Faculty Publications
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Item Microstructure and corrosion behavior of laser processed NiTi alloy(Elsevier Ltd, 2015) Marattukalam, J.J.; Singh, A.K.; Datta, S.; Das, M.; Balla, V.K.; Bontha, S.; Kalpathy, S.K.Abstract Laser Engineered Net Shaping (LENS™), a commercially available additive manufacturing technology, has been used to fabricate dense equiatomic NiTi alloy components. The primary aim of this work is to study the effect of laser power and scan speed on microstructure, phase constituents, hardness and corrosion behavior of laser processed NiTi alloy. The results showed retention of large amount of high-temperature austenite phase at room temperature due to high cooling rates associated with laser processing. The high amount of austenite in these samples increased the hardness. The grain size and corrosion resistance were found to increase with laser power. The surface energy of NiTi alloy, calculated using contact angles, decreased from 61 mN/m to 56 mN/m with increase in laser energy density from 20 J/mm2 to 80 J/mm2. The decrease in surface energy shifted the corrosion potentials to nobler direction and decreased the corrosion current. Under present experimental conditions the laser power found to have strong influence on microstructure, phase constituents and corrosion resistance of NiTi alloy. © 2015 Elsevier B.V.Item Solid state amorphization of Mg-Zn-Ca system via mechanical alloying and characterization(Elsevier B.V., 2017) Manne, B.; Bontha, S.; Ramesh, M.R.; Krishna, M.; Balla, V.K.Magnesium based bulk metallic glasses have attracted significant attention of researchers due to better mechanical and corrosion properties when compared to their crystalline counterparts especially for biomedical applications. Scaling up the part size and production volumes of such materials through liquid metallurgy route is challenging. In this work amorphous Ca5Mg60+xZn35?x (X = 0, 3 and 7) alloys have been successfully synthesized through solid state amorphization using a high energy planetary ball mill. X-ray diffraction was used to identify the crystalline phases of the powder during reaction. Evolution of amorphous phase was analysed using a parameter involving the ratio of integral area of peaks to the integral area of background (IPB) obtained from XRD patterns. Results showed reaction time increases with decreasing Zn content in Ca5Mg60+xZn35?x (X = 0, 3 and 7) alloy to obtain maximum amorphous structure with a small amount of residual crystalline phase. Prolonged milling of these powders, to eliminate residual crystalline phases, resulted in the nucleation of Mg102.08Zn39.6 phase. The composition dependent characteristic temperatures and thermal stabilities were studied using differential scanning calorimetry. © 2016 The Society of Powder Technology JapanItem Effect of heat treatment on microstructure, corrosion, and shape memory characteristics of laser deposited NiTi alloy(Elsevier Ltd, 2018) Marattukalam, J.J.; Balla, V.K.; Das, M.; Bontha, S.; Kalpathy, S.K.The aim of this work is to study the effect of heat treatment on the microstructure, phase transformations, shape memory characteristics and corrosion behaviour of laser deposited equiatomic NiTi alloy. Dense samples of NiTi alloy were fabricated using Laser Engineered Net Shaping (LENS™) with two different laser energy densities by varying the scan speed and laser power. These samples were annealed for 30 min at 500 °C and 1000 °C in flowing argon, followed by furnace-cooling to room temperature. The resulting microstructures and properties were compared with the corresponding as-deposited samples. Microstructural analysis after heat treatment showed needle-shape martensite in the samples processed at lower laser energy density of 20 J/mm2, and lenticular or plate-like martensite in the samples processed at 80 J/mm2. The XRD results revealed relatively high concentration of martensite (B19?) in heat-treated NiTi alloy compared to as-processed samples. Furthermore, the heat treatment decreased the forward and reverse transformation temperatures of NiTi alloy from 80 – 95 °C to 20–40 °C, presumably due to annihilation of thermally induced defects. Interestingly, the samples annealed at 500 °C showed a measurable increase of 1–2% in the shape memory recovery, from the net recovery of 8% exhibited by the as-processed NiTi alloy. The corrosion resistance of laser-processed NiTi alloy decreased upon annealing. © 2018 Elsevier B.V.Item Surface design of Mg-Zn alloy temporary orthopaedic implants: Tailoring wettability and biodegradability using laser surface melting(Elsevier B.V., 2018) Manne, B.; Thiruvayapati, H.; Bontha, S.; Motagondanahalli Rangarasaiah, R.; Das, M.; Balla, V.K.Magnesium-based alloys have attracted significant attention for biomedical applications due to its biodegradability as well as density and elastic modulus which are close to those of human bone. However, the uncontrolled biodegradation and hydrogen evolution are of major concern. In this work, laser surface melting (LSM) has been carried out to tailor initial corrosion rates of Mg-2.2Zn alloy implants. Melt pool dimensions, microstructure and surface topography of the LSM samples were analysed. The wettability and in vitro degradation characteristics of untreated and treated alloy were compared. LSM resulted in much finer cellular microstructural features than as-cast alloy and the melted region depths between 65 and 115 ?m. Higher treatment depths helped to extend the corrosion protection time by suppressing the corrosion front movement. Polished LSM samples resulted in overall corrosion rates of 0.5–0.62 mm/year which was about 40%–50% reduction compared to the as-cast alloy. Accelerated biomineralisation of the surface via enhancements in the surface energy due to microstructural refinement as well as microstructural homogeneity and Zn enrichment in ?