Scanning Velocity Research Articles

Computed Three-Dimensional Atomic Force Microscopy Images of Biopolymers Using the Jarzynski Equality.

Three-dimensional atomic force microscopy (3D-AFM) has resolved three-dimensional distributions of solvent molecules at solid–liquid interfaces at the subnanometer scale. This method is now being extended to the imaging of biopolymer assemblies such as chromosomes or proteins in cells, with the expectation of being able to resolve their three-dimensional structures. Here, we have developed a computational method to simulate 3D-AFM images of biopolymers by using the Jarzynski equality. It is found that some parts of the fiber structure of biopolymers are indeed resolved in the 3D-AFM image. The dependency of 3D-AFM images on the vertical scanning velocity is investigated, and optimum scanning velocities are found. It is also clarified that forces in nonequilibrium processes are measured in 3D-AFM measurements when the dynamics of polymers are slower than the scanning of the probe.

Numerical Simulation and Experimental Investigation of SiC/Ti-6Al-4V Metal Matrix Composites Produced by Laser Melt Injection

In this work, a 3D transient finite element (F.E.) model was established to investigate the temperature field distribution in laser processing of the Ti-6Al-4V substrate. The influences of laser power and scanning velocity on the molten pool state were analyzed. In an integrated model considering the length, temperature, and lifetime of the tail area of the molten pool, a laser power of 2.5 kW and a scanning velocity of 60 mm/s are thought to be suitable for laser melt injection. Particle reinforced metal matrix composite coating with a thickness of about 250 μm was prepared on the Ti-6Al-4V surface under the above process. It was found that the microstructure and hardness of the coating gradient varied along the depth direction. The maximum hardness of the coating can reach 1729.5 HV, which is much higher than that of the Ti-6Al-4V substrate.

Optomechanical Analysis and Design of Polygon Mirror-Based Laser Scanners

Polygon Mirror (PM)-based scanning heads are one of the fastest and most versatile optomechanical laser scanners. The aim of this work is to develop a multi-parameter opto-mechanical analysis of PMs, from which to extract rules-of-thumbs for the design of such systems. The characteristic functions and parameters of PMs scanning heads are deduced and studied, considering their constructive and functional parameters. Optical aspects related to the kinematics of emergent laser beams (and of corresponding laser spots on a scanned plane or objective lens) are investigated. The PM analysis (which implies a larger number of parameters) is confronted with the corresponding, but less complex aspects of Galvanometer Scanners (GSs). The issue of the non-linearity of the scanning functions of both PMs and GSs (and, consequently, of their variable scanning velocities) is approached, as well as characteristic angles, the angular and linear Field-of-View (FOV), and the duty cycle. A device with two supplemental mirrors is proposed and designed to increase the distance between the GS or PM and the scanned plane or lens to linearize the scanning function (and thus to achieve an approximately constant scanning velocity). These optical aspects are completed with Finite Element Analyses (FEA) of fast rotational PMs, to assess their structural integrity issues. The study is concluded with an optomechanical design scheme of PM-based scanning heads, which unites optical and mechanical aspects—to allow for a more comprehensive approach of possible issues of such scanners. Such a scheme can be applied to other types of optomechanical scanners, with mirrors or refractive elements, as well.

Nanomechanical tribological characterisation of nanostructured titanium alloy surfaces using AFM: A friction vs velocity study

Medical-grade titanium alloys used for orthopaedic implants are at risk from infections and complications such as wear and tear. We have recently shown that hydrothermally etched (HTE) nanostructures (NS) formed on the Ti6AlV4 alloy surfaces impart enhanced anti-bacterial activity which results in inhibited formation of bacterial biofilm. Although these titanium alloy nanostructures may resist bacterial colonisation, their frictional properties are yet to be understood. Orthopaedic devices are encapsulated by bone and muscle tissue. Contact friction between orthopaedic implant surfaces and these host tissues may trigger inflammation, osteolysis and wear. To address these challenges, we performed simulation of the contact behaviour between a smooth control Ti6Al4V alloy and HTE surfaces against a hardwearing SiO2 sphere using Atomic Force Microscopy (AFM) in Lateral Force Microscopy mode. The friction study was evaluated in both air and liquid environments at high (5 Hz) and low (0.5 Hz) scan velocities. Lower scan velocities demonstrated opposing friction force changes between the two mediums, with friction stabilising at higher velocities. The friction measured on the NS alloy surfaces was reduced by ~20% in air and ~80% in phosphate buffered saline, in comparison to the smooth control surface, displaying a non-linear behaviour of the force applied by the AFM tip. Changes in friction values and cantilever scan velocities on different substrates are discussed with respect to the Stribeck curve. Reduced friction on nanostructured surfaces may improve wear resistance and aid osseointegration.

