Set Of Laser Parameters Research Articles

Effects of scan speed on vapor plume behavior and spatter generation in laser powder bed fusion additive manufacturing

In-situ observation and metrology of the complex phenomena is a critical step to better understanding of the fundamental physics involved in laser powder bed fusion additive manufacturing process. By using high-speed imaging, we observed and analyzed the vapor plume evolution and spatter generation during single track formation of 304 stainless steel within a board range of scan speed. The experiments were performed on both powder bed and on bare substrate under identical laser parameter sets and observation conditions, in order to identify the role of powder particle on spatter ejection and process stability. With video frames superimposed, observation results are presented and analyzed at track level. It is shown that scan speed has a significant influence on the vapor plume behavior and process stability, and the spatter generation during single track formation is shown to be well related with process stability, rather than with the energy input. Additionally, a unique type of droplet spatter was identified at the starting point of track formation as result of the coupling between the melt pool and intensive vapor flux.

Randomized in situ study on the efficacy of CO2 laser irradiation in increasing enamel erosion resistance.

The aim of this double-blind, randomized in situ study was to evaluate the erosion-preventive effect of a specific set of CO2 laser parameters, associated or not with fluoride. Two hundred forty bovine enamel blocks were prepared for individual palatal appliances (n = 6 samples/appliance). The study had four phases of 5days each, with ten volunteers and the following treatments: CO2 laser irradiation (L), fluoride treatment (F), combined fluoride and laser treatment (FL), and no treatment, control (C). Laser irradiation was performed at 0.3J/cm2 (5μs/226Hz/10.6μm) and the fluoride gel contained AmF/NaF (12'500ppmF-/pH = 4.8-6). For erosive demineralization, the appliances were immersed extra-orally in citric acid (0.05M/20min/pH = 2.3) twice daily. Analysis of enamel surface loss was done using a 3D-laser profilometer on 3days. Additionally, fluoride uptake was quantified and scanning electron microscopies were done. Data were analyzed with repeated measures ANOVA and post hoc pairwise comparisons (α = 0.05). At all analyzing days, both laser groups caused the lowest means of enamel loss, which were also statistically significant lower than C (p < 0.05). At day 5, FL means ± SD (33.6 ± 12.6μm) were even significantly lower than all other groups (C 67.8 ± 15.4μm; F 57.5 ± 20.3μm; L 46.8 ± 14.5μm). Significantly increased enamel fluoride uptake was observed for both fluoride-containing groups (p < 0.05) at day 1. Compared to the control, the CO2 laser irradiation with a specific set of laser parameters (0.3J/cm2/5μs/226Hz) either alone or in combination with a fluoride gel (AmF/NaF) could significantly decrease enamel erosive loss up to 5days in situ. Combined CO2 laser-fluoride treatment has a significant anti-erosive effect.

Laser cladding with HA and functionally graded TiO2-HA precursors on Ti–6Al–4V alloy for enhancing bioactivity and cyto-compatibility

In the present study, the morphology and composition of Titanium alloy (Ti–6Al–4 V) surfaces were modified by laser cladding (LC) process to enhance the biocompatibility and bioactivity of orthopaedic implants. Ti–6Al–4 V substrates were processed using a pulsed Nd:YAG laser with various precursors: 100% HA and functionally graded TiO2-HA material (FGM). A set of laser parameters having 21.6 J/mm2 laser energy density or irradiance was applied for cladding, to execute a comparative study. The morphology, phases and wettability with different biological tests including bioactivity, protein adsorption, cell adhesion and cyto-compatibility were performed on cladded samples. The FGM cladding showed higher apatite precipitation than 100% HA cladding. Both LC samples had significantly higher amount of protein adsorption lead to higher cell adhesion and cyto-compatibility compared to non-cladded Ti-6Al-4 V alloy. In between the LC samples, FGM cladding had more protein adsorption and cell adhesion than 100% HA cladding. With six days of incubation, the proliferation rate of FGM cladding (215.6 ± 17.2%) was significantly higher than 100%HA cladding (134.6 ± 9.5%) and non-cladded Ti–6Al–4 V control (100%). All these outcomes reveal that the LC process improves the biocompatibility and bioactivity of Ti–6Al–4 V.

