Skull Melting Research Articles

Ultrahigh-Temperature Nb-Silicide-Based Composites

AbstractThis article reviews the most recent progress in the development of Nb-silicide-based in situ composites for potential applications in turbine engines with service temperatures of up to 1350°C. These composites contain high-strength Nb silicides that are toughened by a ductile Nb solid solution. Preliminary composites were derived from binary Nb-Si alloys, while more recent systems are complex and are alloyed with Ti, Hf, W, B, Ge, Cr, and Al. Alloying schemes have been developed to achieve an excellent balance of room-temperature toughness, fatigue-crack-growth behavior, high-temperature creep performance, and oxidation resistance over a broad range of temperatures. Nb-silicide-based composites are described with emphasis on processing, microstructure, and performance. Nb silicide composites have been produced using a range of processing routes, including induction skull melting, investment casting, hot extrusion, and powder metallurgy methods. Nb silicide composite properties are also compared with those of Ni-based superalloys.

Temperature measurements during induction skull melting of titanium aluminide

An immersion thermocouple and a twin wavelength infrared radiation pyrometer have been used to measure the temperature of titanium aluminide during melting in an induction skull melting furnace. A molybdenum-alumina cermet thermocouple sheath has been found to survive for long enough to enable about three reliable measurements to be made. A significant drift occurred between the temperatures measured using the pyrometer and the thermocouple when melting under a partial pressure of argon, and has been attributed to the generation of fume inside the melting vessel. The drift did not occur when melting was carried out in a continuously pumped vacuum. The superheat obtained increased from ~33°C when melting 4·5 kg of TiAl with 200 kW in a vacuum to ~44°C and ~62°C when melting under argon partial pressures of 20 and 80 kPa respectively. There appears to be little benefit from increasing the applied power from 200 to 350 kW.

Entrained oxide films in TiAl castings

Tilt pouring of TiAl alloy from an induction skull melting (ISM) furnace into a ceramic shell mould has been carried out in this research. Microstructures and casting defects entrained by surface turbulence have been examined using scanning electron microscopy and EDX analysis. Oxide films were found draped over the dendrites of shrinkage porosity and the inner surface of bubbles in the experimental TiAl casting. The entrained oxide films act as heterogeneous nucleation sites, contributing to the formation of centre-line shrinkage porosity in the cast TiAl bars.

Thermophysical properties of a Ti–44%Al–8%Nb–1%B alloy in the solid and molten states

The families of titanium aluminide intermetallic alloys have attractive high temperature mechanical properties which make them potential candidate materials for a wide range of applications, particularly in the aeronautic and automobile sectors. The development of appropriate manufacturing techniques is an essential stage in the engineering exploitation of these materials, e.g., Induction Skull Melting is one of the techniques which needs to be optimised for the casting of titanium aluminides. Research is underway to develop a computer model of this process but data are required for the key thermophysical properties. Pulse-heating techniques have been used to measure properties for the Ti–44Al–8Nb–1B system. Rectangular samples have been prepared and are resistively heated as part of a fast capacitor discharge circuit. Time-resolved measurements with sub-μs resolution of currents through the specimen were made with a Pearson probe current monitor using the induction principle. Voltages across the specimen were determined with knife-edge contacts and voltage dividers, and radiance temperatures of the sample were measured with a pyrometer. These measurements allow the calculation of specific heat and dependencies between enthalpy, electrical resistivity and temperature of the alloy up into the liquid phase. Data for thermal diffusivity have been obtained by using the Wiedeman–Franz relation. The results are compared with those obtained using DSC and the four-probe method to measure the temperature dependence of the resistivity.

The critical pressure and impeding pressure of Al evaporation during induction skull melting processing of TiAl

In this article, two key pressures, the critical pressure Pcrit and the impeding pressure Pimpe of elemental evaporation, are defined and studied based on the calculation relationship between the evaporation loss rate Nm and the chamber pressure P during melting of titanium-aluminum alloys. When the chamber pressure is below Pcrit or above Pimpe, Nm tends to the maximum or the minimum value and remains almost unchanged. However, if Pcrit<P<Pimpe, with the increase of the chamber pressure, Nm declines sharply. A method has been put forward to calculate Pcrit and Pimpe of Al evaporation in a Ti-XAl (at pct) (X=25 to 50) melt. The calculation result shows that Pcrit or Pimpe is a second-order function of the molar percentage of Al and the melt temperature.

