Melt Overheating Research Articles

Вплив температурноспричинених змін будови розплаву системи Al–Si на структуру і електричний опір сплаву в твердому стані

According to the results of previous studies, it was found that the temperature dependence of electrical resistance during heating of the Al + 17,6 % Si melt curve characterized by presence of several sharp inflections. This gave grounds for asserting possibility of temperature-induced changes in structure of specified system melts. However, it remains an open question whether the changes that occurred when melt was overheated are preserved during subsequent cooling. The problem of effect of temperature-induced changes in Al–Si melts structure on the structure and properties of the alloy in the solid state also requires a more detailed study. Resistometric studies of melt were carried out according to the method [1]. The ability to preserve temperature-induced changes in melt was assessed by electrical resistance value during cooling of the overheated Al + 17,6 % Si melt at five characteristic temperature values. To avoid errors cozied by imbalance, only results obtained after stabilization of melt resistance at a given temperature were taken into account. Temperature-induced changes in melt structure were evaluated by structure of solid samples obtained by quenching from the liquid state. The nature of influence of changes that occur when the melt is overheated on solid alloy structure and properties was determined by specific electrical resistance values and material microstructures of samples obtained after melt isothermal holding at temperatures of 720 °C, 880 °C, 960 °C and 1050 °C. The presence of electric resistance significant hysteresis can indicate that the changes that occurred during melt overheating are preserved during its cooling. It was also established that the specified changes affect to properties (in particular, specific electrical resistance) and the microstructure of the solid alloy (first of all, the melt overheating affects to size of primary silicon particles). Keywords: aluminum, silicon, thermodynamic equilibrium, melts, electrical resistance.

Gas atomization of A2 tool steel: Effect of process parameters on powders' physical properties

In this study, powders of the A2 tool steel were produced by close-coupled gas atomization using argon gas aiming their use in additive manufacturing (AM) technologies. The effects of melt overheating and gas atomization pressure on their physical properties were analyzed. The powders were separated into different particle size ranges, and a complete morphological characterization was carried out. The effect of back pressure was verified due to a substantial increase in the gas-to-melt mass flow rates ratio (GMR) with increasing gas pressure. Atomization gas pressure showed a slightly more significant influence on particle size distribution; however, the melt overheating effect was also significant. As the GMR increases, the median particle size decreases to a certain threshold. Above this, particle size cannot be considerably reduced with increasing GMR.

Anisotropic Thermal Expansion and Electronic Structure of LiInSe2.

Optical quality cm-sized LiInSe2 crystals were grown using the Bridgman–Stockbarger method, starting from pure element reagents, under the conditions of a low temperature gradient of 5–6 degrees/cm and a slight melt overheating. The phase purity of the grown crystal was verified by the powder XRD analysis. The thermophysical characteristics of LiInSe2 were determined by the XRD measurements in the temperature range of 303–703 K and strong anisotropy of the thermal expansion coefficients was established. The following values of thermal expansion coefficients were determined in LiInSe2: αa = 8.1 (1), αb = 16.1 (2) and αc = 5.64 (6) MK−1. The electronic structure of LiInSe2 was measured by X-ray photoelectron spectroscopy. The band structure of LiInSe2 was calculated by ab initio methods.

Effect of melt overheating treatment on the microstructure and mechanical properties of Zn–Al alloy

This study focused on the effect of overheating treatment on the microstructure and tensile mechanical property of zinc-aluminum alloy. Ingots with designed composition were manufactured by melting pure metals and cast at different temperatures from 500 °C to 650 °C. Microstructure, strength and fracture mechanism were studied and their relationships with overheating temperature were analyzed. Dendrite size decreased with increasing overheating temperature and turned to equiaxed crystal at 650 °C. The yield stress, ultimate tensile stress and elongation increased by 16%, 31% and 163%, respectively after overheating treatment. It was also found that with the increase of overheating temperature, the brittle fracture mechanism gradually turns to a mixed fracture mechanism.

