ISIJ International
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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559

ISIJ International Vol. 32 (1992), No. 5

  • Melting and Refining of Superalloys and Titanium Alloys

    pp. 557-562

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    DOI:10.2355/isijinternational.32.557

    The processes used for the manufacture of both superalloys and titanium for the aerospace industry i.e. vacuum induction melting, vacuum arc melting and electroslag remelting, are to be considered as mature technologies. They have been developed over many years in regard to both the equipment used and the process techniques. They have been found to have advantages and have been manifestly successful in producing high quality material as attested to by the impressive reliability of turbines in service. However, in the present progress of turbine manufacture we have arrived at a position where the operational improvements in engine performance require alloy components of even higher reliability–beyond that which can be guaranteed by the present processes, for reasons which are discussed below. We are hence at an interesting stage in the development of the melting processes where we have little potential left in the present techniques and must therefore introduce the next stage in the technical development. The discussion below outlines the rationale for the choice of this type of new process, based on both absolute quality and also on quality assurance through process control. It is concluded that for both titanium alloys and superalloys, electron-beam melting can provide the incremental improvement which we need, as is being presently demonstrated by the industrial introduction of electron beam technology.
  • State of the Art of Superalloy Production for Aerospace and Other Application Using VIM/VAR or VIM/ESR

    pp. 563-574

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    DOI:10.2355/isijinternational.32.563

    Vacuum induction melting is indispensable in the manufacture of Ni- and Co-based superalloys and other sophisticated alloys because of t heir reactivity with atmospheric oxygen and nitrogen. The paper describes the technology of melting and refining in a vacuum induction furnace, the programmable furnace control and metallurgical results. The paper also describes subsequent remelting processes like VAR and ESR which make it possible to meet the very high quality requirements for aerospace applications.
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    1. Melting and Refining of Superalloys and Titanium Alloys ISIJ International Vol.32(1992), No.5
  • Continuous Casting of Titanium Alloy by an Induction Cold Crucible

    pp. 575-582

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    DOI:10.2355/isijinternational.32.575

    Titanium alloy was continuously cast by an induction heating making use of a segmented conductive cold crucible. In this process, the electromagnetic coupling between the coil and the metallic change is promoted through the gaps between the segments. A liquid titanium alloy is confined detached from the crucible wall accompanied with the induction heating and the magnetic stirring. The controlled supply of the raw materials and the simultaneous continuous withdrawal of the ingot enabled the stable casting.
    A technique of casting titanium alloy ingot by the supply of titanium turnings mixed with the alloy elements was demonstrated. This is of great advantage to the increase of titanium alloy species which can be cast. There was neither increase in copper contents nor decrease in volatile elements such as aluminum.
    Suitable casting parameters resulted in the crack free surface with the unevenness as flat as ±5 μm. For the achievement of good surface quality, the casting velocity is so chosen that the solidification front on the periphery of the ingot is to be adjusted to the point where the melt and the crucible wall meet each other.
  • The Effect of Beam Oscillation Rate on Al Evaporation from a Ti-6Al-4V Alloy in the Electron Beam Melting Process

    pp. 583-592

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    DOI:10.2355/isijinternational.32.583

    In order to clarify the quantitative effect of the beam oscillation rate on the Al evaporation behavior, small amounts of Ti-6Al-4V alloy were melted in a 30 kW EB furnace. The changes in the temperature on the melt surface, the weight loss and the Al concentration were investigated in this study.
    On the basis of the experimental results, a two dimensional unsteady state heat and mass transfer model was developed. The model was used to evaluate the effect of the beam oscillation rate on the temperature behavior and the evaporation reaction during EB process and also discussion was made on the optimum beam scanning rate.
    It was clearly shown that the evaporative loss of both Ti and Al could be suppressed by the increase of the beam oscillation rate. With the beam oscillation rate at more than 1.0 Hz, however, this effect could not be observed clearly anymore. Therefore, the optimum beam oscillation rate can be considered to be in the range of 1.0-10.0 Hz.
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    1. Aluminum Evaporation from Titanium Alloys in EB Hearth Melting Process ISIJ International Vol.32(1992), No.5
    2. Dissolution of Refractory Elements to Titanium Alloy in VAR ISIJ International Vol.32(1992), No.5
    3. Control of Chemical Compositions of Ti-6Al-4V Alloy during Melting by Electron Beam Furnace ISIJ International Vol.32(1992), No.5
  • Temperature Measurement of Molten Metal Surface in Electron Beam Melting of Titanium Alloys

    pp. 593-599

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    DOI:10.2355/isijinternational.32.593

