Melt Superheating Research Articles

Technical research on recycling waste blades of single crystal superalloy through ultrasonic alkali cleaning combined with electron beam smelting

The waste blades of single crystal superalloys have not been formally applied on a large scale, resulting in serious waste. This study proposes to treat the waste blades through ultrasonic alkali cleaning combined with electron beam smelting (EBS). The theoretical model is proposed to study the elements volatilization, which provides a reference for controlling the alloy composition. After EBS, the secondary dendrite spacing and the size of γ′ phases of DD5 alloy is significantly reduced. The EBS technology can effectively reduce the element segregation in superalloys, especially W and Mo elements. The purity of DD5 revert alloy after EBS is consistent with or even exceeds that of DD5 virgin alloy. Especially for the removal of ceramic shell and core materials in the waste blades, EBS has a new technological advantage. Under the combined action of Marangoni effect and buoyancy, the inclusions floating to the melt surface can not only be directly decomposed by the bombardment of the electron beam, but also can be dissolved under the condition of local melt superheating.

Solidification behavior and microstructure of Ti-(37−52) at% Al alloys synthesized in situ via dual-wire electron beam freeform fabrication

Titanium-aluminide (TiAl) alloys are very promising substitutes for Ni-based superalloys, and additive manufacturing, which is a disruptive technology, is a promising way to fabricate them. Ti-(37−52) at% Al walls were successfully synthesized in situ using dual-wire electron beam freeform fabrication (EBF3), and the solidification behavior was characterized according to their chemical composition, phase identification, microstructure and orientation. The Ti-37 at% Al wall was composed of α2 grains with an average size of 100 µm and a random orientation distribution, indicating that the primary phase was the β phase and that the solid-state β→α transformation occurred. The orientation relationships, namely (0001)α//{111}γ and <112¯0>α//<11¯0]γ, were found in the lamellar structure of the Ti-42 at% Al walls, and the α2 phase and γ grains presented < 0001 > and < 111 > orientations in the deposition direction, respectively. This suggests that the formation of a lamellar structure was the result of the solid-state α → α2 + γ transformation, where the α phase was the primary phase. The solid-state α →γ lamellae and α →γ massive transformations in the Ti-49 at% Al wall caused the massive γ phases to have a random orientation and the lamellar γ phases to be oriented along the < 012 > and < 010 > directions in the building direction. The γ dendritic structure, a characteristic solidification structure, indicated that the solidification path of the Ti-52 at% Al wall was L→γ. Solidification during the EBF3 process deviated from the thermodynamic equilibrium state, and the metastable phase formed in the liquid phase as the primary phase. The solidification conditions, including the thermodynamic and kinetic conditions and cooling rate, were analyzed. The results indicate that the primary phases were selected by undercooling, and the microstructure of Ti-xAl (x = the actual Al content in at%) alloys during the EBF3 process was determined to correspond to an estimated undercooling temperature of 240–247 K. However, the cooling rate during solidification was 214.36 K/s, which was not enough to produce such a high undercooling temperature. The high undercooling temperature was the result of melt superheating and electron beam oscillation. A novel solidification process with a low cooling rate and a high undercooling temperature occurred during EBF3.

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.

An investigation of direct-chill cast 2024 aluminum alloy under the influence of high shearing with regards to different shear positions

Abstract The 2024 aluminum alloy ingot with a diameter of 300 mm was prepared by a direct-chill (DC) cast with the application of a high-shear unit in the sump to study the influence of the intensive melt shearing on the 2024 aluminum alloy. During the DC casting process, the head of the high-shear unit was set to different positions in the sump and the corresponding temperature curves were recorded to further study the role of the position of the high shear unit in the DC casting process with the influence of the intensive melt shearing. The experimental results showed that the intensive melt shearing during the DC casting process results in uniform temperature distribution in the sump, significant grain refinement and reduction of negative centerline macrosegregation. The average grain size in the center of the ingot was reduced from 739.7 μm to 193.8 μm, and the negative centerline macrosegregation of Cu element was reduced from -0.094 to -0.041. It was also found that the shear position plays a significant role in uniforming temperature field, eliminating melt superheating and refining grains. When the high shear position lowered to slurry region, the morphology of grain in the center of the ingot transformed from equiaxed dendritic into finer globular equiaxed grain. The reduction of negative centerline macrosegregation is believed to be due to the forced convection caused by the intensive melt shearing and consequently refined microstructure and uniform distribution of floating grains through the whole section of the ingot.

