Beam Scanning Strategy Research Articles

Crack-free manufacture of single weld tracks on aluminum alloy 6013 with the usage of laser beam shaping and oscillation strategies

The present paper investigates the application of laser beam shaping and laser beam oscillations (wobbling) for laser processing of the crack-prone aluminum alloy 6013, used in automotive and aerospace applications. A comparison of different laser beam profiles, such as the commonly used Gaussian profile, a ring-core distribution with the intensity of 50 % in the ring and 50 % in the core, and a ring-shaped beam profile shows different cracking behavior of the material. The ring-shaped beam profile shows the most promising results due to a reduction of the thermal gradient G and an enhancement of the growth rate R, which isalso stated by the state of the art. A combination of laser beam shaping and laser beam oscillations shows reproducible crack-free processing of Al6013 sheets at all three beam profiles at different parameter combinations. The crack elimination can be attributed to the emergence of a more pronounced equiaxed grain structure in the fusion zone of the weld with the application of laser beam oscillations and laser beam shaping. Thus, the temperature gradient G, the growth rate R and, therefore, the cooling rate can be controlled with the presented variation of the laser beam shapes and scanning strategies. Furthermore, the penetration depth of the laser at a Gaussian beam profile can be reduced using laser beam shaping, showing shallower melt pools with a lower depth-to-width aspect ratio, also suitable for the process of powder bed fusion of metals using a laser-based system (PBF-LB/M).

An Experiment-Based Variable Compensation Method to Improve the Geometric Accuracy of Sub-mm Features Fabricated by Stereolithography (SLA)

In this paper, we present an experimental procedure to enhance the dimensional accuracy of fabrication via stereolithography (SLA) of features at the sub-mm scale. Deviations in sub-mm hemispherical cavity diameters were detected and measured on customized samples by confocal microscopy. The characterization and experimental observations of samples allowed the identification of inaccuracy sources, mainly due to the laser beam scanning strategy and the incomplete removal of uncured liquid resin in post-processing (i.e., IPA washing). As a technology baseline, the measured dimensional errors on cavity diameters were up to −46%. A compensation method was defined and implemented, resulting in relevant improvements in dimensional accuracy. However, measurements on sub-mm cavities having different sizes revealed that a constant compensation parameter (i.e., C = 85, 96, 120 μm) is not fully effective at the sub-mm scale, where average errors remain at −24%, −18.8%, and −16% for compensations equal to 85, 96 and 120 μm, respectively. A further experimental campaign allowed the identification of an effective nonlinear compensation law where the compensation parameter depends on the sub-mm feature size C = f(D). Results show a sharp improvement in dimensional accuracy on sub-mm cavity fabrication, with errors consistently below +8.2%. The proposed method can be extended for the fabrication of any sub-mm features without restrictions on the specific technology implementation.

Multiphysical simulation of hot cracking in Laser-Based Powder Bed Fusion

This study extends an existing comprehensive computational framework to gain insight on hot cracking in the simulation of laser-based additive manufacturing via powder bed fusion. A novel approach to predict hot crack susceptibility based on vapor cavitation, building on a conceptual model akin to the Rappaz-Drezet-Gremaud criterion is introduced. Unlike conventional practices involving ex-situ evaluation of a criterion, the proposed model emerges implicitly from the underlying multiphysical modeling framework. The model exhibits sensitivity to variations in both material attributes (e.g., alloy composition) and processing conditions (e.g., laser beam shape or scanning strategy). Furthermore, non-equilibrium solidification is incorporated in the underlying Mass-of-Fluid framework, and a model for multi-layer printing is introduced to extend the length and time scale, enabling the derivation of detailed thermal histories on near-part-scale level. Consequently, the framework proves pivotal in optimizing process parameters, transcending the limitations inherent in conventional single melt track computational experiments.