Caliceal Fluid Temperature During High-Power Holmium Laser Lithotripsy in an In Vivo Porcine Model.

With increasing use of high-power laser settings for lithotripsy, the potential exists to induce thermal tissue damage. In vitro studies have demonstrated that temperature elevation sufficient to cause thermal tissue damage can occur with certain laser and irrigation settings. The objective of this pilot study was to measure caliceal fluid temperature during high-power laser lithotripsy in an in vivo porcine model. Four female pigs (30-35 kg) were placed under general anesthesia and positioned supine. Retrograde ureteroscopy with entry into upper or middle calices was performed. Thermocouples were placed into the calix by open exposure and puncture of the kidney or retrograde alongside the ureteroscope. A 242 μm laser fiber was positioned in the center of the calix and activated (0.5 J, 80 Hz, 40 W) for 60 seconds with high, medium, or no irrigation delivered in each trial. Finite element simulations of laser-induced heating in a renal calix were also performed. Peak temperatures of 84.8°C, 63.9°C, and 43.6°C were recorded for no, medium, and high irrigation, respectively. Mean time to reach threshold of thermal injury (t43 of 120 minutes) was 12.7 and 17.8 seconds for no and medium irrigation. Thermal damage thresholds were not reached in high-irrigation trials. Numerical simulations revealed similar results with peak spatial average fluid temperatures of >100°C, 58.5°C, and 37.5°C during 60 seconds of laser activation for 0.1, 15, and 40 mL/minute irrigation, respectively. High-power holmium laser settings (40 W) can induce potentially injurious temperatures in the porcine in vivo model, particularly with slower irrigation rates. Characterization of thermal dose across a broader range of laser parameter settings is underway to map out the thermal safety envelope.

Effect of laser parameters on surface roughness of laser modified tool steel after thermal cyclic loading

This study presents the effects of laser parameters on the surface roughness of laser modified tool steel after thermal cyclic loading. Pulse mode Nd:YAG laser was used to perform the laser surface modification process on AISI H13 tool steel samples. Samples were then treated with thermal cyclic loading experiments which involved alternate immersion in molten aluminium (800°C) and water (27°C) for 553 cycles. A full factorial design of experiment (DOE) was developed to perform the investigation. Factors for the DOE are the laser parameter namely overlap rate (η), pulse repetition frequency (fPRF) and peak power (Ppeak) while the response is the surface roughness after thermal cyclic loading. Results indicate the surface roughness of the laser modified surface after thermal cyclic loading is significantly affected by laser parameter settings.

Condensation and subsequent freezing delays as a result of using femtosecond laser functionalized surfaces

In this paper, the authors report on the use of femtosecond laser surface processing (FLSP) to enhance the anti-icing properties of a commonly used aircraft alloy, Al 7075-O Clad. By changing the surface morphology through FLSP and the surface chemistry through siloxane vapor deposition, the wettability of Al 7075-O Clad was altered. Tall mound and short mound FLSP functionalized surfaces were created through two sets of laser parameters. Condensation and the subsequent freezing of condensates on FLSP Al 7075-O Clad was studied. Both structure height and surface wettability were shown to play a role in the delay of freezing. Freezing occurred on the FLSP superhydrophilic surface faster than on the unprocessed Al 7075-O Clad surface; however, freezing was delayed for all superhydrophobic FLSP surfaces. Tall structure height FLSP functionalized surfaces delayed freezing time longer than short structure height FLSP functionalized surfaces although all were superhydrophobic. It was shown that FLSP functionalized surfaces were able to delay freezing by up to 530 s compared with unprocessed Al 7075-O Clad. The authors also report on self-propelled condensate jumping on FLSP surfaces during the condensing process. The self-propelled jumping phenomena provide a means to promote anti-icing of materials, especially where jumping drops can be swept away in flow conditions.

