Department of Mechanical engineering

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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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    Effect of weld thermal cycle on metallurgical and corrosion behavior of friction stir weld joint of AA2014 aluminium alloy
    (Elsevier, 2019-01) Sinhmar, Sunil
    Friction stir welding of AA2014 aluminium alloy was performed at seven different speed combinations. Weld thermal cycles were measured at all the speed parameters and corresponding peak temperatures were observed at higher tool rotation speed and lower welding speed. Hardness and tensile tests were performed to study the mechanical properties of the weld joints. Corrosion behavior was studied using immersion, Tafel and electrochemical impedance spectroscopy tests. Optical microscopy, FESEM, XRD and transmission electron microscopy were used to investigate the metallurgical behavior of the weld joints. Microhardness and corrosion resistance were found higher at low rotation speed and high traverse speed. Corrosion behavior has been discussed in light of microstructure.
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    Investigation on Microwave Joining of Mild Steel Plates at 2.45 GHz and Joint Characterization
    (Springer, 2021-02) Mishra, Radha Raman
    Microwave joining of materials is a recently developed advanced joining process in which electromagnetic energy at 2.45 GHz is used to develop butt joint between different metallic plates. Rapid selective hybrid heating of the targeted area depends upon the location of the metallic samples inside the applicator cavity during microwave exposure. A high electric field intensity location for rapid hybrid heating of samples was identified inside the resonating cavity with the help of the COMSOL Multiphysics 5.2. Accordingly, experimentation was done to develop joints of mild steel (MS) samples using microwave energy at 2.45 GHz and input power of 900 W. The nickel powder was used as an interface material between metallic plates. The fabricated mild steel butt joints were characterized to analyse the microstructures and the micro indentation hardness of the joints. The microstructural characterization of the joints revealed complete melting of nickel powder and its fusion with base mild steel plates. The presence of iron in the joint zone indicated a metallurgical fusion of the interface layer with the base metal; however, oxides and carbides presence in the joint indicated interaction with atmospheric oxygen and carbon in the susceptor. The hardness of the developed joint zone was 405 ± 12 Hv which is higher as compared to the base metals (211 ± 12 Hv).
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    Effect of input microwave power and insulation on microstructure and tensile properties of cast Al 7039 alloy produced at 2.45 GHz
    (Taylor & Francis, 2020-10) Mishra, Radha Raman
    In the present work, microwave energy was used for casting Al 7039 alloy at 2.45 GHz in the ambient cavity environment. Effects of input power and insulation of the mould assembly during irradiation on charge melting and mould preheating were studied. Five different casts were produced at 1000 W, 1200 W, 1400 W, 1400 W with an insulated pouring basin and 1400 W with insulated mould assembly. Melting time of the charge was the least while using 1400 W with insulated mould assembly, whereas preheating of the mould was observed minimum during casting at 1400 W inside an insulated pouring basin. Cast microstructures revealed that less preheating of the mould resulted in finer grains and intermetallics, which improve tensile properties of the cast. Fractographic analyses showed the presence of coarse intermetallics in the casts produced with insulated mould assembly, which resulted in significant reduction of tensile properties.
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    Multi-physics simulation of in situ microwave casting of 7039 Al alloy inside different applicators and cast microstructure
    (Sage, 2018-06) Mishra, Radha Raman
    In the present study, finite element models of three different applicators (A1, A2, and A3) having different power densities were developed to study melting of the charge and solidification of the melt during in situ microwave casting. Multi-physics simulations were carried out to understand the effect of applicator specific processing conditions on the distribution of electric field inside the cavities at 2.45 GHz for Al 7039 alloy as charge. The alloy was cast inside the selected applicators and the mold temperature was monitored. The experimental results showed reasonable agreement with the simulation data. Simulation results revealed that the distribution of electromagnetic field inside A3 offers the lowest melting time of the charge (141% less than A1); however, it also caused the highest preheating of the graphite mold with respect to A1 (30% higher) and A2 (25% higher). It was found that the applicator-specific solidification conditions affect grain structure, intermetallic precipitation, and their distribution inside the casts. Coarser intermetallic phases (57 µm) and grains (97 ± 54 µm) were present in the Cast 3 developed using A3 due to higher preheating of the mold and slower cooling rate of the melt as compared to that in A1 and A2.