Градієнтність структури і характеристики руйнування сталевих виливків

The effect of melt overheating over the equilibrium liquidus to temperatures of 1570 ºС, 1620 ºС and 1670 ºС and heat removal conditions (normal and rapid cooling) on the formation of a gradient structure and fracture characteristics over the cross section of castings of 25L and 30HGSL steels was investigated. The differentiated phase-structural state of rectangular castings (60x60x100 mm) was provided by the predominant one-sided heat removal in foundry molds with different heat removal capacity and average cooling rate 2 – 5 ºС/s and 300 – 350 ºС/s. Regular changes in the length of the main macrostructural zones in castings are established depending on the crystallization conditions, in particular, an increase in the zone of near-surface small crystals and the transcrystallization zone during rapid cooling of the melt. It is shown that depending on the temperature conditions of melt preparation and cooling during crystallization, the grain size in the cross section of castings varies by 4-5 numbers (steel 25L) and by 6-8 numbers (steel 30HGSL). The rapid cooling of the melt causes a significant increase in the impact strength of the cross section of the castings in the range of 50-35 J/cm2 (steel 25L) and 40-20 J/cm2 (steel 30HGSL), even under conditions of significant overheating of the melt. Under normal cooling conditions, there is a significant decrease in the values of impact strength in the cross section of the castings from 30 to 15 J/cm2. According to the results of determining the toughness in castings with differentiated structure depending on the crystallization conditions and test temperature (+20 ºС… -60 ºС) the regression equation for changing its values in the range of viscous-brittle transition temperatures and brittleness temperature (Tbrtl50) of steels in the volume of castings. The obtained results create preconditions for the development of technological principles of engineering of cast steel products to ensure the specified properties in accordance with the requirements of their operation. Keywords: steel, crystallization, gradient structure, toughness.

Градієнтність структури і властивості сталевих виливків

The influence of melt overheating in the range of 50-150 °C on the equilibrium liquidus and its cooling rate during crystallization of castings on the formation of macrostructural zones along their cross section, on the change of grain dispersion, dendritic structure characteristics and mechanical properties was investigated on the example of 25L steel. It is established that the macrostructure of castings in the direction of unilateral heat removal as it moves away from the cooled surface consists of four main structural zones - small coaxial crystals, columnar, branched and large coaxial crystals, the length and morphology of which naturally change depending on thermokinetic conditions of crystallization. The decisive role of the cooling rate at significant overheating of the melt to increase the number of crystallization nuclei, the formation of a more dispersed cast structure by increasing the degree of supercooling of the melt during crystallization is shown. The regularities of quantitative characteristics change of microstructure and dendritic structure depending on change of temperature-time parameters of crystallization in different structural zones of castings and their connection with characteristics of mechanical properties of steel are established. On the basis of mathematical processing of experimental data by linear regression analysis interpolation models and their graphical interpretations are obtained, which allow to quantify and predict the change of mechanical properties in different structural zones of gradient castings depending on melt overheating temperature and cooling modes within the investigated factor space. Keywords: gradient structure, structural zones, melt, mechanical properties.

Establishing patterns in the effect of temperature regime when manufacturing nanocomposites on their heat-conducting properties

This paper reports the experimental study carried out to establish the dependence of the thermal conductivity of polypropylene-based nanocomposites filled with carbon nanotubes on the main parameter of the temperature regime of their manufacturing ‒ the level of overheating a polymer melt relative to its melting point. The study has been conducted for nanocomposites that were manufactured by applying a method based on the mixing of components in the polymer melt applying a special disk extruder. During the composite manufacturing process, the level of melt overheating varied from 10 to 75 K, with the mass share of filler ranging from 0.3 to 10.0 %. It is shown that increasing the overheating of a polymer melt causes an increase in the thermal conductivity of the composites. However, when the overheating has reached a certain value, its further growth does not increase the thermal conductivity of nanocomposites. Based on the established pattern, the rational level of this overheating has been determined. That resolves the tasks of manufacturing highly heat-conducting nanocomposites and implementing appropriate energy-saving technology. Data have been acquired on the effects of the impact of the amount of polymer melt overheating on the values of the first and second percolation thresholds for the examined nanocomposites. It is established that the value of the first percolation threshold is more sensitive to the specified amount of overheating. The dependences of the density of the examined composites on the level of polymer melt overheating have been derived. The correlation between a given dependence and the nature of a corresponding change in the thermal conductivity of the composites has been established. Applying the proposed highly heat-conducting nanocomposites is promising for micro and nanoelectronics, energy, etc.

