculating radiation within the cylindrical chamber enhanced measurement accuracy of emissivity. To validate the reliability of this apparatus, normal spectral emissivity measurements were conducted on liquid nickel. The results demonstrate that comprehensive uncertainty associated with liquid nickel's spectral emissivity does not exceed 1.82 %. Furthermore, verification based on the Drude-Roberts model confirms normal spectral emissivity values for liquid nickel as well.

Numerical and experimental investigation of electromagnetic cold crucible used for emissivity measurement of molten material

The electromagnetic cold crucible levitation heating technology has tremendous potential in the spectral emissivity measurement of molten metal for its high melting temperature and no contamination. In this work, we designed a dedicated cold crucible with a hemispherical bottom to establish the spectral emissivity measurement apparatus at high temperature. The effects of different numbers of segments, slit width, wall thickness and height-diameter ratio on the magnetic permeability of crucible were studied by the finite element method. The results indicate that increasing the number of crucible segments and the slit width can effectively improve the magnetic permeability of cold crucible, while increasing the wall thickness and the ratio of height to diameter will reduce its magnetic permeability. Based on the simulation result, the cold crucible with optimal parameters was made and used for studying the spectral emissivity of molten metal. The spectral emissivity of iron, cobalt and nickel was measured successively to prove the availability of cold crucible in emissivity measurement. The results agree well with the literature data, and the combined uncertainty of the molten iron is 2%. This work demonstrates that electromagnetic cold crucibles have a promising prospect in emissivity measurement of molten metal.

Microstructure transition of γ-TiAl alloys with abrupt cross-sections in yttria ceramic moulds during directional solidification by electromagnetic cold crucible

Directional solidification of γ-TiAl alloys with abruptly varying cross-sections was performed in yttria molds using an electromagnetic cold crucible. The solid–liquid interface at different growth positions was revealed by quenching using a liquid metal coolant, and the temperature field was mapped with the assistance of a finite element simulation. On the basis of these findings, the process of grain growth was summarized. The columnar–equiaxed transition was found to occur during directional solidification, and a criterion for continuous growth was established, which demonstrates that increasing the withdrawing rate or decreasing the nucleation rate can promote continuous growth. The continuity of columnar grains is confirmed by analysis in orientation evolution which also reconstructs the orientation of α and β grains at elevated temperatures. The degree of directionality of microstructures at elevated temperatures is determined from the angle between the preferential growth direction and the actual growth direction of the β phase. It is found that the degree of directionality increases with increasing withdrawing rate and nucleation rate.

Microstructure refinement and improvement of mechanical properties of Ti46Al8Nb0.9B alloys by electromagnetic cold crucible continuous solidification

Microalloying with boron is effective and economical for refining microstructures of as-cast TiAl alloy. However, microstructural refinement is easily affected by the alloy composition and cooling rate, leading to inhomogeneous grain size in different areas of the ingot. In this study, the Ti46Al8Nb0.9B alloys are continuously solidified by the electromagnetic cold crucible, aiming to obtain homogeneous and fine microstructure. After electromagnetic cold crucible continuous solidification (EMCS), the obtained ingot shows finer and more homogeneous microstructure with lamellar colony size of about 50 μm. The results of tensile test show that the EMCS ingot exhibited improved properties with yield strength (σ0.2) of 548 MPa and plastic elongation of 0.6%.

Microstructure, mechanical and tribological performances of a directionally solidified γ-TiAl alloy

In the present study, Ti-47Al-2Nb-2Cr-2Mn directionally solidified by electromagnetic cold crucible was mainly composed of alternatively arranged (α 2 + γ) lamellae, and exhibited a room-temperature tensile ductility of 1.55%, and an ultimate strength of 587 MPa. At 800 °C ultimate strength remained high at 512 MPa, with 4.33% ductility. The crack propagated around B2 phase at room temperature, but cut though B2 phase at elevated temperature. Both dislocation slip and twinning governed the room-temperature plastic deformation, and the twin boundary acted as the obstacle to dislocation slip. The friction coefficient of the N-orientation (70°-90°) TiAl sample was the highest, followed by the I-orientation (35°-50°) and P-orientation (0°-15°) samples, while their wear rates presented an opposite trend. The P-orientation sample was more liable to form the tribo-oxide layer. Moreover, the local plastic deformation during dry sliding resulted in pile-ups along the edge of wear tracks. • Cracks bypass (room temperature) and cut through (elevated temperature) B2 phase. • Dislocation- and twinning-controlled deformation mechanisms at room temperature. • Lowest friction coefficient and highest wear rate for P-orientation sample. • Tribo-oxide layer preferentially formed in P-orientation sample.

