Laser Spot Size Research Articles

Surface texturing, residual stresses and edge treatment of hardmetal tools by means of femtosecond pulsed laser

Laser induced periodic surface structures (LIPSS) have been produced at the surface of a WC-Co turning insert by femtosecond pulsed laser ablation. Wavelengths of approx. 500 nm have been obtained with a 1030 nm incident radiation and with a spot size of 30 μm. No thermal damage is induced by following this procedure, although a slight surface stress relaxation is detected as fluence increases. The higher material removal needed for chamfering operations requires accumulated fluences over 1 kJ/cm2. At lower energy densities, material removal is not completed. If fluence is increased by reducing the laser spot size, material removal is also inefficient since signicant amount of debris is accumulated on the irradiated areas. Best chamfering strategy involves the combination of relatively large spot sizes and fluences. Form factors (Κappa) of the chamfered cutting edges range from 4 to 10 although the laser is oriented at 45° of the rake face. Lower Κ values can be obtained by adjusting the laser focus along the machining process.

Scaling laws for the additive manufacturing of the AISI 316 L deposited by laser surface cladding and direct metal deposition methods

This study presents a comparative analysis of the shape and microstructure of individual tracks formed from AISI 316 L stainless steel using laser surface cladding (LSC) and direct metal deposition (DMD) methods. The research aims to identify the relationship between laser radiation parameters (laser spot size, radiation power, scanning speed) and geometric characteristics of created tracks. The result shows that the change in the geometric characteristics of single tracks (depth, width, and height) can be related to two dimensionless parameters: normalized enthalpy H and Peclet number Pe. Based on the experimental data summary, the limiting values of the conditions for realizing keyhole melting (H > 2, Pe < 10) and heat conduction (H < 2, Pe > 10) were determined.

Research on the core factors affecting the temperature rise of dichroic mirrors in wavelength beam combination system under high-power CW laser

The wavelength beam combination is considered an important method for improving the source power of high-energy laser weapons while maintaining high beam quality. The dichroic mirror is the key component that decisively determines the laser beam quality of the wavelength beam combining system. However, the temperature rise of the dichroic mirror due to continuous high-power laser irradiation can significantly deteriorate the beam quality and decrease the efficiency of the systems. Along these lines, in this work, the factors that influence the temperature rise of the dichroic mirror under high power CW laser were systematically examined. The temperature field equation of the dichroic mirror under transmission and reflection laser was also calculated. The variation of the dichroic mirror temperature by the laser power, laser spot size, diameter, and thickness of substrates, as well as material of substrates and thin-film absorption was simulated. The simulated results were verified by performing experiments on both sapphire and quartz substrates. The transmittance absorption was proved to be the main cause of the temperature rise and the laser power had a linear relationship with the maximum temperature. Interestingly, the temperature decreased with the thermal conductivity and the laser spot size. As a result, the optimization of the substrate size can significantly reduce the temperature of the dichroic mirror. Our work provides valuable insights into the design and increase in performance of dichroic mirrors in the wavelength combination system.

Depth profile analyses by femtosecond laser ablation (multicollector) inductively coupled plasma mass spectrometry for resolving chemical and isotopic gradients in minerals

