Browsing by Author "Prabhu, K."

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A dimensional parameter for prediction of cooling performance of quenchants
(ASTM International, 2014) Prabhu, K.; Ramesh, G.
Computer aided cooling curve analysis was carried out during immersion quenching of ISO/DIS 9950 quench probe. Water, brine solutions, polymer solutions, mineral oils, and vegetable oils were used as quench media. The results showed that the quench medium used had a significant effect on the quench probe cooling curve parameters. An empirical correlation was proposed to predict the average cooling rate from surface tension, wetting angle, thermal conductivity, and kinematic viscosity of the quench medium. © 2014 by ASTM International.
Assessment of axial and radial heat transfer during immersion quenching of Inconel 600 probe
(2014) Ramesh, G.; Prabhu, K.
The time-temperature data at axial and radial locations were measured during immersion quenching of Inconel 600 probe in a mineral oil quench medium. The cooling of probe was not uniform during quenching. The variation of cooling rate along the axial direction was found to be higher than around the radial location. Inverse heat conduction problem (IHCP) was solved for estimating heat flux transients from the temperature data and thermo-physical properties of the Inconel probe. Single and multiple unknown heat fluxes were assigned on the metal/quenchant boundary. The error between the estimated and measured temperatures reduced significantly with increase in number of unknown surface heat flux components. The peak heat flux was about 50% lower for assignment of single unknown heat flux compared to multiple unknown heat fluxes at the metal/quenchant boundary. A plot of isotherms indicated gradual and uniform cooling of the quench probe when single heat flux boundary was used. With increase in the number of heat flux components, non-uniform and large temperature variations in the quench probe were observed. The present work outlines the importance of estimation of spatially dependent boundary heat flux transients during quench heat treatment. © 2014 Elsevier Inc.
Assessment of latent heat and solid fraction of Al-22Si alloy using Newtonian and Fourier analysis techniques
(Trans Tech Publications Ltd ttp@transtec.ch, 2015) Vijayan, V.; Prabhu, K.
Computer aided cooling curve analysis (CACCA) is an online prediction tool for the determination of solidification characteristics of metals or alloys. The results of CACCA can be used to accurately determine latent heat and solid fraction needed for modeling of the solidification process. Newtonian and Fourier analysis techniques adopt a data base line fitting technique to the first derivative curve for calculation of the solid fraction and latent heat of solidification. This paper describes the theoretical and experimental procedures involved Newtonian and Fourier analysis techniques with reference to an Al-22% Si alloy. The correlations between the solid fraction and temperature/time for the alloy were determined. Â© (2015) Trans Tech Publications, Switzerland.
Assessment of wetting kinematics and cooling performance of select vegetable oils and mineral-vegetable oil blend quench media
(Trans Tech Publications Ltd ttp@transtec.ch, 2015) Pranesh Rao, K.M.; Prabhu, K.
Quench hardening is a process where an alloy is heated to solutionizing temperature and held for a definite period, and then rapidly cooled in a quenching medium. Selection of quenchant that can yield desired properties is essential as it governs heat extraction process during quenching. In the present work, the cooling performance of vegetable oil and mineral-vegetable oil blend quench media was assessed. The vegetable oils used in this work were olive oil, canola oil and rice bran oil. The mineral-vegetable oil blends were prepared by blending 10 and 20 vol. % of rice bran and canola oil in mineral oil. Inconel probe of 12.5mm diameter and 60mm height, instrumented with thermocouples were used to characterize quenchants. The probe was heated to 850Â°C and quenched in the oil medium. The cooling curves at different locations in the probe were used to study wetting kinematics. Inverse modelling technique was used to estimate spatially dependent metal-quenchant interfacial heat flux. It was found that the vegetable oils exhibited very short vapour blanket stage compared to mineral oil and blends. Faster wetting kinematics obtained with blends resulted in uniform heat transfer compared to that of mineral oil. The temperature distribution in the probe quenched in vegetable oils and blends was more uniform compared to that in mineral oil. It is expected that the parts quenched in vegetable oils and blends would lead to better hardness distribution compared to mineral oils. Â© (2015) Trans Tech Publications, Switzerland.
