Faculty Publications
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Item Friction factor of CP aluminium and aluminium-zinc alloys(2006) Vidhya Sagar, N.; Anand, K.S.; Mithun, A.C.; Srinivasan, K.Friction factor has been determined for CP aluminium and aluminium-zinc alloys using ring compression test at different temperatures from 303 K to 773 K. It is found that CP aluminium exhibits sticking whereas Al-Zn alloys do not exhibit sticking at elevated temperatures. Hot working of Al-Zn alloy is easier than that of CP aluminium at 773 K. As zinc content increases up to 10 wt% the friction factor decreases up to 0.02. © Indian Academy of Sciences.Item Determination of spread activation energy and assessment of wetting behavior of solders on metallic substrates(American Society of Mechanical Engineers (ASME), 2010) Prabhu, K.N.; Kumar, G.The effects of substrate material, substrate surface roughness, and operating temperature on the wetting behavior of Sn-37Pb, Sn-3.5Ag, and Sn-9Zn eutectic solders on metallic substrates were investigated. Solder spreading kinetics was successfully represented by the exponential power law (EPL): ?=exp (-K ?n). The EPL parameter K has the significance of accelerating the kinetics of relaxation while the parameter n represents the resistance to spreading process (spread resistance parameter). EPL parameters exhibited a decreasing trend with an increase in surface roughness. Estimated activation energies for solder spreading were found to be in between those reported for inert and highly reactive spreading systems. © 2010 American Society of Mechanical Engineers.Item Electrolytic preparation of cyclic multilayer Zn-Ni alloy coating using switching cathode current densities(2010) Venkatakrishna, K.; Hegde, A.C.Cyclic multilayer alloy (CMA) coating of Zn-Ni was developed on mild steel using single bath technique, by proper manipulation of cathode current densities. The thickness and composition of the individual layers were altered precisely and conveniently by cyclic modulation of cathode current densities. Multilayer coatings, having sharp change in compositions were developed using square current pulses. Gelatin and sulphanilic acid (SA) acid were used as additives. Laminar deposits with different configurations were produced, and their corrosion behaviors were studied, in 5% NaCl solution by electrochemical methods. It was observed that the corrosion resistance of CMA coating increased progressively with number of layers (up to certain optimal numbers) and then decreased. Cyclic voltammetry study demonstrated the role of gelatin and SA in multilayer coating. The coating configuration has been optimized for the peak performance against corrosion. The substantial decrease of corrosion rate, in the case of multilayer coatings was attributed to the changed intrinsic electric properties, evidenced by Electrochemical Impedance Spectroscopy (EIS) study. The surface morphology and its roughness were examined by Atomic Force Microscopy (AFM). The surface and cross-sectional view of coatings were examined, using Scanning Electron Microscopy (SEM). X-ray photoelectron spectrum (XPS) study was carried out for surface analysis. The relative performance of pure Zn, monolithic and CMA coatings were compared and discussed. © 2010 Springer Science+Business Media B.V.Item Electroplating and characterization of Zn-Ni, Zn-Co and Zn-Ni-Co alloys(2010) Eliaz, N.; Venkatakrishna, K.; Hegde, A.C.Zn-Ni, Zn-Co and Zn-Ni-Co coatings were electrodeposited on mild steel from an acidic chloride bath containing p-aminobenzenesulphonic acid (SA) and gelatin. These additives changed the phase content in the coatings, most likely as a result of their adsorption at the surface of the cathode. The effect of gelatin was more pronounced than that of SA. The Faradaic efficiency was higher than 90%. As the current density was increased or the bath temperature was decreased, the concentration of the nobler metal in the coating increased. Both concentrations of Ni and Co in the ternary alloy increased as the applied current density was increased. Nickel and cobalt were found to have a synergistic catalytic effect. The thickness of all coatings increased as the applied current density was increased. The hardness increased with current density to a peak value, and then decreased. The rate of Zn deposition was heavily influenced by mass-transport limitation at high applied current densities, while the rates of Ni and Co deposition were not. The anomalous codeposition was explained by the great difference between the exchange current densities of Zn and the iron-group metal. Potentiodynamic polarization scans and electrochemical impedance spectroscopy showed that the corrosion resistance of the ternary Zn-Ni-Co alloy coatings was approximately 10 times higher than that of Zn-Ni and 7 times higher than that of Zn-Co. The improved corrosion resistance of the ternary alloy was attributed to its surface chemistry, phase content, texture, and surface morphology. The ternary Zn-Ni-Co coating may thus replace the conventional Zn-Ni and Zn-Co coatings in a variety of applications. © 2010 Elsevier B.V.Item Wetting behaviour and evolution of microstructure of Sn-Ag-Zn solders on copper substrates with different surface textures(ASTM International, 2011) Satyanarayan, S.; Prabhu, K.N.The effect of surface roughness on wetting behaviour and evolution of microstructure of two lead-free solders (Sn-2.625Ag-2.25Zn and Sn-1.75Ag-4.5Zn) on copper substrate was investigated. Both solders exhibited good wettability on copper substrates having rough surface and lower wettabilty on smooth surfaces. The contact angles of solders decreased linearly with increase in surface roughness of the substrate. The exponential power law, ?