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.

Carbon fibre reinforced polymer, or CFRP composite materials, play an increasingly important role in modern manufacturing. They are widely used in aerospace, and their use is currently spreading to other industries where high strength-to-weight ratios are required. However, machining of composites is still a challenging task and often hampered by poor quality. Despite the extensive research that was conducted on the machining of composite materials over the last few years, mechanical drilling still suffers from delamination, fibre pull-out and poor surface finish, whereas laser cutting produces microstructured defects and a taper problem. This thesis reports on the drilling of CFRP composites by demonstrating the possibility of drilling small diameter holes (i.e. 8mm) into 25.4mm thick carbon fibre reinforced polymer composites (CFRPs) using mechanical drilling and laser drilling as stand-alone processes and as a sequential combination. The research involved four main phases of experimental testing. The first part of Phase 1 involved!preliminary experiments of drilling thick CFRP to identify the most suitable drilling strategy. Three mechanical drilling strategies conducted in the same parameter by using a 2-flute uncoated WC twist drill that was assessed with respect to feasibility of drilling thick CFRP. The results showed that the single-step strategy was the most feasible strategy to drill thick CFRP compared to 2- and 4-peck drilling strategies. The second part of Phase 1 concerned the influence of speed-feed combinations on hole quality by utilising three twist drills with different materials and geometries in both an uncoated and coated condition. The results indicated that a significant increase in peel-up delamination was found with increasing feed rate. In contrast, using a constant feed rate but increasing the spindle speed seemed to reduce peel-up delamination. Furthermore, the hole entry for 2-flute uncoated WC drill bits was an uncommon study finding because most of the previous researchers experienced more damages at the hole exit and their investigation focused on the hole exit only. Currently, implementation of laser technology in cutting and drilling composites is becoming popular as an alternative solution. Various experiments were conducted with the goal of identifying the effects of machining parameters on key output measures (i.e. heat affected zone (HAZ), hole depth and other damages) in drilling of 25.4 mm thick CFRP by using a fibre laser. Phase 2 involved a number of machining parameters selected to identify the potential of a fibre laser in drilling thick CFRP composites (i.e. laser power, scanning speed, focal point plane position (FPP), assisted-gas type and gas pressure). The results proved that a fibre laser could penetrate thick CFRP to a 22mm depth only. Moreover, the spiral trepanning strategy was able to penetrate 80% out of the total thickness of the CFRP in continuous wave (CW) mode, whereas the modulated laser beam (i.e. laser pulse mode) can penetrate 67% only. This result was a major recorded breakthrough because previous research attempts cut up to 5mm only. Laser power proved to be the most influential factor for hole depth in laser drilling of thick CFRP when the spiral trepanning strategy was applied. Machining trials were conducted in Phase 3 by using a 16kW fibre laser in modulated pulsed laser mode. In this phase, laser power of more than 1kW was attempted to cut the whole thickness of CFRP composites in CW mode, but it was unsuccessful. However, a new parameter was discovered (i.e. the cooling time between passes in modulated pulsed mode), which proved a considerable reduction of HAZ when the higher cooling time was imposed. Finally, phase 4 involved the experiments of sequential laser-mechanical drilling. A 1kW fibre laser was selected as a pre-drilling or initial step and followed by mechanical drilling as the final step. The sequential drilling method successfully reduced thrust force and torque for mechanical drilling by an overall average of 61%, resulting in high productivity and decreasing the thermal and mechanical stresses in the cutting tool and, in turn, promoting higher tool life. The highest delamination factor (Fda) ratio was experienced by the sequential laser 8mm – mechanical 8mm for both tools (i.e. 2- and 3-flute uncoated tungsten carbide) and laser pre-drilling strategies (i.e. single- and double-side). Thus, a novel laser-mechanical sequential drilling technique was developed, evaluated and tested in the drilling of thick CFRP composites; this is the first time ever in drilling thick CFRP (i.e. 25.4mm).

