Laser Scanning Pattern Research Articles

Synchronous 3D Tracking and Imaging at Multiple Scales to Overcome Spatiotemporal Disparity

Single particle tracking methods have been used to dissect the viral infection process step-by-step and elucidate new quantitative understanding of the interactions and dynamics between virus and host. The process begins when a virus diffusing rapidly through solution impacts and binds to the cell. The disparate spatial scales between the virus and a host cell make capturing these moments of transition between fast, 3D viral diffusion and cell-bound interaction an impossible imaging challenge that no single microscope is built to task for. Instead, tradeoffs are required between imaging the narrow field of view required to target-lock a solution-phase virus or imaging the larger cellular environment, temporally limited to confined viral dynamics inside cells after binding. In this work, we introduce the 3D Tracking and Imaging, (3D-TrIm) microscope which merges the best of both worlds; allowing for large-scale, 3D multiphoton volumes to be simultaneously acquired and co-localized with single-particle trajectories obtained via active feedback tracking capable of target-locking freely-diffusive virus-scale particles. This feat is accomplished by the integration of 3D-Dynamic Photon Localization Tracking (3D-DyPLoT), and the 3D Fast Acquisition Scan via z-Translating Raster (3D-FASTR) point-scan imaging pattern. Both microscopes utilize dynamic 3D laser scan patterns to survey two disparate fields of view, coupled through the same objective lens and piezoelectric stage which serves as the reference spatial coordinate system and allows point-scanned multiphoton voxels to be rapidly localized without motion smearing. The ability to quickly and simultaneously image surrounding 3D cellular regions during tracking can provide new insight into the nature of viral activities spanning the range of spatiotemporal regimes in the infection process. Such activities can yield new insight into the critical moments of the process missed by only having access to single spatiotemporal imaging regimes and modalities.

Numerical Evaluation of Advanced Laser Control Strategies Influence on Residual Stresses for Laser Powder Bed Fusion Systems

Process-dependent residual stresses are one of the main burdens to a widespread adoption of laser powder bed fusion technology in industry. Residual stresses are directly influenced by process parameters, such as laser path, laser power, and speed. In this work, the influence of various scan speed and laser power control strategies on residual stresses is investigated. A set of nine different laser scan patterns is printed by means of a selective laser melting process on a bare plate of nickel superalloy 625 (IN625). A finite element model is experimentally validated comparing the simulated melt pool areas with high-speed thermal camera in situ measurements. Finite element analysis is then used to evaluate residual stresses for the nine different laser scan control strategies, in order to identify the strategy which minimizes the residual stress magnitude. Numerical results show that a constant power density scan strategy appears the most effective to reduce residual stresses in the considered domain.

In-Process Fabrication of Carbon-Dispersed Aluminum Matrix Composite Using Selective Laser Melting

We have investigated the basic characteristics of C/Al composites prepared in-process via selective laser melting (SLM) using a mixed powder of pure aluminum and short carbon fiber. Initially, the relationship between the relative density of the SLM composites and laser scan conditions was systematically investigated. The SLM composites were densified by applying laser scan conditions with high input energy density (>100 J/mm3). The densified SLM composite showed excellent hardness together with low thermal conductivity, due to the generation of an Al4C3 phase and increased solid-solution carbon in the α-Al matrix via the reaction between aluminum and carbon during laser irradiation. This reaction could be inhibited in SLM composites fabricated from another mixed powder of copper-plated carbon fiber and pure aluminum powder since laser absorptivity significantly decreased due to the high reflectivity of the copper plate on the carbon fiber. By investigating the Cu plated C/Al SLM composites, we demonstrated that the thermal management material having anisotropic thermal conductivity could be fabricated by controlling the carbon dispersion by using a unidirectional laser scanning pattern.

