Laser Scanning Process Research Articles

Research about the microcrack mechanisms in molybdenum and corresponding suppression strategies during laser additive manufacturing process

In this study, the mechanism of microcracks in the laser powder bed fusion of molybdenum was analyzed, and the corresponding solutions were proposed. Through specially designed single-track experiment and finite element simulation analyses, we found that thermal stress was the main cause of cracking during the laser powder bed fusion process. The cracks were generated almost synchronously with the laser scanning process, and thus optimizing the process parameters alone could not inhibit the occurrence of cracks. Herein, alloying strategy was proposed and proved to be an effective way to solve the cracking problems. Specifically, by adding Cr and Ti elements, the high-temperature mechanical properties of Mo were improved through solid solution strengthening mechanism, hence the thermal stress induced cracking was suppressed effectively. In addition, according to the thermodynamic calculations, it was found that Cr and Ti had an inhibitory effect on the segregation of impurity at the grain boundaries and this conduce to improve the strength as well.

Effects of Laser Scanning Strategy on Bending Behavior and Microstructure of DP980 Steel

Laser bending is a kind of cumulative forming technology and bending efficiency is one of its most important indexes. This study investigates the bending behavior and the microstructure of DP980 steel plates under different laser scanning strategies, using an IPG laser system. Two sets of experiments varied the accumulated line energy density (AED) by altering the laser scanning velocity and number of scans. The results show that, for the single laser scanning process, the bending angle of the plate increases with AED, due to a larger temperature gradient through the thickness direction; however, this relationship is nonlinear. A higher AED led to a sharper initial increase in bending angle, which then plateaued. Under the same AED conditions, the bending angle of the plate undergoing multiple laser scans increases by at least 26% compared to the single one, due to the microstructure changes. It is revealed that the bending efficiency is affected by both the AED and the resultant microstructure evolution in the DP980 steel. Higher AED values and appropriate peak temperatures facilitate better bending behavior due to the formation of uniform martensite and grain refinement. Conversely, excessive peak temperatures can hinder bending due to grain growth.

Controlling the morphologies of femtosecond laser-induced periodic surface structure on silicon by combining deep learning with energy deposition model

As an important patterning method, femtosecond laser-induced periodic surface structure (LIPSS) has attracted widespread attention in recent years. Due to the complex physical processes involved in the femtosecond laser scanning process, it is difficult to predict the required LIPSS morphologies, which hinders rapid customization of the large-area LIPSS patterns. In this study, a structural optimization method combining data-driven deep learning with energy deposition model was proposed to control LIPSS morphologies of silicon. After femtosecond laser processing under dynamic irradiation condition, four LIPSS morphological types were well defined based on experimental results. Deep learning model was constructed to extract data features by labeling and encoding sample data. The accuracy of self-correlation validation reached 98.0% and the accuracy of cross-correlation validation reached 91.9%. Our results show that the distribution of LIPSS morphological types exhibits the dependence of energy deposition, which is the joint effect of the effective pulse number and accumulative fluence. Moreover, the period of groove (super-wavelength LIPSS) increases with the increase of energy deposition, while the period of LSFL (low spatial frequency LIPSS) slightly decreases with the increase of energy deposition. Through this structural optimization method, customization and online monitoring of large-area LIPSS patterns in the future industrial applications.

Effect of nanosecond pulse laser power on welding interface and mechanical properties of AZ31B Mg/6061 Al

AZ31B magnesium alloy and 6061 aluminum alloy of 0.2 mm thickness were joined by nanosecond pulsed laser welding technology, and the effects of nanosecond pulsed laser power on the formation of weld seam, interface microstructure and mechanical properties were investigated, and numerical simulation of the temperature field during the welding process was carried out. The results show that when the laser power is 50 W, more defects such as cracks, pores and sags appeared in the joints due to the high heat input, and these defects resulted in a smaller effective joint area between the metals, which is very unfavorable to the strength of the joints. As the laser power decreases, at a laser power of 25 W, fewer defects are exhibited and the effective joint area becomes larger. As the laser power continues to decrease, the low heat input makes metal melting and joining very difficult. At low laser power, the Mg element intrudes backwards into the aluminum, causing the joint boundary interface to take on a reverse “V” shape. According to the results of the tensile shear experiments, the maximum tensile force of the joint was the highest at a laser power of 25 W, which reached 76.5 N. In addition, it was observed that the effective joint area, the number of oxides and their inclusions, and the degree of angular deformation had a very important effect on the mechanical properties of the joints. The results of temperature field numerical simulation experiments show that the heat is accumulated from outside to inside during the laser scanning process, and whether the temperature in the center of the molten pool reaches the melting point of the alloy can be used as an important basis for analyzing the degree of welding.