A rational approach to beam path planning in additive manufacturing: the inverse heat placement problem

High demand for components with complex geometries at macro and micro levels drives the development of additive manufacturing (AM). However, the scientific basis for designing energy beam scanning strategies (e.g. beam scanning speed, beam path, beam power) still relies on trial and error approaches (i.e. experimental/simulation of predefined beam trajectories) followed by the evaluation of process outcomes (e.g. structural/metallurgical properties of the built parts); this is the Direct Problem. To address such drawbacks, this paper reports, for the first time, a mathematical model for selecting key parameters related to beam exposure time in AM processes as an attempt to improve the build part's uniform properties, i.e. the Inverse Heat Placement Problem. Our algorithm yields variable beam scanning speeds and optimized beam paths for achieving a desired maximum temperature distribution (uniform or target pattern) and is suitable for different circumstances and scanning strategies dependent on the print part configuration. Here, raster and spiral predefined beam paths are chosen as examples. Variable beam scanning speeds and optimized beam paths obtained from our algorithm are able to induce a desirable uniform maximum temperature distribution compared with the conventional approach of constant beam scanning speeds and a predefined beam path.

Scan strategies in EBM-printed IN718 and the physics of bulk 3D microstructure development

Three-dimensional (3D) characterization provides opportunities for understanding processing-structure relationships in additively manufactured (AM) materials. Bulk samples of Inconel 718 were fabricated via electron beam melting (EBM) in order to study microstructural development as a function of energy input and beam scan strategy. TriBeam tomography of bulk Inconel 718 microstructures built under steady-state growth conditions reveals the sensitivity of microstructure formation and evolution to machine process parameters. In this study, samples manufactured using a narrow range of energy input per unit build area result in varied grain morphologies and crystallographic textures. Using TRUCHAS, a thermal simulation software, the thermal history of bulk scan strategies was predicted, and combined with a calibrated microstructure-processing map to accurately predict bulk grain morphologies. The solidification parameters and the 3D measured nucleation density are used to predict the transition between columnar and equiaxed grain morphologies, providing a process map to guide AM parameter choices to locally control as-printed microstructure. A two-dimensional metric for characterizing bulk grain morphology was also found to agree well with predictions from the process map calibrated by 3D data. Combined with 3D tomography and thermal modelling, the physics of structure development were understood at a new level of detail with respect to the competing processes of grain nucleation and epitaxial growth.

The chiral coating on an achiral nanostructure by the secondary effect in focused ion beam induced deposition

Optical metamaterials are widely used in electromagnetic wave modulation due to their sub-wavelength feature sizes. In this paper, a method to plate an achiral nanopillar array with chiral coating by the secondary effect in focused ion beam induced deposition is proposed. Guided by the pattern defined in a bitmap with variable residence time, the beam scan strategy suppresses the interaction between adjacent nanostructures. A uniform chiral coating is formed on the target nanostructure without affecting the adjacent nanostructure, under carefully selected beam parameters and the rotation angle of the sample stage. Energy dispersive x-ray spectroscopy results show that the chiral film has high purity metal, which enables the generation of localized surface plasmon resonances and causes the circular dichroism (CD) under circularly polarized light illumination. Finally, the tailorable CD spectrum of the coated array is verified by the finite difference time domain method.

Unconventional milling of zirconia-based bioceramic material with nanosecond pulsed laser

Fabrication of dental restorations made of zirconia-based biomaterials with enhanced mechanical and tribological properties together with customized surface topography and microstructure for promoting enamel adhesion and cell proliferation while hindering bacterial spreading is a very challenging task to accomplish through conventional machining operations. In fact, traditional milling processes of sintered ceramics are typically labor-intensive, therefore expensive and time-consuming, and these aspects can be further exasperated when microsized features are required. On the other hand, unconventional techniques such as laser milling can represent a suitable solution to produce miniaturized individualized structures on advanced ceramics since it is a non-contact thermal process that ensures the elimination of cutting forces and allows hard and brittle materials to be machined without the need for special equipment which requires high investments and long processing times. In this context, this study aims to investigate the capability of a nanosecond pulsed fiber laser to machine samples made of yttria-stabilized zirconia through a systematic experimental approach in order to identify the most suitable process operational parameters combinations that ensure the obtainment of specific surface topographies while minimizing the machining time. The milling process is carried out by using a 30 W Q-switched Yb:YAG fiber laser by controlling laser beam scan speed, scan strategy and hatch distance and following a multi-level factorial design-based experimentation. The adoption of the laser technology to machine yttria-stabilized zirconia results in a high-repeatable, accurate and time-saving process allowing easy control of the process outcomes.

Influence of Selective Laser Melting Technology Process Parameters on Porosity and Hardness of AISI H13 Tool Steel: Statistical Approach.

