Early Stages Of Solidification Research Articles

Recent Continuous Casting Technologies of Steel in Japan

Recent development in continuous casting technologies in Japan is reviewed mainly from the stand-point of high casting speeds. Heat flux to the mold increases inevitably with higher casting speed and is consistent in both near net shape castings and conventional ones. Heat transfer to the mold is discussed in detail. Especially the selection and control of mold fluxes are of great importance. Stirring and braking of molten steel in the mold by an applied electromagnetic field have been successfully performed to reduce internal inclusions, unsteady casting parts and also to prevent longitudinal cracks. An electromagnetic pressure can impose an early stage of solidification of continuously cast steels which results in enhancing the surface qualitics remarkably.

Thermal properties of poly(ester-imide)s with C12H24 and C22H44 groups

The thermal properties of poly(4,4′-phthaloimidobenzoyl-n-methyleneoxycarbonyl) with n =12 and 22, abbreviated as PEIM-12 and PEIM-22, respectively, have been studied using differential scanning calorimetry (DSC). The heat capacities of the solid states of both polymers were measured and compared to computed heat capacities from approximate vibrational spectra. The deviations from the vibrations-only heat capacity were used to identify large-amplitude, conformational motions. The heat capacities of the liquid states were described as linear functions of temperature. They agreed with the liquid heat capacities generated from the ATHAS addition scheme using group contributions derived from polymers containing the same chemical segments as the PEIM-ns. Knowing the heat capacities for the solid and liquid, the transition parameters could be separated and enthalpies, entropies, and free enthalpies obtained. With these data, the change of the crystallinity with temperature could be computed. In the early stages of solidification both compounds contain significant entropy contributions from conformational ordering of the flexible spacer and little from the rigid, aromatic segments. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 319–328, 2000

Estimation of thermal contract resistance during the first stages of metal solidification process: II—experimental setup and results

This paper presents an experimental study of transient heat transfer in the early stages of solidification of some pure metals on water cooled substrate, quantified in terms of two parameters: thermal contact resistance at the interface and heat flux. Experiments were performed to measure the thermal contact resistance during solidification of tin, lead and zinc drop on nickel substrate. The study is focused on the heat transfer aspects of this process and the identification of parameters affecting the heat transfer mechanism. To this end, the influence of casting process variables such as substrate surface roughness, casting material, metal superheat and lubricant on heat transfer has been analyzed.

Equiaxed-grain size analysis in the mushy zone during solidification via an in-situ method based on the electrical sensing zone principle

This paper addresses dendritic equiaxed growth during grain structure formation in alloy solidification, and focuses on the growth and interaction of primary dendrite arms, which drive and limit grain development, respectively. Dedicated solidification experiments on Al–10 wt.% Cu alloy were carried out at the European Synchrotron Radiation Facility, enabling live observation by in situ and real-time synchrotron X-ray radiography. Equiaxed solidification was achieved by cooling nearly isothermal samples, and the growth of the grains was continuously monitored from the very early stages of solidification up to a state where changes in grain structure were no longer visible to the observer’s naked eye. The time evolution of individual equiaxed grains is analysed through measurements of the lengths and tip velocities of the primary dendrite arms driving grain growth. It is found that equiaxed dendrite propagation is characterized by two growth regimes. The first regime, in which a gradual increase in the tip growth rate is observed, corresponds to free dendritic growth, when the relative distance between grains is large enough to consider a grain as isolated. During this regime, the far-field liquid concentration ahead of the advancing dendrite tip remains constant, equal to the initial alloy composition, and the driving force for growth is the undercooling applied by cooling of the sample, in agreement with three-dimensional models of free dendritic growth. The second regime shows a decrease in the tip velocity towards zero and is attributed to grain interaction when the solute diffusion field in front of the advancing dendrite tip under consideration overlaps with the solute field associated with the growth of the grain(s) facing it. Solute concentration profiles are measured in the extra dendritic liquid ahead of dendrite tips interacting with neighbour grains, from which the far-field solute concentration and neighbour–interaction distance are deduced. These measurements confirm that impingement of the solute boundary layers is the main cause of dendrite growth being slowed down and stopped.

In-Situ Measurement of Fracture Strength of Solidifying Steel Shells to Predict Upper Limit of Casting Speed in Continuous Caster with Oscillating Mold.

Preventing crack formation of solidifying steel shell at increasing speed in continuous slab casting operation is a prerequisite to increase the ratio of hot charge rolling and hot direct rolling of cast slabs. Crack formation is influenced by mechanical properties of the steel shell near its solidus temperature. Thus, fracture strength of solidifying steel shells containing 0.004 to 0.70 mass% C has been measured in-situ by using a method similar to "Submerged Split Chill Tensile (SSCT)" test by Ackermann, Kurz and Heinemann. The fracture strength at an early stage of solidification, when the shell thickness is 2 to 5 mm, has been found to be 1 to 3.5 MPa which is lower than that determined by conventional hot tensile tests, since the crack initiates at and propagates in interdendritic region in the shell in the in-situ SSCT test. Comparing the fracture strength of the shell with the frictional force at the shell/mold interface, upper limit of casting speed of slab caster is calculated to be around 8.5 m/min.