Effects of Melt Superheating on the Microstructure and Tensile Properties of a Ternary Al-15 Wt Pct Si-1.5 Wt Pct Mg Alloy

Unsteady solidified microstructures of the Al-15 wt pct Si-1.5 wt pct Mg alloy under two degrees of melt superheating are examined: 4 and 21 pct above the alloy liquidus temperature. The dendritic array and the eutectic mixture have been investigated, which are affected not only by the solidification kinetics but also by the melt superheating. The directional solidification experiments permitted that aligned Al-rich dendrites could be formed for both high- and low-melt superheatings. These dendrites appear as ‘islands’ isolated from each other and dispersed within the α-Al+Si+Mg2Si eutectic matrix. Such configuration arises in samples under the entire range of examined cooling rates, i.e., from 0.5 to 50 K/s. Directionally solidified samples having different α-Al dendrite interphase spacings have been characterized and subjected to tensile tests. Such microstructural spacings translate the effectiveness in blocking the dislocations motion during loading, which is promoted mainly by the dendritic/eutectic boundaries. This mechanism is especially operative since the applied tensile load is perpendicular to the dendritic growth path. As such, the evolution of tensile properties as a function of these spacings was assessed. Both elongation (δ) and the ultimate strength (σu) are enhanced nearly by 40 pct due to the reduction in the dendrite interphase spacing. The highest properties (σu: ~ 310 MPa and δ: ~ 7 pct) are associated with a microstructure formed by aligned dendrites with 70 μm in spacing embedded in a eutectic mixture. Three factors appear to contribute to the higher strength values observed for the alloy processed under lower-melt superheating, i.e., higher fraction of dendrites, solid solution strengthening of Mg in α-Al, and finer eutectic Si spacing.

Phase control and formation mechanism of Al–Mn(–Fe) intermetallic particles in Mg–Al-based alloys with FeCl3 addition or melt superheating

In this study, we identify the crystal structures and elucidate the formation mechanism of Al–Mn(–Fe) intermetallic particles formed in Mg–Al-based alloys with varied Al contents, FeCl3 addition or melt superheating. The formed Al–Mn(–Fe) particles were collected through a newly developed particle extraction method for structural, chemical and morphological characterizations. It is found that increasing the Al content from 3 wt% to 9 wt% changes the major Al–Mn phase from τ-Al0.89Mn1.11 in AZ31 to γ2-Al8Mn5 in AZ91, and both FeCl3 addition and melt superheating promote the formation of γ2-Al8(Mn,Fe)5 in AZ31 and AZ63. In addition, through tuning FeCl3 addition level or melt superheating temperature, different morphologies of the γ2-Al8(Mn,Fe)5 particles are formed in AZ31 and AZ63, including quasi-cubes, octa-pod microstructures and polyhedral particles. This study provides several important approaches to control the crystal structure and morphology of Al–Mn(–Fe) intermetallic particles formed in Mg–Al-based alloys.

Melt superheating on the microstructure and mechanical properties of diecast Al-Mg-Si-Mn alloy

The application of aluminium alloys in automotive body structure is one of the main developments in recent years. The increase of the mechanical properties of the ductile die-casting is one of the most critical topics for the application. In this work, the effect of melt superheating on the morphology, size and distribution of α-Al phase and Fe-rich intermetallics, and on the mechanical properties of the Al-Mg-Si-Mn diecast alloy was investigated. The results showed that the refined microstructure could be obtained through melt superheating. The volume fraction of dendritic α-Al phase and the Fe-rich intermetallic phase formed in the shot sleeve was significantly reduced, resulting in the refined microstructure. Overall, the melt superheating could improve the mechanical properties of the yield strength, ultimate tensile strength, and elongation of the diecast Al-Mg-Si-Mn alloy. And, the Fe-rich intermetallic phase formed in the shot sleeve with the coarse compact morphology and formed in the die cavity with the fine compact particles were identified as the same α-Al12(Fe,Mn)3Si composition in the present experimental conditions, which was not affected by the melt superheating.

