Laser Scanning Path Research Articles

An adaptive second-order element-free Galerkin method for additive manufacturing process

Element-free Galerkin (EFG) method is applied for the first time to numerical modeling of additive manufacturing (AM) process, in which the superiorities of the EFG method in implementing adaptive computation and in constructing high order approximation are exploited. Second-order moving-least squares (MLS) approximation is constructed for both temperature and displacement. A non-linear coupled thermo-mechanical model considering moving heat sources, radiation boundaries, elastoplastic materials and temperature-dependent parameters is adopted. Adaptive coarsening based on the background integration mesh is developed to efficiently model the layered deposition process. In addition, an efficient integration scheme using background hexahedral elements is proposed to evaluate the domain integrals of the weak forms. Numerical results show that the developed method is able to effectively model the AM process with substantially reduced number of approximation nodes. The effects of laser power, scan speed and scan path on temperature and distortion are investigated. The improved accuracies due to the proposed integration scheme and the adopted second-order approximation are also demonstrated.

Material removal and surface formation mechanism of C-plane sapphire in multipass ablation by a nanosecond UV laser

The material removal and the surface formation mechanism of C-plane sapphire in multipass ablation using a nanosecond laser were investigated. The surface morphology and the width and depth of the grooves under different laser-processing parameters were characterised and investigated based on an analysis of the laser energy fluence distribution in time and space, which determines the obtained depth and width of the groove. The results show that the material removal mechanism in the sapphire processed using a nanosecond UV laser was a mixture of thermal and photochemical effects. The laser thermal effect caused sublimation, vaporisation, melt, and sputtering by plasma and was characterised by the resolidified bulk area and the deposition of fine particles. Surface cracking in a certain direction to the laser scanning path was also observed because of the gradient-temperature-induced thermal stress. In addition, the photochemical effect was revealed by the obtained X-ray photon spectra, in which the appearance of a metal Al peak and a decreasing O1s peak after laser ablation indicated the photolysis of Al2O3.

Correlation of SWIR imaging with LPBF 304L stainless steel part properties

In the Laser Powder Bed Fusion (LPBF) process, the local thermal history can vary significantly over a part as the heat transfer characteristics and the laser scan path are geometry dependent. The variations introduce the potential for defects that lead to part failure, some of which are difficult to identify non-destructively with common ex-situ evaluation techniques. These defects include significant microstructural and mechanical property differences in the part interiors. In this paper, thermal features are extracted from in-situ Short-Wave Infrared (SWIR) imaging measurements to compile voxel based part representations and understand how the complexities in the thermal history affect part performance. The deviations in thermal features due to different laser processing parameters and complex scan pathing are explored. Empirical correlations are developed to map thermal features with the engineering properties (bulk yield strength, area percentage porosity, and local state) of 304L stainless steel parts manufactured by LPBF. Processing modes (insufficient melting and keyholing) are determined by mapping part property measurements with multiple thermal features. Generating the relationships between thermographic measurements and resulting SLM part properties lays the foundation for in-situ part qualification.

Effects of scanning paths on laser welding

Laser biological tissue welding is a potential alternative technology from traditional suturing skills. In this study, we explored the influence laws of laser scanning path on in-vitro skin tissue performance and designed four different scanning paths, including heartbeat, circle, peano20-2, and toothed paths. Then the influence laws of laser scanning paths on temperature distributions, mechanical properties and microstructure characteristics were discussed based on the simple factor design of experiments. The samples under the heartbeat path reached the highest peak temperature and the highest tensile strength, but possessed sparse microstructure. The peak temperature of the samples under the circle path was close to that of the heartbeat samples but the tensile strength was only 8.667 N/cm2. The temperature of the toothed samples was much lower and obviously fluctuated. The toothed samples had very bad mechanical properties, but had uniform microstructure. The Peano20-2 samples achieved the best comprehensive microstructure characteristics and tensile strength was up to 19.939 N/cm2 and suffered no color deepening or scars at the macroscale.

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%.

