Developing Recipe from Local Ceramic Raw Materials for Making Crucibles in Ghana

The study experimented with using local ceramic raw materials (white clay, kaolin and silica or quartz) found in AssinFosu in the Central Region of Ghana to manufacture crucibles for melting metals and other precious minerals. Various physical tests were conducted on the materials to arrive at the body compositions. The compositions were also investigated for their elemental components by using X-ray fluorescence (XRF). The results revealed that the composition of Cruc containing 70% of white clay, 20% of kaolin, 8% of quartz and 2% of white grog;sintered at 1500˚C was very successful and therefore used to develop the recipe to manufacture the proposed crucibles. The “throwing” technique was employed to fabricate the crucibles. Test for thermal expansion was conducted for the manufactured crucibles at 1000˚C for thermal shock and microcracking tests. It was found out among others, that the recipe developed had very good physical and chemical properties of alumina silicate refractory materials and was fit for use at any high-temperature application. The study also recommended among others, that the researchers and institutions responsible for clay research such as Ghana Geological Survey Authority (GGSA) and Centre for Scientific and Industrial Research (CSIR) collaborate to improve upon this innovative idea.

Effect of configuration on magnetic field in cold crucible using for continuous melting and directional solidification

To improve the power efficiency and optimize the configuration of cold crucible using for continuous melting and directional solidification （DS）, based on experimental verification, 3D finite element （FE） models with various configuration-elements were developed to investigate the magnetic field in cold crucible. Magnetic flux density （B） was measured and calculated under different configuration parameters. These parameters include the inner diameter （D2）, the slit width （d）, the thickness of crucible wall, the section shape of the slit and the shield ring. The results show that the magnetic flux density in z direction （Bz） both at the slit and at the midpoint of segment will increase with the decrease of D2 or with the increase of the width of the slit and the section area of wedge slit or removing the shield ring. In addition, there is a worst wall thickness that can induce the minimum Bz for a cold crucible with a certain outer diameter.

Temperature field calculation on cold crucible continuous melting and directional solidifying Ti50Al alloys

In order to optimize technological parameters and realize directional solidification,temperature fields of cold crucible continuous melting and directional solidifying Ti50Al（mole fraction,%） at different parameters were calculated.Continuous casting of the model is achieved by distinguishing the moving unit at different positions.The calculation results show that the feeding rod is entirely melted at 200 s,the melt of feeding rod has some superheat degree at 300 s under the conditions of 52 kW and 3.0 mm/min.Both the superheat degree and the molten zone of the feeding rod reduce,the solid-liquid interface becomes concave with increasing velocity from 1.2 mm/min to 6.0 mm/min when the power is 52 kW,and the outside layer of the rod cannot be melted at the velocity of 6.0 mm/min.Both superheat degree and the molten zone of the feeding rod increase,the solid-liquid interface descends and becomes concave with increasing power from 48 to 58 kW at velocity of 3.0 mm/min,and the rod cannot be melted entirely when the power is 48 kW.Cold crucible continuous melting and directional solidification of TiAl alloys will be achieved successfully when the pulling velocity and the power are matched appropriately.

Process on cold crucible electromagnetic casting for titanium alloy

The parameters and factors that influence the surface quality and macrostructure of titanium alloy with reactive properties under liquid state were studied experimentally using a cold crucible electromagnetic casting method. The variations in the process parameters have great impact on the surface quality and macrostructure of cast billets. Billets with crack free and smooth surfaces as well as directional solidified primary structures were obtained after the selection of optimized process parameters. The formation mechanisms of defects such as cracks and non-directional structural morphology were interpreted briefly. Finally,the casting of billets with good outer qualities and inner column grains has been attained successfully,which in turn gives a solid foundation for further development of the technology.