Methodology for retrospectively developing a BIM model from point cloud scans using ongoing building project as case study

Purpose The purpose of this study is to explore the suggestions that construction processes could be considerably improved by integrating building information modelling (BIM) with 3D laser scanning technologies. This case study integrated 3D laser point cloud scans with BIM to explore the effects of BIM adoption on ongoing construction project, whilst evaluating the utility of 3D laser scanning technology for producing structural 3D models by converting point cloud data (PCD) into BIM. Design/methodology/approach The primary data acquisition adopted the use of Trimble X7 laser scanning process, which is a set of data points in the scanned space that represent the scanned structure. The implementation of BIM with the 3D PCD to explore the precision and effectiveness of the construction processes as well as the as-built condition of a structure was precisely captured using the 3D laser scanning technology to recreate accurate and exact 3D models capable of being used to find and fix problems during construction. Findings The findings indicate that the integration of BIM and 3D laser scanning technology has the tendency to mitigate issues such as building rework, improved project completion times, reduced project cost, enhanced interdisciplinary communication, cooperation and collaboration amongst the project duty holders, which ultimately enhances the overall efficiency of the construction project. Research limitations/implications The acquisition of data using 3D laser scanner is usually conducted from the ground. Therefore, certain aspects of the building could potentially disturb data acquisition; for example, the gable and sections of eaves (fascia and soffit) could be left in a blind spot. Data acquisition using 3D laser scanner technology takes time, and the processing of the vast amount of data acquired is laborious, and if not carefully analysed, could result in errors in generated models. Furthermore, because this was an ongoing construction project, material stockpiling and planned construction works obstructed and delayed the seamless capture of scanned data points. Originality/value These findings highlight the significance of integrating BIM and 3D laser scanning technology in the construction process and emphasise the value of advanced data collection methods for effectively managing construction projects and streamlined workflows.

Enhanced mechanical property through high-yield fabrication process with double laser scanning method in two-photon lithography

Two-photon lithography (TPL) is a representative fabrication process for micro 3D structures with a sub-micron fabrication resolution. As the TPL process is developed and commercialized, the TPL process requires higher process yield to be a considered reliable manufacturing technique for mass production. However, delamination is one of the main factors that reduce the yield of the TPL process. In this study, a double-step laser scanning method is proposed as a reliable high-yield TPS process to reduce the delamination effect. The method entails a simple process that polymerizes the initial interfacial layer between the structures and substrate twice. The additional laser scanning step induces a strong crosslinking in the polymer, and thereby preventing swelling and delamination. Furthermore, the effectiveness of the method is evaluated with large-scale microlens arrays. Although the process time is slightly increased, the process yield is significantly improved through the use of the double-step laser scanning method. Hence, the double-step laser scanning process can be employed as a reliable additive manufacturing process for various 3D microscale devices, and more importantly, it can be used to increase the manufacturing yield through the reduction of defects caused by surface delamination.

Archival plans, alterations, and 3D laser scanning of Erik Gunnar Asplund’s Stockholm Public Library

Erik Gunnar Asplund’s Stockholm Public Library is considered an important building in the history of twentieth-century architecture, yet relatively few archival plans have been saved from the library’s extended design process from 1919–25 and its subsequent construction until opening in 1928. This work presents the first systematic review of the available, digitised archival plans from Sweden’s National Centre for Architecture and Design (ArkDes), and explains Asplund’s gradual design and development of the project. Subsequent alterations in the library are briefly summarised, followed by the results from an extensive 3D laser scanning process throughout and around the building as it undergoes a significant period of renovation and maintenance. The results from 3D laser scanning create the first comprehensive and detailed record of the building for supporting future research, teaching, and renovation work. This study emphasises the mutual benefits of combining historical and technological approaches, and conducting academic research in parallel with contemporary renovation projects of historical architecture.

