A Mathematical Model of Laser Drilling

AbstractLaser drilling has been widely used for producing small diameter holes in hard-to-machine materials for decades. Of particular interest is laser drilling of cooling holes in aircraft turbine blades. In order to enhance the cooling efficiency, these cooling holes need to be produced to a high degree of accuracy and with least defects. In this paper, a mathematical model of laser drilling is developed. The model includes effects of the vapour pressure, exothermic energy and O2 assist gas. The analysis is based on transient heat conduction in solid and liquid regions with appropriate boundary and initial conditions at the solid-liquid and liquid-vapour interfaces. Comparison with the experimental data is presented to validate the model. The developed model enables the prediction of the hole depth, hole profile and recast layer thickness. Effects of the laser peak power and assist gas pressure are also investigate

Simulation of laser drilling of Inconel X-750 and Ti-5Al-2.5Sn sheets using COMSOL

This study aims to investigate the simulation of laser drilling processes on Inconel X-750 and Ti-5Al-2.5Sn sheets. To this end, COMSOL Multiphysics 5.2 software was employed to carry out the virtual experiments. A JK 701 pulsed Nd:YAG laser was used for drilling through the entire depth of Inconel X-750 and Ti-5Al-2.5Sn plates with a thickness of 2 mm and 3 mm, using laser pulses of a millisecond in time. The laser parameters varied in different combinations for well-controlled drilling through the entire thickness of the plates. Effects of laser peak power (10-20 kW) and pulse duration (0.5-2.5 ms) have been determined via studying the temperature distribution on the cross-section of the images taken in the simulation tests. Characterizing the optimum conditions obtained from the combination of parameters that improve the hole quality is an essential objective in this paper. The results suggest that the hole's diameter and depth have increased linearly as the laser beam's peak power and pulse duration are elevated. An improvement in the hole's taper ratio (the best value is 0.72) was observed as the laser beam pulse duration was longer, since an isosceles trapezoid shape was formed instead of a conical. The pulse duration exhibited more impact on the crater depth progression than the peak power. This work's outcomes might be helpful for researchers in terms of the optimum parameters proposed when studying the laser drilling of the mentioned alloys experimentally. The procedure and findings of this study are not presented elsewhere

3D Micromachining of Optical Devices on Transparent Material by Ultrafast Laser

Ultrafast lasers, also referred to as ultrashort pulse lasers, have played an important role in the development of next generation manufacturing technologies in recent years. Their broad range of applications has been investigated in the field of microstructure processing for the biomedical, optical, and many other laboratory and industrial fields. Ultrafast laser machining has numerous unique advantages, including high precision, a small heat affected area, high peak intensity, 3D direct-writing, and other flexible capabilities When integrated with optical delivery, motion devices and control systems, one-step fabrication of assemble-free micro-devices can be realized. In particular, ultrafast lasers enable the creation of various three-dimensional, laser-induced modifications using an extremely high peak intensity over a short time frame, producing precise ablation of material and a small heat affected area in transparent materials. In contrast, lasers with longer pulse durations are based on a thermal effect, which results in significant melting in the heat affected area. In general, ultrafast laser micromachining can be used either to subtract material from or to change the material properties of both absorptive and transparent substances. Recently, integrated micro-devices including optical fiber sensors, microfluidic devices, and lab-on-chips (LOC) have gained worldwide recognition because of their unique characteristics. These micro-devices have been widely used for a broad range of applications, from fundamental research to industry. The development of integrated glass micro-devices introduced new possibilities for biomedical, environmental, civil and other industries and research areas. Of these devices, optical fiber sensors are recognized for their small size, accuracy, resistance to corrosion, fast response and high integration. They have demonstrated their excellent performance in sensing temperature, strain, refractive index and many other physical quantities. In addition to the all-in-fiber device, the LOC is another attractive candidate for use in micro-electro-mechanical systems (MEMS) because it includes several laboratory functions on a single integrated circuit. LOCs provide such advantages as low fluid volume consumption, improved analysis and response times due to short diffusion distances, and better process control, all of which are specific to their application. Combining ultrafast laser micromachining techniques with integrated micro-devices has resulted in research on a variety of fabrication methods targeted for particular purposes. In this dissertation, the direct creation of three-dimensional (3D) structures using an ultra-fast laser was investigated for use in optical devices. This research was motivated by the desire to understand more fully the relationship among laser parameters, material properties and 3D optical structures. Various all-in-fiber sensors in conjunction with femtosecond laser ablation and irradiation were investigated based on magnetic field, temperature and strain application. An incoherent optical carrier based microwave interferometry technique was used for in-situ weak reflector fabrication and a picosecond laser micromachining technique was introduced for developing LOCs with unlimited utilization potential

Recent efforts on model-based simulation of engineering problems : multiphysics and multiphase interactions