On-the-fly laser drilling provides a highly productive method for producing hole clusters (pre-defined groups of holes to be laser drilled) on freeform surfaced parts, such as gas turbine combustion chambers. Although the process is capable of achieving high throughputs, current machine tool controllers are not equipped with appropriate trajectory functions that can take full advantage of the achievable laser drilling speeds. While the problem of contour following has received previous attention in time-optimal trajectory generation literature, on-the-fly laser drilling presents different technological requirements, needing a different kind of trajectory optimization solution, which has not been studied prior to this thesis. The duration between consecutive hole locations, which corresponds to the laser pulsing period, has to be kept constant, ideally throughout the part program. However, the toolpath between the holes is not fixed and can be optimized to enable the shortest possible segment duration. To preserve the dynamic beam positioning accuracy and avoid inducing excessive vibrations on the laser optics, the axis velocity, acceleration, and jerk profiles need to be limited. Furthermore, to ensure that hole elongation does not violate the given part tolerances, the orthogonal component of part velocity relative to the laser beam needs to be capped. All of these requirements have been fulfilled in the trajectory optimization algorithm developed in this thesis. The hole locations are provided as pre-programmed sequences by the Computer Aided Design/Manufacturing software (CAD/CAM). A time-optimized trajectory for each sequence is planned through a series of time-scaling and unconstrained optimization operations, which guarantees a feasible solution. The initial guess for this algorithm is obtained by minimizing the integral square of the fourth time derivative (i.e. ‘snap’). The optimized trajectories for each cluster are then joined together or looped onto themselves (for repeated laser shots) using a time-optimized looping/stitching (optimized/smooth toolpath to repeat/loop a cluster or connect/stitch between consecutive clusters) algorithm. This algorithm also minimizes the integral square of jerk in the faster axes. The effectiveness of the overall solution has been demonstrated in simulations and preliminary experimental results for on-the-fly laser drilling of a hole pattern for a gas turbine combustion chamber panel. It is shown that the developed algorithm improves the cycle time for a single pass by at least 6% (from kinematic analysis of the motion duration), and more importantly reduces the integral square of jerk by 56%, which would enable the process speed to be pushed up further.

Το αντικείμενο της παρούσας διατριβής είναι η διερεύνηση και η ανάπτυξη μεθόδου παρακολούθησης της διεργασίας διάτρησης με παλμούς δέσμης Laser (Laser drilling). Η διεργασία διάτρησης με δέσμη Laser περιλαμβάνει την χρήση μιας εστιασμένης δέσμης Laser υψηλής ενεργειακής πυκνότητας για την διάτρηση του υλικού μέσω της τήξης ή ακόμα και εξάτμισης του. Η παρούσα διατριβή επικεντρώνεται στην ανάπτυξη ενός συστήματος παρακολούθησης ικανό να προβλέψει σε πραγματικό χρόνο τα διάφορα γεωμετρικά χαρακτηριστικά του αποτελέσματος της διεργασίας αξιοποιώντας οπτικές αλλά και ακουστικές εκπομπές προερχόμενες από την περιοχή κατεργασίας. Τα σήματα που καταγράφονται μέσω φωτοδιόδων και μικροφώνων, σχετίζονται με την εξέλιξη της διεργασίας και κατά συνέπεια μπορούν να χρησιμοποιηθούν για την αξιολόγηση της. Θεωρητική ανάλυση της απόκρισης των αισθητήρων σε σχέση με την γεωμετρία της οπής που δημιουργείται, οδήγησε στην δημιουργία θεωρητικών μοντέλων ικανών να προβλέψουν άμεσα τα γεωμετρικά χαρακτηριστικά της οπής, όπως είναι το βάθος και η διάμετρος εισόδου, χρησιμοποιώντας τα σήματα που λαμβάνονται κατά την διάρκεια της διεργασίας. Παράλληλα, η ανάπτυξη εμπειρικών προτύπων συσχέτισης των γεωμετρικών χαρακτηριστικών με τις παραμέτρους της διεργασίας, προκειμένου να αξιοποιηθούν για τον προγραμματισμό και τον έλεγχο της διεργασίας, αποτέλεσε αντικείμενο έρευνας της παρούσης διατριβής. Το σημαντικότερο συμπέρασμα που προκύπτει από την συγκεκριμένη διατριβή είναι ότι τόσο τα οπτικά όσο και τα ακουστικά σήματα που δημιουργούνται κατά την διάρκεια της διεργασίας διάτρησης με δέσμη Laser μπορούν να αξιοποιηθούν για την παρακολούθηση της διεργασίας σε πραγματικό χρόνο και κατά συνέπεια για τον άμεσο έλεγχο της διασφαλίζοντας έτσι