    The objective of this study was to establish the technology for measuring the surface temperature of the metal pool in Electron Beam Melting (EBM) of titanium and titanium alloys.
    The experiments were carried out using the 80 kW×2 EB furnace. Under various pressures in the chamber, the temperature of titanium or titanium alloy pools were measured by a two color pyrometer, with two wave lengths for the measurements were 500 and 580 nm; and by an optical spectrometer with range from 400 to 800 nm. Tungsten-Rhenium (5/26) thermocouples were used for the correction of the optically measured temperature by directly immersion in the metal pool.
    Results obtained were as follows;
    (1) Only when the pressure in the chamber was below 0.04 Pa, the two color pyrometer could be used for the temperature measurements of titanium pool.
    (2) In case of pressure above 0.04 Pa, the reading of the two color pyrometer was disturbed by emitted light especially with the spectral-line of 500 nm in titanium vapor. In these cases, the temperature should be computed from the base line of the visible light spectrum.
    (3) The relationship between the surface temperature of the titanium alloy pool in the hearth and EB irradiation power could be well expressed by a simple heat balance model.
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    1. Control of Chemical Compositions of Ti-6Al-4V Alloy during Melting by Electron Beam Furnace ISIJ International Vol.32(1992), No.5
    2. Continuous Casting of Titanium Alloy by an Induction Cold Crucible ISIJ International Vol.32(1992), No.5
    3. Electron Beam Melting and Refining of Metals and Alloys ISIJ International Vol.32(1992), No.5
  • Dissolution of Refractory Elements to Titanium Alloy in VAR

    pp. 600-606

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    DOI:10.2355/isijinternational.32.600

    In order to clarify the dissolving aspects of refractory elements (Ta, Mo, V) which are components of beta titanium alloys, the experiments of VAR with a mold of 100 mm diameter were carried out. In the experiments, the initial diameters of those elements were changed and their particles were observed after melting. Then, the experimental results were discussed by the mathematical model for dissolving. The following results were obtained.
    (1) In the case of Ta, the dissolving was complete by the single VAR in the size of the initial particles under 200 mesh (74 μm), while in the case of Mo, the dissolving was complete under 100 mesh (149 μm). In the case of V, the dissolving was complete even under 6 mesh (3 360 μm).
    (2) In the case of V, the dissolving was almost finished in the molten layer at the electrode top. But, in the case of Ta and Mo, the dissolving mainly made progress in the molten metal pool of ingot.
    (3) The mathematical model well explained the experimental results, and the time needed for complete dissolving (tN) obtained by this analysis is thought to be an index of difficulty of the dissolving.
    (4) The dissolving time in the molten metal pool was estimated. Compared with tN, it was found out that there exists the critical value of the initial diameter of refractory element for the complete dissolving. It is advisable that the initial diameter close to this critical value should be chosen even in a larger scale of VAR than that of these experiments.
  • Aluminum Evaporation from Titanium Alloys in EB Hearth Melting Process

    pp. 607-615

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    DOI:10.2355/isijinternational.32.607

    In order to investigate Al evaporation behavior in the EBCHR process, Ti alloys were remelted in a 250 kW EB furnace. Ti alloys used in this experiment were Ti-64, Ti-811 and SP700, a newly developed Ti alloy at NKK. In this experiment, EB power was kept almost constant and round ingots with approximately 136 mm in diameter were cast at a different casting rate. During melting operation, in-situ temperature measurement was carried out by a two-color thermometer. For determining an Al evaporation site in this process, both feedstocks and metal in the hearth after melting operation were analyzed in detail together with ingots.
    It was found that the Al yield, the ratio of the final Al concentration to the initial Al concentration, increased with the casting rate. Under the present experimental condition, the Al evaporation reaction took place mainly in the hearth, which was found to be a complete mixing reactor.
    Based on the above experimental results, the rate equation of Al evaporation was considered by taking account of both reaction at the interface and mass transfer in the melt. Calculation explained the experimental results well when the time-averaged temperature in the hearth was in the range of 1 800 and 1 900°C, which was in a relatively good agreement with temperature measurement results.
  • Changes in Oxygen Contents of Titanium Aluminides by Vacuum Induction, Cold Crucible Induction and Electron Beam Melting

