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Experimental study on the effect of liquid metal superheat and casting height on interfacial heat transfer coefficient
Abstract
In this article, the interfacial heat transfer coefficient (IHTC) is investigated as a function of superheat temperatures (100°C, 150°C, and 200°C) and casting heights (100, 150, and 200 mm). The experiments were conducted for a liquid alloy (Al-Si 12.9%) on water-cooled copper chill during vertically upward solidification of a eutectic Al-Si alloy casting. A finite difference method (FDM) is applied for the numerical used for solution of inverse heat conduction problem (IHCP), the so-called Beck's method. Computer-guided thermocouples were connected with the chill and casting, at six positions and the time-temperature data were recorded automatically. As the lateral surfaces are very well heat isolated, the unidirectional solidification process started vertically upward at the interface surface. The measured time-temperature data files were used by FDM using explicit technique. The experimental and calculated temperatures have shown excellent agreement. The IHTC increases as superheat temperatures increases. However, the casting height (100, 150, and 200 mm) has no significant effect on the IHTC. It changed only maximum peak values of the IHTC and increased air gap formation time with increasing casting height.
... Большое количество работ посвящено определению коэффициента теплопередачи между сплавом А356 (АК7ч) и формой из стали. Максимальное значение коэффициента теплопередачи изменяется в интервале от 1300 до 3000 Вт/м 2 К [21][22][23][24][25], но в работе [25] и работе [24] максимальные значения коэффициента теплопередачи составляют 2600 и 1700 Вт/м 2 К соответственно, что близко к значению найденному в данной работе (2050 Вт/м 2 К). Возможные причины такого несоответствия между максимальными значениями коэффициента теплопередачи могут заключаться в различной шероховатости использованных форм, различным составом сплавов и неодинаковой величиной перегрева над температурой ликвидуса [22,26]. ...
... Максимальное значение коэффициента теплопередачи изменяется в интервале от 1300 до 3000 Вт/м 2 К [21][22][23][24][25], но в работе [25] и работе [24] максимальные значения коэффициента теплопередачи составляют 2600 и 1700 Вт/м 2 К соответственно, что близко к значению найденному в данной работе (2050 Вт/м 2 К). Возможные причины такого несоответствия между максимальными значениями коэффициента теплопередачи могут заключаться в различной шероховатости использованных форм, различным составом сплавов и неодинаковой величиной перегрева над температурой ликвидуса [22,26]. По этой причине сравнение коэффициентов теплопередачи следует производить в одинаковых или близких к одинаковым условиям. ...
Steel is one of the widespread materials for producing of permanent molds for aluminium casting. At the same time the materials that can promote higher cooling rate are available. One of that type of materials is graphite. In our work the comparison of cooling conditions for A356 aluminium ingots casting with different diameters into the steel and graphite molds using simulation are made. In order to obtain properly simulation results of heat transfer from ingot to mold the interface heat transfer coefficient (IHTC) on the ingot/mold interface were determined. The experimental casting of ingots with 30 mm diameter into steel molds and 50 mm ingots into graphite mold with simultaneous recording of the temperature curves both in the ingot and in the mold are made. Determination of IHTC were carried out using trial and error method by selection of IHTC at which the difference between the experimental and simulated temperature curves in the ingot and in the mold was minimal. The IHTC versus to ingot surface temperature curves were obtained. It was established that maximal IHTC between A356 alloy ingot and steel mold was 2050 W/mK, but between the ingot from the same alloy and graphite mold the IHTC was 2.5 times higher and has a value 4700 W/mK. The cooling rate at different ingot diameters and mold wall thickness was calculated. It was shown that at small ingot diameter (10 mm) the cooling rate of A356 alloy ingot casting into steel mold was 10.3 °C/s. At the same time for ingot from the same alloy casting into graphite mold the cooling rate near 2.5 times higher and have a value 24.2 °C/s. For ingot with diameter of 100 mm the cooling rate at steel mold casting was 0.8 °C/s, but at graphite mold casting the cooling rate was near two times higher and have a value 1.4 °C/s.