την επίτευξη των στόχων και απαιτήσεων που ορίζονται, όσον αφορά τα γεωμετρικά χαρακτηριστικά της οπής. / The main objective of the present study is the investigation and development of a real-time monitoring method of the percussion laser drilling process. Percussion laser drilling process involves the utilization of a focused laser beam, presenting high power density to melt of vaporize the material. The present work is focalized in the development of a process monitoring system capable to predict the geometrical characteristics of the drill in real time utilizing optical and acoustic emissions originating from the processing zone. The signals acquired by means of photodiodes and microphone, are correlated with the process evolution and consequently can be used for the evaluation of the process output. Theoretical analysis of sensors’ response in relation with the geometry of the drill led in the development of sensor models able to predict the geometrical characteristics of the drill, such as the depth and entrance diameter, utilizing the signals acquired during the process. Additionally, the development of empirical models, correlating hole’s geometrical characteristics with process parameters, able to be utilized for the off-line programming as also and the control of the process, comprised research topic of the present dissertation. The main conclusion of the present work is that both optical and acoustic signals originating from the processing zone during the laser drilling process, can be utilized for real-time process monitoring and consequently for process control purposes ensuring that the final product of the process is within the defined requirements, as far as geometry of the hole is concerned.

On-the-fly laser drilling provides a highly productive method for producing hole clusters (pre-defined groups of holes to be laser drilled) on freeform surfaced parts, such as gas turbine combustion chambers. Although the process is capable of achieving high throughputs, current machine tool controllers are not equipped with appropriate trajectory functions that can take full advantage of the achievable laser drilling speeds. While the problem of contour following has received previous attention in time-optimal trajectory generation literature, on-the-fly laser drilling presents different technological requirements, needing a different kind of trajectory optimization solution, which has not been studied prior to this thesis. The duration between consecutive hole locations, which corresponds to the laser pulsing period, has to be kept constant, ideally throughout the part program. However, the toolpath between the holes is not fixed and can be optimized to enable the shortest possible segment duration. To preserve the dynamic beam positioning accuracy and avoid inducing excessive vibrations on the laser optics, the axis velocity, acceleration, and jerk profiles need to be limited. Furthermore, to ensure that hole elongation does not violate the given part tolerances, the orthogonal component of part velocity relative to the laser beam needs to be capped. All of these requirements have been fulfilled in the trajectory optimization algorithm developed in this thesis. The hole locations are provided as pre-programmed sequences by the Computer Aided Design/Manufacturing software (CAD/CAM). A time-optimized trajectory for each sequence is planned through a series of time-scaling and unconstrained optimization operations, which guarantees a feasible solution. The initial guess for this algorithm is obtained by minimizing the integral square of the fourth time derivative (i.e. ‘snap’). The optimized trajectories for each cluster are then joined together or looped onto themselves (for repeated laser shots) using a time-optimized looping/stitching (optimized/smooth toolpath to repeat/loop a cluster or connect/stitch between consecutive clusters) algorithm. This algorithm also minimizes the integral square of jerk in the faster axes. The effectiveness of the overall solution has been demonstrated in simulations and preliminary experimental results for on-the-fly laser drilling of a hole pattern for a gas turbine combustion chamber panel. It is shown that the developed algorithm improves the cycle time for a single pass by at least 6% (from kinematic analysis of the motion duration), and more importantly reduces the integral square of jerk by 56%, which would enable the process speed to be pushed up further.