Microstructure refinement approaches of melt-grown Al2O3/YAG/ZrO2 eutectic bulk

Microstructure developments of melt-grown Al2O3/YAG/ZrO2 ceramic bulks were investigated by controlling composition, cooling rate, heterogeneous nucleation sites and melt superheating treatment. The solidification microstructure of sample with hypereutectic composition (ZrO2 20 mol%) is finer than that with hypoeutectic or eutectic ones. With increasing the cooling rate, microstructure of melt-grown samples develops from colony to dendrite and finally to cell. The microscopy and the components of samples vary with the melt superheating temperature and the type of heterogeneous nucleation sites. The microstructure evolutions of melt-grown Al2O3/YAG/ZrO2 eutectic relate to the melt undercooling level and the solid–liquid interfaces stability.

Microstructure and Mechanical Behaviors of AZ91 Magnesium Alloy with Thermal Rate Treatment Technique

Microstructure and mechanical behaviors of AZ91 cast magnesium alloys with melt superheating and thermal rate treatment technique were studied. Alloy grains thicken at 820°C and refine at 870°C when they contains Mn element and the crucible had no coating on the internal surface. Moreover, the strengthening γ phase is more dispersive and uniform at 870°C. Thermal rate treatment significantly improved the microstructure, mechanical behaviors and casting quality of AZ91 alloy as microstructure of AZ91 alloy reserves some characteristics of the high temperature melt.

Microstructure and Properties of &lt;i&gt;In Situ&lt;/i&gt; Al-Si-Mg&lt;sub&gt;2&lt;/sub&gt;Si Composite Prepared by Melt Superheating

Mg2Si particle reinforced in-situ Al-Si composite is prepared by melt superheating, and the microstructure and properties of the composite are researched. The results indicate that it is feasible that the melt superheating is applied to preparing Mg2Si particle reinforced in-situ Al-Si composite, and the suitable preparing parameters of melt superheating on the alloy are obtained. When the superheating temperature is 860°C and holding time is 25 min, the average size of Mg2Si particle in the alloy will be reduced to 11μm compared with 34μm without melt superheating, and the mechanical properties of the composite is obviously improved.

Hypereutectic Al-Si-Mg &lt;i&gt;In Situ&lt;/i&gt; Composite Prepared by Melt Superheating

Mg2Si particle reinforced hypereutectic Al-Si-Mg alloy composite is prepared by melt superheating, and the effects of the main processing parameters, such as superheating temperature and holding time, on the microstructure of the composite are studied. The results indicate that it is feasible to prepare Mg2Si particle reinforced Al-18%Si-Mg alloy composite by melt superheating. The suitable preparing parameters of melt superheating on the alloy are obtained. When the superheating temperature is 860°C and holding time is 25 min, the average size of Mg2Si particle in the alloy will be reduced to about 11μm compared with 34μm without melt superheating.

Effect of growth conditions on the compositional homogeneity of Cd1 − x Zn x Te crystals

A crystal growth unit has been designed for Cd1 − x Zn x Te growth by the axial-heat-flux-close-to-the-phase-interface (AHP) method at argon overpressures of up to 12 MPa. The influence of melt superheating, growth rate, melt layer thickness, argon pressure, and growth mode on the Zn distribution in 21- to 45-mm-diameter crystals and defect formation in Cd1 − x Zn x Te ingots has been investigated. The results demonstrate that the AHP method offers the possibility of effectively controlling the shape of the solid-liquid interface and ensures low fluid velocity and high radial compositional homogeneity. The feeding method used markedly improves the axial Zn profile and ensures low dislocation density (5 × 103 cm−2) in the grown crystal.

Grain refinement effect of a pulsed magnetic field on as-cast superalloy K417

The grain refinement effect of a pulsed magnetic field on superalloy K417 was studied. The experimental results show that fine equiaxed grains are acquired with proper thermal control under the pulsed magnetic field. The refinement effect of the pulsed magnetic field is affected by the melt cooling rate and the melt superheating. The refinement effect of the pulsed magnetic field is attributed to the dissociation of nuclei from the mold wall by melt vibration and the subsequent dispersion of nuclei by melt convection. The Joule heat and the melt convection caused by the pulsed magnetic field may defer the formation of solidified shell, which prolongs the continuous refinement process. The decrease of melt cooling rate reduces the number of nuclei produced on the mold wall but prolongs the duration for the nuclei to depart from the mold wall and disperse in the melt, which enhances the refinement effect of the pulsed magnetic field. The increase of melt superheating lessens the survival probability of the nuclei in the melt, which weakens the refinement effect of the pulsed magnetic field.