THz generation by laser coupling to carbon nanotube array

A viable scheme of THz radiation generation by beating of two lasers (ω1, k→1; ω2, k→2) in a nanotube array, mounted on a dielectric substrate, is proposed and studied. The free electrons of the nanotubes acquire a large oscillatory velocity and experience a beat frequency ponderomotive force that turns nanotubes into oscillating dipole antennae emitting THz radiation. The THz power peaks in directions where a phase difference between fields due to successive nanotubes is integral multiple of 2π. The THz power is large when the beat frequency equals ωp/2 (where ωp is the electron plasma frequency) and surface plasmon resonance occurs. For our set of laser and carbon nanotube parameters, the generated THz is about 0.1 kW for CO2 laser power of 10 GW and pulse length of a few picoseconds.

Technique developments and performance analysis of chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering combustion thermometry.

This work characterizes the state of the art in the analysis of high-repetition-rate, ultrafast combustion thermometry using chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs-CARS). Several key aspects of the CARS spectroscopy system are described, including: (1)the ultrafast laser source, (2)use of the frequency-doubled idler versus signal from the optical parametric amplifier, (3)the geometry constraints for phase matching, and (4)spectral fitting for single-shot temperature measurements. A frequency-dependent instrument response function (IRF) for the detection system was modeled as a variable-width Gaussian and implemented through a frequency convolution of synthetic spectra. Proper accounting of the IRF increased spectral fitting performance in the high-frequency region where signal oscillations are weaker and narrower. Aggregated data from 25 system performance assessments taken over four months yielded accuracy and precision of 2.7% and ±3.5% for flame temperatures, and 9.9% and ±6.1% at room temperature, using the commonly reported method. A new processing technique, based on the statistical method of maximum likelihood, was implemented for turbulent flames where strong fluctuations in expected temperatures necessitate use of multiple temperature calibrations. Results from multiple sets of laser parameters are combined to generate an error-weighted temperature from the top-performing calibrations. A testing procedure was designed to characterize system performance when the range of expected temperatures is unknown, simulating the random temperature field of a highly turbulent flame. Accuracy error of the CPP fs-CARS system increased in this more-stressing test at all temperatures, but precision was significantly affected only at room temperature. System stability is characterized, and the contribution from shot-to-shot laser fluctuations on measurement precision is quantified. Finally, the near-adiabatic and steady assumptions for the Hencken burner calibration flame are examined in an axial scan; significant deviations from ideal behavior were observed only at heights of more than four diameters above the burner surface.

Steering population transfer between electronic states of the Na2 molecule beyond the rotating wave approximation

By utilizing the time-dependent wavepacket method, we studied the population transfer between the electronic states of the Na2 molecule driven by femtosecond laser pulses. Three electronic states, , and , are taken to be the initial, the intermediate and the target states, respectively. Two classes of calculations are performed for comparison: one is to invoke the rotating wave approximation (RWA), and the other is more accurate calculations without consideration of RWA (non-RWA). It is found that for this three-state-model, the RWA fails to describe the accurate population transfer process in femtosecond-pulse fields and that the adiabatic passage is hard to be constructed in the non-RWA calculations. Based on the non-RWA calculations, we determined a set of laser parameters, including the peak intensity, detuning, pulse duration and time delay, for efficient population transfer.