... These parameters will vary depending on temperature * corresponding author; e-mail: muratkoru@sdu.edu.tr and time throughout the process. During this process, while heat is transferred by conduction significantly, it also can be transferred by radiation and convection due to high temperature [6][7][8][9][10]. On the other hand, by the evacuation of the air in the mold cavity before the injection process, it is possible to improve interfacial contact between the casting and mold. ...
... On the other hand, in studies of Hallam and Griffiths [16], IHTC has increased with the increase of mold temperature. When the casting is made at room temperature, IHTC values are 1700-2250 W/m 2 K, at mold temperature of 573 K these values are 1900-2080 W/m 2 K. Akar et al. [10] have determined that maximum IHTC at mold temperature of 373 K is 9749 W/m 2 K, at 423 K it is 14790 W/m 2 K and at 473 K it is 17300 W/m 2 K. 6 show that when the casting temperature increases, IHTC and heat flux are increasing by 3% in experimental results and by 2% in simulation results. Kim et al. [3], have investigated casting temperature as function of mold-casting interfacial heat. ...
... Reasons for the observed discrepancies in the IHTC include variations in the roughness of the mold, different alloy compositions, and melt superheating. 29,35 Thus, the IHTC should be compared under similar casting conditions. Therefore, to compare cooling rates during ingot casting in graphite and steel molds, the IHTCs for the alloy/mold interfaces needs to be obtained first. ...
Many types of alloys can be cast in graphite molds. The advantage of using graphite molds is that graphite has a higher thermal conductivity than steel and provides a higher alloy cooling rate. In this work, the solidification conditions in graphite and steel molds are compared. Cylindrical ingots of A356 aluminum and AZ81 magnesium alloys were cast into steel and graphite molds, and the temperature was recorded by thermocouples positioned in both the ingots and the molds. Using the error function, which is a measure of the difference between the experimental and simulated temperatures, the interfacial heat transfer coefficient (IHTC) versus the ingot surface temperature curves were obtained. The peak IHTC for ingot casting in the graphite mold was 2–3 times higher than that for the steel mold for both alloys. The solidification time and cooling rate for the solidification of the A356 and AZ81 ingots with varied sizes in the graphite and steel molds were calculated based on the simulation results. The duration of the solidification process in the steel mold was nearly two times longer than that for ingots in the graphite mold. The cooling rate in the graphite mold was nearly 1.5–2 times higher than that in the steel molds.
... The increase may result in better contact between the melt and chill surface with a higher superheat, because the melt with a higher superheat has a small viscosity and surface tension. Thus, more heat flows through the chill due to the higher initial temperature difference [30]. ...
An experimental apparatus for simulation of continuous casting process of GCr15 bearing steel billet is established. With the apparatus, the billets of diameter 140 mm are casted in various superheats and cooling conditions. The solidification macrostructure, dendrite morphology, segregation and carbide are investigated. It is shown that melting superheats and cooling conditions remarkably influence the microstructure and solute segregation. It is found that the secondary dendrite arm spacing of the steel increases with the increase of the superheat temperature, and with decrease of the cooling rate. Lower superheat with higher cooling rate promotes the refining of the microstructure. Refining equiaxed grains structure in the centre of the billet leads to lower segregation of carbon. Furthermore, with increasing cooling rates, the spacing of the pearlite laminar is refined and the precipitation of proeutectic carbides is suppressed.
... They highlighted a linear increase of the IHTC with the contact pressure up to 10 MPa. Akar et al. [3] have also investigated the IHTC evolution during the casting process, because the IHTC is an important factor for reliable results on the numerical simulation of casting processes. The IHTC also plays a crucial role in hot/warm sheet metal forming processes, since the heat exchanges on the interface of the sheet are important for the temperature control and, consequently, the mechanical properties of the final part [21,29,33]. ...