Effect of Melt Overheating on Structure and Mechanical Properties of Al-Mg-Si Cast Alloy

The paper discusses the complex effect of melt overheating with subsequent fast cooling down to the pouring temperature on the crystallization process, microstructure and mechanical properties of Al-Mg-Si aluminum alloy. The results obtained facilitated the establishment of rational modes of melt overheating, leading to a significant change in the dispersion and morphology of structural components. In particular, with an increase in the melt overheating temperature to 900 °C with holding and subsequent rapid cooling to the casting temperature, a decrease in the average size of dendritic cells of the aluminum solid solution from 39 μm to 13 μm was observed. We also noticed the refinement of eutectic inclusions of the Mg2Si phase with compact morphology. An increased level of mechanical properties was noted; the maximum values of tensile strength and elongation reached 228 MPa and 5.24%, respectively, which exceeded the initial values by 22.5% and 52.3%, correspondingly. The microhardness of the aluminum solid solution sequentially increased from 38.21 to 56.5 HV with an increase in the temperature during melt overheating. According to the EDS linear scanning, an increase in the superheat temperature of the melt is accompanied by an increase in the degree of saturation of the solid solution with magnesium.

ЧИСЕЛЬНЕ МОДЕЛЮВАННЯ ТЕПЛОВОГО СТАНУ ІНДУКЦІЙНОЇ КАНАЛЬНОЇ ПЕЧІ

A mathematical model that describes heat and mass transfer processes in an induction channel furnace is presented. Using the example of a two-phase channel furnace with standard form of channels, the influence of the phase angle between the voltages supplying the furnace inductors on the thermal state of the liquid metal is considered. Values of this angle, which provide minimal melt overheating in channels in comparison with furnace bath have been determined. Ways of intensification of heat and mass transfer processes in two-phase channel furnace have been suggested. References 9, figures 4.

Influence of melt superheating treatment on cast structure of Al–Sn alloys

The study covers the effect of Melt Superheating Treatment (MST) for Al–Sn alloys. To determine the optimal superheating temperature, the authors measured the temperature dependences of the kinematic viscosity, electrical resistivity, density and surface tension of Al–Sn melts with tin contents of 10, 20, 30, 40, and 50 wt.%. According to the measurement results, the temperature t* was determined for each Al–Sn alloy sample. Heating to this temperature breaks down the micro-inhomogeneous state and leads to the heterogeneous liquid – homogeneous liquid structural transition. Melt superheating (MST) results in a decrease in melt viscosity. It was found that the temperature t* rises with increasing tin concentration in the Al–Sn melt. An increase in the tin content in the Al–Sn melt also leads to a decrease in the absolute values of kinematic viscosity and surface tension, while the electrical resistivity and density increase accordingly. Thus, the Melt Superheating Treatment (MST) mode for Al–Sn alloys was determined. The effect of MST of Al–50wt.%Sn melt on the microstructure and mechanical properties of the ingot was studied in order to determine the structural sensitivity to the degree of melt overheating, and to find a new strategy to improve the shaping ability of the Al–Sn alloy two-phase structure. The results demonstrated that the method of resistivity and viscosity determination are more sensitive and effective for melt superheating temperature (MST mode) evaluation. In addition, the desired modified Al–Sn ingot structure can be formed under normal casting conditions; MST can contribute to the modified ingot structure formation by increasing the solidification time and decreasing the average solidification rate by reducing melt viscosity after superheating.

