Laser Scanning Pattern Research Articles

Laser scanning patterns induced surface properties variations of Ti-V-Hf-Zr non-evaporable getter film with 316L stainless steel substrates

The effects of laser treatment scanning patterns on the properties of Ti-V-Hf-Zr non-evaporable getter (NEG) film on 316L stainless steel (316L SS) substrates were analyzed. 316L SS substrates were laser-treated with different patterns, which resulted in spherical structure and protrusions on the surface of the substrates. Subsequently, Ti-V-Hf-Zr NEG films were deposited on ultrasonically cleaned laser-treated 316L SS by a direct current (DC) magnetron sputtering machine. Based on the XPS results of Ti-V-Hf-Zr, it has been revealed that the atomic proportions of film elements on the surface of samples #1-1∼#3–1 were about 2.2 (Ti):1.7 (Zr):0.9 (V):1.0 (Hf). In this paper, the Ar+-induced secondary electron yield (SEY) and surface resistivity of Ti-V-Hf-Zr NEG are discussed for the first time, and it has been observed that the laser treatment leads to an increase of the substrate surface roughness, which decreases the SEY and increases the surface resistivity.

Mitigation of bio-corrosion characteristics of coronary artery stent by optimising fs-laser micromachining parameters

Cardiovascular diseases, particularly coronary artery disease, pose big challenges to human life. Deployment of the stent is a preferable treatment for the above-mentioned disease. However, stents are usually made up of shape memory alloy called Nitinol. The poorer surface finish on the machined nitinol stents accelerates the migration of Nickel ions from the implanted nitinol stent, which is considered toxic and can lead to stenosis. The current study deals with controlling surface quality by minimising surface roughness and improving corrosion resistance. Femtosecond laser (fs-laser 10−15 s) micromachining was employed to machine the Nitinol surface to achieve sub-micron surface roughness. The Grey relational analysis (GRA)-coupled design of the experimental technique was implemented to determine optimal levels of four micromachining parameters (laser power, pulse frequency, scanning speed, and scanning pattern) varied at three levels to achieve minimum surface roughness and to maximise the volume ablation. The results show that to yield minimum surface roughness and maximum volume ablation, laser power and scanning speed are in a higher range. In contrast, the pulse frequency is lower, and the scanning pattern is in a zig-zag manner. ANOVA results manifest that scanning speed is the predominant factor in minimising surface roughness, followed by pulse frequency. Furthermore, the corrosion behaviour of the machined nitinol specimens was evaluated, and the results show that specimens with lower surface roughness had lower corrosion rates.

Crystal plasticity simulations with representative volume element of as-build laser powder bed fusion materials

Additive manufacturing of as-build metal materials with laser powder bed fusion typically leads to the formations of various chemical phases and their corresponding microstructure types. Such microstructures have very complex shape and size anisotropic distributions due to the history of the laser heat gradients and scanning patterns. With higher complexity compared to the post-heat-treated materials, the synthetic volume reconstruction of as-build materials for accurate modelling of their mechanical properties is a serious challenge. Here, we present an example of complete workflow pipeline for such nontrivial task. It takes into account the statistical distributions of microstructures: object sizes for each phase, several shape parameters for each microstructure type, and their morphological and crystallographic orientations. In principle, each step in the pipeline, including the parameters in the crystal plasticity model, can be fine-tuned to achieve suitable correspondence between experimental and synthetic microstructures as well as between experimental stress–strain curves and simulated results. To our best knowledge, this work represents an example of the most challenging synthetic volume reconstruction for as-build additive manufacturing materials to date.