Probing vacuum birefringence under a high-intensity laser field with gamma-ray polarimetry at the GeV scale

Probing vacuum structures deformed by high intense fields is of great interest in general. In the context of quantum electrodynamics (QED), the vacuum exposed by a linearly polarized high-intensity laser field is expected to show birefringence. We consider the combination of a 10 PW laser system to pump the vacuum and 1 GeV photons to probe the birefringent effect. The vacuum birefringence can be measured via the polarization flip of the probe $\gamma$-rays which can also be interpreted as phase retardation of probe photons. We provide theoretically how to extract phase retardation of GeV probe photons via pair-wise topology of the Bethe-Heitler process in a polarimeter and then evaluate the measurability of the vacuum birefringence via phase retardation given a concrete polarimeter design with a realistic set of laser parameters and achievable pulse statistics.

Laser processing of thin-film multilayer structures: comparison between a 3D thermal model and experimental results.

In this research, a numerical model is introduced for simulation of laser processing of thin film multilayer structures, to predict the temperature and ablated area for a set of laser parameters including average power and repetition rate. Different thin-films on Si substrate were processed by nanosecond Nd:YAG laser pulses and the experimental and numerical results were compared to each other. The results show that applying a thin film on the surface can completely change the temperature field and vary the shape of the heat affected zone. The findings of this paper can have many potential applications including patterning the cell growth for biomedical applications and controlling the grain size in fabrication of polycrystalline silicon (poly-Si) thin-film transistors (TFTs).

Effects of surface laser treatments on microstructure, tension, and fatigue behavior of AISI 316LVM biomedical wires

Recent developments in minimally invasive surgical techniques have led to a surge in advancing alternative manufacturing routes. In order to meet the functional needs of such small devices as cardiovascular stents, and guide wires, the use of new materials and delicate geometries has increased creating a new challenge for manufacturing and machining. In this work, two sets of laser parameters leading to different laser power output were employed to study the effects of systematic changes in microstructure and mechanical characteristics of AISI 316LVM biomedical grade wires. Uniaxial tension and cyclic strain-controlled fatigue with the use of flex bending machines were used in order to evaluate the mechanical properties. Surface quality, heat affected zone (HAZ), and subsequent mechanical response were also investigated. Improvements in the fatigue behavior of laser-treated wires were obtained but depended on the choice of laser parameters. Low power Nd:YAG laser-treated AISI 316LVM annealed wires exhibited better fatigue performance in both high cycle fatigue and low cycle fatigue compared to the annealed AISI 316LVM wires. In contrast, high power Nd:YAG laser-treated annealed wires exhibited poorer fatigue performance in both high cycle and low cycle fatigue. Nd:YAG laser-treated AISI 316LVM hard wires exhibited poorer fatigue performance in both high cycle fatigue and low cycle fatigue compared to the AISI 316LVM hard wires. The effects of laser processing conditions on the microstructural features responsible for these changes in fatigue behavior are discussed.

Propagation characteristics of Hermite-cosh-Gaussian laser beam in a rippled density plasmas

AbstractIn the present research work, the authors have investigated the self-focusing and defocusing of Hermite-cosh-Gaussian laser (HChG) beam in an inhomogeneous rippled density plasmas. By taking the relativistic non-linearity into account, an equation for envelope is set up and solved using Wentzel–Kramers–Brillouin and the paraxial ray approximation. An ordinary non-linear differential equation governing the beam width parameter as a function of propagation distance is set up for different mode structures of the beam. Further, a numerical study of this differential equation is carried for suitable set of plasma and laser parameters. The beam undergoes periodic self-focusing/defocusing due to relativistic non-linearity. We also report the comparison between self-focusing/defocusing of HChG beam in the absence and presence of density ripple. Presence of ripple does not only leads to substantial increase in self-focusing length, but also results in oscillatory character with decreasing f. In a relativistic case, strong oscillatory self-focusing and defocusing is observed. Further, self-focusing is enhanced with increased value of decentered parameter.