A novel path strategy design for precise 2D and 3D laser tube forming process; experimental and numerical investigation

Laser tube forming is a flexible process in which the heat generated by the laser beam scanning on the tube surface results in bending. Unlike laser sheet forming, few studies have been carried out on laser tube forming. This originates from the complexity of this process compared to the sheet forming, as well as the limited applicability of the forming strategies. In this research, a new idea is proposed based on the circular scanning method, which is presented in the form of two step-by-step and reverse schemes. Using this new strategy, it would be easy to precisely extract the laser scanning path for all 2D and 3D shapes. Since this strategy is programmable, it can be designed and implemented in a variety of forms only by changing the input parameters. In order to form a desired 2D and 3D shapes on a tube based on this idea, the proposed strategy was verified with both finite element and experimental methods. Fiber laser was used to form the AISI 304L tubes with certain parameters to achieve a specific bend angle. The experimental results are in a very good agreement with the designed path strategy and FE simulations, which confirms the efficiency and effectiveness of the path scanning strategy proposed in this paper.

Patterned arrays of assembled nanoparticles prepared by interfacial assembly and femtosecond laser fabrication

Creating shape-defined structures of inorganic nanoparticles in a maskless and template-free fashion would advance the engineering of nanoparticle-based devices and structures with desired configurations for various applications. In this work, a novel fabrication protocol combining bottom-up interfacial assembly and subtractive laser patterning was developed for creating patterned arrays of assembled nanoparticles. A solid film of magnetic nanoparticles (10 nm, monodisperse CoFe2O4) was assembled as a nanoparticle film (thickness less than 100 nm) on liquid interface under guiding field, and it was further transferred to Si substrate followed by selective material removal using femtosecond laser pulses, producing patterned arrays (typical size of 3 μm) of assembled nanoparticles. The size, shape, and arrangement of the patterned arrays were finely regulated by adjusting the laser pulse energy and laser scanning path. The magnetization behavior and magnetic anisotropy of the patterned arrays differ from those of the nanoparticle-assembled film, as reflected by the changes of coercivity and squareness along the out-of-plane direction. The presented fabrication protocol is compatible with microelectronic fabrication techniques and can be applied to various inorganic nanoparticles.

Generation of microcones on reaction-bonded silicon carbide by nanosecond pulsed laser irradiation

Silicon carbide (SiC) is heavily used in the industry due to its resistance to chemical wear and excellent mechanical properties such as high hardness and high stiffness. However, these properties make it difficult to produce micro and nanostructures on the surface of SiC by conventional methods. In this study, high-density microcones that protrude ~ 10 μm above the initial surface have been fabricated by nanosecond pulsed Nd:YAG laser irradiation (λ = 532 nm) on reaction-bonded SiC. Geometrically aligned cones were also fabricated by modifying the laser scanning path, and effect of different parameters such as pulse frequency, laser fluence was studied. It was observed that the surface morphology of microcones was affected by the pulse width and beam overlap. X-ray spectroscopy and Raman spectroscopy showed that the microcones were mainly composed of silicon. Formation of these cone structures made the surface highly superhydrophilic with a contact angle of ~ 0°.

Model-based feedforward control of laser powder bed fusion additive manufacturing

Control of laser power to improve part quality is critical for fabrication of complex components via Laser Powder Bed Fusion (LPBF) additive manufacturing (AM) processes. If the laser power is too low, it will result in a small melt pool and lack of fusion; on the other hand, if the laser power is too high, it will result in keyhole and material evaporation. This paper examines a model-based feed-forward control for laser power in LPBF to improve build quality by avoiding the onset of keyhole formation or reducing over-melting. First, an analytical, control-oriented model on the dynamics of melt-pool cross-sectional area in scanning a multi-track part was developed, and then a nonlinear inverse-dynamics controller was designed to adjust laser power such that the melt-pool cross-sectional area can be regulated to a constant set point during the build process. The resulting control trajectory on laser power from the simulated closed-loop controller was then implemented in a LPBF process as a feed-forward (FF) controller for laser power. Multiple bead-on-plate samples of Inconel 625, with different number of tracks and track lengths, were then built on an EOSINT M 280 AM system to evaluate the performance of the resulting FF-Analytic controller. Experimental results demonstrated that the proposed FF-Analytic control of laser power was able to avoid the onset of keyhole formation that occurred under a constant laser power for certain samples. Furthermore, the proposed FF-Analytic control was demonstrated to have significantly reduced over-melting at the returning ends of the laser scan path in scanning a multi-track part compared to applying a constant laser power, albeit with some over-compensation due to modeling imperfection. Overall, the proposed FF-Analytic control of laser power had 23–40% lower average error rate than applying a constant laser power in regulating the melt-pool cross-sectional area to a constant reference value, in terms of measurements of cross-sections at track ends.