The correct setting of laser beam parameters and scanning strategy for Selective Laser Melting (SLM) technology is a demanding process. Usually, numerous experimental procedures must be taken before the final strategy can be applied. The presented work deals with SLM technology and the impact of its technological parameters on the porosity and hardness of AISI H13 tool steel. In this study, we attempted to map the dependency of porosity and hardness of the tested tool steel on a broad spectrum of scanning speed—laser power combinations. Cubic samples were fabricated under parameters defined by full factorial DOE, and metallurgic specimens were prepared for measurement of the two studied quantities. The gathered data were finally analyzed, and phenomenological models were proposed. Analysis of the data revealed a minimal energy density of 100.3 J/mm was needed to obtain a dense structure with a satisfactory hardness level. Apart from this, the model may be used for approximation of non-tested combinations of input parameters.

Microstructure-property gradients in Ni-based superalloy (Inconel 738) additively manufactured via electron beam powder bed fusion

Electron beam powder bed fusion (E-PBF) can produce nickel-based superalloy components with minimal cracking and post-processing. This is due to the reduced thermal gradients and high print temperatures accessible through innovative beam scanning strategies compared to other AM processes. However, variations in thermal signature along the build direction inherent in alloys printed using E-PBF can drive significant changes in the microstructure and associated mechanical properties. In this work, through complementary local property measurements we observed a 127 – 145% increase in mean elastic modulus and 7–9% increase in mean hardness, as a function of build height, of an as-fabricated non-weldable Ni-based superalloy, Inconel 738. These properties were attributed primarily to variations in the γ′ character with build height, revealed through a multi-scale microstructural characterisation. The results highlight the influence of the thermal signatures on the microstructural-property relationships of E-PBF Inconel 738.

3D electron backscatter diffraction characterization of fine α titanium microstructures: collection, reconstruction, and analysis methods

3D electron backscatter diffraction (3D-EBSD) is a method of obtaining 3-dimensional crystallographic data through serial sectioning. The recent advancement of using a Xe+ plasma focused ion beam for sectioning along with a complementary metal-oxide semiconductor based EBSD detector allows for an improvement in the trade-off between volume analyzed and spatial resolution over most other 3D characterization techniques. Recent publications from our team have focused on applying 3D-EBSD to understand microstructural phenomena in Ti-6Al-4V microstructures as a function of electron beam scanning strategies in electron beam powder bed fusion additive manufacturing. The microstructures resulting from this process have fine features, with α laths as small as 1 μm interwoven in a highly complex fashion, presenting a significant challenge to characterize. Over the course of these fundamental works, we have developed best-practice 3D-EBSD collection protocols and advanced methods for 3D data reconstruction and analysis of such microstructures which remain unpublished. These methods may be of interest to the 3D materials characterization community, especially considering the lack of standard commercial software tools. Thus, the current paper elaborates on the methods and analysis used to characterize fine titanium microstructures using 3D-EBSD and presents a detailed description of the new algorithms developed for probing the unique features therein. The new analyses include algorithms for identifying intervariant boundary types, classifying three-variant clusters, assigning grains to variants, and quantifying interconnectivity of branched α platelets.

Unambiguous Direction-of-Arrival Estimation for Improved Scanning Efficiency in Subarray-Based Hybrid Array

Hybrid antenna array consisting of analog subarrays provides a low-cost solution to achieving high beamforming gain in millimeter-wave/terahertz communication systems. For beam adjustment, beam scanning can be readily applied to find the line-of-sight (LoS) path. Conventionally, the beam scanning process is inefficient because the number of grids, together with the scanning period, has to be large to achieve high resolution. To improve the beam scanning efficiency, in this paper we propose an off-gird direction-of-arrival (DoA) estimation method utilizing unequal-sized subarrays. Specifically, we first provide a new solution to overcome the ambiguity problem, i.e., the hybrid antenna array may render the DoA estimate non-unique. That new solution is formed by designing the antenna numbers in the subarrays and can be easily combined with the beam scanning process. We then propose a three-stage beam scanning strategy. At the first stage, coarse scanning is applied to roughly steer the subarrays along the LoS path. At the second stage, a simple DoA estimator is employed to obtain the candidate DoAs. Finally, beam scanning is performed again on the candidate DoAs to obtain the final DoA estimate. Simulation results demonstrate that the proposed method can achieve higher DoA estimation accuracy in shorter scanning period than the conventional beam scanning method; and compared with existing methods developed for equal-sized subarrays, the proposed method is more robust against multi-path interference.