Abstract. Femtosecond laser ablation (fs-LA) coupled to a multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) instrument has been proven to be a powerful means to analyze isotope ratios of “non-traditional” stable isotope systems with high spatial resolution, precision, and accuracy. The technique has been successfully applied, e.g., to investigate diffusion-generated isotopic zoning of the elements Li, Mg, and Fe in magmatic crystals. Here, we present a novel sampling technique employing a fs-LA system that is equipped with a computer numerical control (CNC) laser stage, using the open-source software LinuxCNC. Combining this laser set up with ICP-MS or MC-ICP-MS allows us to perform depth profile analyses of major and trace elements, respectively, as well as metal stable isotope variations of Fe and Mg in olivine crystals and in experimental diffusion couples. Samples are ablated in circular patterns with profile diameters of 100–200 µm using a laser spot size of 25–30 µm. Depending on the scan speed and the repetition rate of the laser, each ablated sample layer is between 300 nm and 3.0 µm thick. The integrated signal of one ablated layer represents one data point of the depth profile. We have tested this technique by analyzing 5–50 µm deep depth profiles (consisting of 15–25 individual layers) of homogeneous and chemically zoned olivine crystal cuboids. The minor and trace element analyses of the zoned cuboids, conducted by fs-LA-ICP-MS, were compared with “horizontal” profiles analyzed in polished sections of the cuboids with electron probe microanalysis (EPMA). Furthermore, we analyzed Fe–Mg isotopic depth profiles of the same cuboids with fs-LA-MC-ICP-MS, of which the chemically zoned ones also showed isotopic zoning at identical scales. Isotopic depth profiles were also conducted on an unzoned olivine cuboid that was coated with a 26Mg- and 56Fe-enriched olivine thin film (of ∼ 800 nm) in order to investigate top-to-bottom contamination during depth profiling. Our results indicate that (i) concentration data acquired by fs-LA depth profiling match well with EPMA data, (ii) precise and accurate Fe and Mg isotopic data can be obtained (i.e., precision and accuracy are ≤ 0.12 ‰ and ≤ 0.15 ‰ for both δ26Mg and δ56Fe, respectively), and (iii) potential top-to-bottom contamination during depth profiling of isotope ratios can be avoided. The technique presented herein is particularly suitable for the investigation of minerals or glasses with chemical and/or isotopic gradients (e.g., diffusion zoning) vertical to planar surfaces. It can also be applied in materials sciences in order to analyze thin films, coatings, or surface contaminations on solids.

Batch-Arrangement of Droplets on Silica Surface Based on Laser Wettability Modification

The laser wettability modification method for a silica surface was applied to the liquid batch-arrangement process. The droplets on the contact angle boundary produced by the modification moved to the more hydrophilic modified area because of the driving force of the surface energy hysteresis. Based on this phenomenon, a substrate with an array of 1-mm square modified areas was dip-coated with different concentrations of glycerin solution to align the droplets in batches. The experimental and analytical results showed that when the amount of liquid adhering to the hydrophilic area was small, the droplet shape became round owing to surface tension, and the reproduction of the modified rectangular shape was difficult. To increase the amount of the adhered liquid, the arrangement was conducted under conditions that increased the capillary number, such as high viscosity and high pull-out speed. The reproducibility of the intended shape was found to increase with an increase in the capillary number. Although the spatial resolution of the arrangement was limited by the laser spot size, it was demonstrated that the proposed method could align droplets onto a silica surface in a batch with the intended geometry.

Natural Ti‐Rich Mineral (Ilmenite, Titanite and Perovskite) Reference Materials for In Situ Ti Isotopic Measurement by LA‐MC‐ICP‐MS

Iron‐titanium oxides such as ilmenite (FeTiO3), titanite (CaTiSiO5) and perovskite (CaTiO3) are the common Ti‐rich mineral phases crystallised during magmatic and metamorphic processes on Earth. Depending on magma types or conditions of phase equilibria, formation of these Ti‐rich minerals can result in Ti isotopic fractionation in the range of ‐1.52‰ to +2.90‰ on δ49TiOL‐Ti (i.e., the per mil difference of 49Ti/47Ti ratio relative to the OL‐Ti reference material), making the Ti isotope ratios of these minerals potential tracers for conditions of magmatism and metamorphism. Due to their resistance to aqueous alteration, these Ti‐rich accessory minerals are also commonly present as pristine, detrital phases in sedimentary rocks, which offer an opportunity to study the evolution of magmatism and metamorphism throughout the Earth's geological history. Here we have developed a novel technique for in situ Ti isotopic measurement in ilmenite, titanite and perovskite using femtosecond laser ablation multi‐collector inductively coupled plasma‐mass spectrometry (fs‐LA‐MC‐ICP‐MS) under wet plasma conditions. Samples were ablated with different laser spot sizes (15–50 μm) and different laser energy densities (0.6–4.2 J cm‐2) to obtain adequate Ti signal intensity at a fixed laser repetition rate of 2 Hz. When 49Ti signal intensity of samples ranged from 0.3 to 3.7 V, no significant signal‐dependent Ti isotopic fractionation was observed under wet plasma conditions. Repeated measurements on twelve Ti‐rich minerals using different analytical protocols provided comparable δ49TiOL‐Ti values within uncertainties, confirming the accuracy of the proposed fs‐LA‐MC‐ICP‐MS method. With one exception (i.e., RUS1), all analysed minerals are homogeneous in their Ti isotopic compositions between individual chips with an intermediate precision of ±0.13‰ to ±0.17‰ (2s) on δ49TiOL‐Ti. Collectively, these minerals record a significant δ49TiOL‐Ti variation ranging from ‐0.46‰ to +2.12‰. These warrant the suitability of these materials as Ti isotopic reference materials for in situ Ti isotopic measurement.