A large format DMLM system using a continuously rotating powder bed

The aircraft engine industry manufactures many ring-like metal parts of large diameter but small cross-sectional area. Designers of these parts require increasingly complex geometries for improved aerodynamic efficiency and cooling while manufacturers of these parts require larger and faster equipment for high productivity and low cost. The combination of these industrial requirements inspired the development of a new Direct Metal Laser Melting (DMLM) architecture, reported here, which incorporates a rotating powder bed. The system coordinates the rotational motion of the powder bed with an ascending laser scanner and recoater to build parts in a helical fashion. A single-point powder feeder delivers metal powder near the inner radius of an annular build volume, and a recoater spreads the powder to the outer radius in a “snow plow” fashion. Because the recoater and laser scanner are installed at different angular positions, they operate independently and simultaneously. Encoder feedback from both the rotational stage and the galvanometers assures accuracy of the laser scan path. A prototype system was built to demonstrate this new concept for an aircraft engine combustor liner (600-mm dia. x 150-mm ht.) and showed continuous laser utilization exceeding 97%. Build rates were shown to triple conventional DMLM systems while powder requirements were decreased by more than 4x.

Improving edge quality and optical transmittance of Ag films on glass substrates by selective nanosecond pulsed laser ablation using various scanning methods

Selective laser ablation (SLA) of silver (Ag) films, which were deposited on glass substrates by radio frequency (RF) magnetron sputtering, was performed by using a 532 nm nanosecond (ns) pulsed laser. Two kinds of simple and mask-free SLA strategies featured by two new scanning paths (i.e. frame-typed serial scanning, FTSS, and frame-line combined scanning, FLCS) were proposed to improve the edge quality and optical transmittances of the SLA patterns. The effects of laser scanning speed (v), hatch distance (HD) and scanning path on edge quality of the laser-ablated grooves and SLA patterns were systematically studied. The results indicated that compared with the conventional line-typed cyclic scanning (LTCS) path, which resulted in unsmooth edge with semicircle-shaped boundaries and some Ag residues, the proposed scanning paths of FTSS and FLCS produced clean and smooth edges without obvious heat-affected zone and Ag residues under the condition of using a v value of 15 mm/s and an HD value of 90 μm. As a result, the as-obtained SLA patterns exhibited relatively high optical transmittances. The simple methods of adopting the FTSS or FLCS path for improving the edge quality during SLA and thus the optical transmittance of SLA patterns may have great potential application in various fields.

An inherent strain based multiscale modeling framework for simulating part-scale residual deformation for direct metal laser sintering

Residual distortion is a major technical challenge for laser powder bed fusion (LPBF) additive manufacturing (AM), since excessive distortion can cause build failure, cracks and loss in structural integrity. However, residual distortion can hardly be avoided due to the rapid heating and cooling inherent in this AM process. Thus, fast and accurate distortion prediction is an effective way to ensure manufacturability and build quality. This paper proposes a multiscale process modeling framework for efficiently and accurately simulating residual distortion and stress at the part-scale for the direct metal laser sintering (DMLS) process. In this framework, inherent strains are extracted from detailed process simulation of micro-scale model based on the recently proposed modified inherent strain model. The micro-scale detailed process simulation employs the actual parameters of the DMLS process such as laser power, velocity, and scanning path. Uniform but anisotropic strains are then applied to the part in a layer-by-layer fashion in a quasi-static equilibrium finite element analysis, in order to predict residual distortion/stress for the entire AM build. Using this approach, the total computational time can be significantly reduced from potentially days or weeks to a few hours for part-scale prediction. Effectiveness of this proposed framework is demonstrated by simulating a double cantilever beam and a canonical part with varying wall thicknesses and comparing with experimental measurements which show very good agreement.

