Browsing by Author "Roy, Tribeni"

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Analysis of material removal mechanism in nanoscale machining of copper
(Sage, 2024-01) Sharma, Anuj; Roy, Tribeni
In the current study, molecular dynamics modeling and simulation are carried out to analyse mechanisms in tool-workpiece interaction in nanoscale cutting. Various combinations of a/r ratios for constant r (r = tool edge radius and a = uncut chip thickness) are considered and different crystal orientations of the workpiece specimen are employed in the nanoscale cutting model. From the simulation, material anisotropy behavior is observed during the nanoscale cutting of copper material. Analysis at the molecular scale reveals that the crystal orientations family {1 1 0}<1 0 0> is hard to machine and the family of crystal planes {1 1 1}<1 1 0> is easiest to cut. While comparing in different crystal planes and directions, it was noticed that the material deformation in nanoscale machining takes place only in slip directions, that is, <1 1 0> family of directions. It is also found that as the uncut chip thickness is decreased, the cutting mechanism changes from shear plane cutting to plowing to sliding in Cu.
Atomistic study on the effect of the size of diamond abrasive particle during polishing of stainless steel
(Sage, 2023-10) Roy, Tribeni; Sharma, Anuj
Prediction of material removal in any machining process is usually based on the input machining parameters. However, apart from controllable parameters, there are various other parameters that needs to be monitored in real time to ensure better prediction of accuracy, especially in random processes. Hence, real time data monitoring using appropriate sensors in machining processes is extremely important as the input parameters cannot predict the output with high efficiency. In Micro EDM (MEDM), real time signal monitoring can yield various time domain features of individual current and voltage pulses that can help to enhance the prediction accuracy of material removal. In this study, an attempt has been made to predict the material removal in single spark MEDM based on two different modelling approaches i.e. multiple linear regression (MLR) and classification and regression tree (CART). A total number of 21 experiments were conducted on a specially designed single spark MEDM machine with input parameters viz. voltage and capacitance. Material removal measurements was carried out using Coherent Correlation Interferometer. Open source software “R-3.4.0” was used for building and prediction of the model. A total of 14 predictors (2-input and 12-time domain extracted predictors) and a single output i.e. material removal was used for prediction. Prediction model by multiple linear regression (MLR) showed root mean square error of 5.82 whereas that by CART showed 12.07. Hence, material removal in single spark MEDM can be predicted by MLR with better accuracy as compared to CART.
A close-packed sphere model for characterising porous networks in atomistic simulations and its application in energy storage and conversion
(Elsevier, 2024-05) Belgamwar, Sachin U.; Mishra, Radha Raman; Roy, Tribeni
Hierarchical (micro, meso & macro) porosity in materials plays a crucial role in influencing the movement of ions which governs the energy and power density during energy storage and conversion. The extant available methods to characterise porosity across scales (nano to meso to macro) lacks rigour and accuracy. Having accurate assessment of the porosity in materials can unlock new designs of electrodes for energy efficient energy storage and conversion devices such as batteries, supercapacitors and fuel cells. Through this work, we report the systematic development of a method to fully characterise the carbon porous networks using a molecular dynamics simulation testbed. Our work entails modelling and simulation of porous carbon structures using quenched molecular dynamics (QMD) simulations using Gaussian Approximation potential (GAP) and benchmarking the results with prior literature. This modelling technique can reliably be used for quantitative characterisation of the interconnectivity in porous structures to study ionic movements and charge transfer mechanisms. A new parameter, namely nearest neighbour search (NNS) coefficient was introduced to quantify homogeneity and networking in the porous structures. NNS coefficient increased from 1.62 to 1.92 with decrease of the annealing temperature from 8000 K to 4000 K in carbon. The procedure outlined was although tested on porous carbon networks, but adaptable to study any other material system at multi-length scales.
