Publications

Femtosecond laser-assisted surface texturing of compression piston rings is proposed and demonstrated. A femtosecond pulsed Ti3+:sapphire laser is used to generate dimples of the same size but with different area density on a moly-chrome ceramic deposited cast iron piston ring. The influence of the surface morphology and tribology operating conditions, such as reciprocating frequency and temperature of the lubricant, on friction and wear characteristics of textured piston rings and plateau honed liner samples is investigated. A decrease in the friction coefficient is observed with the texturing of rings. The dimple area density, which is defined as the ratio of the total textured area and the total area of the surface, of 16% and 27% offered a reduced coefficient of friction and minimize wear on the liner surface.

We report the applicability of the ultrafast pulse laser inscription technique to achieve high ablation depth on uncoated silicon wafer despite its higher surface reflectivity. The proposed methodology of this research work can be an alternative approach for the usual industrial practice of coating silicon surface with highly reflective materials for increasing the absorption phenomenon. To unveil the potential of the proposed methodology, a comparative study was carried out by fabricating microchannels of higher depth on uncoated and coated silicon wafer by varying repetition rate from 10 kHz to 500 kHz at a constant pulse energy of 18 μJ. The formation of ablation depth, ablation width and amorphous layer thickness was taken as the standard for evaluating the effectiveness of the proposed methodology. The experimental results revealed the formation of a higher ablation depth of 6.6 μm and an amorphous layer thickness of 0.039 μm for uncoated silicon material. Whereas, in the case of coated silicon material the ablation depth was found to be 3.199 μm with an amorphous layer thickness of 0.101 μm. This justified the applicability of the ultrafast pulse laser inscription technique for achieving quality microchannels having higher depth on silicon material without any surface coating. The underlying mechanism for the improved performance is due to the low temporal separation (μs) property of ultrafast lasers which results in negligible heat diffusion into the bulk material, thereby minimizing the collateral thermal damages. This was further proved based on an analytical model by evaluating the surface temperature at various repetition rates. The experimental and analytical results from the present work will be highly beneficial for the electronic industry, where the laser micro structuring of MEMS components made of silicon material is highly challenging due to its reflective property.

This paper covers an analytical approach for predicting the bend angle of a laser formed sheet with a single laser scan. The proposed approach is based on the strain energy stored under the laser-irradiated area that causes plastic deformation of sheet due to laser heating. With any particular intensity of laser, the temperature profile established across the sheet thickness is determined by employing a one-dimensional transient heat conduction equation and by considering the loss of heat to the surrounding material by thermal conduction. Based on the temperature gradient across the sheet thickness, thermal strain, strain-induced stress, and bending moment are evaluated. By equating the strain energy stored in the sheet material due to thermal stress and strain with the strain energy stored due to the bending moment, the angle of bend in the sheet is determined. The results obtained with the proposed model are validated with the experiments performed on AISI 304 stainless steel and AA 1100 aluminum alloy sheet material of different thickness using CO2 laser with varying power and scanning speed. A three-dimensional finite element model is developed to validate the temperature profile predicted by the proposed model. The effectiveness of the proposed approach is validated by comparing the predicted bend angle with those predicted by other analytical models. Apart from this, the effect of various process parameters on the laser formed sheet was studied using microstructure and microhardness analysis. Finally, with the application of the proposed model, a process map is generated to achieve pure bending during laser forming.

An integrated computational materials engineering (ICME)-based workflow was adopted for the study of microstructure and property evolution at the heat-affected zone (HAZ) of gas metal arc-welded DP980 steel. The macroscale simulation of the welding process was performed with finite element method (FEM) implemented in Simufact Welding® software and was experimentally validated. The time–temperature profile at HAZ obtained from FEM simulation was physically simulated using Gleeble 3800® thermo-mechanical simulator with a dilatometer attachment. The resulting phase transformations and microstructure were studied experimentally. The austenite-to-ferrite and austenite-to-bainite transformations during cooling at HAZ were simulated using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation implemented in JMatPro® software and with phase-field modeling implemented in Micress® software. The phase fractions and the phase transformation kinetics simulated by phase-field method agreed well with experiments. A single scaling factor introduced in JMatPro® software minimized the deviation between calculations and experiments. Asymptotic homogenization implemented in Homat® software was used to calculate the effective macroscale thermo-elastic properties from the phase-field simulated microstructure. FEM-based virtual uniaxial tensile test with Abaqus® software was used to calculate the effective macroscale flow curves from the phase-field simulated microstructure. The flow curve from virtual test simulation showed good agreement with the flow curve obtained with tensile test in Gleeble®. An ICME-based vertical integration workflow in two stages is proposed. With this ICME workflow, effective properties at the macroscale could be obtained by taking microstructure morphology and orientation into consideration.

The non-equilibrium response of a high entropy alloy CoCuFeNiTa0.5 has been studied using undercooling as a control parameter. The solidification growth rates are rapid (30–50 m/s) at deep undercooling (>150 °C) and are comparable with conventional alloys. The elemental segregation especially that of Cu as predicted by phase field simulations in lower (<50 °C) undercooling regime matches with the experimental observations. This study indicates that even extreme non-equilibrium conditions during solidification could not avoid elemental segregation at the atomic scale.