Improved Active Disturbance Rejection Double Closed-Loop Control of a Rotary-Type VCM in a Moving Mirror Control System.

Aiming to address the problem of moving mirror speed fluctuations in moving mirror control systems, an improved active disturbance rejection double closed-loop controller (IADR-DCLC) is proposed and verified by simulation to realize the high-performance control of a moving mirror control system. First, the mathematical model of a rotary-type voice coil motor (RT VCM) is established, and the relationship between the angular velocity of the RT VCM and the optical path scanning velocity is analyzed. Second, in order to suppress the model uncertainty and external disturbance of the system, an improved active disturbance rejection controller (IADRC) is proposed. Compared with a conventional ADRC, the tracking differentiator of the proposed IADRC is replaced with desired signal optimization (DSO), and the actual speed is introduced to the extended state observer (ESO). The IADRC is used in the position–speed double closed-loop control model. Finally, the simulation results show that the IADR-DCLC has not only a good tracking effect but also a good anti-interference ability and can meet the requirements of the moving mirror control system for the uniformity of optical-path scanning speed and accurate control of the position of the moving mirror.

Anisotropic growth of domain rays in lithium niobate crystal induced by IR laser scanning

Formation of nanodomain structure after linear scanning by IR laser beam in congruent lithium niobate crystal was attributed to switching under the action of pyroelectric field. The complicated domain patterns consisted of strictly oriented narrow stripe domains (width about 200 nm and depth about 20 µm) arising in a wide switched band. Width of the switched band and domain length density at different distances from the center of the irradiated zone depend on the scanning velocity v s. Relevance of the used model was supported by similarity of the experimental and calculated dependences.

Processing of Ti–13Nb–13Zr powder by selective laser melting under a relatively high oxygen atmosphere

Ti–13Nb–13Zr (TNZ) parts were produced by selective laser melting under an atmosphere with a relatively high O content using a comprehensive combination of laser powers and laser beam scanning velocities. The influence of processing conditions on the density, microstructure, interstitial element contents (N and O), and hardness of as-printed TNZ parts were studied. The N and O contents in the TNZ parts increased on increasing the energy density (ED). Despite the high O content in the chamber during laser processing, it was possible to obtain TNZ parts with high density and relatively low N and O contents, depending on the ED applied. Two different types of microstructure were observed in the TNZ parts: α′ martensite and α/α′ + β phases. Depending on its microstructure, the TNZ parts presented interstitial solid solution hardening and second phase hardening.

Multi-Fidelity Surrogate-Based Process Mapping with Uncertainty Quantification in Laser Directed Energy Deposition.

A multi-fidelity (MF) surrogate involving Gaussian processes (GPs) is used for designing temporal process maps in laser directed energy deposition (L-DED) additive manufacturing (AM). Process maps are used to establish relationships between the melt pool properties (e.g., melt pool depth) and process parameters (e.g., laser power and scan velocity). The MFGP surrogate involves a high-fidelity (HF) and a low-fidelity (LF) model. The Autodesk Netfabb finite element model (FEM) is selected as the HF model, while an analytical model developed by Eagar-Tsai is chosen as the LF one. The results show that the MFGP surrogate is capable of successfully blending the information present in different fidelity models for designing the temporal forward process maps (e.g., given a set of process parameters for which the true depth is not known, what would be the melt pool depth?). To expand the newly-developed formulation for establishing the temporal inverse process maps (e.g., to achieve the desired melt pool depth for which the true process parameters are not known, what would be the optimal prediction of the process parameters as a function of time?), a case study is performed by coupling the MFGP surrogate with Bayesian Optimization (BO) under computational budget constraints. The results demonstrate that MFGP-BO can significantly improve the optimization solution quality compared to the single-fidelity (SF) GP-BO, along with incurring a lower computational budget. As opposed to the existing methods that are limited to developing steady-state forward process maps, the current work successfully demonstrates the realization of temporal forward and inverse process maps in L-DED incorporating uncertainty quantification (UQ).

