BITS Faculty Publications

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    Defining the stages of annealing in a moderately deformed commercial Zirconium alloy
    (Elsevier, 2015-11) Kumar, Gulshan
    Fully recrystallized Zircaloy-4 was cold rolled to 20% reduction in thickness. The deformed microstructure had fragmented and non-fragmented grains. Fragmentation represented deformation-induced refinement in grain size. Typically, the fragmented grains had more misorientation and were finer than the as-received grains. The deformed samples were subjected to 650°C annealing for different time periods, followed by water quenching. Based on experimental observations, three distinct stages of annealing were noted. Stage I caused changes in the misorientations of the non-fragmented grains, while the fragmented regions remained unaffected. This was also the most effective stage for residual stress relief. In stage II, discontinuous recrystallization and grain coarsening consumed the fragmented regions. This stage provided the highest softening. Finally, stage III created recovery-induced grain refinement of the larger non-fragmented grains. A combination of indirect and direct observations thus provided a complete picture of the annealing related microstructural changes in a moderately deformed commercial Zirconium alloy.
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    A miniature physical simulator for pilgering
    (Elsevier, 2016-11) Kumar, Gulshan
    Pilgering is a complex incremental manufacturing process for seamless tubes. In this work, a miniature physical simulator for pilgering was designed and fabricated. This miniature simulator employs a grooved roll-die and a mandrel and can impose controlled reductions in both tube diameter and wall thickness. Pilgering deformation over a range of ratios of reductions in wall thickness and in tube diameter, known as the -factor, was imposed on hemi-cylindrical zirconium alloy specimens. The influence of the -factor on the microstructure and deformation texture of the deformed specimens was quantified. A polycrystal plasticity calculation based on the binary tree model was used to simulate texture evolution during the simulated pilgering process. The computer model quantitatively captured the variation with of the Kearns factors, as measured in the physically simulated specimen. The small differences noticed between the predicted and experimental final textures point to unaccounted transverse components of the flow field. These observations suggest that physical and/or computer simulations can form the basis of a rapid methodology for tool selection to realize prescribed post-pilgering textures.
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    Temperature dependence of work hardening in sparsely twinning zirconium
    (Elsevier, 2017-01) Kumar, Gulshan
    Fully recrystallized commercial Zirconium plates were subjected to uniaxial tension. Tests were conducted at different temperatures (123 K - 623 K) and along two plate directions. Both directions were nominally unfavorable for deformation twinning. The effect of the working temperature on crystallographic texture and in-grain misorientation development was insignificant. However, systematic variation in work hardening and in the area fraction and morphology of deformation twins was observed with temperature. At all temperatures, twinning was associated with significant near boundary mesoscopic shear, suggesting a possible linkage with twin nucleation. A binary tree based model of the polycrystal, which explicitly accounts for grain boundary accommodation and implements the phenomenological extended Voce hardening law, was implemented. This model could capture the measured stress-strain response and twin volume fractions accurately. Interestingly, slip and twin system hardness evolution permitted multiplicative decomposition into temperature-dependent, and accumulated strain-dependent parts. Furthermore, under conditions of relatively limited deformation twinning, the work hardening of the slip and twin systems followed two phenomenological laws proposed in the literature for non-twinning single-phase face centered cubic materials.
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    Experimental characterization and finite element modeling of through thickness deformation gradient in a cold rolled zirconium sheet
    (Elsevier, 2017-11) Kumar, Gulshan
    A commercial Zirconium alloy was subjected to different thickness reductions (20%, 40% and 60%) by cold rolling. A through-thickness gradient in microstructure, crystallographic texture and residual stress was observed. This gradient was till 1/8th of the specimen thickness, and implied a corresponding anisotropy in the imposed strain state. An elasto-plastic FE (finite element) model was developed to capture such through thickness deformation gradients. A reasonably good agreement was observed between the experimental and predicted residual stress distributions when the material anisotropy was accounted for. Through-thickness residual stress evolution was shown to be significantly affected by material anisotropy and to a lesser extent by the rolling parameters (coefficient of friction and rotational speed).
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    An assessment of residual stresses and micro-structure during single point incremental forming of commercially pure titanium used in biomedical applications
    (Elsevier, 2020) Kumar, Gulshan
    Single point incremental forming (SPIF) is a branch of incremental sheet forming where a very small portion of the sheet is deformed plastically at any moment. The highly localized point deformation is done by a simple hemispherical tool, whose path is numerically monitored by a Computer numerical control (CNC) machine, performs this progressive extremely localized deformation. Since no die is required during forming, highly customized and user-oriented sheet metal products can be manufactured employing the process. SPIF can be readily employed in the manufacturing of customized orthopaedic implants and braces, e.g., cranial implants, ankle implants, elbow and knee support braces. The forming of these sheets through SPIF would results in the generation of residual stresses in the sheet metal. With time and other physical factors, these residual stresses would be relieved resulting in dimensional inaccuracy. This inaccuracy is highly detrimental in the case of implants and highly undesirable for supporting braces. The objective of this work is to investigate, experimentally, the state and magnitude of residual stresses on commercially pure titanium grade 2 by SPIF for biomedical applications. The important process parameters: forming angle and incremental step depth are used for this investigation in the present study. The X-ray diffraction technique was used for the experimental measurements of the residual stresses. Microstructural behaviour of the final product at different incremental step depth and forming angles are also observed by EBSD (Electron backscattered diffraction) technique. The experimental findings showed the formation of increased tensile residual stresses with an increase in incremental step depth and steepness of forming angles.
