Laser Beam Scanning Research Articles

Method of Suppressing Solidification Cracking by Laser Surface Melting and Epitaxial Growth Behavior for Directionally Solidified 247LC Superalloy

In this study, the relationship between solidification cracking and epitaxial growth behavior with the high-speed laser surface melting of a directionally solidified 247LC superalloys was fundamentally and metallurgically investigated, to develop a successful welding procedure for the next generation of gas turbine blades. Under typical laser surface melting conditions (scan speed: 50 mm/s, heat input: 40 J/mm), severe solidification cracking phenomena occurs. The key metallurgical factors of solidification cracking have been identified as solidification segregation-assisted pipeline diffusion behavior at the solidification grain boundary, and in the randomly formed polycrystalline melting zone microstructure. In addition, under extremely low heat input and high-speed laser beam scan conditions (scan speed: 1000 mm/s, heat input: 2 J/mm), an effective surface melting zone can be obtained within a single directionally solidified grain under a relatively high-energy beam density (65 J/mm2) using the characteristics of single-mode fiber lasers. Results reveal that the laser melting zone successfully shows a 99.9% epitaxial growth achievement ratio. Because of the superior epitaxial growth ratio within the laser surface melting zone, and the rapid solidification phenomena, formation of a solidification grain boundary and solidification segregation-assisted pipeline diffusion behavior can be suppressed. Finally, a solidification crack-free laser melting zone can thus be achieved.

Highly Regular LIPSS on Thin Molybdenum Films: Optimization and Generic Criteria.

A systematic experimental study was performed to determine laser irradiation conditions for the large-area fabrication of highly regular laser-induced periodic surface structures (HR-LIPSS) on a 220 nm thick Mo film deposited on fused silica. The LIPSS were fabricated by scanning a linearly polarized, spatially Gaussian laser beam at 1030 nm wavelength and 1.4 ps pulse duration over the sample surface at 1 kHz repetition rate. Scanning electron microscope images of the produced structures were analyzed using the criterion of the dispersion of the LIPSS orientation angle (DLOA). Favorable conditions, in terms of laser fluence and beam scanning overlaps, were identified for achieving DLOA values <10∘. To gain insight into the material behavior under these irradiation conditions, a theoretical analysis of the film heating was performed, and surface plasmon polariton excitation is discussed. A possible effect of the film dewetting from the dielectric substrate is deliberated.

Heating of a Metal Layer by a Scanning Laser Beam

Heating of a Metal Layer by a Scanning Laser Beam

Liquation crack-free welding strategy for 247LC DS superalloy by control of pipeline diffusion via ultra-high-speed laser beam scanning

Under completely suppressed constitutional liquation of MC carbide, unexplored 247LC superalloy single-mode fibre laser welds experience unexpected liquation cracking. These cracks continuously propagate from a solidification crack, particularly at highly misoriented and epitaxially grown solidification grain boundaries, and dominated by pipeline diffusion of impurities. Two novel metallurgical preconditions were identified which, if satisfied, ensure complete suppression of liquation cracking of 247LC superalloy welds. These preconditions are (i) the constitutional liquation of MC carbide at the heat-affected zone and (ii) pipeline diffusion of impurities at the fusion zone. It is also highly recommended that the welding is carried out in a single directionally solidified grain under scanning conditions, avoiding the formation of highly misoriented solidification grain boundaries.

Laser-treatment-induced surface integrity modifications of stainless steel

Scanning of a high-power laser beam on the surface of martensitic stainless steel (SS420) has been studied, addressing the effect of scanning rate V on integrity modifications in the near-surface regions. Structural, compositional, and crystallographic characterizations revealed the presence of ablations, surface melting/resolidification, surface oxidations, and austenite (γ-phase) precipitations when V ≤ 20 mm s−1. Melt pool (MP), heat affected zone (HAZ), and base material have been clearly distinguished at the cross-section of the slow-scanned samples. Adjacent MPs partially overlapped when V = 5 mm s−1. The γ-phase precipitations solely occurred in the MPs, i.e., of ∼ 400 μm deep for V = 5 mm s−1, while oxidations dominantly occurred in the surface regions of shallower than ∼30 μm within the MPs. Compositional analysis revealed increased Cr-, Mn-, and Si-to-Fe ratios at the laser-scanned surface but without variations along the surface normal direction. The enhanced surface hardness has been achieved up to 805 HV, and the hardness monotonically decreased when moving deeper (i.e., ∼1000 μm) into the base material. These observations shed new light on surface engineering of metallic alloys via laser-based direct energy treatments.

