Laser Paths Research Articles

Optimization of Laser Sintering for Demineralized Bone/Polycaprolactone Composite Powder for Bone Tissue Scaffold

Demineralized bone matrix (DBM) is an excellent bone scaffold material, but is available in only limited sizes. An additive manufacturing (AM) method that retains these properties while enabling customized geometry fabrication would provide bone scaffolds for a larger range of geometries while maintaining the benefits of DBM. This work examines laser sintering (LS) of a blend of demineralized bone matrix (DBM) and polycaprolactone (PCL) using a CO2 laser beam. A comprehensive experimental study was carried out to find the conditions that form defect-free layers while still retaining the favorable biological features of DBM. The results identify a process setting window over which LS can be utilized to constructing complex patient-specific scaffolds. With the identified setting, first, the DBM/PCL blend was fused in the LS machine. Parts were then were further strengthened through a post-processing heat treatment. The shrinkage level, skeletal density, mechanical testing, and porosimetry of the resultant samples were compared to traditional machined DBM blocks. The maximum tensile strength of the samples and post-processing shrinkage depends on heat treatment duration. The tensile strength measurements demonstrate that the post-processing conditions can be tuned to achieve the tensile strength of the demineralized bone strips. Evaluation of the dimensional change suggests that the shrinkage along the laser paths is ~0.3% while thickness shrinks the most (up to ~20%). The porosimetry and density studies showed that the final part achieved over 40% porosity with a density comparable to blocks of DBM.

Real-time simulation for long paths in laser-based additive manufacturing: a machine learning approach

This study predicts in real time the evolution of temperature and density for arbitrary long tracks in laser-based additive manufacturing using artificial neural networks (ANNs). First, a random laser path is transformed appropriately to serve as an ANN input via a custom trajectory decomposition method providing a local description of each trajectory point relative to its surroundings, which is calculated and load-optimized using K-d trees. For each trajectory point, a “Master” ANN calculates the present temperature, using the trajectory descriptors and previous step temperature as inputs. This is made into a parallel procedure by exploiting a “Pilot” ANN without temperature feedback, making a first pass over the whole trajectory to estimate temperature responses. The Master ANN uses these results as a feedback that is iteratively refined. A “Rider” ANN uses the final temperatures as input and calculates local density evolution. Four hundred fifty finite element model experiments of short random walk trajectories were used for ANN training. Results are presented for various types of laser paths, including random, hatch, spiral, fractal, and spline. ANN simulation execution time consistently taking under 50% of the real process time in all validated cases for track length in excess of 100 m proved the ability of the platform to operate at the full layer-scale of LBAM.

Density characterization of discharged gas-filled capillaries through common-path two-color spectral-domain interferometry.

Electrically discharged plasma structures, typically several centimeters in length and sub-millimeter in diameter, have been applied to guide laser pulses in laser plasma accelerators and to focus ion and relativistic electron beams in compact, radially symmetric transport configurations. Knowledge of the on-axis plasma density is critical. Traditional density interferometry has been ineffective for these laser-machined structures, while group velocity delay (GVD) techniques involve combining two laser paths with corresponding alignment complexities and stability sensitivities. Here the GVD technique is advanced to a common-path two-color interferometer configuration performed in the spectral domain of a broad-bandwidth femtosecond laser. Multi-shot tracking of the phase is not required, and the common path assures improved stability. This in situ technique was validated on 15mm long plasma structures, measuring electron densities of 1017-1018 cm-3 for various fill pressures.

Additive Manufacturing of Ti-6Al-4V with Added Boron: Microstructure and Hardness Modification

Titanium alloy composites with titanium boride (TiB) discontinuous reinforcement have shown improved performance in terms of strength, stiffness, and hardness. Producing this composite through selective laser melting (SLM) can combine the advantages of freeform design with the ability to produce TiB reinforcement in-situ. In this study, SLM was used to consolidate a pre-alloyed Ti-6Al-4V (Ti64) and amorphous boron (B) powder mixture with the intent of producing 1.5wt% TiB reinforcement in a Ti64 matrix. The processing parameters of laser power and scanning speed were investigated for their effect on the density, microstructures, and hardness of the composite material. The results showed that the boron and Ti64 composite could achieve a density greater than 99.4%. Furthermore, it was found that processing parameters changed the microstructural features of the material. The higher the energy density employed the more homogenous the distribution of boron modified material. Macro features were also observed with laser paths being clearly evident in the subsurface microstructure. Micro-hardness testing and density measurement also showed a corresponding increase with increasing energy density. Maximum hardness of 392.4HV was achieved in the composite compared to 354.2HV in SLM fabricated Ti64.