Heat flow across the interface of solid bodies in contact is an important aspect in several engineering applications. This work presents a finite element model for the analysis of thermal contact, which takes into account the effect of contact pressure and gap dimension in the heat flow across the interface between two bodies. Additionally, the frictional heat generation is also addressed, which is dictated by the contact forces predicted by the mechanical problem. The frictional contact problem and thermal problem are formulated in the frame of the finite element method. A new law is proposed to define the interfacial heat transfer coefficient (IHTC) as a function of the contact pressure and gap distance, enabling a smooth transition between two contact status (gap and contact). The staggered scheme used as coupling strategy to solve the thermomechanical problem is briefly presented. Four numerical examples are presented to validate the finite element model and highlight the importance of the proposed law on the predicted temperature.
In this study, the time-dependent variation of interfacial heat transfer coefficient (IHTC) for metal molds at different mold temperatures was numerically investigated and the time-dependent interfacial heat transfer mechanisms were determined. For this purpose, IHTC, temperature distribution and heat transfer of aluminum alloy A383 for metal molds were numerically investigated. The interfacial heat transfer coefficient between the mold and the cast part was calculated at a different preheating temperature (493 K, 543 K and 593 K) and the time dependent temperatures of the A383 casting alloy cast at 973 K. The calculated temperatures were used to define the impact of mold temperature on the IHTC. The average IHTC for casting temperatures of 493 K 543 K and 593 K were calculated as 16453 W/m2K, 17441 W/m2K and 18127 W/m2K, respectively. As a result, the IHTC increased with rising up mold temperature.
Pressure casting process, which is based on the principle of filling and solidifying the liquid metal into the mold cavity with the effect of speed and pressure, enables to obtain a serial product. The pressure casting process usually involves a thermal process. Starting with the casting process, the thermal resistances, especially formed at the casting mold interface, and the resultant interfacial heat transfer coefficient (IHTC) are among the most important factors determining the mechanical and physical properties of the produced part. The IHTC depends on the mold temperature, casting temperature, injection pressure, injection rate, vacuum application and many other incalculable parameters. In this study, it was aimed to determine the heat transfer coefficient and heat flux of the casting mold interface which has a significant effect on the quality of parts in the pressure casting of cylindrical mold geometry of AlSi8Cu3Fe aluminum alloy. The study was carried out depending on different casting temperatures, injection pressure, injection speed and vacuum application to the mold cavity. Temperatures were measured with thermocouples placed in the mold and casting material, IHTC and heat flux were calculated with finite difference method by using experimentally measured temperatures. In the application of artificial intelligence methods, casting temperature, injection speed, injection pressure and vacuum conditions are given as input parameters and interfacial flow coefficient and heat flux are accepted as output parameters. With the help of these parameters, DTR, MLR and ANNR deep learning algorithms were used to estimate the interfacial heat transfer coefficient. Among these algorithms, ANNR algorithm was found to be the most accurate estimating model at the rate of 99.9%. For the obtained model, a computer program was prepared for the users to be able to see and follow the experimental results and the results obtained from the model at the same time.
Thermal resistances in the mould, die-mold interface and mould material are most important parameters for produced material's mechanical, physical and solidification properties in die casting. Particularly, determining of heat transfer in die and mould is important for quality of die material. For this aim, die-mould interfacial heat transfer coefficient (IHTC), temperature distribution and heat flux are determined according to different mould initial temperatures during injection process of A413 aluminum-silicon alloy. Temperatures are measured based on time by 24 pieces of thermocouples which are mounted at different depths of die and mould material. Measured and calculated temperature values are concordant. Interfacial heat transfer coefficient (h) and heat flux (q) are calculated with finite difference method (FDM) by FORTRAN programming language using experimentally measured temperatures values. Additionally, dynamic parameters of cold chamber machine (injection pressure and speed) are recorded with dynamic parameter measuring device as a function of time. As a result, interfacial heat transfer coefficient, temperature distribution and heat flux are determined as a function of time. The variations of heat transfer coefficients and heat flux are presented according to mould initial temperatures.