Correlation of Energy Density and Manufacturing Variables of AA6061 through Laser Powder Bed Fusion and Its Effect on the Densification Mechanism

Aluminum alloy processing via additive manufacturing (AM) technologies has increased in usage during the last decade. AM now enables manufacturing complex geometries not previously achieved through traditional manufacturing. Aluminum is usually processed using laser powder bed fusion (LPBF) technologies, which are used to manufacture metallic components of high geometrical complexity and dimensional accuracy and good mechanical, electrical, and chemical properties, which is why this technology is quite popular at industrial levels. To further develop quality control systems and new aluminum alloys using LPBF, there is a need to establish a predictive relationship between the parameters of the material. A study was carried out to investigate the relationship between energy density and process parameters such as laser power, scan speed, hatch spacing, scan pattern, and laser focus and its influence on the densification mechanism in additively manufactured components with aluminum alloys. AA6061 was selected due to its wide usage in different industries, given its low density and high mechanical performance. Relative density was analyzed using the Archimedes principle, and the quality and morphology of the AA6061 powder were analyzed through metallographic analysis. The process parameter selection was performed according to the best results obtained according to the laser power and energy density factors. The best manufactured samples had an energy density between 30 and 40 J/mm3, with relative densities above 99%.

A comparative study of solute trapping in Fe-(33–45 at%) Cu alloys manufactured by laser directed energy deposition

The high cooling rates in laser directed energy deposition (DED-LB) of alloys lead to substantial amounts of solute trapping as solute atoms cannot diffuse away from the solid/liquid interface before it advances. In some concentrated alloys, this results in supersaturated solid phases that form nanoscale hierarchical microstructures when the solute atoms precipitate out during reheating from subsequent laser passes. We choose the Iron-Copper (Fe-Cu) binary alloy as model system as it is chemically homogeneous in the liquid phase and has negligible solid solubility at room temperature. Two alloys with nominal compositions in atomic (at.) %, Fe67Cu33 and Fe55Cu45, were manufactured using DED-LB. Scanning transmission electron microscopy (STEM), energy dispersive spectroscopy (EDS) and wavelength dispersive spectroscopy (WDS) were used to characterize the nanostructures and heterogeneous chemical compositions. A non-equilibrium solute partitioning model was used to compute the supersaturated chemistries of the constituent phases and validated with experimentally measured compositions. The measured phase compositions of the two alloys were very similar, at roughly 12 at% Cu and 4 at% Fe in the α(bcc)-Fe and ε(fcc)-Cu phases respectively, despite having different processing parameters and mechanical behavior. This indicates that the total thermal history, that depends on both the processing parameters and laser scan pattern, plays a stronger role on the final microstructure evolution than just the initial quantity of trapped solute. Additionally, we find that current non-equilibrium solute partitioning models applied on the continuum scale fall short of predicting accurate quantitative phase compositions in concentrated alloys, although the qualitative trends are captured correctly.

Influence of pulsed laser scanning patterns on microstructural evolution and mechanical properties of Inconel 718 in direct laser deposition

The mechanical properties of direct laser deposition (DLD) parts are crucial for engineering applications, which are determined by the microstructure and texture. To achieve fine microstructure and higher mechanical performance, pulsed laser deposition is used to generate less heat accumulation and a higher cooling rate. However, the relationship between microstructure and thermal history is difficult to investigate due to the transient temperature change. In this work, a three-dimensional numerical model is developed to study the thermal behavior of the molten pool in the continuous laser deposition (CLD), the continuous pulsed laser deposition (CPLD), and the interval pulsed laser deposition (IPLD) processes. The microstructure of the deposition samples is investigated by optical microscope and electron back-scattered diffraction. The mechanical response of the deposition samples is investigated by tensile test and microhardness test. Compared with the CLD, CPLD, and IPLD result in higher temperature gradient and cooling rate, long columnar grains, and a strong fiber texture. However, grains with larger sizes and a texture with a herringbone pattern due to the change of heat flow are obtained in CLD samples. The test values of IPLD samples show higher tensile strength, higher yield strength, lower ductility, and higher hardness values. This research brings about a perspective for studying the thermal effects of deposition layers and a novelty deposition process to obtain specific microstructures and better mechanical properties.