Ratiometric, single-dye, pH-sensitive inhibited laser-induced fluorescence for the characterization of mixing and mass transfer

Inhibited planar laser-induced fluorescence (I-PLIF) techniques are widely used for heat and mass transfer studies in fluid mechanics. They allow the visualization of instantaneous two-dimensional field of a passive or reactive scalar, providing that this scalar acts as an inhibitor to the fluorescence of a specific molecule, and that this molecule is homogeneously mixed in the fluid at a known concentration. Local scalar values are deduced from fluorescence recordings thanks to preliminary calibration procedure. When confronted with non-optically thin systems, however, the knowledge of the excitation intensity distribution in the region of interest is also required, and this information is most of the time hard to obtain. To overcome that problem, two-color ratiometric PLIF techniques ( $$\text {I}^\text {r}$$ -PLIF) have been developed. In these methods, the ratio of two different fluorescence wavelengths triggered by the same excitation is used as an indicator of the scalar value. Such techniques have been used for temperature measurements in several studies but never, to the author’s knowledge, for pH tracking and acid–base mixing, despite the frequent use of the one-color version in mass transfer studies. In the present work, a ratiometric pH-sensitive-inhibited PLIF technique ( $$\text {I}_\text {pH}^\text {r}$$ -PLIF) using fluorescein sodium as a single dye and applicable to complex geometries and flows is developed. Theoretical considerations show that the ratio of the two-color fluorescence intensities should only depend on the dye’s spectral quantum yield, itself pH-dependent. A detailed spectrofluorimetric study of fluorescein reveals that this ratio strictly increases with the pH for two well-chosen spectral bands (fluorescence colors). A similar trend is found when using sCmos cameras equipped with optical filters to record fluorescence signals. The method is then experimented on a test flow, a turbulent acidic jet injected in an initially pH-neutral volume of fluid. The results obtained using the ratiometric version are consistent with single-color technique measurements, but excitation intensity heterogeneity is more efficiently accounted for, with a much smaller time needed for data treatment and without requiring the knowledge of laser paths across the fluid. This new technique is also able to reduce the impact of some unwanted experimental features such as time-varying excitation intensity or reflections at interfaces. It can be of great interest for further applications to multiphase mass transfer studies.

Image-Based Slicing and Tool Path Planning for Hybrid Stereolithography Additive Manufacturing

The hybrid stereolithography (SLA) process integrates a laser scanning-based system and a mask projection-based system. Multiple laser paths are used to scan the border of a 2D pattern, whereas a mask image is adopted to solidify the interior area. By integrating merits of two subsystems, the hybrid SLA process can achieve high surface quality without sacrificing productivity. For the hybrid system, closed polygonal contours are required to direct the laser scanning, and a binary image is also needed for the mask projection. We proposed a novel image-based slicing method. This approach can convert a 3D model into a series of binary images directly, and each image is corresponding to the cross section of the model at a specific height. Based on the resultant binary image, we use an image processing method to gradually shrink the pattern in the image. Boundaries of the shrunk image are traced and then restored as polygons to direct the laser spot movement. The final shrunk image serves as the input for the mask projection. Experimental results of test cases demonstrate that the proposed method is substantially more efficient than the traditional approaches. Its accuracy is also studied and discussed.

Laser path calculation method on triangulated mesh for repair process on turbine parts

Laser metal deposition is a very effective and precise technology applied by the manufacturers of turbomachinery parts to repair damages on parts in service or reclaim quality during manufacturing of new parts. The process chains to repair or refurbish turbomachinery parts are based upon a 3D laser scanning process which acquires digitalized models in the format of triangulated-meshes to represent the geometry of parts. This paper firstly studies a discrete geodesic method proposed by Mitchell, Mount and Papadimitriou (MMP) and based on this method proposes an optimized exact algorithm to generate precise laser paths on triangulated-mesh data of part geometry. Furthermore this paper presents an approximate method based on Sethian’s Fast Marching Method (FMM) and constructs vertex-based geodesic distance field on triangulated-meshes to divide the part surface in equal-distant stripes morphed from boundary curves. These two methods are compared and error condition is evaluated. During the research the methods are implemented into an automated LMD process planning software, which is utilized into industrial applications and obtained satisfactory results. The results from application of LMD process planning software are also demonstrated in this paper.