-Mg, favoured improvement of the overall corrosion performance of LSM-treated alloy. © 2018 Elsevier B.V.Item Effect of zinc and rare-earth element addition on mechanical, corrosion, and biological properties of magnesium(Cambridge University Press, 2018) Kottuparambil, R.R.; Bontha, S.; Ramesh, M.R.; Arya, S.; Jana, A.; Das, M.; Balla, V.K.; Amrithalingam, S.; Prabhu, T.R.The present work aims to understand the effect of zinc and rare-earth element addition (i.e., 2 wt% Gd, 2 wt% Dy, and 2 wt% of Gd and Nd individually) on the microstructure evolution, mechanical properties, in vitro corrosion behavior, and cytotoxicity of Mg for biomedical application. The microstructure results indicate that the Mg-Zn-Gd alloy consists of the lamellar long period stacking ordered phase. The electrochemical and immersion corrosion behavior were studied in Hanks balanced salt solution. Enhanced corrosion resistance with reduced hydrogen evolution volume and magnesium (Mg2+) ion release were estimated for the Mg-Zn-Gd alloy as compared to the other two alloy systems. At the early stage of corrosion, formation of the oxide film inhibited the corrosion propagation. However, at the later stages, the breaking of the oxide film leads to shallow pitting mode of corrosion. The ultimate tensile strength of Mg-Zn-Gd-Nd is better than the other two alloys due to the uniform distribution of the Mg12Nd precipitate phase. The moderate strength in the Mg-Zn-Gd alloy is due to the low volume fraction of the secondary phase. The MTT (methylthiazoldiphenyl-tetrazolium bromide) assay study was carried out to understand the cell cytotoxicity on the alloy surfaces. Studies revealed that all three alloys had significant cellular adherence and no adverse effect on cells. © 2018 Materials Research Society.Item Laser surface modification of Mg-Zn-Gd alloy: Microstructural, wettability and in vitro degradation aspects(Institute of Physics Publishing helen.craven@iop.org, 2018) Rakesh, K.R.; Bontha, S.; Ramesh, M.R.; Arya, S.; Das, M.; Balla, V.K.; Srinivasan, A.Mg-Zn-Gd have great potential for biomedical applications owing to excellent bioactivity and non-toxicity properties. In the present study, laser surface melting (LSM) was carried out on newly developed Mg-1Zn -2Gd (wt%) alloy. Effects of laser energy on microstructural evolution, corrosion properties, surface energy, and hardness have been investigated. The surface modified sample processed at different energy densities showed fine grain structure in the melt zone compared to the untreated substrate. Grain refinement in the laser melted region improved the hardness by 60%. The surface roughness was found to be increased with increasing laser energy density. At higher energy density, removal of materials from the surface is enhanced, resulting in deeper grooves and higher surface roughness. The wettability studies indicated that the variations in surface geometry, grain size and surface roughness of LSM samples strongly influence the surface energy and hydrophilicity. Improved wetting of LSM sample was achieved owing to grain refinement and low surface roughness. The corrosion resistance determined by immersion and electrochemical methods of laser melted sample in Hank's balanced salt solution improved considerably due to grain refinement, meltpool depth and uniform distribution of secondary phases. © 2018 IOP Publishing Ltd.Item Laser surface melting of ?-TiAl alloy: An experimental and numerical modeling study(Institute of Physics Publishing helen.craven@iop.org, 2019) Mallikarjuna, M.; Bontha, S.; Krishna, P.; Balla, V.K.The objective of present work is to study the evolution of thermal stresses during laser surface melting (LSM) of ?-TiAl alloy using experimental and numerical modeling approaches. LSM of ?-TiAl alloy samples were carried out at different processing conditions in a controlled atmosphere. Material characterization of the melted region was investigated using scanning electron microscope. It was found that fully lamellar microstructure was transformed into predominantly ?-TiAl with little amount of ?2-Ti3Al. A maximum improvement in hardness of over 72% was noticed in the melted region compared to that of the substrate. Three-dimensional thermomechanical finite element analysis of LSM of ?-TiAl alloy was carried out. Melt pool dimensions, temperature history, and residual stresses were predicted from the finite element models. Measured and predicted values of melt pool depth were in good agreement with a maximum error of 13.6% at P=400Wand V=10mms-1. Predicted residual stress in the melted region exceeded the yield strength of ?