Casting/mould interfacial heat transfer during solidification in graphite, steel and graphite lined steel moulds
(Maney Publishing maney@maney.co.uk, 2003) Prabhu, K.; Mounesh, H.; Suresh, K.M.; Ashish, A.A.
Heat flow between the casting and the mould during solidification of three commercially pure metals, in graphite, steel and graphite lined steel moulds, was assessed using an inverse modelling technique. The analysis yielded the interfacial heat flux (q), heat transfer coefficient (h) and the surface temperatures of the casting and the mould during solidification of the casting. The peak heat flux was incorporated as a dimensionless number and modeled as a function of the thermal diffusivities of the casting and the mould materials. Heat flux transients were normalised with respect to the peak heat flux and modeled as a function of time. The heat flux model proposed was used to estimate the heat flux transients during solidification in graphite lined copper composite moulds.
Centrifugal casting and characterisation of primary silicon and Mg2Si dispersed aluminium functionally graded materials
(Trans Tech Publications Ltd ttp@transtec.ch, 2015) Midhun Krishnan, P.; Sanil, S.; Jayakumar, E.; Rajan, T.P.D.; Prabhu, K.
Aluminium based FGM rings, reinforced by in-situ primary Si and primary Si/ Mg2Si hybrid reinforcement were successfully fabricated by centrifugal casting and micro structural, chemical, hardness and corrosion characteristics were evaluated. It was observed that in Al-20Si ring the primary Si particles were present mostly in inner region and few in outer region where as in Al-20Si-3Mg ring both the primary silicon and Mg2Si were completely found in the inner region only resulting in a graded FGM structure. The hardness values were measured along radial direction of samples and variations corresponding to micro structural variation were analysed. Optical Emission spectroscopic studies have revealed the remarkable compositional changes along radial direction. Corrosion characteristics were also evaluated both in particle rich and depleted regions. Â© (2015) Trans Tech Publications, Switzerland.
Characterization of metal-PCMs for thermal energy storage applications
(Trans Tech Publications Ltd ttp@transtec.ch, 2015) Sudheer, R.; Prabhu, K.
In recent years phase change materials have emerged to be ideal energy storage materials for their higher energy density over sensible heat storing materials. Use of phase change materials (PCM) have been successfully implemented at lower temperature applications with various organic compounds. On the other hand, high temperature applications have been solely dominated by various salts, their eutectics and mixtures as phase change materials. This work discusses the suitability of metals and alloys for thermal energy storage applications as the phase change material. Metals offer superior thermal conductivities with considerable energy density compared to salts. Here, two alloys namely, Sn-0.3Ag-0.7Cu (SAC) solidifying over 212-224Â°C and ZA8 (Zn-8%Al) solidifying over 378-405Â°C have been studied. Thermal analysis of PCMs using Computer Aided Cooling Curve Analysis (CA-CCA) and DSC technique were performed to predict the solidification path. In addition to this, Newtonian technique was employed to estimate the latent heat of fusion for these phase change materials. Cooling rate curves and Fraction Solid curves offered a better insight into their ability to receive and discharge heat over the concerned temperature range. Â© (2015) Trans Tech Publications, Switzerland.
Comparative study of wetting and cooling performance of polymer-salt hybrid quench medium with conventional quench media
(Taylor and Francis Ltd., 2015) Ramesh, G.; Prabhu, K.
Wetting kinetics, kinematics, and cooling performance of a polymer-salt hybrid quenchant were investigated. The rewetting phenomenon for brine, water, polymer, and polymer-salt hybrid solutions was characterized as rapid uniform, fast non-uniform, slow uniform, and fast uniform processes, respectively. A dimensionless rewetting time was proposed to assess the nature of the wetting front. The hybrid quenchant showed higher heat transfer during vapor and transition boiling and lower heat transfer during nucleate boiling and convective cooling. The presence of salt in the hybrid solution resulted in early destabilization of the vapor film and an increase in wetting front velocity and rewetting temperature. The polymer constituent delayed the rewetting phenomenon. © 2015 Taylor & Francis Group, LLC.
Comparison of spreading behaviour and interfacial microstructure in Sn-0·7Cu, Sn-0·3Ag-0·7Cu and Sn-2·5Ag-0·5Cu lead free solder alloys on Fe-42Ni substrate
(Maney Publishing michael.wagreich@univie.ac.at, 2013) Satyanarayan, S.; Prabhu, K.