Changes in Microstructure and Mechanical Properties of 17-4PH Stainless Steel according to the Application of Laser Rotation in the Powder Bed Fusion(PBF) Process

17-4 precipitation hardened stainless steel (17-4PH SS) has been reported to have excellent mechanical properties and excellent corrosion resistance, and is one of the materials used and studied with the powder bed fusion (PBF) method. Powder bed fusion (PBF) is a new manufacturing technology that has recently attracted attention in automotive, aerospace and other industries because of its ability to produce complex geometries for high-strength and lightweight applications. In the PBF process, each layer has a different laser scan length resulting from the application of laser rotation. The laser rotation could affect the laser scan length, which causes a difference in the peak temperature and cooling rate of the deposited layer, resulting in microstructure changes. This work aims to investigate how varying the laser scan pattern in the PBF process affects the microstructure and mechanical properties of 17-4PH SS. A decrease in cooling rate was observed after applying laser scan rotation, resulting in a higher austenite phase fraction. It was confirmed that a transformation induced plasticity (TRIP) phenomenon affects mechanical characteristics. These results could be suggested for fabricating thin wall shaped such as tire blow mold parts in the powder bed fusion process using the 17-4PH SS.

Mechanisms responsible for the onset of selective laser flash sintering of 8‐YSZ

AbstractSelective laser flash sintering (SLFS) is a variation of flash sintering where the only external heat source is a scanning laser. The scanning laser locally heats a region of the sample between two electrodes, initiating measured current flow at a threshold electric field and laser energy density. This study focuses on understanding how charge transport occurs during stage I SLFS in 8 mol% yttria stabilized zirconia. Two potential charge transport mechanisms are considered: (1) a continuous current moving along a hot line between electrodes and (2) charge transported in a discrete bundle within a hot spot, localized to the area under the laser beam. Two laser scan patterns are employed to experimentally determine how charge is transported during stage I SLFS. Numerical modeling is used to estimate the temperatures at which SLFS initiates, which allows the calculation of charge carrier densities and mobilities relevant for the onset of SLFS. Results demonstrate that both a discrete bundle of charges and continuous flow of charge carriers contribute to the current at the onset of SLFS.

In situ characterization of nanoparticle protein corona using real-time 3D single-particle tracking.

Nanoparticles (NPs) adsorb proteins when they are exposed to biological fluids, forming a dynamic protein corona that consists of tightly bound “hard” corona and rapidly exchanged “soft” corona. This protein layer alters the NPs’ surface identity and affects their fate in vivo. However, current methods fail to directly investigate single NP-protein interactions in the solution phase due to the fast NP diffusion and protein exchange dynamics. To address this gap, we introduce a real-time lock-on protein corona spectroscopy based on previously developed 3D single molecule active real-time tracking (3D-SMART), primarily using polystyrene NP (PSNP) and bovine serum albumin (BSA) as a model system. 3D-SMART uses real-time photon arrival information to estimate the PSNP's position within a rapid 3D laser scanning pattern. The position estimate is then used to move a piezoelectric stage to counteract the PSNP's diffusion, keeping the NP-protein complex locked in the center of detection volume for spectral interrogation. Consequently, the 3D trajectory of the NP-protein complex and the fluorescence traces of the associated proteins can be synchronously measured for size measurement and hard corona quantification. Moreover, we applied a lock-in filtering-based algorithm to the protein intensity trace to extract the full protein corona signal in situ at signal-to-background ratios of 1%. It was discovered that the full corona consists of a sub-monolayer of BSA on PSNP, which contains twice the quantity of proteins compared to hard corona measurements, indicating that half of the full protein corona can be classified as soft corona. To expand this method to smaller NP-protein systems, we have further implemented a Galvo mirror as control actuator with response time five times faster than the piezoelectric stage, opening the possibility of studying a wider range of transient NP-protein interaction in vivo.