Laser beam micro-milling of nickel alloy: dimensional variations and RSM optimization of laser parameters

Micro-channels are considered as the integral part of several engineering devices such as micro-channel heat exchangers, micro-coolers, micro-pulsating heat pipes and micro-channels used in gas turbine blades for aerospace applications. In such applications, a fluid flow is required to pass through certain micro-passages such as micro-grooves and micro-channels. The fluid flow characteristics (flow rate, turbulence, pressure drop and fluid dynamics) are mainly established based on the size and accuracy of micro-passages. Variations (oversizing and undersizing) in micro-passage’s geometry directly affect the fluid flow characteristics. In this study, the micro-channels of several sizes are fabricated in well-known aerospace nickel alloy (Inconel 718) through laser beam micro-milling. The variations in geometrical characteristics of different-sized micro-channels are studied under the influences of different parameters of Nd:YAG laser. In order to have a minimum variation in the machined geometries of each size of micro-channel, the multi-objective optimization of laser parameters has been carried out utilizing the response surface methodology approach. The objective was set to achieve the targeted top widths and depths of micro-channels with minimum degree of taperness associated with the micro-channel’s sidewalls. The optimized sets of laser parameters proposed for each size of micro-channel can be used to fabricate the micro-channels in Inconel 718 with minimum amount of geometrical variations.

NiTi shape memory alloy with enhanced wear performance by laser selective area nitriding for orthopaedic applications

Abstract In recent years, near-equiatomic NiTi alloy has been identified as a sound alternative to replace the conventional Ti6Al4V alloy as the next generation orthopaedic biomaterial because of its lower young modulus and unique shape memory effect. The potential problem of using NiTi alloy is the generation of Ni-rich debris when wear occurs. Surface treatment is therefore needed to improve the wear resistance in order to alleviate the impact of the wear. This paper details the surface treatment of NiTi by laser selective area nitriding for enhancing the wear resistance. This was done by a systematic two-step optimization approach: (1) selecting the appropriate set of laser parameters with an L9 Taguchi experiment to optimize the nitride properties and (2) identifying the optimized surface coverage ratio to maximize the wear resistance. The microstructure and surface profiles of the optimized nitride surface was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), and 3-D profile measurement, respectively. The wear resistance of nitrided surfaces with different coverage ratios were then evaluated using reciprocating wear testing against ultra-high-molecular-weight polyethylene (UHMWPE) in simulated body fluid, i.e., Hanks' solution.

Experimental investigation of micro-channels produced in aluminum alloy (AA 2024) through laser machining

Aluminum and its alloys are growingly used in various applications including micro-channel heat exchangers and heat sinks to facilitate heat transfer though micro-fluidic flows. Micro-channels with precise control over geometrical features are very important in order to design micro-fluidic flow dynamics and its characteristics. In this research, Nd:YAG laser beam micro-milling has been utilized to produce micro-channels in aluminum alloy (AA 2024) having cross-sectional size of 400 × 200 µm2. The objective was to control the material removal rate (MRR) of the process in order to get the micro-channels’ geometries (width, depth and taperness of sidewalls) close to the designed geometries. In this context, parametric effects of predominant laser parameters on the process performance have been categorically studied. Quadratic mathematical models have further been developed to estimate the MRR and each geometrical aspect of micro-channels over different levels of laser parameters. Additionally, multi-objective optimization has been performed to get an optimized set of laser parameters generating the accurate machining geometries with appropriate material removal per laser scan. Finally, the models and optimization results were validated through confirmatory experimental tests. The results reveal that the precised micro-channel geometries can be obtained through laser beam micro-milling by selecting the appropriate combination of laser parameters (lamp current intensity of 84.48 %, laser pulse frequency of 35.70 kHz and laser scanning speed of 300 mm/s) that can collectively remove a required amount of material thickness per laser scan.