Growth Peculiarities and Properties of KR3F10 (R = Y, Tb) Single Crystals

Cubic KR3F10 (R = Y, Tb) single crystals have been successfully grown using the Bridgman technique. Growth of crystals of this type is complicated due to the hygroscopicity of potassium fluoride and melt overheating. The solution to the problem of oxygen-incorporated impurities has been demonstrated through the utilization of potassium hydrofluoride as a precursor. In this study, the crystal quality, structure features, and optical, thermal and electrophysical properties of KR3F10 were examined. Data on the temperature dependences of conductivity properties of KTb3F10 crystals were obtained for the first time. These crystals indicated thermal conductivity equal to 1.54 ± 0.05 Wm−1K−1 at room temperature caused by strong phonon scattering in the Tb-based crystal lattice. Ionic conductivities of KY3F10 and KTb3F10 single crystals were 4.9 × 10−8 and 1.2 × 10−10 S/cm at 500 K, respectively, and the observed difference was determined by the activation enthalpy of F− ion migration. Comparison of the physical properties of the grown KR3F10 crystals with the closest crystalline analog from the family of Na0.5−xR0.5+xF2+2x (R = Tb, Y) cubic solid solutions is reported.

SIMULATION OF FLUIDITY AND DENSITY OF CAST STEELS

As a result of the analysis of the formation processes of fluidity and the conditions for feeding the castings during cooling after solidification, a significant effect of the properties of the melt and the dispersion of the primary structure on the fluidity and density of steel was established. It is theoretically substantiated and experimentally confirmed that the level of fluidity and density is determined by the dispersion of the dendritic structure, the magnitude of the melt overheating over the liquidus temperature, as well as the properties of the liquid metal, the thermal conductivity of steel at the solidus temperature, crystallization heat and crystallization interval. The established quantitative laws describe the real process with a probability more than 95% and a high degree of reliability (R = 0,709-0,837; ð = 1,2 – 13,8%). The article shows that in order to increase the effectiveness of the influence of integral factors on the fluidity of structural steels, they can be arranged in the following sequence: thermophysical conditions of solidification, dispersion of the dendritic structure, properties of liquid metal. In this case, an increase in fluidity occurs with an increase in the overheating of the melt above the liquidus temperature, the heat of crystallization and the dispersion of the dendritic structure. An increase in the values of other factors leads to the opposite effect. Alloying elements are arranged in the following sequence: Si, Cr, Mn, C, V, N, V + N according to the specific efficiency of increasing fluidity. The results of the studies performed show that according to the effectiveness of the influence of the considered factors on the steel density, they can be arranged in the following sequence: dispersion of the dendritic structure, properties of liquid metal and thermalphysic conditions of solidification. Alloying elements affect these parameters in such a way that a complex multiextremal change in density is observed during alloying of steel. The general trend is that carbon and chromium decrease, while silicon, manganese, vanadium, nitrogen, and co-alloying with nitrogen and vanadium increase the density of the steel. Alloying elements can be arranged in the following sequence: V, Cr, Mn, Si, N, N + V, C to increase the specific efficiency of changing the density.

Effect of Temperature on the Formation of Stable and Metastable Aluminide Phases in Al‒Zr‒Nb Alloys

The conditions of the formation of stable and metastable aluminides Aln(Zr1 – xNbx) formed during the crystallization of superheated melts of the Al‒Zr‒Nb system are considered. Upon a close zirconium content in the alloys of 0.23–0.25 at %, the niobium content varies from 0.05 to 0.21 at %. Alloys are prepared in a resistance furnace at a temperature of 1230°C in an argon atmosphere in graphite crucibles. The alloys are cast into a bronze mold; the cooling rate is estimated as 200°C/s. The structural features, distribution, and morphology; the composition of matrix, dendritic cells, and aluminides; and the structural type of aluminides in the alloys are studied using scanning electron and optical microscopy, X-ray diffraction, and inductively coupled plasma atomic emission spectroscopy. It is shown that the growth forms of stable aluminides (D023 structural type) change from faceted to dendritic and primary metastable aluminides (L12 structural type) begin to form; in the process of the Al‒Zr‒Nb melt, overheating was 360–365°C above the liquidus temperature. Exclusively metastable aluminides with both polyhedral and dendritic growth forms are formed in the Al‒Zr‒Nb alloys when the overheating of their melt is 390–395°C (as well as at higher overheating) above the liquidus temperature. The near-peritectic niobium composition of the Al‒Zr‒Nb alloy and the zirconium content (more than seven times higher than peritectic) are crucial factors in the formation of a large fraction of metastable AlnZr aluminides having a cubic L12 structure. It is shown that, in accordance with the rules of isomorphism, Nb replaces Zr at equivalent positions of the aluminides crystal lattice. The intensity of isomorphism of the formed Al4(Zr0.79Nb0.21) aluminides increases with the increase in temperature of the melt overheating.

