Laser Scanning Process Research Articles

Experimental and computational study on thermal and fluid behaviours of powder layer during selective laser melting additive manufacturing

The movability of powder layer such as spattering and denudation is a major cause of defect formation and quality uncertainty in selective laser melting (SLM) additive manufacturing. Currently, the detailed thermal and fluid behaviours of the powder layer during laser scan process are still not fully understood. In this work, we employ a combination of high-speed imaging and multi-physics modelling to elucidate the thermal and fluid behaviours of the powder layer during the laser scanning process. The particle spattering speed, particle trajectory, and denudation zone of the powder layer are quantified. The sensitivity of particles’ thermo-fluid behaviours to the laser power intensity is also investigated. The multi-physics simulation is used to reproduce the scanning process of the powder layer with various laser power intensities. Detailed physics that result in the observed powder layer motion, such as hot metal vapour flow, ambient gas flow, drag force and heat flux on particles are investigated. This provides deeper insights into the thermal fluid dynamics of the powder layers. Potential ways to reduce the spattering and denudation of the powder layer during SLM process are discussed.

Effect of scan speed on grain and microstructural morphology for laser additive manufacturing of 304 stainless steel

Abstract The laser scanning parameters used in laser additive manufacturing (LAM) can impact the growth direction of the columnar grains produced during rapid solidification. This growth direction affects the mechanical properties of the manufactured parts after cladding. The effective use of a high laser power and a rapid scanning speed for LAM requires an accurate analysis of the relationships between the laser scanning process parameters and the grain growth direction and microstructural morphology of the scanned material. An experimental study was conducted to determine the macromorphology, microstructural morphology, and grain growth direction of 304 stainless steel material obtained during the laser scanning process at different scan speeds. The impact of the scan speed on different regions in the cladding layer (the clad zone (CZ), the heat affected zone (HAZ), and the dilution zone (DZ)) was determined, as well as on the direction of grain growth, the grain morphology and the grain size (which are the microstructures of the cladded materials), the degree of elemental mixing during laser scanning (which are changes in material composition of cladded material), and the microscopic hardness of the CZ (which is one of the mechanical properties of cladded material). With increasing scan speed, the CZ, HAZ, DZ, and dilution rate (DR) of the material gradually decreased, and grain growth gradually oriented towards the building direction of the cladding layer. At a 16 mm·s−1 scan speed, the angle between the grain growth direction and the scan direction was 84°. Changing the scan speed from 4 to 16 mm·s−1 caused the columnar grain size to gradually decrease from 13.3 to 9.2 μm and the corresponding microhardness to gradually increase.

Damage visualization using laser-generated residual guided waves with optimization of laser scanning path

Laser ultrasonic scanning is intensively used in guided wave-based non-destructive testing due to its non-contact scanning, automated scanning path and full wavefield measurement. However, in practical applications, the selected repetition rate of laser generation is limited due to the existence of residual guided waves generated by consecutive pulsed laser impingements, leading to a low scanning speed. To overcome such limitations, the formation of residual wavefield is investigated and an optimization method of laser scanning path is developed to improve the damage visualization using the residual guided waves. First, the energy partition of residual wavefield is investigated based on diffuse wavefield approximation and transient statistical energy analysis. Then, a virtual superposition method is proposed to obtain the synthetic wavefields with different laser repetition rates. The influence of laser repetition rate on the wavefield directivity and energy distribution is analyzed. Moreover, according to the ultrasonic attenuation property of the test specimen, an optimized sub-block raster scanning path is proposed to minimize the spatial misalignment of the energy distribution in residual wavefield. Experiments are conducted on an aluminum alloy plate with multiple artificial notches using a 1-kHz-repetition-rate scanning laser ultrasonic system. Based on the residual wavefield signal energy, damage visualization results before and after laser scanning path optimization are compared. Results show that the proposed method is not only more efficient during the laser scanning process, but also more capable of distinguishing damaged areas from the intact structure.