The heat transfer coefficient on the casting-mold interface is an important factor which determines the structural and mechanical properties of the casting, and controls the solidification rate of the casting. It is affected by many parameters such as surface roughness, mould material, type of alloy, direction of solidification, mould and casting temperatures, expansion of the mold, contraction of the metal, and air gap on the interface. In this study, it is aimed to investigate the change of the interfacial heat transfer coefficient with temperature of the mold, and to identify the metal-mold interfacial heat transfer mechanisms depending on the temperature of the mold. Timetemperatures relationships were measured with copper chill having different preheat temperatures (323 K, 373 K, and 423 K) in upwards solidifying commercial ETIAL-220 aluminum alloy which is poured at 1023 K. The measured temperatures have been used to determine the effect of mould temperature on the interface heat transfer coefficient. The casting-mold interfacial heat transfer coefficient increased with increasing preheat temperature of the chill (mold). Maximum interface heat transfer coefficients for 323 K, 373 K, 423 K chill temperatures was calculated 12 000 W/m2K, 20 000 W/m2K and 25 000 W/m2K, respectively.
A combined theoretical and experimental program was carried out to study the effect of applied pressure and die coatings on the heat transfer coefficient at the metal-die interface during solidification of AlSi eutectic alloy against an H-13 die. The castings were unidirectionally solidified in a commercial hydraulic press modified to accept a controlled temperature die set. The die thermal behavior was experimentally determined from thermocouples located at different distances from the metal-die interface. A one-dimensional computer heat flow program was used to correlate measured and simulated die temperatures and hence to deduce the heat transfer coefficients. It was found that application of a pressure of 19.6 × 107 Pa increases the heat transfer coefficient from about 3.4 × 103 to 5.25 × 104 W m−2 K−1. The effects of different die coatings on the heat transfer coefficient were also determined.
A study has been made of the effect of casting size, casting alloy, mold (or chill) material, and geometry on the heat transfer coefficient at the casting/mold interface. The effect of geometry has been found to be the overriding factor in determining the interface coefficient. The heat transfer coefficient decreases from a maximum value to a study value if the casting contracts away from the mold. A high value of the coefficient is maintained if geometry causes the casting to remain in intimate contact with the mold. Mold or chill material has little effect on the coefficient of heat transfer. The alloy being cast similarly has only a small effect compared to geometry. Casting size governs the time frame of the transient interface properties. The use of a constant value for the interface coefficient is only valid where the time to reach a steady value is small compared to the total casting freezing time. A correlation was made of the interface heat transfer coefficient with the casting surface temperature and the mold surface temperature at the interface. The coefficient was found to be strongly related to the casting surface temperature but independent of mold surface temperature.
The interfacial heat transfer coefficient plays an important role in the rate of heat transfer from the metal to the mould. The macro and microstructures of castings are mainly related on the heat transfer rate. The objective of the present work was to investigate the effect of surface roughness of chill on the interfacial heat transfer coefficient (IHTC) for vertically upward unidirectional solidification of a eutectic Al-Si casting on water cooled steel chills during solidification. The interfacial heat transfer coefficient measured as a function of different surface roughness of the chill was determined by using finite difference method solving FORTRAN language. The obtained results; the heat transfer coefficients increases as chill surface roughness decreases, the maximum heat transfer coefficient values were obtained; 3320 W/m 2K with 0.15 μm, 3302 W/m 2K with 0.22 μm and 2954 W/m 2K with 2.47 μm chill surface roughness, the interfacial heat transfer coefficients have been expressed as a power function of time, the sensitivity of numerical solution increased as Fourier number decreases, the optimum solution were achieved with Fo=0.0625 and Δx=0.25 mm by considering the calculation time.
