Ingot Surface Research Articles

Numerical Simulation of Solidification Structure of ESR Ingot Using Cellular Automaton Method

The electroslag remelting (ESR) process is a widely used secondary remelting process for the production of high-value-added alloys and steels. The grain structure of ESR ingot has a great effect on the final properties of products. A multiscale mathematical model combining the macroscopic heat transport with the mesoscopic nucleation and grain growth was developed to predict the grain structure evolution of solidification ingot during the ESR process. A moving cell frame, which dynamically defines the calculation domain for grain structure simulation, was proposed to save the computation resources and time. The thermophysical properties of steel related to the solidification of rotor steel 30Cr1Mo1V were adopted in present model and the nucleation parameters, which were suitable for the ESR process, were determined using the trial and error method in numerical simulation. The multiscale mathematical model was validated by the comparison between predicted and experimentally observed grain structure, and the results showed that the model was capable of simulating the grain structure evolution during the ESR process. Finally, the preliminary investigation on the effect of industrial process parameters on the grain structure was carried out and the results showed that increasing melting rate caused finer columnar grain structure and changed the growth direction of columnar grain structure from the axial–radial growth into the radial growth at very high melting rate. Meanwhile, increasing the molten slag temperature made the columnar grain structure finer and reduced the thickness of the refined equiaxed grain layer both at the surface and bottom of the ESR ingot.

Theoretical analysis of the possibility of controlling the solidification conditions in a continuously cast ingot

It is shown that the solidification conditions in an entire continuously cast ingot cannot be controlled by varying the thermal conditions of cooling the ingot surface. The methods that can intensify the heat transfer in the solidifying melt should be applied in the most problematic axial zone of the ingot.

Evaluation of the Silicon Ingot With Addition of SiCl$_{\bf 4}$ in Atmosphere During Unidirectional Solidification

Adding SiCl4 to the atmosphere during the growth of a crystal suppressed crystalline defects and impurities from the edge of silicon ingots. Crystalline silicon ingots with seed crystal were grown by using the unidirectional solidification technique. Most of the grain boundaries were inactive Σ3 in the ingot with the SiCl4 injection. There were no small-angle grain boundaries at the top and edge of ingots. The carbon concentration with SiCl4 was decreased to less than half of that without the SiCl4 injection. Carbon precipitation did not occur even at the surface of ingots and the surface of grown ingots had a metallic luster.

Processing Characteristics Using Phosphorous Dielectric on Wire Electrical Discharge Machining of Polycrystalline Silicon

In this study, wire electrical discharge machining (WEDM) was implemented to process polycrystalline silicon ingot, and the influences on processing characteristics were examined. There are two different dielectrics, pure water and pure water with sodium pyrophosphate powder, that were experimented to compare their effects on cutting speed: kerf loss and surface roughness. In the experiment, sodium pyrophosphate powder has shown that it has enhanced process efficiency and has improved surface smoothness. From the results, it has been found that by using phosphorous dielectric, the cutting speed increased 2.4 times than pure water. Meanwhile, by using phosphorous dielectric, it caused high temperature and brought about the electrolysis effect, which improved its surface roughness enhanced 16% compared with pure water. This also decreased kerf loss and boosted up the material utilized. Through WEDM, high-temperature characteristic forces the phosphorous element of the dielectric to infiltrate on the cutting surface of the polycrystalline silicon ingot. The result proved that using phosphorous dielectric on WEDM could increase the efficiency and surface roughness, and could infiltrate phosphorous element on the surface of the polycrystalline silicon ingot. These findings could be future references on the researches of solar cell in both industrial and academic fields.

Influence of Anvil Shape of Surface Crack Generation in Large Hot Forging Process

In hot free forging process of large products, surface cracks occur occasionally. When these cracks occur, they must be removed by hot scarfing process after stopping forging process temporarily. Final products have no cracks on those surfaces after applying the hot scarfing process. Hot scarfing process, however, cuts into productivity due to additional time consuming. Since surface cracks are regarded as significant defects, suppression of the surface crack generation is important issue. A lot of research has been studied about mechanism and criteria of forging crack generation [1]∼[8]. In hot forging process, however, there are a few researches [9]∼[14]. Then the mechanism and criteria of crack generation during the hot forging process are not clear at this moment. There are various factors for crack generation, such as reduction ratio, forging temperature, anvil shape, ingot surface integrity and so on. The goal of this research is to clarify the mechanism of crack generation in actual forging processes. The derived mechanism is applied to design anvil shapes by using numerical forging simulation.

