Faculty Publications
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Item Influence of Process Parameters on Microstructural Properties of L-DED Produced Ti64 Alloy(Springer Science and Business Media Deutschland GmbH, 2025) Suresh, S.; Kuriachen, B.; Kumar, V.; Bontha, S.; Gurugubelli, R.C.Additive manufacturing (AM) techniques have revolutionized the manufacturing of complex and customized parts across various applications. However, they are known for producing titanium parts with high anisotropy and low ductility, due to high cooling gradient in the build direction and the presence of martensite phase in microstructure respectively. These are inherent problems which limit their application in critical engineering fields. Laser—Direct Energy Deposition (L-DED) produced parts also have the same disadvantages. Thus, the primary objective of this paper is to identify the optimal combination of process parameters for L-DED that can mitigate these inherent limitations. Keeping the parameters such as powder size, orientation angle and hatch angle as constant, the laser power and scan speed are varied to fabricate 9 different sets of samples using L-DED. The research methodology includes an analysis of the microstructure, focusing on grain width, phase distribution, lath characteristics and presence of defects, if any. Microscopy and XRD techniques were used to observe the microstructure. Additionally, hardness studies were performed to evaluate the changes in material hardness. It was noticed that laser power significantly influences β width and α’ length while scan speed has a lesser dominant effect on both of them. The findings will contribute to the development of process-structure-property relations for L-DED-produced Ti64 and further, optimized manufacturing strategies for producing titanium parts with reduced anisotropy and increased ductility. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2025.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 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 Microstructure, mechanical and wear properties of the A357 composites reinforced with dual sized SiC particles(Elsevier Ltd, 2019) Avinash, A.; Bontha, S.; Krishna, M.; Koppad, P.G.; Ramprabhu, T.Current work reports on the development of A357 alloy composite which is reinforced with dual size SiC particles by stir casting route. Influence of different weight fractions (3% coarse+ 3% fine, 4% coarse + 2% fine, and 2% coarse + 4% fine) of dual size SiC particles on mechanical properties and wear resistance of A357 composites is the focus of this work. Hardness and tensile properties were studied for dual size composites and then were compared with A357 alloy. Microstructural study, fractured surface and worn surface investigation were carried out using optical and scanning electron microscopes respectively. Microstructural analysis showed fairly uniform dispersion of dual size SiC particles in A357 matrix with good interfacial bonding. Compared to A357 alloy, the composites showed improvement in hardness, yield, and tensile strength. In particular, composite with 4 wt. % of fine and 2 wt. % of large SiC particles displayed the highest tensile strength while composite with 4 wt. % of large and 2 wt. % of fine SiC particles exhibited high hardness and wear resistance among A357 alloy and dual particle size composites. The strengthening mechanisms that contributed to improvement in strength values were effective load transfer and dislocation strengthening due to thermal mismatch. © 2019 Elsevier B.V.Item The effect of heat treatment on the mechanical and tribological properties of dual size SiC reinforced A357 matrix composites(Elsevier Editora Ltda, 2020) Avinash, A.; Prabhu, T.R.; Babu, U.S.; Koppad, P.G.; Gupta, M.; Krishna, M.; Bontha, S.In the present work, the effect of aging temperature and particle size ratio of SiC particles on the mechanical and tribological properties of A357 composites reinforced with dual particle size SiC were investigated. The composites were prepared by melt-stirring assisted permanent mold casting technique with different weight fractions (3% coarse +3% fine, 4% coarse +2% fine, and 2% coarse +4% fine) of large and small size SiC particles. These three prepared composites are referred as DPS1, DPS2 and DPS3 composites. The solutionizing temperature was maintained constant at 540 ?C for 9 h while the aging was done at 160 ?C, 180 ?C and 200 ?C (T6 treatment) for 6 h. Optical and scanning electron microscopy studies showed fairly uniform dispersion of dual size SiC particles in A357 matrix with good interfacial bonding. High-resolution transmission electron microscopy images showed formation of uniformly dispersed needle-like phase and spherical shaped -Mg2Si precipitates under peak aging conditions. Compared to T6 treated A357 alloy, the T6 treated DPS A357 composites showed improved yield strength, tensile strength, hardness and wear resistance. Among the three composites, hardness and wear resistance of T6 treated DPS2 composite was found to be significantly higher when compared to the other two composites (DPS1 and DPS3). Ratio of large particles to small particles also seems to effect the mechanical and tribological properties. Presence of more small particles was found to be good for strength and ductility whereas more large particles were found to be good for hardness and wear resistance. © 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND licenseItem Degradation, wettability and surface characteristics of laser surface modified Mg–Zn–Gd–Nd alloy(Springer, 2020) K.r, R.; Bontha, S.; M.r, R.; Das, M.; Balla, V.K.This work evaluates the effects of laser surface modification on Mg–Zn–Gd–Nd alloy which is a potential biodegradable material for temporary bone implant applications. The laser surface melted (LSM) samples were investigated for microstructure, wettability, surface hardness and in vitro degradation. The microstructural study was carried out using scanning and transmission electron microscopes (SEM, TEM) and the phases present were analyzed using X-ray diffraction. The in vitro degradation behaviour was assessed in hank’s balanced salt solution (HBSS) by immersion corrosion technique and the effect of LSM process parameters on the wettability was analyzed through contact angle measurements. The microstructural examination showed remarkable grain refinement as well as uniform redistribution of intermetallic phases throughout the matrix after LSM. These microstructural changes increased the hardness of LSM samples with an increase in energy density. The wetting behaviour of processed samples showed hydrophilic nature when processed at lower (12.5 and 17.5 J/mm2) and intermediate energy density (22.5 and 25 J/mm2), which can potentially improve cell-materials interaction. The corrosion rate of as cast Mg–Zn–Gd–Nd alloy decreased by ~83% due to LSM. [Figure not available: see fulltext.]