Laser induced single spot oxidation of titanium

Titanium oxides have a wide range of applications in industry, and they can be formed on pure titanium using different methods. Laser-induced oxidation is one of the most reliable methods due to its controllability and selectivity. Colour marking is one of the main applications of the oxidation process. However, the colourizing process based on laser scanning strategies is limited by the relative large processing area in comparison to the beam size. Single spot oxidation of titanium substrates is proposed in this research in order to increase the resolution of the processed area and also to address the requirements of potential new applications. The method is applied to produce oxide films with different thicknesses and hence colours on titanium substrates. High resolution colour image is imprinted on a sheet of pure titanium by converting its pixels’ colours into laser parameter settings. Optical and morphological periodic surface structures are also produced by an array of oxide spots and then analysed. Two colours have been coded into one field and the dependencies of the reflected colours on incident and azimuthal angles of the light are discussed. The findings are of interest to a range of application areas, as they can be used to imprint optical devices such as diffusers and Fresnel lenses on metallic surfaces as well as for colour marking.

Porous media based bio-heat transfer analysis on counter-current artery vein tissue phantoms: Applications in photo thermal therapy

The present work deals with the determination of transient variation of temperature distributions inside laser irradiated biological tissue phantoms consisting of an artery–vein counter current arrangement. An optical inhomogeneity that is absorbing in nature and embedded inside an otherwise homogenous medium has been employed to model the abnormal cells. The surrounding tissue region has been modeled as a porous medium consisting of solid and fluid matrices. Separate energy equations for solid and fluid matrices of the porous tissue regions have been solved as part of the bio-heat transfer model. A short pulse laser has been employed to irradiate the phantom. A numerical model coupling the Discrete Ordinates Method (DOM) for solving the transient form of Radiative Transport Equation (RTE) with that of the porous media based bio-heat transfer model has been developed and benchmarked. The convective cooling effects of blood flow through a single blood vessel as well as counter current blood vessels under local thermal non-equilibrium (LTNE) conditions have been investigated. Thermal response of the porous tissue phantom with respect to the changes in the Reynolds number (Re=1 and 10) and for varying porosity levels has been presented. The effect of blood vessel diameter on the resultant temperature distribution within the body of the laser irradiated tissue phantom has been studied. Results have been presented in the form of temporal variations of temperature distributions, local variations in the heat transfer rates (Nusselt numbers) along the spatial dimensions of various interfaces present in the physical domain. Finally, the influence of the position of the embedded inhomogeneity with respect to the point of laser irradiation on the temperature rise at the location of the inhomogeneity has been investigated. The study reveals a strong dependence of the maximum possible temperature rise on the relative position of the embedded abnormal cells for a given set of laser parameters.

Laser-welded fused silica substrates using a luminescent fresnoite-based sealant

The laser welding of two fused silica substrates using a fresnoitic glass thin film as a sealant by irradiation with a ns laser is studied. Two different laser parameter sets were compared in terms of bond quality, which include two different laser beam trajectories: linear and wobble (circular) trajectory. The composition of the glass sealant changes with the course of the laser welding, incorporating silica from the substrates. After joining, the bonded samples were exposed to UV light and a very intense emission in the blue spectral range is observed by naked eye, which is due to the crystallization of the fresnoite glass upon the laser irradiation. EDX analysis confirms the crystallization of fresnoite, together with a great enrichment in silica. The formation of a eutectic between both is very plausible. Bond quality and bond strength were evaluated by scanning acoustic microscopy (SAM) and tensile test, which results in a tensile stress of 7MPa.

Laser detection of remote targets applying chaotic pulse position modulation

Chaotic pulse position modulation (CPPM) has been successfully used in robust digital communication for years. We propose to adapt CPPM for laser detection of remote targets to address the issue of noise. Specified in a time-of-flight (TOF) consecutive laser ranging application scenario, the feasibility of laser detection applying CPPM for laser detection is experimentally investigated. The scheme including the adaptive design for laser detection and parameter settings with validation is introduced. Lab-based electrical experiment and a proof-of-concept outdoor TOF experiment are further conducted to verify the feasibility of laser ranging and detection using CPPM through comparison with traditional Lidar detection and other pulse interval patterns. According to experiments and the following analysis, laser ranging using CPPM is feasible and more robust than traditional laser ranging.