Ultra-Short Pulse Laser Cleaning of Contaminated Pleistocene Bone: A Comprehensive Study on the Influence of Pulse Duration and Wavelength

The impact of wavelength and pulse duration in laser cleaning of hard blackish contaminants crust from archaeologically significant Pleistocene bone is investigated in this research. The objective is to determine the practical cleaning procedures and identify adequate laser parameters for cleaning archaeological bone from Sima de los Huesos (Spain) based on conservation and restoration perspectives. Bone surface cleaning was performed utilizing two Q-switched Nd:YAG lasers: sub-nanosecond pulsed lasers with emission wavelengths at 355 nm and 1064 nm, respectively, and a Yb:KGW femtosecond pulsed laser with an emission wavelength in the third harmonic at 343 nm. In all experiments, the laser beam scanning mode was applied to measure cleaning efficiency in removing contaminants and degradation products while assessing the underlying substrate surface damage. Several properties, including wavelength-dependent absorption, pulse repetition rate, and thermal properties of the material, are analyzed when evaluating the ability of these lasers to boost the cleaning efficiency of the deteriorated bone surface. Bone surface morphology and composition were studied and compared before and after laser irradiation, using Optical Microscopy, Scanning Electron Microscopy with Energy Dispersive X-ray Spectrometry (SEM-EDS), Fourier Transform Infrared Spectroscopy (FTIR), and X-ray Photoelectron Spectroscopy (XPS) characterization methods. The results indicate that 238-femtosecond UV laser irradiation with 2.37 TWcm−2 is significantly safer and more efficient toward surface contaminant desorption than sub-nanosecond laser irradiation. The results herein presented suggest that these types of fs lasers may be considered for realistic laser conservation of valuable historic and archaeological museum artifacts.

Ultrashort Laser Texturing of Superelastic NiTi: Effect of Laser Power and Scanning Speed on Surface Morphology, Composition and Wettability

Among metallic biomaterials, near-equiatomic NiTi is one of the most promising intermetallic system applicable for biomedical applications, thanks to its high biocompatibility and unique superelasticity (or pseudoelasticity), offering a complete recoverable strain up to 8%. In the prospective uses for bio-devices, the surface processing of NiTi medical components plays a fundamental role for guaranteeing both a Ti oxide passivizing layer for avoiding Ni ion release into the human body and surface morphology for controlling the cell proliferation. Mechanical polishing, thermal, chemical or electro-chemical treatments are typically performed for surface modifications. Recently, laser texturing has been successfully applied for different materials, included NiTi shape memory alloys, and also for tuning the surface properties, such as wettability. In the present work, ultrashort laser surface modification was carried out, through the use of a femtosecond laser, for the surface texturing of commercial superelastic NiTi plates. The main goal is to investigate the correlation among morphology, chemical composition and wettability with the principal process parameters, such as average power and scanning velocity in high power ultrashort laser texturing. Laser patterned surfaces were characterized by means of scanning electron microscopy, 3D-profilometry, XPS analysis and wetting measurements. After the laser treatments, both surface morphology and Ni/Ti ratio were largely modified from the initial surface, depending on the adopted process parameters. The wettability of the laser textured surfaces can be also varied with respect to the initial surface, due to the roughness values and grooves induced by the laser beam scans. The laser texturing process induced a combination between micro and nano structures, depending on the input energy. In details, the surfaces were tuned to lower roughness values (from 0.4 μm to 0.3 μm) with a laser power of 1 W, while it was increased up to 0.65 μm with a laser power of 13 W. The laser surface modification promoted a change of the contact angle from 70° of the untreated condition up to 135° to the surface laser treated with a power of 13 W.

Intelligent adaptable excitation field for uniform dye photoswitching in SMLM.

In Single Molecule Localization Microscopy (SMLM), the quality of the super-resolved image highly depends on the capacity to generate single molecule imaging conditions. dSTORM in particular strongly relies on the control of the fluorescent dye photophysics. A method, called ASTER (Adaptable Scanning for Tunable Emission Regions) has been recently proposed to generate an effective uniform excitation by introducing a fast scanning (kHz) of the gaussian LASER beam which creates dynamically a top hat excitation. We will see how this technology can be used to understand and quantify the photoswitching rates for several fluorescent dyes.

Rotational dynamics of indirect optical bound particle assembly under a single tightly focused laser.

The optical binding of many particles has the potential to achieve the wide-area formation of a "crystal" of small materials. Unlike conventional optical binding, where the entire assembly of targeted particles is directly irradiated with light, if remote particles can be indirectly manipulated using a single trapped particle through optical binding, the degrees of freedom to create ordered structures can be enhanced. In this study, we theoretically investigate the dynamics of the assembly of gold nanoparticles that are manipulated using a single trapped particle by a focused laser. We demonstrate the rotational motion of particles through an indirect optical force and analyze it in terms of spin-orbit coupling and the angular momentum generation of light. The rotational direction of bound particles can be switched by the numerical aperture. These results pave the way for creating and manipulating ordered structures with a wide area and controlling local properties using scanning laser beams.