Calculated Study of the Influence of Overheating Aluminum Melt on the Dynamics the Granulation Process

A three-dimensional mathematical model of the solidification process of a liquid metal is considered, taking into account the mobility of the boundaries at which the phase transition is carried out (Stefan boundary value problem). The algorithm of calculation is improved, allowing due to the use of the Dirac δ-function in determining the effective heat capacity to take into account the nonlinearity of the equation of unsteady thermal conductivity and the heat of the phase transition. A numerical study of heat transfer during solidification of lead-containing aluminum melt droplets in air and water is carried out. The influence of droplet size and melt overheating on the solidification dynamics of granules has been studied. An approximate ratio based on the square root law is proposed, taking into account the amount of overheating of the liquid phase and linking the thickness of the formed solid phase with the duration of the granulation process

The improvement of surface quality and thickness stability of Fe78Si9B13 melt-spun ribbons by melt overheating

The influence of melt overheating on the viscosity of the melt and surface quality of the Fe78Si9B13 melt-spun ribbons have been studied by using oscillating vessel viscometry, X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscopy. A critical temperature Tc (about 1350 °C) of the melt transformation was determined via viscosity measurements during overheating and subsequent cooling processes. The Fe78Si9B13 amorphous ribbons were produced through planar flow melt spinning in the temperature range of 1300–1550 °C. A detailed analysis of overheating effects on the melt viscosity and surface features of the as-spun ribbons were performed to rationalize the correlation between the melt state and surface quality of the ribbons. It was found that the enhanced homogeneity of the melt can be achieved by the overheating treatment higher than the critical temperature, which in turn improves the surface quality and thickness stability of the Fe78Si9B13 melt-spun ribbons.

Influence of Overheating and Cooling Rate on the Structure and Physicochemical Properties of Al-Cu Alloys

The effect of overheating of the melt and cooling rate of alloys of the Al-Cu system with a copper content of 25.0 – 36.0% ( mass.), the rest of the aluminum is investigated. It is shown that an overheating of the liquid at 50 – 100 K above the liquid-liquid line leads to the formation of a fine-dispersed eutectic structure and the inhibition of the formation of primary aluminum crystals in the pre-evacuation of alloys and the Al2Cu phase in hypereuvtectic alloys, in accordance. An increase in the melt overheating temperature by 150 K above the liquid-liquid line and the subsequent cooling at 103 – 104 K/s leads to the complete inhibition of the formation of primary crystals. An overheating of the melt on 100 – 150 K alloys above the liquid line and subsequent cooling with a velocity of 103 – 104 K /s reduces the rate of corrosion by 30 – 45% and increases the numerical value in 1.3 – 1.45 times the relative wear resistance, and the brittleness of alloys decreases in 1.2 – 1.35 times in comparison with the samples after casting.

