Small Beam Diameter Research Articles

Localized Laser Heat Treatment on AISI 1045 Steel Using a LPBF Laser

ABSTRACTLaser heat treatment (LHT) can provide local microstructure modification for metallic materials. This study investigates the viability of LHT on AISI 1045 steel using a laser designed for laser powder bed processing (LPBF) with a very small beam diameter compared to existing literature. Energy density models from the literature are analyzed and adapted to improve consistency across varying laser properties, mainly focusing on spot size and laser–material interaction time. Fifteen different laser parameter schematics are evaluated, comparing hardness versus depth relations as well as laser‐affected zone (LAZ) profiles. Through adapting the energy density model, new laser parameters are calculated and demonstrated to produce a valid localized heat treatment.

Repairing weld for directly solidified Ni-based superalloy substrates using directed energy deposition of Inconel 625

Although the maintenance, repair, and operation of gas turbine systems are important issues in enhancing the operation of power units, advanced welding repair processes have limitations in depositing directionally solidified Ni-based superalloy components. To overcome this problem, in this study, a directed energy deposition method was applied to repair directionally solidified components using an Inconel 625 alloy. The rapid cooling rate and small laser beam diameter during directed energy deposition minimized the heat-affected zone and duration in the mushy zone, resulting in crack inhibition at the interface. The first laser scan partially melted the substrate and formed a mixing zone with a chemical composition gradient. By combining the chemical compositional gradient and dislocation accumulation due to repetitive solidification, that is, with the melting cycles of directed energy deposition, the thermal stability decreased, resulting in recrystallization and grain growth in the mixing zone. Consequently, the crack-free continuous interface of the partially repaired components did not induce tensile fracture near the interface. Although the mechanical properties of the IN625 deposit layer were degraded by severe heat treatment of the substrate, the results demonstrated that energy deposition with post-heat treatment is an effective approach in repairing Ni-based superalloy components without cracks and inclusion initiation.

Shock Wave Detection for In-Process Depth Measurement in Laser Ablation Using a Photonic Nanojet

Three-dimensional micro- and submicrometer-scale structures exhibit unique functions that cannot be obtained with bulk materials. To create such three-dimensional microstructures with high precision and efficiency, we proposed laser ablation using a photonic nanojet. A photonic nanojet is an optical beam with both a small beam diameter and a large depth of focus, which is obtained by irradiating a dielectric microsphere using a laser beam. In this study, we proposed an in-process depth measurement method to improve the machining accuracy of laser ablation using a photonic nanojet. We focused on the propagation characteristics of the shock waves generated during laser ablation. Shock waves were generated at the deepest point of the machining area and reached the microspheres as the pressure decayed, showing that different machining depths exerted different pressures on the microspheres. The microspheres were displaced by the pressure of the shock wave, and the amount of displacement depended on the pressure. Therefore, microspheres can be used as probes for shock wave detection, and the machining depth can be determined by measuring the displacement of microspheres during photonic nanojet machining. In this study, the displacement of a microsphere was measured simultaneously during photonic nanojet machining using a confocal optical system. From the obtained microsphere vibration data, the effect of the shock wave pressure was extracted, and the displacement of the microsphere due to the shock wave was obtained. When the hole depth varied from 155 to 1121 nm, the displacement of the microspheres varied from 0.58 to 0.03 µm. The experimental results show that the displacement of the microspheres vibrated by the shock wave decreased as the machining depth increased. This was due to an increase in the shock wave propagation distance and a decrease in the pressure of the shock wave as the machining depth increased. In conclusion, in-process depth measurements are possible in laser ablation using a photonic nanojet with a microsphere as a probe to detect shock waves.

Laser Ablation for Nondestructive Sampling of Microplastics in Single-Particle ICP-Mass Spectrometry.