Impact of Pulsed Laser Parameters and Scanning Pattern on the Properties of Thin-Walled Parts Manufactured Using Laser Metal Deposition

The quality of parts manufactured using laser metal deposition (LMD), similar to other additive manufacturing methods, is influenced by processing parameters. Such parameters determine geometric stability, favorable microstructures, and good mechanical properties. This study aimed to investigate the effects of pulsed laser parameters (duty cycle and pulse frequency) and scanning patterns (unidirectional and bidirectional patterns) on the properties of parts fabricated using LMD. Results show that the properties of the LMD-fabricated parts are obviously influenced by pulsed laser parameters and scanning patterns. Using the unidirectional scanning pattern in both pulsed laser parameters enhances the properties of the thin-walled parts prepared using LMD. An increase in duty cycle can improve geometric stability, increase grain size, and reduce microhardness. Furthermore, the geometric stability does not vary considerably with the use of different frequencies, but the microstructure of fabricated parts shows various grain sizes with different pulse frequencies. In addition, the microhardness increases as the frequency increases from 13.33 to 50 Hz. In general, the influence of the duty cycle on geometric properties is greater than that of frequency. Meanwhile, the impact of frequency on microhardness is greater than that of the duty cycle.

Study on the effects of manufacturing parameters on the dynamic properties of AlSi10Mg under dynamic loads using Taguchi procedure

This study presents the effect of additive manufacturing (AM) laser powder bed fusion (LPBF) on the dynamic properties of an AlSi10Mg alloy. AlSi10Mg products manufactured using the LPBF process allow tailoring the dynamic properties to the needs of applications. Hence, we created a design of experiments based on the Taguchi method to study the effects of the most influential LPBF process parameters. The chosen parameters were laser power, scanning speed, hatching distance, scanning pattern, and fabrication orientation. Using a split Hopkinson pressure bar experimental system, the maximum dynamic stress (MDS) and elongation until fracture were characterized. The dynamic properties were significantly affected: up to a 30% increase in the MDS, without a reduction in elongation was observed. Sensitivity analysis revealed that the fabrication orientation significantly influences the dynamic properties. This study quantitatively correlates the macrostructure of the melt pools and their dynamic properties. There is a close to linear dependency between the crack path following the melt pool boundaries to the MDS. The results obtained from this study can assist engineers in estimating their dynamic response to materials by analyzing the LPBF AlSi10Mg macrostructure.

Effects of laser surface modification on stainless steel 316L thin annular discs under radial and cartesian scans

Lasers direct use on metals allows treatment of localized surfaces, without affecting nearby material or the body of the piece. Depending on the expected result, laser surface treatments are classified into hardening, texturing, alloying, coating, and melting. Using the latter is possible to modify the microstructure, dissolve precipitates, and increase hardness and wear resistance, among other applications depending on the material and the parameters used. In this work, it is proposed to empirically study the Laser Surface Melting (LSM) technique on 316L stainless steel annular discs, to evaluate the influence of laser scanning pattern on the results of the laser treatment. The considered scanning patterns correspond to a Cartesian, the most used pattern in the LSM process, also known as “zigzag”, and a Radial pattern, previously proposed in Additive Manufacturing research, which moves the laser in a cylindrical way. The main contribution of this work is to introduce the variable of the scanning pattern in LSM studies, besides, generating a complete characterization of laser treatment on 316L stainless steel annular discs. The results show that, under the same conditions, the variation of the scanning pattern produces a meaningful change in the stress state of the specimen and its deformation after treatment. The main results of LSM treatment were an increase in crystallite size and a decrease in dislocation density, a state of compression, a slight hardening in the melted layer, and a decrease in toughness. This knowledge can be used to encourage new scanning patterns in this field.