Interaction between Local Shielding Gas Supply and Laser Spot Size on Spatter Formation in Laser Beam Welding of AISI 304

Background. Spatter formation at melt pool swellings at the keyhole rear wall is a major issue for laser deep penetration welding at speeds beyond 8 m/min. A gas nozzle directed towards the keyhole, that supplies shielding gas locally, is advantageous in reducing spatter formation due to its simple utilization. However, the relationship between local gas flow, laser spot size, and the resulting effects on spatter formation at high welding speeds up to 16 m/min are not yet fully understood. Methods. The high-alloy steel AISI 304 (1.4301/X5CrNi18-10) was welded with laser spot sizes of 300 μm and 600 μm while using a specially designed gas nozzle directed to the keyhole. Constant welding depth was ensured by Optical Coherence Tomography (OCT). Spatter formation was evaluated by precision weighing of samples. Subsequent processing of high-speed images was used to evaluate spatter quantity, size, and velocity. The keyhole oscillation was determined by Fast Fourier Transform (FFT) analysis. Tracking the formation of melt pool swellings at the keyhole rear wall provided information on the upward melt flow velocity. Results. The local gas flow enabled a significant reduction in the number of spatters and loss of mass for both laser spot sizes and indicated an effect on surface tension by shielding the processing zone from the ambient atmosphere. The laser spot size affected the upward melt flow velocity and spatter velocity.

Optimization of extreme ultra-violet light emitted from the CO2 laser-irradiated tin plasmas using 2D radiation hydrodynamic simulations.

We studied Extreme Ultra-Violet (EUV) emission characteristics of the 13.5 nm wavelength from CO2 laser-irradiated pre-formed tin plasmas using 2D radiation hydrodynamic simulations. Our results indicate that when a CO2 laser irradiates pre-formed tin plasma, the heated plasma expands towards the surrounding plasma, steepening the density at the ablation front and lowering the density near the laser axis due to the transverse motion of the plasma. Consequently, the laser absorption fraction decreases, and the contribution to EUV output from the ablation front becomes dominant over that from the low-density plasmas. We estimated that an EUV conversion efficiency of 10% from laser to EUV emission could be achieved with a larger laser spot size, shortened laser pulse width, and longer pre-formed plasma density scale length. Our results offer one optimizing solution to achieve an efficient and powerful EUV light source for the next-generation semiconductors.

Geometrical and crystal plasticity modelling: Towards the establishment of a process-structure-property relationship for additively manufactured 316L struts

This paper presents a methodology to establish a process structure property (PSP) relationship for the additive manufacturing (AM) of thin AISI 316L struts, as might be used in coronary stent applications. The methodology is based on a new geometrically based process-structure method for AM process variables, and crystal plasticity finite element (CPFE) modelling that includes the representation of melt pool microstructure morphology and texture. The effects of AM process variables are characterised with respect to ductility, yield strength and UTS across a range of process variables, including hatch spacing, layer thickness and build orientation (two orientations are considered: horizontal and vertical). The CPFE-predicted effect of build orientation is shown to be consistent with experimental test results. The present methodology has allowed identification of optimal hatch spacing and layer thickness for a given laser spot size, power and scanning speed. CPFE modelling of AM melt pool texture is shown to be key to successful prediction of the effects of process variables on mechanical behaviour.