Dimensional Errors Due to Overhanging Features in Laser Powder Bed Fusion Parts Made of Ti-6Al-4V

The rise in popularity of Additive Manufacturing technologies and their increased adoption for manufacturing have created a requirement for their fast development and maturity. However, there is still room for improvement when compared with conventional manufacturing in terms of the predictability, quality, and robustness. Statistical analysis has proven to be an excellent tool for developing process knowledge and optimizing different processes efficiently and effectively. This paper uses a novel method for printing overhanging features in Ti-6Al-4V metal parts, by varying process parameters only within the down-facing area, and establishes a methodology for predicting dimensional errors in flat 45° down-facing surfaces. Using the process parameters laser power, scan speed, scan spacing, scan pattern, and layer thickness, a quadratic regression equation is developed and tested. An Analysis of variance (ANOVA) analysis concluded that, within the down-facing area, the laser power is the most significant process parameter, followed by the layer thickness and scan speed. Comparatively, the scanning pattern is determined to be insignificant, which is explained by the small down-facing area where the various scanning patterns play no role. This paper also discusses the interaction effects between parameters. Some thoughts on the next steps to be taken for further validation are discussed.

LaryngoTORS: A Novel Cable-Driven Parallel Robotic System for Transoral Laser Phonosurgery

Transoral laser phonosurgery is a commonly used surgical procedure in which a laser beam is used to perform incision, ablation or photocoagulation of laryngeal tissues. Two techniques are commonly practiced: free beam and fiber delivery . For free beam delivery, a laser scanner is integrated into a surgical microscope to provide an accurate laser scanning pattern. This approach can only be used under direct line of sight, which may cause increased postoperative pain to the patient and injury, is uncomfortable for the surgeon during prolonged operations, the manipulability is poor and extensive training is required. In contrast, in the fiber delivery technique, a flexible fiber is used to transmit the laser beam and therefore does not require direct line of sight. However, this can only achieve manual level accuracy, repeatability and velocity, and does not allow for pattern scanning. Robotic systems have been developed to overcome the limitations of both techniques. However, these systems offer limited workspace and degrees-of-freedom (DoF), limiting their clinical applicability. This work presents the LaryngoTORS, a robotic system that aims at overcoming the limitations of the two techniques, by using a cable-driven parallel mechanism (CDPM) attached at the end of a curved laryngeal blade for controlling the end tip of the laser fiber. The system allows autonomous generation of scanning patterns or user-driven free-path scanning. Path scan validation demonstrated errors as low as 0.054 $\pm$ 0.028 mm and high repeatability of 0.027 $\pm$ 0.020 mm (6 × 2 mm arc line). Ex vivo tests on chicken tissue have been carried out. The results show the ability of the system to overcome limitations of current methods with high accuracy and repeatability using the superior fiber delivery approach.

Thermal field prediction for laser scanning paths in laser aided additive manufacturing by physics-based machine learning

Laser aided additive manufacturing (LAAM) is a key metal 3D printing and remanufacturing technology for fabrication of near-net shape parts. Studying thermal field induced by different scanning strategies is important to evaluate and optimize the resultant residual stress and distortion distribution. However, it is very computationally expensive to simulate multi-bead deposition process using existing numerical model to analyze and select appropriate laser scanning strategies. In this paper, we make use of a recently developed and experimentally validated efficient thermal field prediction numerical model for LAAM to generate training data for a physics-based machine learning algorithm. A combined Recurrent Neural Networks and Deep Neural Networks (RNN–DNN) model was developed to identify the correlation between laser scanning patterns and their corresponding thermal history distributions. Subsequently, the developed RNN–DNN model is able to make thermal field prediction for an arbitrary geometry with different scanning strategies. Comparison between the numerical simulation results and the RNN–DNN predictions showed good agreement of more than 95%.