Effects of Heating Power on Microstructure Evolution and Tensile Properties at Elevated Temperature by Directional Solidification for Ti2AlC/TiAl Composites

To improve the directionally solidified microstructure and tensile properties at elevated temperature, Ti46Al4Nb1C0.8Ta ingots are directional solidified by electromagnetic cold crucible with different heating powers. The microstructure is observed and characterized, tensile properties are tested, and related mechanisms are revealed. Results show that a relatively planar liquid–solid interface is formed under the low heating power (39 kW), which promoted formation of columnar crystals. When the heating power increases from 39 to 48 kW, degree of dent (H1) increases from 8.5 to 18.1 mm and influence length of the initial transition area (H2) increases from 4.3 to 28.6 mm. The formation of a concave liquid–solid interface is due to the high heating power increasing the temperature gradient. High heating power increases the electromagnetic stirring ability, which increases the time to reach stable growth. Tensile properties of the directionally solidified alloy are higher than those of the as‐cast alloy under the same testing temperature, e.g., the tensile strength is 342.5 MPa, an increase of 1.4 times, and the tensile strain is 4.4%, an increase of 1.3 times at 950 °C. This shows that the tensile properties and microstructure of TiAl‐based composites can be improved by the directional solidification technology.

Microstructure and elevated temperature tensile property of Ti–46Al–7Nb-(W,Cr,B) alloy compared with binary and ternary TiAl alloy

The newly developed high-Nb TiAl with a small amount of W, Cr, and B addition prepared by electromagnetic cold crucible has been investigated by comparing with the microstructure and elevated temperature tensile properties of binary and ternary high-Nb TiAl alloy. Results indicate that microstructures of the three alloys are fully lamellar (FL), near fully lamellar (NFL) and near fully lamellar alloy with TiB (NFL + TiB). The dispersed TiB particles in the alloy have the B27 structure and are highly enriched with Nb and W but depleted with Al by the characterization of transmission electron microscopy. The elevated temperature tensile test results show that the high-Nb TiAl alloy with NFL + TiB microstructure exhibits excellent mechanical properties compared with the other two alloys, and the tensile strength and elongation at 800 °C are 648 MPa and 5.73%, respectively. Fracture morphology shows the alloy has a mixed mode of trans-lamellar and inter-lamellar fracture. As the dispersed second phase particles, rod-like TiB can act as the obstacle to crack propagation and dislocation movement. The excellent mechanical properties of the alloy at elevated temperatures are attributed to the combined effects of dislocation block and nanotwins formation during deformation within the γ lamellae. Moreover, the probable generation and propagation mechanism of dislocations and twins in the γ lamellae are further discussed.

Microstructures and mechanical properties of directionally solidified C-containing γ-TiAl alloys via electromagnetic cold crucible

TiAl alloys with nominal compositions of Ti–47Al–2Nb–2Cr-(0.2, 0.5)C were prepared via electromagnetic cold crucible directional solidification. Effects of alloying element carbon on the solidification behavior, microstructure and mechanical properties of the TiAl alloy were investigated. Experimental results revealed that alloying with carbon reduced the B2 phase content and refined the lamellar structure. Also, additions of 0.5 at.% carbon changed the primary phase from single β to the mixture of β and α, and promoted the formation of Ti2AlC particles. The yield strength of the alloy enhanced as the carbon content increased, while the ultimate strength of the alloy containing 0.2 at.% and 0.5 at.% C decreased consequently. Dislocation glide governed the plastic deformation of the two directionally solidified C-containing TiAl alloys. The strain rate sensitivity value and activation volume of the both experimental alloys decreased with the increase of strain. The hindrance of Ti2AlC particles to dislocations movements caused stress concentration and premature cracking.

On the high temperature deformation of a directionally solidified γ-TiAl alloy

The γ-TiAl alloy with a nominal composition of Ti-47Al-2Nb-2Cr-0.2C directionally solidified by electromagnetic cold crucible was isothermally compressed at different temperatures and strain rates. Experimental results revealed that the peak stress decreased with the increased deformation temperature or decreased strain rate. Subgrains were formed in the interior of the lamellae during thermo-compression and the dislocation walls served as the subgrain boundaries. The dynamic recrystallization grain (DRX) size was larger and a single DRX grain grew at the expense of many lamellae. Slip bands formed during thermo-compression process were in a single direction at a small strain, while the increase in the strain led to the intersections of slip bands in two directions. Kink bands were developed as the strain reached or exceed a critical value, acting as the preferred nucleation sites for recrystallization grains. Moreover, the model for the nucleation of recrystallization grains was given.