Comparative analysis of mechanical properties for mono and poly-crystalline copper under nanoindentation – Insights from molecular dynamics simulations
(Elsevier, 2022-02) Roy, Tribeni; Sharma, Anuj
Crystallographic orientation and grain size for monocrystalline and polycrystalline materials respectively play a critical role in defining their mechanical behaviour under nanoindentation. To understand their effects on mechanical properties, molecular dynamics (MD) simulations help in revealing the underlying physical phenomena governing the nanoindentation behaviour. This paper attempts to comparatively analyse and study the effects of crystallographic orientations of monocrystalline copper {(100), (110) and (111)} and critical grain size of polycrystalline copper on the nanoindentation response using MD simulations. The results obtained for indentation load vs. depth curve, hardness, dislocations, and elastic recovery were analysed for comparison. Cu(111) exhibited an average hardness of 12.62 GPa, which is 18.27% more than that of Cupoly. The pile-ups of 8 Å size were observed in Cupoly; and this was higher than any of copper system studied here. The dislocation extraction algorithm (DXA) analysis revealed that the total dislocations in Cu(111) was 34.23% and 153.8% lower than that of Cu(110) and Cupoly, respectively. Cu(111) comprised of highest Stair-rod dislocation along with LC and Hirth locks. Furthermore, a prismatic loop comprised of sessile dislocations also appeared in Cu(111). The elastic depth recovery rate for Cu(100) was 52.75%, 41.60% and 40.66% higher than that of Cu(110), Cu(111) and Cupoly, respectively. This study revealed that the nanoindentation based mechanical performances of monocrystalline copper systems, specifically Cu(111) were superior to any other copper systems.
Corrigendum to “Postulation of optimal charging protocols for minimal charge redistribution in supercapacitors based on the modelling of solid phase charge density
(Elsevier, 2021-08) Roy, Tribeni
The authors regret that the model equations and the model codes used in the original article contained errors. Specifically, Eq.(20) of the original article omitted a liquid phase electrical potential term. Further, the parameters used in the original model codes were subject to input errors which were later determined to produce erroneous results. The model equations, input parameters and model codes have been corrected and are presented in this Corrigendum. For the most part, the qualitative results and conclusions are similar to those presented in the original article. However, the corrections have led to significant changes to the results on which the analysis conducted in section 3.5 was based. As a consequence, the results presented and the conclusions drawn therein have been subject to major revisions.
Dataset for "Strain induced electrochemical behaviours of ionic liquid electrolytes in an electric double layer capacitor: Insights from molecular dynamics simulations"
(Zenodo, 2020-10) Roy, Tribeni
The datafile contains molecular dynamics simulation results for analysing the electrochemical behaviour of ionic liquid based EDLC under compression and tension.
Debris based discharge segregation in reverse micro EDM
(Elsevier, 2020-03) Roy, Tribeni
Segregation of discharge pulses in micro electrical discharge machining (MEDM) and its variants viz. wire EDM, reverse micro EDM (RMEDM) etc. is mostly carried out on the basis of voltage and current signals. With increase in the machining time, more and more debris accumulate thereby causing abnormal discharges which degrades the machining process. This paper attempts to model discharges in the presence of debris, which hitherto has not been established in literature. Two different simulations were carried out (a) to segregate discharges based on electric field intensity in the presence or absence of debris and (b) to determine the minimum size of debris agglomeration required for a full discharge at increasing machining time where open circuit voltage (OCV) reduces. Based on the magnitude of electric field intensity, the discharge pulses are segregated into three stages: primary or normal discharge without debris, secondary discharge with singular debris particles and higher order discharge in the presence of debris agglomerates. Segregation of discharge pulses in case of experiments is done based on magnitude of voltage which shows that primary discharges dominate initially, however, with increase of machining time, the number of secondary and higher order discharges increase as compared to the primary discharges. A good agreement was established between simulation (segregation of discharges based on magnitude of electric field intensity) and experiments (segregation of discharges based on magnitude of voltage). Moreover, to initiate a full discharge at reduced OCV (higher machining time), a higher size of agglomerated debris is required as compared to the case with rated OCV; wherein full discharge occurs in the absence of debris. The size of agglomerated debris required for initiating a full discharge at lower OCV is very small as compared to the size of actual agglomerated debris, thereby, confirming that number of primary discharges are comparatively lower than the secondary or higher order discharges at higher machining time.
