Browsing by Author "Kumar, Gulshan"

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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.
Biocorrosion studies of a novel Mg70Al18Zn6Ca4Y2 low entropy multicomponent alloy in different simulated body fluids
(Springer, 2022-02) Kumar, Gulshan
In the present investigation, biocorrosion behaviour of novel Mg-based multicomponent low-entropy alloy (Mg70Al18Zn6Ca4Y2 LEA), fabricated using disintegrated melt deposition technique has been evaluated. The electrochemical behaviour of this Mg LEA alloy at different simulated body fluid solutions, such as [artificial blood plasma (ABP), pH 7.4], [phosphate buffer saline (PBS), pH 7.4], [Hank’s balanced salt solution (HBSS), pH 7.4] and artificial saliva solution (ASS, pH 6.2) has been studied using Potentio-dynamic polarization test followed by qualitative analysis attained from electrochemical impedance spectroscopy (EIS). Further, the surfaces of the corroded samples were examined using scanning electron microscopy and its chemical composition were analysed using energy-dispersive X-ray spectroscopy. The results show significant passive behaviour and improved corrosion resistance for sample in ASS solution compared to the ABP, PBS and HBSS. The rank of the Mg LEA alloy based on the corrosion resistance is in the following order: ABP
Burst ductility of zirconium clads: the defining role of residual stress
(Springer, 2016-05) Kumar, Gulshan
Closed end burst tests, using room temperature water as pressurizing medium, were performed on a number of industrially produced zirconium (Zr) clads. A total of 31 samples were selected based on observed differences in burst ductility. The latter was represented as total circumferential elongation or TCE. The selected samples, with a range of TCE values (5 to 35 pct), did not show any correlation with mechanical properties along axial direction, microstructural parameters, crystallographic textures, and outer tube-surface normal (σ 11) and shear (τ 13) components of the residual stress matrix. TCEs, however, had a clear correlation with hydrostatic residual stress (P h), as estimated from tri-axial stress analysis on the outer tube surface. Estimated P h also scaled with measured normal stress (σ 33) at the tube cross section. An elastic–plastic finite element model with ductile damage failure criterion was developed to understand the burst mechanism of zirconium clads. Experimentally measured P h gradients were imposed on a solid element continuum finite element (FE) simulation to mimic the residual stresses present prior to pressurization. Trends in experimental TCEs were also brought out with computationally efficient shell element-based FE simulations imposing the outer tube-surface P h values. Suitable components of the residual stress matrix thus determined the burst performance of the Zr clads.
Characterization of heavy ion induced defects in Zr-2.5 wt. % Nb by grazing incidence X-ray diffraction
(Bhabha Atomic Research Centre, 2015-07) Kumar, Gulshan
Zr-2.5 wt. % Nb alloy is used as a pressure tube material in pressurized heavy water reactor (PHWR). It is one of the most critical component which decides the life of the reactor. The in-reactor degrading phenomenon of prime concern is dimensional changes caused by irradiation induced creep and growth processes. The present study aims to understand the mechanism of irradiation damage by irradiating the alloy with heavy ion. Heavily charged Ar9+ ions have been used, as it leads to dense cascade similar to those observed in neutron beam damage. Such type of irradiation study would facilitate larger damage of material in a shorter time. Zr-2.5 %Nb alloy samples were irradiated using 315 keV Ar9+ ion for different durations. The irradiation doses were varied from 3.1x1015 to 4.17x1016 Ar9+/cm2. Grazing incidence X-ray diffraction (GI-XRD) is known as a direct, non-destructive, surface-sensitive technique for characterization of materials. Using GI-XRD domain size, micros train by Williamson hall method, dislocation density by line profile analysis (LPA), residual stress, and depth profiles of irradiated phase distributions were calculated. The analysis revealed that there is a significant decrease in domain size with corresponding increase in microstrain and dislocation density with irradiation dose.
A comparative study on misorientations to determine the extent of recrystallization in pure ETP copper
(Springer, 2021) Kumar, Gulshan
In electron backscatter diffraction (EBSD), kernel average misorientation (KAM), grain average misorientation (GAM), and grain orientation spread (GOS) are considered as the reflection of the extent of recrystallization. This work presents a comparative study of KAM, GAM, and GOS to bring out the best-suited parameter to determine the extent of recrystallization in pure copper. The pure ETP (electrolytic tough pitch) copper samples were characterized through EBSD at three different states: (i) deformed (ii) partially recrystallized and (iii) fully recrystallized. The result shows that the GOS found to be dominating over KAM and GAM in distinguishing the strain-free and deformed grains for pure ETP copper. The cut-off point for delineating the deformed and the strain-free grains has also been determined and applied to low percentage deformation study where higher mechanical strength and electrical conductivity is achieved than the as-received sample.