In this work, laser ablation (LA) was characterized as a method for sampling and introducing microplastic particles (MPs) into an inductively coupled plasma (ICP) for subsequent 13C+ monitoring using an ICP-mass spectrometer operated in single-event mode. MPs of different types (PS, PMMA, and PVC) and sizes (2-20 μm) were introduced intactly. The laser energy density did not affect the particle sampling across a wide range (0.25-6.00 J cm-2). Single-shot analysis separated clustered MPs (2-7 MPs per cluster) during the LA and particle transport processes, allowing the temporally resolved analysis of the individual constituting MPs. Line scanning showed superior performance when using a small laser beam diameter combined with a high repetition rate. The 13C+ signal intensity correlated linearly (R2 >0.9945) with the absolute C mass in a 2-10 μm size range, while the use of He in the collision-reaction cell (CRC) allowed extension of the linear range to 20 μm. The LA approach generated narrower 13C+ signal distributions than the traditional solution-based approach (dry versus wet plasma conditions) and proved successful for the analysis of a mixed suspension (containing four sizes of PS MPs in a 2-5 μm size range) and for sampling MPs from PVDF and glass microfiber filters, with the latter offering a lower background.

Design Rules for Resistors, Capacitors and Inductors Fabricated From Single Layer Y-Ba-Cu-O Thin Films With Focused Helium Ion Beam Irradiation

Yttrium barium copper oxide (YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-<inline-formula><tex-math notation="LaTeX">$\delta$</tex-math></inline-formula></sub> or YBCO) Josephson junctions fabricated using focused helium gas field ion sources show great promise for high transition temperature superconducting electronics. A unique feature of this technology is that it can also be utilized for the fabrication of resistors, capacitors and inductors in the same material and process step. The key to this method is that the electrical resistivity of high critical temperature superconductors is very sensitive to ion irradiation. The ions introduce disorder into the material which causes it to undergo a metal-to-insulator transition. By carefully controlling the position of the beam and ion dose delivered to the material, insulating pathways and reduced critical temperature metallic regions can be fabricated to construct the aforementioned passive components. We have prepared this manuscript to serve as a guide for circuit designers utilizing this process. We discuss the capabilities and limitations of helium ion beam lithographic patterning of YBCO and estimate the practical ranges of the values that can be obtained for resistors, capacitors and inductors in single layer thin films. Estimates are provided for two processes: one with the highest resolution with the smallest beam diameter (0.5 nm) and another based on the highest source current (100 pA) for fastest write time.

Review of quality issues and mitigation strategies for metal powder bed fusion

PurposeSelective laser melting and electron beam melting processes are well-known for the additive manufacturing of metal parts. Metal powder bed fusion (MPBF) is a common term for them. The MPBF process can empower the manufacturing of intricate shapes by reducing the use of special tools, shortening the supply chain and allowing small batches. However, the MPBF process suffers from many quality issues. In literature, several works are recorded for qualification of the MPBF part. The purpose of this study is to recollect those works done for quality control and report their helpful findings for further research and development.Design/methodology/approachA systematic literature review was conducted to highlight the major quality issues in the MPBF process and its root causes. Further, the works reported in the literature for mitigation of these issues are classified and discussed in five categories: experimental investigation, finite element method-based numerical models, physics-based analytical models, in-situ control using artificial intelligence (AI) and machine learning (ML) methods and statistical approaches. A comparison is also prepared among these strategies based on their suitability and limitations. Additionally, improvements in MPBF printers are pointed out to enhance the part quality.FindingsAnalytical models require less computational time to simulate the MPBF process and need a smaller number of experiments to confirm the results. They can be used as an efficient process parameter planning tool to print metal parts for noncritical applications. The AI-ML based quality control is also suitable for MPBF processes as it can control many processing parameters that may affect the quality of the MPBF part. Moreover, capabilities of MPBF printers like thinner layer thickness, smaller beam diameter, multiple lasers and high build temperature range can help in quality control.Research limitations/implicationsThis study converts the piecemeal data on MPBF part qualification methods into interesting information and presents it in tabular form under each strategy. This tabular information provides the basis for further quality improvement efforts in the MPBF process.Originality/valueThis study references researchers and practitioners on recent quality control efforts and their significant findings for a better quality of MPBF part.