Anomalous refinement and uniformization of grains in metallic thin films

AbstractWhen a laser beam writes on a metallic film, it usually coarsens and deuniformizes grains because of Ostwald ripening, similar to the case of annealing. Here we show an anomalous refinement effect of metal grains: A metallic silver film with large grains melts and breaks into uniform, close-packed, and ultrafine (∼ 10 nm) grains by laser direct writing with a nanoscale laser spot size and nanosecond pulse that causes localized heating and adaptive shock-cooling. This method exhibits high controllability in both grain size and uniformity, which lies in a linear relationship between the film thickness (h) and grain size (D), D ∝ h. The linear relationship is significantly different from the classical spinodal dewetting theory obeying a nonlinear relationship (D ∝ h5/3) in common laser heating. We also demonstrate the application of such a silver film with a grain size of ∼ 10.9 nm as a surface-enhanced Raman scattering chip, exhibiting superhigh spatial-uniformity and low detection limit down to 10−15 M. This anomalous refinement effect is general and can be extended to many other metallic films.

Development of automated microplastic identification workflow for Raman micro-imaging and evaluation of the uncertainties during micro-imaging

In this study, an automated identification workflow for Raman micro-imaging (RMI) was developed, and the performance was evaluated by artificial samples of microplastic (MP) microsphere with different sizes and types. Theoretical detection rate and estimated particle size were derived and compared with experimental data. Results show that the proposed workflow can identify plastic types and estimate the size of the MP microspheres under different conditions for most cases. However, size of laser spot and discrepancy between sample surface and focal plane can influence RMI results in two ways. Firstly, small particles are more likely to be detected. Secondly, estimated sizes of particles are more likely to be overestimated. The derived uncertainties can serve as a reference for future experimental design and further investigation of more complex situations. The workflow is accessible online, and interested researchers can adjust the parameter values as necessary to suit their specific circumstances.

Additive manufacturing by laser powder bed fusion and thermal post-treatment of the lunar-regolith-based glass-ceramics for in-situ resource utilization

Powder bed fusion is one of the most promising additive manufacturing approaches for in-situ resource utilization (ISRU) in the following lunar exploitation missions. However, the harsh conditions for energy supply and equipment operation on the lunar surface and the amorphization of materials due to rapid heat-cooling cycles could hinder the efforts to produce high-quality lunar regolith-based products. Laser powder bed fusion (LPBF) with large laser spot sizes and low scan speeds were used to print the glassy lunar regolith simulant (LRS) precursors. Then the thermal post-treatment was carried on to transform the LRS precursors into glass–ceramic products. The precursors and glass–ceramic samples' microstructure, mechanical properties, and thermophysical properties were investigated. The results indicated that the acceptable range of volumetric energy density (VED) for the LPBF process of lunar regolith was from 3.5 to 4.3 J/mm3. The porous structure resulting from the large molten pools leads to low thermal conductivities and an acceptable strength compromise. The glassy precursors formed augite phase-dominated LRS glass-ceramics at 823 °C during thermal post-treatment and augite-plagioclase polycrystalline LRS glass-ceramics with better mechanical properties at 1100 °C. The increase of the augite phases precipitation contributed to the strength enhancement, and the grain coarsening caused by the prolonged crystallization process at 1100 °C would reduce the strength. The average compressive strength, fracture toughness, and Vickers hardness of the LRS glass-ceramics could reach 50.71 MPa, 1.49 MPa·m1/2, and 897.91 HV1/15, respectively. This multi-process method of laser powder bed fusion of glass-ceramics can maximize the application potential of the ubiquitous lunar regolith in future in-situ additive manufacturing to produce lunar regolith-based building materials for different stages of future lunar exploitation missions.

Influence of laser spot size on structure and properties of high-temperature CompoNIAL-M5-3 alloy produced by selective laser melting

