Multi-pass Scanning Research Articles

Nanosecond laser processing and surface quality of SiC particle-reinforced AA2024 composite, homogeneous AA2024 aluminum alloy, and SiC: A comparative experimental study

This study performed comparative experiments on ablation behavior and surface quality obtained by nanosecond laser processing of silicon carbide (SiC) particle-reinforced 2024 aluminum alloy matrix composite (SiCp/AA2024), homogeneous SiC, and 2024 aluminum alloy. A nanosecond laser with a pulse fluence range of 0.68 ∼ 6.83 J/cm2 was used at irradiate samples of these materials, which were then ablated by various numbers of scanning passes (1 to 10) at a fluence of 6.83 J/cm2. The material melting could be achieved only for homogeneous AA2024. The ablation depth for SiC exceeded that of the composite, and the former could be ablated with a pulse fluence exceeding 3.42 J/cm2. The composite ablation threshold was higher than 4.10 J/cm2. Due to the thermal transition between the reinforcement and matrix, the melting threshold of the composite’s AA2024 matrix was lower than that of homogeneous AA2024 alloy. Moreover, the ablation threshold of SiC particles exceeded that of homogeneous SiC. The ablated surface roughness for the homogeneous material showed a sudden increase because of melting or oxidization, while that of the composite exhibited a gradual increase. For homogeneous SiC, oxidization accompanied by ablation and the accumulation of oxidization products on the surface deteriorated the laser pulse processing quality, especially in multipass scanning. In contrast, the dispersed distribution of SiC particles in the composite improved the composite’s machining feasibility.

Effect of scanning spacing on the efficiency of CFRP hole-cutting with multi-pass scanning strategy by nanosecond laser

Short-pulse laser processing of CFRP can reduce heat damage, but the efficiency is limited. In this paper, the efficiency of CFRP hole-cutting with multi-pass scanning strategy by nanosecond laser was investigated. The differences in material removal mechanisms at different scanning spacing and the effect of different scanning spacing on processing efficiency were investigated. The results show that the material is removed by the combined effect of laser ablation and mechanical erosion effect with different scanning spacing, and the appropriate scanning spacing and number of passes can effectively utilize the effect of mechanical erosion to improve processing efficiency. With a repetition frequency of 80 kHz (pulse energy is 141.25 μJ), the shortest cutting time is 73.3 s, and the optimal scanning spacing is 0.06 mm (focused spot diameter is 22 μm), which is 26.8 % more efficient compared to the scanning spacing of 0.08 mm and number of passes with 3; As laser repetition frequency increases, the optimal scanning spacing for processing efficiency increases and the optimal number of passes decreases.

Experimental Study on Femtosecond Laser Processing Performance of Single-Crystal Silicon Carbide

Femtosecond laser processing technology offers a promising technique for the preparation of micro and nanostructures of single-crystal silicon carbide (SiC), thanks to its high precision and non-destructive processing. However, further research is needed to optimize processing parameters, as well as improve efficiency and quality of the process. This study conducts experiments to explore the effects of femtosecond laser ablation on single-crystal SiC. The influence and significance of parameters, such as fluence (F), repetition rate, scan speed (S), multipass scanning (c) and numerical aperture on the performance of grooves, including groove depth, groove width, heat-affected zone (HAZ) width, material removal rate (MRR) and side wall inclination angle, were studied. The results show that the influence of fluence and numerical aperture on groove depth, groove width, HAZ width, MRR and side wall inclination angle is very significant. The scan speed has a very significant effect on the groove depth, groove width, HAZ width and side wall inclination angle but has insignificant effect on the MRR. Repetition rate and multipass scanning have a very significant effect on groove depth, HAZ width, MRR and side wall inclination angle and a moderately significant effect on groove width. The experimental methods of increasing the aspect ratio and reducing the HAZ width were studied, and a significance analysis was carried out. Fluence, multipass scanning and z-layer feed have significant effects on groove depth, groove width, aspect ratio, HAZ width and MRR. The influence of polarization angle on groove depth, groove width, aspect ratio and MRR is insignificant.

