Deepak Marla

Critical assessment of the issues in the modeling of ablation and plasma expansion processes in the pulsed laser deposition of metals

This paper presents a review on the modeling of ablation and plasma expansion processes in the pulsed laser deposition of metals. The ablation of a target is the key process that determines the amount of material to be deposited; while, the plasma expansion governs the characteristics of the deposited material. The modeling of ablation process involves a study of two complex phenomena: (i) laser-target interaction and (ii) plasma formation and subsequent shielding of the incoming radiation. The laser-target interaction is a function of pulse duration, which is captured by various models that are described in this paper. The plasma produced as a result of laser–target interaction, further interacts with the incoming radiation, causing the shielding of the target. The shielding process has been modeled by considering the various photon absorption mechanisms operative inside the plasma, namely: inverse Bremsstrahlung, photoionization, and Mie absorption. Concurrently, the plasma expands freely until the ablated material gets deposited on the substrate. Various models describing the plasma expansion process have been presented. The ability of the theoretical models in predicting various ablation and plasma characteristics has also been compared with the relevant experimental data from the literature. The paper concludes with identification of critical issues and recommendations for future modeling endeavors.This paper presents a review on the modeling of ablation and plasma expansion processes in the pulsed laser deposition of metals. The ablation of a target is the key process that determines the amount of material to be deposited; while, the plasma expansion governs the characteristics of the deposited material. The modeling of ablation process involves a study of two complex phenomena: (i) laser-target interaction and (ii) plasma formation and subsequent shielding of the incoming radiation. The laser-target interaction is a function of pulse duration, which is captured by various models that are described in this paper. The plasma produced as a result of laser–target interaction, further interacts with the incoming radiation, causing the shielding of the target. The shielding process has been modeled by considering the various photon absorption mechanisms operative inside the plasma, namely: inverse Bremsstrahlung, photoionization, and Mie absorption. Concurrently, the plasma expands freely until the abla...

Models for predicting temperature dependence of material properties of aluminum

A number of processes such as laser ablation, laser welding, electric discharge machining, etc involve high temperatures. Most of the processes involve temperatures much higher than the target melting and normal boiling point. Such large variation in target temperature causes a significant variation in its material properties. Due to the unavailability of experimental data on material properties at elevated temperatures, usually the data at lower temperatures is often erroneously extrapolated during modelling of these processes. Therefore, this paper attempts to evaluate the variation in material properties with temperature using some general and empirical theories, along with the available experimental data for aluminum. The evaluated properties of Al using the proposed models show a significant variation with temperature. Between room temperature and near-critical temperature (0.9Tc), surface reflectivity of Al varies from more than 90% to less than 50%, absorption coefficient decreases by a factor of 7, thermal conductivity decreases by a factor of 5, density decreases by a factor of 4, specific heat and latent heat of vapourization vary by a factor between 1.5 and 2. Applying these temperature-dependent material properties for modelling laser ablation suggest that optical properties have a greater influence on the process than thermophysical properties. The numerical predictions of the phase explosion threshold in laser ablation are within 5% of the experimental values.

Modeling of electrochemical micromachining: comparison to experiments

A detailed theoretical modeling of the electrochemical micromachining (EMM) process has been conducted here. The model takes into account a very small interelectrode gap thickness (order of a few micrometers) under which the electric field between the electrodes becomes a predominant factor in governing the machining process. The effect of electric double layer (EDL) also becomes significant under such conditions. The governing equations of the material removal rate are derived by considering a one-dimensional electric field, which is perpendicular to the cross section of the electrode. The model also incorporates the applied pulsed voltage used in EMM. A comparison between the theoretical and the experimental results indicate that, under high voltage and small interelectrode gap, the electric field and the formation of EDL have a significant effect on the material removal rate.

A Computational Model to Study Film Formation and Debris Flushing Phenomena in Spray-Electric Discharge Machining

A computational model to investigate the flushing of electric discharge machining (EDM) debris from the interelectrode gap during the spray-EDM process is developed. Spray-EDM differs from conventional EDM in that an atomized dielectric spray is used to generate a thin film that penetrates the interelectrode gap. The debris flushing in spray-EDM is investigated by developing models for three processes, viz., dielectric spray formation, film formation, and debris flushing. The range of spray system parameters including gas pressure and impingement angle that ensure formation of dielectric film on the surface is identified followed by the determination of dielectric film thickness and velocity. The debris flushing in conventional EDM with stationary dielectric and spray-EDM processes is then compared. It is observed that the dielectric film thickness and velocity play a significant role in removing the debris particles from the machining region. The model is used to determine the spray conditions that result in enhanced debris flushing with spray-EDM.

