Laser Scanning Parameters Research Articles

Microstructures and mechanical properties of aluminum-transition metal binary alloys (Al-Fe, Al-Mn, and Al-Cr) processed by laser powder bed fusion

Transition metals added as alloying elements to aluminum improve high-temperature strength. To design heat-resistant aluminum alloys for additive manufacturing, we have focused on the basic transition metals of iron (Fe), manganese (Mn), and chromium (Cr). The present study systematically examined the aging characteristics and the relationship between the microstructures and mechanical properties of aluminum-transition metal alloys, specifically Al-3mass%Fe, Al-3mass%Mn, and Al-3mass%Cr, fabricated by laser powder bed fusion (LPBF). All the LPBF samples were densified by optimizing laser scan parameters, achieving a relative density greater than 99.9%. The hardness values of the LPBF samples, especially Al-3mass%Fe, increased as a result of the aging heat treatment, leading to excellent high-temperature strength. From the extensive microstructural investigation, the peak aged Al-3mass%Fe and Al-3mass%Mn LPBF samples had microstructures with distinct network Al-Fe and granular/platy Al3Mn compounds with high-temperature stability, respectively. The improvement of mechanical properties in the Al-3mass%Fe and Al-3mass%Mn LPBF samples was thus attributed to the microstructural composite and dispersion hardening along with the grain boundary reinforcement. Meanwhile in the Al-3mass%Cr LPBF sample, chromium had a tendency to form a solid solution in the α-Al matrix, leading to solid-solution hardening. These results indicate that the strengthening factors significantly differed depending on the added transition metals.

Controlling the geometry of laser ablated microneedle cavities in different mould materials and assessing the replication fidelity within polymer injection moulding

Microneedle arrays are minimally invasive devices which enable self-administration of drugs in a straightforward and efficient way. Recently, a low-cost mass production method to produce solid polymer microneedles using laser ablated moulds in an injection moulding process has been developed. In this context, it is key to understand how the geometry of the microneedle cavities can be changed and what parameters affect the replication fidelity during injection moulding. In this study, two laser scanning parameters, being the programmed base diameter and the number of layers were varied in four levels and their effects on the geometry of the microneedle cavities were evaluated in tool steel, copper alloy, and aluminium alloy inserts. Afterwards, the microneedle cavities were replicated in polypropylene and polycarbonate using injection moulding. The study showed how the depth and base diameter of the microneedle cavity can be changed by varying laser scanning parameters. Moreover, the moulds in the three materials exhibited different microneedle cavity shapes and dimensions, due to differences in material properties. As to the injection moulding process, strong linear relationships were found between the aspect ratio of the microneedle cavities and the replicated thermoplastic microneedles, where a higher aspect ratio resulted in a lower replication. The replication fidelity for the polypropylene microneedles was in all cases higher compared to the polycarbonate needles. Between the three mould materials, a comparable replication fidelity was found for tool steel and copper alloy and a lower replication for the aluminium alloy, due to differences in thermal and wetting properties.

Prediction of residual stress and distortion in laser powder bed fusion additive manufacturing process of Inconel 718 alloy

Rapid heating, melting, solidification and cooling cycles generates high residual stress in additively manufactured parts, causes to part distortion. Finite Element Method (FEM) is one of the common and practical ways to predict residual stress and distortion taking place during additive manufacturing. Thus, it is possible to determine appropriate parameters to eliminate such problems. In this study, laser powder bed fusion (LPBF) additive manufacturing (AM) of Inconel 718 alloy is considered for simulation. Case studies on the residual stress and distortion of arch-shaped structure parts were presented through comparing the simulation results in present study and experimental data borrowed from the literature. The influence of laser scanning parameters such as laser power, scanning speed, layer thickness, scanning strategy on residual stress and distortion of the arch-shaped structure was investigated and compared with the borrowed experimental results. Besides, the influence of heat treatment on residual stress and distortion is also simulated and predicted results are presented. This study reveals that the presented FEM based simulation is capable of capturing experimental data reasonably well.

Expedited Transition in the Wettability Response of Metal Meshes Structured by Femtosecond Laser Pulses for Oil-Water Separation.