Formation process of Si3N4 particles on surface of Si ingots grown using silica crucibles with Si3N4 coating by noncontact crucible method

A noncontact crucible method was used to investigate the process by which a Si3N4 coating material forms Si3N4 particles or precipitates on the surface of Si melts and ingots. Si ingots were grown using crucibles with and without a mixture of α- and β-Si3N4 particles. The oxygen and nitrogen concentrations in the ingots were measured by Fourier transform infrared spectrometry analysis. The nitrogen concentration in the ingots grown using crucibles with a Si3N4 coating was significantly higher than that in ingots grown using crucibles without a Si3N4 coating because the nitrogen from the Si3N4 coating material dissolved into the Si melt. From orientation image maps analyzed using electron backscattering diffraction patterns of SixNy particles on the surface of the ingots, it was clarified that most of the SixNy particles were β-Si3N4. This was also confirmed by X-ray diffraction measurements. The Si3N4 particles on the surface of the ingots had several morphologies such as needle-like, columnar, leaf-like, and hexagonal structures. There were two cases in which floating Si3N4 particles were formed on the surface of the Si melts, i.e., the removal and dissolution of the Si3N4 coating material. The removed or dissolved Si3N4 coating materials, which consisted of a mixture of α- and β-Si3N4 particles, are considered to have finally changed into β-Si3N4 in the form of transformers or precipitates on the surface of the Si melt, and these β-Si3N4 particles became attached to the surface of the ingots.

Determination of Bulk Lifetime and Surface Recombination Velocity of Silicon Ingots From Dynamic Photoluminescence

This paper elaborates upon the theory of self-consistent minority carrier bulk lifetime measurements of silicon ingots via time-modulated photoluminescence and presents an experimental proof of concept. For silicon ingots, the solution of the continuity equation at harmonic time modulation of excess carrier generation is shown to generally reveal a remarkably pronounced contrast with respect to minority carrier bulk lifetime. We combine our dynamic self-consistent approach with an analysis of surface recombination velocity from photoluminescence intensity ratios upon irradiation with different laser wavelengths. This combined analysis enables an accurate simultaneous determination of bulk lifetime and surface recombination velocity. For sufficiently high or accurately known ingot surface recombination velocities, this approach could likewise be used for an accurate determination of the minority carrier diffusion coefficient and of minority carrier mobility in novel silicon materials.

A Roll Caster to Cast Clad Strip

A tandem-type roll caster that can cast a three-layered clad strip was developed by mounting one twin roll caster on another twin roll caster. In this caster, the base strip is cast by the upper twin roll caster, and the overlay strips are cast by the lower caster. The three strips are metallurgically bonded by the lower caster. This study investigated three aspects of this caster. First, the clad ratio could be controlled by the solidification lengths of strips from the upper and lower twin roll casters, and a clad ratio of 1:8:1 was attained. Second, although it is known that fabrication of clad strips from Al-Mg alloy and other Al alloys is very difficult, the clad strip with the Al-Mg alloy as the base strip or the overlay strip could be cast. Finally, by adding scrapers, the caster could cast the clad strip with a base strip having a lower melting point than the overlay strip. Element strips of the clad strip are made by many processes, such as direct chill (DC) casting, scraping of the ingot surface, heat treatment, hot rolling, and cold rolling. Typically, surface treatment and hot rolling are used to clad the strips. Since many processes are required, clad strips require consume much energy. Therefore, producing clad strips is expensive. A vertical-type tandem twin roll caster was developed to cast clad strips. This caster has the advantages of process saving and energy saving, and so can fabricate economical clad strips. In the fabrication of clad strips, control of the clad ratio is very important. In the brazing sheet for automobile radiators, the base strip is made from AA3003 and the overlay strips are made from AA4045, and the clad ratio is usually 1:8:1. In the present study, a clad strip with a clad ratio of 1:8:1 was attained. The cladding by hot rolling of Al-Mg and other aluminum alloys, which is considered to be a very difficult process, was also investigated. Clad strips with either a base strip or an overlay strip of an Al-Mg alloy were cast by the roll caster. Although the cladding was not easy, the Al-Mg alloy could be cast into the clad strip. In addition, a clad strip with a base strip having a lower melting point than that of the overlay strip was investigated. Such cladding cannot be cast by the vertical-type tandem twin roll caster as mentioned above because the base strip is re-melted from the heat of the overlay strips. In the present study, a scraper was developed and adopted to cast a clad strip with a base having a lower melting point than that of the overlay strips. This type of clad strip could be cast because the scrapers prevented the re-melting of the base strip. In this paper, these three aspects of fabrication are reported.

