Transient thermal analysis of solidification in a centrifugal casting for composite materials containing particle segregation

One-dimensional heat-transfer analysis during centrifugal casting of aluminum alloy and copper base metal matrix composites containing Al₂O₃, SiC_p, and graphite particles has been studied. The model of the particle segregation is calculated by varying the volume fraction during centrifugal casting, and a finite difference technique has been adopted. The results indicate that the thickness of the region in which dispersed particles are segregated due to the centrifugal force is strongly influenced by the speed of rotation of the mold, the solidification time, and the density difference between the base alloy and the reinforcement. In the case where the base alloy density is larger than that of the particles, the thickness of the particle-rich region near the inner periphery decreases with an increase in speed, thereby increasing the volume fraction of dispersion. The solidification time of the casting is also dependent upon the speed of rotation of the mold, and it decreases with an increase in speed. This study also indicates that the presence of particles increases the solidification time of the casting.     

Analytical,numerical, and experimental analysis of inverse macrosegregation during upward unidirectional solidification of Al-Cu alloys

The present work focuses on the influence of alloy solute content, melt superheat, and metal/mold heat transfer on inverse segregation during upward solidification of Al-Cu alloys. The experimental segregation profiles of Al 4.5 wt pct Cu, 6.2 wt pct Cu, and 8.1 wt pct Cu alloys are compared with theoretical predictions furnished by analytical and numerical models, with transient h _i profiles being determined in each experiment. The analytical model is based on an analytical heat-transfer model coupled with the classical local solute redistribution equation proposed by Flemings and Nereo. The numerical model is that proposed by Voller, with some changes introduced to take into account different thermophysical properties for the liquid and solid phases, time variable metal/mold interface heat-transfer coefficient, and a variable space grid to assure the accuracy of results without raising the number of nodes. It was observed that the numerical predictions generally conform with the experimental segregation measurements and that the predicted analytical segregation, despite its simplicity, also compares favorably with the experimental scatter except for high melt superheat.     

The movement of the concave casting surface during mushy-type solidification and its effect on the heat-transfer coefficient

The air gap formation process at the casting/mold interface of a hollow cylinder casting was investigated for alloys solidifying in a mushy type by measuring the displacements of the casting and the mold surfaces during solidification. The formation process of the air gap between the convex casting surface and the outer mold and the heat-transfer coefficient through the gap have been well documented by previous publications. However, the air gap between the concave casting surface and the inner mold, or the core, was found to form differently during mushy solidification, in which the air gap formed during solidification, reached a maximum gap distance, and then decreased due to the contraction of the solidified casting on the expanding inner mold. The gap formation was caused by an inward collapse of the coherent dendrite networks at the concave interface because of low pressure inside of the casting due to solidification shrinkage. The coherent dendrite networks at the convex interface did not collapse inward. The heat-transfer coefficients estimated by measuring the air gap thickness showed a similar tendency to the calculated values obtained by the inverse heat-conduction analysis.     

The heat-transfer coefficient during the unidirectional solidification of an Al-Si alloy casting

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.     

Effect of interactions between bubbles and graphite particles in copper alloy melts on microstructure formed during centrifugal casting: Part I. Theoretical analysis

Frequently, particles get associated with gas bubbles in a melt and their interaction influences the final distribution of particles and porosity in the casting. An analytical model for the separation of a particle from a bubble in melts containing dispersed particles and bubbles is proposed. During centrifugal casting of alloys containing dispersed particles, both the particles and gas bubbles present in the melt move with the centrifugal forces. Using the force balance between surface tension and net centrifugal forces (centrifugal force minus buoyancy force), the critical rotational speed of the mold for the separation of the particles and the bubbles during centrifugal casting is calculated. The critical rotational speed of the mold to separate the particle from the bubble is lower for a small particle attached to a larger bubble, as compared to the case when a large particle is attached to a smaller bubble. For a given bubble size, the critical rotational speed of the mold to separate the bubble from the particle decreases with increasing particle size. For the specific case of spherical 5-μm radius graphite particles dispersed in copper alloy melt, it was found that even at a low semiapical angle of about 9 deg, the critical rotational speed needs to be around 5000 rpm for a bubble size of 500-μm radius and 0.09-m-diameter mold. The rotational speed decreases to 1000 rpm when the graphite particle radius increases to 100 μm for the same bubble size in copper alloy melt.     