Wavelet estimation and nonstretching NMO correction

Normal-moveout (NMO) correction is an important step applied to common-midpoint (CMP) gathers for subsequent stacking. However, conventional NMO correction methods often suffer from the problem of NMO stretching, which nonlinearly increases with offsets and decreases with zero-offset traveltime. The NMO stretching can be quantified by frequency distortion, so stretching is confined mainly to large offsets and shallow times. To solve this problem, we have proposed a wavelet-based method with the following four steps. First, we estimate a wavelet from the CMP gather by using the NMO stretching appearing in the conventional NMO correction. Second, we deconvolve the original CMP gather based on the estimated wavelet. This step of removing the wavelet from the CMP gather is helpful for the next steps of NMO velocity scan and NMO correction. Third, we apply an improved NMO correction to the deconvolved CMP gather and obtain flattened reflectivities. Finally, we convolve the flattened and deconvolved gather with the estimated wavelet back to obtain an NMO-corrected gather without stretching artifacts. In our method, by using a deconvolved CMP gather, we are able to calculate a high-resolution semblance velocity spectrum that benefits from the NMO velocity picking. In addition, applying the NMO correction to the deconvolved CMP gather, instead of the original gather, is helpful in reducing the NMO stretching related to the wavelet distortion. Tests on synthetic and field data find that our new NMO correction method can estimate an accurate seismic wavelet and obtain an NMO-corrected gather without NMO stretching. Reducing the NMO stretching can significantly improve the resolution of shallow layers at far offsets and preserve the spectral bandwidth.

Surface Laser Treatment of Cast Irons: A Review

Heat treatments are frequently used to modify the microstructure and mechanical properties of materials according to the requirements of their applications. Laser surface treatment (LST) has become a relevant technique due to the high control of the parameters and localization involved in surface modification. It allows for the rapid transformation of the microstructure near the surface, resulting in minimal distortion of the workpiece bulk. LST encompasses, in turn, laser surface melting and laser surface hardening techniques. Many of the works devoted to studying the effects of LST in cast iron are diverse and spread in several scientific communities. This work aims to review the main experimental aspects involved in the LST treatment of four cast-iron groups: gray (lamellar) cast iron, pearlitic ductile (nodular) iron, austempered ductile iron, and ferritic ductile iron. The effects of key experimental parameters, such as laser power, scanning velocity, and interaction time, on the microstructure, composition, hardness, and wear are presented, discussed, and overviewed. Finally, we highlight the main scientific and technological challenges regarding LST applied to cast irons.

Precise laser-induced local modification of AZO:Ag films and their optical properties

The paper presents precise modifications of ZnO:Al thin films with silver nanoparticles by CW laser leading to the improvement of the composite structure and optical properties. The violet laser wavelength (405 nm) was chosen to be far from the plasmon resonance wavelength. The morphology of the obtained ZnO:Al thin films with silver nanoparticles was investigated. It was detected that under an exposure by laser power densities I0 = 0.89 MW/cm2 and I0 = 1.07 MW/cm2 and scanning velocities Vsc = 100 μm/s and Vsc = 50 μm/s, the film structure became denser, the size of ZnO:Al crystallites increased, and the size of silver nanoparticles decreased. The effects of laser radiation on the optical characteristics of the samples were then investigated. Particularly, a blue shift of the plasmon resonance peak and an increase in the bandgap from 3.27 eV to 3.36 eV were obtained. Additionally, numerical modeling was performed based on a photothermal film processing mechanism and clearly explained the observed effects.

Laser assisted surface conditioning of a residual pattern on single crystal silicon induced by ultra-precision cutting

Fabrication of a silicon lens using an ultra-precision machining tool generates a periodic cutting texture, which can cause light diffraction and scattering, resulting in a significant degradation of the optical performance. This phenomenon usually appears in the form of rainbow patterns. In this study, a single-crystal silicon wafer, after diamond turning, is polished by a nanosecond laser mounted on a four-axis ultra-precision machine lathe. It’s found that the rainbow pattern has been effectively eliminated after laser irradiation through the observation of surface topography. Effects of defocus, scanning velocity, and pitch on polishing were investigated. This polishing method is able to obtain surface roughness Sa better than 1 nm for silicon surface without removing or adding material to the workpiece, and high throughout can be achieved through easy integration with the existing cutting process.

Secondary electron yield reduction of copper after 355 nm ultrashort pulse laser ablation

Nanostructured surfaces exhibit remarkable chemical, physical and microbiological properties and have therefore various technical and industrial applications. The ultrashort laser pulse irradiation (wavelength λ = 355 nm, pulse duration Δtp = 12 ps, repetition rate f = 100 kHz) of copper samples with appropriate laser parameters results in the formation of a micro- and nanostructured surfaces. The influence of these hierarchically textured surfaces on the secondary electron yield (SEY) was studied especially with regard to their morphological and geometrical properties. Specific SEY changes are caused by both, the shape and the depth of the microstructures, as well as the morphology of the formed nanostructures; that can be either compact flower head-like nanostructures, non-compact filament-shaped nanostructures, molten and resolidified spherical structures, or minor modified surfaces. The measured SEY as a function of the primary electron energy is correlated with the surface topography that forms upon laser irradiation. The SEY decreases with increasing accumulated laser fluence and ablated volume, respectively. Especially flower-head-like nanostructures can be produced at high laser power (P ≥ 400 mW) and low scanning velocity (v ≤ 10 mm/s) and represent a surface with strongly reduced SEY maximum as low as 0.7.