In the present work, spreading behaviour and development of interfacial microstructure in Sn- 0·7Cu, Sn-0·3Ag-0·7Cu and Sn-2·5Ag-0·5Cu lead free solder alloys on Fe-42Ni substrates having two different surface textures were investigated. Smooth textured surfaces yielded better wettability than rough surfaces particularly for Sn-0·7Cu solder alloy. Wettability of Sn-0·7Cu solder on rough textured surfaces was found to be poor compared to other solder alloys. Spreading of Sn-0·7Cu solder on substrate surface showed longer viscous regime, Sn-2·5Ag- 0·5Cu solder exhibited shortest viscous regime. Sn-0·3Ag-0·7 solder showed intermediate behaviour. Sn-Cu solder alloy exhibited needle and coarse shaped (Cu,Ni)6Sn5 intermetallics at the interface and in the matrix of the solder alloy on smooth substrate, whereas on rough substrate, formation of only coarse shaped (Cu,Ni)6Sn5 intermetallic compounds (IMCs) was observed. For Sn-0·3Ag-0·7Cu solder alloy, Fe-Ni-Sn and FeSn2 IMCs identified at the interface. (Cu,Ni)6/Sn5 IMCs were found to be less coarser than as observed at Sn-0·7Cu/substrate interface. Sn-2·5Ag-0·5Cu alloy exhibited (Cu,Ni) 3Sn4 and (Cu,Ni)6 Sn5 IMCs at the interface and in the bulk of solder alloy. © 2013 Institute of Materials, Minerals and Mining.
Computer aided cooling curve analysis and microstructure of cerium added hypereutectic Al-Si (LM29) alloy
(Springer India sanjiv.goswami@springer.co.in, 2014) Vijayan, V.; Prabhu, K.
Thermal analysis of LM29 alloy and Ce added LM29 alloys was carried out. The effect of cerium addition on solidification parameters and microstructural features of hypereutectic Al-Si (LM29) alloy was studied using Newtonian analysis technique. Thermal analysis parameters such as primary and eutectic phase nucleation and solidus temperatures were determined. The addition of Ce to LM29 alloy decreased the nucleation temperature of primary silicon and eutectic silicon. The microstructural examination of Ce added LM29 alloys revealed the presence of a polyhedral shaped Al-Si-Ce compound that might have caused the refinement of primary and eutectic silicon. The dendrite coherency point temperature of LM29 alloy was found to be suppressed on addition of Ce. © 2014 Indian Institute of Metals.
Cooling performance of select mineral oil and polymer quenchants
(ASTM International, 2014) Tiwary, V.; Prabhu, K.
In the present investigation, quench severity was determined for industrial oil quenchants and varying concentrations of PAG polymer. Viscosity, thermal conductivity, density, flash point, and fire point were measured for all quenchants. Cooling curve parameters were determined based on cooling curve analysis ISO/DIS 9950 technique. An Inconel 600 probe was used for this purpose. Severity of quenching was determined based on the Grossmann technique. Viscosity and thermal conductivity had a significant effect on quench severity. Heat flux and HTC at the metal/quenchant interface were computed by both lumped capacitance and Inverse modeling techniques. The effect of ultrasonic and Tensi agitation on cooling curve parameters was assessed. Polymer quenchants of lower concentrations showed marginally higher heat transfer rates compared to water particularly during unagitated condition. © © 2014 by ASTM International.
Dimensionless cooling performance parameter for characterization of quench media
(2013) Ramesh, G.; Prabhu, K.
The effect of varying thermal properties and boundary heat transfer coefficients on temperature profiles inside cylindrical quench probes was simulated during immersion cooling. The results of simulation indicated that, for assessment of the cooling performance of the quench media, the ratio of the quench probe diameter to its thermal conductivity should be less than 0.0005 m2K/W. A simple dimensionless cooling parameter (D 2CR/??T) was proposed to assess the cooling performance of quench media. © 2013 The Minerals, Metals & Materials Society and ASM International.