During the solidification of metal castings, an air gap usually develops at the interface between the solidifying metal and the surrounding mold or chill. This condition occurs in most casting geometries except in cases such as the cast metal solidifying around a central core. For the purpose of heat transfer calculations, the effect of this air gap is traditionally described by an overall heat transfer coefficient between the casting and the mold (chill). The present study describes two independent means for determining the time varying interfacial heat transfer coefficient for a simple casting geometry with one dimensional heat flow. These two methods are 1) computer solution of the inverse heat conduction problem using thermocouple measurements at selected locations in the casting and mold or chill and 2) measurement of the variation of interfacial gap size with time and, deriving the interfacial heat transfer coefficient from heat transfer data across a static gap. (Edited author abstract. )
The interfacial heat transfer coefficient was measured for a typical diecasting situation in which alloys of Al-4.5wt%Cu were unidirectionally solidified by casting onto a copper chill. To investigate the mechanism of heat transfer at the casting-chill interface the surface roughness of the chills was varied to obtain Ra values of from 0.02 to 0.47 μm. Anodising the casting surfaces revealed the features associated with initiation of solidification and the structures responsible for surface roughness. The cast surfaces had Ra values of from 0.7 to 3.0μm, consistently greater than the surface roughness of the chill surface against which they formed. Moreover, the casting surfaces were convex towards the plane surface of the chill on a scale of from 6 to 23μm. These metallographic examinations of the casting surface and measurements of its surface roughness and planeness were used to interpret the mechanisms occurring at the casting-chill interface which influenced the value of the heat transfer coefficient measured during the course of solidification.
1. Foundations of Heat Transfer. 2. Steady Conduction. 3. Unsteady/Steady, Multidimensional Conduction. 4. Computational Conduction. 5. Foundations of Convection. 6. Correlations for Convection. 7. Heat Exchangers. 8. Foundations of Radiation. 9. Enclosure Radiation. 10. Gas Radiation. 11. Phase Change.
Heat transfer at the metal/chill interface during solidification of commercially pure aluminium square bar castings with cast iron chill at one end was investigated. Experiments were carried out for different chill thicknesses and superheats. The inner surface temperature of the chill initially was found to increase at a faster rate for higher superheats. The effect of chill thickness on the inner surface temperature of the chill was observed only after the heat from the solidifying casting had sufficient time to diffuse to the interior of the chill material.Inverse analysis of the non-linear one-dimensional Fourier heat conduction equation indicated the occurrence of peak heat flux at the end of filling of the mould. The effect of superheat on heat flux was minimal after filling. However, the effect of chill thickness had a significant effect on the heat flux after the occurrence of peak heat flux. Higher heat flux transients were estimated for castings poured at higher superheats. The corresponding heat transfer coefficients were also estimated and reported. The heat flux model presented in this work can be used for determination of casting/chill interfacial heat flux as a function of chill thickness and superheat. These heat flux transients could be used as boundary conditions during numerical simulation of solidification of the casting.
An experimental and numerical study were made on the time-varying heat transfer coefficient h(t) between a tube-shaped casting and metal molds. One dimensional treatment was adopted in analyzing the heat flows between the casting and the inner and the outer mold. The sequential function specification method was employed to solve the nonlinear inverse heat conduction problem. In order to investigate the different behavior of h(t) for different alloys, casting experiments were carried out with three Al-base alloys and pure Al having different types of solidification behavior. It was found that the temperature change of the outer mold showed a normal heating and cooling curve. However, that of the inner mold was unusual especially for the alloys with a wide solidification range, i.e. the temperature increases first rapidly, then halts for a while and then increases again showing finally a regular heating and cooing curve. The resulting heat transfer coefficient at the interface to the inner mold hi(t) decreases temporarily and then increases, while the one at the interface to the outer mold ho(t) decreases monotonously to a quasi steady state. The abnormal heat transfer phenomenon at the inner interface for the alloys with a wide solidification range was concluded to be caused by a slight movement of the semi-solid inner wall at the inside of the tube-shaped casting due to the solidification contraction of the casting freezing in a mushy type.