Ultrafast femtosecond laser micro-marking of single-crystal natural diamond by two-lens focusing system

The inscription of unique logo and security marking on diamonds and gemstones is in high demand by worldwide manufacturers and businesses for anti-counterfeiting purposes and traceability. Short pulsed lasers enable marking of transparent materials, challenge remains to produce digital security micro-features on thin facets of the natural diamond in a non-intrusive manner. We propose the design and demonstration of a novel two-lens focusing system to inscribe diamond at microscale and high throughput by an ultrafast laser scanning process. A threshold laser fluence of ∼2.5 J/cm2 and a scan speed of 2 mm/s realized writing of high-contrast data matrix code and serial number without inducing defects and cracking in the diamond. Characterization revealed smooth ablation depth profile with distinct laser-induced periodic surface structures (LIPSS) in the laser inscribed regions determined through scanning electron microscopy (SEM) and 3D optical microscopy. Raman spectroscopy revealed diamond cubic structure dominating with mixing of graphitic structure in the laser markings at various scanning speeds. Comparing with the 0.05 NA f-theta lens, the two-lens focusing system offered 7x improvement in the marking resolution (3 μm in line-width) in addition to its simplicity and add-on flexibility to industrial laser marking systems.

Laser ultrasonic imaging of wavefield spatial gradients for damage detection

This article describes a new damage visualization method to investigate and analyze propagating guided Lamb waves using analyses of wavefield spatial gradients. A laser ultrasonic interrogation system was used to create full-field ultrasonic data measurements for ultrasonic wavefield imaging. The laser scanning process was performed based on both a raster scan and a circle scan. From the high-resolution wavefield data, a spatial gradient–based image processing technique was developed using gradient vectors to extract features sensitive to defects. Local impedance changes at the damaged area would result in a local distortion of the waveform which was captured and quantified by the variation of the gradient vectors in the scanning area as time evolves. Such variation was accumulated over time with a statistical threshold filter to generate a gradient-orientation map for damage visualization. The proposed algorithm was capable of producing distinctive damage patterns when tested experimentally on a 3-mm aluminum plate with multiple simultaneous simulated defects. Compared to conventional techniques like local wavenumber estimation, the generation of the accumulated orientation map involves no filtering process in the frequency or wavenumber domain, at the expense of more accurate shaping of the defect. A spatial covariance analysis was adopted to locate damage from the results as well as to evaluate the correlation between different kinds of defects. Combining the proposed approach with conventional laser ultrasonic imaging techniques enables a fast and robust damage identification and characterization process which requires lower computational burden and practical operation.

Dynamic Motion Simulation: Applications in Forensic Engineering

A worker was injured when a large sculpture overturned while it was being transferred on a wheeled cart from a delivery truck onto a dock lift. This paper introduces the use of dynamic motion simulation (DMS) soft-ware as a forensic engineering tool for analyzing and simulating motion/contact between multiple interacting physical objects. Important inputs to the software include the mass properties of the objects — in this case, a very irregularly shaped sculpture. For simple shapes, the distribution of mass can easily be approximated by manually discretizing the object into several smaller, simpler shapes. Accurate determination of the mass dis-tribution of an irregular shape (such as a sculpture) can be aided by measurement methods such as the laser scanning process used in this case. The resulting scan data was used to create a 3-D computer model that was processed using conventional mechanical computer-aided design (CAD) and DMS software to determine the mass properties and ultimately to simulate the dynamic motion.

Recreating Historical Malay Architecture with BIM Process

In South-East Asia, the Malay architecture is among the popular subject of research because of its historical importance within the region, apart from many others. To some researchers, the Malay architecture is unique because of its intangible meaning and historically rich design characters. It is difficult to be reproduced, and only limited numbers of people are acknowledged as experts. With the introduction of technology such as BIM, it is hypothesized that the gap can be minimized. The idea of this paper is to outline the process of recreating cultural architectural design using a modern process such as Building Information Modelling (BIM) platform in specific, from data collection using Terrestrial Laser Scanning (TLS) and digitalization process in Revit software. This paper employs observation approach using data from laser scanner collected from case study and content analysis technique. While normally most of the cultural architecture is undocumented, the findings of this activity are aimed to provide guideline to develop geometrical information for heritage-enthusiast in practicing their undertakings. It is hoped that more historical and cultural architecture can be recreated and appreciated for the use and inspiration of current construction industry

Embedding sensors using selective laser melting for self-cognitive metal parts

We devised a novel method to embed sensors or integrated circuit (IC) chips into metal components by using a selective laser melting (SLM) process. The concept of a protective layer is introduced to fabricate all parts without damaging the sensors during the laser scanning process. The operation of sensors in the parts is analyzed from a computational analysis on the thermal influence of laser heat. The fabricated metal parts show continuous microstructures including grains and phases between the base part and the new part formed after embedding the sensor despite the intermittent SLM process. The embedded sensor operates properly when compared to bare sensors. Plastic circuit board-based IC components were embedded into an Inconel 718C turbine blade, which accurately distinguished three-dimensional vibration along the X, Y, and Z axes. Our results imply that the proposed process can open new avenues for SLM technology to realize metal components with a self-cognitive ability using integrated sensors.