Transient Analysis of Laser Ablation Process With Plasma Shielding: One-Dimensional Model Using Finite Volume Method

This paper presents a comprehensive transient model of various phenomena that occur during laser ablation of TiC target at subnanosecond time-steps. The model is a 1D numerical simulation using finite volume method (FVM) on a target that is divided into subnanometric layers. The phenomena considered in the model include: plasma initiation, uniform plasma expansion, plasma shielding of incoming radiation, and temperature dependent material properties. It is observed that, during the target heating, phase transformations of any layer occur within a few picoseconds, which is significantly lower than the time taken for it to reach boiling point (~ns). The instantaneous width of the phase transformation zones is observed to be negligibly small (<5nm). In addition, the width of the melt zone remains constant once ablation begins. The melt width decreases with an increase in fluence and increases with an increase in pulse duration. On the contrary, the trend in the ablation depth is exactly opposite. The plasma absorbs about 25–50% of the incoming laser radiation at high fluences (20-40 J/cm2), and less than 5% in the range of 5-10 J/cm2. The simulated results of ablation depth on TiC are in good agreement at lower fluences. At moderate laser fluences (10-25 J/cm2), the discrepancy of the error increases to nearly ±7%. Under prediction of ablation depth by 15% at high fluences of 40 J/cm2 suggests the possibility of involvement of other mechanisms of removal such as melt expulsion and phase explosion at very high fluences.

A study on curved surface laser ablation using beam profile approach

Laser ablation process was used to generate micro-scale curved surfaces that find applications such as micro-lens and hydrophobic surfaces. Curved surface laser ablation to produce micro-scale features generally involves altering the spatial beam profiles using masks with varying spatial opacity. The surface profile obtained mainly depends on the altered laser beam profile. This work aimed at understanding the influence of laser beam profile on the generated surface in curved surface laser ablation. The study was based on a 2D finite element model of laser ablation that considered target heating, heat transfer and vaporisation on a poly methyl methacrylate (PMMA) target. Using this model, surface profile was predicted for various profiles of laser intensity and a qualitative analysis of curved surface profiles was carried out. The quantitative comparison of the ablation depths for uniform beam was used to validate the numerical model. The results of ablation depth were found to be in good agreement with that of the experiments.

Atomized Dielectric Spray-Based Electric Discharge Machining for Sustainable Manufacturing

A novel method of using atomized dielectric spray in micro-electric discharge machining (EDM) (spray-EDM) to reduce the consumption of dielectric is developed in this study. The atomized dielectric droplets form a moving dielectric film up on impinging the work surface that penetrates the interelectrode gap and acts as a single phase dielectric medium between the electrodes and also effectively removes the debris particles from the discharge zone. Single-discharge micro-EDM experiments are performed using three different dielectric supply methods, viz., conventional wet-EDM (electrodes submerged in dielectric medium), dry-EDM, and spray-EDM in order to compare the processes based on material removal, tool electrode wear, and flushing of debris from the interelectrode gap across a range of discharge energies. It is observed that spray-EDM produces higher material removal compared to the other two methods for all combinations of discharge parameters used in the study. The tool electrode wear using atomized dielectric is significantly better than dry-EDM and comparable to that observed in wet-EDM. The percentage of debris particles deposited within a distance of 100 μm from the center of EDM crater is also significantly reduced using the spray-EDM technique.

Atomized Dielectric Spray-Based Electric Discharge Machining (Spray-EDM) for Sustainable Manufacturing

A novel method of using atomized dielectric spray in EDM to reduce the consumption of dielectric is developed in this study. The atomized dielectric droplets form a moving dielectric film up on impinging the work surface that penetrates the inter-electrode gap and acts as a single phase dielectric medium between the electrodes and also effectively removes the debris particles from the discharge zone. EDM experiments are performed using three different dielectric supply methods, viz., conventional wet-EDM (electrodes submerged in dielectric medium), dry-EDM and spray-EDM in order to compare the processes based on material removal, tool electrode wear and flushing of debris from the inter-electrode gap across a range of discharge energies. It is observed that spray-EDM produces higher material removal compared to the other two methods for all combinations of discharge parameters used in the study. The tool electrode wear using atomized dielectric is significantly better than dry-EDM and comparable to that observed in conventional wet-EDM. The percentage of debris particles deposited within a distance of 100 μm from the center of EDM crater is also significantly reduced using the atomized dielectric spray EDM technique.Copyright