Development of a Clinical Neutron Source for Boron Neutron Capture Therapy

<h3>Purpose/Objective(s)</h3> In boron neutron capture therapy (BNCT), a source of epithermal (1 eV to 30 keV) neutrons activates high concentrations of ¹⁰B, selectively accumulated in tumor cells from infusion of a non-radioactive pharmaceutical, whose high LET reaction products fully deposit their energy within 10 microns of the reaction site. Historically, only nuclear reactors provided sufficient neutron intensity and beam quality for clinically acceptable BNCT treatments, but hospital-based BNCT is now feasible using compact accelerator sources. The present work has designed a system to produce an epithermal neutron beam using a tandem electrostatic accelerator, proton beam line, neutron-producing targets, neutron beam shaping assembly (BSA) and collimators. Testing was required to verify that this design can produce a clinically viable neutron source for BNCT in terms of total neutron yield, output reproducibility and operational stability. <h3>Materials/Methods</h3> The present neutron source uses neutrons from the ⁷Li(p,n)⁷Be reaction at nominal proton energy and current of 2.5 MeV and 10 mA. A negative hydrogen ion source is pre-accelerated to 200 keV and injected into the tandem accelerator, which increases the negative ion kinetic energy to half the goal proton energy at its center. A charge exchange target strips the electrons to create positively-charged protons that are further accelerated to the full operational energy at the tandem exit. The proton beam is focused and may be directed to up to three treatment rooms, each with a copper-backed target of natural lithium metal under vacuum. The maximum neutron energy from 2.5 MeV protons slowing down in lithium is 800 keV and requires moderation to the desired epithermal range. A static beam BSA surrounds the target and provides efficient neutron moderation and reflection using carefully chosen materials. The BSA's 25 cm circular exit port may be collimated to small circular epithermal beam diameters as small as 5-8 cm. <h3>Results</h3> A tandem accelerator, proton beam line and lithium target have been installed and began initial testing. Proton energy and current up to 2.3 MeV and 7 mA have been achieved for continuous operating periods exceeding 30 minutes with no energy or current degradation. Raster patterns have been designed to scan the 1-cm diameter proton beam over a 10-cm circular area on the lithium target, and water cooling of the copper backing has maintained maximum target temperatures below 150°C. The measured lithium deposition uniformity is expected to translate to a neutron yield uniformity better than 5%. <h3>Conclusion</h3> This BNCT neutron source has demonstrated operational stability and reproducibility of proton energy, proton current and neutron production. These measurements support an expectation that the equipment currently undergoing evaluation, in combination with the planned BSA, will meet the necessary requirements for a hospital-based, clinical BNCT system.

TV and video game streaming with a quantum receiver: A study on a Rydberg atom-based receiver's bandwidth and reception clarity

We demonstrate the ability to receive live color analog television and video game signals with the use of the Rydberg atom receiver. The typical signal expected for traditional 480i National Television Standards Committee format video signals requires a bandwidth of over 3 MHz. We determine the beam sizes, powers, and detection method required for the Rydberg atoms to receive this type of signal. The beam size affects the average time the atoms remain in the interaction volume, which is inversely proportional to the bandwidth of the receiver. We find that small beam diameters (less than 100 μm) lead to much faster responses and allow for color reception. We demonstrate the effect of the beam size on bandwidth by receiving a live 480i video stream with the Rydberg atom receiver. The best video reception was achieved with a beam width of 85 μm full-width at half-max.

Non-Invasive Laser-Cross Calibration For Volumetric Flow Measurements In Closed Channels

The calibration of a multi-camera system is a crucial step of a 3D particle tracking velocimetry (3D-PTV) measurement and other optical velocimetry techniques, which are based on multi-camera setups. Conventional calibration methods for multi-camera systems are based on recording hardware targets in the cameras’ fields of view. In measurement volumes that are difficult to access, the insertion of such hardware targets is associated with an increased technical or mechanical effort or is eventually not possible. This work presents a calibration method without the use of a hardware target. Instead, crossing laser beams are introduced into the volume, which create unique calibration points due to their beam paths. The underlying algorithm for detecting the laser beams and the optimization of the calibration is explained. The influence of the beam diameter and positioning errors on the calibration quality are examined by using synthetic data, which is created in Blender (Free and Open 3D Creation Software under GNU GPL). The new laser cross calibration is compared to the calibration with a two-plane target by synthetic data and experimental data from a 3D-PTV measurement in a difficult-to-access volume in a transparent manifold. For the comparison, the synthetic data and the therefore known absolute positions of the lasercross allow to calculate the reconstruction error in addition to the widely used reprojection error. The ransac algorithm is used for the detection of the beam paths from the raw images. The ransac algorithm is well suited for this application, as it is stable against parasitic reflections while its subpixel accuracy is sufficient for this application with small beam diameters. The non-invasive calibration techniques have different advantages over the usage of a hardware target. Due to the in-situ calibration it is not necessary to move the camera setup, which can otherwise lead to errors in the alignment of the cameras to each other. The installing and deinstalling process of the calibration plate is not needed, which reduces the effort for the construction and further reduces errors. Hardware-targets need to be homogenously illuminated, which is not necessary with "self-illuminating" calibration targets like the laser cross. When having opposing cameras, hardware-targets need to be two-faced with front- and back-side being perfectly parallel and the calibration points need to be in defined geometric relation. Deviations can introduce systematic errors. With a non-invasive method like the laser cross there is no obstruction by the calibration plate itself.

