Faculty Publications
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Item Heat flux transients and casting surface macro-profile during downward solidification of Al-12% Si alloy against chills(American Foundry Society, 2011) Prabhu, K.N.; Sharath, K.; Ramesh, G.Heat flux transients were estimated during downward solidificationofAl- 12%Sialloy(A413)againstaluminumand graphite chills. The thermal plot of graphite chill indicated one-dimensional heat flow in the initial stages which then changes to two-dimensional heat transfer. The heat transfer becomes one-dimensional again during the final stages of solidification. In aluminum chill, heat flow was nearly one- dimensional. Experiments were designed to verify whether the peak heat flux is an artifact of the experiment. The results clearly showed that the occurrence of the peak in the heat flux transients is not an artifact of the inverse model or the experimental technique. The macro-profile of the casting surface in contact with the chill revealed the occurrence of crests and troughs. A mechanism based on the convection within the liquid metal below the solid shell was proposed to account for the formation of wavy casting surface. Copyright © 2011 American Foundry Society.Item Effect of modification melt treatment and chilling on eutectic arrest temperature and time during solidification of A357 alloy(2011) Prabhu, K.N.; Hegde, S.Thermal analysis technique has been recognised as an efficient non-destructive tool to assess the degree of modification in Al-Si alloys. Apart from chemical modification, chilling refines the microstructure. This is particularly significant as majority of Al-Si alloys are cast in metallic moulds. In the present study, the interaction between chilling and modification melt treatment is investigated to assess their effect on thermal analysis parameters using computer aided cooling curve analysis. For modified alloys, the depression of the eutectic arrest temperature was significant at higher cooling rates. The eutectic arrest temperature and time were correlated with the cooling rate using a power law. High cooling regime in thermal analysis plots was attributed to the combined effect of chilling and modification melt treatment on heat transfer. © 2011 Institute of Materials, Minerals and Mining.Item Heat transfer during solidification of chemically modified Al-Si alloys around a copper chill(2011) Prabhu, K.N.; Hegde, S.The solidifying metal/chill contour will significantly affect the boundary heat transfer coefficients, and solidification modellers should be aware of the casting conditions for which the heat transfer coefficients are determined. The previous work carried out on solidification of Al-Si alloys in a metallic mould and solidification against bottom/top chills has shown that modification and chilling have synergetic effect resulting in a significant increase in the heat flux transients at the casting/chill interface. In the present work, the heat transfer during solidification of unmodified and chemically modified Al-Si alloys around a cylindrical copper chill was investigated. Heat flux transients were estimated using lumped heat capacitance method. Lower peak heat flux was obtained with chemically modified alloy. This is in contrast to the results reported for alloys solidifying against chills and in metallic moulds. The chill thermal behaviour and heat transfer to the chill material when surrounded by modified and unmodified alloys were explained on the basis of the decrease in the degree of undercooling in the case of modified alloy as compared to unmodified alloy and the change in contact condition and shrinkage characteristics of the alloy due to the addition of chemical modifiers. © 2011 Institute of Materials, Minerals and Mining.Item Effect of chemical modification of Al-Si alloys on thermal diffusivity and contact heat transfer at the casting-chill interface(2012) Prabhu, K.N.; Jayananda; Hegde, S.The heat flow during the unidirectional downward solidification of Al-7Si and Al-12Si alloys was analyzed using thermal analysis technique and inverse modeling. Chills instrumented with thermocouples were brought into contact with a small pool of liquid metal so as to minimize the effect of convection caused by pouring and temperature gradients. Modification melt treatment resulted in an increase in the cooling rate of the solidifying casting near the casting-chill interfacial region. The corresponding interfacial heat flux transients were also found to be higher. The thermal diffusivities of alloys were measured using a laser pulse technique and were found to be higher for modified alloys. However, the increase in the heat flux transients was attributed mainly to the improvement in the casting-chill interfacial thermal contact condition brought about by the decrease in the surface tension of the liquid metal upon the addition of sodium. Copyright © 2012 by ASTM International.Item The effect of simultaneous refinement and modification by cerium on microstructure and mechanical properties of Al-8% Si alloy(Taylor and Francis Ltd. maney@maney.co.uk, 2016) Vijayan, V.; Prabhu, K.N.The effect of cerium melt treatment on microstructure and mechanical properties of gravity die cast Al-8% Si alloy was studied. The addition of Ce transformed the large columnar ?-Al grains into fine equiaxed grains and modified the acicular eutectic Si into fine fibrous form. The addition of Ce resulted in the formation of needle-shaped complex Ce intermetallic along the grain boundaries. Electrical conductivity measurements are used for the first time to reveal the effect of Ce addition on modification of the eutectic Si. The electrical conductivity of the alloy increased with Ce melt treatment. Scanning Electron Microscopy results suggests that the Ce particles did not heterogeneously nucleate the ?