Application of bridge information modelling using laser scanning for static and dynamic analysis with concrete damage plasticity

A new proposal has been applied using Bridge Information Modeling associated to Terrestrial Laser Scanning TLS and photogrammetry to obtain the digital model of an existing bridge. For the laser scanning process. Thus, it was also possible to identify the pathologies and damages in the concrete of the bridge. Furthermore, it was shown how geo-information with remote sensing using the station Trimble SX 10 can be applied in area of bridge projects with less time and lower cost, in relationship to a conventional survey Asbuilt with manual survey in field. One of the innovations of this paper was to obtain the digital model of the bridge by integration of TLS and photogrammetry to obtain the 3D digital model. The new methodology consisted of acquisition of the bridge scenes through points previously implanted, making the polygonal contour of the entire bridge. Furthermore, the execution of the field survey from georeferenced points to capture the scenes during the sweep with TLS facilitated the refinement and, consequently, it was possible to obtain the digital model with the concrete damage. It was also possible to perform a complex nonlinear analysis with Finite Element with the digital model of the bridge. Therefore, it was applied a reverse analysis of the strength of the structure using the Concrete Damage Plasticity Model using ABAQUS software. Two numerical analyses were performed on the bridge; one considering only the static behavior of the bridge and another considering the dynamic behavior. In the static analysis, the behavior of the dapped-end beam of the bridge, was investigated. On the other hand, in the dynamic evaluation, the natural frequency and damping ratio for bridges with Dapped-end beams was obtained in ABAQUS through Rayleigh damping and compared with the technical literature, which presented satisfactory results.

Effects of shell scanning and build orientation on dynamic properties of laser powder bed fusion AlSi10Mg alloy

Additively manufactured (AM) products use two scanning strategies to create the central core region and the inner region near the free-surface circumference i.e. the shell. Manufacturers usually machine the shell to meet standard mechanical property testing. However, it's essential to determine how the shell affects an AM product's mechanical properties.This study focused on the dynamic tension and compression mechanical properties of an AlSi10Mg alloy made using the laser powder bed fusion technique. Machined and non-machined samples with and without shells of different build sizes and orientations underwent testing using split Hopkinson bar systems.Machined products displayed anisotropic dynamic behavior, favoring vertically built samples for maximum dynamic stress and horizontally built samples for elongation. However, samples with a shell showed apparent isotropic dynamic behavior due to pore distribution in the shell regime. The high porosity in the shell region resulted from the high intensity of heating energy input during scanning. Differences in shell shape between Z (load parallel to the direction of the building) and XY (load tangential to the direction of the building) samples and a slower, repeated laser scanning process in the shell region contributed to differences in porosity distribution between orientations. High porosity negatively impacted mechanical behavior and reduced dynamic mechanical properties.

Multi-approach integration of ALS and TLS point clouds for a 3-D building modeling at LoD3

Registering, documenting, updating, revitalizing, expanding, and renovating old urban buildings require proper documentation. The adoption of 3D survey techniques is essential to grant efficiency and agility to such purposes. This article discusses a multi-approach integration of Light Detection and Ranging (LiDAR) data collected by aerial and terrestrial platforms, meant for the 3D modeling of a building at Level of Detail 3. The selected building presents challenging elements for modeling, such as blocks with different heights and indented facades. It is located on the campus of the Federal University of Paraná (UFPR) in Curitiba, Brazil, on a site with irregular terrain and surrounded by trees, what made the terrestrial laser scanning process difficult. For its three-dimensional reconstruction, data from an Aerial Laser Scanning system were integrated with data from a Terrestrial Laser Scanner (TLS). Based on the 3D modeling, an as-is Building Information Modeling model of the building’s exterior was created. To validate the results, measurements of the building were obtained by means of an Electronic Distance Measurement (EDM) device and they were then compared with measurements extracted from the point cloud-based BIM model. The results demonstrate that there was a correspondence between the EDM and the LiDAR-derived measures, attaining a satisfactory statistical agreement. The article focuses on the accuracy of LiDAR models for the cadastral update of buildings, providing information for decision making in documentation projects and construction interventions. The main contribution of this work consists in a multi-approach workflow for delivering an effective and precise solution for accomplishing an as-is BIM documentation, highlighting advantages, drawbacks, and the potential of this set of methods for integrating multi-source LiDAR point clouds.