A model for inverse segregation: The case of directionally solidified AlCu alloys

A model for predicting macrosegregation during solidification of casting alloys has been developed. The continuum formulation was used to characterize the macroscopic mass, energy and momentum transports for two-phase systems. It was assumed that the interdendritic liquid flow was driven by thermal and solutal buoyancy, as well as by solidification contraction. The movement of free surface during solidification contraction and of precipitated solid phases in the early stage of solidification was taken into account. Validation of this model was performed for the case of inverse segregation in a directionally solidified Al-4.0% Cu casting with equiaxed grains. Calculated results show that positive segregation is formed in the vicinity of the bottom chill, associated with negative segregation in the mushy zone.

Heat transfer and microstructure during the early stages of metal solidification

Transient heat transfer in the early stages of solidification of an alloy on a water-cooled chill and the consequent evolution of microstructure, quantified in terms of secondary dendrite arm spacing (SDAS), have been studied. Based on dip tests of the chill, instrumented with thermocouples, into Al-Si alloys, the influence of process variables such as mold surface roughness, mold material, metal superheat, alloy composition, and lubricant on heat transfer and cast structure has been determined. The heat flux between the solidifying metal and substrate, computed from measurements of transient temperature in the chill by the inverse heat-transfer technique, ranged from low values of 0.3 to 0.4 MW/m2 to peak values of 0.95 to 2.0 MW/m2. A onedimensional, implicit, finite-difference model was applied to compute heat-transfer coefficients, which ranged from 0.45 to 4.0 kW/m2 °C, and local cooling rates of 10 °C/s to 100 °C/s near the chill surface, as well as growth of the solidifying shell. Near the chill surface, the SDAS varied from 12 to 22 (µm while at 20 mm from the chill, values of up to 80/smm were measured. Although the SDAS depended on the cooling rate and local solidification time, it was also found to be a direct function of the heat-transfer coefficient at distances very near to the casting/chill interface. A three-stage empirical heat-flux model based on the thermophysical properties of the mold and casting has been proposed for the simulation of the mold/casting boundary condition during solidification. The applicability of the various models proposed in the literature relating the SDAS to heat-transfer parameters has been evaluated and the extension of these models to continuous casting processes pursued.

Solidification Velocity and Temperature Gradient in Infinitely Thick Alloy Castings

A classical rigorous solution of solidification of pure metals as modified for alloys with a finite solidification temperature range was used to examine the behavior of temperature gradient G (K.m -1 ) at the solidus in the mushy zone and solidification velocity V (m.s -1 ), assuming an infinitely thick mold and casting. The ratio G/V, which controls porosity formation, constitutional supercooling or columnar crystal growth, is constant independent of time. The ratio increases by slow cooling through the use of a poor conducting mold material or at a high mold temperature. The relation among G, V and other major thermal variables was presented in dimensionless forms. Since the discussion is based on the assumption of infinite thickness, the results are applicable to real castings only in the early stage of solidification

Convective Transport Phenomena and Macrosegregation During Solidification of a Binary Metal Alloy: I—Numerical Predictions

A continuum model is used to simulate transient convective transport phenomena numerically during solidification of a Pb-19 percent Sn alloy in an experimental test cell. Solidification occurs in an axisymmetric, annular mold of stainless steel, cooled along its outer vertical wall. Results show that, during early stages of solidification, double-diffusive convection and liquid exchange between melted and mushy zones are responsible for the formation of channels in the outer periphery of the ingot, which ultimately lead to a form of macrosegregation known as A-segregates. During intermediate stages of solidification, solutally driven natural convection spawns a cone segregate in the interior region of the ingot. The final macrosegregation pattern is characterized, in general, by increasing Sn concentration with increasing height throughout the ingot and by increasing Sn concentration with decreasing radius in the upper portion of the ingot.

Convective Transport Phenomena and Macrosegregation During Solidification of a Binary Metal Alloy: II—Experiments and Comparisons With Numerical Predictions

Experiments involving the solidification of a Pb-19 percent Sn alloy in an axisymmetric, annular mold of stainless steel are performed, and results are compared with numerical predictions. Agreement between measured and predicted cooling curves is reasonable, although undercooling and recalescence, which occur in the experiments, are not predicted by the model. Measured temperatures also indicate that dendrite fragments are transported throughout the mold cavity during early stages of solidification, and convection patterns similar to those predicted by numerical simulation can be inferred from the measurements. While there is general agreement between measured and predicted trends for macrosegregation, the numerical simulation, which assumes axial symmetry, cannot predict the inherently three-dimensional nature of solute redistribution. Conclusions drawn from temperature and composition measurements are supported by metallographic examinations of experimental ingot specimens.