Ｓｉ‐４５ｍａｓｓ％Ｎｉ合金融液からのシリコン結晶育成

Silicon crystal pulling from the melt of Si–45mass%Ni alloy is performed as a model experiment of a silicon solidification refining process with silicon alloy melts. A needle-like silicon crystal is successfully grown, using a seed crystal with a ‹211› crystallographic orientation to the pulling direction, which is poly-crystalline contains a large number of twins inside. A rod-type crystal with Si–Ni eutectics engulfed inside is also obtained. Appropriate ranges of melt superheating and pulling velocity for crystal growth are estimated to be from one to five degree Celsius and from 0.018 to 0.15 mm/min, respectively. It is necessary to decrease the pulling velocity to less than 0.03 mm/min for obtaining the sufficient epitaxial growth on the seed crystal.

Shape memory behaviour of Cu–Al wires produced by horizontal in-rotating-liquid-spinning

Rapidly quenched Cu–Al-based thin wires of approximately 100 μm in diameter produced by in-rotating-liquid-spinning process (INROLISP) exhibit shape memory behaviour. In contrast to the production mode used for preparing thin Cu–Al wires showing the shape memory effect described in recent publications, a horizontal arrangement of the INROLISP process was used for the fabrication on a laboratory scale. The effects of the process parameters on the quality of the thin wires have been studied. Scanning electron microscopy was performed to investigate the influence of rapid solidification on the grain size. In order to characterize the shape memory behaviour, the characteristic transformation temperatures were determined by differential scanning calorimetry and tensile tests. In performing these measurements, the INROLISP process parameters like melt jet velocity (Reynolds’ number), flow velocity of the coolant, melt superheating and alloy compositions were taken into account.

A novel approach to the mechanism for the grain refining effect of melt superheating of Mg–Al alloys

The possible grain refining candidates for Mg–Al alloys that may occur in the superheating process have been systematically studied from a crystallographic perspective. The edge-to-edge matching model was used to investigate and compare the grain refining efficiency of various (Al, Mn) containing phases existing in the melt. The results show that the metastable τ-AlMn phase possesses significantly better crystallographic matching to the Mg matrix than the other Al–Mn intermetallic phases and could therefore serve as an effective heterogenous nucleation site. However, since the metastable τ-AlMn phase can only be generated from the high temperature ε-AlMn phase through a massive transformation, overheating the melt is necessary to produce the high temperature ε-AlMn phase. During the subsequent cooling to pouring temperatures, the ε-AlMn phase will partially or fully transform into the τ-AlMn phase, which in turn can act as a nucleation site. The rapid cooling and the short isothermal holding time prevent it from further transforming into the equilibrium Al 8Mn 5 and β-Mn phases.

Iron-rich intermetallic phases and their role in casting defect formation in hypoeutectic Al-Si alloys

Iron is the most common and detrimental impurity in aluminum casting alloys and has long been associated with an increase in casting defects. While the negative effects of iron are clear, the mechanism involved is not fully understood. It is generally believed to be associated with the formation of Fe-rich intermetallic phases. Many factors, including alloy composition, melt superheating, Sr modification, cooling rate, and oxide bifilms, could play a role. In the present investigation, the interactions between iron and each individual element commonly present in aluminum casting alloys, were investigated using a combination of thermal analysis and interrupted quenching tests. The Fe-rich intermetallic phases were characterized using optical microscope, scanning electron microscope, and electron probe microanalysis (EPMA), and the results were compared with the predictions by Thermocalc. It was found that increasing the iron content changes the precipitation sequence of the β phase, leading to the precipitation of coarse binary β platelets at a higher temperature. In contrast, manganese, silicon, and strontium appear to suppress the coarse binary β platelets, and Mn further promotes the formation of a more compact and less harmful α phase. They are therefore expected to reduce the negative effects of the β phase. While reported in the literature, no effect of P on the amount of β platelets was observed. Finally, attempts are made to correlate the Fe-rich intermetallic phases to the formation of casting defects. The role of the β phase as a nucleation site for eutectic Si and the role of the oxide bifilms and AlP as a heterogeneous substrate of Fe intermetallics are also discussed.