The microstructure and mechanical property of high niobium TiAl alloy prepared by electromagnetic cold crucible

With the development of aerospace industry, higher requirements are placed on the performance of high temperature structural materials. TiAl alloy, which has low density, high specific strength and specific modulus, good oxidation resistance and creep resistance, has a potential to be widely used in the future. Especially the high niobium TiAl alloys that has been developed in recent years will have more broad application than ordinary TiAl alloys because of higher operating temperature and greater oxidation resistance. In this paper, high niobium TiAl alloys are prepared by electromagnetic cold crucible (EMCC) multifunctional continuous casting technique. The microstructure, element distribution, room-temperature tensile property and high-temperature tensile property of as-cast and the alloy prepared by EMCC are investigated, respectively. The microstructure of high niobium TiAl-based alloys mainly consists of (α2+γ)lamellae, B2 and blocky γ phase. The concentration of Nb in the B2 phase is higher than in the other two phases. Appreciable room-temperature tensile properties, including an ultimate tensile strength of 622 MPa compared with 450 MPa in as-cast condition, were achieved at optimal solidification parameters. As the test temperature increases, the ultimate tensile strength of the alloy has a tendency to increase first at 700ºC and then decrease, which is related to dislocation motion and slip characteristics of intermetallic compound. The results show that the alloy prepared by EMCC has more uniform microstructure, less segregation and great improvement in mechanical properties.

A Novel Directional Solidification of TiAl-Based Alloys by Electromagnetic Cold Crucible Zone Melting Technology with Y<sub>2</sub>O<sub>3</sub> Moulds

A Novel Directional Solidification of TiAl-Based Alloys by Electromagnetic Cold Crucible Zone Melting Technology with Y<sub>2</sub>O<sub>3</sub> Moulds

Dimensionless parameters controlling fluid flow in electromagnetic cold crucible

A 3-D numerical model was established for predicting the flow field in a square electromagnetic cold crucible (EMCC) used for melting and directionally solidifying TiAl alloys. Four dimensionless parameters that characterise the melt flow in the EMCC were derived, those being the Hartman (Ha), magnetic Reynolds (Rω), coils-melt position (h) and the ratio of the melt height to length (H/L) numbers. Parametric simulations and experiments were carried out to understand the effects of processing parameters such as the intensity and frequency of the current, the relative coils-melt position and the melt shape on the flow field. The meridional flow normally consists of two vortices in the half meridian plane, the lower vortex decreases with increasing Ha, Rω and h (hb>hm), as well as decreasing H/L. Higher Ha, H/L and lower h induce intensive fluid flow in the melt due to the stronger EM coupling, which could promote the uniformity of solute in the melt. The turbulence kinetic energy is significantly influenced by the length scale of the turbulent flow and the flow velocity in the melt, it increases with increasing Ha, h and H/L, while reduces and tends to be stable at higher Rω. Relatively higher flow velocity and turbulence kinetic energy can be obtained when Rω is close to 10. The weakened flow in the vicinity of solid/liquid interface under lower Ha and Rω, as well as higher h and H/L is beneficial for continuous growth of columnar crystals during the directional solidification process.

Dominant dimensionless parameters controlling solute transfer during electromagnetic cold crucible melting and directional solidifying TiAl alloys

Electromagnetic cold crucible (EMCC) is widely applied to melt and solidify refractory and reactive materials, the electromagnetically driven flow in the melt leads to intensive transfer behaviors of solute and strongly affects crystal growth. In this paper, a 3-D numerical model for predicting the solute transfer behaviors in a square EMCC used for melting and directional solidifying was established and verified. A two-way coupling method was proposed to calculate the electromagnetically driven flow and its effects on solute transfer. Influences of the dominant dimensionless parameters on solute transfer behaviors in the EMCC were examined, those being the Hartman (Ha), magnetic Reynolds (Rω), coils-melt position (h) and the ratio of the melt height to length (H/L) numbers. Results demonstrate that the solute segregation tends to appear in the vicinity of solid/liquid (S/L) interface, the top of meniscus and the confluence of two eddies near the wall of meniscus. The solute segregation degree (Se=Cmax−CminC0) decreases with increasing Ha, which contributes to the augment of flow intensity in the melt. Moreover, the Se decreases linearly with increasing Rω owing to the intensive oscillation of melt flow. The enhanced EM coupling with increasing H/L results in the decrease of Se in the melt. However, the solute segregation in the corner of the meniscus gradually aggravates with increasing h, which results from the enlarging of lower eddies in the bulk of the melt. The solute segregation in the vicinity of S/L interface is strongly influenced by the scale and the flow intensity of lower eddies, which could change the phase transition path and morphology of crystal during directional solidification process. Larger value of Ha, Rω and H/L, as well as smaller h are beneficial to alleviate the solute segregation in the vicinity of S/L interface.