The CompoNiAl-M5-3 high-temperature alloy based on nickel monoaluminide was obtained by selective laser melting (SLM) of a spheroidized powder with particle size in the range of 20 – 45 μm. The powder was manufactured using an integral technology including self-propagating high-temperature synthesis (SHS), briquette grinding, sieve and air classification followed with spheroidization of powder particles in a thermal plasma flow and ultrasonic purification of spheroidized particles from nanofraction. Using parametric studies, the SLM modes were tested on SLM 280H and TruPrint 1000 machines. Mechanical tests of the samples were carried out using the uniaxial compression scheme with the strain rate dε/dt = 10–4 s–1 in the temperature range 1023 – 1273 K. Scanning and transmission electron microscopy methods were used to study the influence of laser spot size on the evolution of microstructure and thermomechanical properties of the SLM-consolidated material in comparison with that obtained by hot isostatic pressing (HIP). The authors established the effect of HIP + HT (aging in vacuum) post-treatment on the structure and mechanical properties of the material. The yield strength at 1073 K of the alloy built on the additive machine with a laser spot diameter of 38 μm after SLM + HIP + HT was 500 MPa, which exceeded the yield strength of the HIP-samples by 220 MPa.

Betatron Radiation and Bremsstrahlung in the Interaction of Intense Laser Pulse with Solid Target

X-ray generation via synchrotron radiation and bremsstrahlung in the interaction of short laser pulses with a solid target is of much current interest owing to its numerous applications. The efficiency of laser to X-ray energy conversion is thus a crucial factor. We found that the energy conversion efficiency of synchrotron radiation and bremsstrahlung is mainly governed by the ratio of the laser pulse width to the preplasma width, which is in turn governed by the laser profile, intensity, and spot size. Synchrotron radiation dominates when the ratio is less than unity, otherwise bremsstrahlung dominates. The type of radiation can thus be controlled by tailoring the laser parameters.

Printing regime for single metal microdroplet deposition in laser-induced forward transfer

Laser-induced forward transfer (LIFT) is proposed as an additive manufacturing technique for high-resolution metallic microstructures. However, the printing regime of single microdroplet deposition is lacking, making it difficult to determine the processing parameters for single microdroplet printing. In this study, both LIFT experiments and thermal simulations are carried out to investigate the printing behaviors of a single microdroplet under different laser fluences and spot sizes. From the LIFT experiments, three printing behaviors are observed, including the non-release deposition, the single microdroplet deposition, and the splash deposition. The printing regime for single metal microdroplet deposition is obtained via varying laser pulse energy and spot size, where three printing behaviors are bounded by three thresholds. Lower threshold of the laser fluence for single microdroplet deposition is required to be constant regardless of the spot size, owing to the induced kinetic energy just overcoming the surface energy, which satisfies the ejection condition of Weber number greater than 1. Upper threshold of the laser fluence for single microdroplet deposition is observed to linearly increase with spot size, indicating that the rupture mode of the molten film determined by the nominal capillary number is closely related to the laser spot size. By analyzing the physical basis of microdroplet ejection at the threshold, Weber number and nominal capillary number are proposed as the criteria for the low and high thresholds of single microdroplet deposition.

Evaluation of carbonification of coals using a portable Raman spectrometer

AbstractThe estimation of the degree of carbonification of coals can be investigated using the Raman microspectroscopy applied for the analysis of grains of selected macerals (altered remains of organic particles). This approach allows for the best control of the analyses such as precise selection and orientation of the maceral grains to be analysed. On the other hand, the miniaturized, especially handheld Raman spectrometers typically offer a significantly less controlled environment with the issues including large laser spot sizes and the focusing issues. However, the advantages of the miniaturized spectrometers include fast analyses and the possibility of in situ analyses. This study illustrates that it is possible to discriminate among the coals of different degrees of maturation of the carbonaceous matter and other types of carbonaceous matter using the data acquired with a portable Raman spectrometer using spectroscopic parameters: D and G band widths and wavenumber positions. The issues inherent to the analyses of such heterogeneous materials such as coals using a portable spectrometer were addressed. Moreover, the variability in the Raman spectroscopic backgrounds of a wide range of coals from lignite to anthracite necessitated the utilization of a robust background removal function. This study shows that modern portable Raman spectrometers with the appropriate excitation are capable of basic discrimination among the different types of carbonaceous matter, with implications towards fast in situ analyses of carbonaceous matter in rocks as an additional tool for a geologist.