In the casting processes, the heat transfer coefficient at the metal/mould interface is an important controlling factor for the solidification rate and the resulting structure and mechanical properties. Several factors interact to determine its value, among which are the type of metal/alloy, the mould material and surface conditions, the mould and pouring temperatures, casting configuration, and the type of gases at the interfacial air gap formed. It is also time dependent. In this work, the air gap formation was computed using a numerical model of solidification, taking into consideration the shrinkage and expansion of the metal and mould, gas film formation, and the metallostatic pressure. The variation of the air gap formation and heat transfer coefficient at the metal mould interface are studied at the top, bottom, and side surfaces of Al and Al–Si castings in a permanent mould in the form of a simple rectangular parallelepiped. The results show that the air gap formation and the heat transfer coefficient are different for the different casting surfaces. The bottom surface where the metallostatic pressure makes for good contact between the metal and the mould exhibits the highest heat transfer coefficient. For the sidewalls, the air gap was found to depend on the casting thickness as the larger the thickness the larger the air gap. The air gap and heat transfer coefficient also depend on the surface roughness of the mould, the alloy type, and the melt superheat. The air gap is relatively large for low values of melt superheat. The better the surface finish, the higher the heat transfer coefficient in the first few seconds after pouring. For Al–Si alloys, the heat transfer coefficient increases with increasing Si content.
In this work, metal–mold heat transfer coefficients (h) are determined during unidirectional solidification of Al–Cu and Sn–Pb alloys. The effects of casting assembly (horizontal and vertical), alloy composition, material and thickness of the mold and melt superheat are investigated. By using measured temperatures in both casting and metal, together with numerical solutions of the solidification problem, metal–mold heat transfer coefficients are quantified based on solution of the inverse heat conduction problem. Experimental temperatures are compared with simulations furnished by an explicit finite difference numerical model, and an automatic search selects the best theoretical–experimental fitting from a range of values of metal–mold heat transfer coefficients. Experiments were conducted to analyze the evolution of h during solidification of Al–2, 4.5, 5, 8, 10, 15, 33 wt% Cu alloys and Sn–5, 10, 15, 20, 30, 39 wt% Pb in horizontal and vertical steel chills. The results permitted the establishment of expressions as a power function of time, for different alloy compositions, casting assembly material and thickness of the mold and melt superheat.
Solidification of metal castings inside moulds is mainly dependent on the heat flow from the metal to the mould which is in turn proportional to an overall heat transfer coefficient h which includes all resistances to heat flow such as the presence of an air gap. In the present work the heat transfer coefficient is determined using a directional solidification set-up with end chill for solidifying commercial-purity aluminium with different superheats (40 K and 115 K) against copper chill. A computer program solving the heat conduction and convection in the solidifying metal is used together with the experimental temperature history in order to determine the heat transfer coefficient at the interface. The variation of h as a function of time, surface temperature and gap temperature for each melt superheat is found. The results indicate that h reaches a maximum value for surface temperature close to the liquidus. The analysis of heat flux from the metal to the mould indicates that it is mainly by conduction. The air gap size is evaluated with time, surface temperature and with melt superheat. It is found that higher h values and smaller gap sizes are obtained with higher superheats.
Interfacial heat-transfer coefficients were measured during the solidification of Al-Si alloys against coated die steel chills
with varying chill temperature, coating thickness and coating type. Two principal resistances to heat transfer across the
casting-chill interface were identified, namely, (1) the resistance to heat transfer of the coating itself and (2) the resistance
to heat transfer of a layer of gas, (assumed to be air), trapped between the coating and casting surfaces by virtue of their
roughness. These thermal resistances were evaluated by measurement of the coating thermal conductivity and determination of
the thickness of the applied coatings and the thickness of the layer of air between the coating and casting surfaces. This
produced a simple equation to predict the interfacial heat-transfer coefficient during the solidification of Al alloy die
castings, which produced values that were found to agree well with the experimentally determined results. This equation was
used to interpret the experimentally measured heat-transfer coefficients and to explain their variation with the different
experimental conditions employed. A simple modification of the equation can also take into account the formation of an air
gap, where the casting locally retreats away from the die surface, leading to a local reduction in the heat-transfer coefficient.