Steel casting by diffusion solidification

A new process for casting and welding carbon steels is described in which carbon diffuses isothermally or adiabatically within an intimate mixture of solid low carbon steel and high carbon liquid iron to effect solidification and subsequent homogenization with respect to carbon. Advantages over conventional casting processes and products result from 1) 150 to 200‡C lower casting temperature; 2) reduced solidification shrinkage, obviating the need for risers in most cases; and 3) more rapid solidification, especially for castings with large ratios of volume to area. In its most versatile form the process involves low pressure forced infiltration of a mold filled with preheated spherical low carbon steel particles by a higher-carbon liquid. The process can reliably produce castings with greater than 99 pct of theoretical density; solidification times typically range from a few seconds to several minutes; and tensile strengths as high as 185 ksi with 15 pct reduction of area to break have been attained. The ductility of such castings is approximately one order of magnitude more sensitive to total oxygen content than the ductility of wrought steels, probably because of cavitation nucleated by oxides during solidification of the pools of liquid trapped between the shot particles. An analysis of the kinetics of the infiltration and solidification is per-formed for steel casting by diffusion of carbon, manganese or heat in iron. The iron-carbon system is most tractable; steel casting by thermal diffusion has also been demonstrated but no attempt was made to test the iron-manganese system. GEORGE LANGFORD, formerly with the Monsanto Triangle Park Development Center, Inc.     

Steel casting by diffusion solidification

A new process for casting and welding carbon steels is described in which carbon diffuses isothermally or adiabatically within an intimate mixture of solid low carbon steel and high carbon liquid iron to effect solidification and subsequent homogenization with respect to carbon. Advantages over conventional casting processes and products result from 1) 150 to 200°C lower casting temperature; 2) reduced solidification shrinkage, obviating the need for risers in most cases; and 3) more rapid solidification, especially for castings with large ratios of volume to area. In its most versatile form the process involves low pressure forced infiltration of a mold filled with preheated spherical low carbon steel particles by a higher-carbon liquid. The process can reliably produce castings with greater than 99 pct of theoretical density; solidification time typically range from a few seconds to several minutes; and tensile strengths as high as 185 ksi with 15 pct reduction of area to break have been attained. The ductility of such castings is approximately one order of magnitude more sensitive to total oxygen content than the ductility of wrought steels, probably because of cavitation nucleated by oxides during solidification of the pools of liquid trapped between the shot particles. An analysis of the kinetics of the infiltration and solidification is performed for steel casting by diffusion of carbon, manganese or heat in iron. The iron-carbon system is most tractable; steel casting by thermal diffusion has also been demonstrated but no attempt was made to test the iron-manganese system. GEORGE LANGFORD, formerly with the Monsanto Triangle Park Development Center, Inc.     

冷冻铸造A356铝合金微观组织分析

基于数字化无模冷冻铸造精密成形技术实现了冷冻砂型的快速成形,对其浇注A356高温铝合金获得冷冻铸造平板试件。采用电子探针显微分析技术对冷冻铸造和树脂砂型铸造铸件微量元素的分布进行了表征,同时对冷冻铸造和树脂砂型铸造铸件断裂形貌进行了分析。结果表明,冷冻铸造Si元素在铝基体相中的溶解度较树脂砂型铸造显著提高,冷冻铸造较树脂砂型铸造试件中Mg元素分布均匀,树脂砂型铸造试件中出现较多的Mg元素成分偏析区;冷冻铸造试件断裂方式为韧性和脆性的混合断裂模式,树脂砂型铸造试件的断裂形貌为解理台阶破坏形貌和长方状的撕裂结构形貌,合金偏向于脆性断裂。      

Effect of interactions between bubbles and graphite particles in copper alloy melts on microstructure formed during centrifugal casting: Part II. Experiments

During centrifugal casting of copper alloys containing graphite particles, both particles and bubbles move under the influence of centrifugal forces and influence the final microstructure, including porosity and the distribution of graphite. The movement of graphite particles and bubbles in the melts of copper alloys, originally containing 7 and 13 vol pct graphite particles and centrifugally cast at 800 and 1900 rpm in horizontal rotating molds, has been examined. Microstructural observations of sections of these centrifugal castings show that the graphite particles are segregated near the inner periphery and the amount of porosity in the graphite-rich zone is higher than the porosity in the graphite-free and transition zones. The intimate association of porosity with graphite particles in the graphite-rich zone was explained on the basis of attachment of graphite particles to bubbles in the melt and the viscosity of the melt, which increases with increasing concentration of graphite particles near the inner periphery of the castings. It was found that the amount of the porosity in the graphite-rich zone increases with volume fraction of graphite particles used in this study; the size of the porosity in the graphite-rich zone also increases with increasing rotational speed of the mold. This suggests that the graphite particles and bubbles were attached to each other in the melt and they did not get separated during centrifugal casting conditions of the present study. The present experiments qualitatively confirm theoretical computations.     

