Low Laser Energy Density Research Articles

Controllable plasmonic nanostructures induced by ultraviolet laser

The plasmonic resonance express as strong extinction band and great enhancement of the electromagnetic field in the vicinity of the nanoparticles, which have been exploited in various applications such as data storage, super-resolution imaging, light collimation, solar cells, biosensors and absorbers. However, a simple and effective route to obtain uniform metal nanoparticles is still a challenge. Here, we have demonstrated a cost-efficient, lithography-free, and facile methods to obtain controllable gold nanoparticles (GNPs) for structure color by direct ultraviolet (UV) laser writing. The thermals dewetting process realized by UV laser direct writing, greatly simplifies the manufacturing process of plasmonic nanoparticles, lowers the bar for equipment, and realizes the structure color in selected areas. The direct laser writing process is performed on a 4 nm-thin island-like Au films atop dielectric-metal sandwiches. By local laser heating, the semi-continue gold film merges and reshapes into approximately spherical structure, generating brand colors. The structure color changes as a function of the laser energy density, low laser energy density to green and high laser energy density to red. Especially for lager laser energy density, the processed area shows a high saturation Numerical simulations confirm that the reflectance of the processed area is strongly affected by the absorption of the bottom metal layer and the local surface plasmon resonance of the top GNPs. In addition, the top GNPs play a major role in tunning the peak of reflection spectrum.

Influence of Low Energy Density Laser Re-Melting on the Properties of Cold Sprayed FeCoCrMoBCY Amorphous Alloy Coatings

Fe-based amorphous alloys (FAA) have excellent anti-corrosion and anti-abrasive comprehensive performances. However, sprayed thin FAA coatings with high porosity cannot provide efficient protection, or even accelerate the corrosion rate of the substrate due to galvanic corrosion. Laser re-melting densifying is usually used to improve the anti-corrosion performance of sprayed coatings. There are two disadvantages of the common laser re-melting method, including crystallization and residual stress. In the present paper, a low density energy laser re-melting method was used to improve the performance of cold spraying (CS) FeCoCrMoBCY FAA coating on 40Cr substrate. The results show that the CS FAA coatings were crystallized partially during the melting process. The hardness of the coating is improved at the melting zone after laser re-melting, which improves the anti-abrasive performance. Potentiodynamic test results show that laser re-melting can decrease the corrosion rate, but the salt spray test indicates that low energy density re-melting cannot eliminate penetrated diffusion passage. Further optimization should be conducted to improve the anticorrosion performance for this method.

Extremely Robust and Reliable Transparent Silver Nanowire‐Mesh Electrode with Multifunctional Optoelectronic Performance through Selective Laser Nanowelding for Flexible Smart Devices

As the demand for foldable or rollable smart electronic devices increases, the issue of damage caused by cyclic fatigue of the flexible transparent electrode (FTE) due to repeated bending stress at extremely low bending radius is emerging. Herein, it is demonstrated that a mesh electrode composed of silver nanowires (Ag NW–mesh) with multifunctional optoelectronic performance microfabricated without photomasking via selective laser nanowelding (SLNW) followed by soft‐etching of highly ductile Ag NWs on a flexible polymer film exhibits long‐term cycling stability at bending stresses of a 1 mm radius of curvature (ROC). In the optimal NW microgrid pattern design, the cycle fatigue resistance of NW–mesh electrodes considerably increases with increase in NW areal mass density (AMD) and decreasing microgrid spacing due to the improved weldability and networkability of NWs at a low laser energy density of 1 J cm−2. A diagonally gridded NW–mesh electrode with 30 μm line spacing exhibits extremely high reliability with little change in the performance of Joule heating and electromagnetic interference shielding under long‐term bending stresses of 300 000 cycles at 1 mm ROC. Being a simple process without photomasking, this method can easily use a multifunctional FTE with ultrahigh fatigue resistance and reliability suitable for highly flexible smart devices.

Machinability of INCONEL718 Alloy with a Porous Microstructure Produced by Laser Melting Powder Bed Fusion at Higher Energy Densities.

Products produced by additive manufacturing (AM) seek to exploit net shape manufacturing by eliminating or minimizing post-process stages such as machining. However, many applications which include turbo machinery components with tight dimensional tolerances and a smooth surface finish will require at least a light machine finishing stage. This paper investigates the machinability of the additively fabricated INCONEL718 (IN718) alloy produced by laser melting powder bed fusion (LM-PBF) with different levels of spherical porosity in the microstructure. The literature suggests that the band width for laser energy density, which combines the various scan process parameters to obtain a low spherical type porosity in the LM-PBF IN718 alloy (~1%), has wide breadth. With the increasing laser energy density and above a threshold, there is a rapid increase in the spherical pore size. In this paper, three tube samples each with different levels of spherical porosity were fabricated by varying the laser energy density for LM-PBF of the IN718 alloy within the stable and higher energy density range and the porosity measured. A low laser energy density was avoided due to balling up, which promotes highly irregular lack of fusion defects and poor consolidation within the alloy microstructure. An orthogonal turning test instrumented, with a three-component dynamometer to measure the cutting forces, was performed on AM produced IN718 tube samples under light cut conditions to simulate a finish machining process. The orthogonal turning tests were also performed on a tube sample obtained from the wrought extruded stock. The machining process parameters, which were studied include varying the cutting speed at three levels, at a fixed feed and under dry cut conditions for a short duration to avoid the tool wear. The results obtained were discussed and a notable finding was the higher rate of built-up-edge formation on the tool tip from the AM samples with a higher porosity and especially at a higher cutting speed. The paper also discusses the mechanisms that underpin the findings.