A fiber-coupled laser diode design for reflection mode optical resolution photoacoustic microscopy

Pulsed Laser Diodes (PLD) are compact and high pulse repetition rate laser sources that show a great potential for low-cost Optical Resolution Photoacoustic Microscopes (OR-PAM). Nevertheless, their non-uniform multimode laser beams are of low quality so that high lateral resolutions with tightly focused beams are difficult to realize at long focusing distances, as required for reflection mode OR-PAM devices of clinical application. A new strategy based on homogenizing and shaping the laser diode beam with a square-core multimode optical fiber allowed to attain competitive lateral resolutions while keeping one centimeter working distance. The theoretical expressions for the laser spot size, determining optical lateral resolution, and the depth of focus are also written for general multimode beams. An OR-PAM system was built in confocal reflection mode with a linear phased-array as the ultrasound receiver in order to test its performance, first, on a resolution test target and, afterwards, on ex vivo rabbit ears to show the system potential for subcutaneous imaging of blood vessels and hair follicles.

Systematic approach to process parameter optimization for laser powder bed fusion of low-alloy steel based on melting modes

In the metal additive manufacturing (AM) process of laser powder bed fusion (LPBF), there are a limited number of materials suitable for producing parts with high density and desired mechanical properties. To establish novel materials, it is essential to determine optimized process parameters in order to overcome process-related challenges and mitigate defects such as lack of fusion, keyholing, and balling. Scaling laws based on thermophysical properties and process parameters can be used to transfer knowledge from other materials or LPBF systems. In this work, a scaling law is used to adjust process parameters for single-track experiments over a wide range, which are laser power PL (100–1000 W), scan speed vs (300–2500 mm/s), and laser spot size ds (0.08–0.25 mm). Compared to existing studies, the parameter range is thus extended towards large laser spot sizes and high laser powers. The scaling law used is based on the calculation of the normalized enthalpy frac{Delta H}{{h}_{s}}. The ratio of the deposited energy density DeltaH and the melting enthalpy hs correlates with the dimensions of the melt pool. According to the aspect ratio {delta }_{mathrm{c}} of the melt pool of each single track, the respective melting mode—conduction, transition, and keyhole mode—was identified. The process parameters of the single tracks in transition mode were used to optimize the density of the LPBF specimens with varying hatch distance hd (0.06–0.12 mm), resulting in specimens with a relative density of > 99.8%. The proposed methodology can accelerate the process parameter finding for new alloys and avoid process-related defects.Graphical

An optimization method of rough surface adapted small rectangular LD laser source based on combination of Powell prism and cylindrical lenses

A combination optimization method was proposed to design a small-size rectangular laser spot and decrease the measurement error caused by the uneven intensity distribution of LD laser source. First, the intensity distribution of LD was analyzed and the combination lens structure of Powell prism and cylindrical lenses was proposed. The equations were established to calculate the profile parameters of lenses. And an illuminance loss model δΔED±R was built to determine the values of laser spot size, working reference and range. Secondly, the specific design process of our method was given, the 3D model was established and simulated in ZEMAX to further improve the imaging uniformity. Finally, the collimation, uniformity and location deviation of the spot were discussed. The average uniformity was 95.626%, and the difference of location deviation on shot peened surfaces was 0.004 μm in x direction and 0.121 μm in y direction when compared with the white ceramic surface.

Raman spectroscopy of lithium niobate (LiNbO3) − Sample temperature and laser spot size effects

The Raman spectrum of lithium niobate (LiNbO3) was investigated in the 83–823 K temperature range and as a function of different laser spot sizes. The measurements considered a Z-cut (congruent, undoped) LiNbO3 crystal, HeNe laser excitation (632.8 nm photons), and the zx,xyz¯ geometry. The main LiNbO3-related phonon modes were identified and analyzed in terms of their peak position (ω) and linewidth (γ), according to which it was possible to identify the contributions originating from anharmonic phonon-coupling and thermal lattice-expansion processes. The analyses of the temperature-dependent ωT and γT data took into account the 3- and 4-phonon model and the experimental Grüneisen and linear thermal expansion coefficients − as available from literature. Among all phonon modes of LiNbO3, the A1(TO4) mode presented a peculiar behavior (at room temperature) that depends on the laser spot size during the Raman measurements. The development of this forbidden mode has been associated with the congruent-photorefractive nature of LiNbO3 as well as with small variations in the geometry of polarization (as a result of changes in the laser spot size).