Microstructure evolution and mechanical properties of laser metal deposition of Invar 36 alloy

Laser metal deposition (LMD) technology was used to fabricate single-track-multi-layers samples using Invar 36 alloy powder by two different deposition patterns: one-way laser scanning pattern and back and forth laser scanning pattern. The microstructure characteristics, grain morphology, formation mechanism and mechanical properties of the samples fabricated in two different deposition patterns were systematically investigated. It was shown that there was a thin layer of equiaxed grains in the top zone, an alternative arrayed grain morphology of columnar and cellular grains in the middle and periodic circular growth of small columnar grains at the bottom, which was generated by different temperature gradient in different location. The crystal orientation of columnar grains was distinct due to different deposition patterns. X-ray diffraction (XRD) and Energy Dispersive Spectrometer (EDS) showed that the deposited specimen was composed of austenite-single phase. Samples built by back and forth laser scanning pattern showed higher microhardness and tensile strength but lower elongation compared with samples built by one-way pattern, the horizontally built sample showed higher Ultimate tensile strength and yield strength but lower elongation compared with vertically built sample in the same laser scanning pattern. Laser metal deposited Invar 36 showed more excellent mechanical properties compared to the forged and casted counterparts.

Experimental Investigation of Laser Surface Hardening of AISI 4340 Steel Using Different Laser Scanning Patterns

Laser surface transformation hardening becomes one of the most modern processes used to improve fatigue and wear properties of steel surfaces. In this process, the material properties and the heating parameters are the factors that present the most significant effects on the hardened surface attributes. The control of these factors using predictive modeling approaches to achieve desired surface properties leads to conclusive results. However, when the dimensions of the surface to be treated are larger than the cross-section of the laser beam, various laser-scanning patterns are involved. This paper presents an experimental investigation of laser surface hardening of AISI 4340 steel using different laser scanning patterns. This investigation is based on a structured experimental design using the Taguchi method and improved statistical analysis tools. Experiments are carried out using a 3 kW Nd: YAG laser source in order to evaluate the effects of the heating parameters and patterns design parameters on the physical and geometrical characteristics of the hardened surface. Laser power, scanning speed and scanning patterns (linear, sinusoidal, triangular and trochoid) are the factors used to evaluate the hardened depth and the hardened width variations and to identify the possible relationship between these factors and the hardened zone attributes. Various statistical tools such as ANOVA, correlations analysis and response surfaces are applied in order to examine the effects of the experimental factors on the hardened surface characteristics. The results reveal that the scanning patterns do not modify the nature of the laser parameters’ effects on the hardened depth and the hardened width. But they can accentuate or reduce these effects depending on the type of the considered pattern. The results show also that the sinusoidal and the triangular patterns are relevant when a maximum hardened width with an acceptable hardened depth is desired.

Numerical Investigation of Laser Surface Hardening of AISI 4340 Using 3D FEM Model for Thermal Analysis of Different Laser Scanning Patterns

Laser surface hardening is becoming one of the most successful heat treatment processes for improving wear and fatigue properties of steel parts. In this process, the heating system parameters and the material properties have important effects on the achieved hardened surface characteristics. The control of these variables using predictive modeling strategies leads to the desired surface properties without following the fastidious trial and error method. However, when the dimensions of the surface to be treated are larger than the cross section of the laser beam, various laser scanning patterns can be used. Due to their effects on the hardened surface properties, the attributes of the selected scanning patterns become significant variables in the process. This paper presents numerical and experimental investigations of four scanning patterns for laser surface hardening of AISI 4340 steel. The investigations are based on exhaustive modelling and simulation efforts carried out using a 3D finite element thermal analysis and structured experimental study according to Taguchi method. The temperature distribution and the hardness profile attributes are used to evaluate the effects of heating parameters and patterns design parameters on the hardened surface characteristics. This is very useful for integrating the scanning patterns’ features in an efficient predictive modeling approach. A structured experimental design combined to improved statistical analysis tools is used to assess the 3D model performance. The experiments are performed on a 3 kW Nd:Yag laser system. The modeling results exhibit a great agreement between the predicted and measured values for the hardened surface characteristics. The model evaluation reveals also its ability to provide not only accurate and robust predictions of the temperature distribution and the hardness profile as well an in-depth analysis of the effects of the process parameters.