During the solidification of metal castings, an interfacial heat transfer resistance exists at the boundary between the metal
and the mold. This heat transfer resistance usually varies with time even if the cast metal remains in contact with the mold,
due to the time dependence of plasticity of the freezing metal and oxide growth on the surface. The present work has studied
interfacial heat transfer on two related types of castings. In the first type, a copper chill was placed on the top of a cylindrical,
bottom gated casting. Using the techniques of transducer displacements and electrical continuity, a clearance gap was detected
between the solidified metal and the chill. The second type of casting had a similar design except that the chill was placed
at the bottom. Owing to the effect of gravity, solid to solid contact was maintained at the metal-chill interface, but the
high degree of interface nonconformity resulted in a relatively low thermal conductance as indicated by solution of the inverse
heat conduction problem. Finally, the influence of interfacial heat transfer on solidification time with three mold ma-terials
is compared by a numerical example, and criteria for utilizing Chvorinov's rule are discussed.
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.
The formation process of the air gap at the casting-mold interface and the heat transfer mechanism through the gap were investigated
by measuring the displacement of, and the temperature in casting and mold for cylindrical and flat castings of aluminum alloys.
The thickness of the air gap was measured as the difference between the location of the casting surface and that of the mold
inner surface. For cylindrical castings, the mold began to move outward immediately after pouring, while the casting stayed
until solidification progressed to a great extent. For flat castings, the mold began to move greatly toward the casting pushing
the casting immediately after pouring and moved reversely after a maximum appeared. It was possible to calculate the displacement
of the mold by thermal expansion. It was found that when the thickness of the air gap was not large, the heat through the
gap was transferred mainly by heat conduction.
Great advances in casting modeling/simulation software have been made in the past several years. However, software predictions
are only as accurate as the material and processing properties used as inputs. The process of low-pressure semipermanent mold
casting is widely used by the automotive industry in the production of aluminum cylinder head castings. However, the process
is highly sensitive to various casting parameters and many of the influences are not well understood. This report presents
results from casting experiments and simulations using a specially designed casting, the “wing casting.” The casting experiments
demonstrated the variation in temperature change in different parts of the casting for different processing parameters. Boundary
conditions were determined for different areas on the mold surface and those parameters were discussed in terms of temperature
sensitivity in the casting behavior. It was also demonstrated that boundary conditions play a major role in the thermal behavior
in the casting during solidification. Finally, changes in the mold/casting surface geometry were found to have a large influence
on the value of the boundary conditions.
The heat-transfer coefficient was measured during the unidirectional solidification of Al-7 wt pct Si alloy castings against
a water-cooled Cu chill. Heat-transfer coefficients in the range of 2.5 to 9 kW m−2 K−1 were obtained with solidification vertically upward associated with higher values than solidification vertically downward.
Horizontal solidification was associated with intermediate values. Profiles taken across the diameters of the casting surfaces
at the interface with the chill showed them to be convex toward the chill by an amount which would produce a mean gap between
the casting and the chill that would account for a significant proportion of the measured heat-transfer coefficient. The convex
casting surfaces were attributed to the deformation of the initial casting skin by the thermal stress produced during solidification.
Heat transfer during casting solidification is shown to be a complex mechanism controlled by the microscale surface roughness
of the respective surfaces, mesoscale deformation of the casting skin by thermal stress, and macroscale movements of the casting
and the chill due to their relative thermal expansion and contraction.