Numerical Research on Magnetic Field, Temperature Field and Flow Field During Melting and Directionally Solidifying TiAl Alloys by Electromagnetic Cold Crucible

The electromagnetic cold crucible (EMCC) technique is an effective method to melt and directionally solidify reactive and high-temperature materials without contamination. The temperature field and fluid flow induced by the electromagnetic field are very important for melting and controlling the microstructure. In this article, a 3D EMCC model for calculating the magnetic field in the charges (TiAl alloys) using the T-Ω finite element method was established and verified. Magnetic fields in the charge under different electrical parameters, positions and dimensions of the charge were calculated and analyzed. The calculated results show that the magnetic field concentrates in the skin layer, and the magnetic flux density (B) increases with increasing of the frequency, charge diameter and current. The maximum B in the charge is affected by the position of the charge in EMCC (h 1) and the charge height (h 2), which emerges at the middle of coils (h c) when the relationship of h c < h 1 + h 2 < h c + δ is satisfied. Lower frequency and smaller charge diameter can improve the uniformity of the magnetic field in the charge. Consequently, the induced uniform electromagnetic stirring weakens the turbulence and improves temperature uniformity in the vicinity of the solid/liquid (S/L) interface, which is beneficial to forming a planar S/L interface during directional solidification. Based on the above conclusions, the TiAlNb alloy was successfully melted with lower power consumption and directionally solidified by the square EMCC.

Method of recycling titanium scraps via the electromagnetic cold crucible technique coupled with calcium treatment

In this study, the electromagnetic cold crucible (EMCC) technique coupled with a Ca treatment method was used to test its potential for recycling Ti scraps. Deoxidation of molten Ti scraps was studied at Ca concentrations ranging from 0 to 0.5 wt%. With the addition of Ca, the contents of O, C, and H in the Ti scraps decreased, whereas the N content increased. Ca-based compounds were only observed on the top surface of the Ti ingots. The grain size and hardness of the fabricated Ti ingots before and after Ca addition were similar among the specimens. Noticeably, the ultimate tensile strength and the strain at fracture of the fabricated Ti ingots significantly increased from 641 ± 23 to 774 ± 8 MPa and from 5.8 ± 0.8% to 19.5 ± 5.1%, respectively, with the addition of Ca.

Experimental and numerical investigation on mass transfer induced by electromagnetic field in cold crucible used for directional solidification

Electromagnetic stirring induced by alternating magnetic fields could generate complex mass transfer in the liquid metals. In this paper, the flow field and the mass transfer behavior induced by the high frequency magnetic field in a square electromagnetic cold crucible (EMCC) used for directional solidification was experimentally and numerically investigated. By using tungsten particles as the tracer, the experiments under different operating powers and times were carried out, and the distribution of the particles in melt after cooling were analyzed. The results show that two mean eddies exist in the half meridian plane of the molten pool, and the skin effect results in a stronger convection within the skin layer. The mass moves following the mean eddies and emerges preferentially in the skin layer; when the mass is transported to the region between the upper and the lower eddies, which is homogenized in axial by the strong oscillations of convection. However, there is a concentration gradient along the radial in melt due to the skin effect. It found that the mass distributes more uniform with higher operating power or longer time. Moreover, the inhomogeneous convection at the vicinity of solid-liquid interface leads to the heterogeneous distribution of the mass, which aggravates the deflection of solid-liquid interface and changes the phase transition path during crystal growth.

Dependency of microstructure and microhardness on withdrawal rate of Ti-43Al-2Cr-2Nb alloy prepared by electromagnetic cold crucible directional solidification

The intermetallic Ti-43Al-2Cr-2Nb (at.%) alloy was directionally solidified in an electromagnetic cold crucible with different withdrawal rates (V) ranging from 0.2 to 1.0 mm∙min-1, at a constant temperature gradients (G=18 K∙mm-1). Macrostructures of the alloy were observed by optical microscopy. Microstructures of the alloy were characterized by scanning electron microscopy (SEM) in back-scattered electron mode and transmission electron microscopy. Results showed that morphologies of macrostructure depend greatly on the applied withdrawal rate. Continuous columnar grains can be obtained under slow withdrawal rates ranging from 0.2 to 0.6 mm∙min-1. The microstructure of the alloy was composed of α2/γ lamellar structures and a small number of mixtures of B2 phases and blocky γ phases. The columnar grain size (d) and interlamellar spacing (λ) decrease with an increasing withdrawal rate. The effect of withdrawal rate on microhardness was also investigated. The microhardness of the directionally solidified Ti-43Al-2Cr-2Nb alloy increases with an increase in withdrawal rate. This is mainly attributed to the increase of B2 and α2 phases as well as the refinement of lamellae.