Interfacial reactions between Ti-1100 alloy and CaO crucible during casting process

Ti-1100 alloys were melted in a controlled atmospheric induction furnace equipped with a Ca O crucible. The microstructure, chemical composition, microhardness and metal-crucible interfacial reactions were systematically investigated. The results demonstrate that the primary solidification microstructure in the as-cast alloys was the typical Widmansttten structure. The interactions between crucible and molten alloys are attributed to slight chemical dissolution and weak physical erosion. According to the line scanning analysis, the interfacial layer（α-case） thicknesses of Ti-1100 samples in the bottom and side wall are about 18 and 17 μm, respectively, which are slightly lower than those presented from microhardness tests（25 and 20 μm）. The formation of α-case was caused by interstitial oxygen atoms. The standard Gibbs energy of reaction Ca O（s）=Ca＋O for Ti-1100 alloy was also determined. The equilibrium constant and the interaction parameter between calcium and oxygen were obtained as lg K=-3.14 and eCa O =-3.54.

Numerical calculation of flow field inside TiAl melt during rectangular cold crucible directional solidification

Numerical investigations on the flow field in Ti-Al melt during rectangular cold crucible directional solidification were carried out. Combined with the experimental results, 3-D finite element models for calculating flow field inside melting pool were established, the characteristics of the flow under different power parameters were further studied. Numerical calculation results show that there is a complex circular flow in the melt, a rapid horizontal flow exists on the solid/liquid interface and those flows confluence in the center of the melting pool. The flow velocity v increases with the increase of current intensity, but the flow patterns remain unchanged. When the current is 1000 A, the vmax reaches 4 mm/s and the flow on the interface achieves 3 mm/s. Flow patterns are quite different when the frequency changes from 10 kHz to 100 kHz, the mechanism of the frequency influence on the flow pattern is analyzed, and there is an optimum frequency for cold crucible directional solidification.

Influence of crucible material on inclusions in 95Cr saw-wire steel deoxidized by Si–Mn

To investigate the interaction mechanism between 95 Cr saw-wire steel and different refractories,we conducted laboratory experiments at 1873 K.Five crucible materials(SiO2,Al2 O3,MgO·Al2 O3,MgO,and MgO-CaO)were used.The results indicate that SiO2,Al2 O3,and MgO·Al2 O3 are not suitable for smelting low-oxygen,low-[Al]s 95 Cr saw-wire steel,mainly because they react with the elements in the molten steel and pollute the steel samples.By contrast,MgO-CaO is an ideal choice to produce 95 Cr saw-wire steel.It offers three advantages:(ⅰ)It does not decompose by itself at the steelmaking temperature of 1873 K because it exhibits good thermal stability;(ⅱ)[C],[Si],and[Mn]in molten steel cannot react with it to increase the[O]content;and(ⅲ)it not only desulfurizes and dephosphorizes but also removes Al2 O3 inclusions from the steel simultaneously.As a result,the contents of the main elements([C],[Si],[Mn],[Cr],N,T.O(total oxygen))in the steel are not affected and the content of impurity elements([Al]s,P,and S)can be perfectly controlled within the target range.Furthermore,the number and size of inclusions in the steel samples decrease sharply when the MgO-CaO crucible is used.

Interaction mechanism between niobium-silicide-based alloy melt and Y_2O_3 refractory crucible in vacuum induction melting process

The Y2O3 crucibles were introduced in the study as an alternative to the traditional ceramic ones in vacuum induction melting of multi-component Nb-16Si-22Ti-2AI-2Hf-17Cr （at.%） alloys, to reveal the possible interactions between the alloy melt and the refractory crucible. Multiple melting time lengths and two cooling schemes were designed and used for the experiments. The chemical composition and microstructure of the tested alloy and the melt-crucible interaction were investigated and evaluated. In the experiments,Y2O3 crucible displays good physical-chemical compatibility. The results indicate that the increment of O element in the as-cast ingot iS： 0.03at.%-0.04at.% （72-97 ppm） and the increment of Y element is very insignificant. The key features of the alloy melt interacting with Y2O3 ceramics are analyzed and concluded in the paper. As a result of the dissolution reactiofi xY2O3 （in molten alloy） ＋ （1-x）HfO2 （impurity） →,Hf1-xY2xO2-x, a continuous double-layer solid film consisted of HfO2 solid solution （-2 pm） and pure HfO2 （-5μm） is formed on the surface of the test ingot after cooled down in the crucible. The experimental results show that theY2O3 crucible is applicable to the vacuum induction melting of Nb＇ Si based alloys.