Segregation of the Eu impurity as function of its concentration in the melt for growing of the lead telluride doped crystals by the Bridgman method

Behavior of a rare earth impurity of Eu in the PbTe single crystals grown by the Bridgman method from the melt with different initial concentrations of impurity NEuint(ml) of about 1×1020cm−3 and less is investigated with X-ray fluorescent element analysis, Secondary Neutral Mass Spectroscopy (SNMS), and magnetic measurements. The impurity distributions along and across the doped ingots are established. It is revealed that doping impurity enters into the bulk of doped crystals only if its initial concentration in the melt is high enough, approximately 1×1020cm−3. If this concentration is lower, about 1×1019cm−3 and less, the doping Eu impurity is pushed out onto the surface of doped ingot. The thickness of the doped surface layer is estimated to be in the order of several microns or somewhat more. The longitudinal distributions of Eu impurity along the axis of doped ingot–for NEuint(ml)=1×1020cm−3, as well as the transverse one in the surface layer where entire doping impurity is pushed out–for NEuint(ml)=1×1019cm−3, are strongly non-monotonic. Possible reasons for this unusual behavior of Eu doping impurity during the growth of PbTe:Eu crystals from the melt are analyzed.

Flux Entrapment and Titanium Nitride Defects in Electroslag Remelting of INCOLOY Alloys 800 and 825

Electroslag remelted (ESR) ingots of INCOLOY alloys 800 and 825 are particularly prone to macroscale slag inclusions and microscale cleanliness issues. Formation of these structures near the ingot surface can cause significant production yield losses (~10 pct) due to the necessity of extensive surface grinding. Slag inclusions from near the outer radius of the toe end of alloy 800 and 825 ingots were found to be approximately 1 to 3 mm in size and have a multiphase microstructure consisting of CaF2, CaTiO3, MgAl2O4, MgO, and some combination of Ca12Al14O32F2 and/or Ca12Al14O33. These inclusions were often surrounded by fields of 1- to 10-μm cuboidal TiN particles. A large number of TiN cuboids were observed in the ESR electrode with similar size and morphology to those observed surrounding slag inclusions in the ESR ingots, suggesting that the TiN particles are relics from the ESR electrode production process. Samples taken sequentially throughout the AOD processes showed that the TiN cuboidals that are found in ESR ingots form between tapping the AOD vessel into the AOD ladle and the casting of ESR electrodes.

Effect of Out-Phase Electromagnetic Field on the HDC Casting Process of Commercial Aluminum

Commercial aluminum ingot with the diameters of 100 mm is produced by conventional out-phase electromagnetic field HDC casting process. The out-phase electromagnetic field is applied on the HDC casting process. The effect of out-phase electromagnetic filed on the ingot surface and structure is investigated. The result shows that the out-phase electromagnetic field is more effective in improving the upper surface, while, has little function on the bottom surface. The macro/microstructure of the ingot is changed by the out-phase electromagnetic field. Very long and coarse columnar grains evolve to short and thin columnar grains with the application of the out-phase electromagnetic field.

Superconductivity of the surface layers of the doped PbTe:Eu crystals

We present evidence of superconductivity of the surface layers of PbTe crystals grown from melt and doped with Eu during growth. We have studied two different type of the powder samples made out of the surface layer and from bulk of the PbTe:Eu ingots. The magnetization in the range of magnetic field 0–5T and the magnetic susceptibility (MS) in the temperature range 1.7–400K were investigated. At low temperatures, a strong diamagnetism of the surface layers of the PbTe:Eu ingot is observed whereas at high temperatures a transition to strong paramagnetism takes place. This effect is attributed to the transition of the PbTe:Eu surface layers to state of superconductivity at low temperature. The superconductivity of the doped surface layers is explained by formation of the Pb-based inclusions within the ingot surface.

Effect of the Silicon Nitride Coating of Quartz Crucible on Impurity Distribution in Ingot‐Cast Multicrystalline Silicon

Multicrystalline silicon ingots were fabricated via the directional solidification method in a vacuum induction furnace using silicon nitride (Si3N4) as quartz crucible coating. The effect of Si3N4 coating on the impurity content, transference, and distribution in Si ingots was investigated. The results can be summarized as follows: The impurities contained in Si ingots, including Fe, Ca, Mn, Cu, P, and B, were found to decrease at varying degrees when the inner surface of the quartz crucible was coated with Si3N4. A mixed layer composted by Si3N4 and Si was observed on the surface of the Si ingot. This mixed layer provided microdefects for the nucleation of metallic impurities. The Fe and N contents in the Si/Si3N4 mixed layer changed with the same tendency, and FeSi2 particles formed on the Si/Si3N4 crystalline boundary. Both Fe and Ca precipitated in the SiC particles near the Si ingot/Si3N4‐coating interface.