Oil-water separation using super-wetting and the selective permeability of membranes for oil or water has great ecological and economic significance. We report on the transition of wettability response, from superhydrophilic underwater-superoleophobic to superhydrophobic-superoleophilic state, by nanostructuring stainless steel and copper meshes using ultrashort femtosecond laser pulses. Our approach is environment-friendly, chemical free, and efficient as it exploits the benefit of aging the processed samples in a high vacuum environment. We optimized the laser scanning parameters, mesh pore size, and aging conditions to produce membranes exhibiting an extraordinary separation efficiency of 98% for the oil-water mixture. A variation in the water and oil contact angles for different meshes is presented as a function of the laser scanning speed. Stainless steel meshes with 150 μm pore size and copper meshes with 100 μm pore size have demonstrated an excellent wettability response for oil and water phases. Vacuum aging causes rapid chemisorption of hydrocarbons on laser-structured surfaces in the absence of water molecules, rapidly transforming the wetting state from superhydrophilic to superhydrophobic.

Laser nitriding of titanium surfaces for biomedical applications

In this paper, titanium nitrides were produced on commercially pure titanium samples by laser nitriding processes, with the goal of improving bio-compatibility and corrosion resistance. To identify the critical set of laser nitriding processing parameters (a combination of both laser scanning parameters and the N2 content processing gas), mouse MLO-Y4 cells were utilized for bio-compatibility evaluation, and simulated body fluid was used for electrochemical behavior examination to characterize corrosion performance. The modified surfaces were also characterized with scanning electron microscope, X-ray diffraction, and nano-indentation techniques to reveal the connections between the bio-compatibility/corrosion resistance performance and the nitrided layer composition, thickness, and surface roughness. For improved bio-compatibility and corrosion resistance performance, a dense and thick titanium-nitride dendritic layer is desirable, which can be formed in a pure N2 environment, together with a high laser energy density.

Particle-based simulation, dimensional analysis and experimental validation of laser absorption and thermo-viscous flow during sintering of polymers

Smoothed Particle Hydrodynamics simulations are used to study the thermo-viscous flow on the powder particle length scale. The effects of laser absorption, latent heat, thermal diffusion, melting and re-solidification, viscous diffusion, surface tension, Marangoni currents and gravity are considered in the model. Influences of varied laser scanning parameters are analyzed. The strongly different time scales of laser absorption, heat conduction and viscous flow as well as the transient behavior of temperature and strain rate observed in the simulations are predicted with good accuracy by a dimensional analysis. The simulation results for the transient surface temperature are validated by infrared camera measurements.

Electrical Discharge Machining using Copper Electrode made by Additive Manufacturing

In electrical discharge machining (EDM), it is important to evacuate EDM debris from the discharge gap for machining stability and accuracy. Especially in the processing of deep slots with high aspect ratio, EDM debris accumulated in the bottom of the groove often causes short circuit and false discharge, resulting in process instability. The tool electrode having micro holes for the machining fluid, which flush debris out of the discharge gap, enables efficient EDM machining of the deep slots. However, in conventional machining, it is difficult to drill high aspect ratio micro holes. Recently, the additive manufacturing (AM) technologies with the selective laser melting (SLM) method using metal powder attracts attention as a promising technique which can produce various complicated shapes efficiently. By using the AM method, it is possible to fabricate a high aspect ratio EDM electrode with micro holes inside. This paper describes the application of electrodes fabricated by the AM method to electrical discharge machining. Almost fully dense additive manufactured copper electrode (AM-Cu electrode) having more than 99.7% relative density was obtained by optimizing the laser scanning parameters for Cu-1.3mass%Cr (Cu-Cr) alloy powder. EDM properties of the AM-Cu tool electrode and the machinability of deep rib slots by using that electrode with micro holes inside were investigated. As a result, it was confirmed that EDM properties of the AM-Cu tool electrode were almost the same as those of conventional Cu tool electrode. Moreover, it was shown that use of the AM-Cu tool electrode with micro holes inside was found to shorten the processing time and improve machining accuracy significantly compared with the conventional solid Cu tool electrode.

Experimental observation of stress formation during selective laser melting using in situ X-ray diffraction

Despite the ongoing success of metal additive manufacturing and especially the selective laser melting (SLM) technology, process-related defects, distortions and residual stresses impede its usability for fracture-critical applications. In this paper, results of in situ X-ray diffraction experiments are presented that offer insights into the strain and stress formation during the manufacturing of multi-layer thin walls made from Inconel 625. Using different measuring modes and laser scanning parameters, several experimental observations are discussed to validate and extend theoretical models and simulations from the literature. As a sample is built-up layer by layer, the stress state changes continuously up until the last exposure. The localized energy input leads to a complex stress field around the heat source that involves alternating tensile and compressive stresses. The correlation of temperature and yield strength results in a stress maximum at a certain distance to the top layer. The present study demonstrates the potential of high-energy synchrotron radiation diffraction for in situ SLM research.