Controllable fabrication and adjustment of optical anti-reflectance micro-nano structures on NiTi alloy surface made by nanosecond laser circular scanning

The scanning path plays a very important and novel breakthrough in preparing optical anti-reflectance micro-nano structures on the surface of NiTi alloy. Compared with simple grids and grating structures, circular structures are more likely to form closed structures under high heat accumulation nanosecond laser processing conditions. These relatively closed circular structures will further help reduce optical reflection performance. In this study, some designed surface micro-nano structures are prepared by nanosecond laser circular scanning, which are the scanning pitch (plan A), the scanning angle (plan B), and the multi-pass scanning (plan C), respectively. The influence of the different micro-nano structures, surface roughness and elemental content on the surface optical anti-reflectance are also investigated. The results indicate that the best optical anti-reflectance micro-nano structures is obtained by 10-pass scanning among plan C. and which is 5% during the measurement wavelength of 400 nm ∼ 800 nm. At the same time, the mechanism is also discussed for changes in the light trapping effect of the laser textured surfaces.

Laser cutting of carbon fiber reinforced plastic components for remanufacturing

Carbon fiber reinforced plastic (CFRP) is extensively used in automotive and aerospace industries with the aim to achieve reduction on emissions by reducing weight. Due to governmental regulations to reduce the environmental impact and to reduce waste, the need for remanufacturing CFRP is becoming an interesting area of application with economic benefits to industry. This is important as manufacturing carbon fiber is a costly process and remanufacturing CFRP is more cost effective and reduces the dependency on virgin materials. Processing CFRP to meet demands, for fast and high-quality cuts, can impose problems for conventional methods. The use of multi-pass scanning technique in laser cutting CFRP is investigated using a 1.5 kW fiber laser with assist gas pressure of 16 bar and gas flow rate of 126 lt/min. Using multi-pass technique, a through cut can be obtained by repeating the beam travel more than once. The advantage of laser cutting when compared with traditional CNC, is the low cost of maintenance over time due to the non-contact nature of the process i.e. no wear of tool at contact area. And due to the small beam spot size of the laser small and complex shapes can be cut. The aim of the paper is to determine how the process performs in terms of cutting speed and fiber damage. Average power was used to carried out experimental tests. A fiber damage below 100 µm with laser cutting speed of 2.5 m/min and above was obtained. Thermal effects were analyzed using scanning electron microscope (SEM) and optical microscope (OM). The fiber damage was further optimized using specialist methods such as double aperture nozzle and trenching. The use of trenching and double aperture further reduces the fiber damage to 10 µm and 50 µm, respectively with laser cutting speed of 7.5 m/min and 3.33 m/min.

High-resolution ptychographic imaging enabled by high-speed multi-pass scanning

As a coherent diffraction imaging technique, ptychography provides high-spatial resolution beyond Rayleigh's criterion of the focusing optics, but it is also sensitively affected by the decoherence coming from the spatial and temporal variations in the experiment. Here we show that high-speed ptychographic data acquisition with short exposure can effectively reduce the impact from experimental variations. To reach a cumulative dose required for a given resolution, we further demonstrate that a continuous multi-pass scan via high-speed ptychography can achieve high-resolution imaging. This low-dose scan strategy is shown to be more dose-efficient, and has potential for radiation-sensitive sample studies and time-resolved imaging.

Experimental investigation and optimization of femtosecond laser processing parameters of silicon carbide–based on response surface methodology

The femtosecond laser, with its extremely high peak power and ultrashort pulse duration, is widely used in silicon carbide (SiC) processing. Herein, the response surface method was used to develop four quadratic regression models to predict the groove depth, width, heat affected zone (HAZ), and material removal rate (MRR). The effects of each parameter were investigated using variance analysis. In addition, the interaction of processing parameters with groove depth, width, HAZ and MRR was investigated, which provided an important analysis for the precise selection of processing parameters. The laser fluence is shown to significantly impact the groove depth, width, HAZ, and MRR. The scan speed significantly influences the groove depth, width, and HAZ but not the MRR. The repetition rate and multipass scanning significantly influence the groove depth, HAZ, and MRR but only moderately influence the groove width. The models show that changing the energy received by the groove can change the groove depth, width, HAZ, and MRR. The groove depth, width, and HAZ increase as the laser fluence, repetition rate, and multipass scanning increase; and they decrease as the scan speed increases. The MRR increases as laser fluence and multipass scanning increase. Meanwhile, the MRR varies slightly with repetition rate and scan speed. A nonlinear functional relationship exists in the interaction process of processing parameters. The processing parameters were optimized using response surface methodology with the maximum MRR and minimum HAZ as the objective function. Within the range of the processing parameters used, the average relative error between the experimental and predicted values was less than 5%. Consequently, the established regression models can accurately predict the groove depth, width, HAZ, and MRR in SiC femtosecond laser processing.