    pp. 616-624

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    DOI:10.2355/isijinternational.32.616

    Fundamental studies on the process of melting titanium aluminides, TiAl, have been pursued from the viewpoint of contamination.
    Three processes–vacuum induction melting (VIM) with a calcia crucible, cold crucible induction melting (CCM) with a water-cooled copper crucible, and electron beam melting (EBM), –were investigated and compared to determine the behavior of impurity elements, especially oxygen.
    Experiments using the conventional VIM method revealed that an increase in oxygen content from the calcia crucible during melting was unavoidable. With CCM, oxygen content did not change, while in EBM, it decreased abruptly with the evaporation of aluminum; the aluminum segregation, however, was found accumulated in ingots. The cleanest ingot of 0.02 mass% oxygen was obtained using the combined processes of EB and CCM (EB-CCM). Mechanical properties of the ingots were also examined, and elongation of up to 1.0% was recognized at ambient temperature.
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    3. Melting and Refining of Superalloys and Titanium Alloys ISIJ International Vol.32(1992), No.5
  • Control of Chemical Compositions of Ti-6Al-4V Alloy during Melting by Electron Beam Furnace

    pp. 625-629

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    DOI:10.2355/isijinternational.32.625

    Strict control of chemical compositions is quite important during melting and casting of Ti-6Al-4V alloy using an electron beam furnace because of the high vacuum accompanying the vaporization of elements with high vapor pressure. Thermochemical parameters of alloying elements have been determined and vaporization of Al from the molten pool under high vacuum and high temperature has been investigated in this paper. In addition, a method is proposed to forecast the yield of Al added to the molten pool just before casting to compensate vaporization loss using the change in surface temperature observed at addition of Al as an index.
  • Removal of Boron from Metallurgical-grade Silicon by Applying the Plasma Treatment

    pp. 630-634

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    DOI:10.2355/isijinternational.32.630

    The removal of boron from metallurgical-grade silicon was investigated by applying an Ar/H2O plasma treatment to develop a new production technique for low cost solar-grade silicon (SOG-Si).
    The concentration of boron in MG-Si decreased from 35.7 to 0.4 ppmw, satisfying the requirement for SOG-Si, using Ar+1.24vol%H2O plasma gas for 25 min of melting time. It has been demonstrated that the overall rate of elimination of boron is controlled by the diffusion of boron in silicon melts. The removal of boron is affected by the kind of plasma operating gas with the highest elimination rate of boron by Ar/H2O plasma. The experimental findings support that boron in silicon reacts with oxygen in gas only at the plasma-impinging area.
    It may be said that only this technique has the possibility to lower boron content of silicon down to the required 0.1 ppm for SOG.
  • Purification of Metallurgical Silicon for Solar-grade Silicon by Electron Beam Button Melting

    pp. 635-642

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    DOI:10.2355/isijinternational.32.635

    Behavior of various impurities such as carbon, phosphorus, boron, calcium, aluminum, iron and titanium in metallurgical-grade silicon has been investigated during electron beam button melting with various modifications. Carbon, phosphorus, calcium and aluminum were removed by Electron Beam Remelting (EBR) treatment under 10–2 Pa for 30 min. Ninety percent of carbon, 75% of aluminum, 89% of calcium and 93% of phosphorus was removed. The lowest content of these impurities were 15 ppmw C, 470 ppmw Al, 150 ppmw Ca and 3 ppmw P, respectively. First order rate equation was used for the removal of carbon, calcium and aluminum, and second order equation fit for the dephosphorization. Rate constants for calcium, aluminum and carbon ranged from 0.01 to 0.1 min–1. That of phosphorus was from 0.003 to 0.01 ppmw–1·min–1.
  • Production of High Purity Silicon by Carbothermic Reduction of Silica Using AC-arc Furnace with Heated Shaft

    pp. 643-649

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    DOI:10.2355/isijinternational.32.643