Automated high-throughput tensile testing reveals stochastic process parameter sensitivity

The mechanical properties of additively manufactured metals tend to show high variability, due largely to the stochastic nature of defect formation during the printing process. This study seeks to understand how automated high throughput testing can be utilized to understand the variable nature of additively manufactured metals at different print conditions, and to allow for statistically meaningful analysis. This is demonstrated by analyzing how different processing parameters, including laser power, scan velocity, and scan pattern, influence the tensile behavior of additively manufactured stainless steel 316L utilizing a newly developed automated test methodology. Microstructural characterization through computed tomography and electron backscatter diffraction is used to understand some of the observed trends in mechanical behavior. Specifically, grain size and morphology are shown to depend on processing parameters and influence the observed mechanical behavior. In the current study, laser-powder bed fusion, also known as selective laser melting or direct metal laser sintering, is shown to produce 316L over a wide processing range without substantial detrimental effect on the tensile properties. Ultimate tensile strengths above 600 MPa, which are greater than that for typical wrought annealed 316L with similar grain sizes, and elongations to failure greater than 40% were observed. It is demonstrated that this process has little sensitivity to minor intentional or unintentional variations in laser velocity and power.

Epitaxial laser deposition of single crystal Ni-based superalloy: Variation of stray grains

A successful repair of single crystal Ni-based superalloy components requires maintaining the stability of the laser cladding process. So, it is critical to understand the relationship between stray grains and the actual repair condition including cooling condition, laser beam scanning pattern and unstable powder feeding. In this study, the variation of stray grains was investigated in terms of volume, crystal orientation, and distribution through experimental approaches in the laser cladding of single crystal superalloy. The results indicated that the formation of stray grains, low angle boundaries, and dendritic deflections or rotations could be suppressed on the condition of continuous substrate cooling and unidirectional laser beam scanning. In particular, a single crystal deposit with no stray grains was fabricated in a liquid nitrogen cooling condition. Besides, the dendritic deflection and rotation, which were harmful to epitaxial growth, were affected by the laser scanning pattern. Additionally, it was proved that stray grains preferably appeared at the entrance of powder into the molten pool. The offset of powder relative to the laser beam reshaped the distribution of stray grains in the molten pool, which would provide a new strategy to repair single crystal with laser cladding.

Shear strength of bonded joints of carbon fiber reinforced plastic (CFRP) laminates enhanced by a two-step laser surface treatment

The apparent shear strength of single-lap bonded joints of carbon fiber reinforced plastic (CFRP) can be enhanced by laser surface treatment. In this work, a two-step laser surface treatment was used to improve the single-lap bonded joint performance of CFRP-laminates. STEP-1, the resin at the outer surface of laminates was completely removed by an optimal laser etching process to provide neat carbon fiber fabric. STEP-2, the as-obtained exposed carbon fiber fabric was further irradiated to produce a series of mini-grooves on its surface. Laser scanning parameters, i.e., spot distance, scanning angles, groove distance and patterns, were investigated to analyze their effects on the morphologies of generated grooves. Single-lap shear tests were also carried out to assess the mechanical performance of these laser processing bonded joints. Experimental results indicated that crossed grooves significantly enhanced the shear strength of the CFRP-laminates. Failure morphologies of shear tests were observed to analyze the strengthening mechanism of grooved-CFRP by laser surface treatment. Using optimal processing parameters during laser surface treatment, the apparent shear strength of single-lap bonded joints of CFRP-laminates reached 18.58 MPa, implying a 40.8% enhancement compared with smooth laminates.

Influence of scan pattern and geometry on the microstructure and soft-magnetic performance of additively manufactured Fe-Si

The influence of geometry and scan pattern on the microstructure evolution and magnetic performance of additively manufactured Fe-3Si components was investigated. To reduce eddy current losses, novel geometries were designed and built and the microstructure and properties of these samples were characterized. The laser scan pattern was shown to strongly influence both the as-built grain structure and strength of the crystallographic texture, resulting in measurable changes in the as-built magnetic performance. In thin wall samples, heat treatment resulted in an increase in the maximum relative magnetic permeability and decrease in power losses in most samples, consistent with grain growth. However, decreases in the spacing between thin walls to increase the stacking factor of the cross-section was shown to result in unwanted electrical shorting between walls and an increase in eddy current losses. Compared to simple parallel plate construction and a mesh structure, a novel cross-section design based on the Hilbert space filling curve was found to produce the lowest power losses. The mechanisms behind these results were explored using a combination of heat conduction and electromagnetic simulations, providing a route for future component and process optimization.