During soldering, the solder/substrate thermal contact conductance must be high enough to prevent the separation of the solidified shell from the substrate. In the present work, the effect of thermal contact conductance on temperature distribution inside a 60Sn-40Pb solder alloy solidifying against metallic chills was simulated. The results of the simulation indicated that the thermal contact conductance plays a major role during solidification in the solder/substrate interfacial region of the casting particularly when the substrate material has a high thermal conductivity. The influence of solder/chill contact conductance on solidification decreased with increase in the distance from the solder/chill interface.
(Effect of thermal contact conductance on the solidification of a Pb-Sn solder alloy) Prabhu, K.; Kumar, S.T.; Venkataraman, N.
2002
Effect of Ce melt treatment on solidification path of ZA8 alloy
(Institute of Physics Publishing michael.roberts@iop.org, 2016) Sudheer, R.; Vijayan, V.; Prabhu, K.
The solidification path of ZA8 alloy with Ce addition was characterized using Newtonian technique of thermal analysis. The solidification events were determined using cooling curve and its first derivative curve. The microstructure and chemical composition of various phases in the alloy were studied using EDS, SEM and XRD techniques. It was found that the addition of Ce did not cause formation of new phases. However, it hinders the nucleation of stable Î² dendrites in the alloy. The presence of Ce promotes the eutectoid phase transformation and increases the hardness of the alloy. Latent heat of solidification and heat of eutectoid transformation were found to increase on Ce addition. The upward solidification of the alloy against Cu chill was analysed. Chilling had significant influence on solidification parameters, and caused refinement of the microstructure. The addition of Ce to the melt had no effect during chill casting of the alloy.
Effect of cerium addition on casting/chill interfacial heat flux and casting surface profile during solidification of Al-14%Si alloy
(Institute of Physics Publishing michael.roberts@iop.org, 2016) Vijayan, V.; Prabhu, K.
In the present investigation, Al-14 wt. % Si alloy was solidified against copper, brass and cast iron chills, to study the effect of Ce melt treatment on casting/chill interfacial heat flux transients and casting surface profile. The heat flux across the casting/chill interface was estimated using inverse modelling technique. On addition of 1.5% Ce, the peak heat flux increased by about 38%, 42% and 43% for copper, brass and cast iron chills respectively. The effect of Ce addition 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 an air gap at the periphery of the casting. The arithmetic average value of the profile departure from the mean line (Ra) and arithmetical mean of the absolute departures of the waviness profile from the centre line (Wa) were found to decrease on Ce addition. The interfacial gap width formed for the unmodified and Ce treated casting surfaces at the periphery were found to be about 35Î¼m and 13Î¼m respectively. The enhancement in heat transfer on addition of Ce addition was attributed to the lowering of the surface tension of the liquid melt. The gap width at the interface was used to determine the variation of heat transfer coefficient (HTC) across the chill surface after the formation of stable solid shell. It was found that the HTC decreased along the radial direction for copper and brass chills and increased along radial direction for cast iron chills.
Effect of chilling and cerium addition on microstructure and cooling curve parameters of Al-14%Si alloy
(Maney Publishing maney@maney.co.uk, 2015) Vijayan, V.; Prabhu, K.
Al-14%Si alloys, with and without cerium, were cast at varying cooling rates by solidifying them in a crucible and against chills. The effect of melt treatment and chilling on microstructure and cooling curve parameters of the alloy was assessed. Ce treated alloys solidified in clay graphite crucible at a slow cooling rate showed refinement of primary silicon and the formation of Al-Si-Ce ternary intermetallic compound. The addition of Ce to the alloy solidified against chills resulted in simultaneous refinement and modification of primary and eutectic silicon. Nucleation temperatures of both primary and eutectic silicon decreased on addition of cerium. The formation of the intermetallic compound decreased with increase in cooling rate, leading to the modification of the eutectic silicon. The increase in the degree of modification of the eutectic Si was associated with the decrease in the volume fraction of the intermetallic compound formed. © 2015 Canadian Institute of Mining, Metallurgy and Petroleum Published by Maney on behalf of the Institute.
Effect of cooling rate on joint shear strength of Sn-9Zn lead-free solder alloy reflowed on copper substrate
(ASTM International, 2017) Tikale, S.; Sona, M.; Prabhu, K.