Study of high–low KPFM on a pn-patterned Si surface

Comparative measurements between frequency modulation Kelvin probe force microscopy (FM-KPFM) using low frequency bias voltage and heterodyne FM-KPFM using high frequency bias voltage were performed on the surface potential measurement. A silicon substrate patterned with p- and n-type impurities was used as a quantitative sample. The multi-pass scanning method in the measurements of FM-KPFM and heterodyne FM-KPFM was used to eliminate the effect of the tip–sample distance dependence. The measured surface potentials become lower in the order of the p-type region, n-type region and n+-type region by both FM-KPFM and heterodyne FM-KPFM, which are in good agreement with the order of the work functions of the pn-patterned Si sample. We observed the difference in the surface potentials due to the surface band bending measured by FM-KPFM and heterodyne FM-KPFM. The difference is due to the fact that the charge transfer between the surface and bulk levels may or may not respond to AC bias voltage.

Laser Processing of Hard and Ultra-Hard Materials for Micro-Machining and Surface Engineering Applications.

Polycrystalline diamonds, polycrystalline cubic boron nitrides and tungsten carbides are considered difficult to process due to their superior mechanical (hardness, toughness) and wear properties. This paper aims to review the recent progress in the use of lasers to texture hard and ultra-hard materials to a high and reproducible quality. The effect of wavelength, beam type, pulse duration, fluence, and scanning speed is extensively reviewed, and the resulting laser mechanisms, induced damage, surface integrity, and existing challenges discussed. The cutting performance of different textures in real applications is examined, and the key influence of texture size, texture geometry, area ratio, area density, orientation, and solid lubricants is highlighted. Pulsed laser ablation (PLA) is an established method for surface texturing. Defects include melt debris, unwanted allotropic phase transitions, recast layer, porosity, and cracking, leading to non-uniform mechanical properties and surface roughness in fabricated textures. An evaluation of the main laser parameters indicates that shorter pulse durations (ns—fs), fluences greater than the ablation threshold, and optimised multi-pass scanning speeds can deliver sufficient energy to create textures to the required depth and profile with minimal defects. Surface texturing improves the tribological performance of cutting tools in dry conditions, reducing coefficient of friction (COF), cutting forces, wear, machining temperature, and adhesion. It is evident that cutting conditions (feed speed, workpiece material) have a primary role in the performance of textured tools. The identified gaps in laser surface texturing and texture performance are detailed to provide future trends and research directions in the field.

Fabrication of high-aspect-ratio grooves with high surface quality by using femtosecond laser

PurposeThe purpose of this study is to fabricate high-aspect-ratio grooves with high surface quality by femtosecond laser (FS) to improve the machinability of silicon carbide (SiC) and optimize the process parameters in micromechanical applications.Design/methodology/approachFour contrast experiments are reported to characterize the FS laser grooving process for SiC with polarization direction, crystal orientation, multi-pass scanning and z layer feed, respectively. The effects of different experimental conditions on the groove characteristics, material removal rate (MRR), aspect ratio, heat affected zone (HAZ) and surface roughness Ra are analyzed.FindingsThe influence of increasing laser fluence and multi-scanning pass on the groove depth is greater than on the groove width. The MRR, aspect ratio, HAZ and Ra increased with the increase of laser fluence and multi-scanning pass. The direction of laser polarization affects the direction of hot electron injection but has little effect on the material characteristics. FS laser ablation is an isotropic process and there is no obvious change in different crystal orientations. The z-layer feed can significantly increase the groove width and depth and reduce HAZ and Ra. The maximum aspect ratio of 82.67% was fabricated.Originality/valueThe results contribute to the understanding of the removal mechanism and reduce the friction of the microfluidic device and improve the flowability in the FS laser ablation of SiC. This paper provides suggestions for the selection of suitable process parameters and provides a wider possibility for the application of micro-texture on SiC.