=exp(-KT -1), was used to model the relaxation behaviour of solders. A high intermetallic growth was observed at the interface particularly on copper substrates with rough surface texture. A thin continuous interface showing scallop intermetallic compounds (IMC) was obtained on smooth surfaces. With an increase in surface roughness, the IMC morphology changed from scallop shaped to needle type at the Sn-2.625Ag-2.25Zn solder/ substrate interface and nodular to plate like IMCs for Sn-1.75Ag-4.5Zn solder matrix. Copyright © 2010 by ASTM International.Item Development of nano-structured cyclic multilayer Zn-Ni alloy coatings using triangular current pulses(2011) Bhat, R.S.; Hegde, A.C.Cyclic multilayer alloy (CMA) deposits of Zn-Ni were developed on mild steel from sulphate bath having thiamine hydrochloride (THC) and citric acid (CA) as additives. CMA coatings were developed galvanostatically using triangular current pulses, under different conditions of cyclic cathode current density (CCCD's) and number of layers. The corrosion behaviors of the coatings were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy methods, and were compared with that of monolayer Zn-Ni alloy of same thickness. At optimal configuration, CMA coating represented as, (Zn-Ni)2.0/5.0/300 was found to exhibit ?40 times better corrosion resistance compared to monolayer alloy, (Zn-Ni)3.0. Cyclic voltammetry study demonstrated that THC and CA have improved the appearance of the deposit by complexation with metal ions. The corrosion protection efficacy of CMA coatings was attributed to the difference in phase structure of the alloy in successive layers, evidenced by XRD analysis. The formation of multilayer and corrosion mechanism was analyzed by Scanning Electron Microscopy (SEM) study. © 2011 Allerton Press, Inc.Item Corrosion stability of electrodeposited cyclic multilayer Zn-Ni alloy coatings(2011) Bhat, R.S.; Udupa, K.R.; Hegde, A.C.This paper reports on a study of electrodeposition and characterisation of cyclic multilayer coatings of Zn-Ni alloy from a sulphate bath. Cyclic multilayer alloy coatings were deposited on mild steel through the single bath technique by appropriate manipulation of cathode current densities. The thickness and composition of the individual layers of the CMA deposits were altered precisely and conveniently by cyclic modulation of the cathode current during electrodeposition. Multilayer deposits with sharp change in composition were developed using square current pulses, using thiamine hydrochloride and citric acid as additives. Laminar deposits with different configurations were produced and their corrosion behaviours were studied by AC and DC methods in 5%NaCl solution. It was observed that the corrosion resistance of the CMA coating increased progressively with the number of layers (up to certain optimal numbers) and then decreased. The decrease in corrosion resistance at high degree of layering was attributed to interlayer diffusion due to less relaxation time for redistribution of metal ions at cathode during deposition. The coating configurations have been optimised for peak performance of the coatings against corrosion. It was found that CMA coating developed at cyclic cathode current densities of 3.0/5.0 A dm-2 with 300 layers showed the lowest corrosion rate (0.112×10-2 mm/year) which is ?54 times better than that of monolithic Zn-Ni alloy, deposited from the same bath. The protection efficacy of CMA coatings is attributed to the difference in phase structure of the alloys in successive layers, deposited at different current densities, evidenced by X-ray diffraction analysis. The formation of multilayers and corrosion mechanism were examined by scanning electron microscopy. © 2011 Institute of Metal Finishing.Item Effect of cooling rate during solidification of Sn-9Zn lead-free solder alloy on its microstructure, tensile strength and ductile-brittle transition temperature(Elsevier Ltd, 2012) Prabhu, K.N.; Deshapande, P.; Satyanarayan, S.Solidification rate is an important variable during processing of materials, including soldering, involving solidification. The rate of solidification controls the metallurgical microstructure at the solder joint and hence the mechanical properties. A high tensile strength and a lower ductile-brittle transition temperature are necessary for reliability of solder joints in electronic circuits. Hence in the present work, the