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.
Deformation twinning in zirconium: direct experimental observations and polycrystal plasticity predictions
(Springer, 2015-08) Kumar, Gulshan
Deformation twinning was directly observed in three commercial zirconium alloy samples during split channel die plane-strain compression. One pair of samples had similar starting texture but different grain size distributions, while another pair had similar grain size distribution but different starting textures. Extension twinning was found to be more sensitive to the starting texture than to the grain size distribution. Also, regions of intense deformation near grain boundaries were observed. A hierarchical binary tree-based polycrystal plasticity model, implementing the Chin-Hosford-Mendorf twinning criterion, captured the experimentally observed twinning grains’ lattice orientation distribution, and the twin volume fraction evolution, provided the critical resolved shear stress for extension twinning, was assumed much larger than any of the values reported in the literature, based on the viscoplastic self-consistent model. A comparison of the models suggests that obtained using the present model and the viscoplastic self-consistent models physically correspond to the critical stress required for twin nucleation, and twin growth, respectively.
Effect of heat treatment on wire + ARC additive manufactured aluminum 5356 alloy: mechanical properties and microstructure correlation
(Springer, 2023) Kumar, Gulshan
In this work, the aluminum 5356 (Al5356) component was fabricated by Wire Arc Additive Manufacturing (WAAM) process and subjected to heat treatment at three different temperatures i.e., 450, 525, and 600°C. Detailed mechanical and microstructural characterization was performed on the as-fabricated and heat-treated samples to correlate the change in mechanical properties with its corresponding microstructure. The mechanical properties were estimated using the tensile and Rockwell hardness tester, and the microstructural characterization was performed using Scanning Electron Microscopy (SEM) and Electron Backscattered Diffraction (EBSD) technique. The samples heat-treated at 450°C show superior strength (i.e. UTS 260 MPa) as compared to as-fabricated, heat-treated at 525 and 600°C due to the beneficial evolution of fine second phase particles after heat treatment.
Effect of pre-annealing strains on annealing texture developments in commercially pure (CP) titanium
(Taylor & Francis, 2015-03) Kumar, Gulshan
Hexagonal commercially pure titanium (cp-titanium) plates were subjected to unidirectional-rolling (rolling), accumulative roll-bonding (ARB) and cross-rolling in a laboratory rolling mill. Rolling and cross-rolling were carried out to impart 90% reduction in thickness and ARB processing was performed for six passes. The deformed plates were then subjected to annealing at 600 °C for a large range of soaking time starting from 0.17 min (10 s) to 30 min. It was observed that the samples were fully recrystallized after 5 min of annealing, irrespective of the rolling processes employed in this study. Also, the samples were seen to develop almost similar texture when annealing was carried out beyond 5 min of annealing time. However, before annealing, the texture development was seen to be different in the respective samples subjected to different rolling processes. The initial texture present in the deformed structure got strengthened during annealing of the samples under investigation. It was also observed that the texture development was insignificant in ARB-processed samples after annealing.
Experimental and numerical assessment of residual stresses in aa6061 after surface treatment by deep cold rolling
(Springer, 2019-10) Kumar, Gulshan
Deep cold rolling is a mechanical method of surface treatment used to adapt the desired surface properties in machine components for aerospace industries. In this process, localized plastic deformation is rendered on the surface by rolling a form tool wheel on the specimen to condense high dislocation density which imparts high compressive residual stresses resulting in increased tensile and fatigue strength. The main focus of the present study is to characterize the distribution of residual stresses on the surface and subsurface of an 8-mm-thick plate made of an aluminium alloy AA6061-O, after deep cold rolling operation to different depths of 0.5, 1, 1.5 and 2.0 mm. The observed residual stresses are correlated with the microstructural features in the deformed depths of the specimens. The residual stress and grain size in the deformed specimens are examined by using grazing incidence X-ray diffraction and electron backscatter diffraction technique, respectively. It is observed that deep cold rolling leads to significant grain refinement, resulting in high values of residual stresses through thickness. The compressive residual stresses are found to be higher on the surface of the specimen and reduce gradually with the penetration depth. The experimental results are compared with the results obtained by FE analysis using ABAQUS. A good agreement is observed between the experimental and the simulated values of residual stresses and the trends of stress distribution pattern.
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).