Remote access technology offers tremendous possibilities to the cytometry community

THE opportunity for low cost, high quality training and supervision is an attractive one to most scientists in the cytometry field. Many have had little if any training, no expertise based locally, or may not have the funds to attend regional or international training courses. In addition, rather than general cytometry they may require assistance with a particular application, which could be accomplished in a number of hours through support from an expert. Separately, shared resource scientists, also known as core facility staff, are generally responsible for the performance and upkeep of multiple complex instruments, as well as monitoring, training etc. Some acquisitions or cell sorts may take hours for a single run, and with limited staff the scientist is usually multitasking on an hourly basis. Also, if users of the core facility are having problems and staff is unavailable, valuable samples or time may be lost. There is potential for error at many steps of flow cytometry, including sample preparation, experimental design and fluorochrome selection, instrument set up, compensation, gating, analysis, and interpretation. External/offsite tracking may improve monitoring, and perhaps limit such errors. Also interand intra-instrumentation variations can lead to different results for the same sample. A means of unifying performance, setup, and interpretation is vital, in particular with multicenter collaborations. Peer-review of results and analysis, as well as hierarchical approval systems, provides some solutions, but further stream-lining is clearly beneficial. Remote access technology for cytometry (or remote cytometry) may offer solutions to many such issues and can be performed at very low costs to the institution. There is potential for such systems to facilitate instrumentation monitoring, control equipment in containment areas, speed up vendor service, improve training courses, increase collaborations, and further promote cytometry globally.

An Innovative Computerized Control System in Transoral Laser Surgery of the Larynx

Objectives: 1) Describe a new computerized system in transoral laser surgery of the larynx based on a tablet stylus interface. The mechanical joystick interface currently used is the major limiting factor for surgical precision, quality, and safety, requiring extensive surgeon practice. 2) Analyze the statistical reduction of absolute and root-mean-square error (RMS error) with the stylus interface in path-follow trials. Methods: A new robotic system for transoral laser surgery has been created to improve the surgeon’s control. It offers a more ergonomic, intuitive, and programmable laser control device: a touch interface with dynamic intraoperative planning capabilities. The system is based on a large-range laser scanner fully controlled by a stylus on a tactile sensor. It aims the laser in real-time over the whole surgical field and allows path memorization, providing accurate repetitions of surgeon-defined laser paths. This enables pre-execution surgical action tracking and unmatched control for long cuts. Paths of any shape and length are dynamically planned too with the plain stylus device. Results: Trials on ex-vivo pig larynges and artificial targets performed by experienced and non-experienced surgeons demonstrate a drop of 80% in max absolute error and about 50% in RMS error with the stylus interface in path-follow trials. Dynamic planning has also demonstrated its benefits. No error was observed in trials, getting only .03 mm mean RMS error under a 40× microscope magnification. Conclusions: Trials so far have assessed the new system as highly intuitive and effective. It improves surgical precision, quality, and safety.

Concentration and Temperature Tomography at Elevated Pressures

Computer simulations are used to introduce a new approach to measure temperature fields of a gas using laser attenuation measurements, which exhibits good theoretical accuracy despite very high static pressures. Near-infrared laser light is used to target specific molecular absorption lines of water vapor, whose strength depends (nonlinearly) on the gas temperature. This temperature can be inferred when multiple laser paths coincide nearby by tomographic reconstruction of the attenuation coefficient and then spectral fitting to local temperature, species concentration, and gas pressure. Temperature phantoms (invented distributions for the purpose of numerical testing) are used to simulate experimental results, which are then contaminated with Gaussian noise and used to reconstruct the temperature field. The root-mean-square reconstruction temperature error varied from ~0.4%, in no Gaussian noise and at 1 bar, to 2.2%, in 5% noise and at 50 bar.

Accuracy of micrometeorological techniques for detecting a change in methane emissions from a herd of cattle

