Publications

Cooling holes in turbine blades made of high-temperature materials such Titanium alloys are produced by laser processing. A priori knowledge on laser interaction with material will be useful in the selection of laser parameters in practice. In the present work, two-temperature model consisting of a set of coupled Partial Differential Equations in spatial and time domain is used to study ultra-short pulse laser-matter interaction. The model is solved using finite element simulation available in COMSOL Multi-physics software. The present approach is validated taking gold as bench mark material as results for 1D and 2D cases are already reported in literature. The simulation approach is then extended to titanium alloy (Ti6Al4V), the material under investigation in our present work. The simulation results are obtained for 2.00 mm thick Ti6Al4V using 2D axi-symmetric two-temperature model. In order to compare the results, single-shot laser ablation experiments are carried out at laser fluency ranging from 0.84 to 8.4 Jcm−2. A method has been proposed in this work for assessing the crater depth and diameter uniquely from the images of the ablated specimens obtained using laser scanning confocal microscope. The simulation and experimental results are presented and discussed.

Design and development of microscale features are found to be an evolving field of interest in various manufacturing industries including aerospace, automobile, spacecraft, and biomedical. Even though there are various advancements in ultraprecision machining techniques, accomplishment of microscale features with higher geometrical quality is still found to be the critical area of research, which needs to be explored as it affects the performance of the micro-components. Hence, in the present work a detailed investigation on the lasing parameter with respect to the surface integrity of microfeature has been carried out, and it is discussed in detail. Microfeatures in the form of channels and circular profile were machined on Ti6Al4V using ultra-short pulsed laser trepanning technique at various scan speeds. All the laser processed surfaces were analyzed using an optical microscope and 3D profilometer to evaluate the formation of heat-affected zone. Experimental results show a significant reduction in the width of heat-affected zone with the increase in scan speed from 2 to 2000 mm/s. Further analysis on the profile of the microfeature depicted the occurrence of higher order distortions at scan speed of 2 mm/s, which can be attributed to the occurrence of re-solidification layer and debris entrapment. A benchmark can be set from the current observations for the future investigations in selecting the optimal scan speed for achieving high-quality microfeatures on Ti6Al4V.

High-aspect-ratio high-quality microholes are required in turbine blades to improve cooling performance. These cooling holes are drilled by pulsed laser and hence dimensional as well as geometrical tolerances like circularity and cylindricity are important. The measurement of geometrical features of the microholes is a very challenging task without destroying the components. In the present work, the microholes are produced on Ti6Al4V alloy by ultra-short pulse laser. The geometrical features of microholes are then captured using a non-destructive technique, namely computed tomography. CT-scanned 3D data is directly used for geometrical analysis using open-source software, GOM Inspect. Since algorithms used in the GOM Inspect are proprietary in nature, the extracted coordinate data are also analyzed using the computational methods developed by the authors based on least squares technique. The dimension, circularity, and cylindricity of microholes are compared with the results obtained from GOM Inspect software and a close match is found.

In the current work, multipoint single layer brazed diamond dressers were indigenously developed by high vacuum brazing technology using Ni-Cr and Ag-Cu-2Ti alloy. The vacuum level was maintained in the range of 10−6–10−7 mbar. To assess the quality, the developed dressers were subjected to eight dressing cycles with increasing grit penetration depth. The Ag-Cu-2Ti alloy, possessing a lower liquidus temperature of 820 °C, showed a superior grit retention ability compared to Ni-Cr alloy, having a higher liquidus temperature of 1050 °C. Higher thermal residual stresses developed in the case of Ni-Cr alloy, due to higher brazing temperature and difference in Young’s moduli and the coefficient of thermal expansion of the diamond grit, filler alloy, unfavorably led to the premature bond level failure of brazed diamond grit under lower specific loads. Elevated brazing temperature and formation of unfavorable intermetallic phases were found impairing the effectiveness of Ni-Cr alloy and caused severe graphitization of diamond. On the other hand, the silver-based bond experienced a potential threat of rapid wear by the hard bond-abrasive system of the grinding wheel during dressing. Localized “scooping” of bond material was observed in Ag-based bond layer securing diamond grits. Through a separate pin-on-disc test, it was demonstrated that this alloy received a substantial wear rate of 3.364 mm3/min and exhibited deterioration of frictional behavior, despite the presence of silver. The Ag-Cu-2Ti alloy with lower brazing temperature and lower tendency of graphitization has otherwise a more promising scope but only if the composition is suitably altered to bring in significantly higher abrasion resistance in its mechanical characteristics.

A heat transfer model of nanosecond laser irradiation on a copper thin film coated on a polyimide substrate is developed and studied to understand the thermal influence on thin films during the laser ablation. Temperature distribution inside the target is calculated using heat conduction equation considering hydrodynamic boundary conditions. Effect of plasma shielding on the ablation process due to inverse bremsstrahlung is considered in the model. The study clearly shows that the shielding effect of plasma decreases the laser energy reaching the target surface and therefore reduces the depth of micro crater formed by a single pulse. A comparative study has been made for depth achieved for micro-craters fabricated on copper thin films in air using a Q-switched Nd3+:YAG laser with 6 ns pulse duration for various laser intensities. The simulation results are compared with experimental data for the laser ablation depth which is measured using 3D optical non-contact profilometer.