-Al grains, instead the fine equiaxed grains were formed through an invariant reaction between the liquid melt and Ce phase. The ultimate tensile strength, % elongation and hardness of the alloy significantly improved due to simultaneous modification and refinement. © 2016 Informa UK Limited, trading as Taylor & Francis Group.Item The effect of Sr modification on thermal diffusivity of Al–8Si alloy(Taylor and Francis Ltd. michael.wagreich@univie.ac.at, 2018) Vijayan, V.; Prabhu, K.N.Newtonian and Fourier analysis techniques were used to calculate the solid fraction and latent heat during the solidification of Al–8Si alloy and the subsequent results were used to study the effect of Sr modification on thermal diffusivity of the solidifying melt. The results indicate that the thermal diffusivity increased significantly with increase in Sr content. The increase in the thermal diffusivity was attributed to the increase in the electronic conduction of the modified alloy due to the decreased activity of the bifilm. The results also indicate that the latent heat evolved during solidification increased with increase in Sr concentration and was associated with the increase in the solidification range that occurred during the modification process. © 2017 Informa UK Limited, trading as Taylor & Francis Group.Item Thermal Analysis of Cerium-Treated Chill-Cast Al-23 Si Alloy(Springer New York LLC barbara.b.bertram@gsk.com, 2018) Vijayan, V.; Prabhu, K.N.The influence of elemental cerium addition on the cooling curve parameters and microstructure of hypereutectic Al-23Si alloy was investigated in this work. The cooling rate of the treated and untreated alloy samples was varied by solidifying the melt against chills of different materials in a stainless-steel tube. The cooling curve was recorded and nucleation temperatures of various phases in the alloy were measured. The results show that the chilling improves the effectiveness of the modifier. The two main phases of the alloy, primary silicon and eutectic silicon, were nucleated at temperatures of about 661.2 and 571.6 °C, respectively. The addition of Ce to slowly cooled alloys resulted in an increase in undercooling temperature of both the phases, with a decrease in its nucleation temperature, whereas the same additions increased the nucleation temperatures in chilled alloys. Based on the thermal analysis results, a relation between thermal analysis parameter (? TG) and silicon particle size of the alloy was proposed. © 2018, ASM International.Item Wettability and Interfacial Heat Transfer During Solidification of Al–Si Alloy (A413) Melt Droplets on Metallic Substrates(Springer Science and Business Media Deutschland GmbH, 2024) Shamil, K.M.; Kamala Nathan, D.; Prabhu, K.N.Wetting and heat transfer during solidification were studied with Al–Si alloy (A413) melt droplets on copper and stainless steel cylindrical substrates. As the molten metal cools down and solidifies, the interface changes from the initial liquid/solid contact to solid/solid contact, leading to variations in the rate of interfacial heat transfer. The effect of substrate roughness and melt superheat on the droplet contact angle and heat flux was investigated. A smooth substrate surface resulted in higher casting surface roughness, and the contact angle of solidified droplets increased with an increase in substrate roughness. Surface profile analysis indicated the presence of an air gap between the substrate and solidified droplet. The gap width was used to determine the variation of heat transfer coefficient (HTC) along the radial direction of the substrate surface at the end of solidification. Deformation of the solidified droplet was inverse for copper versus stainless steel resulting in an increase in HTC for copper and a decrease for stainless steel. © 2023, American Foundry Society.Item Effect of Mold Contour on Interfacial Heat Transfer During Solidification of AlSi11Cu3Fe Alloy (ADC-12)(Springer Science and Business Media Deutschland GmbH, 2024) Kamala Nathan, D.K.; Prabhu, K.N.The present work investigated the effect of flat, concave, and convex mold contours on heat transfer during the solidification of an aluminum AlSi11Cu3Fe (ADC-12) alloy. Experiments were designed with copper/steel cylindrical and flat molds to study the effect of convex and flat casting/mold interface on heat transfer. To examine the effect of a concave and flat interfaces, an experimental setup consisting of a cylindrical and square bar chill was fabricated. Casting/mold (chill) interfacial heat flux was estimated by solving an inverse heat conduction problem (IHCP). The temperatures measured at locations inside the mold/chill were used as input to the inverse solver. It was observed that the flat contour yielded higher heat flux than a convex contour for both copper and steel molds. Although the volume to surface area (V/A) ratio for castings solidified against a flat and convex interface are the same, the larger mold volume associated with the flat interface yielded higher heat flux transients. Experiments involving chills suggested that the flat interface resulted in higher heat transfer when the (V/A) ratio for the chill was the same. To study and compare the combined effect of mold material and contour on heat transfer during casting solidification, the molds must have the same volumetric thermal effusivity per unit surface area available for heat transfer. © American Foundry Society 2023.Item Effects of Phosphorus Treatment on Cooling Behavior, Heat Transfer, Microstructure, and Mechanical Properties of Hypereutectic Al-23%Si Alloy(Springer, 2025) Vijayan, V.; Prabhu, K.N.The influence of phosphorus (P) treatment on the microstructure, cooling behavior, interfacial heat flux, and mechanical properties of hypereutectic Al-23 Si alloy is investigated in the present work. Computer-aided cooling curve analysis revealed that higher cooling rates suppressed silicon cluster agglomeration and promoted nucleation of primary silicon at lower undercooling. The addition of P increased the nucleation temperature of primary silicon and resulted in finer silicon crystals. Eutectic silicon nucleation was facilitated by P treatment, with refined primary silicon acting as nucleation sites. Interfacial heat flux analysis demonstrated that P addition decreased the heat flux, attributed to the presence of less conductive primary silicon, and unmodified eutectic silicon microstructure. Microstructural analysis revealed the refinement of primary silicon and transformation of its morphology to polyhedral shape with P treatment. Heat treatment improved tensile properties, with refined primary silicon and copper precipitation contributing to enhanced strength. The morphology and composition of copper intermetallic varied with P and Cu content, influencing mechanical properties. These findings provide insights into optimizing alloy compositions and processing conditions for hypereutectic Al-Si alloys in various industrial applications. © ASM International 2023.