Finite element analysis and experimental investigation in incremental sheet metal forming of composite matrix of Grade-V titanium
(Springer, 2024-07) Kumar, Gulshan
The single point incremental forming (SPIF) method is well-suited to meet the demands of the biomedical and aerospace sectors and a wide range of consumer preferences due to its notable attributes. The forming time and corrosion behaviour of the SPIF process using a composite matrix sheet of Ti-6Al-4 V were examined in this study. This paper also simulates a truncated conical, hemisphere, and hyperbolic geometry using finite element analysis and founds compressive residual stresses in a truncated conical shape. The effects of various process parameters, i.e. sheet thickness, tool diameter, spindle speed, step size, feed rate, and wall angle on these aspects were examined to optimize parameter levels to achieve the lowest forming time with the aid of the design of experiments (DOE) using Taguchi analysis. The tool diameter, sheet thickness, and incremental depth are the three most significant parameters that have the most effects on the forming time, according to the analysis’s results. The forming time is predicted using an artificial neural network (ANN). ANN anticipates the forming time with 98% accuracy. A salt spray tester was used in the present study for the observation of corrosion behaviour at different time intervals. It was found that there was no white rust or no red rust after multiple intervals. Based on the corrosion behaviour shown in this study, it can be suggested that composite matrix grade-V titanium sheet material is suitable for biomedical and aerospace applications.
Finite element analysis of residual stresses during incremental sheet forming of Ti-6Al-4V alloys using different tool path profiles
(Springer, 2023-08) Kumar, Gulshan
The incremental sheet metal forming is highly flexible and a die-free production method for fabricating various sheet metal components using a CNC spindle tool. Compared to the conventional process, it is beneficial for small-batch components. In biomedical and aeronautical sectors, titanium grade-5 (Ti-6Al-4V) is highly recommended due to its optimal specific strength, biomedical applications, and excellent resistance rate against corrosion. This paper simulates a truncated conical, hemisphere, and hyperbolic geometry in the incremental sheet metal forming process. MATLAB programming is then used to compute the profile, modify, and export the data to the Abaqus input file format for further FE analysis. This research uses an explicit-based computational approach to simulate SPIF and determine the output response parameters such as residual stresses, von Mises stress distribution, and variation in sheet thickness along the deforming depth. The Johnson–Cook (J-C) parameters have been used for carrying out the incremental forming simulations. Compared to other tool path profiles, more compressive stresses were observed in the conical shape profile. The distribution of effective residual stresses and part thickness were also explored in a detailed comparison of various tool path profile predictions.
Identification and prioritization of critical success factors of a lean six sigma–industry 4.0 integrated framework for sustainable manufacturing using topsis
(MDPI, 2025-02) Kumar, Gulshan
The relationship between Lean Six Sigma, Industry 4.0 and sustainable manufacturing has been evaluated only to a limited extent within this domain of the published literature. A DMAIC-DMADV-based framework along with a phase-by-phase implementation path is proposed in this study to integrate Lean Six Sigma and Industry 4.0 technologies for achieving sustainable manufacturing. The paper also focused on identifying and prioritizing the critical success factors for the implementation of the proposed framework. The critical success factors identified through a literature review are ranked using the multi-decision criteria technique TOPSIS, with input from selected experts across various manufacturing companies. The results highlight that the most important enablers set clear sustainability goals, regularly monitor progress and have a skilled workforce. The findings provide actionable guidance for practitioners, and the study contributes to the existing body of knowledge by offering a comprehensive methodology to integrate Lean Six Sigma and Industry 4.0 for sustainable manufacturing. Further research must focus on the validation of the framework in diverse industrial settings and refining the sustainability assessment model to enhance its adaptability.
Influence of non-covalent modification of multiwalled carbon nanotubes on the crystallization behaviour of binary blends of polypropylene and polyamide 6
(RSC, 2014-12) Kumar, Gulshan
Blends of polypropylene (PP) and polyamide 6 (PA6) with multiwalled carbon nanotubes (MWNTs) were prepared using different processing strategies in a twin-screw micro-compounder. The effect of MWNTs on the crystallization behaviour of the PP phase and the PA6 phase of the blend has been investigated through non-isothermal crystallization studies by differential scanning calorimetric analysis. Furthermore, the effect of the addition of the compatibilizer (PP-g-MA) and the modification of MWNTs (m-MWNTs) with a non-covalent organic modifier (Li-salt of 6 amino hexanoic acid, Li–AHA) has also been studied in context to the crystallization behaviour of the PP and PA6 phase in the blend. The crystallization studies have indicated a significant increase in bulk crystallization temperature of the PP phase in the blend in the presence of MWNTs. Moreover, the formation of ‘trans-lamellar crystalline’ structure consisting of PA6 ‘trans-crystalline lamellae’ on MWNTs surface was facilitated in the case of blends prepared via ‘protocol 2’ as compared to the corresponding blends prepared via ‘protocol 1’. Wide angle X-ray diffraction analysis has showed the existence of a β-polymorph of the PP phase due to incorporation of the PA6 phase in the blend. Addition of MWNTs in the blends has facilitated further β-crystalline structure formation of the PP phase. In the presence of m-MWNTs, a higher β-fraction was observed in the PP phase as compared to the blend with pristine MWNTs. Addition of PP-g-MA has suppressed the β-phase formation in the PP phase in the blend. X-ray bulk texture analysis revealed that incorporation of PA6 as well as pristine/modified MWNTs has influenced the extent of orientation of the PP chains towards specific crystalline planes in various blend compositions of PP and PA6.