    To produce inexpensive solar-grade silicon of high purity, an AC-arc furnace has been developed based on the thermodynamic consideration of carbothermic reduction of silica to metallic silicon. The furnace features a closed type shaft and a hearth with arc electrodes and feeding nozzles. Carbon pellets as a reducing agents are fed from the top of the shaft, and silica powder is transferred to the hottest arc spot in the hearth. The resulting species of SiO and SiC, generated through the reactions of SiO2+C→SiO+CO and SiO+2C→SiC+CO, react in the lower part of the shaft to yield silicon, SiO+SiC→2Si+Co. Melting silicon is accumulated in the hearth and pulled out from the tapping hole.
    The productivity has been about 2 kg/hr and Si yield has been found to be typically 83%. Analyses have shown that the impurities in the silicon were below 0.1 ppmw for B, 12 ppmw for Fe and below 5 ppmw for the other elements. Single-crystalline solar cells fabricated starting from this silicon after purifications of decarburization and unidirectional solidification have recorded a conversion efficiency of 16.5%. This value was equivalent to that of solar cells made from semiconductor-grade silicon.
  • Electron Beam Melting and Refining of Niobium

    pp. 650-655

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    DOI:10.2355/isijinternational.32.650

    With respect to the importance of remelting of the ATR-Nb for the future large scale production of niobium applied to corrosion resistance materials and superconducting magnets with Nb-Ti alloys, the electron beam melting and refining have been examined to the following items:
    1) Process for the winning of niobium, 2) Thermochemical background for the melting and refining of niobium in vacuum, 3) Electron beam cold hearth remelting of niobium and 4) Purification of the ATR-Nb.
    In this process, the electron beam furnace can act not only as remelting but also as refining apparatus. The experimental results showed its feasibility for obtaining niobium ingot of more than 99.8 mass% purity from crude raw materials.
  • Electron Beam Cold Hearth Melting and Refining (EB-CHR) of Refractory Metals

    pp. 656-663

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    DOI:10.2355/isijinternational.32.656

    Electron beam cold hearth melting and refining (EB-CHR) of refractory metals (Nb, Mo, Ta) and an alloy (Nb-1%Zr) was examined for its ability to produce homogeneous materials in the 2 000 and 400 kW EB-CHR furnaces and the 70 kW EB furnace.
    Behavior of gaseous elements was discussed with taking the volatile parameter R into the consideration.
    In the production of Nb-1%Zr in the 400 kW EB-CHR furnace, zirconium content was controllable. No serious segregation was found. These results show that the mechanism of zirconium loss in the EB-CHR process depends on both the evaporation and segregation of zirconium.
    Heat balance in molybdenum melting was measured in the 2 000 kW EB-CHR furnace. The obtained heat efficiency was about 70% and energy loss of EB agreed with the ratio of backscattered electron energy to whole EB well energy input, which is about 30%.
    Mechanical processing tests involving forging, hot-rolling and cold-rolling were carried out. Material property such as workability was evaluated. EB-CHR processed materials gave better workability than others. Plates and sheets of some refractory metals were successfully produced.
  • Composition Control of Refractory and Reactive Metals in Electron Beam Melting

    pp. 664-672

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    DOI:10.2355/isijinternational.32.664

    Aiming at controlling the composition of refractory and reactive metals in electron beam melting, melting experiments were carried out for commercially pure titanium (C.P.Ti), Ti-6Al-4V alloy and pure niobium and the following results were obtained:
    (1) In the melting of C.P.Ti, the addition of TiO2 powder as an oxygen source and independent control of the feeding rates of titanium sponge and TiO2 enabled the fine composition adjustment of [O]. The properties of the materials produced from the ingots obtained were the same as those of materials produced by the conventional VAR-forging process.
    (2) In the melting of Ti-6Al-4V alloy, it was confirmed that the evaporation of Al could be rationally expressed by the Langmuir equation. Also, in the independent control of the feeding rates of titanium sponge and Al-V alloy, the control of molten pool temperature made the uniform composition adjustment of [Al] and [V] possible.
    (3) In the melting of pure Nb, deoxidation and denitrification ratio were improved as melting energy increased, and these reactions were considered to be rate-controlled by degassing reactions. In addition, the relation between residual resistance ratio (RRR) and the contents of impurities was quantified.
  • Electron Beam Melting and Refining of Metals and Alloys

    pp. 673-681

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    DOI:10.2355/isijinternational.32.673