The effect of beam scan strategies on the microstructure and mechanical properties of additive manufacturing Ti-6Al-4V builds

The effect of beam scan strategies on the microstructure and mechanical properties of additive manufacturing Ti-6Al-4V builds

Elongated cavities during keyhole laser welding

During laser beam deep penetration welding, the characteristic keyhole occurs that enables efficient energy absorption due to multiple reflections. Since this welding mode usually shows high dynamic behaviour, often a high amount of weld imperfections like spattering or porosity occur. Elongated keyholes during buttonhole, a.k.a donut welding, have shown the possibility to create smooth welding tracks when controlled variations of energy input were induced, e.g. beam oscillation. It was observed that the elongated keyholes can maintain themselves due to the balancing effects of the melt pool surface tension, which can form a catenoid shape that does not require any additional forces to be stable. Basic theoretical descriptions of the catenoid shape keyholes were derived. However, the complex system requires a more detailed investigation of the effect of the metal plume and the melt flows in order to explain the opening and maintenance effects of the elongated keyholes.Therefore, this work examined different modes of elongated keyholes, comparing traditional non-oscillating keyholes to keyholes formed by different beam scanning strategies. Analysis was made mainly by observations with high-speed-imaging and theoretical considerations to explain the phenomena of elongated keyholes. The observations showed that extended keyholes are not necessarily related to a reduced occurrence of imperfections and not either to the formation of perfect catenoid shapes. The keyhole front is deformed where the laser beam impacts and the rear wall is impacted where the vapour plume strikes. The opening of an elongated keyhole is likely related to the keyhole vapour plume that pushes the keyhole rear wall rearwards, while the non-affected rear wall parts do not collapse the keyhole due to the balancing nature of the second curvature of the rear wall.

In Situ Observation of γ′ Phase Transformation Dynamics During Selective Laser Melting of CMSX‐4

Additive manufacturing (AM) of superalloys has been attracting increasing interest. While most studies focus on the processability and mechanical properties of the finished product, it is also necessary to understand the phase transformations during the consecutive melting processes. Herein, the precipitation and dissolution of the γ′ phase in the Ni‐base superalloy CMSX‐4 in a selective laser melting process is reported. These phase transformations are studied in situ by small‐angle X‐ray scattering (SAXS) during AM. Concurrent wide‐angle X‐ray scattering (WAXS) provides information on the evolution of lattice parameters and temperature during the process. Additional thermal and thermodynamic simulations are carried out to support the experiments. The investigations are focused on the influence of different beam scanning strategies as well as the effect of laser power and scanning speed on the phase transformation dynamics. Due to the high cooling and heating rates inherent to AM, phase transformations occur far off equilibrium. Both precipitation and dissolution of γ′ phase are observed. The scan strategies are shown to have a considerable effect on the phase transformation dynamics, which exceed the impact of the beam parameters. The capability of combined SAXS and WAXS for the in situ study of phase transformations in AM processes is demonstrated.

Influence of laser beam scanning strategy in the selective laser melting of 316L steel powder on macrostress

The use of selective laser melting (SLM) leads to the formation of a complex material structure with high residual elastic stresses. The work investigates the distribution of macrostresses in SLM samples using five different scanning strategies. The stresses were measured by X-ray method and by recording the change in the shape of the specimens as a result of their separation from the platform using digital correlation image method. The combined use of the two methods provides more reliable results for SLM samples. It is shown that depending on the scanning strategy, the nature and magnitude of macrostresses change which are distributed more isotropically and have a smaller value when using scanning strategies with different directions of the laser beam movement.