The interfacial heat transfer between a solidifying molten metal and a metallic substrate is critical in many processes such as strip casting and spray deposition. As the molten metal cools down and solidifies, the interface undergoes a change from the initial liquid/solid contact to a solid/solid contact, leading to very dynamic variations in the rate of interfacial heat transfer. This article presents the results of an experimental study of the contact heat transfer when molten nickel or copper droplets are dropped on an inclined metallic substrate. The interfacial heat transfer coefficient, h, between the melt and the substrate is evaluated by matching model calculations with the top splat surface temperature history measured by a fast-response pyrometer. The results suggest that a high value of the interfacial heat transfer coefficient h (104 to 3×105 W/m2 K) is achieved when the molten splat is in contact with the substrate, followed by a smaller value (<104 W/m2 K) during the later stages of solidification and the solid cooling phase. A parametric study was performed to investigate the effect on h of the metal/substrate materials combination, the melt superheat, and the substrate surface roughness, and some of the results are also presented.
The present work focuses on the determination of transient mold–environment and metal–mold heat transfer coefficients during solidification. The method uses the expedient of comparing theoretical and experimental thermal profiles and can be applied both to pure metals and metallic alloys. A solidification model based on the finite difference technique has been used to provide the theoretical results. The experiments were carried out by positioning the thermocouples in both metal and mold. The comparison between experimental and theoretical results is made by an automatic search of the best fitting among theoretical and experimental cooling curves simultaneously in metal and in mold. This has permitted the evaluation of the variation of heat transfer coefficients along the solidification process in unsteady state unidirectional heat flow of Al–Cu and Sn–Pb alloys, as well as the analysis of the effects of the material and the thickness of the mold and melt superheat.
The estimation of the surface temperature or heat flux density utilizing a measured temperature history inside a heat-conducting solid is called the inverse heat conduction problem. This problem becomes nonlinear if the thermal properties are temperature-dependent. A new finite-difference method is given. It is based in part upon the concepts of a general technique for solving inverse problems called nonlinear estimation. The method (or family of methods) estimates the components of the heat flux one at a time and thus, may be considered an on-line method. Another method is outlined for which all of the components of the heat flux are found simultaneously. As suggested by the developments in nonlinear estimation, the sensitivity coefficients can be utilized to gain insight into these methods. The sensitivity coefficients help indicate that the on-line method requires "future" temperatures when small time steps are to be used. Several examples of the use of the on-line method are given. These examples are for cases with non-exact data. The results demonstrate a method that is rather remarkable in its ability to extract information about the surface condition from experimental measurements that lag and are damped compared to the surface condition.
In this study, the interfacial heat transfer coefficient (IHTC) for vertically upward unidirectional solidification of a eutectic Al–Si casting on water cooled copper and steel chills was measured during solidification. A finite difference method (FDM) was used for solution of the inverse heat conduction problem (IHCP). Six computer guided thermocouples were connected with the chill and casting, and the time–temperature data were recorded automatically. The thermocouples were placed, located symmetrically, at 5 mm, 37.5 mm and 75 mm from the interface. As the lateral surfaces are very well heat isolated, the unidirectional solidification process starts vertically upward at the interface surface. The measured time–temperature data files were used by a FDM using an explicit technique. A heat flow computer program has been written to estimate the transient metal–chill IHTC in the IHCP. The experimental and calculated temperatures have shown excellent agreement. The IHTC during vertically upward unidirectional solidification of an Al–Si casting on copper and steel chills have varied between about 19–9.5 kW/m2 K and 6.5–5 kW/m2 K, respectively.
In the numerical simulation of casting solidification, the thermal behavior of the casting/mold interface is characterized by the interfacial heat transfer coefficient, ‘h’. The determination of h is difficult as it involves the solution of the Inverse Heat Conduction Problem (IHCP). One of the satisfactory solution procedures for solving the IHCP is the Beck's non linear estimation procedure. In this work, this procedure has been used successfully by the authors for the determination of h in steady state unidirectional heat flow.
Figure 5. Evolution of the metal/mold heat flow as a function of superheat temperatures. Figure 6. Evolution of the metal/mold heat flow as a function of casting height
- H M Sah
- In
- Al
H. M. ¸ SAH ˙ IN ET AL. Figure 5. Evolution of the metal/mold heat flow as a function of superheat temperatures. Figure 6. Evolution of the metal/mold heat flow as a function of casting height.
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