Designing porous electrode structures for supercapacitors using quenched MD simulations
(Elsevier, 2022) Mishra, Radha Raman; Belgamwar, Sachin U.; Roy, Tribeni
Recently, supercapacitors with hierarchical porous structured electrodes are gaining a lot of research interest due to their unique qualities such as high power, durability and eco-friendly nature. In this study, porous structured electrodes were generated using quenched molecular dynamics (QMD) simulations, that can provide high energy density by virtue of high porosity. Here, three different quench rates (16, 8 and 4 K/ps) were applied on liquid carbon system to generate different porous structures. It was observed that at 4000 K, the carbon atoms become disorderly bonded and arranges themselves in an ordered hexagonal ring sheets after the completion of quenching process at 300 K. The porous carbon structures were visualized by contour surface mesh. The pore size distribution showed an increase of 62% on decreasing the quench rate from 16 K/ps to 4 K/ps. These light-weight porous carbon structures may also be tested for mechanical and electrical performances, which can have future implications as electrodes for supercapacitor.
Diamond turned hierarchically textured surface for inducing water repellency: Analytical model and experimental investigations
(Elsevier, 2021-03) Roy, Tribeni
Functional surfaces have gained a lot of interest due to enhancement in water repellency for advanced engineering applications. In the present era, single and multi-level texturing is usually carried out on surfaces for inducing complete water repellency. The role of a hierarchical textured surface for enhancing the contact angle (CA), however, has not been extensively studied and understood. In the present work, the physical interpretation of water droplet activity on a hierarchical textured surface has been studied by the development of a mathematical model taking into account different forces responsible for inducing water repellency. Subsequently, hierarchical textured surface i.e., arrays of major pillars with rectangular sub-micron minor pillars over its top surface, were fabricated with high precision by using a diamond turn machining (DTM) process. The fabricated hierarchical textured surface was investigated based on the structural dimension and static CA. Also, the effects of variation of spacing between major pillars (p), the ratio of spacing between minor pillars to the minor pillar width and ratio of minor pillar to major pillar width with CA were studied. The developed mathematical model was able to predict the water CA with dimensions similar to experimentally fabricated hierarchical textured surface with an error of 6.67%. The model is capable of designing and optimizing the hierarchical textured surface of various sizes, which enables their manufacturing in a cost-effective way.
Effect of various factors influencing the generation of hemispherical micro features using non-conformal RMEDM
(Sage, 2019-04) Roy, Tribeni
Non-conformal reverse micro electrical discharge machining (NC-RMEDM) is a variant of conventional RMEDM developed by the present authors wherein modification in the tool is carried out to generate different shapes of micro features. In this study, the effect of various factors like flat bottom and taper bottom hole, inversing the position of tool and workpiece and changing the hole depth have been experimentally investigated to determine the optimal combination required for generating hemispherical shaped micro features. It was found that hemispherical shaped micro feature can be generated by employing tapered bottom blind hole as tool. Buoyancy assisted machining (BAM) with traverse of workpiece (anode) downwards into the tool (cathode) and vice versa, i.e., buoyancy opposed machining (BOM) with traverse of tool downwards into the workpiece were carried out to study the generation of hemispherical micro feature based on inversion of electrode positions. Although both BAM and BOM generated hemispherical shaped feature, BAM is preferred due to reduced machining time as opposed to BOM. Also, increasing the hole depths led to altering the shape of micro feature from hemispherical to cylindrical with hemispherical end and coni-spherical end. An array of hemispherical micro features was fabricated based on the finding from this study, and surface roughness analysis was carried out which showed that irrespective of the position of micro feature on the array, surface roughness at the tip and base of the micro feature is lower as compared to side portion.
Efficient fabrication of zero taper µ-electrode using novel dry µ-electrical discharge turning
(Sage, 2025-01) Roy, Tribeni
Micromachining is essential for producing components smaller than 1000 µm and is increasingly significant due to the trend toward miniaturizing industrial products. Tool-based micromachining techniques like µ-turning, µ-grinding, µ-EDM, and µ-ECM offer benefits in productivity, efficiency, adaptability, and cost-effectiveness. Modern industrial products require high dimensional accuracy and superior surface finishes. µ-EDM is particularly promising for economically producing complex microfeatures with high precision. However, the high tool wear rate in µ-EDM requires on-machine microelectrode fabrication, which is essential for creating micro-sized holes and channels during µ-EDM operations. A major challenge lies in achieving high dimensional accuracy and minimal taper in microelectrodes. Additionally, there are health concerns related to hydrocarbons produced from liquid dielectrics. This research explores fabrication of microelectrodes using dry µ-ED Turning. Five machining strategies were employed, using stationary block electrical discharge turning (SB-EDT), moving block electrical discharge turning (MB-EDT), and their combinations to produce microelectrodes with improved surface finishes and minimal taper. Gaseous dielectrics significantly reduce harmful emissions and contamination, creating a safer working environment while minimizing environmental pollution. The taper issue was significantly mitigated in microelectrodes fabricated using MB-EDTd, achieving nearly zero taper with a surface finish of 2.04 µm. MB-EDTd also achieved highest material removal rate, producing a microelectrode with an average diameter of 428 µm. Scanning electron microscope (SEM) micrographs and surface finish analyses were conducted to examine the effects of various parameters, such as discharge currents and linear feed speed. The findings highlight the potential of µ-EDM with gaseous dielectrics to enhance microelectrode fabrication process.