. © 2020, Springer Science+Business Media, LLC, part of Springer Nature.Item Tailoring surface characteristics of bioabsorbable Mg-Zn-Dy alloy using friction stir processing for improved wettability and degradation behavior(Elsevier Editora Ltda, 2021) Rokkala, U.; Bontha, S.; Ramesh, M.R.; Balla, V.K.; Srinivasan, A.; Kailas, S.V.Magnesium (Mg) and its alloys are currently under consideration for use as temporary implants. However, early degradation and maintaining mechanical integrity is a significant concern. Surface modification techniques are used to improve mechanical and corrosion properties of Mg based alloys. In the present study, friction stir processing (FSP) was used to tailor the surface characteristics of Mg-1Zn-2Dy (wt.%) alloy for temporary implant applications. The FSPed alloy was characterized using EBSD to understand the influence of FSP on crystallographic texture, grain size and grain boundaries and thereby their effect on corrosion, wettability and hardness. Results showed that the grain size of stir zone (SZ) was refined to less than 3 ?m, as a result of dynamic recrystallization (DRX) during FSP and the FSPed alloy exhibited better wettability than as-cast alloy. An increase in the hardness (11.7%) and elastic modulus (6.84%) of FSPed alloy were also observed. Electrochemical corrosion and weight loss methods were conducted in Dulbecco's Modified Eagle's Medium (DMEM) with, 10% Fetal Bovine Serum (FBS) physiological solution. The lower degradation rate (0.72 mm/yr) of FSPed alloy has been attributed to the fine grains and evenly distributed secondary phase particles. Further, the influence of grain boundary characteristics and crystallographic texture on the corrosion behavior have been investigated. © 2021 The Author(s).Item Characterization and thermal analysis of laser metal deposited ?-TiAl thin walls(Elsevier Editora Ltda, 2021) Mallikarjuna, B.; Bontha, S.; Krishna, P.; Balla, V.K.The present work focuses on investigating the effect of process variables (power, travel speed, powder flow rate) on microstructure and mechanical properties of Laser Metal Deposited (LMD) ?-TiAl thin walls. To this end, LMD technique was used to deposit ?-TiAl thin walls at different processing conditions. Microstructures of as-deposited samples were investigated using both optical and scanning electron microscopy. X-ray diffraction (XRD) technique was used to determine the phases present. Microhardness measurements were carried out along both longitudinal and build directions. Microstructural analysis of as-deposited samples revealed a fine lamellar structure comprising of ? and ?2 phases. Colony size of 30–60 ?m and lamellar spacing between 0.1 and 0.7 ?m were observed. XRD analysis confirmed the presence of ? and ?2 phases. Comparison of elemental analysis results on both powder and as-deposited samples revealed a negligible loss of Al and no oxygen pick up in the deposited thin walls. Hardness values were found to decrease with an increase in wall height, and hardness values increased marginally (5%) with an increase in travel speed. Further, 3D transient thermal analysis was also carried out to complement the LMD of thin walls in terms of melt pools and cooling rates. It was found that the melt pool depth (MPDc = 0.266 mm) is smaller at the centre than the edge (MPDe = 0.513 mm) of the wall. A higher cooling rate of 1.05 × 105 °C/s near the wall substrate was found for 200–12. © 2021Item Effect of Heat Treatment on Microstructure and Dry Sliding Wear Behavior of Laser Directed Energy Deposited Inconel 625(Springer, 2025) Praharaj, A.K.; Chaurasia, J.K.; Suvin, P.S.; Narayanan, J.A.; Paul, C.P.; Balla, V.K.; Chakrapani, S.K.; Bontha, S.Laser directed energy deposition (LDED) is a promising technology for manufacturing and repair of Inconel 625 (IN625) components used in critical sectors requiring enhanced tribological performance due to harsh operating environments. Hence, the current work focuses on the evaluation of the tribological performance of LDED-built IN625 with the implementation of different heat treatment methods, i.e., solution treatment (ST), direct aging (AG), and solution treatment + aging (ST + AG). A detailed microstructural analysis, hardness, and wear testing were performed for the as-deposited (AD) and heat-treated (HT) samples, and the results were compared. The analysis revealed coarser grains in the case of ST and ST + AG samples, whereas finer grains for AD and AG samples, indicating grain coarsening after solution treatment. Further, the brittle laves phase gets dissolved after ST, whereas the AG and ST + AG samples resulted in the precipitation of metal carbides and strengthening phases. The microhardness of the ST sample (193.2 HV) was lower compared to the AD (211.6 HV) sample, whereas the AG and ST + AG samples exhibited 25.6 and 9.3% higher hardness than the AD sample. Considering tribological performance, the AG sample illustrated a maximum reduction of 61.4% in the coefficient of friction (COF) and 36.5% in wear rate when compared to the AD sample. This could be attributed to the presence of finer grains and strengthening phases. © ASM International 2025.