Fabrication of Mesh Patterns Using a Selective Laser-Melting Process

The selective laser-melting (SLM) process can be applied to the additive building of complex metal parts using melting metal powder with laser scanning. A metal mesh is a common type of metal screen consisting of parallel rows and intersecting columns. It is widely used in the agricultural, industrial, transportation, and machine protection sectors. This study investigated the fabrication of parts containing a mesh pattern from the SLM of AISI 304 stainless steel powder. The formation of a mesh pattern has a strong potential to increase the functionality and cost-effectiveness of the SLM process. To fabricate a single-layered thin mesh pattern, laser layering has been conducted on a copper base plate. The high thermal conductivity of copper allows heat to pass through it quickly, and prevents the adhesion of a thin laser-melted layer. The effects of the process conditions such as the laser scan speed and scanning path on the size and dimensional accuracy of the fabricated mesh patterns were characterized. As the analysis results indicate, a part with a mesh pattern was successfully obtained, and the application of the proposed method was shown to be feasible with a high degree of reliability.

Multiphysics modelling of lack-of-fusion voids formation and evolution in IN718 made by multi-track/multi-layer L-PBF

Laser-based powder bed fusion (L-PBF) is a branch of additive manufacturing technology which is considered to be a superior process due to its capability of producing complex designs with low material waste. Despite L-PBFs various unique characteristics, manufactured parts still suffer from a wide variety of defects, among which porosity is one of the most important. In this paper, a multiphysics numerical model for the multi-track/multi-layer L-PBF is developed and used for analysing the formation and evolution of voids caused by lack of fusion and improper melting. The multiphysics model is in meso-scale and is used to track and observe the formation of porosities, and considers phenomena such as multi-phase flow, melting/solidification, radiation heat transfer, capillary and thermo-capillary (Marangoni effect) forces, recoil pressure, geometry dependant absorptivity and finally evaporation and evaporative cooling. A novel methodology has been introduced to model the two subsequent powder-laying and fusion processes, for each layer, by means of a discrete element method (DEM) in a Lagrangian framework and a computational fluid dynamics (CFD) model, both implemented in Flow-3D. The results for the investigated process parameters indicate that the porosities (voids) are mainly formed in between the tracks, largely due to improper fusion of the particles. Moreover, it is observed that the pores are mostly elongated in the direction parallel to the laser scanning paths, as expected. The probability of the presence of pores is also observed to be higher in the first layer, where the average layer temperature is lower as well. Furthermore, the lack of fusion zones are seen to become smaller in the subsequent layers, largely due to better fluid flow and higher temperatures, because of heat accumulation in those layers.

Thermal analyses for optimal scanning pattern evaluation in laser aided additive manufacturing

Laser aided additive manufacturing (LAAM) is one of the key metal 3D printing technologies for surface cladding or fabrication of near-net shape parts. The study of LAAM scanning pattern is important to understand their relationship with residual stress and part distortion. This paper proposed a framework to both simulate and evaluate laser scanning paths. Firstly, an efficient 3D thermal history analysis finite element (FE) model was developed to predict temperature field evolution for arbitrary scanning patterns. Subsequently, a thermal field based evaluation method was established to determine the optimal scanning pattern with minimal distortion. The effectiveness of the framework was validated experimentally by depositing a rectangular clad on a cuboid substrate with five different scanning patterns. Experiments showed width-wise Zigzag scanning pattern yielded largest distortion. This method with 5 criteria and two evaluation levels were effective to identify an improved width-wise Zigzag scanning pattern by adjusting the deposition paths sequence, while determining the length-wise scanning as the optimal pattern. This work also demonstrated that the temperature field can be used to make qualitative evaluation of LAAM induced distortion which further reduces computational costs.

Numerical simulation of the growth characteristics of laser chemical vapor deposition of silicon carbide

A thermal model, which involves heat transfer in substrate and gases, mass transfer in gases, and chemical reaction on the top surface of the substrate, is set up to simulate the Laser Chemical Vapor Deposition (LCVD) process of Silicon Carbide (SiC) by a finite volume method. Methyltrichlorosilane (MTS; SiCl3CH3) and hydrogen (H2) are chosen as precursor and carrier gas, respectively. A designed set of model cases is executed for both stationary and moving laser beams. For the cases of stationary laser beam, the shape of the SiC deposits is higher and wider with increasing laser power. For the cases of moving laser beam, a narrow strip of SiC deposits is formed along the laser scanning path. Due to the low sticking coefficient of SiC deposits at high temperature, the volcano-like defects occur on the top center of the SiC deposits for both stationary and moving laser beams.