Micrometeorological techniques are effective in measuring methane (CH4) emission rates at the herd scale, but their suitability as verification tools for emissions mitigation depends on the uncertainty with which they can detect a treatment difference. An experiment was designed to test for a range of techniques whether they could detect a change in weekly mean emission rate from a herd of cattle, in response to a controlled change in feed supply. The cattle were kept in an enclosure and fed pasture baleage, of amounts increasing from one week to the next. Methane emission rates were measured at the herd scale by the following techniques: (1) an external tracer-ratio technique, releasing nitrous oxide (N2O) from canisters on the animals’ necks and measuring line-averaged CH4 and N2O mole fractions with Fourier-transform infra-red (FTIR) spectrometers deployed upwind and downwind of the cattle, (2) a mass-budget technique using vertical profiles of wind speed and CH4 mole fraction, (3) a dispersion model, applied separately to CH4 mole fraction data from the FTIR spectrometers, the vertical profile, and a laser system measuring along four paths surrounding the enclosure. For reference, enteric CH4 emissions were also measured at the animal scale on a daily basis, using an enteric tracer-ratio technique (with SF6 as the tracer). The animal-scale technique showed that mean CH4 emissions increased less than linearly with increasing feed intake. The herd-scale techniques showed that the emission rates followed a diurnal pattern, with the maximum about 2h after the feed was offered. The herd-scale techniques could detect the weekly changes in emission levels, except that the two vertical-profile techniques (mass-budget technique and dispersion model applied to profile) failed to resolve the first step change. The weekly emission rates from the external tracer-ratio technique and the dispersion model, applied to data from either the two FTIR paths or the four laser paths, agreed within ±10% with the enteric tracer-ratio technique. By contrast, the two vertical-profile techniques gave 33–68% higher weekly emission rates. It is shown with a sensitivity study that systematically uneven animal distribution within the enclosure could explain some of this discrepancy. Another cause for bias was the data yield of the vertical-profile techniques being higher at day-time than at night-time, thus giving more weight to times of larger emission rates. The techniques using line-averaged mole fractions were less sensitive to the exact locations of emission sources and less prone to data loss from unsuitable wind directions; these advantages outweighed the lack of a method to calibrate CH4 mole fractions in situ.

5 Axes computer aided laser milling

In this paper a 5 axes CAM procedure for the laser milling of free form surfaces has been developed and experimentally verified. The laser beam is deflected by a galvanometric scanning head and is directly moved on the working surface by the CNC controlled axes of a machine center. The procedure has been implemented in a software called CALM (computer aided laser manufacturing) able to generate the laser paths and the movements of the controlled axes reducing the defects on the workpiece. The approach is based on the sequential overlapping of the scanning passes on the working area. The different working areas in every laser displacement are obtained directly from the triangulation of the whole surface to machine.

Single Molecule Myosin V Dynamics Using High Time Resolution Polarized TIRF

Translocation of the molecular motor myosin along its actin filament track was studied using polarized total internal reflection (polTIRF) microscopy, a technique that determines the 3D orientation and dynamic wobble of single fluorophores. The maximum time resolution of the experimental setup was increased 50-fold by cycling through 8 different input laser paths and linear polarizations in approximately 1 ms. The arrival time and polarization state of each photon was recorded using a modified time-correlated single photon counting module. A new analysis, the Multiple Channel Change Point algorithm, was developed to detect changes in molecular orientation and wobble using the raw time-stamped data with no user-defined bins or thresholds. The analysis objectively identified changes in orientation and wobble of each fluorophore with a time resolution limited only by the photon counting rate. The polarized fluorescence from single rhodamine probes bifunctionally attached to one of the calmodulins on the myosin V lever arm was recorded as the motor translocated along a fixed actin filament in the presence of ATP. At the low (10 uM) ATP concentrations used here, the time intervals between sudden polarization changes averaged 120 ms and corresponded to stable orientations of the lever arm after each step. Occasionally, 10–80 ms substeps with increased probe wobble were detected between the longer duration dwells, possibly corresponding to the putative diffusive search of the detached head for its next actin binding site. Periods of large wobble were also detected at the end of many runs and when molecules were attached to actin in the absence of ATP. In both situations motors are probably bound to actin by only one head. This work was supported by NSF grants DMR-0832802, EF-0928048, IGERT DGE02-21664 and NIH grant GM-086352.

3316 Measurement of Geometric Deviation on a Machining Center : Measurement Method of Roll using Laser

On a machine tool, geometric deviations (positional and angular deviation) occur according to the movement of table, column, and so on. On the other hand, a laser measurement system has become popular, since it performs highly accurate measurement of the each geometric deviation except for roll deviation. In this paper, we focus on the improvement of measurement sensitivity of roll. The system consists of a bi-prism, a double-path optical heterodyne laser interferometer and a V-shaped reflector which composed of two plane mirrors. For improvement of measurement sensitivity, reflexive reflecting prisms are attached on the rear-side of the interferometer to make number of laser paths increase comparing with the conventional system. From the experimental results, it is confirmed that measurement sensitivity is increased in proportion to the number of laser paths.