Influence of laser powder bed fusion scanning pattern on residual stress and microstructure of alloy 718

A comprehensive investigation is undertaken on the effect of laser scanning pattern on the microstructure of cylindrical samples made of Alloy 718 processed by Laser Powder Bed Fusion. It is observed that the common alternate direction scanning of the laser results in a more homogeneous microstructure than the less common concentric line scans where significant microstructural heterogeneities are seen between the edges and the center of the sample. The investigation focuses on the precipitation, crystallographic texture, grain size, grain morphology and residual stresses utilizing synchrotron X-ray diffraction, neutron diffraction and electron microscopy. The heterogeneous microstructure of the sample processed with the concentric laser pattern influences the chemical composition of the matrix, which alters the reference “strain free” interplanar spacing used for evaluating the residual strain. The investigation underlines the significance of the processing parameters on the homogeneity of the microstructure and the effect of the chemical variations on the determination of residual stresses in materials such as Alloy 718, where strong local chemical variations occur because of different types and extent of precipitation.

Effect of laser scanning pattern on residual stress in Al-10%Si-0.5%Mg objects fabricated by laser powder bed fusion

Effect of laser scanning pattern on residual stress in Al-10%Si-0.5%Mg objects fabricated by laser powder bed fusion

Effect of laser scan pattern in laser powder bed fusion process: The case of 316L stainless steel

Recent developments in additive manufacturing technologies allows generating different type of microstructure by modifying the additive manufacturing parameters. Indeed, with the laser powder bed fusion process, the laser power, energy density or laser scan strategies are such of parameters which affect size, morphology and orientations of grains. This allows to build microstructures which were no accessible with conventional processes. Furthermore, the control of laser scan strategies allows to significantly reducing the size and density of defects inherent to the process (gas pores, lack of fusion). These advances make it possible to study the role of microstructure and defect on fatigue crack initiation behaviour for additive manufactured materials. This paper aims to take this opportunity for studying the 316L stainless steel built with the laser powder bed fusion process.

Experimental Investigation and Optimization of Laser Parameters on Tensile Strength of Sintered PP and PP/CNT Composite in Selective Laser Sintering

Selective laser sintering (SLS) is an additive manufacturing technology whereby parts are built through selective consolidation of a powder by a laser beam. Recently, Polypropylene (PP) has been used in the SLS because of its appropriate mechanical performance as well as low cost and density. In the SLS, laser parameters are very effective on the mechanical properties of the sintered parts. In this study, the effects of the laser parameters including power, the laser scanning speed, and the laser scanning pattern on the tensile strength of the sintered samples were experimentally investigated. The PP and PP/CNT (carbon nanotube) composites were used as raw materials and the results were compared. Taguchi method was employed as the experimental design, and optimal levels of parameters were extracted using signal-to-noise analysis. The main effects of factors and interactions were considered in this paper. The results show that the laser scanning pattern and the laser power have the most effects on the tensile strength of the produced samples. In addition, the comparison between the results of the experiments demonstrates an increase in the tensile strength, which is 15% in maximum value, by adding CNT to PP.

An Efficient Track-Scale Model for Laser Powder Bed Fusion Additive Manufacturing: Part 2—Mechanical Model

This is the second of two manuscripts that presents a computationally efficient full-field deterministic model for laser powder bed fusion (LPBF). The Hybrid Line (HL) thermal model developed in part I is extended to predict the in-process residual stresses due to laser processing of a nickel-based superalloy, RENÉ 65. The computational efficiency and accuracy of the HL thermo-mechanical model is first compared to the exponential decaying heat input model on a single-track simulation. LPBF thin-wall builds with three different laser powers and four printing patterns are evaluated in this study and compared with part-scale simulations. The simulations show good agreements with the experimental X-Ray diffraction measured residual stresses. Compared to the laser power, the scanning pattern is demonstrated to have significant effects on residual stresses. Laser scan patterns utilizing short laser paths generate lower tensile stress along the longitudinal direction of the part and higher compressive stress along the build direction.