Modeling of the formation of under-riser macrosegregation during solidification of binary alloys

The formation of macrosegregation in a rectangular ingot with reduced cross section from the riser to the casting, chilled from the bottom, has been studied numerically. In addition to positive inverse segregation occurring near the chilled surface, very severe negative segregation around the under-riser region and moderate positive segregation near the top corners of the casting were found. Although large circulating vortexes are created by natural convection in the under-riser region during the early stage of solidification, the fluid flow in the mushy zone is dominated by solidification shrinkage. As a result, the final solute distribution in the casting is determined by the flow of solute-rich liquid in the mushy zone owing to the combined effects of solidification shrinkage and change of cross section from casting to riser. Detailed explanations regarding the effect of different flow phenomena on the formation of the segregations are provided. The effects of riser size and cooling condition at the bottom of the ingot on the formation of macrosegregation also were studied. The predicted negative and positive macrosegregations in the casting compared very well with the available experimental data.

SOLIDIFICATION OF MOLTEN METAL DROPLETS IMPINGING ON A COLD SURFACE

Two experiments are conducted to study flow and heat transfer behavior in a molten metal droplet on a cold surface following impingement. Laser holographic interferometry is employed to visualize molten metal-solid contact state in real time over the entire solidification process by means of a high-speed video camera. The formation of two distinct solidified layers is disclosed. One, formed in the early stage of solidification, adheres to the cooling surface and is latent heat controlled. The other, formed in the later stage, is nonadhesive and radial-shaped, and is dominated by a slip flow of the molten phase accompanied by phase change. The latter is verified by a separate experiment using thermocouple measurements and inverse conduction theory. These two different heat transfer mechanisms may cause transient thermal and residual stress problems, which would affect the quality of solidified metal products. Results for solidification of a single molten metal drop on quartz and copper surfaces, representi...

Initial stage of rapid solidification of 18-8 stainless steel

The early stage of solidification of 18-8 stainless steel droplets on substrates was studied. A temperature-measuring system has been developed to trace the thermal history of the droplet surface. Factors influencing the degree of undercooling at the surface were investigated, the resulting microstructures and phase selection being affected by the degree of undercooling. The experimental results are compared with theoretical predictions using a recent model of rapid dendritic growth.

Fluid evolution in submarine magma‐hydrothermal systems at the Mid‐Atlantic Ridge

Fluid inclusions in a suite of gabbro, quartz‐breccia, and metabasalt samples recovered from the MARK area on the Mid‐Atlantic Ridge are the product of a complex hydrothermal history involving late stage magmatic fluids at temperatures >700°C and penetration by modified seawater at 300–400°C. The evolution of volatiles during the early stages of solidification and cooling of magma bodies near the ridge‐transform intersection is marked by exsolution of a CO2 fluid, entrapped within primary inclusions in fluorapatites. Attendant with progressive melt fractionation, residual evolved melts reached water saturation, and locally, supercritical CO2+H2O+NaCl±Fe brines (>50 wt % NaCl) and cogenetic H2O+CO2‐rich vapors (1–2 wt % NaCl) were exsolved as immiscible phases. Concomitant or subsequent fracturing, perhaps in response to volatile exsolution from the melts, allowed migration of these fluids along microfracture networks at >700°C. Trondhjemitic‐hosted inclusions, which homogenize by halite dissolution, indicate that the last fluids exsolved from the melts may have been 35–40 wt % brines. The transition from magmatic to seawater‐dominated hydrothermal conditions in the gabbros is marked by initial penetration of lower salinity fluids (1–7 wt % NaCl) at temperatures in excess of 400°C, with the general cessation of fluid flow occurring at minimum temperatures of ≈ 250°C. The relative enrichment and depletion of NaCl with respect to seawater in these fluids may record supercritical phases separation of seawater or boiling of hydrothermal fluids enriched in NaCl. Migration along microfracture networks of CH4‐rich, 350°C fluids, may reflect deeper seated hydrothermal processes involving hydration of underlying mantle material in response to fluid flow along deeply penetrating fault systems. In shallow crustal rocks, circulation of seawater‐derived fluids occurred at temperatures up to 400°C, with subsequent collapse of the active hydrothermal system at minimum temperatures of 200–250°C. In fault‐related upflow zones, multiple hydrothermal pulses involving 180–340°C and 3.5–10 wt % NaCl fluids, pervasively altered bounding wall rocks, forming chlorite‐rich, pyrite‐ and chalcopyrite‐bearing breccias. At shallow crustal depths, fluids reached temperatures of 150–300°C and contained salinities of 3.8–6.9 wt % NaCl. Following collapse of the axial‐related hydrothermal system, the plutonic and shallow crustal rocks were uplifted and emplaced as allocthonous blocks attending formation of the ridge‐transform intersection massif.

