Laser-workpiece Interaction Research Articles

The Caputo‐Fabrizio heat transport law in vibration analysis of a microscale beam induced by laser

AbstractIn the application of pulsed laser heating, such as laser hardening of metallic surfaces, conduction limited process is the dominant mechanism during the laser–workpiece interaction. As a consequence, time unsteady analysis of this problem becomes necessary. The present study examines the effect of ultra‐short‐pulsed laser heating in coupled thermoelastic vibration of a microscale beam resonator. The heat transport equation for the current problem is defined by incorporating the three‐phase‐lag heat transport law in the context of Caputo‐Fabrizio (CF) derivative. Finite sinusoidal Fourier and Laplace transform techniques have been employed to determine the lateral vibration of the beam and the temperature increment within the microscale beam. According to the graphical representations corresponding to the numerical results, conclusions about the new theory is constructed. Excellent predictive capability is demonstrated due to the presence of energy absorption depth and the order of CF derivative also.

A memory response in the vibration of a microscale beam induced by laser pulse

In the application of pulsed laser heating, such as the laser hardening of metallic surfaces, the conduction limited process is the dominant mechanism during the laser-workpiece interaction. As a consequence, time unsteady analysis of this problem becomes necessary. The present study examines the effect of ultra-short-pulsed laser heating in the problem of coupled thermoelastic vibration of a microscale beam resonator. Due to the shortcomings of power law distributions in fractional derivatives, the usage of some other forms of derivatives with some other kernel functions is proposed. With this motivation, the heat transport equation is defined in an integral form of a common derivative on a slipping interval by incorporating the three-phase-lag memory-dependent heat transfer. Finite sinusoidal Fourier and Laplace transform techniques are then employed to determine the lateral vibration of the beam and the temperature increment within the medium. According to the graphical representations corresponding to the numerical results, conclusions about the new theory are drawn. An excellent predictive capability is demonstrated due to the presence of the energy absorption depth, memory dependent derivative, and delay time.

A study of surface swelling caused by graphitisation during pulsed laser ablation of carbon allotrope with high content of sp3 bounds

Experiments and theory are employed to investigate the laser ablation of boron doped diamond and tetrahedral amorphous carbon using nanosecond laser pulses. For a single laser pulse at low values of fluence, the laser induces a swelling of the surface due to graphitisation, whilst a high level of fluence leads to recession of the surface due to vaporization. To understand and investigate the underlying phenomena during the diamond-laser interaction, a model has been developed to reliably and quickly predict the behaviour of the surface and the thickness of the heat affected zone. The model is based on conservation of heat and mass during the laser-workpiece interaction. It consists of a one-dimensional system of non-linear equations that models the material heating, evaporation, graphitisation and plasma shielding. There is excellent agreement between numerical and experimental results for the position of the interfaces up to a high laser fluence. This model is the first to investigate the ablation of diamond that is able to capture surface swelling due to the graphitisation of the diamond layer, the graphite thickness and the amount of ablated material within a single framework. Furthermore, the model provides a novel methodology to investigate the thermal stability of diamond-like carbon films. The activation energy for tetrahedral amorphous carbon is obtained using the model with an accuracy of eV.

Laser ablation of a Cu–Al–Ni combinatorial thin film library: analysis of crater morphology and geometry

The conventional approach to studying laser–workpiece interaction in the ablation regime is to vary beam parameters used on a specimen of uniform chemical composition. The current work instead utilises a pulsed laser beam of constant parameters to ablate a ternary alloy thin film where the chemical composition of the sample varies continuously; this will enhance the understanding of pulsed laser ablation by means of a combinatorial approach. The analysis of the studied workpiece (a Cu–Al–Ni thin film deposited by magnetron sputtering) revealed the presence of both compositional and morphological gradients. Variation in the surface morphology was correlated with aluminium content. Single-pulse laser ablation (Nd:YAG, 1064 nm, 30 ns, 4.54 J/cm2) of the surface resulted in different crater features, geometry and volume. Two characteristic regions separated by a transition zone were identified based on the craters’ geometrical and morphological characteristics. The ablated volume increases with the atomic percentage of aluminium up to a threshold value of roughly 30 at.% after which the ablation volume slowly declines. This phenomenon may be attributed to plasma absorption and heat dissipation in the thin film.

LASER HEATING AND THERMAL STRESSES TIME EXPONENTIALLY HEATING PULSE CASE

Model studies of laser heating process minimize the experimental time and cost and give insight into the laser workpiece interaction mechanism. In the present study laser pulse heating is modelled and the governing equation of heat transfer, including phase change process, and thermal stresses are solved numerically using a control volume approach. In order to account for the time variation of the laser heating pulse, time exponentially varying pulse intensity is employed in the analysis. Since the heating conditions are considered to be axisymmetric, two dimensional case is introduced in the analysis. The temperature and stress fields are simulated for steel. It is found that temperature level attains considerably high values in the melt zone and pulse parameter (β/γ) with small values (1/2) results in large evaporated zones. Equivalent stress level increases rapidly in the region close to the melt surface and two stress peaks are developed in the radial direction. The location of stress peaks remains same with progressing heating period; however, the magnitude of second peak reduces with advancing heating periods.