Metal-Mold interfacial heat transfer

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. Formerly Graduate Student.     

A mold simulator for the continuous casting of steel: Part I. The development of a simulator

Surface defects, such as oscillation marks, ripples, and cracks that can be found on the surface of continuously cast steel, originate in the continuous casting mold. Therefore, a detailed knowledge of initial solidification behavior of steel in a continuous casting mold is necessary because it determines the surface quality of continuously cast slabs. In order to develop an understanding of the initial solidification of continuous cast steels, a “mold simulator” was designed and constructed to investigate heat-transfer phenomena during the initial phase of strand solidification. The mold simulator was used to obtain solidified steel shells of different grades of steel under conditions similar to those found in industrial casting operations. The resulting cast surface morphologies were compared with industrial slabs and were found to be in good agreement, indicating that it is possible to simulate the continuous casting process by a laboratory scale simulator.     

The air-gap formation process at the casting-mold interface and the heat transfer mechanism through the gap

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.     

Prediction of the As-Cast Structure of Al-4.0 Wt Pct Cu Ingots

A two-stage simulation strategy is proposed to predict the as-cast structure. During the first stage, a 3-phase model is used to simulate the mold-filling process by considering the nucleation, the initial growth of globular equiaxed crystals and the transport of the crystals. The three considered phases are the melt, air and globular equiaxed crystals. In the second stage, a 5-phase mixed columnar-equiaxed solidification model is used to simulate the formation of the as-cast structure including the distinct columnar and equiaxed zones, columnar-to-equiaxed transition, grain size distribution, macrosegregation, etc. The five considered phases are the extradendritic melt, the solid dendrite, the interdendritic melt inside the equiaxed grains, the solid dendrite, and the interdendritic melt inside the columnar grains. The extra- and interdendritic melts are treated as separate phases. In order to validate the above strategy, laboratory ingots (Al-4.0 wt pct Cu) are poured and analyzed, and a good agreement with the numerical predictions is achieved. The origin of the equiaxed crystals by the “big-bang” theory is verified to play a key role in the formation of the as-cast structure, especially for the castings poured at a low pouring temperature. A single-stage approach that only uses the 5-phase mixed columnar-equiaxed solidification model and ignores the mold filling can predict satisfactory results for a casting poured at high temperature, but it delivers false results for the casting poured at low temperature.     

The critical amount of nitrogen on the formation of nitrogen gas pores during solidification of 25Cr-7Ni duplex stainless steels

The effects of casting thickness, nitrogen contents, cooling rate, and Mn contents on the formation of nitrogen gas pores during solidification of 25Cr-7Ni-1.5Mo-3W duplex stainless steels (DSS) were quantitatively investigated. In the case of a sand mold, the formation of nitrogen gas pore was not affected by the thickness of castings, which ranged from 13 to 52 mm, and the critical initial nitrogen content for the formation of gas pore was 0.30 wt pct. In the case of the molds made of a stainless steel (STS) and water-cooled Cu, the critical initial nitrogen content did not change much compared to the sand mold. The amount of nitrogen gas pores increased with initial nitrogen contents of castings. The segregation of nitrogen and alloying elements was calculated with Thermo-Calc. The calculated data and the experimental results were compared to estimate the critical nitrogen partial pressure in the residual melt for the nucleation of gas pores. The effect of Mn content on the formation of gas pores was also investigated. The increase of Mn content from 1 wt pct to 2.6 wt pct changed the critical initial nitrogen content 0.30 wt pct to 0.40 wt pct.     

Numerical Simulation and Cold Modeling experiments on Centrifugal Casting

In a centrifugal casting process, the fluid flow eventually determines the quality and characteristics of the final product. It is difficult to study the fluid behavior here because of the opaque nature of melt and mold. In the current investigation, numerical simulations of the flow field and visualization experiments on cold models have been carried out for a centrifugal casting system using horizontal molds and fluids of different viscosities to study the effect of different process variables on the flow pattern. The effects of the thickness of the cylindrical fluid annulus formed inside the mold and the effects of fluid viscosity, diameter, and rotational speed of the mold on the hollow fluid cylinder formation process have been investigated. The numerical simulation results are compared with corresponding data obtained from the cold modeling experiments. The influence of rotational speed in a real-life centrifugal casting system has also been studied using an aluminum-silicon alloy. Cylinders of different thicknesses are cast at different rotational speeds, and the flow patterns observed visually in the actual castings are found to be similar to those recorded in the corresponding cold modeling experiments. Reasonable agreement is observed between the results of numerical simulation and the results of cold modeling experiments with different fluids. The visualization study on the hollow cylinders produced in an actual centrifugal casting process also confirm the conclusions arrived at from the cold modeling experiments and numerical simulation in a qualitative sense.     