Femtosecond laser-induced large area of periodic structures on chalcogenide glass via twice laser direct-writing scanning process

We presented a theoretical and experimental study for the evolution of femtosecond laser direct-writing induced large area of periodic surface structures on As2S3 glass. The laser direct-writing induced large area of low spatial frequency LIPSS (LSFL) structures can be fabricated with the overlap rate increases from 65% to 80%. The evolution process can be defined by the competition between the laser fluence and pulse overlap rate. What is more important is that, based on secondary femtosecond laser scanning process, the interaction between the initial LSFL structures induced by the first scanning and the laser pulse of the second scanning process can induce local field enhancement effect, which can directly split the surface periodic structures of LSFL. Such new subsequent high spatial frequency LIPSS (HSFL) fabrication process is quite different from the traditional multi-pulse induced HSFL and can be used to fabricate large-area HSFL structures.

Ultrafast multi-focus 3-D nano-fabrication based on two-photon polymerization

Two-photon polymerization (TPP) is the most precise 3-D printing process that has been used to create many complex structures for advanced photonic and nanoscale applications. However, to date the technology still remains a laboratory tool due to its high operation cost and limited fabrication rate, i.e., serial laser scanning process. Here we present a revolutionary laser nanofabrication process based on TPP and an ultrafast random-access digital micromirror device (DMD) scanner. By exploiting binary holography, the DMD scanner can simultaneously generate and individually control one to tens of laser foci for parallel nanofabrication at 22.7 kHz. Complex 3-D trusses and woodpile structures have been fabricated via single or multi-focus processes, showing a resolution of ~500 nm. The nanofabrication system may be used for largescale nano-prototyping or creation of complex structures, e.g., overhanging structures, that cannot be easily fabricated via conventional raster-scanning-based systems, bringing significant impact to the world of nanomanufacturing.

THE FACADE’S DOME OF THE ST. ANTHONY’S BASILICA IN PADUA

Abstract. The basilica of St. Anthony in Padua (13th–14th cent.) is one of the most remarkable pilgrimage sites in Italy. To date, the monument itself has never been subject to a comprehensive stratigraphic analysis. Important information about the construction sequence of the building may be conserved in the domed roofs protecting the inner masonry shells.The present paper will focus on the dome next to the facade. During the survey, data acquisition via laser scanner have been flanked by standard tasks. Specifically, the stratification analysis of the timber framework of the dome requires to measure the entire structure, including parts with difficult access, and calls for many scan bases to go further the sight obstacles represented by the rafters and the horizontal collar-beams. Therefore, application of laser scanning might appear difficult at first sight.The authors will show that the approach confirms the suitability of the laser scanner technology in facing the general complexity of the structure. The development of a graphic documentation in CAD environment entailed a manageable complexity in terms of time-consumption and precision in data processing. So far, the plans reveal the irregular profile of the dome in its inner masonry shell, and of the outer masonry drum. The sections show a two-centre curvature of the elevation of the outer timber shell. However, the joints among the rafters, ribs, and tie-beams still require a series of traditional in-depth assessments acquired in close-range access.Nevertheless, the pragmatic investigative modus operandi, tested up to now, does represent a fixed protocol suitable to be iterated and perfected for each cupola. In such complex structures, the laser scanning process confirms to be a valid strategy to reach a good compromise between time consumption, human effort, and millimetre precision. In this way, the collected material provides a first contribution to acquire knowledge on this Italian medieval masterpiece, which stands out on the international scenario for its historical richness and architectural complexity.

A Case Study on the Restoration of A Historical Masonry Building Based on Field Studies and Laboratory Analyses