On the Morphology Changes of Al and Al-Cu Powder After Laser Melting

Gas-atomized powders are commonly used in additive manufacturing, specifically laser powder bed fusion, due to their high flowability during recoating. Morphological changes can occur in particles that are irradiated by the laser during additive manufacturing, but are not incorporated into the melt pool. These irradiated particles will affect the rheology of the recycled powder in subsequent builds, potentially leading to failures due to uneven powder flow or spatial distribution. Thus, a better understanding of mechanisms that degrade the sphericity of powder after being laser irradiated is needed. This research examines morphological changes in Al and Al-Cu eutectic powders after laser melting. Two complementary approaches were taken. First, particles found along the edges of line scans following high-power (300 W) laser irradiation were characterized. The collected particles displayed morphological anomalies not observed in the as-received powder. Then, to gain a more quantitative and controlled perspective on morphological evolution, the same base powders were dispersed onto glass substrates and irradiated with a low-power (6.5 W) CW laser diode. This approach, which permits characterization of specific particles before and after laser irradiation, clearly shows laser-induced changes in the surface morphology of particles in the form of dents and rifts. These results suggest that isolated melting and resolidification of particles contained within their respective oxide shells can occur at the relatively low laser energy densities present at the edges of laser melt tracks. Thermal stresses developing in the oxide shell during cooling can account for the observed morphological changes in the context of shell-buckling theory.

Effects of laser-energy density and build orientation on the structure–property relationships in as-built Inconel 718 manufactured by laser powder bed fusion

This study investigates the effects of build orientation and laser-energy density on the pore structure, microstructure, and tensile properties of Inconel 718 manufactured by laser powder bed fusion. Three different build conditions were selected for comparison based on previous research (namely, the conditions that resulted in the worst and best fatigue lifetimes): 0° build orientation and 38 J/mm3 laser-energy density, 0° build orientation and 62 J/mm3 laser-energy density, and 60° build orientation and 62 J/mm3 laser-energy density. Differences in porosity were measured between each build condition. In terms of microstructure, all three conditions exhibited a predominantly 〈001〉 texture in the build direction, grains elongated in the build direction, and a sub-grain structure oriented with the build direction that consisted of dislocation networks decorated with nano-scale precipitates. Build orientation (0° versus 60° with respect to the build plate) produced a difference in yield strength due to anisotropic grain morphology and effective grain size. The low laser-energy density specimens showed a significant decrease in all mechanical properties compared to the high laser-energy density specimens because the amount (6.91% by volume) and size of the lack-of-fusion porosity (from insufficient melting) surpassed a level at which microstructure (the grain and sub-grain structure) no longer governs quasi-static mechanical properties. This work provides insight that could enable the tunability of structure-property relationships in as-built Inconel 718 by optimizing laser-energy density and build orientation.

Structural and Optical Properties of Self-Assembled Epitaxially Grown GaN Nanorods and Nanoporous Film on Sapphire (0001) Using Laser Molecular Beam Epitaxy.

The GaN nanoporous-film (NPF) and nanorods (NRs) were grown on sapphire (0001) using laserassisted molecular beam epitaxy (LMBE) technique by laser ablating solid GaN target at different laser energy density. The interconnected GaN NPF was grown at low laser energy density of ˜4 J/cm² whereas vertically aligned GaN NRs was obtained at high laser energy density of ˜7 J/cm². The pore size of the GaN NPF structure is in range of 40-120 nm. The GaN NRs possess hexagonal shape with six sidewall facets and truncated top facet. The length, width and density of GaN NRs are 600-900 nm, 150-250 nm and ˜2.5×107 cm-2, respectively. The X-ray rocking curve full width at half maximum values along GaN (0002) and (1012) planes for GaN NRs obtained to be 0.41 and 0.53°, respectively. The biaxial stress in hetero-epitaxially grown GaN was investigated with Raman spectroscopy and it was found that GaN NRs possesses a very low in-plane compressive biaxial stress of 0.09 GPa. The photoluminescence study exhibits a sharp band-to-band emission at 3.4 eV with a peak line width of 140 meV, signifying the good optical quality of the LMBE grown GaN NRs on sapphire (0001).

