Faculty Publications

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Author: search.filters.author.Prabhu, K.N.Subject: search.filters.subject.Modification

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    Microstructure evolution in AI-7Si-0.3Mg alloy during partial melting and solidification from melt: A comparison
    (2006) Nyamannavar, S.; Ravi, M.; Prabhu, K.N.
    In the present work, a comparative study of microstructure evolution in Sr modified AI-7Si-0.3Mg alloy isothermally held at semi-solid state under conditions of (i) cooling from liquid state and (ii) partial melting from solid state to the semi-solid temperature was carried out. The effect of cooling rate (0.01 to 100 K/s) on the microstructure during solidification of semisolid alloy is studied. Partial melting of alloy results in the fine and more spherical solid phase compared to cooling of the same alloy from liquid state. Chemical modification of the eutectic silicon by Sr addition was found to remain same for both cooling the melt from liquid as well as partial melting from solid state, contrary to the reported results. The morphology of eutectic silicon corresponding to the liquid entrapped in solid phase is finer compared to that in interconnected liquid channel.
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    The effect of strontium modification on casting/chill interfacial heat flux and casting surface profile during solidification of Al-7Si alloy
    (Institution of Engineering and Technology jbristow@theiet.org, 2013) Vijayan, V.; Prabhu, K.N.
    The effect of Sr modification of Al-7%Si alloy on casting/chill interfacial heat flux transients was investigated. The alloy was solidified against polished surfaces of copper, brass and cast iron chills. The heat flux across the casting/chill interface was estimated using inverse modeling technique. On modification melt treatment, the peak heat flux increased by about 70%, 24% and 22% for copper, brass and cast iron chills respectively. The effect of Sr modification on casting surface texture was analyzed using a surface profilometer. The surface profile of the casting and the chill surfaces clearly indicated the formation of anair gap at the periphery of the casting. The arithmetic average value of the profile departure from the mean line, within a sampling length(Ra) and arithmetical mean of the absolute departures of the waviness profile from the centre line within a sampling length(W a) were reduced by 50% on modification. The enhancement in heat transfer achieved due to modification was attributed to the decrease in gap width formed at the interface. The width of the gap formed for the unmodified and modified casting surfaces at the periphery were found to be about 35μm and 19μm respectively. The gap width was used to determine the variation of heat transfer coefficient (HTC) acrossthe chill surface after the formation of stable solid shell.It was found that the HTC decreased along the radial direction.
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    Effect of Ni and Sr additions on the microstructure, mechanical properties, and coefficient of thermal expansion of Al-23%Si alloy
    (Elsevier Ltd, 2021) Vijayan, V.; Ravi, M.; Prabhu, K.N.
    In this paper, the combined and individual effect of strontium (Sr) and nickel (Ni) additions on the microstructure and mechanical properties of hypereutectic Al-23Si alloy was studied using an optical microscopy, universal tensile testing machine and Brinell hardness tester. The wear properties and coefficient of thermal expansion of the melt treated samples were determined using pin on disc wear testing machine and linear expansion measurement unit respectively. The results indicate that the Ni additions significantly enhanced the mechanical properties of the alloy, whereas, presence of Sr was detrimental. It was observed that the improvement in mechanical properties due to Ni was primarily due to refinement and transformation of the star shaped primary silicon into polyhedral crystals. However, the addition of Sr, both as separate and when combined with Ni, transformed the primary silicon into a faceted crystal and hence decreased the mechanical property. Among all, the Ni melt treated alloy samples showed lowest wear rate and coefficient of thermal expansion. © 2021 Elsevier Ltd. All rights reserved.
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    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.
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    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.
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    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.
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    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.