Simulation-Based Engineering Science (SBES) is playing a more important role in gaining new knowledge and providing guidance for engineering activities, in particular, in the fields in which time scale and/or spatial scale make physical experiments dramatically expensive or even impossible. The success of SBES heavily relies on the development of algorithms that provide the bridge between the models describing physical and engineered systems and the computational devices that generate the digital representations of simulations. My efforts on the development of algorithms that simulate multiphysics and multiphase flow are presented in this dissertation. The first part of the dissertation describe the algorithms for the multiphysical model that simulates the laser drilling process. During laser drilling, heat conduction, melt flow, and vaporization occur in a very short time period. Vaporization also produces the recoil pressure that drives melt flow and complicates the heat transfer and material removal rate. To get a more realistic picture of the melt flow, a series of differential equations were developed that govern the process from pre-heating to melting and evaporation. In particular, the Navier-Stokes equation governing the melt flow is solved with the use of the boundary layer theory and integral methods. Heat conduction in a solid is investigated by using classic solutions with the corrections that reflect the change in boundary condition from constant heat flux to Stefan condition. The dependence of saturation temperature on the vapor pressure is taken into account by using the Clausius-Clapeyron equation. Both constantly rising radial velocity profiles and rising-fall velocity profiles are considered. In spite of the assumed varying velocity profiles, the new model predicts that the drilling hole profiles are very close to each other in a specific super alloy for given laser beam intensity and pulse duration. The numerical results show that the effect of melt flow on material removal can be ignored in some cases. The solutions derived can be applied to new cases to determine the role of melt flow and vaporization on laser drilling profile evolution and to study the solid material removal efficiency. The second part of this dissertation describes a new method that simulates the interaction between fluid and solid elements. The discrete element method (DEM) has been used to deal with the interactions between solid elements of various shapes and sizes, while the material point method (MPM) has been developed to handle the multi-phase (solid-liquid-gas) interactions involving failure evolution. A combined MPM-DEM procedure is proposed to take advantage of both methods so that the interaction between solid elements and fluid particles in a container could be better simulated. In the proposed procedure, large solid elements are discretized by the DEM, while the fluid motion is computed using the MPM. The contact forces between solid elements and rigid walls are calculated using the DEM. The interaction between solid elements and fluid particles are calculated via an interfacial scheme within the MPM framework. The proposed procedure is illustrated by representative examples. The convergence of numerical solutions and the factors affecting the simulation fidelity is also discussed

Simulation-Oriented Methodology for Distortion Minimisation during Laser Beam Welding

Distortion is one of the drawbacks of any welding process, most of the time needed to be suppressed. One doubtful factor that could affect welding deformation is the shape of the liquid melt pool, which can be modified via variation of process parameters. The aim of this work was to numerically study the dynamics of the weld pool and its geometrical influence on welding distortion during laser beam welding. To achieve such a goal, a promising novel process simulation model, employed in investigating the keyhole and weld pool dynamics, has successfully been invented. The model incorporated all distinctive behaviours of the laser beam welding process. Moreover, identification of the correlation between the weld pool geometry and welding distortion as well as, eventually, weld pool shapes that favour distortion minimisation has also been simulatively demonstrated

Simulation-Oriented Methodology for Distortion Minimisation during Laser Beam Welding

Distortion is one of the drawbacks of any welding process, most of the time needed to be suppressed. One doubtful factor that could affect welding deformation is the shape of the liquid melt pool, which can be modified via variation of process parameters. The aim of this work was to numerically study the dynamics of the weld pool and its geometrical influence on welding distortion during laser beam welding. To achieve such a goal, a promising novel process simulation model, employed in investigating the keyhole and weld pool dynamics, has successfully been invented. The model incorporated all distinctive behaviours of the laser beam welding process. Moreover, identification of the correlation between the weld pool geometry and welding distortion as well as, eventually, weld pool shapes that favour distortion minimisation has also been simulatively demonstrated

Mathematical modelling of multiple pulsed laser percussion drilling

In laser percussion drilling, a series of laser pulses with specified energies and durations irradiate the workpiece surface to gradually heat, melt, and vaporise material until a hole with-required depth-and-diameter-is-achieved. Despite being the quickest technique for producing small diameter holes, laser percussion drilling regularly suffers from difficulties in controlling the hole quality such as hole circularity, hole taper and recast layer. Therefore, in order to produce holes to a specific requirement at minimum cost and time, it is crucial to fully understand the effects of each parameter on hole quality. In this research, a new mathematical model for multiple pulsed laser drilling is developed to predict the hole depth, hole taper, and recast layer thickness, and to investigate the effects of key laser parameters on hole dimensions. The new model accounts for recoil pressure, melt ejection, O2 assist gas effects, as well as solidification of the melt. The development of-the new model is divided into two stages; pulse on stage where interaction between laser beam-material takes place, and pulse off stage where solidification of the melt is modelled. Governing equations are established from heat conduction, energy, and mass equations at the solid-liquid and liquid-vapour interfaces with appropriate boundary and initial conditions. Analytical solutions are derived by using Mathematica 7 software as a tool to solve the system of non-linear equations. To validate the model, experimental work has been conducted and the measured results are compared to those calculated from the model. It is shown that the new model gives a good prediction of the hole depth and acceptable prediction of the recast layer thickness. Laser peak power and pulse width are shown to have a significant influence over the drilled hole quality whereas the changes due to pulse frequency are less pronounced.EThOS - Electronic Theses Online ServiceRoyal Thai GovernmentGBUnited Kingdo

Assistive control for non-contact machining of random shaped contours

Recent achievements in robotics and automation technology has opened the door towards different machining methodologies based on material removal. Considering the non force feedback nature of non-contact machining methods, careful attention on motion control design is a primary requirement for successful achievement of precise cutting both in machining and in surgery processes. This thesis is concerned with the design of pre-processing methods and motion control techniques to provide both automated and human-assistive non-contact machining of random and complex shaped contours. In that sense, the first part of the thesis focuses on extraction of contours and generation of reference trajectories or constraints for the machining system. Based on generated trajectories, two different control schemes are utilized for high precision automated machining. In the first scheme, preview control is adopted for enhancing the tracking performance. In the second scheme, control action is generated based on direct computation of contouring error in the operational space by introducing a new coordinate frame moving with the reference contour. Further, non-contact machining is extended for realization in a master/slave telerobotic framework to enable manual remote cutting by a human operator. With the proposed approach, the human operator (i.e. a surgeon) is limited to conduct motion within a desired virtual constraint and is equipped with the ability of adjusting the cutting depth over a that contour providing advantage for laser surgery applications. The proposed framework is experimentally tested and results of the experiments prove the applicability of proposed motion control schemes and show the validity of contributions made in the context of thesis