Resolution adjustable Lissajous scanning with piezoelectric MEMS mirrors.

We previously designed a dual-axis piezoelectric MEMS mirror with a low crosstalk gimbal structure, which is utilized as the key device for further research for laser beam scanning. This paper mainly focuses on studying the Lissajous scanning resolution of this MEMS mirror with frequency ratio and phase modulation. For accurately evaluating the scanning resolution, the center angular resolution of Lissajous scanning is redefined by theoretical calculation and verified with experimental measurement. Meanwhile, the scanning nonlinearity of MEMS mirror is studied carefully. Finally, the MEMS mirror works at the state of pseudo-resonance, and the center angular resolution better than 0.16° (H) × 0.03° (V) is achieved at a scanning Field of view (FoV) of 35.0° (H) × 16.5° (V). Moreover, a feasible route of resolution adjustable Lissajous scanning is provided by optimization of frequency ratio and phase modulation, which is helpful for high definition and high frame rate (HDHF) laser scanning imaging with the dual-axis mirror.

High-flexibility and high-accuracy phase delay calibration method for MEMS-based fringe projection systems.

Microelectromechanical system (MEMS) mirror based laser beam scanning (LBS) projectors for fringe projection profilometry (FPP) are becoming increasingly popular attributing to their small size and low cost. However, the initial phase of the scanning MEMS mirror employed in an LBS projector may vary over time, resulting in unstable and distorted fringe patterns. The distorted fringe patterns will largely decrease the accuracy of the three-dimensional (3D) topographic reconstruction. In this paper, an efficient phase delay calibration method based on a unique fringe projection sequence and a corresponding image processing algorithm is proposed. The proposed method can compensate the phase uncertainty and variation with no need to add any extra components. One LBS projector has been constructed using a uniaxial electrostatic MEMS mirror that has a mirror size of 2.5 mm × 2.5 mm and a scanning field of view of 60 ∘ at its resonance of 1523 Hz. 3D reconstruction experiments are conducted to study how the 3D reconstruction results are affected by the phase delay. The standard deviation of a sphere reconstruction is improved from 2.05 mm to 0.20 mm after the positive phase delay deviation of 5 μs is compensated using this new calibration method.

Management of Repeated Denture Fracture Through Direct Metal Laser Sintering Metal Reinforced Denture

Direct Metal Laser Sintering (DMLS) is a type of additive metal production that is simpler, more precise, and healthier than traditional manufacturing, while being highly cost-effective. In this process, the metal powder which is free of binder or fluxing agent, is completely melted by the scanning of a high-power laser beam. The resulting part has properties like the original material. In removable prosthodontics, the fracture of heat cure acrylic resin dentures is an unsolved problem. The most common issue is a midline fracture of the maxillary denture. Among midline fractures, 71% were reported in maxillary complete dentures and 29% in mandibular dentures. Patients with heavy masticatory loads and those with parafunctional habits are more likely to experience this. In such circumstances, a metal framework base improves a prosthesis’s fracture resistance, stability, and retention. A 45- year-old female patient reported with a chief complaint of repeated fracture of maxillary complete denture and gave the history of midline fracture of upper denture three times in last 2 years. Intraoral examination revealed v shaped deep hard palate and severely resorbed mandibular ridge. DMLS metal reinforced maxillary denture and metal mesh reinforced mandibular denture was planned to prevent fracture of the denture in future. Metal framework reinforcement of complete denture is a simple, quick, and cost-effective technique to decrease the chances of denture fracture in future, thus beneficial for both patient and the dentist. One year follow-up has been done and patient is comfortable with the denture.

Finite element analysis of deformation behaviour of circular sheet during laser forming process

Laser forming of sheet metal involves inducing a bending process through the application of thermal stress using a defocused laser source. In laser forming the formation of bowl-shaped shapes requires the input of various laser process parameters. In this study, we investigate the thermo-mechanical deformation behaviour of a circular plate subjected to laser irradiation scanning. To analyse this phenomenon, we employ the Finite Element Method (FEM) with the aid of the software ABAQUS 6.10. The moving laser heat source is modelled as Gaussian heat flux, and DFLUX subroutine is written in FORTRAN. The work piece material AH36 is considered. The Finite Element Method (FEM) proposed model is verified against experimental results. Additionally, a parametric study has been conducted to assess the impact of laser process parameters on the temperature distribution of AH 36 material. The temperature distribution is noted to rise with an increase in laser power. However, as the scanning speed and laser beam diameter of the circular laser heat source increase, both temperature and bend angle exhibit a continuous decrease. Further, it is found the plastic strain is measured along the laser irradiation path, therefore the averaged material properties are not altered of the dome shaped obtained by laser forming. Based on the parametric studies, the optimum laser process parameters are also determined to get the dome shaped of the circular sheet.