effect of cooling rate during solidification on microstructure, impact and tensile properties of Sn-9Zn lead-free solder alloy was investigated. Four different cooling media (copper and stainless steel moulds, air and furnace cooling) were used for solidification to achieve different cooling rates. Solder alloy solidified in copper mould exhibited higher cooling rate as compared to other cooling media. The microstructure is refined as the cooling rate was increased from 0.03 to 25 °C/s. With increase in cooling rate it was observed that the size of Zn flakes became finer and distributed uniformly throughout the matrix. Ductile-to-brittle transition temperature (DBTT) of the solder alloy increased with increase in cooling rate. Fractured surfaces of impact test specimens showed cleavage like appearance and river like pattern at very low temperatures and dimple like appearance at higher temperatures. The tensile strength of the solder alloy solidified in Cu and stainless moulds were higher as compared to air and furnace cooled samples. It is therefore suggested that the cooling rate during solidification of the solder alloy should be optimum to maximize the strength and minimize the DBTT. © 2011 Elsevier B.V.Item Effect of Mn on cooling behaviour and microstructure of chill cast Zn-Al (ZA8) alloy(2012) Ramesh, G.; Vishwanatha, H.M.; Prabhu, K.N.In the present work, the effect of manganese addition to ZA8 alloy on thermal analysis parameters, heat transfer and microstructure was investigated. The thermal analysis parameters were found to be significantly affected by chemical modification of ZA8 alloy. Cooling curve and differential scanning calorimetry analyses of modified alloy showed nucleation of new phase other than b dendrites. Chilling of modified alloy resulted in decreased liquidus temperature and enhanced eutectoid transformation. Further, chilling avoids the formation of intermetallic compounds in modified alloy. The heat flux transients were estimated using inverse modelling during solidification of unmodified and modified alloys against different chills. The peak heat flux decreased on addition of Mn to ZA8 alloy. Differential scanning calorimetry analysis indicated that the addition of Mn to ZA8 alloy decreases the heat of solidification. The addition of Mn to ZA8 alloy increased the contact angle, indicating decreased wettability of the modified alloy on the chill surface. The microstructure of ZA8 with Mn showed an increased amount of b phase and a decreased amount of eutectic. X-ray diffraction analysis confirmed the formation of MnAl6 intermetallics in Mn added ZA8 alloy. Chilling with chemical modification resulted in enhanced decomposition of b phase. © 2012 Institute of Materials, Minerals and Mining.Item Magnetically induced electrodeposition of Zn-Ni alloy coatings and their corrosion behaviors(Elsevier B.V., 2013) Rao, V.R.; Bangera, K.V.; Hegde, A.C.The less magnetic features of Zn-Ni alloy compared to Fe-Ni and Fe-Co alloys made it interesting to develop them under the influence of applied magnetic field. In this regard, the effects of a magnetic field (B) applied in a direction parallel and perpendicular to the nominal current, during electrodeposition process of Zn-Ni alloy have been investigated by means of X-ray diffraction and EDX analysis. The modification of crystal orientation by superimposition of a varying magnetic field is studied for alloys of constant nickel content (8 a %.), deposited at optimal current density (j) of 3.0 A dm-2. The effect of magnetic field on crystallographic orientation and hence the corrosion behaviors of the coatings were studied. The preferential orientations (101) and (002) of the zinc phase and (330) ?-Ni 5Zn21 phase are always favored to exist with parallel and perpendicular magnetic field. The preferential (321) ?-Ni 5Zn21 orientation is found to be the characteristic of perpendicular magnetic field. Further, Zn (100) orientation is found to be non-responsive to the effect of parallel magnetic field. The coatings developed using perpendicular magnetic field is more corrosion resistant compare to that for parallel magnetic field. This is attributed to the additional (321) ?-Ni5Zn21 orientations. The changes in the phase structure of the coatings deposited at different magnetic field are attributed to the effect caused by the magnetic convection induced in the electrolytic solution, called MHD effect (magneto-hydrodynamic effect). The chemical composition of the alloy was found to be same in both natural and magnetically induced deposition due to constant Ni content in the bath. The variation in the surface morphology of the coatings was studied by scanning electron microscopy (SEM). The Zn-Ni alloy coating deposited at 0.8 T perpendicular B showed the highest corrosion resistance (with corrosion rate=0.26 × 10-2 mm y-1) compared to the one with no B (corrosion rate = 14.46 × 10-2 mm y-1). The improved corrosion resistance of the coatings was discussed in the light of magnetic field effect on crystallographic orientation. © 2013 Elsevier B.V. All rights reserved.