Heat impact during laser ablation extraction of mineralised tissue micropillars

The underlying constraint of ultrashort pulsed laser ablation in both the clinical and micromachining setting is the uncertainty regarding the impact on the composition of material surrounding the ablated region. A heat model representing the laser-tissue interaction was implemented into a finite element suite to assess the cumulative temperature response of bone during ultrashort pulsed laser ablation. As an example, we focus on the extraction of mineralised collagen fibre micropillars. Laser induced heating can cause denaturation of the collagen, resulting in ultrastructural loss which could affect mechanical testing results. Laser parameters were taken from a used micropillar extraction protocol. The laser scanning pattern consisted of 4085 pulses, with a final radial pass being 22 upmu {text {m}} away from the micropillar. The micropillar temperature was elevated to 70.58 ^{circ }{text {C}}, remaining 79.42 ^{circ }{text {C}} lower than that of which we interpret as an onset for denaturation. We verified the results by means of Raman microscopy and Energy Dispersive X-ray Microanalysis and found the laser-material interaction had no effect on the collagen molecules or mineral nanocrystals that constitute the micropillars. We, thus, show that ultrashort pulsed laser ablation is a safe and viable tool to fabricate bone specimens for mechanical testing at the micro- and nanoscale and we provide a computational model to efficiently assess this.

A Coupled DEM/SPH Computational Model to Simulate Microstructure Evolution in Ti-6Al-4V Laser Powder Bed Fusion Processes

A new multi-stage three-dimensional transient computational model to simulate powder bed fusion (L-PBF) additive manufacturing (AM) processes is presented. The model uses the discrete element method (DEM) for powder flow simulation, an extended smoothed particle hydrodynamics (SPH) for melt pool dynamics and a semi-empirical microstructure evolution strategy to simulate the evolving temperature and microstructure of non-spherical Ti-6Al-4V powder grains undergoing L-PBF. The highly novel use of both DEM and SPH means that varied physics such as collisions between non-spherical powder grains during the coating process and heat transfer, melting, solidification and microstructure evolution during the laser fusion process can be simulated. The new capability is demonstrated by applying a complex representative laser scan pattern to a single-layer Ti-6Al-4V powder bed. It is found that the fast cooling rate primarily leads to a transition between the β and α martensitic phases. A minimal production of the α Widmanstatten phase at the outer edge of the laser is also noted due to an in situ heat treatment effect of the martensitic grains near the laser. This work demonstrates the potential of the coupled DEM/SPH computational model as a realistic tool to investigate the effect of process parameters such as powder morphology, laser scan speed and power characteristics on the Ti-6Al-4V powder bed microstructure.

Microstructure and properties of additively manufactured Al–Ce–Mg alloys

Additive manufacturing of aluminum alloys is largely dominated by a near-eutectic Al-Si compositions, which are highly weldable, but have mechanical properties that are not competitive with conventional wrought Al alloys. In addition, there is a need for new Al alloys with improved high temperature properties and thermal stability for applications in the automotive and aerospace fields. In this work, we considered laser powder bed fusion additive manufacturing of two alloys in the Al–Ce–Mg system, designed as near-eutectic (Al–11Ce–7Mg) and hyper-eutectic (Al–15Ce–9Mg) compositions with respect to the binary L → Al + Al11Ce eutectic reaction. The addition of magnesium is used to promote solid solution strengthening. A custom laser scan pattern was used to reduce the formation of keyhole porosity, which was caused by excessive vaporization due to the high vapor pressure of magnesium. The microstructure and tensile mechanical properties of the alloys were characterized in the as-fabricated condition and following hot isostatic pressing. The two alloys exhibit significant variations in solidification structure morphology. These variations in non-equilibrium solidification structure were rationalized using a combination of thermodynamic and thermal modeling. Both alloys showed higher yield strength than AM Al-10Si-Mg for temperatures up to 350 °C and better strength retention at elevated temperatures than additively manufactured Scalmaloy.