Modelling macrosegregation in a 2.45 ton steel ingot

A three phase model for the mixed columnar-equiaxed solidification was proposed by the current authors [Wu and Ludwig 2006 Metall. Mater. Trans. 37A 1613-31]. The main features of the mixed columnar-equiaxed solidification are considered: the growth of the columnar dendrite trunks from the ingot surface, the nucleation and growth of the equiaxed crystals, the sedimentation of the equiaxed crystals, the thermal and solutal buoyancy flow and its interactions with the growing crystals, the solute partitioning at the solid-liquid interface during solidification, the solute transport due to melt convection and equiaxed sedimentation, the mechanical interaction/impingement between columnar and equiaxed crystals and the columnar-to-equiaxed transition (CET). However, due to the model complexity and the limited computational capability the model has not yet applied to the large steel ingots of engineering scale. This paper is going to simulate a 2.45 ton big-end-up industry steel ingot, for which some experimental results were reported [Marburg 1926 Iron Steel Inst. 113 39-176]. Here a simplified binary phase diagram for the steel (Fe-0.45 wt. %C) is considered. Comparison of the modelling results such as as-cast columnar and equiaxed zones, macrosegregation with the experimental results is made. Details about the formation sequence of the distinguished crystal zones and segregation patterns are analyzed.

Microstructure and mechanical properties of the Mg–Gd–Y–Zn–Zr alloy fabricated by semi-continuous casting

a b s t r a c t Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy ingot with diameter of 280 mm and length of 2940 mm was success- fully fabricated by semi-continuous casting. The microstructure and mechanical properties of the large ingot at different locations were investigated. The concentration of the RE and Zn elements distributed almost homogeneously through the whole alloy ingot. The average grain size decreased from the center to the surface of the alloy ingot, while the strength and ductility were improved gradually. The main eutectic compounds located at the grain boundaries were (Mg,Zn)3(Gd,Y) phase, adjacent to these eutec- tic compounds the 14H-type long period stacking ordered (LPSO) phase formed. Furthermore, some block-shaped Mg24(Gd,Y)5 phase and isolated Zr cores can be recognized in the interior of the grains. © 2012 Elsevier B.V. All rights reserved.

A Simple Model of the Mold Boundary Condition in Direct-Chill (DC) Casting of Aluminum Alloys

An accurate thermofluids model of aluminum direct-chill (DC) casting must solve the heat-transfer equations in the ingot with realistic external boundary conditions. These boundary conditions are typically separated into two zones: primary cooling, which occurs inside the water-cooled mold, and secondary cooling, where a film of water contacts the ingot surface directly. Here, a simple model for the primary cooling boundary condition of the steady-state DC casting process was developed. First, the water-cooled mold was modeled using a commercial computational fluid dynamics (CFD) package, and its effective heat-transfer coefficient was determined. To predict the air-gap formation between the ingot and mold and to predict its effect on the primary cooling, a simple density-based shrinkage model of the solidifying shell was developed and compared with a more complex three-dimensional (3-D) thermoelastic model. DC casting simulations using these two models were performed for AA3003 and AA4045 aluminum alloys at two different casting speeds. A series of experiments was also performed using a laboratory-scale rectangular DC caster to measure the thermal history and sump shape of the DC cast ingots. Comparisons between the simulations and experimental results suggested that both models provide good agreement for the liquid sump profiles and the temperature distributions within the ingot. The density-based shrinkage model, however, is significantly easier to implement in a CFD code and is more computationally efficient.

Effect of low frequency electromagnetic field on as casting structure and surface quality of horizontal direct chill casting 7075 alloy

Horizontal direct chill (HDC) casting is a well established production route for aluminium alloy ingots; however, the ingots may suffer from inhomogeneous microstructures and serious macrosegregation due to the unbalanced cooling condition and gravitational effect. The present study considers the HDC casting of 7075 aluminium alloy ingot under the influence of a low frequency electromagnetic field (LFEF). Temperature measurement is employed to analyse the effect of LFEF on the HDC casting process. Surface topography and structure observation are conducted to analyse the effect of LFEF on the surface and interior quality of the HDC ingot. The results show that the LFEF can effectively uniformalise the thermal distribution, flatten the sump shape, improve the ingot surface quality, promote a transition from columnar/feathery to equiaxed dendritic growth and has a distinct grain refining effect.