Effect of melt superheating on the morphology of MC carbide in a cast Ni-base superalloy M963

The effect of the melt superheating temperature on the as-cast microstructure of a cast nickel-base superalloy M963 has been investigated. The results show that the as-cast microstructure of the alloy consists of γ solid solution matrix, γ′ precipitate in cubic shape, (γ+γ′) eutectic and MC carbide, and the morphology of MC carbide in the microstructure can be varied from coarse scriplike, fine scriptlike to fine cubelike or discontinuous particles by increasing the melt superheating temperature. The mechanism of melt superheating is discussed by means of differential thermal analysis (DTA) technique.

Experimental studies of oxidic molten corium–vessel steel interaction

The experimental results of molten corium–steel specimen interaction with molten corium on the ‘Rasplav-2’ test facility are presented. In the experiments, cooled vessel steel specimens positioned on the molten pool bottom and uncooled ones lowered into the molten pool were tested. Interaction processes were studied for different corium compositions, melt superheating and in alternative (inert and air) overlying atmosphere. Hypotheses were put forward explaining the observed phenomena and interaction mechanisms. The studies presented in the paper were aimed at the detection of different corium–steel interaction mechanisms. Therefore certain identified phenomena are more typical of the ex-vessel localization conditions than of the in-vessel corium retention. Primarily, this can be referred to the phenomena of low-temperature molten corium–vessel steel interaction in oxidizing atmosphere.

Effect of cooling rate and Sr-modification on porosity and Fe-intermetallics formation in Al-6.5% Si-3.5% Cu-Fe alloys

Together with porosity, the problems created by the formation of iron intermetallics in Al-Si alloys pose serious concerns in regard to their use in structural components. The presence of these intermetallics, particularly the brittle β-Al5FeSi phase, tends to reduce the alloy properties and casting quality. In order to optimise the iron intermetallics in Al-6.5% Si-3.5% Cu-Fe alloys, the effects of cooling rate, Sr modification, other melt additions, and heat treatment were studied, with a view to determining the means of their formation and dissolution, in relation to their effect on porosity formation and alloy properties. The iron levels selected for study were 0.5%, 1.0% and 1.5%, covering the range of levels normally observed in foundry alloys. Castings were prepared using a cooling fin sand mould (DAS range: 25–85 μm) for microstructural analysis, and a Stahl permanent mould for tensile testing purposes. Samples of molten metal quenched in cold water provided much smaller DASs. Microstructural characterisation was carried out using optical microscopy and thermal, image and electron microprobe analyses. Tensile properties (T6 condition) were determined at room temperature and a strain rate of 4 × 10−4/s using an Instron Universal testing machine. The results show that at any Fe level, the average β-Al5FeSi platelet length is strongly determined by the DAS. Strontium (250–350 ppm) is effective in reducing the β-platelet size through its fragmentation/dissolution effect on the latter. At higher Sr concentrations, both coarsening of the β-platelets (i.e., “overmodification”) and their decomposition (through Si rejection ahead of the platelets) occur. Addition of P increases β-phase formation (due to the presence of AIP particles that act as possible nucleation sites). Melt superheating (900 °C) thereafter reduces the effect. At Fe levels of 1% or more, α-Chinese script or sludge particles form only at slow cooling rates or with superheating. Tensile properties of alloys containing up to 1 % Fe are improved with Mn, Mn + Cr, Sr or Be additions. Best overall properties are achieved with Mn at Mn/Fe = 0.7. Also, Sr additions of 200–300 ppm are best, higher levels giving rise to increased porosity levels and, hence, lower properties. For a given Fe level, the optimum Sr concentration depends on the cooling rate and Si content of the alloy. Iron levels of 1.5% are not recommended for Al-6.5% Si-3.5% Cu (319.2) alloys as the properties cannot be improved by any of the above means.