Electrical Discharge Diamond Grinding
(CRC Press, 2022) Roy, Tribeni
The current requirement for complex systems has prompted the development of novel materials that are both strong and lightweight. Machining complex shapes on these materials is always a difficulty. Also, it is challenging to achieve satisfactory results with a single machining process. As a result, hybrid machining processes are commonly utilized to achieve excellent precision and polish. Electrical discharge diamond grinding (EDDG) is an example of a hybrid process that combines the benefits of both EDM and mechanical grinding. Hard and brittle conductive materials are softened by electrical discharges, which is followed by mechanical grinding to remove the softened layer. EDDG eliminates the difficulties associated with EDM (such as recast layer, heat-affected zone) and scratching due to excessive grinding forces. This chapter provides an overview of mathematical models used in the EDDG process to predict cutting forces and material removal.
Fabrication, characterization and comparative analysis of mechanical properties of micro features generated by reverse micro EDM
(Springer, 2019-07) Roy, Tribeni
Moderate to high aspect ratio micro features (projected) finds wide application in MEMS components, fuel cells for micro reactors, sensors and actuators, micro optical components etc. Reverse micro electro discharge machining (RMEDM) is one of the extensively used process for fabricating such micro features. The present authors have already developed a technique for generating different shapes and sizes of micro features by suitable modification of tool in RMEDM. Mechanical properties like hardness and elastic modulus of these micro features play a significant role in determining their suitability for various applications. Therefore, the objective of this paper is (a) to generate projected micro features corresponding to tapered blind hole depths of 0.1 mm and 0.3 mm using RMEDM and (b) to determine the effect of tapered blind hole depth on the hardness and elastic modulus of the projected micro features. Hardness and elastic modulus were measured using nano indentation technique. Results indicate that starting from the periphery, both hardness as well as elastic modulus first increase (presence of recast layer) and then decrease (presence of heat affected zone) for projected micro feature corresponding to 0.1 mm hole depth before attaining the properties of the parent material. Due to high amount of debris agglomeration in case of 0.3 mm hole depth, high amount of abnormal discharges occur which do not provide sufficient time for the formation of thermally softened layer as was in the case of micro feature corresponding to 0.1 mm hole depth. Hardness, therefore, is always high starting from recast layer to the HAZ before finally attaining the hardness of the parent material. Debris agglomeration did not have much influence on the elastic modulus and the variation of elastic modulus of the micro feature corresponding to 0.3 mm hole depth remains fairly uniform a compared to the parent material.
Feasibility of Inducing Superhydrophobicity on Laser-Textured Surfaces: Development of Mathematical Model and Experimental Investigations
(World Scientific, 2022) Roy, Tribeni
Hydrophobicity is a prominent characteristic of a surface that governs its applications in domains such as wear reduction by lubrication retention, self-cleaning surfaces, fluid drag reduction, viscosity testing, development of oleophobic coatings, etc. A superhydrophobic surface exhibits a water contact angle (CA) of 150∘ or larger. High surface energy of nontextured surface limits its wettability. Texturing of a surface imparts low surface energy which proves to be favorable for enhancing the overall surface hydrophobicity. Research and analysis done to fathom an optimum method by which surfaces accomplish superhydrophobicity is still miniscule. It is challenging to fabricate superhydrophobic surfaces by micro-machining due to the expansive range of the features involved. To minimize the exorbitant costs incurred due to trial-and-error-based experimentation, a mathematical model with >90% accuracy has been developed in this study, which would help determine the closest ranges of values of parameters like micro-dimple diameter and areal density responsible for inducing superhydrophobic properties on a micro-dimpled specimen. The exceptionality of this study lies in the fact that though mathematical models are available for textures like micro-grooves and micro-pillars, but miniscule research is available for micro-dimpled surfaces with hardness greater than 55 HRC.