Structural and electrical properties of Pt film fabricated by laser scanning and reduction of its oxide film

A thin platinum (Pt) oxide film was deposited on a polypropylene substrate, and a part of the film was removed and reduced by a laser scanning process using a commercially available apparatus. A Pt film that is electrically isolated from the surroundings was successfully obtained, and the potential of using it as a thermometer was demonstrated by measuring its resistance as a function of temperature in the vicinity of RT. The results indicate that a device structure (thermometer) can be prepared using a Pt oxide film, which is applicable when using a flexible substrate.

Mechanical and Surface Geometric Properties of Reinforcing Bars and Their Significance for the Development of Near-Surface Notch Stresses

Due to the production process, reinforcing steel bars possess an inhomogeneous microstructure associated with different material properties over the cross-section (e.g., hardness, ductility or strength). Furthermore, the surface required for the bond has a negative effect on the fatigue behavior. The first investigations were carried out in the 1970s and detected the fillet radius r as a key influencing factor. Until now, few studies had been carried out that investigate the quantification of the surface properties on the fatigue behavior, and none of them compared these properties with the local strengths of the material. The current paper presents the first results of a reverse-engineered reinforcing steel bar based on a previously performed laser scanning process. The rebar models were used to calculate the notch stress factors for different diameters based on von Mises stresses taken from FEM simulations. The notch stress factors showed a functional relationship with the fillet radius, which was already shown in the literature. Further experimental investigations on the fatigue and tensile behavior of the structural components in the investigated Tempcore® rebars were carried out on microstructure specimens eroded by WEDM. The results of the tensile tests were used to derive a yield and tensile strength distribution in the cross-section. Depending on the microstructure, a yield strength between 415 N/mm2 (ferrite/pearlite core) and 690 N/mm2 (tempered martensite surface) was found. The acting notch stresses show a logarithmic dependency of the fillet radius, but do not reach the material strength of the surface.

Numerical Simulation in the Melt Pool Evolution of Laser Powder Bed Fusion Process for Ti6Al4V.

In order to track the free interface of the melt pool and understand the evolution of the melt pool, the flow of fluid, and the interface behavior of gas and liquid, a physical model is developed by using the VOF method in this paper. Its characteristics are a combined heat source model, including a parabolic rotation and a cylindrical distribution, and a powder bed stochastic distributed model with powder particle size. The unit interface between the metallic and gas phase in the laser-powder interaction zone can only be loaded by the heat source. Only the first and second laser scanning tracks are simulated to reduce the calculation time. The simulation results show that process parameters such as laser power and scanning speed have significant effects on the fluid flow and surface morphology in the melt pool, which are in good agreement with the experimental results. Compared with the first track, the second track has larger melt pool geometry, higher melt temperature, and faster fluid flow. The melt flows intensely at the initial position due to the high flow rate in the limited melt space. Because there is enough space for the metal flow, the second track can obtain smooth surface morphology more easily compared to the first track. The melt pool temperature at the laser beam center fluctuates during the laser scanning process. This depends on the effects of the interaction between heat conduction or heat accumulation or the interaction between heat accumulation and violent fluid flow. The temperature distribution and fluid flow in the melt pool benefit the analysis and understanding of the evolution mechanism of the melt pool geometry and surface topography and further allow regulation of the L-PBF process of Ti6Al4V.

In Situ Alloying of Fe-Cr-Co Permanent Magnet by Selective Laser Melting of Elemental Iron, Chromium and Cobalt Mixed Powders

Fe-25Cr-15Co (wt.%) permanent magnets were fabricated via selective laser melting (SLM) and in situ alloying from a blend of Fe, Cr and Co elemental powders. Under the optimal laser scanning process, the as-built Fe-25Cr-15Co alloy has a homogeneous composition distribution without defects such as holes or un-melted particles, and presents a single α phase with the bcc crystal structure. The density of as-built samples was 7.705 g/cm3 (the relative density is 99.32%). The preferred magnetic properties of the sample in the isotropic state were obtained as Hc = 22.84 kA/m, Br = 0.86 T and (BH)max = 7.98 kJ/m3. The hardness and yield strength of Fe-25Cr-15Co permanent magnets are above 331.5 HV and 800 MPa, respectively. The results of this study verified the feasibility of fabricating Fe-Cr-Co permanent magnets by SLM in situ alloying and can be extended to a wide range of applications that require complex shapes with variable magnetic circuit characteristics or gradient structures.