ABSTRACT In historical masonry buildings, the durability of materials used for the structural members can be impaired by climatic conditions and user interventions, and the building may lose its authenticity as a result of functional changes made over time. To restore structures to their original form, the historical changes that have been made over time are researched, and by identifying the original material characteristics it becomes possible to preserve the load-bearing members, thus prolonging their service life. This paper presents a detailed description of the restoration works, which were based on the original geometric and material properties, carried out on a historical masonry building that started out in life as a hospital building in the 1840s, but was later remodeled to become a military prison building and currently serves as the Faculty of Political Sciences of Istanbul University. The study is presented in three stages, namely: i. a research of the historical timeline of alterations of the examined building, ii. a field study, and iii. laboratory analyses. The historical research identifies the structural changes that the building has undergone since its construction, while the field study includes a three-dimensional laser scanning process and the taking of representative samples from different points of the building. In the laboratory stage, the gathered samples are subjected to chemical and mineralogical analyses. The mortar and plaster mixture ratios used in the construction were ascertained through physical and chemical tests to samples taken from the building, and subsequently, all non-original binder materials have been removed and the original mortar-plaster mixture ratios have applied. After a review of the data collected within the scope of this study, it was determined that the two reinforced concrete buildings to the north and south of the site, which were built later in the history of the investigated building, as well as the temporary partition walls to each story, should be removed. Furthermore, it is recommended that the modified facade and interior window forms, together with the blocked-off doorways, should be restored to their original state. The contribution this study makes to literature is based on its focus on a historical masonry building that has lost its original form over time, having been refunctioned for different purposes, that is to be restored back to its original form in terms of its geometric and material properties. The building itself is of particular interest, being the first on the historical peninsula of Istanbul to be constructed using industrial bricks during the Ottoman period. The proposed restoration project can be considered a novel, practical and appropriate approach to authentic restoration.

Novel laser-based metasurface fabrication process for transparent conducting surfaces

Transparent conducting film provides key functions for various optoelectronic devices. Existing manufacturing processes of a transparent conducting film are usually very costly in terms of materials or processing time. The goal of this research is to develop a new surface engineering method for low-cost and high-throughput fabrication of large-size, transparent conducting glass windows. A novel laser-based metasurface fabrication process is presented in this work, which comprises two steps: (1) evaporating the glass substrate by an ultrathin metal film with a thickness on the order of 10 nm and (2) laser patterning the coated surface using a nanosecond pulsed laser (1064 nm wavelength) with a typical feature size of hundreds of micrometers. During the second step of the laser scanning process using an appropriate pulse energy density, the metal film absorbs most of the laser pulse energy and is patterned through laser material ablation, while little damage will be induced on the substrate since its absorptivity at the laser wavelength is low. Experimental results have shown that a transparent conducting film with an average visible transmittance of ∼67% and a sheet resistance of ∼20 Ω/sq can be successfully fabricated. Compared with the other existing methods, this novel laser surface patterning process significantly improves the processing efficiency and reduces the production cost that renders practical treatment of glass materials or transparent ceramics to produce transparent conducting surfaces.

Edge Effect Investigation of DP980 Steel Sheet in Multiple Laser Scanning Process

Due to free spring back, laser forming is an effective non-contact forming technique for hard forming materials. A desired shape is obtained by residual plastic strain which is induced by thermal stresses depending on multiple laser scanning. In this study, the combination of experimental and numerical analysis is employed to investigate the temperature distribution and bending angle of the dual phase (DP980) high strength steel sheet. The edge effect of six cases of laser scanning process involving 1, 2, 3, 4, 5 and 10-time laser scanning are investigated by analyzing their bending angle relative variations and transverse plastic strains. It is found that the edge effect at the middle part of laser scanning line reduces with the increasing number of laser scanning cycles. It is attributed to the fact that the relative variation of the residual plastic strain reduces with the increase of laser scanning cycles. To efficiently reduce the edge effect of DP980 steel sheet, a locally repeated scanning strategy is proposed. The numerical simulation results indicate that the edge effect reduces significantly in the 10-time laser scanning process and the average bending angle simultaneously increases around 20% under the new laser scanning strategy.

Lamellar-structured Ni-silicide film formed by eutectic solidification

Abstract Pt-doped NiSi‒NiSi2 thin films in a uniform lamellar structure with a periodicity on the scale of a few tens of nanometers were formed on Si(001) substrates using a continuous laser scanning process. When the Pt-doped NiSi film was melted at high temperatures and was supercooled at high solidification rates (a high scanning speed of over 200 mm/s), a NiSi‒NiSi2 lamellar structure evolved while interacting with the underlying Si substrate and following the classical eutectic solidification path. The lamellar spacing could be easily controlled by the laser scanning speed. In addition, the periodically formed, nearly single-crystalline NiSi and NiSi2 phases exhibited epitaxial relationships with each other and also with the Si(001) substrate. It is believed that this novel NiSi‒NiSi2 lamellar structure can be used as a template for application areas requiring an electrode with a line/space pattern on the scale of a few tens of nanometers that can be prepared without using costly photolithographic processes.