On the role of volumetric energy density in the microstructure and mechanical properties of laser powder bed fusion Ti-6Al-4V alloy

The volumetric energy density (EV) is considered as a comprehensive index to evaluate the complex parameters in laser powder bed fusion (L-PBF) of metallic parts. This work aims to investigate the influence of varying laser power and scanning velocity coupling under a specific EV value on the microstructure, texture, and tensile properties of L-PBF Ti-6Al-4 V (Ti64) alloy to illustrate the limitation of EV. Three sets of laser parameters under a specific EV of 58.8 J/mm3 were employed to fabricate dense parts. Analyses by electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) revealed that the same EV in the L-PBF process can result in distinct thickness of α′ lath, crystallographic texture and tensile behavior. The α′-lath was refined from 0.89 µm to 0.66 µm while laser power increased from 180 W to 240 W. Meanwhile, the crystallographic orientation of prior-β grains was oriented < 111 >β direction to parallel building direction (BD) from < 001 >β//BD, which determined the orientations of α′ lath. Tensile test results exhibit that the yield strength, ultimate tensile strength and elongation to fracture all increase with the linearly increases in laser power and speed. An excellent balance between yield strength of ~1042.2 ± 37.1 MPa and total elongation of ~ 10.8 ± 1.1% was obtained after the variant optimization. The finite element analysis shows that even under a constant EV value, varying laser power and scanning velocity coupling will carry out distinct thermal histories, leading to the grain refinement and higher dislocation density. This work shows the limited significance of EV in the guidance of the L-PBF process with complex physics. More efforts should be made on microstructural control via the combination of specific laser parameters.

The Atacama Cosmology Telescope: Modeling bulk atmospheric motion

Fluctuating atmospheric emission is a dominant source of noise for ground-based millimeter-wave observations of the cosmic microwave background (CMB) temperature anisotropy at angular scales $\ensuremath{\gtrsim}0.5\ifmmode^\circ\else\textdegree\fi{}$. We present a model of the atmosphere as a discrete set of emissive turbulent layers that move with respect to the observer with a horizontal wind velocity. After introducing a statistic derived from the time-lag dependent correlation function for detector pairs in an array, referred to as the pair-lag, we use this model to estimate the aggregate angular motion of the atmosphere derived from time-ordered data from the Atacama Cosmology Telescope (ACT). We find that estimates derived from ACT's CMB observations alone agree with those derived from satellite weather data that additionally include a height-dependent horizontal wind velocity and water vapor density. We also explore the dependence of the measured atmospheric noise spectrum on the relative angle between the wind velocity and the telescope scan direction. In particular, we find that varying the scan velocity changes the noise spectrum in a predictable way. Computing the pair-lag statistic opens up new avenues for understanding how atmospheric fluctuations impact measurements of the CMB anisotropy.

Printing of NiTinol parts with characteristics respecting the general microstructural, compositional and mechanical requirements of bone replacement implants

NiTinol is considered to be an attractive candidate alloy for biomedical applications. The aim of this research was to obtain porous NiTinol parts that respected the general microstructural (having 30–80 vol% interconnected pores and pore sizes in the range of 100–600 μm), compositional (an impurity content of less than 500 ppm) and mechanical (an elastic modulus close to that of cortical bone (10–20 GPa) or cancellous bone (<3 GPa), higher strength than 100 MPa at 2% strain and adequate fracture elongation of at least 8%) prerequisites of implants used in bone replacement applications. This was done by printing parts using a laser powder bed fusion technique and by employing a slightly Ni-rich pre-alloyed NiTinol powder having ∼55.9 wt% Ni in its composition. In the next setup, different post-printing heat treatments were employed to adjust the microstructural, compositional and mechanical characteristics of the printed parts step-by-step with the mentioned ones required for the bone replacement implants. The effects of different printing parameters and conducted heat treatment procedures on the microstructure, composition, phase transformation characteristics and mechanical properties of dense and porous printed parts were also systematically discussed and results were correlated together. The developed porous parts (with ∼33 vol% interconnected porosity and ∼600 μm pore width size) were printed using the processing parameters: laser power = 120 W, scanning velocity = 1100 mms−1, hatch spacing = 0.08 mm and layer thickness = 0.03 mm. Regardless of being heat treated or not, the parts generally respected the mentioned targeted properties.