    The increasing need for improved refractory and reactive high strength materials has led to the development of special production processes. This article will consider electron beam melting and refining which is playing an important role especially in the production of nickel base superalloys, specialty steels, refractory metals such as tantalum, niobium, tungsten, and molybdenum and reactive metals such as hafnium, vanadium, zirconium, and titanium and their alloys. The drip and cold hearth melting and refining techniques including the electron beam heat sources are described. Process data and results for various materials are given.
  • The Use of Ceramic Foam Filters in the Production of High Integrity Steels and Ni-base Alloys

    pp. 682-684

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    DOI:10.2355/isijinternational.32.682

    Inclusion removal using ceramic foam filtration in the casting industry is well known, however the effect of ceramic foam filters on removing inclusions from 'clean' metal is not so well documented. This communication provides a preliminary report of the findings to date. Ceramic foam filters have been used in the production of approximately 3.3 tonne casts using the new VIDP furnace at Special Melted Products Ltd. Clean metal is supplied by the use of a specially designed launder system preheated to 900°C prior to teeming.
    Examination of impregnated filter cross sections from an alloy 718 and FV458 casts have revealed the ability of ceramic foam filters to remove inclusions in the <20 μm size range.
    The types of inclusion that are removed using this method include, from alloy 718, Titanium Nitride and Primary deoxidation products containing aluminium and magnesium. Examination of FV458 shows deoxidation products containing aluminium and calcium.
  • Evaporation of Alloying Elements and Behavior of Degassing Reactions of High Chromium Steel in Electron Beam Melting

    pp. 685-692

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    DOI:10.2355/isijinternational.32.685

    Electron beam melting enables to evaporate the volatile alloying elements and remove carbon, oxygen and nitrogen by high-temperature, high-vacuum melting. A rod melting technique with 250 kW EB furnace was applied to stainless steel and high chromium steel.
    Evaporation reaction of alloying elements during melting was of the first order and was considered to be rate-controlled by free evaporation. Rate of evaporation of alloying elements was higher for element with higher vapor pressure. Further the speed of evaporation was in proportion to the square root of melting energy of electron beam, and this relation was possible to be derived from Langmuir's equation. Deoxidation and decarburization reactions were enhanced by CO degassing reaction. The removal of nitrogen was remarkably enhanced with increasing amount of the CO degassing reaction, and removal ratio of higher than 30% was possible.
    Based on the results above, controlling compositions of melted ingots was made possible, and achievement of high purification of [O]+[C]+[N]≤50 ppm in 25% chromium steel was made possible.
  • Removal of Inclusions in Stainless Steel by Electron Beam Melting

    pp. 693-699

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    DOI:10.2355/isijinternational.32.693

    Electron beam melting is thought to deoxidize and remove inclusions by high-temperature and high-vacuum melting. A rod melting technique using 250 kW EB furnace was applied to stainless steels and resulted in the following:
    Deoxidation is enhanced by the CO degassing reaction, and the adjustment of feeding material composition and the controlling of melting energy allow a total oxygen content (T.[O]) of 15 ppm or less. The removal of inclusions occurs as the deoxidation reaction proceeds, resulting in a reduction in their amount to 1/10 per EB melting. The effect of removal is remarkable with increasing size of the inclusions. It is considered that the majority of inclusions can be removed by the decrease in oxygen accompanied by their decomposition and then by the reaction of decomposed oxygen with carbon.
    The results confirmed higher cleanliness for inclusions than achieved by the conventional methods and production of an ultra-fine wire of 30 μm diameter without breakage during drawing.

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