3D laser scanning in civil engineering - measurements of volume of earth masses

Considering the constant drive to improve and accelerate building processes as well as possible applications of the latest technological achievements in civil engineering practice, the author has proposed to use 3D laser scanning in the construction industry. For example, data achieved through a 3D laser scanning process will facilitate making inventories of parameters of buildings in a very short time, will enable one to check irregularly shaped masses of earth, heavy and practically impossible to calculate precisely using traditional techniques. The other part of the research, performed in the laboratory, consisted of measurements of a model mound of earth. All the measurements were made with a 3D SkanStation C10 laser scanner manufactured by Leica. The data were analyzed. The results suggest that there are great opportunities for using the laser scanning technology in civil engineering

Close-range laser scanning in forests: towards physically based semantics across scales.

Laser scanning with its unique measurement concept holds the potential to revolutionize the way we assess and quantify three-dimensional vegetation structure. Modern laser systems used at close range, be it on terrestrial, mobile or unmanned aerial platforms, provide dense and accurate three-dimensional data whose information just waits to be harvested. However, the transformation of such data to information is not as straightforward as for airborne and space-borne approaches, where typically empirical models are built using ground truth of target variables. Simpler variables, such as diameter at breast height, can be readily derived and validated. More complex variables, e.g. leaf area index, need a thorough understanding and consideration of the physical particularities of the measurement process and semantic labelling of the point cloud. Quantified structural models provide a framework for such labelling by deriving stem and branch architecture, a basis for many of the more complex structural variables. The physical information of the laser scanning process is still underused and we show how it could play a vital role in conjunction with three-dimensional radiative transfer models to shape the information retrieval methods of the future. Using such a combined forward and physically based approach will make methods robust and transferable. In addition, it avoids replacing observer bias from field inventories with instrument bias from different laser instruments. Still, an intensive dialogue with the users of the derived information is mandatory to potentially re-design structural concepts and variables so that they profit most of the rich data that close-range laser scanning provides.

Femtosecond pulsed laser textured titanium surfaces with stable superhydrophilicity and superhydrophobicity

A facile and highly-efficient laser scanning process coupled with a simple silanization modification was used to prepare textured titanium (Ti) surfaces with stable superhydrophilicity and superhydrophobicity. Femtosecond pulsed laser scanning along two mutually perpendicular directions led to the formation of binary structures featuring micrometer-scale spikes covered with nanometer-scale ripples. The period of the spikes significantly increased and the period of the ripples irregularly changed in the narrow range of 550–600nm with the increase of laser fluence. The obtained laser-textured Ti surfaces were hydrophilic or even superhydrophilic, and the superhydrophilic laser-textured Ti surface using a laser fluence of 1.5J/cm2 was observed to retain its wetting property after 30days of storage in ambient atmosphere. After silanization, all the laser-textured Ti surfaces exhibited high hydrophobicity or superhydrophobicity, and the superhydrophobic laser-textured Ti surfaces using laser fluences of 1.5 and 1.8J/cm2 remained stable when stored in air for over 30days. The results imply the potential applications of these surfaces in a variety of fields.

Thinking Outside of the Box: The Potential of 3D Printing in Veterinary Medicine

The purpose of this project was to gain a better perspective of how 3D printing technology works by getting a first-hand look at this process and then strategize ways that this technology can potentially be used in veterinary medicine. A metal orthopedic plate and a humerus from a dog were obtained from the Veterinary Teaching Hospital at Tuskegee University College of Veterinary Medicine. A 3D Computer Aided Design (CAD) model of the metal orthopedic plate was made and printed using a plastic Polylactic Acid (PLA) filament. 3D CAD models of the metal plate and the humerus were also generated using a 3D Laser Scanning process. Using the CAD images from laser scanning, the metal plate was printed using aluminum material and the humerus was printed using nylon material. The 3D printed objects were very accurate in comparison to the original objects. In conclusion, this project demonstrated the value and potential in having the technology to duplicate anatomical structures and surgical instruments, which can lead to a variety of useful and unique applications in the academic and clinical realm of veterinary medicine.

CAD-Based Path Planning for 3D Laser Scanning of Complex Surface

A CAD-based approach to plan the laser scanning path of complex surfaces is presented. Firstly, in the laser scanning process the issues of the positional relationship between scanning direction and normal vectors of sampling points of the measured surface, sensor work area check and surface occlusion judgment are researched and a method for generating a single arbitrary surface scanning path is developed. Secondly, through the analysis of surface segmentation, scanning directions and displacements synthesis during the multi-surface scanning process, a scanning path planning method based on any complex multi-curved surface is established. Finally, through a simulation example, the feasibility of the method is verified.