High-speed laser micro polishing of TiAl6V4

In the present paper, laser micro polishing of TiAl6V4 is investigated with main focus on increasing the area rate by using high pulse energy. First, the influence of the laser beam size, the scanning velocity, and the track distance on the final roughness is examined since these parameters determine the area rate. The results of these investigations are used to identify a set of process parameters that allow increasing the area rate and achieving a surface roughness comparable to state-of-the-art laser micro polishing. The results show that the laser beam size must exceed a critical size; otherwise, smoothing of the surface is limited, which leads to higher final roughness. The scanning velocity and the track distance do not have a great effect on the final roughness for the investigated regimes. However, with decreasing track distance, the number of undesirable surface structures, presumably resulting from evaporating vanadium compounds, is reduced. Additionally, the heat input into the surface decreases with increasing laser beam size, scanning velocity, and track distance. The results of these investigations are used to significantly increase the area rate to 19.1 cm2/s, which is a factor of 64 times faster than the state of the art. The final surface roughness is comparable to state-of-the-art laser micro polishing in this case.

Influence of γ/γ′ morphology evolution on the mechanical properties of Ni-based single crystal superalloys formed by laser metal deposition

The variation in hardness and tensile properties of Ni-based single crystal (SX) superalloy DD6 during laser metal deposition processing has been investigated, and the effects of laser scanning velocity and post heat-treatment on the evolution of microstructure and its interaction with mechanical properties of deposited region were determined. It is found that the microstructure of as-deposited Ni-based single crystal superalloy mainly consists of columnar dendrite epitaxially growing from the substrate. The fine γ′ phase microstructure and dense γ/γ′ phase interfaces contribute more to the microstructural evolution and mechanical properties of deposited part formed by laser metal deposition. Furthermore, the heat treatment reduces the mechanical properties of deposited sample due to the introducing of stray grains that destroys the original single crystallinity. The mechanism unraveled here could shed light on the reliability evaluation and parameter selection of the laser metal deposition technique and other directed energy deposition technology.

Evolution of Surface Topography and Microstructure in Laser Polishing of Cold Work Steel 1.2379 (AISI D2) Using Quadratic, Top-Hat Shaped Intensity Distributions.

Within the scope of this study, basic experimental research was carried out on macro-laser polishing of tool steel 1.2379 (D2) using a square intensity distribution and continuous wave laser radiation. The influence of the individual process parameters on surface topography was analyzed by a systematic investigation of a wide range of process parameters for two different, square laser beam diameters. Contrary to a typical laser polishing approach, it was shown that short interaction times (high scanning velocity and small laser beam dimensions) are required to reduce both micro-roughness and meso-roughness. A significant reduction of surface roughness of approx. 46% was achieved from Raini = 0.33 ± 0.026 µm to Ramin = 0.163 ± 0.018 µm using a focused square laser beam with an edge length of dL,E = 100 µm at a scanning velocity of vscan = 200 mm/s, a laser power PL = 60 W and n = 2 passes. However, characteristic surface features occur during laser polishing and are a direct consequence of the laser polishing process. Martensite needles in the micro-roughness region, undercuts in the meso-roughness region, and surface waviness in the macro-roughness region can dominate different regions of the resulting surface roughness spectrum. In terms of mechanical properties, average surface hardness was determined by hundreds of nano-indentation measurements and was approx. 390 ± 21 HV0.1 and particularly homogeneous over the whole laser polished surface.

On the laser forming process of copper/aluminum bi-metal sheets with a functional thickness

Bi-metal sheets with a functional thickness are the heterogeneous materials in which physical/mechanical properties of the sheet change from upper layer to the bottom layer. The scope of this work is to investigate the forming of copper/aluminum bi-metal sheets with a functional thickness using laser forming process. In fact, the combined properties of a bi-metal sheet make it complicated to predict the forming behavior of these materials. For this reason, laser forming process of bi-metal sheets has been simulated using a DFLUX subroutine to define the continuous scan path of laser beam. A detailed discussion has been presented on how to activate the temperature gradient mechanism during the process. Furthermore, a set of experimental tests was also performed to examine the validity of the numerical tests. Thus, the effect of each input process parameter including laser power, scan velocity, and beam diameter has been examined. A full-factorial design of experiment has been utilized to specify the quantitative influences of the parameters on the final bending angle. A statistical model has been also derived for predicting the bending angle in laser forming of bi-metal sheets. The numerical results show that by increasing the laser power and scan velocity and decreasing the beam diameter a maximum forming magnitude would be achieved.