High efficiency focusing double-etched SiN grating coupler for trapped ion qubit manipulation

A one-dimensional focusing grating coupler array based in silicon nitride (SiN) was proposed for trapped ion qubit manipulation. By applying inverse design optimization with a double-etched grating structure, a directionality of 98% was achieved. A small beam diameter of 2.5 μm on the target ion with a low crosstalk of −36 dB was attained. Additionally, the impact of fabrication errors was investigated through a Monte Carlo simulation; within the accuracy of an electron beam lithography-based process, the output efficiency was maintained at 93%.

Analysis of ICNIRP 2020 Basic Restrictions for Localized Radiofrequency Exposure in the Frequency Range above 6 GHz.

ICNIRP 2020 guidelines have defined a practical temperature elevation threshold for human health effects, namely the operational adverse health effect threshold that forms the basis of the absorbed power and energy density basic restrictions. These basic restrictions for localized exposures at frequencies above 6 GHz were evaluated by comparing numerically computed temperature rise against the target temperature rise of 2.5 o C, which is the operational adverse health effect threshold divided by the occupational safety factor of 2. The numerical model employs the maximum absorbed power and energy density levels allowed by the occupational basic restriction for both pulsed and continuous wave exposures. These analyses were performed considering 3- and 4-tissue layer models and a variety of beam diameters, frequencies, and exposure durations. The smallest beam diameters were based on a study of theoretically achievable beam widths from half-wave resonant dipoles and show the impact of the averaging area on the computed temperature elevation. The results demonstrated that ICNIRP's assumed occupational safety factors in the frequency range above 6 GHz were not sufficiently maintained for all exposure scenarios and particularly for short pulse exposures at frequencies of 30 GHz or higher with small beam diameters. Worst-case tissue temperature elevations were estimated to be as much as 3.6 times higher than ICNIRP's target temperature increases. Consequently, the authors suggest a small modification in the application of the ICNIRP 2020 localized basic restrictions, thereby limiting the worst-case tissue temperature increases to 1.4 times the target value.

Intracavity spherical aberration for selective generation of single-transverse-mode Laguerre-Gaussian output with order up to 95

We investigate the generation of single-transverse-mode Laguerre-Gaussian (LG) emission from a diode-end-pumped Nd:YVO4, 1064 nm laser using mode selection via intracavity spherical aberration (SA). We present both theoretical and experimental investigations, examining the limits of the order (both radial and angular indices) of the LG modes which can be produced, along with the resultant output power. We found that in order to generate single-mode emission of low-order LG modes which have relatively small beam diameters, lenses with shorter focal-length were required (to better differentiate neighboring LG modes via SA). The converse was true of LG modes with high-order. Through appropriate choice of the focal length of the intracavity lens, we were able to generate single-mode, LG0,±m laser output with angular indices m selectable from 1 to 95, as well as those with non-zero radial indices p of up to 4.