Reliability of the solder joint largely depends on mechanical strength, fatigue resistance, coefficient of thermal expansion, and intermetallic compound formation. Cooling rate significantly affects the physical properties of an alloy and influences the mechanical behavior of solder joints. In the present study, Sn-9Zn lead-free solder alloy was solidified on Cu substrate under furnace cooling (0.04°C/s), air cooling (0.16°C/s), and water cooling (94°C/s) conditions. The effect of varying cooling rates on the intermetallic compound (IMC) formation at the interface and the resulting joint shear strength was studied. A microstructure study revealed the presence of Cu5Zn8 and CuZn5 intermetallic compounds at the solder-substrate interface. The IMC layer thickness at the interface increased with a decrease in the cooling rate. The joint shear strength increased with an increase in the cooling rate. The air and furnace cooling resulted in the formation of a thick IMC layer. The IMC obtained from the furnace cooling was associated with micro-cracks leading to a decrease in the joint shear strength. © Copyright 2017 by ASTM International.
Effect of purging gas on wetting behavior of Sn-3.5Ag lead-free solder on nickel-coated aluminum substrate
(Springer Science and Business Media, LLC, 2013) Prabhu, K.; Varun, M.; Satyanarayan, S.
The wetting characteristics of Sn-3.5Ag lead-free solder alloy on nickel-coated aluminum substrates in air (ambient), nitrogen, and argon atmospheres were investigated. The contact angles for the solder alloy obtained under air and argon atmospheres were in the range of 36 -38. With nitrogen atmosphere the contact angle was found to be significantly lower at about 26. Solder solidifying in air exhibited needle-shaped tin-rich dendrites surrounded by a eutectic matrix. The amount of tin dendrites decreased in argon atmosphere. However, the morphology of tin dendrites transformed from needle-shaped to nearly non-dendritic shape as the soldering atmosphere was changed from air to nitrogen. The interfacial microstructures revealed the presence of Ni 3Sn and Ni3Sn4 IMCs at the interface. The enhanced wettability observed under nitrogen atmosphere is attributed to the higher thermal conductivity of nitrogen gas and the formation of higher amount of Ni3Sn IMCs at the interface compared to air and argon atmospheres. © 2012 ASM International.
Effect of reflow temperature and substrate roughness on wettability, IMC growth and shear strength of SAC387/Cu bonds
(2014) Bhat, K.N.; Prabhu, K.; Satyanarayan, S.
The effect of reflow temperature and substrate surface roughness on wettability, intermetallics and shear strength of Sn-3.8Ag-0.7Cu solder alloy on copper (Cu) substrate was studied. It was found that increase in reflow temperature and substrate surface roughness improved the wettability of solder alloy. The size of needle shaped Cu6Sn5 IMCs (intermetallic compounds) increased with increase in temperature. The morphology of IMCs transformed from long to short needles with increase in substrate roughness. Shear strength and shear energy of the solder bond on rough Cu surfaces were found to be higher than that on smooth Cu surfaces. However, the sheared surfaces of the solder bond on rough Cu surface exhibited a transition ridge characterised by sheared IMCs whereas solder bond on smooth Cu surfaces exhibited completely ductile failure. Although, rough surface exhibited higher shear strength and shear energy, smoother surface is preferable due to its predominant bond failure in the solder matrix. © 2013 Springer Science+Business Media New York.
Effect of section thickness and modification on thermal analysis parameters of A357 alloy
(2006) Hegde, S.; Kumar, G.; Prabhu, K.
Thermal analysis technique relies on the cooling curve obtained when the sample is cooled in a sampling cup. This may not represent the cooling behaviour of the real casting. The microstructure developed during solidification depends not only on the nucleation and modification potential of the melt but also on the thermal gradient imposed during solidification by the mould. The factors affecting the thermal gradient are the mould material and casting section thickness. In the present investigation the effect of modification melt treatment, cooling rate and casting section thickness on the thermal analysis parameters of A357 alloy was studied. It is found that the dimensionless heat flux parameter is high for small section thickness castings. The metal/mould interfacial heat flux is high in a copper mould. Thermal analysis parameters of A357 alloy are found to be affected significantly by the combined action of modification, chilling and section thickness. © 2006 W. S. Maney & Son Ltd.