Experimental and numerical analysis of the selective laser sintering (SLS) of PA12 and PEKK semi-crystalline polymers

A dual experimental-numerical approach was carried out to estimate thermal cycles and resulting fusion depths obtained during the selective laser sintering (SLS) of two polymers: PA12 and PEKK. The validation of thermal cycles was obtained by considering fusion depths on single layers for different experimental conditions and temperature measurements with IR thermal camera. It was shown that a simple Beer-Lambert’s heat deposit equation incorporating an extinction coefficient determined experimentally, and an efficiency ratio including both laser absorption and diffusion in the powder bed were sufficient for determining accurately fusion depths, and heat cycles for the two polymers. This allowed determining optimum process conditions for manufacturing additive layers on a specifically-designed SLS set-up.FUI Fadiplas

Study of laser interaction in water flow confinement at high repetition rate

This paper presents a study on the confined interaction with water flow for two successive laser pulses. The dynamic of the renewal of water films after shock produced by the laser is observed using a high speed camera. Pressure produced by the two pulses is measured from rear free surface velocity measurements using a velocimeter interferometry system for any reflector. The results show a threshold delay between the two laser pulses for which laser/target coupling of the second pulse decreases. This depends on the spot diameter, the laser intensity, and flow rate. This threshold can be calculated from the maximum jet diameter and flow rate. At an incident power density of 3 GW/cm2, a spot diameter of 1 mm, and a flow rate of 10 m/s, the maximum repetition rate ensuring target/coupling of successive laser pulses can be 1 kHz. The results open perspective for laser shock peening at high repetition rates

Quantitative evaluation of the mechanical strength of titanium/composite bonding using laser-generated shock waves

Intense acoustic shock waves were applied to evaluate the mechanical strength of structural epoxy bonds between a TA6V4 titanium alloy and a 3D woven carbon/epoxy composite material. Two bond types with different mechanical strengths were obtained from two different adhesive reticulations, at 50% and 90% of conversion, resulting in longitudinal static strengths of 10 and 39 MPa and transverse strengths of 15 and 35 MPa, respectively. The GPa shock waves were generated using ns-scale intense laser pulses and reaction principles to a confined plasma expansion. Simulations taking into account the laser-matter interaction, plasma relaxation, and non-linear shock wave propagation were conducted to aid interpretation of the experiments. Good correlations were obtained between the experiments and the simulation and between different measurement methods of the mechanical strength (normalized tests vs laser-generated shock waves). Such results open the door toward certification of structural bonding

Novel Confinement Possibility for Laser Shock: Use of Flexible Polymer Confinement at 1064 nm Wavelength

Through the years, laser shock peening became a treatment of choice in the aerospace industry to prolong the life of certain critical pieces. Water flow is commonly used as a confinement to improve the process capability but some applications cannot allow for water presence in the area of interest. In a previous article, an alternative to the water confinement was presented, a flexible polymer confinement was used and demonstrated the production of pressures equivalent to the water configuration treatment. However, laser parameters have been restricted to a wavelength in the visible range at 532 nm. In this paper, the study is extended to 1064 nm which is commonly used in LSP applications and with two different pulse durations. A 1064 nm near infra-red laser is used to do pressure characterization of shots with polymer confinement through Velocity Interferometer System for Any Reflector (VISAR) measurements coupled with Finite Element Modelling on Abaqus software. The results show that the pressures produced by the confinement is slightly lower with the 1064 nm wavelength, similar to what is observed with the classic water confined regime when switching from 532 nm to a near infra-red wavelength. Nevertheless, the high level of pressure produced by laser shock under the polymer confinement configuration allows for the treatment of common types of metal alloys used in the aerospace industry. Although the use of such a confinement has yet to be applicable to peening setups, it has already uses in some single shot configurations such as LasAT where it allows the avoidance of the water flow optimization

Laser induced plasma characterization in direct and water confined regimes: new advances in experimental studies and numerical modelling

Optimization of the laser shock peening (LSP) and LASer Adhesion Test (LASAT) processes requires control of the laser-induced target's loading. Improvements to optical and laser technologies allow plasma characterization to be performed with greater precision than 20 years ago. Consequently, the processes involved during laser-matter interactions can be better understood. For the purposes of this paper, a self-consistent model of plasma pressure versus time is required. The current approach is called the inverse method, since it is adjusted until the simulated free surface velocity (FSV) corresponds to the experimental velocity. Thus, it is not possible to predict the behavior of the target under shock without having done the experiments. For the first time, experimental data collected in different labs with the most up-to-date laser parameters are used to validate a self-consistent model for temporal pressure-profile calculation. In addition, the parameters characterizing the plasma (temperature, thickness and duration) are obtained from the ESTHER numerical code, together with the amount of ablated matter. Finally, analytic fits are presented that can reproduce any pressure-temporal profiles in the following domains of validity: Intensities, I, ranging from 10 to 500 GW cm-2 and pulse durations, T pul, between 5 and 40 ns for the direct-illumination regime at 1053 nm, I ranging from 1 to 6 GW cm-2 and T pul between 10 to 40 ns in the water-confined regime at 1053 nm, and I from 1 to 10 GW cm-2 and T pul between 7 and 20 ns in the water-confined regime at 532 nm. These temporal pressure profiles can then be used to predict the aluminum target's behavior under laser shock using mechanical simulation software

Impact of laser fiber tip cleavage on power output for ureteroscopy and stone treatment