The selection of laser beam diameter in directed energy deposition of austenitic stainless steel: A comprehensive assessment

One focus of laser directed energy deposition (L-DED) is to increase productivity, and this can be achieved by using a larger beam diameter and higher laser power. However, it is difficult to build complex metallic components with fine structures using a laser beam of large diameter. Under the trade-off between formability and productivity, the choice of beam diameter needs to be thoroughly evaluated. Herein, three beam diameters (0.75 mm, 1.5 mm, and 3 mm) in building austenitic stainless steel parts have been investigated and compared in four aspects: microstructure, mechanical properties, powder utilization, and as-built part surface quality. Specifically, cooling rate and powder splashing in the building process, and phase composition, grain size and dislocation density of as-built parts are revealed by means of numerical simulation, high-speed camera, electron backscattered diffraction, and X-ray diffractometry. Results show that for a small beam diameter, the microstructure formed has finer grains and less amount of ferrite due to higher cooling rate of the molten pool, and grain refinement and high density of dislocations lead to superior mechanical properties of the as-built part; conversely, for a larger beam diameter, more stable molten pool and less amount of powder splashing brings higher material utilization rate and better surface quality. A brief evaluation of beam diameter selection is accordingly proposed, and a novel hybrid beam diameters building strategy is presented for building complex metallic components with high quality and efficiency.

Total-reflection high-energy positron diffractometer at NEPOMUC - Instrumentation, simulation, and first measurements.

We report the instrumentation of a new positron diffractometer that is connected to the high-intensity positron beam at the neutron induced positron source Munich. Crucial elements for the adaption of the positron beam are presented, which include the magnetic field termination, the optional transmission-type remoderator for brightness enhancement, and the electrostatic system for acceleration and beam optics. The positron trajectories of the remoderated and the twofold remoderated beam have been simulated to optimize the system, i.e., to obtain a coherent beam of small diameter. Within a first beamtime, we tuned the system and characterized the direct beam. For the twofold remoderated beam of 10keV energy, we experimentally observe a beam diameter of d < 1.3mm, which agrees well with the simulation.

Photothermocapillary Method for the Nondestructive Testing of Solid Materials and Thin Coatings.

The photothermocapillary (PTC) effect is a deformation of the free surface of a thin liquid layer on a solid material that is caused by the dependence of the coefficient of surface tension on temperature. The PTC effect is highly sensitive to variations in the thermal conductivity of solids, and this is the basis for PTC techniques in the non-destructive testing of solid non-porous materials. These techniques analyze thermal conductivity and detect subsurface defects, evaluate the thickness of thin varnish-and-paint coatings (VPC), and detect air-filled voids between coatings and metal substrates. In this study, the PTC effect was excited by a “pumped” Helium-Neon laser, which provided the monochromatic light source that is required to produce optical interference patterns. The light of a small-diameter laser beam was reflected from a liquid surface, which was contoured by liquid capillary action and variations in the surface tension. A typical contour produces an interference pattern of concentric rings with a bright and wide outer ring. The minimal or maximal diameter of this pattern was designated as the PTC response. The PTC technique was evaluated to monitor the thickness of VPCs on thermally conductive solid materials. The same PTC technique has been used to measure the thickness of air-filled delaminations between a metal substrate and a coating.

Effect of higher layer thickness on laser powder bed fusion built single tracks of Ni-Cr-Fe-Nb-Mo alloy

Laser Powder Bed Fusion (LPBF) is one of the revolutionary technologies that can fabricate complex-shaped components by selective melting of the pre-placed powder layer, using high-power laser as directed by the input digital files. Generally, research on the LPBF process is called out for layer thickness (LT) up to 50 µm and smaller beam diameter (≤100 µm), but it has lower productivity. In LPBF, higher productivity can be achieved with higher LT (&gt;50 µm), but it consists of various process instabilities. In the present work, parametric studies are performed by laying Ni-Cr-Fe-Nb-Mo single tracks, using LPBF at higher LT. The process parameters such as laser power ( P), scan speed ( v), and LT are varied among 150–450 W, 0.04–0.1 m s−1, and 80–160 µm, respectively, at three levels each. For the range of parameters under investigation, the maximum track width of 610 µm and aspect ratio of 7.63 are achieved at a P of 450 W and v of 0.04 m s−1 at 80 µm LT. It is observed that an increase in the energy density and layer thickness resulted in the reduction of track width and aspect ratio due to material vaporization occurring from poor heat conductivity due to unconventionally high powder layer thickness. It is also observed that the build rate increases with an increase in P, v, and LT. As single tracks are basic building blocks, the obtained results can provide an insight into the effect of process parameters on LPBF-built single tracks at higher LT for building engineering components of required width with higher build rate. Furthermore, the track dilution is also found to increase with the increase in P and decrease in v.