Improving two-photon excitation microscopy for sharper and faster biological imaging.

Two-photon excitation laser scanning microscopy (TPLSM) has provided many insights into the life sciences, especially for thick biological specimens, because of its superior penetration depth and less invasiveness owing to the near-infrared wavelength of its excitation laser light. This paper introduces our four kinds of studies to improve TPLSM by utilizing several optical technologies as follows: (1) A high numerical aperture objective lens significantly deteriorates the focal spot size in deeper regions of specimens. Thus, approaches to adaptive optics were proposed to compensate for optical aberrations for deeper and sharper intravital brain imaging. (2) TPLSM spatial resolution has been improved by applying super-resolution microscopic techniques. We also developed a compact stimulated emission depletion (STED) TPLSM that utilizes electrically controllable components, transmissive liquid crystal devices, and laser diode-based light sources. The spatial resolution of the developed system was five times higher than conventional TPLSM. (3) Most TPLSM systems adopt moving mirrors for single-point laser beam scanning, resulting in the temporal resolution caused by the limited physical speed of these mirrors. For high-speed TPLSM imaging, a confocal spinning-disk scanner and newly-developed high-peak-power laser light sources enabled approximately 200 foci scanning. (4) Several researchers have proposed various volumetric imaging technologies. However, most technologies require large-scale and complicated optical setups based on deep expertise for microscopic technologies, resulting in a high threshold for biologists. Recently, an easy-to-use light-needle-creating device was proposed for conventional TPLSM systems to achieve one-touch volumetric imaging.

Processes of ablation and structures growth under the action of femtosecond laser pulses on the gallium surface in an ammonia medium

In this paper, we present the results of processing metallic gallium in an ammonia vapor medium at 2 bar pressure by femtosecond laser pulses. The influence of the ammonia concentration and the mode of laser beam scanning on the result of laser action is considered. It has been established that an increase in the concentration of ammonia vapor and a change in the scanning regime lead to a radical change in the laser ablation process. A decrease in the scanning speed leads to the cessation of the ablation process and the development of the nitridation process of the gallium surface, accompanied by the formation of columnar structures up to 12 mm long and about 100 μm in diameter. The synthesized nanoparticles and structures were studied using scanning electron microscopy, Raman spectroscopy, and X-ray analysis. Keywords: laser ablation, gallium nitride, gallium nitride nanoparticles manufacturing, ablative synthesis of gallium nitride nanoparticles.

Процессы абляции и роста структур при воздействии фемтосекундных лазерных импульсов на поверхность галлия в среде аммиака

In this paper, we present the results of processing metallic gallium in an ammonia vapor medium at 2 bar pressure by femtosecond laser pulses. The influence of the ammonia concentration and the mode of laser beam scanning on the result of laser action is considered. It has been established that an increase in the concentration of ammonia vapor and a change in the scanning regime lead to a radical change in the laser ablation process. A decrease in the scanning speed leads to the cessation of the ablation process and the development of the nitridation process of the gallium surface, accompanied by the formation of columnar structures up to 12 mm long and about 100 μm in diameter. The synthesized nanoparticles and structures were studied using scanning electron microscopy, Raman spectroscopy, and X-ray analysis.

Angle amplifier in a 2D beam scanning system based on peristrophic multiplexed volume Bragg gratings

Abstract In this paper, a 2D angle amplifier based on peristrophic multiplexed volume Bragg gratings is designed and prepared, in which a calculation method is firstly proposed to optimize the number of channels to a minimum. The induction of peristrophic multiplexing reduces the performance difference in one bulk of the grating, whereas there is no need to deliberately optimize the fabrication process. It is revealed that a discrete 2D angle deflection range of ±30° is obtained and the relative diffraction efficiency of all the grating channels reaches more than 55% with a root-mean-square deviation of less than 3.4% in the same grating. The deviation of the Bragg incidence and exit angles from the expected values is less than 0.07°. It is believed that the proposed 2D angle amplifier has the potential to realize high-performance and large-angle beam steering in high-power laser beam scanning systems.