Microstructure and texture development during cold rolling in UNS S32205 and UNS S32760 duplex stainless steels
(Springer, 2017-03) Kumar, Gulshan
In the present study, microstructure and texture evolution during cold rolling in UNS S32205 and UNS S32760 duplex stainless steel was investigated. Both steels were unidirectionally cold rolled up to 80 pct thickness reduction. Scanning electron microscopy and electron backscattered diffraction (EBSD) were used for microstructural characterization, while X-ray diffraction (XRD) was used for the measurement of bulk texture. Strain-induced martensite (SIM) was identified and quantified with the help of magnetic measurements (B–H curve and magnetization saturation). With the increase in plastic strain, the grains became morphologically elongated along the rolling direction with the reduction in average band thickness and band spacing. SIM increased with the increase in deformation and was found to be a function of strain and the SFE of austenite. The increase in SIM was much more pronounced in UNS S32205 steel as compared to UNS S32760 steel. After cold rolling, strong α-fiber (RD//〈110〉) texture was developed in ferrite, while brass texture was dominant in austenite for both steels. The strength of texture components and fibers was stronger in UNS S32760 steel. Another significant feature was the development of weak γ-fiber (ND//〈111〉) in UNS S32760 steel at intermediate deformation.
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.
Observation of enhanced magnetic anisotropy in PLD YIG thin film on GGG (1 1 1) substrate
(Elsevier, 2019-08) Kumar, Gulshan
Magnetic properties and FMR of pulsed laser deposited Y3Fe5O12 (YIG) thin film on Gd3Ga5O12 (1 1 1) substrate have been investigated in the temperature range 4.2 ≤ T ≤ 300 K. The effective saturation magnetization (4πMeff), obtained from Ferromagnetic Resonance at room temperature, is found to be 2600 G, which is higher than SQUID measured 4πMS value of 1770 G. This implies that the value of 4πMeff can only be accounted by considering a negative anisotropy field (HU) of around 830 Oe at room temperature. Such an anisotropy is attributed to a compressive stress (−1.70 × 1010 dyne/cm2) induced in the film closer to the film-air interface. The presence of this stress in YIG film was confirmed using multiple {hkl} stress measurements with grazing incidence X-ray diffraction performed at different depth of penetration on the YIG film.
Orientation dependent mechanical properties of commercially pure (cp) titanium
(2014-12) Kumar, Gulshan
The present investigation is an attempt at correlating the crystallographic orientation and mechanical properties of hexagonal commercially pure titanium (cp-titanium). Annealed cp-titanium sheets are subjected to tensile deformation along the rolling direction, along 45° to the rolling direction and along 90° to the rolling direction respectively. Crystallographic textures and mechanical properties of these cp-titanium samples are investigated in the present study. The hardness of different grains/orientations is estimated through nanoindentation, grain average misorientation, orientation estimated elastic stiffness and Taylor factor measurements. It is observed that the hardness of the grains close to basal orientation is higher compared to non-basal orientations. It is further observed that the estimated bulk mechanical properties of cp-titanium have a direct relationship with the volume fraction of basal grains/orientations.
Orientation-dependent solid solution strengthening in zirconium: a nanoindentation study
(Springer, 2020) Kumar, Gulshan
Orientation-dependent solid solution strengthening was explored through a combined microtexture plus nanoindentation study. Pure zirconium (6N purity crystal-bar Zr) and commercial Zircaloy-2 were investigated for comparison. Local mechanical properties were estimated through finite element (FE) simulations of the unloading part of the nanoindentation load–displacement response. Combinations of ‘averaging’ scheme and constitutive relationship were used to resolve uncertainty of FE-extracted mechanical properties. Comparing the two grades, non-basal oriented grains showed an overall hardening and increase in elastic modulus. In contrast, insignificant change was observed for basal (or near-basal) oriented grains. The strengthening of non-basal orientations appeared via elimination of the lowest hardness/stiffness values without a shift in the peak value. Such asymmetric development brought out the clear picture of orientation-dependent solid solution strengthening in zirconium.