Microstructures and properties of investment castings of γ-titanium aluminide

γ-TiAl castings have been produced by induction skull melting (ISM), and their microstructures, hardness and tensile properties have been studied in the as-cast, hot isostatically pressed (HIPped) and HIPped+heat treated conditions. The equiaxed rosette grains found in the cast structure suggest that the alloy was cast in a partially solid condition. HIPping and heat treatment for 24 h at 1300 °C resulted in a duplex structure and decreased the hardness and tensile properties compared with the as-HIPped condition. The hardness and tensile properties also decreased with increasing bar diameter. The main fracture modes in duplex structures were transgranular (TG) in equiaxed γ-grains and translamellar (TL) in lamellar grains. The presence of an α 2/γ lamellar structure can improve strength and ductility.

Composition control of a TiAl melt during the induction skull melting (ISM) process

The properties of intermetallic compounds are sensitive to alloying composition determined by the melting process. This article describes research that aims to control the composition of a TiAl melt during the ISM process. Firstly, the vapor pressures of Al and Ti in the TiAl melt were established and the results show that P Al/ P Ti is over 300. The ratio points out that Al will evaporate more predominantly than Ti in the TiAl melt during the vacuum melting process. Then, the mass transport coefficient K and the mass evaporation rate N m were calculated. The vacuum pressure has the same influence on K and N m. There are two contraflexure points of the vacuum pressure, namely the critical point and the impeding point. When the vacuum pressure surpasses the impeding point, the evaporation of Al can be neglected. When the vacuum pressure is below the critical point, the evaporation of Al will take on a free evaporation and the Al content will decreases to 40at% from 48at%. The calculated results were compared with the experimental data, which verified the calculation results.

Molding of temperature field for the induction skull melting process of Ti-47Ni-9Nb

Based on the direct finite-difference method, a numerical model for simulating the temperature field in the charge during the induction skull melting (ISM) process has been developed in this article. The related factors, including water-cooling boundaries, the induction-heating electromagnetic stirring hump, and the power distribution in the charge, had been analyzed. The interfaces between the charge and crucible were treated as radiation boundaries before the charge was melted and as a combined boundary of radiation and conduction after the charge was melted. The free surface was treated as a radiation boundary. The relationship between the height of the electromagnetic stirring hump, charge weight, and melting power was derived based on the law of mass conservation. Based on the law of energy conservation, the distribution density of the induction current was attained. With the complied program, the temperature fields in the charge of Ti-47Ni-9Nb under different melting powers and/or charge weights were calculated. The results show that increasing the rate of the melting power has no influence on melt temperature. The ratio of melt weight to charge weight is determined by the melting power and charge weight. The melt temperature can be expressed as T=A0+A1×P+A2×P2, in which the coefficients A0, A1, and A2 are cubic functions of the charge weight, and P is the melting power.

Temperature control of TiAl melt during induction skull melting

The properties of intermetallic compounds are sensitive to alloy composition and interstitial element content determined by the melting process. Induction skull melting is one of the best methods for melting reactive alloys. For induction heating under vacuum, melt temperature control is problematic. A numerical model for simulating temperature field during induction skull melting has been developed using the direct finite difference method. Factors such as water cooling boundaries, electromagnetic stirring meniscus, and power distribution in the charge were analysed. The chargecrucible interface was taken as a radiation boundary before melting and as a combined radiation and conduction boundary after melting. The free surface was taken as a radiation boundary. The relationship between the height of electromagnetic stirring meniscus and charge weight and melting power was reduced based on the conservation of mass. Based on the conservation of energy, the distribution density of induction current was ascertained. With the program, the relationship between melting power, charge weight, and melt temperature was established. During induction skull melting of gamma-TiAl based alloys, the melt temperature was measured carefully. The theoretical and experimental results were found to be in agreement.