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Le perçage par laser est le procédé majoritairement utilisé pour la fabrication des trous de refroidissement des chambres à combustion des moteurs d'avion. La maîtrise de ce procédé, afin de limiter les écarts de géométrie des trous ainsi que les défauts microstructuraux induits, est une problématique qui a un fort enjeu industriel.L'interaction entre un faisceau laser et une matière métallique absorbante dans les régimes de perçage laser implique des phénomènes thermiques et hydrodynamiques dont le rôle sur le perçage n'est pas encore complétement expliqué. Ces travaux de thèse apportent des éléments de réponse par une approche de simulation numérique.Une investigation expérimentale couvrant une large gamme de paramètres opératoires a permis de quantifier la vitesse d'éjection liquide, la pression de recul et la température de surface en cours de procédé et d'analyser l'influence de la puissance crête du faisceau laser et de propriétés physiques matériau sur le perçage. Plusieurs méthodes expérimentales originales d'observations in situ et post-procédé ont été mises en œuvre dans cette investigation.Le modèle physique du perçage laser qui a été retenu pour la simulation intègre les changements de phase solide/liquide et liquide/vapeur, la mobilité des interfaces, l'éjection de la phase liquide et les échanges de chaleur par conduction et convection. Il est résolu en 2D axisymétrique via un code de calcul développé entièrement durant la thèse. Ce développement se base sur une méthode numérique récente, appelée méthode des éléments naturels contraints (CNEM en anglais), qui permet une description lagrangienne des interfaces mobiles et de l'écoulement du liquide. Cette description facilite l'application des conditions aux frontières. Les paramètres et données d'entrée du modèle sont tous issus de la littérature et/ou de mesures expérimentales.Les simulations réalisées ont permis de prédire la plupart des mesures sans aucun ajustement de paramètres. Les écarts observés ont donné des informations inédites sur la contribution de la répartition spatiale du faisceau laser et de la phase vapeur sur la géométrie des trous. L'étude des cycles thermiques a permis de mieux comprendre les transformations métallurgiques induites au cours du perçage laser. / The laser drilling process is the main process used in machining procedures on aeronautic engines, especially in the cooling parts of the engine. The industrial stake is to reduce geometrical deviations of the holes and defects during production.The interaction between a laser beam and an absorbent metallic matter in the laser drilling regime involve thermal and hydrodynamical phenomenon. Their role on the drilling is not yet completely understood. This thesis work is attached to give some responses to these questions with a simulation approach.An experimental investigation was first set up in order to estimate the velocity of the liquid, the vapor pressure and the temperature of the surface and to characterize the influence of the laser power and some material properties on the drilling.The physical model of the laser drilling used for simulations include solid/liquid and liquid/vapor phase transformations, the liquid ejection and the convective and conductive thermal exchanges. It is solved using a homemade calculation code and 2D axisymmetric formulation, developed during the thesis. The development is based on a recent numerical method called CNEM (Constrained Natural Element Method). This method allows us to use a lagrangian representation of the moving boundaries and the liquid flow and so facilitates the application of the boundary conditions. The model parameters were taken from literature or from measures.Simulations results predicted most of measurements without identifying any parameters. The deviations between experiments and simulations gave new discussions on the influence of the laser beam space repartition and on the contribution of the metallic vapor phase on the hole geometry. The thermal cycles were studied clarifying the metallurgical transformations induced by laser drilling.