Surface Structuring by Laser Remelting (WaveShape): Microstructuring of Ti6Al4V for a Small Laser Beam Diameter and High Scan Speeds.

The appearance of a surface is a crucial characteristic of a part or component. Laser-based micromachining gets increasingly important in generating tailored surface topographies. A novel structuring technique for surface engineering is surface structuring by laser remelting (WaveShape), in which surface features are created without material loss. In this study, we investigated the evolution of surface topographies on Ti6Al4V for a laser beam diameter of 50 µm and scan speeds larger than 100 mm/s. Surface features with aspect ratios (ratio of height to width) of almost 1:1 were achieved using the WaveShape process. Furthermore, wavelengths smaller than 500 µm could be effectively structured using scan speeds of up to 500 mm/s. The experimental results showed further that the efficiency of the WaveShape process in terms of achieved structure height per unit time significantly increases for high scan speeds.

Simulations of the Self-Focused Pseudospark-Sourced Electron Beam in a Background Ion Channel

Using pseudospark discharge sourced electron beams for the generation of high-peak-power millimeter and terahertz radiation has attracted increasing research interest in recent years. However, one of the crucially important and hitherto unanswered questions is what is the upper-frequency limit at which millimeter-wave devices can be driven by pseudospark discharge sourced electron beams?. In this paper, we studied this question from the perspective of beam transportation in a plasma background, more specifically an ion channel using particle-in-cell simulations to find the limitations. The parameter ranges of the beam transportation with small oscillations in the beam diameter were investigated and summarized, through simulations of beam propagation in a large diameter drift tube with different ion densities, plasma electron densities, beam density distributions, and beam energies. The beam transportation in a small diameter beam tunnel was also simulated. It showed the maximum beam current with a small velocity spread that can be transported in the beam tunnel was determined by the diameter of the beam tunnel and the ion density. High injected current will cause significant beam loss and reduce the overall efficiency. The simulation results indicate a minimum diameter of the beam tunnel in a millimeter-wave circuit that can be effectively driven by a pseudospark-sourced electron beam. The equivalent upper limit in the operating frequency is about 400 GHz.

Modification of exposure conditions by the magnetic field configuration in helicon antenna-excited helium plasma

Modification of exposure conditions downstream in the diffusion chamber has been performed in helicon antenna-excited helium plasma by adjusting the magnetic field (intensity and geometry). In the inductively coupled mode (H mode), a reduction in ion and heat fluxes is found with increasing magnetic field intensity, which is further explained by the more highly magnetized ions off-axis around the last magnetic field lines (LMFL). However, in helicon wave mode (W mode), the increase in magnetic field intensity can dramatically increase the ion and heat fluxes. Moreover, the effect of LMFL geometry on exposure conditions is investigated. In H mode with contracting LMFL, off-axis peaks of both plasma density and electron temperature profiles shift radially inwards, bringing about a beam with better radial uniformity and higher ion and heat fluxes. In W mode, although higher ion and heat fluxes can be achieved with suppressed plasma cross-field diffusion under converging LMFL, the poor radial uniformity and a small beam diameter will limit the size of samples suitable for plasma irradiation experiments.

Filament-wound composite pressure vessel inspection based on rotational through-transmission laser ultrasonic propagation imaging

In this paper, we introduce a rotational through-transmission ultrasonic propagation imaging system to inspect the full thickness of the entire cylindrical section of filament-wound composite overwrapped pressure vessels. This system can achieve high signal-to-noise ratio throughout the cylindrical section of pressure vessels by using a rotational scanning method. Based on the advantages of the laser ultrasonic system that can generate and detect ultrasound waves by using a small-diameter laser beam and making an inspection path in a narrow environment, this system directs the laser beam inside the pressure vessel via a mirror to enable the through-transmission mode for vessel inspection. A type-3 pressure vessel was inspected using the system developed, which successfully detected two impact damage spots and showed the filament winding direction clearly. The test results show that the system developed is highly reliable and can replace the current hydrostatic sample test for pressure vessel safety certification.