Design and optimization of projection stereolithography additive manufacturing system with multi-pass scanning

Purpose Scanning projection-based stereolithography (SPSL) is a powerful technology for additive manufacturing with high resolution as well as large building area. However, the surface quality of stitching boundary in an SPSL system has been rarely studied, and no positive settlement was proposed to address the poor stitching quality. This paper aims to propose an approach of multi-pass scanning and a compensation algorithm for multi-pass scanning process to address the issue of poor stitching quality in SPSL systems. Design/methodology/approach The process of multi-pass scanning is realized by scanning regions repeatedly, and the regions can be cured simultaneously because of the very short repeat exposure time and very fast scanning. Then, the poor stitching quality caused by the non-simultaneous curing can be eliminated. Also, a compensation algorithm is designed for multi-pass scanning to reduce the stitching errors. The validity of multi-pass scanning is verified by curing depth test, while the performance of multi-pass scanning as well as proposed compensation algorithm is demonstrated by comparing with that of a previous SPSL system. Findings The results lead to a conclusion that multi-pass scanning with its compensation algorithm is an effective approach to improve the stitching quality of an SPSL system. Practical implications This study can provide advice for researchers to achieve the satisfactory surface finish with SPSL technology. Originality/value The authors proposed a process of multi-pass scanning as well as a compensation algorithm for SPSL additive manufacturing (system to improve the stitching quality, which has rarely been studied in previous work.

Evolution of micro/nano-structural arrays on crystalline silicon carbide by femtosecond laser ablation

An experimental study is conducted to investigate the multi-length scale evolution from micro/nano-structural arrays to V-grooves by femtosecond laser-irradiations, considering the effects of laser polarization and the number of scans on the periodic features and the micro/nano-structures. It is found that the evolution from micro/nano-structural arrays to V-grooves is gradual as the laser fluence, scan speed or the number of scans is increased. Laser-induced surface instability, hatching effects, Coulomb explosions, melting, and evaporation are found to be the underlying mechanisms for the micro/nano structural formation. As the laser fluence increases, the agglomerated nanoparticles change from sparse to dense, so that the boundary between the LIPSS (laser-induced periodic surface structures) becomes less obvious. The oxygen (O) elements attached to the surface of nanoparticles increase with an increase in laser fluence. A high-spatial-frequency LIPSS (HSFL) can transform to low-spatial-frequency LIPSS (LSFL) as the laser fluence excesses the ablation threshold, while the spatial period of the HSFL and LSFL is independent of fluence. It is shown that the irradiated surface evolves from a near-damage-free zone to one with a recast layer and thermal-induced micro-cracks, defined as heat-affected zones (HAZ). Near thermal damage-free micro/nano-structural arrays with different orientations on the sidewalls of the grooves are fabricated at close to the laser ablation threshold by multi-pass scanning. A self-organizing model is developed which shows that laser polarization leads to the asymmetry of energy distribution and the structural array is arranged along the main direction of energy flow. The model analysis shows a good agreement with experimental results and provides an efficient means to understand the interaction between femtosecond laser and silicon carbide (SiC).

Atomic-Force Microscopy in the Study of the Tribological Characteristics of Thin Al−Si−N Coatings

Results of the atomic-force microscopic determination of the surface microstructure and roughness, friction forces and coefficients of nanocrystalline AlSiN films in the initial state and after annealing are presented. A procedure is proposed to determine the friction coefficients by atomic-force microscopy with multi-pass scanning. The dependences of the friction coefficients on the number of passes are obtained. A significant effect of annealing on a decrease in the friction coefficients of AlSiN films is detected.

Heat accumulation between scans during multi-pass cutting of carbon fiber reinforced plastics

Matrix evaporation caused by heat accumulation between scans (HAS) was studied in the case of multi-pass scanning of a laser beam over the surface of carbon fiber reinforced plastic (CFRP). The experiments were performed in two regimes, namely, in the process of CFRP cutting and in the regime of low-fluence irradiation avoiding ablation of carbon fibers. The feature of the ablation-free regime is that all absorbed energy remains in the material as heat, while in the cutting regime the fraction of residual heat is unknown. An analytical model based on two-dimensional (2D) heat flow was applied to predict the critical number of scans, after which the HAS effect causes a distinct growth of the matrix evaporation zone (MEZ). According to the model, the critical number of scans decreases exponentially with increasing laser power, while no dependence on the feed rate is expected. It was found that the model fits well to the experimental data obtained in the ablation-free regime where the heat input is well defined and known. In the cutting regime the measured significant reduction of the critical number of scans observed in deep grooves may be attributed to transformation of the heat flow geometry and to an expected increase of the residual heat fraction.