Thermal Expansion of Gases at the Metal/Mould Interface in Permanent-mould Casting

AbstractThe contact resistance at the metal/die (mould) interface during the early stages of solidification in gravity-dies (permanent moulds) is explained by a gas film theory. The theory, which a...

Magnetically Damped Convection During Solidification of a Binary Metal Alloy

The transient transport of momentum, energy, and species during solidification of a Pb-19 percent Sn alloy is numerically simulated with and without magnetic damping. The system is contained in an axisymmetric, annular mold, which is cooled along its outer vertical wall. Since thermosolutal convection accompanies solidification and is responsible for final macrosegregation patterns, application of a steady magnetic field, which is parallel to the axis of the mold, has the potential to reduce macrosegregation by damping buoyancy-driven flow during solidification. Results show that, during early stages of solidification, the magnetic field significantly affects thermally driven flow in the melt, as well as interactions between thermally and solutally driven flows. However, interdendritic flows and macrosegregation patterns are not significantly altered by moderate magnetic fields. Scaling analysis reveals that extremely strong fields would be required to effectively dampen convection patterns that contribute to macrosegregation.

Microstructure of Ni-10Co-8Cr-4W-13Zr alloy and its bonding behaviour for single-crystal nickel-base superalloy

In the later stages of solidification of zirconium-containing superalloys, the concentration of zirconium in the interdendritic melts is above 10 wt%. The γ dendrites formed in the early stage of solidification may be considered to be joined by the interdendritic zirconium-rich melt. Based on the composition of the zirconium-rich melt, an Ni-10Co-8Cr-4W-13Zr (wt %) alloy was selected as an interlayer alloy for brazing and transient liquid-phase (TLP) bonding of single crystal superalloys. All the elements in the interlayer alloy are beneficial to the single-crystal superalloys. A bonding microstructure which is very similar to that of the base alloy was obtained by means of the TLP process using this interlayer alloy. In the present work, the microstructural characteristics of the interlayer alloy and the phase relationships in the bond during brazing and TLP processes were investigated.

Behavior of metal alloys in the semisolid state

During dendritic solidification of castings and ingots, a number of processes take place simultaneously within the semisolid region. These include crystallization, solute redistribution, ripening, interdendritic fluid flow, and solid movement. The dendritic structure which forms is greatly affected by convection during the early stages of solidification. In the limit of vigorous convection and slow cooling, grains become spheroidal. Alloys with this microstructure possess rheological properties in the semisolid state which are quite different from those of dendritic alloys. They behave thixotropically, and viscosity can be varied over a wide range, depending on processing conditions. The metal structure and its rheological properties are retained after solidification and partial remelting. The semisolid alloys can be formed in new ways, broadly termed «semisolid metal (SSM) forming processes». Some of these are now employed commercially to produce metal components and are also used to produce metal-matrix composites

NUMERICAL ANALYSIS OF BINARY SOLID-LIQUID PHASE CHANGE WITH BUOYANCY AND SURFACE TENSION DRIVEN CONVECTION

The effects of thermo/diffusocapillary convection on the solidification of aqueous NH4Cl in a rectangular cavity have been simulated numerically using a newly developed continuum model. Diffusocaptttary convection is negligible relative to thermocapillary convection, and for a 20 × 20 mm cavity in a one-gravity environment, thermocapillary effects are most pronounced during the early stages of solidification, when flow conditions are characterized by three major cells. One cell, driven by solutal buoyancy forces, extends from the mushy region to the melt and separates top and bottom melt region cells driven primarily by surface tension and buoyancy forces, respectively. With increasing time, however, the top cell strengthens and eventually envelops the entire melt. In terms of the strength of the flow, the liquidus front morphology, and the amount of solid formed, final conditions differ only slightly from those predicted for pure thermal/solutal convection.

Al-Mg合金による鋳塊のチャンネル型偏析の生成に関する実験的研究

Effects of solidification condition and morphology of growing crystals on the formation of channel-type segregation were investigated by using unidirectionally solidified Al-Mg alloys, in which denstiy change in interdendritic liquid is similar to that in steel.Under any casting condition, channels formed at any growth rates less than a critical value R*=(2.0±0.2)×10−4 (m·s−1). Channels were mainly generated from a mixed zone of branched columnar and equiaxed crystals near equiaxed sedimental zone, and did not form in a sedimental zone. In the vicinity of the boundary region, the distribution of dendrite crystals were not uniform, and larger liquid pools existed at an early stage of solidification. The channel-type segregation was considered to originate from the stronger fluid flow through the pools. The segregation took the form of channel at fraction of solid fs=0.2-0.3.