Application of Deep Learning Approach for the Classification of Buildings’ Degradation State in a BIM Methodology

Currently, there is extensive research focused on automatic strategies for the segmentation and classification of 3D point clouds, which can accelerate the study of a landmark and integrate it with heterogeneous data and attributes, useful to facilitate the digital management of architectural heritage data. In this work, an automated image-based survey has been exploited a Region- Based Convolutional Neural Network. The training phase has been executed providing examples of images with the anomalies to be detected. At the same time, a laser scanning process was conducted to obtain a point cloud, which acts as a reference for the BIM process. In a final step, a process of projecting information from the images onto the BIM recreates the pathology shapes on the model’s objects, which generates a decision support system for the built environment. The innovation of this research concerns the development of a workflow in which it is possible to automatize the recognition and classification of defects in historical buildings, to finally interpolate this geometric and numerical information with a BIM methodology, obtaining a representation and quantification of the information adapted to the facility management process. The use of innovative techniques such as artificial intelligence algorithms and different plug-ins becomes the main strength of this project.

Resonance colors in an amorphous-Si thin layer irradiated by an ultraviolet nanosecond laser

The optical resonances of nanoparticles have drawn considerable attention in academia and industry; however, the formation of uniform nanoparticles has been challenging, hindering their application. Herein, we propose that reflective-type color filters can be achieved using the optical resonance effect arising from Si nanoparticles created on an amorphous Si thin film via laser irradiation. Multiple equally spaced irradiations of an amorphous-Si surface by a Gaussian laser beam created Si nanoparticles with sizes of 100–200 nm at laser fluences in the range of 150–200 mJ/cm2. Furthermore, we demonstrated that the resonance colors could be tuned to red, green, and blue by adjusting the laser fluence and scan speed. The optical properties of the nanoparticles were analyzed using dark-field microscopy, scanning electron microscopy, and transmission electron microscopy. According to the measured nanoparticle geometries and spectral data, the Si nanoparticles are spheroidal shapes embedded in the Si layer, and simulations indicate that the resonance colors originate from electric and magnetic dipoles inside the Si nanoparticles. The results could inform the fabrication of color filters in reflective display device or sensor applications through a simple laser scanning process.

Self‐Aligned Laser‐Induced Periodic Surface Structures for Large‐Area Controllable Nanopatterning

AbstractLaser‐induced periodic surface structures (LIPSS) have become an important avenue towards surface nanopatterning and certain device applications due to their subwavelength feature size and versatility with different materials. However, the uncontrollable non‐uniformity in achievable nanostructures presents a limit towards its practical application. Here, a robust approach is proposed to obtain controllable nanostructures with long‐range order based on a new insight into one of the electromagnetic origins of nonuniformity of LIPSS, namely a half‐periodic mismatched optical enhancement (h‐MOE) effect. The h‐MOE effect originates from the interference enhancement of surface plasmon polaritons (SPPs) located between tips of every two adjacent ripples in laser scanning process. It is found that LIPSS can be self‐aligned and highly controllable if the h‐MOE effect is elaborately modulated through designed laser scanning strategies especially sequential scanning paths. The new laser nanopatterning approach based on h‐MOE demonstrates controllable patterning ability to theoretically infinitely large‐area super‐straight gratings, orientation‐controllable gratings, and half‐periodic mismatched nanohole arrays.