Femtosecond laser inscription waveguides in Nd:GdVO4crystal

Channel waveguides in Nd:GdVO4 crystals were fabricated by using low-repetition-rate (1 kHz) femtosecond laser inscription. The influences of pulse energy and scanning velocity were investigated. Guiding occurs in the surroundings of the focal spot. Single-line, double-line, and double-layer waveguides were realized. The near-field mode images, refractive-index profile reconstruction, and mode analysis of waveguides are presented. The double-line waveguides show strong guidance at 632.8 nm with a loss of about 0.5 dB/mm and an index increase of 3×10−4. The mode profile asymmetry of single-line waveguide has been improved by using multipass scanning.

A predictive divergence compensation approach for the fabrication of three-dimensional microstructures using focused ion beam machining

In this paper, the major factors which cause fabrication divergence in the focused ion beam (FIB) milling process are discussed. A divergence compensation approach is outlined which calculates the corrected dwell time in order to allow for the divergence caused by the FIB milling process; this is due to overlap effects and the angular-dependent sputter yield. A multi-pass scanning method is used to reduce the fabrication divergence precipitated by atom redeposition. Microstructures, such as parabolic, hemispherical and sinusoidal shapes and a nano hemispherical structure have been produced during the FIB milling experiments using conventional bitmap milling and proposed divergence compensation approaches. A flat top/bottom surface is obtained in convex/concave structures when using the conventional bitmap FIB milling approach. Further research shows that the reasons for this phenomenon are mainly related to both the aspect ratio of the structures and the existence of an oxide layer on the substrate surface. The flat top/bottom phenomenon can be removed by using the combination of a novel divergence compensation approach and the removal of the oxide layer from the substrate surface prior to FIB machining. The experimental results show that the surface form accuracy has been greatly improved by using this method and the overlap effect can be suppressed by carefully choosing the normalized pixel spacing.

XPath Evaluation Oriented XML Data Stream Compression

Because XML (extensible markup language) is self-described, there is much redundant structural information in XML data stream. How to compress XML data so as to reduce the network transfer cost and support XPath evaluation on the compressed data is a new area of research. The existing methods on XML compression require the multi-pass scan on data or can not support real time query processing on compressed data. In this paper, a novel compression method XSC (XML stream compression) is proposed to compress and decompress XML stream in real time. XSC constructs XML element event sequence dictionary and outputs the related index dynamically. When DTD is available, XSC can generate the XML element event sequence graph for producing more reasonable encoding before XML data stream is processed. The compressed XML data stream can be decomposed directly for XPath evaluation. Experimental results show that XSC outperforms other methods in compression ratio

MRI fast tree log scanning with helical undersampled projection acquisitions

Magnetic Resonance Imaging opens an alternative way to analyze wood structures using a non-destructive technology. It provides high resolution, compound-based contrast manipulation and increased data acquisition flexibility. The technique is particularly useful for tree logs, since they present several characteristics that can be used to reduce the long scan time. This study proposes a method that takes advantage of the log cylindrical symmetry, acquiring transverse 1-D projections with a helical and undersampled pattern. Linear interpolation is used to estimate the skipped data and slice images are reconstructed by filtered backprojection. The sequence is improved using selective multi-pass scanning, without major variations of the scan time. Computer simulations and experimental results show that the proposed technique can increase the scan speed by a factor of 6, while maintaining the ability to identify typical tree log characteristics.

A new approach of surface treatment: micro-beam plasma arc remelting

Abstract A new method of surface treatment, micro-beam plasma arc melting/remelting, is proposed, and is applied to Fe–C–Cr–Si–B overlay in the paper. The surface of overlay was remelted by multi-pass scanning of micro-beam plasma arc and self-quenched by the underlying substrate. With this technique a microcrack-free remelting layer was produced for the iron-base overlay despite that it contained great amounts of carbon (4.2 wt%) and chromium (24.66 wt%), and even amorphous phase was found through transmission electron microscopy (TEM) on the top surface of remelting layer. The experimental results show that the micro-beam plasma arc melting/remelting is a unique technique of surface treatment with a significant application potential.