Picosecond Laser-Induced Hierarchical Periodic Near- and Deep-Subwavelength Ripples on Stainless-Steel Surfaces.

Ultrafast laser-induced periodic surface subwavelength ripples, categorized based on the ripple period into near-subwavelength ripples (NSRs) and deep-subwavelength ripples (DSRs), are increasingly found in the variety of materials such as metals, semiconductors and dielectrics. The fabrication of hierarchical periodic NSRs and DSRs on the same laser-irradiated area is still a challenge since the connection between the two remains a puzzle. Here we present an experimental study of linearly polarized picosecond laser-induced hierarchical periodic NSRs and DSRs on stainless-steel surfaces. While experiencing peak power density higher than a threshold value of 91.9 GW/cm2, in the laser-scanned area appear the hierarchical periodic NSRs and DSRs (in particular, the DSRs are vertically located in the valley of parallel NSRs). A large area of the uniformly hierarchical periodic NSRs and DSRs, with the spatial periods 356 ± 17 nm and 58 ± 15 nm, respectively, is fabricated by a set of optimized laser-scanning parameters. A qualitative explanation based on the surface plasmon polariton (SPP) modulated periodic coulomb explosion is proposed for unified interpretation of the formation mechanism of hierarchical periodic NSRs and DSRs, which includes lattice orientation of grains as a factor at low peak power density, so that the initial DSRs formed have a clear conformance with the metallic grains.

Modeling of thermal behavior and microstructure evolution during laser cladding of AlSi10Mg alloys

An improved three-dimensional finite element model has been proposed for studying the thermal behavior and microstructure evolution during laser cladding of AlSi10Mg alloys. Different material properties between AlSi10Mg powders and AlSi10Mg alloys are distinguished from the experiment and theoretical calculation to provide more reliable material parameters for simulation. In order to investigate the melting and solidification process during the formation of cladding layers, a temperature selection judgment mechanism is established to simulate the evolution of AlSi10Mg powders from the powder state to melting state and alloy state. In addition, to simulate the complex thermal behavior associated with powder particles and the voids between particles, a simplified exponential attenuation model is used for correcting the heat source. A complex asymmetric heat source considering about the different material properties and laser absorptivity on both sides of the remelting zone is used for multi-track cladding process.By simulating the temperature distribution of molten pool, the improved FEM could be used to predict the geometric shape of cladding layers (ignoring the effect of melting flow) and the temperature history. The simulation results show that the heat tends to diffuse to the unmelted powder owing to the asymmetric heat source during multi-track cladding, which leads to the asymmetry of cladding layers along the width direction. Based on the results of the temperature field simulations and the solidification characteristics of AlSi10Mg powders, the temperature gradient (G), solidification growth rate (R), cooling rate (G*R) and G/R are investigated to predict the morphology and size of the solidification microstructure under different laser scanning parameters. The scanning speed mainly determines the cooling rate during the laser cladding process, which results in different microstructures. Higher scanning speed leads to higher cooling rate, corresponding to a finer microstructure. Coarse dendrites are generated at the bottom of the molten pool, while finer dendrites are formed at the top.

Picosecond laser texturing on titanium alloy for biomedical implants in cell proliferation and vascularization.

Introducing specific textures to titanium alloy implant surface is helpful to modify the surface properties of materials. In this article, biomedical TC4 (Ti-6Al-4V) alloy was textured by a 10-ps infrared laser. Laser parameters that directly affected the detailed dimension of textures and its characteristics were optimized within laser power, defocusing amount, and scanning parameters via response surface methodology. These textures consisted of groove array about 30-90 μm in depth and 100 μm in width were prepared and their surface property (including surface morphology, element composition, wetting behavior, and biocompatibility) was analyzed. Surface characteristic analysis indicated that picosecond laser texturing improved surface properties and biocompatibility mainly by altering the microstructure and morphology of materials. In addition, laser textured groove array promoted contact area and hydrophobicity of material surface. Cell culture experiments and animal studies showed that titanium alloy implants with 30- and 60-μm-deep groove arrays on the surface-enhanced cell proliferation and adhesion. Meanwhile, compared to the polished samples, these groove arrays promoted the growth of new blood vessels and enhanced the combination of blood vessel and implants in vivo. That is, the deeper groove array was, and the better vascularizing effect the blood vessel exhibited.