-TiAl alloy and resulted in cracking of the melted region at all process conditions. ©2019 IOP Publishing Ltd.Item Laser surface melting of Mg-Zn-Dy alloy for better wettability and corrosion resistance for biodegradable implant applications(Elsevier B.V., 2019) K.r, R.; Bontha, S.; M.r, R.; Das, M.; Balla, V.K.In order to improve the performance of magnesium (Mg) for resorbable implant applications, Mg-1Zn-2Dy alloy was developed and the surface of the alloy has been modified by melting using lasers. Laser melted samples, at different laser energy density, were then subjected to microstructural, hardness, wettability and in-vitro degradation assessment. The microstructure of the Mg-Zn-Dy alloy mainly consisted of ?-Mg and eutectic phase (Mg 8 ZnDy). The melted region of the alloy surface evolved with fine grain microstructure at the near surface region and columnar grains near to the liquid solid substrate. The degree of grain size refinement obtained at the melted zone in the order of 1–2 ?m. The cross sectional microhardness of the modified zone was measured by Vickers microhardness tester. Due to these microstructural refinements and solid solution strengthening the surface hardness of laser treated alloy increased by two-fold. It was found that as the energy density increased the surface roughness along with the surface energy also increased. The wetting behaviour of the surface was estimated through measuring the contact angle by dropping the polar and non-polar liquid. Results showed that the surface energy is also found to change with LSM due to changes in the surface morphology, microstructure and chemical composition of the material. The detailed degradation study was carried out by immersing the samples in hanks balances salt solution (HBSS).The improvement in the degradation behaviour followed by laser surface melting is related to the microstructural refinement as a result of rapid heating and cooling of the melted zone. © 2019 Elsevier B.V.Item Prediction and validation of residual stresses generated during laser metal deposition of ? titanium aluminide thin wall structures(Institute of Physics Publishing helen.craven@iop.org, 2019) Mallikarjuna, M.; Bontha, S.; Krishna, P.; Balla, V.K.The focus of the current work is to predict and validate residual stresses developed during Laser Metal Deposition (LMD) of Gamma Titanium Aluminide (?-TiAl) alloy by using a combination of numerical modeling and experimental methods. Laser Engineered Net Shaping (LENS), which is one of the commercially available LMD techniques, was used to fabricate ?-TiAl alloy thin wall structures at various processing conditions. These deposits are expected to develop residual stresses due to the rapid heating and cooling cycles involved in the LMD process. 3D transient thermomechanical finite element analysis was used to simulate the LMD process. Thermal gradients and residual stresses were predicted from the thermomechanical models. It was found that the magnitude of thermal gradients increases with the addition of each deposited layer. Tensile residual stresses were observed at the edges of the thin-wall, while compressive residual stresses were observed at the center of the wall as well as in regions away from the edges. Residual stresses in the deposited samples were also measured using the x-ray diffraction technique. Reasonable agreement was observed between the predicted and measured values of residual stresses. © 2019 IOP Publishing Ltd.Item Eco-friendly lightweight filament synthesis and mechanical characterization of additively manufactured closed cell foams(Elsevier Ltd, 2019) Patil, B.; Bharath Kumar, B.R.; Bontha, S.; Balla, V.K.; Powar, S.; Hemanth Kumar, V.H.; Suresha, S.N.; Doddamani, M.Environmentally pollutant fly ash cenospheres (hollow microballoons) are utilized with most widely consumed, relatively expensive high density polyethylene (HDPE) for developing lightweight eco-friendly filament for 3D printing of closed cell foams. Cenospheres (20, 40 and 60 by volume %) are blended with HDPE and subsequently extruded in filament to be used for 3D printing. Cenosphere/HDPE blends are studied for melt flow index (MFI) and rheological properties. MFI decreases with cenospheres addition. Complex viscosity, storage and loss modulus increase with filler loading. DSC results on the filament and printed samples reveal increasing crystallization temperature and decreasing crystallinity % with no appreciable change in peak melting temperature. Cooling rate variations exhibit crystallinity differences between the filament and the prints. CTE decreases with increasing cenosphere content resulting in lower thermal stresses and under diffusion of raster leading to non-warped prints. Micrography on freeze fractured filament and prints show cenospheres uniform distribution in HDPE. Intact cenospheres lower the foam density making it lightweight. Tensile tests are carried out on filaments and printed samples while flexural properties are investigated for 3D prints. Cenospheres addition resulted in improved tensile modulus and decreased filament strength. Tensile and flexural modulus of printed foams increases with filler content. Results are also compared with injection molded samples. Printed foams registered comparable tensile strength. Specific tensile modulus is noted to be increased with cenospheres loading implying weight saving potential of 3D printed foams. Property map reveals printed foams advantage over other fillers and HDPE composites synthesized through injection and compression molding. © 2019 Elsevier Ltd
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