Microstructure evolution of rare earth rich phase for rapidly-solidified TiAl based alloys

Much attention has been paid to rapid solidification (RS) of titanium alloys these years. As potential structure material used at elevated temperatures, TiAl based alloys have been given much emphasis in the last two decades. There are two main ways of improving the poor ductility at room temperatures (RT) of TiAl based alloys. They are alloying technique and RS technique. In recent years, rare earth elements have been added to titanium alloys so as to enhance the properties at elevated temperatures through the forming of dispersed second phase particles. Rare earth rich phases are either rare earth oxides (RE2O3) or some intermetallics formed by rare earth elements and other metals such as AlCe [1–4]. In the experiment, rare earth mixture was added to TiAl based alloys. The purpose of adding rare earth elements to TiAl based alloys is obtaining fine dispersed second phase microstructure through RS techniques in order to improve ductility (RT) and mechanical propertied at elevated temperatures. Main constituent of the rare earth mixture is cerium and lanthanum. Table I is chemical analysis result of the alloy used in the experiment. Induction skull melting (ISM) was used in obtaining as-cast TiAl-RE alloy ingot. Through published papers [5, 6] we know that oxygen content in the alloy obtained by ISM can be easily controlled at the scope of 0.04–0.07 wt%. In the experiment, specimens used in RS technology were cut from the as-cast ingot. Two RS techniques employed in the experiment are melt spinning (MS) technique and hammer-and-anvil (HA) technique. The MS device used is 5 T Melt Spinner which was made by Marco Corp. of U.S.A. Ribbons of 50–80 μm thick and 30– 50 mm long were obtained. The cooling rate of MS technique for TiAl based alloys is estimated at the scope of 102–104 T/s. In the HA process, TiAl based alloy cubic of 2 mm × 2 mm × 2 mm was put on the copper anvil, after being arc-melted, the hammer was driven by high-pressure gas and hit the anvil with very high speed, finally, a thin foil (30–100 μm thickness) was obtained. The cooling rates can reach as high as 107 T/s [7]. Gas pressure of protective argon in the furnace is 0.03–0.05 Mpa. Though scanning electron microscopy (SEM) and transmission electron microscopy (TEM) microstructures of as-cast specimens and RS specimens were observed.

Examination of defects in gamma titanium aluminide investment castings

Induction skull melting (ISM) and vacuum induction rollover (VIR) furnaces have been used to melt a γ-TiAl alloy prior to casting into ceramic shell moulds. Castings produced by ISM contained only a slightly higher oxygen content than the charge material, but the melt was cast at a relatively low superheat resulting in numerous turbulence-induced gas bubbles in the castings. Although higher superheat could be achieved by VIR, the use of refractory crucibles resulted in castings with a high oxygen content. Castings poured turbulently under an argon atmosphere were liable to misrun and contained gas bubble defects. These defects were eliminated by pouring the melt into the moulds in a quiescent manner. Casting in a vacuum helped to reduce misrun even when the mould was filled quickly in a turbulent manner. Zirconia cannot be used as a crucible material for melting γ-TiAl alloys, whereas high purity (>98%) calcia is more suitable.

Evaporation behavior of aluminum during the cold crucible induction skull melting of titanium aluminum alloys

Taking the Ti-Al binary alloy as an example, this article studied the evaporation behavior of Al during the cold crucible induction skull melting (ISM) process of titanium alloys. A formula was deduced to predict the activity of Al in a molten Ti-Al binary system. The calculated activity of Al negatively deviates from an ideal solution. A model was established to judge the evaporation controlling mode and, on this basis, several conclusions were obtained. (1) The evaporation controlling mode of Al in molten Ti-Al transfers from the evaporation reaction controlling mode to the double controlling mode (diffusion and evaporation reaction) with increasing melt temperature (Tms) and/or Al content (xAl) and/or decreasing pressure (P) in the melting chamber. (2) The expression P≤Pcrit (Pcrit≈0.44 Pe(Al)) is a criterion used to judge whether the evaporation is in the state of free evaporation. (3) The term Pimpe (Pimpe=(3.5 to 4) Pe(Al)) is a critical value which impedes the evaporation loss. Almost all of common used ternary additions could enhance the activity of Al in molten Ti-Al and, accordingly, aggravate the evaporation of Al, except for Zr. The enhancing sequence is Y, Ni, Nb, Mn, V, Fe, Cr, Mo, Cu, Si, W, Mg, B, and Sn. The Al evaporation mass-transfer losses, measured from the melting experiments of several titanium aluminum alloys, were in reasonable agreement with the calculated results.