Microstructure and compressive properties of directionally solidified Er-bearing TiAl alloy using cold crucible

Dual phase γ-TiAl of Ti-47Al-2Nb-2Cr was alloyed by erbium and then prepared by directional solidification using electromagnetic cold crucible. Compression test was performed to determine the work hardening capacity in accordance with microstructures regarding erbium additions. The directionally solidified columnar grains were refined by 0.2at.% erbium in comparison to those without erbium, while columnar to equiaxed transition (CET) happened in the case of 0.8at.% erbium. Since oxygen has extremely strong affinity to erbium, oxygen dissolved in the TiAl matrix decreased from 940ppm to 560ppm by internally oxidizing of erbium. The scanning electron microscopy characterization revealed that the Er2O3 particles were dispersed uniformly in the (α2+γ) lamellae. Arising from the loss of interstitial oxygen in the matrix, stacking fault energy of the h.c.p. phase (α2 or α) was decreased which resulted in increasing nucleation sites of γ phase. Then the γ lamellae were refined in the Er-bearing alloys. During the compression test, the work hardening capacity (Hc) of the 0.2at.% Er-bearing alloy was improved at strain rate ε̇ of 10−1–10−3s−1, which could be fitted as Hc=1.66ε̇−0.05. Besides, the governing deformation mechanism of the Er-bearing TiAl alloy was the same as the Er-free TiAl alloy according to the strain rate sensitivity results.

Electromagnetic Cold Crucible Technology Applied for Producing Big-Sized γ-TiAl Based Ingots with Directional Growth Structure

Developing high performance aero-engines usually depends on the development of new candidate materials with charming properties such as relatively light-weight, good high temperature strength and environmental resistance. Fortunately, γ-TiAl alloys were followed into this catalogue that they offer a significant potentiality for weight savings. Recently, in order to improve their low room temperature plasticity, a motive technology called electromagnetic cold crucible directional solidification technology for preparation γ-TiAl alloy ingots with directional growth structure was put forward by our group. By the approach, TiAl ingots with controllable lamellar structures and less impurity contaminations are obtained. Typically, Ti-46Al-0.5W-0.5Si and Ti-47Al-2Cr-2Nb were performed with respect to compositions and solidification parameters. As a result, Ti-46Al-0.5W-0.5Si DS-samples exhibit good combination of mechanical properties at room temperature in TS of 500MPa and EL of 2% in average. Meanwhile TS of DS Ti-47Al-2Cr-2Nb reaches 650 MPa and EL exceeds 3%.

Uniformity analysis of magnetic field in an electromagnetic cold crucible used for directional solidification

PurposeThe purpose of this paper is to introduce a numerical calculation method to study the uniformity of the magnetic field in a cold crucible used for directional solidification (DS) and provide information for designing a cold crucible that can induce a uniform magnetic field.Design/methodology/approachTo obtain the characteristics of the magnetic field in a cold crucible and its influence on the directional solidification processing, based on experimental verification, 3‐D finite element (FE) models with different crucible configuration‐elements and power parameters were established to study the uniformity of the magnetic field in a cold crucible. In addition, different TiAl ingots were directionally solidified with different cold crucibles, and the solid/liquid (S/L) interfaced were examined to investigate the effect of the magnetic field on the macrostructure of those ingots.FindingsThe uniformity of the magnetic field in a given domain can be quantitatively analyzed by statistical methods. Numerical calculation results showed that the uniformity of the magnetic field can be improved by optimizing the crucible configuration and adopting lower frequency. Better uniformity of the magnetic field in a cold crucible is beneficial to directional solidification.Originality/valueThe calculation of the uniformity of the magnetic field is proposed as a method for quantitative study of the distribution characteristics of the magnetic field in a cold crucible. The relationship between the S/L interfaces of TiAl ingots and the uniformity of the magnetic field is initially characterised; additionally, techniques for improving the uniformity of the magnetic field in a cold crucible are suggested.