Time-dependent 3-D modelling of laser surface heating for the hardening of metallic materials

A numerical code for the time-dependent three-dimensional modelling of the laser surface heating for the hardening of metallic materials has been developed by the authors. The temperature-dependence of the thermal properties of the material (stainless steel) is taken into account in the frame of a heating process that doesn't lead to material melting or evaporation. Calculations have been carried out for various dimensions of the parallelepiped-shaped and of the square-shaped spot of the laser beam, as well as for different scanning velocity and for different levels of the laser source power. Various patterns of the laser spot path have also been studied, including a single-pass hardening pattern, a double-pass hardening pattern with and without overlapping, multiple discontinuous and continuous hardening patterns and spiral hardening patterns. The presented results show how the proposed model can be usefully employed in the prediction of the time-evolution of temperature distribution which arises in the workpiece as a consequence of the laser-workpiece interaction under operating conditions typically encountered in industrial applications of the laser hardening process.

LASER HEATING OF A MOVING SLAB IN THE PRESENCE OF AN IMPINGING GAS JET: INFLUENCE OF SLAB VELOCITY

Modeling of the laser induced heating process can yield much insight into the phenomena occurring within the laser-workpiece interaction region. This study investigates the influence of the workpiece speed on the resulting temperature profiles during the gas-assisted laser heating process. Air is considered as impinging onto a steel workpiece coaxially with the laser beam, while the workpiece is assumed to move with a constant speed. The governing three-dimensional flow and energy equations are simulated numerically using a control volume approach. The low Reynolds number k- k model is introduced to account for the turbulence. During the simulations variable properties of gas and solid are considered. It is found that the workpiece speed has a significant effect on the resulting temperature profiles; specifically, an increase in the workpiece speed results in a decrease of the maximum temperature attained at the workpiece surface.

Closed Form Solution for Exponentially Decaying Laser Pulse Heating: Evaporation at the Surface

The modeling of the laser heating process gives insight into the laser workpiece interaction and minimizes the experimental cost. In the present study, an analytical solution for the laser pulse heating process is considered and the closed form solution for the temperature rise due to the time exponentially varying pulse is obtained. In the analysis, evaporation of the surface is taken into account. A Laplace transformation method was used when formulating the closed form solution for the temperature profiles. The effects of the boundary condition at the surface and pulse parameters on the temperature profiles are examined in detail. It is found that the surface temperature profile is very sensitive to the convective boundary condition at the surface. In this case, as the convective cooling of the surface proceeds, the oscillation in the temperature profiles is observed around the evaporation temperature. Moreover, as the power intensity profile moves towards the pulse beginning (0.07≤β′≤0.11), the peak surface temperature increases.

Analytical solution for laser evaporative heating process:time exponentially decaying pulse case

The modelling of the laser heating process gives insight into the laserworkpiece interaction and minimizes the experimental cost. In the presentstudy, analytical solution for the laser pulse heating process is consideredand the closed form solution for the temperature rise due to timeexponentially varying pulse is obtained. In the analysis, evaporation of thesurface is taken into account. A Laplace transformation method was used whenformulating the closed form solution for the temperature profiles. Theeffect of pulse parameters on the temperature profiles is examined indetail. It is found that the closed form solution derived from the presentstudy reduces to the previously obtained analytical solution when thesurface recession velocity is set to zero in the closed form solution. Moreover, the predictions of numerical simulation and closed form solutionare found to be in good agreement.

Laser pulse heating of steel surface and flexural wave analysis

Laser gas-assisted material processing finds wide application in industry. The modelling of heating, elastic response of the substrate material, and the wave analysis gives insight into the laser workpiece interaction. In the present study, laser gas-assisted heating of steel is considered. The normal component of the thermal stress is taken as the source of load for the flexural wave generation in the material. The flexural wave generated is simulated and the wave characteristics are analyzed at four locations at the workpiece surface. The numerical scheme employing a control volume approach is introduced when solving the governing equations of flow and heat transfer while finite element and spectran element methods are used when solving the stress and wave equations. It is found that the normal component of the stress is tensile. The dispersion effect of the workpiece material, interference of the reflected beam, and partial overlapping of second mode of the travelling wave enable to identify a unique pattern in the travelling wave in the substrate.

System for monitoring the focal position in laser material processing

A novel system for on-line measurement of the distance from the melt pool to the lens in laser material processing is described. A spectrometer analyzes the broadband radiation that is produced at the laser-workpiece interaction zone and then focused by the effector optics in the direction of the laser source. From the influence of chromatic aberration on the detected radiation spectrum, the distance from the workpiece to the lens can be derived. An analytical calculation of the shift of the melt pool spectrum that depends on the distance between the molten zone and the focusing lens shows good agreement with the measured results.