Experimental Study on Laser-Induced Surface Damage of a Single-Crystal Nickel-Based Superalloy Under Continuous Wave Fiber Laser Scanning Process

Abstract Single-crystal (SC) nickel-based superalloy castings offer high-temperature mechanical properties that result in superior gas turbine engine performance and durability. These castings undergo various precision machining operations to remove a significant amount of material while manufacturing. Here, nickel-based superalloys are one of the most difficult materials to be cut. Therefore, novel concepts are being employed to improve their machinability including lowering their surface strength. This paper presents the introduction of laser-induced surface damage (LISD) on a second-generation SC nickel-based superalloy using a continuous wave (CW) fiber laser. Laser scanning experiments were performed on SC specimens in the as-cast condition with a laser power of 1000 W, a beam diameter of 1.2 mm, and scanning speeds from 5.5 mm/s to 16.5 mm/s. The cross-sections of the laser-irradiated surfaces were investigated by measuring the irradiated geometries (IRG), microstructural changes, microsegregations, solidification cracking, and heat affected zone (HAZ). The IRG shows the conduction mode of penetration with a high width-to-depth ratio under a bigger beam diameter and top-hat type beam profile. The IRG boundaries have irregular profiles due to the dissolution of interdendrite regions and eutectic phases. The IRG showed fine dendrites and solidification cracks with reduced microsegregation levels. The solidification cracking is mainly attributed to thermal stresses and the microcracking in HAZ is attributed to the dissolution of low melting Mo and Ti eutectics. The evolved HAZ ranges from 15% to 20% of the IRG depth. The LISD volume is evaluated as IRG plus HAZ for removal by machining process.

Laser-induced microjet-assisted ablation for high-quality microfabrication

Liquid-assisted laser ablation has the advantage of relieving thermal effects of common laser ablation processes, whereas the light scattering and shielding effects by laser-induced cavitation bubbles, suspended debris, and turbulent liquid flow generally deteriorate laser beam transmission stability, leading to low energy efficiency and poor surface quality. Here, we report that a continuous and directional high-speed microjet will form in the laser ablation zone if laser-induced primary cavitation bubbles asymmetrically collapse sequentially near the air-liquid interface under a critical thin liquid layer. The laser-induced microjet can instantaneously and directionally remove secondary bubbles and ablation debris around the laser ablation region, and thus a very stable material removal process can be obtained. The shadowgraphs of high-speed camera reveal that the average speed of laser-induced continuous microjet can be as high as 1.1 m s−1 in its initial 500 μm displacement. The coupling effect of laser ablation, mechanical impact along with the collapse of cavitation bubbles and flushing of high-speed microjet helps achieve a high material removal rate and significantly improved surface quality. We name this uncovered liquid-assisted laser ablation process as laser-induced microjet-assisted ablation (LIMJAA) based on its unique characteristics. High-quality microgrooves with a large depth-to-width ratio of 5.2 are obtained by LIMJAA with a single-pass laser scanning process in our experiments. LIMJAA is capable of machining various types of difficult-to-process materials with high-quality arrays of micro-channels, square and circle microscale through-holes. The results and disclosed mechanisms in our work provide a deep understanding of the role of laser-induced microjet in improving the processing quality of liquid-assisted laser micromachining.

Laser-Sculptured Hierarchical Spinous Structures for Ultra-High-Sensitivity Iontronic Sensors with a Broad Operation Range.

Tactile pressure sensing over a wide operation range (>1 MPa) is challenging for a variety of applications in fields such as aviation, oceanography, and biomedicine. Recently, innovative strategies have been utilized to improve the performances of tactile sensors using specially designed structures, dielectric layers, and electrodes. Here, a hierarchical structural design based on ionic gel films has been utilized to build iontronic pressure sensors with ultrahigh sensitivities and broad operation ranges. Sculptured patterns made by a controlled CO2 laser scanning process have been produced on polyimide films to achieve two kinds of protrusion structures for high specific surface areas and strength to withstand high pressure. The iontronic sensor has been constructed by adding two screen-printed electrodes of high surface areas to achieve an ultrahigh sensitivity of 2593 kPa-1 and a wide pressure range from 0 Pa to 3.36 MPa. The prototype device also has a fast response and recovery time of 26 and 13 ms, respectively, and an excellent mechanical durability in the endurance test of over 2700 repeated loading and unloading cycles under a pressure of 1 MPa. Several application examples have been demonstrated, including the detection of physiological signals on human volunteers, the feedback control of intelligent robots, the grasping operation of underwater soft grippers, and the environmental wind-speed monitoring. As such, this work demonstrates a versatile and economical methodology to produce high-performance flexible sensors for various potential applications.