Laser surface alloying of 1045 AISI steel using Ni–CrB2 powder

Laser surface alloying is a process whose purpose is to improve the surface properties by incorporating alloying elements into the surface. The advantages of using laser for surface treatment are: formation of a non-equilibrium or amorphous phase as well as homogenisation and refinement of the microstructure, all without affecting the substrate properties. Powder (50 wt-%Ni–50 wt-%CrB2) was injected into a melt pool created by a CW–CO2 laser on AISI1045 steel plates. In order to alloy the entire surface, the sample was scanned at scan speeds in the range of 600–6000 mm min–1 and the laser power was in the range of 1750–2500 W. The powder feed rate was 1·6 g min–1, the laser beam was 2 mm in diameter, with 60% overlap between successive laser paths. Metallographic cross-sections were made of the samples. For each sample the following properties were characterised: layer depth, microhardness (HV), layer microstructure and composition. It has been found that the scan speed and the laser power affect the depth of the melt pool, the microstructure, the hardness and the treated layer composition. The laser boronised surface exhibits better wear resistance than D2 tool steel hardened to 59 ± 1 HRC. This will be discussed based on numerical analysis of the laser/material interaction.

Meshing strategies for the efficient computation of laser beam forming processes on large aluminium plates

Due to its enormously high flexibility laser beam forming has been gaining importance during the last years. However, this high flexibility demands very precise irradiation strategies. This challenge is nowadays approached by process simulations mainly based on the finite element method (FEM) and thus demanding a high grid density to achieve reliable simulation results. For complex processes on large plates this is hardly feasible due to the limited computation power of current work stations. Thus, the simulation of the laser beam forming process on large plates demands intelligent meshing strategies decreasing the number of knots on the one hand and guaranteeing accurate simulation results on the other. The moving mesh method where exclusively a small area around the current laser position is meshed finely and is moved synchronously to the laser beam meets these demands. The applicability of the moving mesh method is proven and applied to large aluminium plates. Straight and diagonal laser paths are accomplished as well as two moving meshes at a plate with non-constant thickness and saddles, realising simulations in minimum times which had not at all been possible before.

Generation of 1.5–12 ns width-tunable 532 nm pulses by adopting laser-induced plasma shutter technique

The pulse-shaping technique has found widespread applications in nonlinear optics and material processing. Experimental research on laser-induced plasma shutter to control the 532 nm pulse width is conducted. The impacts of the total pulse output energy on pulse compression are investigated, and a useful conclusion can be drawn that there exists an optimal value of pulse energy at which the shortest output pulse of 3.23 ns can be obtained without a device for delay-time. Once the device for delay-time is employed to change the optical differences between two laser paths, the pulse width can be further shortened to 1.51 ns. In short, the 1.5–12 ns width-tunable 532 nm laser pulses have been obtained by adopting the laser-induced plasma shutter technique.

Laser-guided, intersecting discharge channels for the final beam transport in heavy-ion fusion

Ion-beam transport in space charge neutralizing discharge channels has been proposed for the final focus and chamber transport in a heavy-ion fusion reactor. A driver scenario with two-sided target illumination requires a system of two intersecting discharges to transport beams of the same charge from opposite sides towards the fusion target. In this article we report on experiments on the creation of free-standing, intersecting high-current discharge channels. The discharges are initiated in ammonia gas (NH3) in a metallic chamber by two perpendicular CO2-laser beams, which resonantly heat and subsequently rarefy the gas along the laser paths before the breakdown. These low density channels guide the discharges along the predefined paths and also around the 90° angles without any mechanical guiding structures. In this way stable X-, T-, and L-shaped discharges with currents in excess of 40 kA, at pressures of a few mbar were created with a total length of 110 cm. An 11.4 A MeV Ni+1258 beam from the UNILAC (Universal Linear Accelerator) linear accelerator was used to probe the line-integrated ion-optical properties of the central channel in a T-shaped discharge.

Herriott cell interferometry for millimeter-scale plasma measurements

A Herriott cell consists of two concave mirrors positioned on opposite sides of plasma so as to create multiple laser paths through the plasma. Added to a traditional interferometry diagnostic, the Herriott cell multiplies the effective path length through the medium and thereby increases instrument resolution. Previous work used a planar geometry to validate the use of Herriott cells in interferometer applications where the numerous mirror reflections will significantly degrade the phase front quality. The current work extends the Herriott cell capability to a point configuration. In this geometry the multipass beams converge near a single point within the plasma, useful for performing density measurements in very small scale length plasmas. Ray tracing analysis is used to illustrate example measurement geometries attainable with the instrument. It is shown that the configuration results in two convergence points for the laser paths, which somewhat complicates the interpretation of the experimental data. In addition, the multitude of passes through the high density, small scale length passes increases the likelihood for deleterious refractive effects. The diagnostic capability is demonstrated with measurements of the electron and neutral densities in the plasma exhaust of a Micro Pulsed Plasma Thruster. The measurements are validated with similar, lower resolution measurements, obtained using a single-pass interferometer.