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.

Pursuit of optimal synthetic conditions for obtaining colloidal zero-valent iron nanoparticles by scanning pulsed laser ablation in liquids

Liquid-Assisted Pulsed Laser Ablation (LA-PLA) is a promising top-down method to directly synthesize colloidal dispersions of nanoparticles in a eco-friendly manner. However, the role of LA-PLA synthesis parameters is not yet fully agreed. This work seeks to optimize the production of nanoscale zero-valent iron (nZVI) particles suitable for biomedical or environmental applications using nanosecond LA-PLA on iron targets with different ablation media, laser and target scanning parameters. The use of alcohols as solvents produces iron-iron oxide core-shell nanoparticles with amorphous cores, except for a small crystalline fraction corresponding to the biggest core sizes. Decreasing carbon chain length and complexity leads to a thinning of the carbonaceous material coatings and an increase of the colloidal stability and the nanoparticle productivity. Moreover, a decrease of solvent density and surface tension allows obtaining reduced sizes and polydispersity values. Among, laser and scanning parameters, the pulse accumulation per spot displayed a clear effect in boosting size and productivity. As main outcome, aqueous dispersions with suitable colloidal properties are obtained, either by transferring to water of optimized nZVI particles produced in ethanol, or by direct formation of nZVI particles and in situ coating with hydrophilic molecules in aqueous solutions of these molecules.

Nanosecond laser texturing with hexagonal honeycomb micro-structure on Titanium for improved wettability and optical properties

Nanosecond laser provided an efficient and flexible solution for surface texturing on different metals. In this work, a hexagonal honeycomb model was designed on Titanium by laser processing. The surface texturing experiment was carried out by a nanosecond pulsed fiber laser with different scanning speeds. The Scanning Electron Microscopy and White Light Interferometer were used to characterize the surface morphology and roughness. Besides, the contact angle instrument and the Fourier Transform Infrared Spectrometer were used to obtain the effects of surface modification on wettability and optical properties of the Titanium. The wettability and optical reflectivity of the surface changed by controlling the side length of the hexagonal honeycomb model and the laser scanning parameters. The contact angle reached a maximum of 147.4°, the contact angle hysteresis was found to lie between 1.6° and 7.1°, and the optical reflectance for titanium surface decreased by more than 90% after laser processing. The approach presented in this study provided a feasible and efficient method to improve the wettability and optical absorption of titanium surface.

Influence of selective laser melting process parameters on texture evolution in pure copper

In the present work, Crack-Free pure copper parts with relative densities exceeding 98% are processed by selective laser melting (SLM) using a high-power fiber laser. The influence of different laser scan parameters such as hatch spacing, laser power and scan speed on the texture evolution in the manufactured parts is determined. On the top surfaces, a clear interrelationship between laser scan parameters and crystallographic texture is established. This shows that the texture at the top surface can be very strong especially when a high laser power and a low scanning speed is used. However, a random texture exists in the core of the sample due to successive re-melting and altered heat gradients induced by the application of a 90° scan rotation between subsequent layers. A maximum electrical conductivity of 88% of the International Annealed Copper Standard (IACS) is achieved in the as-built state. Damage to the optical mirror of the SLM machine is observed and recommendations for sustainable up-scalability are proposed.

Laser-Induced Reduction of Graphene Oxide by Intensity-Modulated Line Beam for Supercapacitor Applications.

Supercapacitors are irreplaceable energy-storage devices for high power output and rapid charge/discharge of electrical energy. In this study, the laser-based fabrication of reduced graphene oxide (rGO) electrodes for supercapacitors is demonstrated with several new features of laser irradiation. A conventional CO2 laser irradiation system is equipped with (1) a nitrogen blower to avoid combustion of the GO paper, (2) a cylindrical lens for producing a wide line beam, and (3) an optical chopper system for generating an intensity-modulated laser beam. Scanning of the intensity-modulated line beam transforms an extended area of GO into chemically reduced and physically porous graphene. The effects of the laser beam modifications and scanning parameters on the electrochemical performance of the rGO electrode are investigated. The rGO electrode exhibits a high specific capacitance (up to ∼130 F/g) at a current density of 1 A/g. This work can serve as a reference for the process optimization of laser-induced GO reduction.

Temperature Modeling of AISI 1045 Steel during Surface Hardening Processes.