The Influence of the Near-Meniscus Zone in Continuous Casting Mold on the Surface Quality of the Continuous Casting Ingots

The Influence of the Near-Meniscus Zone in Continuous Casting Mold on the Surface Quality of the Continuous Casting IngotsThe physical, chemical and mechanical phenomena which take place in the near-meniscus zone of continuous casting mold are the significant factors influencing the quality of CC ingot and especially the quality of its surface. Such phenomena consist of the following processes: lubrication of the ingot surface by the liquid slag-forming phase of mold powder, creation of meniscus, formation of the specific kind of galvanic cell and connected with this cell ions migration of liquid mold powder. Application of the mold powders is the commonly used lubrication method of the surface of CC ingots in mold (in near-meniscus zone). According to the ionic structure theory of the liquid metallurgical slags the following thesis can be formulated: the liquid slag-forming phase of mold powder is the ionic liquid. The ionic liquid occurs between two metals: the copper wall of mold and the steel surface of ingot can create a specific kind of galvanic cell in the upper part of mold (the near-meniscus zone of mold). The paper presents results of industrial research of low-carbon steel continuous casting. The electromotive force of galvanic cell situated in the upper (near-meniscus) part of CC mold was measured. Moreover, the influence of applied powders with different alkalinity on the character of oscillatory marks forming on the ingot surface was considered. The galvanic cell, which is created in the upper part of mold in the near-meniscus zone, can cause the essential change of the chemical composition of electrolyte (liquid phase of mold powder) in the near-electrodes zones. So in the process the condition of lubrication and character of obtained oscillatory mark can also be changed.

Studies of the conditions of formation of transverse corner cracks on the surface of continuous-cast slabs

A statistical analysis is made of production data to determine the effect of the main process parameters in the continuous casting of slabs on the probability of formation of edge cracks on the surface of the slabs. The parameters that have the greatest effect on crack formation are identified and several methods are proposed for changing these parameters to make the formation of edge cracks less likely. Corner cracks located on the surface of the casting are one type of defect commonly encountered on slabs of crack- resistant steels cast on curvilinear continuous casters. Up to 10% the slabs may be damaged by such cracks. The length of these defects ranges from several millimeters to 15-20 mm, while their depth reaches 5-10 mm. All types of surface cracks are stress concentrators that can lead to fracture of the material during rolling or the formation of different types of surface defects on flat-rolled product (1). Conditioning of the surface - hot scarfing or mechanical scarfing - is often used to remove cracks from the surface of slabs. These measures make it possible to improve the quality of the rolled product but also increase labor costs, and there is always the possibility that a defect may be missed or not completely repaired. All these con- siderations make it a matter of practical importance to re-examine the continuous-casting process in order to reduce the dam- age to slabs by corner cracks. To resolve the problem, it is necessary to make a detailed study of the conditions under which the edges of slabs are formed and cooled. It is noted in the literature that transverse cracks are formed on surface of slabs at their edges due to hanging of the ingot in the corners of the mold, the formation of shrinkage stresses in the longitudinal direction when the edges of the ingot undergo excessive cooling, or tearing of an excessively cool ingot skin during its bending or unbending (2). Thus, the condi- tions under which edge cracks are formed in the mold are related to the quality of its working surface, the effect of the slag- forming mixtures that are used, and the effect of the mold's conicity. Both of the broad surfaces of the slab are subjected to mechanical tensile stresses (the most dangerous type of stresses, leading to loss of integrity of the metal) in the secondary cooling zone (SCZ) of a radial continuous caster. The surface on the large-radius side undergoes tension as the slab is bent under the mold, while the surface on the small-radius side undergoes tension as the slab is bent in the opposite direction dur- ing its entry onto the horizontal section of the caster. It is impossible to eliminate the effects of these stresses in continuous casters of the given design. Thus, the only way to make crack formation on these sections less likely is to vary the chemical composition of the metal by changing its chemical composition and the temperature of the ingot surface. The possibilities for varying the chemical composition of the metal are severely limited by the requirements established for the given grade of

Effect of casting speed on surface quality of horizontal direct chill casting 7075 aluminum alloy ingot

7075 aluminum alloy ingot with the diameter of 100 mm has been produced by horizontal direct chill casting in difierent casting speed. The efiect of the casting speed on the ingot surface and subsurface layer was studied by surface observation and subsurface structure analysis. It was found that increasing the casting speed results in the adding of segregation knots in the ingot surface. The thickness of the dendrite microstructure layer in the subsurface reduces with increasing the casting speed. And the elements of Zn, Cu and Mg enrich in the coarse dendrite microstructure layer of the ingot. KEY WORDS Horizontal direct chill casting; Casting speed; 7075 aluminum alloy; Surface quality