>Magister Scientiae - MSc / This dissertation consists of two main sections. The first section focuses on generating zero order Bessel beams using axicons. An axicon with an opening angle y = 5⁰ was illuminated with a Gaussian beam of width ω₀ = 1.67 mm from a cw fiber laser with central wavelength λ = 1064 nm to generate zero order Bessel beams with a central spot radius r₀ = 8.3 ± 0.3 μm and propagation distance ½zmax = 20.1 ± 0.5 mm. The central spot size of a Bessel beam changes slightly along the propagation distance. The central spot radius r₀ can be varied by changing the opening angle of the axicon, y, and the wavelength of the beam. The second section focuses on applications of the generated Bessel beams in laser microdrilling. A Ti:Sapphire pulsed femtosecond laser (λ = 775 nm, ω₀ = 2.5 mm, repetition rate kHz, pulse energy mJ, and pulse duration fs) was used to generate the Bessel beams for drilling stainless steel thin sheets of thickness 50 μm and 100 μm and microscopic glass slides 1 mm thick. The central spot radius was r₀ = 15.9 ± 0.3 μm and ½zmax = 65.0 ± 0.5 mm. The effect of the Bessel beam shape on the quality of the holes was analysed and the results were discussed. It was observed that Bessel beams drill holes of better quality on transparent microscopic glass slides than on stainless steel sheet. The holes drilled on stainless steel sheets deviated from being circular on both the top and bottom surface for both thicknesses. However the holes maintained the same shape on both sides of each sample, indicating that the walls are close to being parallel. The holes drilled on the glass slides were circular and their diameters could be measured. The measured diameter (15.4±0.3 μm) of the hole is smaller than the diameter of the central spot (28.2 ± 0.1 μm) of the Bessel beam. Increasing the pulse energy increased the diameter of the drilled hole to a value close to the measured diameter of the central spot.

Le perçage est un procédé de fabrication répandu dans le milieu industriel. Le secteur aéronautique a recours à ce procédé pour usiner de nombreuses pièces. Les chambres de combustions en sont un exemple. Exposées à des cycles thermiques sévères, elles sont percées d’une multitude de trous de refroidissement. Chez Safran Helicopter Engines, motoriste spécialisé dans la conception de turbines à gaz pour hélicoptères, ce perçage est réalisé à l’aide d’un procédé d’usinage unique : le perçage par percussion laser à la volée. Ce procédé phare leur permet de réaliser des milliers de trous inclinés dans des tôles en matériaux réfractaires. Safran Helicopter Engines cherche à maintenir son avance dans la production de turbines en s’appuyant sur les innovations constantes du procédé de perçage. Ce travail de thèse s’est articule autour du développement d’un nouveau procédé de perçage laser. De manière à définir un procédé fiable et efficace, plusieurs stratégies de perçage sont étudiées via les possibilités offertes par un laser à fibre milliseconde de haute puissance. Les résultats obtenus laissent entrevoir des perspectives intéressantes en termes d’augmentation de cadence de production à qualité d’usinage équivalente. L’amélioration des performances a été démontrée expérimentalement sur un banc d’essais représentatif des machines de production. Des trous de 400 μm de diamètre incliné à 60o ont pu être réalisés avec un cycle de perçage raccourci au minimum d’un facteur 2. Cette thèse présente également des éléments de compréhension des phénomènes de formation d’un trou. Pour cela, une modélisation des processus thermo hydrodynamiques impliqués dans le procédé de perçage laser a été développée. Ce modèle basé sur une approche par éléments finis repose sur l’étude thermique au cours de l’interaction entre le faisceau laser et la matière. Une partie du modèle inclut les phénomènes de propagation du faisceau dans la cavité en cours de formation afin de traduire de manière plus réaliste le dépôt d’énergie. / Drilling is a well-known manufacturing process. The aeronautic industry uses this process to machine a lot of elements. Among them, we can mention turbine blades, fuel injectors, combustion chambers. The latter are exposed to serious thermal stresses. To limit their impacts, combustion chambers are drilled with multiples cooling holes. At Safran Helicopter Engines, engines manufacturer specialized in engines design for helicopters, drilling is performed with a unique machining process : on the fly laser percussion drilling. This flagship process allows drilling thousands of tilted holes on refractory materials. Safran Helicopter Engines seeks to preserve its leading position in turbo engines manufacturing by looking for steady innovative drilling process. This thesis is structured around the development of a new laser drilling process. To define a reliable and efficient process, several drilling strategies were studied through the possibilities offered by a high-power millisecond fiber laser. The presented results allow catching a glimpse of the possibilities of production improvement. The performance enhancement was established through experimentation with a test bench. Holes of 400 μm diameter with an angle of 60o were drilled with a reduced processing time thanks to this new drilling process. In this thesis, we also propose a theoretical approach describing the hole formation. A mathematical model, accounting for all the physical process taking place during the drilling process is introduced. This model is based on finite elements method and considers the laser beam propagation within the drilled holes by solving Maxwell equations