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    A review of the applications of machine learning for prediction and analysis of mechanical properties and microstructures in additive manufacturing
    (ACM Digital Library, 2024-12) Challa, Jagat Sesh; Singh, Amit Rajnarayan
    This article provides an insightful review of the recent applications of machine learning (ML) techniques in additive manufacturing (AM) for the prediction and amelioration of mechanical properties, as well as the analysis and prediction of microstructures. AM is the modern digital manufacturing technique adopted in various industrial sectors because of its salient features, such as the fabrication of geometrically complex and customized parts, the fabrication of parts with unique properties and microstructures, and the fabrication of hard-to-manufacture materials. The functioning of the AM processes is complicated. Several factors such as process parameters, defects, cooling rates, thermal histories, and machine stability have a prominent impact on AM products’ properties and microstructure. It is difficult to establish the relationship between these AM factors and the AM end product properties and microstructure. Several studies have utilized different ML techniques to optimize AM processes and predict mechanical properties and microstructure. This article discusses the applications of various ML techniques in AM to predict mechanical properties and optimization of AM processes for the amelioration of mechanical properties of end parts. Also, ML applications for segmentation, prediction, and analysis of AM-fabricated material’s microstructures and acceleration of microstructure prediction procedures are discussed in this article.
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    Synthesis of nanoscale oxide scaffold on Nitinol surface using hydrothermal treatment
    (Taylor & Francis, 2015-04) Roy, Banasri
    Nanostructured scaffolds were synthesized on the surface of equiatomic NiTi alloy (Nitinol) via hydrothermal treatment at 120 ± 1°C and 250 kPa using alkali (NaOH) solution of different strength. The scaffolds were found to be composed of intermingled nanopetals with varying morphology and phase content depending on the treatment time and alkali concentration. Single or mixed Ni3Ti3O, NiTiO3, H2Ti3O7 and TiO2 (anatase and rutile) phases were observed in the scaffold by X-ray diffraction study. Standard hemolysis testing showed significant biocompatibility improvement of the scaffolds grown in low strength alkali. Measurement of Ni release in the simulated body fluid (SBF) revealed that Ni release can be decreased from ∼60 μg L− 1 for the mechanically polished bare NiTi surface to ∼2·7 μg L− 1 for the scaffolded surface (scaffolds grown in low strength alkali).
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    Synthesis of Mixed-Phase TiO2 Powders in Salt Matrix and Their Photocatalytic Activity
    (Taylor & Francis, 2016-05) Roy, Banasri
    Three mixed-phase TiO2 powders, containing ∼80 volume % anatase and ∼20 volume % rutile, were prepared from amorphous titanium hydroxide and three different salt matrices—pure sodium chloride, pure Na2CO3, and pure disodium hydrogen phosphate (DSP). Amorphous titanium hydroxide and salt mixtures were heat treated at 875°C in a rapid thermal annealing system for different times, according to the time–temperature phase transformation graphs. Time-dependent UV degradation of aqueous solutions of methylene blue dye (15 ppm) in the presence of mixed-phase powders was used to monitor the activity of the catalysts. Microstructural study of the powders by scanning electron microscope and transmission electron microscope combined with phase analysis by XRD and optical absorbance by UV-absorption spectroscopy indicated that the higher photocatalytic activity of the powder obtained from pure DSP salt could be explained by its smaller rutile particle size and anatase–rutile interparticle bonding.
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    Synthesis of Al-doped Nano Ti-O scaffolds using a hydrothermal route on Titanium foil for biomedical applications
    (Elsevier, 2016-09) Roy, Banasri
    Al-doped and undoped Ti-O nanowire scaffolds were fabricated on the surface of titanium foil via a one-step hydrothermal reaction in the presence of an alkali solution at 120±1 °C temperature and at 15 psi pressure. Grazing Incidence X-Ray and electron microscopy analyses confirmed that the phase composition, length and diameter of the nanowires depend on alkali concentration and reaction time, and the Al doping. Whereas, Al doping retarded the oxide phase formation and transformation rate and changed the morphology of the nanowires. Preliminary hemolysis test showed better biocompatibility of the Al-doped scaffolds compared to the undoped ones.
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    Investigating the effect of dopant type and concentration on TiO2powder microstructure via rietveld analysis
    (Elsevier, 2018-02) Roy, Banasri; Rao, N.V.M.
    The influence of dopant types (anion: Cl and F; cation: Fe and Zn) on the phase transformation, crystallite size, lattice parameters, and lattice strain of TiO2 powder is investigated by Rietveld refinement of X-ray powder diffraction (XRD) data. Undoped and doped powders are synthesized by using a sol-gel route and heat treated for different times and temperatures. In general, dopants diminish the phase transformation rate and decrease particle size (FESEM data support), but F doping demonstrates the strongest effect. Cation doping induces defects and distortion in the lattice and increases strain both in anatase and rutile phases, but anion doping enhances strain in anatase only. The decreasing order of the dopants inducing strain in anatase and rutile phases is observed as F > Zn10% > Cl > Zn5%> Fe10% > Fe5% ≈Zn1%>Fe1%>UD, and Zn10% > Fe10% > Zn5%>Fe5%> Zn1%> Fe1%> UD > Cl > F, respectively. This could be explained from EDX study, which shows that the anion dopants, irrespective of the amount, abandon the material at a treatment temperature ≥400 °C. This may create high defect density in anatase, influence phase transformation, and particle size. But, the high temperature ion mobility annihilates the point defects and shows less strain in rutile. Whereas, the cations assimilate in structure and show similar effects in both the phases.