Heterogeneously tempered martensitic high strength steel by selective laser melting and its micro-lattice: Processing, microstructure, superior performance and mechanisms

Herein, we report the selective laser melting of AISI 4130 high strength steel and its micro-lattice with superior energy absorption capabilities based on a dual material processing and structural design approach. Bulk 4130 was printed to high part qualities with an excellent combination of tensile properties of 1243 ± 25 MPa yield strength, 1449 ± 19 MPa ultimate tensile strength and 15.5 ± 1.5% fracture elongation. Such performance derives from its unique microstructure consisting of an alternating enhanced tempered and well-retained martensitic network. Experiments and simulation reveal the unique microstructure to result from a single-step fusion and quenching process followed by an in-situ rapid dynamic tempering that is associated with the laser scanning patterns. Based on these mechanical properties, orthogonally isotropic micro-lattices were designed and structurally optimized through finite element modelling. Superior per unit weight and volume energy absorption are measured; ranging from 13 to 35 J/g and 12–76 J/cm3 for relative densities of 10–30% respectively along with high energy absorption efficiencies of ~80%. These excellent properties in turn derive from the synergistic design and material properties. This work demonstrates the potential combination of additive manufacturing and design to create microstructure-geometric specific lattice materials for high performance energy absorption applications.

A comprehensive study on variability of relative density in selective laser melting of Ti-6Al-4V

ABSTRACT In this research, the effects of Selective Laser Melting (SLM) process parameters comprising laser power, scan speed, hatch space and scan pattern angle on the formation of porosity and subsequently density have been analysed. To improve the mechanical properties, post-processes (heat treatment) must be performed. Therefore, heat treatment was added to the design of experiment to analyse the effect of this process coupling with SLM process parameters on the value of density. A comprehensive design set with five levels for each parameter was selected so Taguchi L25 was used as the design of experiment. The significance of each parameter on obtained results was examined using statistical analysis (F-Test) and numerical model (interrogator analysis). The correlation between two process parameters was discussed by using 3D analytical and contour plots and the mechanisms behind these were discussed in depth. The contribution of this paper is a deep investigation of the relation of process parameters and heat treatment on density based on the Artificial Neural Network model. Results showed that better density is obtained with lower scan speed, laser power and scanning pattern angle. Meanwhile, for heat treatment and hatch space, the best density was obtained in their optimum range.

Modeling analysis of grain morphologies in Directed energy deposition (DED) coating with different laser scanning patterns

Directed energy deposition (DED) is often utilized to coat metallic part with complex geometries in a variety of anti-corrosion materials or high-temperature materials. Computer-controlled scanning patterns always affect the grain morphology in coating, which can further influence coating’s mechanical properties. In this study, a Kinetic Monte Carlo (KMC) Potts model is applied to predict the grain morphology under two different frequently-used laser scanning patterns: unidirectional and bidirectional raster patterns. Then the predicted grain morphologies in coatings with these two scanning patters were quantified and analyzed respectively. The simulated results were verified with an electron backscatter diffraction (EBSD) image of SS304 DED part reported by a literature. It was found that the primary grain morphology in coating was columnar shape. Some fine equiaxed grains appeared near the center line of scanning path. However, the columnar grains had crescentic shapes under unidirectional pattern, which was different from the linear shape of columnar grains under bidirectional pattern.