대면적 나노구조 패턴 제작을 위한 위상 마스크 기반 스캐닝 근접장 홀로그래픽 리소그래피

Near-filed holographic nanolithography is a method to fabricate periodic nanostructures using a phase mask comprised of nanogratings, where this method results in twice the pattern density of the phase mask. However, the practical use of this method has been limited due to low throughput and scalability. In this study, the drawbacks of the conventional holographic nanolithography were overcome by using a scanning laser beam interference lithography for making a large area phase mask, and by utilizing large area exposure with superimposition of laser beam. To confirm the feasibility of the proposed near-field holographic nanolithography, we fabricated a nanograting having a line width of 88.8nm and a pitch of 170nm onto an entire 4-in wafer from a phase mask having a pitch of 338nm.

Nanoindentation plasticity and loading rate sensitivity of laser additive manufactured functionally graded 316L and 410L stainless steels

The present work is the first to show the effect of processing cooling rate on phase structure and the resulting nano-scale deformation mechanisms in functionally graded stainless steels produced by directed energy deposition. To this end, fabrication by laser additive manufacturing involved implementing a closed-loop feedback control approach to monitor and control the peak temperature of the molten pool and the corresponding cooling rate. Single-track wall structures were produced from 316-L austenitic and 410-L martensitic stainless steels deposited using open-loop conditions with varying processing parameters (laser power and beam scanning speed) to produce graded microstructures. These were then compared to uniformly deposited layers where a constant cooling rate was maintained to generate homogenous microstructures using these two alloys, containing well-aligned columnar austenitic grains and random martensitic packets. Microstructural aspects (grains and crystallographic texture) across different layers (bottom, middle, and top) of the consolidated stainless steel walls (austenitic and martensitic) were compared under various processing conditions (without and with closed-loop or feedback control) and characterized using electron backscattering diffraction analysis. The nano-scale plastic deformation of these austenitic and martensitic stainless steel structures were assessed using nanoindentation testing over a range loading rates from 1 to 50 mN/s. The indentation response using varying strain rates was investigated based on the plastic flow indenting transition behavior at a high loading rate in order to correlate their dependence on microstructural features (predominantly for the martensitic phase). According to the rate sensitivity analyses, the gradient in grain structure (size and morphology) in the case of additively manufactured austenitic microstructure was not sensitive to loading rate, while the martensitic grains had a high local loading rate sensitivity.

Impacts of microsecond control in laser powder bed fusion processing

Lasers used in standard metal Laser Powder Bed Fusion (LPBF) machines are focused at the build plane with typical beam diameters less than 100 µm. This results in millions of individual laser beam scanning vectors when fabricating complex metal parts, and in most build scenarios, the laser turns on and off at the beginning and end of each vector, respectively. The standard configuration for LPBF machines includes a laser beam steering system with two mirrors attached to galvanometers, called the scanner. Most commercial lasers have response times for beam power control below 10 µs yet acceleration/deceleration times of commercial scanners are an order of magnitude slower (due to the dynamics of the mechanical system). A common strategy for allowing the scanner to “catch up” with the commanded location and speed is to employ “skywriting” with additional built-in time delays to allow for the scanner to maintain constant velocity while the laser is on. These delays along with skywriting are intended to increase resolution and reduce defects, although the actions and times are typically not accessible by the user. In fact, the millions of laser on/off events result in additional sources for defect generation to include lack of fusion voids, keyhole voids, surface roughness, and spatter. Ultimately, this results in differences in part performance within builds (build location) and between LPBF platforms. Recent work has identified methods to eliminate some forms of laser on/off defects by controlling the laser power ramp rate. To the authors knowledge, laser power ramping is not currently implemented on any commercial LPBF machine, and the use of laser power ramping has yet to be evaluated on realistic, complex parts. As a result, the current work implemented laser power ramping on a commercial LPBF machine using a complicated geometry with 1,309,338 laser on/off events (vectors) to analyze the holistic impact of fabricating real parts on defects, mechanical properties, and spatter. Ramping the laser power over 300µs eliminated the formation of back spatter, and a demonstration build of a complex qualification test artifact using IN718 reduced total spatter by nearly 5%. However, this laser power ramp change over 300µs (i.e., linearly varying laser power from 0 W to the desired power set point over 300µs) increased lack of fusion defects near part edges that reduced the tensile ductility from 17% to 12% without affecting tensile strength. The research highlighted the financial challenges imposed when scrutinizing microsecond level interactions, and as a result, a low-cost method to identify minimum ramp rates to prevent back spatter using a smart phone and an optical microscope was demonstrated. Microsecond control over the laser must be accompanied by microsecond control of the scanner and the impacts must be quantified holistically over a variety of response variables to broadly apply the developments to industry applications.1