Desulphurization during VIM Refining Ni-base Superalloy using CaO Crucible

The variation of S content during VIM refining Ni-base superalloy using CaO crucible was studied. It was foundthat the desulphurization process could not be carried out by only using CaO crucible. The role of Al additionto desulphurization was also studied. Combining with the results of XRD and composition analysis of the CaOcrucible, the mechanism of desulphurization was proposed. Thermodynamical calculation about the reaction betweenthe interface of CaO crucible and liquid metal has been discussed. This work indicated that under proper refiningtechnology the S content in the liquid Ni-base alloy could be reduced from 3×10-5 to 2×10-6~4×10-6.

ELECTROMAGNETIC MODEL OF LEVITATION MELTING WITH COLD CRUCIBLE

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Electromagnetic characteristics of square cold crucible designed for silicon preparation

In the present work, the strength and distribution of electromagnetic field in the square cold crucible that designed for casting multicrystalline silicon were measured and analyzed by using a small coil method. The results show that in the perpendicular direction the maximum of magnetic flux density （B） appears at the position slightly above the middle of the coil, and then B attenuates toward both sides, and decreases more to the bottom of the crucible. In the horizontal direction, from the edge （comer） to the center, B firstly decreases gradually, and then slightly increases in the center. While along the inner sides of the crucible, the distribution is relatively uniform, especially in the effective acting range. B increases with the increasing of the input power. Moving the coil to the top of the crucible, B increases and the effective acting range of the electromagnetic field becomes bigger. For the coils with different turns, the five turns coil can induce the highest magnetic flux density.

THE ELECTROMAGNETIC FIELD CALCULATION AND ANALYSIS OF LEVITATION MELTING WITH COLD CRUCIBLE

In this paper, using the quasi-3D coupled current method, the influences of structure of cold crucible, the power frequency, the electricity property of melt, the coil position and current on the electromagnetic field (EMF) and the levitation characteristics in the melting processes are analyzed. It is shown that in the processes of levitation melting with cold crucible, the power frequency and cold crucible structure are the decisive factors for the ability of magnetic flux penetrating into cold crucible. The magnetic flux density in cold crucible is reduced as the increasing of power frequency, and this tendency becomes stronger when the power frequency is higher than 100kHz. The segmented structure of cold crucible can reduce the induction eddy in itself effectively, and the higher the power frequency is, the better the result is. So, a cold crucible can be segmented into 16-20 sectors for high frequency electromagnetic field and/or 8-12 sectors for lower frequency one. It is also shown that the levitation force of melting charge is related to coil current as a parabolic function.

Characterization of CdZnTe crystal grown by bottom-seeded Bridgman and Bridgman accelerated crucible rotation techniques

The growth of CdZnTe crystals with diameter up to 60 mm using bottom-seeded Bridgman method as well as Bridgman accelerated crucible rotation technique (ACRT-B) was investigated. Both ingots exhibit high yields, where single crystal with the volume exceeding 200 cm3 is produced. The crystal properties of two ingots were compared in the aspects of yields, crystalline quality and composition uniformity. For CdZnTe ingot grown by bottom-seeded Bridgman method, the full width at half-maximum (FWHM) of X-ray rocking curve was determined to be 36″, indicating a better crystalline quality than ingot grown by ACRT-B method, which gave FWHM of 56″. The composition distribution of Zn and In in CdZnTe was determined by using electron probe microanalysis (EPMA) and inductively coupled plasma mass spectrometry (ICP-MS), respectively. The effective segregation coefficients of Zn kZn and In kIn in the two ingots were evaluated by fitting the experimental data with the Pfann equation.

Effect of power parameter and induction coil on magnetic field in cold crucible during continuous melting and directional solidification

Bottomless electromagnetic cold crucible is a new apparatus for continuous melting and directional solidification;however,improving its power efficiency and optimizing the configuration are important for experiment and production.In this study,a 3-D finite element (FE) method based on experimental verification was applied to calculate the magnetic flux density (Bz).The effects of the power parameters and the induction coil on the magnetic field distribution in the cold crucible were investigated.The results show that higher current intensity and lower frequency are beneficial to the increase of Bz at both the segment midpoint and the slit location.The induction coil with racetrack section can induce greater Bz,and a larger gap between the induction coil and the shield ring increases Bz.The mechanism for this effect is also discussed.