Numerical Investigation into the Influence of Overheating of Aluminum Melt on the Heat Exchange in the Continuous Combined Casting and Pressing

The results of a numerical investigation into the temperature–temporal dependences in continuous combined casting and pressing of the AK12 experimental aluminum alloy at a different overheating temperature from startup to the instant the steady-state thermal conditions are reached by the installation are reported. Calculations are performed based on a three-dimensional computer model of a complex heat exchange in an installation of a new design equipped by a horizontal rotary crystallizer. Theoretical investigations into the effect of overheating of the poured aluminum melt on the temperature-dependent heat exchange processes are performed. The influence of the character of the heat exchange in the transient thermal mode on the temperature field of the solidified melt for its different remoteness from the pouring point is determined. It is shown that the asymmetry of the temperature field in the control metal cross section near the pressing tool with the maximal temperature to the contacting crystallizer surface increases during crystallizer heating in the transient process. It is established that the duration of the transient process during the installation startup from the cold state until the steady-state thermal mode is attained depends on the temperature of the melt being poured. The maximal limit of overheating of the poured metal is determined, above which, when implementing the continuous combined casting–pressing technology, the aluminum melt does not solidify in the crystallizer and forced cooling of installation elements should be organized. The influence of melt overheating on the character of the temperature field along the crystallizer cross section is evaluated for the entire period of the transient thermal process. The design measures ensuring the rational temperature working conditions of bearings during the installation operation are given.

Influence of temperatures of melt overheating and pouring on the quality of aluminum alloy lost foam castings

Lost foam casting (LFC) is currently one of the most efficient and promising methods of fabricating high-quality thin-wall castings possessing specified dimensional accuracy, required surface roughness, and other properties. This technology is widely used in the production of aluminum alloy products. To minimize costs in the fabrication of wares and to fabricate high-quality castings, it is reasonable to use an increased amount of secondary materials in the charge, herewith paying attention to the melt overheating temperature and holding time. The results of studying the temperature modes of smelting pouring aluminum alloys in the LFC are presented. The most efficient modes in manufacturing conditions under consideration which provide the best quality characteristics of leak-tight castings by dimensional accuracy and surface roughness were as follows: the melt overheating temperature is 880–890°C and the melt pouring temperature into the casting mold is 820–830°C. The influence of various variants of temperature parameters of smelting and pouring the melt of the AK7 composition during the LFC on the content of nonmetallic inclusions in the cast state is investigated. It is revealed that the minimal γ-Al2O3 content in the final alloy is provided by a melt overheating temperature of up to 880–890 or 940–950°C and a melt pouring temperature into the casting mold of 820–830°C.

Effect of Melt Overheating Treatment on the Melt Structure and Solidified Structures of Al<sub>75</sub>Bi<sub>9</sub>Sn<sub>16</sub> Immiscible Alloy

In this paper, the liquid phase separation and solidification process of the Al75Bi9Sn16 immiscible alloy were studied with calorimetric and resistivity methods to make the melt superheated treatment process. The impact of melt overheating treatment (MOT) on the phase constitution and solidification microstructures were investigated using X-Ray diffraction (XRD) and field emission scanning electron microscope (FESEM) to determine the structural sensitivity to the melt superheated degree, and find a new strategy for improving the forming ability of the core-shell structure of the Al75Bi9Sn16 alloy. The results show that: the liquid phase separation and precipitation of primary (Sn) phase occur in 1039K-880K and 460K-403K; the core-shell structure with Sn-Bi-rich core and Al-rich shell can be formed under conventional casting conditions; the melt overheating treatment (MOT) can promote the formation of core-shell structure by increasing solidification time t0 and decreasing the average solidification rate v.

Influence of crystallization conditions on the structure and modifying ability of Al–Sc alloys

The influence of the modes of thermal-and-temporal treatment and cooling rate of metallic alloys on crystallization regularities of Al–Sc alloys and their structure, properties, and modifying ability are established. Castings of Al–Sc alloy, which were prepared by the electrolysis of salt melts KF–NaF–AlF3–Sc2O3 at 820–850°C, are used as the initial charge for casting. It is established that, by varying the magnitude of melt overheating and casting temperature, it is possible to vary the crystal shape, amount, and size in wide limits. The modifying action of cast and rapidly quenched master alloys, as well as the master alloy produced by electrolysis, is tested for Al–4.5% Cu alloy. The largest effect of milling the structure of the Al–4.5% Cu–0.4% Sc alloy is attained when using the rapidly quenched master alloy.