From data to alloys predicting and screening high entropy alloys for high hardness using machine learning
(2025-09) Roy, Tribeni
The growing need for structural materials with strength, mechanical stability, and durability in extreme environments is driving the development of high entropy alloys. These are materials with near equiatomic mixing of five or more principal elements, and such compositional complexity often leads to improvements in mechanical properties and high thermal stability, etc. Thus, high-entropy alloys have found their applications in domains like aerospace, biomedical, energy storage, catalysis, electronics, etc. However, the vast compositional design and experimental exploration of high-entropy alloys are both time consuming and expensive and require a large number of resources. Machine learning techniques have thus become essential for accelerating high entropy alloys discovery using data driven predictions of promising alloy combinations and their properties. Hence, this work employs a machine learning framework that predicts high entropy alloy hardness from elemental descriptors such as atomic radius, valence electron count, bond strength, etc. Machine learning regression models, like LightGBM, Gradient Boosting Regressor, and Transformer encoder, were trained on experimental data. Additionally, a language model was also fine tuned to predict hardness from elemental descriptor strings. The results indicate that LightGBM has better accuracy in predicting the hardness of high entropy alloys compared to other models used in this study. Further, a combinatorial technique was used to generate over 9 million virtual high entropy alloy candidates, and the trained machine learning models were used to predict their hardness. This study shows how machine learning-driven high throughput screening and language modelling approaches can accelerate the development of next generation high entropy alloys.
Fundamental insights of mechanical polishing on polycrystalline Cu through molecular dynamics simulations
(Elsevier, 2022-08) Belgamwar, Sachin U.; Roy, Tribeni
Mechanical polishing is an ultra-precision class finishing process to achieve a nanoscale surface finish. During nano-polishing of any engineering materials, the material removal takes place in the form of atomic-clusters. However, quantifying the process characteristics becomes difficult from a mechanical and metallurgical point of view. To understand the mechanism of material removal on polycrystalline material during nano-polishing through abrasives, a molecular dynamics simulation has been implemented. This simulation work investigates nano-polishing on polycrystalline copper (p-Cu) with two different cases of abrasive sizes at different cutting velocities. Results were analysed for the temperature, percentage of material removal, shear slip planes, dislocations, and interaction forces between abrasive and p-Cu workmaterial. Increased abrasive sizes and velocities resulted in increasing interaction forces during nano-cutting until shear slip formation. The p-Cu grains adjacent to the abrasive undergoes different elastoplastic deformation due to the shear slip and sessile dislocations, which affected the surface finish. This work is helpful to utilize mechanical nano-polishing or diamond turning process parameters for efficient material removal from polycrystalline surface.
Impedance Response of Ionic Liquids in Long Slit Pores
(IOP, 2022-12) Roy, Tribeni
We study the dynamics of ionic liquids in a thin slit pore geometry. Beginning with the classical and dynamic density functional theories for systems of charged hard spheres, an asymptotic procedure leads to a simplified model which incorporates both the accurate resolution of the ion layering (perpendicular to the slit pore wall) and the ion transport in the pore length. This reduced-order model enables qualitative comparisons between different ionic liquids and electrode pore sizes at low numerical expense. We derive semi-analytical expressions for the impedance response of the reduced-order model involving numerically computable sensitivities, and obtain effective finite-space Warburg elements valid in the high and low frequency limits. Additionally, we perform time-dependent numerical simulations to recover the impedance response as a validation step. We investigate the dependence of the impedance response on system parameters and the choice of density functional theory used. The inclusion of electrostatic effects beyond mean-field qualitatively changes the dependence of the characteristic response time on the pore width. We observe peaks in the response time as a function of pore width, with height and location depending on the potential difference imposed. We discuss how the calculated dynamic properties can be used together with equilibrium results to optimise ionic liquid supercapacitors for a given application.