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.     

Determination of interface heat transfer coefficient between aluminum casting and graphite mold

To produce castings of titanium, nickel, copper, aluminum, and zinc alloys, graphite molds can be used, which makes it possible to provide a high cooling rate. No die coating and lubricant are required in this case. Computer simulation of casting into graphite molds requires knowledge of the thermal properties of the poured alloy and graphite. In addition, in order to attain adequate simulation results, a series of boundary conditions such as heat transfer coefficients should be determined. The most important ones are the interface heat transfer coefficient between the casting and the mold, the heat transfer coefficient between the mold parts, and the interface heat transfer coefficient into the environment. In this study, the interface heat transfer coefficient h between the cylindrical aluminum (99.99%) casting and the mold made of block graphite of the GMZ (low ash graphite) grade was determined. The mold was produced by milling using a CNC milling machine. The interface heat transfer coefficient was found by minimizing the error function reflecting the difference between the experimental and simulated temperatures in a mold and in a casting during pouring, solidification, and cooling of the casting. The dependences of the interface heat transfer coefficient between aluminum and graphite on the casting surface temperature and time passed from the beginning of pouring are obtained. It is established that, at temperatures of the metal surface contacting with a mold of 1000, 660, 619, and 190°C, the h is 1100, 4700, 700, and 100 W/(m² K), respectively; i.e., when cooling the melt from 1000°C (pouring temperature) to 660°C (aluminum melting point), the h rises from 1100 to 4700 W/(m² K), and after forming the metal solid skin on the mold surface and decreasing its temperature, the h decreases. In our opinion, a decrease in the interface heat transfer coefficient at casting surface temperatures lower than 660°C is associated with the air gap formation between the surfaces of the mold and the casting because of the linear shrinkage of the latter. The heat transfer coefficient between mold parts (graphite–graphite) is constant, being 1000 W/(m² K). The heat transfer coefficient of graphite into air is 12 W/(m² K) at a mold surface temperature up to 600°C.     

Interface heat transfer in investment casting of aluminum alloys

Via design of experiments and using a newly developed inverse method, the heat-transfer boundary conditions in the investment casting process have been studied. It has been shown in the past that these conditions, expressed as interface heat transfer coefficients (HTCs), vary during alloy solidification and cooling. In this work, the authors have studied the additional effects of alloy solidification range, metallostatic head, investment shell thickness, preheat, and interface geometry. This provides an improved set of relationships from which to build realistic boundary conditions into computer simulations of shape casting. Using axisymmetric solidification experiments and numerical inverse analysis, it is shown that the effect of metallostatic head is only significant for long freezing-range alloys. Increasing shell mold thickness and preheat also have effects that are alloy-dependent, and significant differences in thermal behavior are reported between the alloy/mold interface and the alloy/core interface. The four alloys used in the experiments are aluminum-based and vary from short freezingrange commercially pure to an alloy with a freezing range of 120 °C.     

A correlation to describe interfacial heat transfer during solidification simulation and its use in the optimal feeding design of castings

It is known from experimental data that for pure aluminum castings manufactured via the gravity die casting process, the interfacial heat-transfer coefficient can vary in the range 500 to 16,000 W/m² K. These coefficients are of significant importance for the numerical simulation of the solidification process. The experimentally determined variation of interfacial heat-transfer coefficients with respect to time has been recalculated to highlight the variation with respect to casting temperature at the interface. This variation was observed to be of an exponential nature. Also, the pattern of variation was found to be similar in all the experimental results. It has been found that all these patterns of interfacial heat-transfer coefficient variation can be matched by a unique equation that has been proposed as a correlation to model the metal-mold interfacial heat transfer. The benefit of this correlation is in its ability to approximate the combined effects of geometry variation, insulation, chills, die coatings, air gap formation, etc. during the numerical simulation and its use in the optimal design of heat transfer at the metal-mold interface.