Pressurization control system for low pressure crucible casting

A new compact pressurization control system of the low pressure casting machine for crucible pressure casting has been developed. It is especially designed for the production of high-quality aluminum or magnesium alloy parts with low input cost. This machine with such a system has the virtue of economical and compact, and combines the Fuzzy-PID technology and achieves accuracies of ±2.5 mbar. At present, this machine has been adopted by several users in China for the production of aluminum alloy castings with high property requirements. Furthermore, for magnesium alloy castings, this machine can be used with the gas protect unit.

Electromagnetic characteristics of cold crucible in an induction coil for melting purpose

The electromagnetic characteristics of cold wall crucible, especially its field strength and distribution, were evaluated experimentally, in which the effects of parameters as input power and position were also discussed. The results showed that the magnetic induction intensity （B） increased with the increase of the input power. Along the radial direction inside the cold crucible, B is the strongest at the edges, and gradually attenu- ated from the edges to the center. While along the axial direction, B is the strongest at the center, and gradually attenuated from the center to the edges. Corresponding with the calculation on electromagnetic field distribution inside the coil without cold crucible, the effects of cold crucible on the electromagnetic field have been reduced by comparing measured and calculated results. However, it was also found that the value of B strength will be decreased to about 50% of its original proposition without cold crucible due to the electromagnetic shielding effect, which will be the understanding for mehing turns in cold crucible.

Microstructure and mechanical properties of cast Ti-47Al-2Cr-2Nb alloy melted in various crucibles

The main factors limiting the mass production of TiAI-based components are the high reactivity of TiAl- based alloys with the crucible or mould at high temperature. In this work, various crucibles （e.g. CaO, Y203 ceramic crucibles and water-cooled copper crucible） were used to fabricate the Ti-47Al-2Cr-2Nb alloy in a vacuum induction furnace. The effects of crucible materials and melting parameters on the microstructure and mechanical properties of the alloy were analyzed by means of microstructure observation, chemical analysis, tensile test and fracture surface observation. The possibilities of melting TiAI alloys in crucibles made of CaO and Y2O3 refractory materials were also discussed.

Progress in research on cold crucible directional solidification of titanium based alloys

Cold crucible directional solidification(CCDS)is a newly developed technique,which combines the advantages of the cold crucible and continuous melting.It can be applied to directionally solidify reactive,high purity and refractory materials.This paper describes the principle of CCDS and its characteristics;development of the measurement and numerical calculation of the magnetic field,flow field and temperature field in CCDS;and the CCDS of Ti based alloys.The paper also reviews original data obtained by some scholars,including the present authors,reported in separate publications in recent years.In Ti based alloys,Ti6Al4V,TiAl alloys and high Nb-containing TiAl alloys,have been directionally solidified in different cold crucibles.The crosssections of the cold crucibles include round,near rectangular and square with different sizes.Tensile testing results show that the elongation of directionally solidified Ti6Al4V can be improved to 12.7%from as cast5.4%.The strength and the elongation of the directionally solidified Ti47Al2Cr2Nb and Ti44Al6Nb1.0Cr2.0V are 650 MPa/3%and 602.5MPa/1.20%,respectively.The ingots after CCDS can be used to prepare turbine or engine blades,and are candidates to replace Ni super-alloy at temperatures of 700 to 900°C.

WATER-COOLING CRUCIBLE VACUUM INDUCTION MELTING OF γ-TiAl BASED ALLOYS

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Continuous melting and directional solidification of silicon ingot with an electromagnetic cold crucible

In order to avoid contamination from the crucible and to modify the structures,a new solidification method based on cold crucible technology was used to prepare silicon ingots.A silicon ingot with square cross section was directionally solidified with a cold crucible.The mechanism of the cold crucible directional solidification of silicon ingot was revealed.Due to the induction heat that was released in the surface layer and the incomplete contact between the crucible and the melt,the lateral heat loss was reduced and the silicon ingot was directionally solidified.The structures,dislocation defects and the grain growth orientation of the ingot were determined.The results show that neither intergranular nor intragranular precipitates are found in the ingot,except for the top part that was the last to solidify.The average dislocation density is about 1 to 2×106 cm-2.The grains are preferentially <220> orientated.