Measurement of laser beam transmittance through laser produced vapour plume

Laser produced plume, consisting of vapor front and ejected substrate on the workpiece surface, is an intermediate between the incident laser beam and the workpiece on which the beam is directed. It partially blocks, defocuses, absorbs, scatters, and deflects the incident beam and thereby reduces the laser energy reaching the workpiece. However, plume additionally acts as a heat source enhancing the machining. Consequently, laser induced plume plays an important role in laser machining process. The present study investigates the transmittance of a reference beam by the plume generated during the laser-workpiece interaction. To achieve the transmittance measurements, a Nd:YAG laser was used and four different materials were employed in the experiment. To obtain realistic values of the plume transmittance in relation to the laser machining process, the reference beam was sampled from the incident laser beam. The study was extended to include the effect of the laser pulse parameters on the transmission process. It is found that about 10–30% of the transmittance of the reference beam through a vapor plume, produced due to laser ablation, occurs close to the workpiece surface.

Quality optimisation for laser machining under transient conditions

Quality improvements in laser machining have been achieved by a newly developed model-based optimisation strategy. The specific aims of such efforts are to assure machining quality right up to boundaries or pre-machined sections, which are inherent in intricate part geometry. Such boundaries frustrate heat-transfer and result in bulk heating of the workpiece. This in turn leads to a degradation of the machining quality. In order to achieve such optimisation, transient heat-transfer is modeled. Close inspection of the laser–workpiece interaction zone reveals that the machining front exhibits dynamic behaviour, and such mobility plays a significant role in temperature determination. Non-linear parameter adaption profiles are generated via the optimisation strategy in order to stabilise the machining front temperatures. Currently, trial-and-error based experimentation is needed in order to improve machining quality in such regions. Thus model-based optimisation has the added benefit of reducing this step whilst leading to an optimal solution. Experimental results are presented and it is demonstrated that such process manipulation can lead to significant quality improvements.

Analytical solution for time unsteady laser pulse heating of semi-infinite solid

In the application of pulsed laser heating, such a laser hardening of metallic surfaces, conduction limited process is the dominant mechanism during the laser-workpiece interaction. Consequently, time unsteady analysis of this problem becomes necessary. The present study examines the time unsteady analysis of the conduction limited process for a time dependent laser pulse power intensity. It is shown that the conditions for thermal integration to occur requires a minimum pulse rate of 100 kHz and in the limit the solution obtained in the present case reduces to that obtained for a step input.

Laser heating process and experimental validation

Laser machining of engineering materials requires deep investigation into the laser-workpiece interaction mechanism, which is generally complicated and depends on the laser and workpiece properties. The present study examines the heat transfer mechanism, including conduction, phase change and convection processes taking place during Nd YAG laser irradiation of steel workpieces. Gaussian profile is considered for the spatial distribution of the laser output power intensity, while a time-dependent profile resembling the actual laser output pulse is introduced in computation. The thermal properties of the substance is considered as temperature dependent. Mass removal from the laser irradiated spot is modeled considering momentum and continuity equations. To validate the theoretical predictions, experimental measurements of surface temperature and evaporating surface velocities are carried out. Optical method and streak photography techniques are employed for this purpose. It is found that surface temperatures and evaporating front velocities are in good agreement with the experimental results.

Heating mechanism in relation to the laser machining process

Laser workpiece interaction mechanism is an important phenomenon which will assist in the development of laser machining systems. The interaction mechanism is generally complicated and depends on the laser and workpiece properties. In the present study a mathematical analysis for the laser material removal by evaporation and radial ejection of liquid is carried out. In the analysis the time unsteady problem is solved and nucleation explosions are predicted.

Analytical solution for the heat conduction mechanism appropriate to the laser heating process

In the application of pulsed laser material processing, such as laser hardening of metallic surfaces, conduction limited heating is the dominant mechanism during laser-workpiece interaction. Consequently, unsteady analysis of this problem becomes necessary. The present study examines the unsteady analysis of the conduction limited heat transfer process for an exponential pulse input with time dependent intensity. It is shown that, in the limit, the solution obtained in the present case reduces to that obtained for a step input.

Measurement of temperature-dependent reflectivity of Cu and Al in the range 30-1000 degrees C

In the CO2 laser cutting process it is important to predict the absorption mechanism during laser-workpiece interaction, since the form of the dependence of absorption on temperature greatly affects the form of the resulting temperature profile in the material. In order to be able to use any of the theoretical models for absorption with any degree of confidence, it appears necessary to obtain experimental data on the subject. Consequently an experiment to determine the variation of absorption with temperature is necessary. A reflectivity method is employed to measure the temperature-dependent reflection coefficient experimentally. The results obtained from the experiment for aluminium and copper samples are then compared with the theoretical results. In the analysis, high-temperature surface oxidation is also included.

Fundamentals of laser microprocessing of metals

The physical processes involved in laser microprocessing of metals are reviewed like the intensity-dependent absorption, the laser workpiece interaction with optical feedback, and the plasma generation with all the drawbacks to different modes of operation as well as to the performance of the laser tool itself. The laser-solid interaction, the hydrodynamics of the molten pool, the optical feedback, the absorption and the recoil of the laser-included plasma govern the resulting surface finish of laser materials processing. For achieving the optimum processing results any laser tool has to be matched carefully to the thermophysical properties of matter and to the physical mechanisms of processing that lasers offer economic as well as technical advantages compared to other processing methods.