Process Optimization for 100 W Nanosecond Pulsed Fiber Laser Engraving of 316L Grade Stainless Steel

High average power (>50 W) nanosecond pulsed fiber lasers are now routinely available owing to the demand for high throughput laser applications. However, in some applications, scale-up in average power has a detrimental effect on process quality due to laser-induced thermal accumulation in the workpiece. To understand the laser–material interactions in this power regime, and how best to optimize process performance and quality, we investigated the influence of laser parameters such as pulse duration, energy dose (i.e., total energy deposited per unit area), and pulse repetition frequency (PRF) on engraving 316L stainless steel. Two different laser beam scanning strategies, namely, sequential method (SM) and interlacing method (IM), were examined. For each set of parameters, the material removal rate (MRR) and average surface roughness (Sa) were measured using an Alicona 3D surface profilometer. A phenomenological model has been used to help identify the best combination of laser parameters for engraving. Specifically, this study has found that (i) the model serves as a quick way to streamline parameters for area engraving (ii) increasing the pulse duration and energy dose at certain PRF results in a high MRR, albeit with an associated increase in Sa, and (iii) the IM offers 84% reduction in surface roughness at a higher MRR compared to SM. Ultimately, high quality at high throughput engraving is demonstrated using optimized process parameters.

Effect of laser beam trajectory on pocket geometry in laser micromachining

Abstract The article presents the problem of planning the laser beam trajectory for the laser micromachining process. The article concerns on the ablative laser micromachining issues. Different effects of laser beam trajectory on pocket geometry in laser micromachining were investigated. The results of experimental tests are presented. Based on the research, potential causes of different effects of the laser beam for various trajectories were formulated. Several different types of trajectories for the assumed shape were developed for the purposes of the research. Laser micromachining was performed with fixed parameters of the laser device using different trajectories. The article indicates the significant impact of the laser beam trajectory on the effect of interaction on matter during the laser milling process, which is not often mentioned in scientific reports. The article presents the basic geometrical measurements indicating the need to determine the leading of the laser beam. Studieswere conducted using the microscopic observation methods and interferometric methods for estimating the surface condition. The article indicates the need for extensive research focusing on the mechanism of the impact of the laser beam scan strategy on the effect on the material during ablative laser machining. The article summarizes the analysis and discussion of research results.

The effect of beam scan strategies on microstructural variations in Ti-6Al-4V fabricated by electron beam powder bed fusion

In electron beam melting (EBM) powder bed fusion (PBF), variations in electron beam scan strategies can be used to control thermal transients in the additive manufacturing process, both in the melt pool and in previously deposited layers. In this study, three different EBM beam scan strategies, i.e. the standard raster scan, ordered spot scan, and random spot scan patterns, were used to fabricate three identical Ti-6Al-4V blocks. Using scanning electron microscopy and electron backscatter diffraction, variations in microstructure and crystallographic texture, such as α lath thickness, prior beta β grain size and orientation, α/α lath boundary (LB) distributions, are investigated with respect to the applied scan strategy. Both spot scan strategies result in coarser α laths and smaller prior β grains with width and height < 1/3 of the value of the typical large columnar grains, observed in raster scan samples. The combined fraction of type 2 and type 4 α/α LBs measured in the three samples was found to be between 0.50 and 0.85, which is greater than the expected combined fraction of ~0.36 for a random distribution of α variants. This suggests the presence of a mild to weak variant selection in EBM Ti64.

The Effect of Beam Scan Strategies on the Microstructure of EBM Additively Manufactured Inconel 738

The Effect of Beam Scan Strategies on the Microstructure of EBM Additively Manufactured Inconel 738

Investigation of an interlaced laser beam scanning method for ultrashort pulse laser micromachining applications

Abstract This article investigates picosecond and sub-picosecond laser micromachining of Borofloat®33 glass and provides clear evidence that a simple modification of the laser beam scanning strategy can lead to significant improvement of machining efficiency and hence process throughput. Besides studying the impact of the fundamental laser machining parameters, such as laser fluence, pulse overlap, pulse repetition frequency (PRF), pulse duration and laser spot diameter, on the machined depth, surface roughness and material removal rate (MRR), it also compares the machining results for two different laser beam scanning strategies, called here sequential” method (SM) and “interlaced” method (IM). By changing the scanning strategy from SM to IM, the MRR can be significantly increased because IM allows high-quality machining of the glass at higher PRF values. The experimental results show that this simple, cost-free modification allows the MRR value to be increased by more than 4 times, i.e. from 0.12 mm3/s to 0.53 mm3/s. Moreover, by using a Phantom V2512 high-speed camera, the picosecond laser micromachining process using both SM and IM was filmed. The videos show that SM leads to the accumulation of glass particles within the laser-machined area, whereas in IM the glass material is removed layer by layer which leads to the generation of “cleaner” and deeper areas. The mechanisms associated with these machining improvements are discussed.