Influence of gaseous dielectrics on the wettability of Al-6061 alloy using dry µ-electrical discharge milling
(Elsevier, 2025-03) Roy, Tribeni
Superhydrophobic surfaces are essential for applications such as self-cleaning and corrosion resistance. This study explores the fabrication of superhydrophobic surfaces on Al-6061 alloy using dry micro electrical discharge milling (µEDM milling) and investigates the effects of gaseous dielectrics on wettability under atmospheric conditions. Surfaces machined in an oxygen environment showed a surface area roughness (Sa) of 4.68 µm, 73.3 % higher than those machined in argon, attributed to enhanced metal oxide formation. Energy Dispersive X-ray Analysis (EDAX) indicated a significantly elevated O/Al atomic ratio of 0.80 for oxygen-machined samples, which was 73.9 % greater than argon-machined samples, suggesting the presence of hydrophilic compounds such as AlO(OH) and Al2O3. Additionally, X-ray Photoelectron Spectroscopy (XPS) revealed that Al2O3 peaks on oxygen-machined surfaces were broadened and shifted to higher binding energies, correlating with decreased contact angles and increased surface hydrophilicity. These results suggest that using oxygen as a dielectric in the electrical discharge machining (EDM) process promotes oxide layer formation, resulting in hydrophilic characteristics, while machining in argon encourages organic adsorption and greater hydrophobicity. The findings of this research provide insights that can inform future efforts to tailor surface properties for advancements in aerospace, automotive, and biomedical engineering applications.
Influence of ion-rich plasma discharge channel on unusually high discharging points in reverse micro electrical discharge machining
(Springer, 2020-01) Roy, Tribeni
Different discharges occurring during machining in micro electrical discharge machining (MEDM) and its variants, viz. reverse MEDM (RMEDM) and wire EDM, affect the shape, size, surface roughness, and surface integrity of the generated surface. Of these different discharges, there are certain discharges with unusually high discharging points (well above open circuit voltage) during machining that have not been analysed till date. This study primarily aims in understanding the physical phenomenon behind occurrences of these unusual discharges specific to RMEDM process. This understanding should also hold good for MEDM and other electric discharge-based machining processes. A numerical model was developed taking into account the movement of ions and electrons in the dielectric during machining. The model predicted that the presence of ions only in the plasma discharge channel for a very short period of time during machining leads to occurrences of these unusually high discharging points (~170% of open circuit voltage, OCV) which was also verified from experiments (162–164% of OCV).
Influence of micro-textures on wettability and antibacterial behavior of Titanium surfaces against S. aureus and E. coli: in vitro studies
(Springer, 2023-04) Roy, Tribeni
Bacterial adhesion to the surface can quickly lead to the development of biofilms, which can create a variety of economic and health issues. In the marine industry, Biofouling causes sailing resistance, resulting in higher fuel consumption and waste emissions for boats, ships, and submarines. Metals such as titanium and its alloys along with stainless steel are commonly used in orthopedic implants and the most common complications seen after the implantation are bacterial infections acquired through invasive and post-operative medical procedures. This type of infection damages the bone and surrounding tissues. Separating a biofilm from an implant surface is time-consuming because even minor biofilm residues left on the implant surface might cause the infection to reappear. Implant replacement or long-term antibiotic therapy are usually suggested in such circumstances, which is not optimal for individuals with co-morbid conditions. In the case of long-term use of antibiotics, antibiotic resistance is a serious issue. As a result, several techniques aimed at lowering the risk of bacterial infections in bio-implants are critical. Surface textures on a micro scale can help in the fight against Biofouling by increasing antibacterial properties, preventing bacterial adhesion, or killing or inactivating adherent microorganisms and creating an inapt environment for biofilm formation. As a result, the capacity to generate passive antibacterial surfaces on components has far-reaching consequences in practically every industry. In this study, micro-textures were generated on the surfaces of titanium (Grade 5), aluminum, and stainless steel. The topographical characteristics of the textured surfaces were studied, and a comparative study of wettability between textured and non-textured surfaces was conducted. S. aureus and E. coli, being the most common infection-causing bacteria in implants and were used to investigate the antibacterial properties. The results show that the textured surface lowers bacterial adhesion and growth up to 91.57% in comparison to the non-textured samples for E. coli and 51.40% for S. aureus, which can be related to the surface topography and the presence of peaks and troughs, which also contribute to the surface’s Hydrophilicity