Microstructures and mechanical properties of an investment cast gamma titanium aluminide

Investment castings have been produced in γ-TiAl of composition Ti–48Al–2Nb–2Mn (at.-%) using induction skull melting. The microstructures of the bars were studied in the as cast condition and after hipping and heat treatment. Heat treatment at 1200°C led to a near γ structure whereas treatment at 1350°C resulted in a nearly lamellar structure. However, a duplex structure was retained after treatment at 1300°C. Tensile, fracture toughness, and fatigue crack growth resistance tests have been carried out on specimens machined from different sized bars. The tensile properties increased with decreasing bar diameter but, conversely, both the fracture toughness and fatigue crack growth resistance improved as the bar diameter increased. It has been found that the fracture toughness and fatigue crack growth resistance in nearly lamellar structures were better than those in near γ structures, whereas duplex structures had intermediate properties. However, the tensile properties of duplex structures were better than both near γ and nearly α2 /γ lamellar structures, with optimum values at 35 ± 5% α2 /γ lamellae of ∼400 MPa 0·2% proof strength, 470 MPa tensile strength, and 0·9% elongation.

Mechanism of skull formation during induction skull melting of intermetallic compounds

The forming mechanism of skull during induction skull melting (ISM) of TiAl intermetallic compounds was investigated by chemical composition analysis, Optical Microscopy (OM), Electron Probe Micro Analyzer (EPMA) and ESCA survey. The results showed that the forming process of skull during the ISM process of Ti + Al alloy could be divided into three stages. The distribution of the alloying elements in skull was not homogeneous and the skull bottom had an important influence on composition. The size of skull during the ISM melting processing is smaller than that which remained in the crucible after pouring.

Possible use of Skull melting under microgravity

Skull melting is a special kind of direct inductive heating which allows quasi-crucible-free processing of materials with very high melting points (3000 K and above). Under terrestrial gravity, the temperature gradients essential to the Skull technique cause buoyancy driven convection in the melt, which might interfere with crystal growth. Skull melting could therefore benefit from processing in a microgravity environment. We report on results of a joint project under contract with the European Space Research and Technology Centre (ESTEC), which has evaluated the possible use of Skull melting in microgravity from two main viewpoints: the power efficiency and the expected effects of μg environment on melt convection.

Power Consumption of Skull Melting, Part I: Analytical Aspects and Experiments

An analytical model for a Skull melting set-up is presented, which describes the power absorption in the melt and the losses in the inductor coil and the cold crucible. A strong dependence of the relative size of these quantities on the applied frequency has been found. Above a characteristic frequency, where the skin depth equals about one half of the melt radius, only a small percentage of the overall power (typically 10% for oxide melts) is lost in the copper coil and the crucible. Below this frequency, the relative part of these losses increases considerably and can prevent a successful Skull melting experiment. Analytical results are in good agreement with experimental (calorimetric) measurements of the power consumption in a typical Skull run (cubic stabilized zirconia), and with corresponding numerical simulations of the Skull process. The thermal conductivity of the Skull crust was estimated from experimental data.

Power Consumption of Skull Melting, Part II: Numerical Calculation of the Shape of the Molten Zone and Comparison with Experiment

A mathematical model for the numerical calculation of the shape of the molten zone during high frequency (HF) inductive Skull melting of ceramic materials in the cold crucible is presented. The mutual interaction between the molten zone and the HF electromagnetic field of the inductor is considered by an axisymmetric model because of the good electromagnetic transparency of the slitted crucible wall (see part I). By means of the developed corresponding Finite-Element program package the shape of the molten zone and corresponding power consumption of the system are calculated. The dependence of the shape of the molten zone on system parameters is analysed numerically and compared with experimental data. Additionally, some further investigations on stability of the molten zone are performed.

Power Consumption of Skull Melting, Part II: Numerical Calculation of the Shape of the Molten Zone and Comparison with Experiment

Power Consumption of Skull Melting, Part II: Numerical Calculation of the Shape of the Molten Zone and Comparison with Experiment

Power Consumption of Skull Melting, Part I: Analytical Aspects and Experiments

Power Consumption of Skull Melting, Part I: Analytical Aspects and Experiments

高周波誘導加熱・スカル融解におけるスカルおよびプール形成の数学的モデル

高周波誘導加熱・スカル融解におけるスカルおよびプール形成の数学的モデル