Exploring Convergence of Snake-Skin-Inspired Texture Designs and Additive Manufacturing for Mechanical Traction

Abstract This research explores the convergence of a 3D printing process with snake-skin-inspired textures for mechanical traction. The design attributes of snake skin scales were studied via the shed skin of species Python Regius . Snake skin design, especially on ventricular side, consists of hexagonal scale patterns with denticulations and fibrils as hierarchical structures of the design. The snake-skin-inspired design was then explored for printing using the laser-powder bed fusion (L-PBF) manufacturing technique on 420 stainless steel. It was discovered that the surface roughness generated by the laser scanning path during the 3D printing technique closely mimics the hierarchical micro-texture of snake skin scales. The texture of the 3D printed part surface was measured using the JIS B 06061-2001 standard by a laser scanning microscope and this manuscript presents a comparison of the 3D printed texture with the shed snake skin surface. This paper also discusses directional traction properties of the printed surface using tribological study.

Femtosecond pulsed laser fabrication of a novel SCD grinding tool with positive rake angle

Ablating a single-crystal diamond (SCD) plate with a femtosecond laser to fabricate a novel abrasive regularly arranged binder-less face grinding tool with acute inclination angle at the edge of the micro grains was proposed in this paper. A Ti:Sapphire femtosecond laser with a wavelength of 1030 nm and a pulse width of 250 fs was used for the ablation experiments. The effects of processing parameters, especially the ablation track pitch on the ablation surface quality of the diamond, were investigated in advance. Based on the optimal processing parameters, an array of square frustum microabrasive with grain inclination angle of approximately 100° was fabricated on the surface of SCD according to the predesigned laser scanning path. The microabrasive array was subsequently ablated to reduce the inclination angle at the edge of the micrograins to approximately 80°, and a novel abrasive regularly arranged binder-less diamond face grinding tool with positive rake angle which was expected to decrease the grinding force and improve the surface quality of hard and brittle materials had been fabricated for the first time.

Estimation of forest leaf area index using terrestrial laser scanning data and path length distribution model in open-canopy forests

Terrestrial Laser Scanning (TLS) is an active technology that can acquire the finest characteristics of canopy structure and plays an increasing role in estimating Leaf Area Index (LAI) in forest canopies. However, 3D information is not directly used in conventional TLS-based methods using the gap fraction theory. In addition, quantifying clumping effect within canopies is still a difficult task. In this paper, we presented a method to reduce clumping effect and estimate LAI using TLS data. Our recently proposed path length distribution model was applied to TLS data. Instead of converting 3D points to 2D image, the path length distribution can be extracted using the TLS-recorded 3D data and the crown models built with the alpha shapes algorithm. Two simulated scenes and one actual forest plot were utilized for validation. The results of the proposed method agree well with both the true LAI (in the simulated scenes) and the extracted PAI by the digital hemispherical photography (in the actual plot). This LAI estimation method using TLS and the path length distribution model provides a novel way for ground-based LAI measurements and shows its great potential.

Laser forming of glass laminate aluminium reinforced epoxy (GLARE): On the role of mechanical, physical and chemical interactions in the multi-layers material

The present work concerns the laser forming of Fiber-Metal Laminates (FMLs) by a high power diode laser source. FMLs are made of different layers of metal and composite material. The wide difference in the thermo-mechanical and physical properties of the different layers within FMLs makes them extremely problematic to be reprocessed. In particular, the economic shaping of FMLs in complex shapes is practically impossible, limiting greatly their field of application. Therefore, this manuscript explores the possibility of using laser forming to reprocess FMLs and obtain final components with precisely controlled bending radii. For this purpose, the effect of the laser operating parameters, namely laser power, scanning speed and number of passes, on the final shapes of FMLs was investigated. Secondly, further tests were also performed to obtain other complex shapes on FMLs from multiple laser scanning paths, side by side and equidistant on the substrates. An analysis of the failure of the substrates during the shaping process was also performed, with particular emphasis on deformation mechanisms, interfacial delamination and thermal alteration of the layers. The experimental results showed the good viability of laser reprocessing to shape FMLs, thus opening up new possible applications of this class of materials in aeronautical and aerospace applications.