A Coupled thermo-mechanical finite element model was employed to simulate the possible effects of varying laser scanning parameters on the surface hardening process for AISI 1045 and AISI 4140 steels. We took advantage of the high-power density of laser beams to heat the surface of workpieces quickly to achieve self-quenching effects. The finite element model, along with the temperature-dependent material properties, was applied to characterize the possible quenching and tempering effects during single-track laser surface heat treatment. We verified the accuracy of the proposed model through experiments. The effects of laser surface hardening parameters, such as power variation, scanning speed, and laser spot size, on the surface temperature distribution, hardening width, and hardening depth variations during the single-track surface laser treatment process, were investigated using the proposed model. The analysis results show that laser power and scanning speed are the key parameters that affect the hardening of the material. The numerical results reveal that the proposed finite element model is able to simulate the laser surface heat treatment process and tempering effect of steel.

Microstructure and mechanical properties of X65MoCrWV3-2 cold-work tool steel produced by selective laser melting

In this study, martensitic cold-work tool steel X65MoCrWV3-2 was processed by selective laser melting (SLM) by varying the laser scanning parameters and baseplate preheating temperatures. Porosity as well as crack density of the SLM-densified steel were determined by quantitative image analysis. The resulting microstructure and the associated local mechanical properties were characterized, and the hardness-tempering behavior of the SLM-densified steel was compared to the behavior of the conventionally manufactured X65MoCrWV3-2 steel in the cast and hot-formed condition. Regardless of the preheating temperature, SLM-densified X65MoCrWV3-2 possesses a porosity of less than 0.5 vol.-%. The crack density was reduced significantly by means of a higher preheating temperature. The microstructure after SLM densification shows a fine, equiaxed cellular-dendritic subgrain structure, superimposed by lath- or needle-like martensite. The martensite morphology appeared to be finer at a lower preheating temperature, whereas the observed subgrain structure did not seem to be influenced by the preheating temperatures. Microhardness measurements indicated tempering effects in first solidified layers caused by the densification of subsequently deposited layers. Peak hardness after tempering of the SLM-densified steel was found to be higher compared to the maximum hardness in the X65MoCrWV3-2 steel in the cast condition.

Numerical simulation and process optimization of laser surface treatment on duplex stainless steel

Duplex stainless steel (DSS) has excellent mechanical properties and corrosion resistance. The excellent performance is due to an approximately equal ratio of ferrite and austenite, 50:50. Influenced by the welding thermal cycle, the ratio of ferrite and austenite shows an imbalance in DSS welded joints, resulting in the loss of performance. Laser surface treatment (LST) has marvelous advantages of high flexibility and efficiency to restore corrosion resistance of DSS welded joints. The effect of LST is dependent on laser scanning parameters, material composition, specimen geometry, and so on. The resulting phase ratio is very sensitive to those parameters. In this paper, three different types of DSS, 2304, 2205, and 2507, were used as test pieces. The temperature field under different LST parameters was calculated by finite element analysis, which helped to optimize the LST processing window. The phase transformation of DSS was divided into three steps: austenite to ferrite and ferrite grain growth in the heating stage, ferrite to austenite in the cooling stage. The effect of thermal history on the ratio of ferrite and austenite was investigated by thermal simulation experiment and numerical calculation. The calculation results were in good agreement with metallographic measurement. It was concluded that the established numerical simulation is capable of optimizing LST processing parameters and predicting the phase ratio of DSS after certain thermal cycle considering different material conditions and thermal conditions.

Surface modification on AISI 316L stainless steels by nanosecond laser with boron nitride powders

Austenitic stainless steels AISI 316L is widely used in chemical and food manufacturing because they possess a unique combination of mechanical and corrosion resistance properties. However, applications of 316L stainless steels are limited due to their relatively low yield strength and low hardness (200 HV). Austenitic stainless steels can not be transformation hardened by heat treatment. Therefore, this work investigates the effect of nanosecond laser surface treatment on 316L stainless steel with boron nitride (BN) powder coating to improve its hardness and wear properties. The boron nitride power was used to improve its wear resistance by lower surface friction. The laser heating effect can be controlled by adjusting the laser power and scanning speed. We measured the laser treated layer thickness and the hardness along its cross-section by a micro Vickers hardness machine. The results showed that the laser process hardened the top surface to 290 HV hardness up to 250 μm depth. The structure of the laser treated region was strongly affected by the laser scanning parameters and create smaller grain size long the laser track lines. The BN powders did not have significant effect on hardness. The laser treated region had material redeposit on the surface, leading to rougher surface and higher corrosion rate measured by a potentiodynamic test.