Effect of process parameters and electropolishing on the surface roughness of interior channels in additively manufactured nickel-titanium shape memory alloy actuators

Recently, laser-powder bed fusion (L-PBF) has been utilized to produce a NiTi shape memory alloy actuator with embedded channels for liquid metal forced fluid convection to increase actuator heat transfer rates. To enable further increases in performance, it is critical to characterize and control the surface quality of fully interior channels which have higher surface roughness compared to exterior top surfaces. This work utilizes a design of experiments methodology by varying laser power, scan speed, hatch space, scan pattern, channel orientation, and channel diameter on the as-fabricated surface roughness of the overhangs and walls of interior channels in NiTi. To enable post-process increases in surface quality, the channels are subjected to an electropolishing treatment and further characterized. Internal channel surfaces are characterized using optical profilometry and SEM imaging. It is concluded that channel orientation plays a prominent role in determining the surface roughness of as-fabricated interior channels, and a lower laser energy density results in the highest reduction in surface roughness after an electropolishing treatment.

Corrosion behaviour of 316L stainless steel manufactured by selective laser melting

AbstractThis work investigates the electrochemical behaviour of an AISI 316L stainless steel produced by selective laser melting (SLM) and compares its behaviour with that of wrought stainless steel with similar chemical composition. The SLM stainless steel specimens are tested in the as‐produced condition without stress relief or recrystallization heat treatments. The electrochemical tests are carried out in two electrolytes: 3.5 wt% NaCl solution with neutral pH and with pH of 1.8.At the macroscale, the microstructure of the SLM specimens is determined by the laser scanning pattern and displays an overlapping network of melt pools. At the microscale, the SLM specimens exhibit a cellular/columnar dendritic structure with submicrometric cell size. Electrochemical measurements highlight a more extended passive range for SLM stainless steel in both neutral and acid electrolytes indicating higher protective properties of the oxide film on SLM specimens. In contrast to the wrought material, the refined microstructure of the SLM specimens promotes a very shallow morphology of attack without deep penetration in the bulk.

Selective growth of single-grain crystal in Al thin film by micron chevron-shaped laser beam scanning

Polycrystalline Al film has been widely used in electronics and plasmonics, but the high density of grain boundaries causes conductivity deterioration, electromigration failure or incompatibility with fabrication of complex nano-circuits. In this letter, we propose an alternative approach to overcome the problems encountered with the use of polycrystalline Al film. Single-grain crystal of Al has been grown selectively in Al thin film by micron chevron-shaped laser beam scanning. The single-grain crystal, which is free of grain boundary, is of about 9-μm width and its length is comparable with the laser scan distance. A detailed investigation shows that the width of the single-grain crystal depends mainly on the laser power, whereas the crystal orientation depends on both the laser power and scan pattern. A wide-area of single-grain stripes can also be realized by a two-dimensional scan. The approach could be applicable to other metallic films, which are widely used in microelectronic and plasmonic devices.

A comprehensive study on microstructure and tensile behaviour of a selectively laser melted stainless steel

316L stainless steel samples have been prepared by selective laser melting (SLM) using a pulsed laser mode and different laser powers and scanning patterns. The as-fabricated samples were found to be dominated by clusters of nano-sized γ needles or cells. TEM imaging shows that these needles contain a high population of dislocations while TEM-EDX analysis reveals high chemical homogeneity throughout the as-fabricated samples as evidenced by the fact that there is even no micro-/nano-segregation at interfaces between neighbouring γ needles. The good chemical homogeneity is attributed to the extremely high cooling rate after SLM (>106 °C/s) and the formation of Si- and Mn-oxides that distribute randomly in the current samples. The laser-processed samples show both superior strength and ductility as compared with conventionally manufactured counterparts. TEM examination on the deformed specimens reveals a significantly high density of dislocations and a great number of twinning within nano-needles, suggesting that the plastic deformation has been governed by both gliding of dislocations and twinning deformation, which is believed to be responsible for the simultaneous acquisition of superior strength and ductility. Finally, laser power shows a much more dominant role than laser scanning pattern in porosity and grain size development for the SLM-processed 316L stainless steel samples.