Purpose: Holmium:YAG laser is the most used laser for urolithiasis. Generally, we use metallic scissors to cut the fiber tip to restore its effectiveness. Many cleaving methods have been described to avoid fiber damage and to restore its greatest power to the fiber. There is a lack of information regarding which cleaving method should be used and its effect on the fiber. In order to compare these effects, we studied different cleavage methods in terms of power output and its effects on the fiber. Methods: New single-use 272-μm fibers were used with a holmium:YAG laser lithotripter. Five kinds of fiber tips were compared: a new intact fiber, cleaved with ceramic scissors, cleaved with metallic scissors, first cleaved then stripped and first stripped then cleaved. The fibers were used against synthetic stones (BegoStone ® ) similar to calcium oxalate monohydrate, with fragmentation (SP, 5 Hz, 1.5 J) and dusting (LP, 15 Hz, 0.5 J) settings. We measured power output at 0, 1, 5, 10 and 15 min. Results: For fragmentation parameters, there was a statistical difference between the 5 groups at 0 and 1 min of laser use (p < 0.05) and none for time period over 1 min (p = 0.077–0.658). For dusting parameters, there was a statistical difference between the 5 groups at 0 min of laser use (p < 0.05) and none for time period over 0 min (p = 0.064–1). Conclusion: Cleaving the fiber tip may restore its effectiveness to the fiber, but only for a limited time, although it may preserve the scopes from damage

Beam size dependency of a laser-induced plasma in confined regime: Shortening of the plasma release. Influence on pressure and thermal loading

Processes using laser-shock applications, such as Laser Shock Peening or Laser Stripping require a deep understanding of both mechanical and thermal loading applied. We hereby present new experimental measurements of the plasma pressure release regarding its initial dimension, which depends on the laser beam size. Our data were obtained through shock waves’ velocity analysis and radiometric assessments. A new model to describe the adiabatic release behavior of a laser-induced plasma with a dependency to the beam size is developed. The results and the associated model exhibit that the plasma release duration is shortened with smaller laser spots. As a consequence, with chosen smaller laser spots (0.6 mm to 1 mm), the thermal loading applied during the plasma lifetime will also decrease. These new results shall help for a better understanding of laser-matter interaction for laser-shock applications by giving more accurate plasma profiles. Thus, process simulations can be improved as well. Eventually, by considering recent developments with high-power Diode Pumped Solid-State lasers (DPSS), we now expect to develop a new configuration for LSP which could be applicable both without any thermal coating and deliverable by an optical fiber.This research was funded by Thales company, institutions (CEA,NRS, ENSAM), and by the ANR (Agence Nationale de la Recherche), Forge Laser Project (Grant No.: ANR-18-CE08-0026)

Towards selective laser paint stripping using shock waves produced by laser-plasma interaction for aeronautical applications on AA 2024 based substrates

Laser stripping is a process which typically includes different forms of ablation phenomena. The presented work investigates a mechanical stripping process using high pressure laser-induced shock waves in a water confined regime. Power density is studied as a parameter for selective laser stripping on painted specimens and for adhesion relations with single layer epoxy targets. A flashlamp-pumped Nd:YAG laser with fixed spot size (4 mm) is shot on single layer epoxy and several layers of polymeric paint applied on a AA 2024-T3 (Aluminium) substrate. After laser treatment, samples are investigated with optical microscopy, profilometer and chemical analysis (FTIR & TGA). The results show that selective laser stripping is possible between different layers of external aircraft coatings and without any visual damage on the substrate material. In parallel to the experimental work, a numerical model has been developed to explain the background of the physical mechanisms and to qualitatively evaluate the detailed stress analysis and interfacial failure simulation for a single layer of epoxy on an aluminium substrate. The predicted failure patterns agree with the surfaces of the tested specimens observed by a microscope

Laser shock peening: toward the tse of pliable polid polymers for confinement

This paper presents the first extensive study of the performances of solid polymers used as confinement materials for laser shock applications such as laser shock peening (LSP) as opposed to the exclusively used water-confined regime up to now. The use of this new confinement approach allows the treatment of metal pieces needing fatigue behavior enhancement but located in areas which are sensitive to water. Accurate pressure determination in the polymer confinement regime was performed by coupling finite element simulation and experimental measurements of rear free-surface velocity using the velocity interferometer system for any reflector (VISAR). Pressure could reach 7.6 and 4.6 GPa for acrylate-based polymer and cross-linked polydimethylsiloxane (PDMS), respectively. At 7 and 4.7 GW/cm2, respectively, detrimental laser breakdown limited pressure for acrylate and PDMS. These results show that the pressures produced were also as high as in water confinement, attaining values allowing the treatment of all types of metals with LSP and laying the groundwork for future determination of the fatigue behavior exhibited by this type of treated materials

Development and optimization of Laser Shock Repeated Dense Peening (LSRDP) using most advanced laser architectures

The laser shock peening process (LSP), used to reinforce metals, currently has two major configurations with limitations. (1) Laser irradiation with large spot sizes, but with the need to use a thermal protective coating to avoid detrimental thermal damage (which increases the overall cost of the process) or (2) laser irradiation without thermal coating but with very small spot sizes and high overlap ratios, thus increasing the amount of time required to treat a given surface. In this study, we develop a new faster configuration for the LSP process, which can be applied without a thermal coating, but is still effective regarding surface treatment time. A new laser system has been developed for this faster configuration and has been used to perform the LSP treatment of aluminum alloys at a high-repetition rate. This new DPSS Q-switched Nd:YAG laser, delivers 1 J of energy with a pulse duration from 7 to 21 ns at a very high frequency of 200 Hz. We also studied the laser/matter interaction, according to the laser pulse duration, energy, and its wavelength. The water confinement (ejection and renewing) was monitored while an air-blowing system was implemented to manage water issues identified with this new configuration. Altogether, we demonstrated that such a configuration is fully operational