Microstructures and unique low thermal expansion of Invar 36 alloy fabricated by selective laser melting

Invar 36 alloy, which presented extremely low coefficient of thermal expansion (CTE), was fabricated by selective laser melting with island scanning strategy. The microstructures and CTE were systematically characterized. The as-SLMed Invar 36 is mainly composed of fcc γ phase and sparse bcc α precipitates, which are consistent with the phases of the wrought one. The grains grow along the maximum temperature gradient directions, as the microstructure on the side surface is dominated by large columnar grains. The cross-section shows a large number of small columnar grains in the melt pool. The low laser energy density results in the lack-of fusion pores and corresponding high level porosity up to 15%. When the laser energy density is 99.2 J/mm3 and 198.4 J/mm3, excellent quality is obtained, as the porosity is only within 0.2–0.4%. Due to the porosity and nickel element evaporation, the CTE is 1.72–1.96 × 10–6 °C−1 that is lower than that of wrought one. Besides, lower than 99.2 J/mm3, the decreasing laser energy density leads to the reduction of CTE, since the pores provide extra space for internal thermal expansion. Higher than 99.2 J/mm3, the metal vaporization rate and vapor pressure are enhanced with the increasing laser energy density, and then the loss of nickel element leads to the reduction of CTE.

Photobiomodulation Therapy on Orthodontic Movement: Analysis of Preliminary Studies with a New Protocol.

This study aimed to investigate the effectiveness of photobiomodulation therapy (PBMT) on the acceleration of orthodontic movements, deriving from its biostimulating and regenerative capacity on soft tissues, consequent to the increase in differentiation, proliferation, and activity of cells that are involved with alveolar bone remodeling. The present randomized controlled trial was conducted on six patients who required extractive orthodontic therapy because their ectopic canines had erupted. A total of eight canines were analyzed, four of which received laser irradiation (i.e., experimental group). Two weeks after the extractions, all canines of the experimental and placebo groups were distalized simultaneously and symmetrically with the laceback retraction technique. The PBMT protocol consisted of four cycles of laser applications, one each on days 0, 3, 7, and 14 of the study, with session treatment durations of 2–4 min. The results of the descriptive analysis on the distal displacement speed of the canines after 1 month of follow-up indicate an average displacement of 1.35 mm for the non-irradiated group and 1.98 mm for the irradiated group. Through inferential analysis, a statistically significant difference (p < 0.05) was found between the average speed of the irradiated canines and the control canines. The low energy density laser used in this study, with the parameters set, was found to be a tool capable of statistically significantly accelerating the distal displacement of canines.

Low-toxicity FePt nanoparticles for the targeted and enhanced diagnosis of breast tumors using few centimeters deep whole-body photoacoustic imaging

A considerable amount of early breast tumors grown at a depth over 2 cm in breast tissues. With high near-infrared absorption of iron-platinum (FePt) nanoparticles, we achieved few centimeters deep photoacoustic (PA) imaging for the diagnosis of breast tumors. The imaging depth can extend over 5 cm in chicken breast tissues at the low laser energy density of 20 mJ/cm2 (≤ ANSI safety limit). After anti-VEGFR conjugation and the tail-vein injection, we validated their targeting on tumor sites by the confocal microscopy and PA imaging. Using a home-made whole-body in vivo PA imaging, we found that the nanoparticles were rapidly cleared away from the site of the tumor and majorly metabolized through the liver. These results validated the clinical potential of the FePt nanoparticles in the low-toxicity PA theragnosis of early breast cancer and showed the capacity of our whole-body PA imaging technique on monitoring the dynamic biodistribution of nanoparticles in the living body.

Simple gold nanoparticles production method by ablated laser: Diameter modification

Recently, an improved interest in gold nanoparticles (AuNPs) with an interesting phenomenology such as plasmon resonance, has been aroused. There are many methods to synthesized AuNPs. We have successfully fabricated AuNPs base on laser ablation method. The pulsed Nd:YAG laser ablated at 1064 wavelength to a pure gold plate. The frequency was adjusted in low range to produce low energy density laser beam. Three different solution (aqueous, polyvinyl alcohol and polyethylene glycol) was applied to modify AuNPs diameter. The modified nanoparticles diameter was observed by mean UV-Vis spectroscopy exhibit different absorption peak wavelength. For comprehensive discussion, the spectrum was compared to simulation tool on MATLAB. Interestingly, AuNPs diameters have been modified by using different solution.

长脉冲激光辐照单晶硅的热损伤

In view of the thermal damage law and mechanism of monocrystalline silicon for millisecond pulsed laser, the temperature of monocrystalline silicon irradiated by millisecond pulsed laser is measured by high precision point temperature meter and spectral inversion system. Then the temperature evolution process is analyzed. Also, the temperature state during the whole process of thermal damage of monocrystalline silicon irradiated by millisecond pulsed laser and the corresponding damage structure are studied. The results of this study show that the peak temperature of laser-induced monocrystalline silicon increases with the increase of energy density when the pulse width is fixed, When the pulse width is between 1.5 ms-3.0 ms, The temperature decreases with the increase of pulse width. Temperature rise curve shows inflection point when it is close to the melting point (1687 K), the reflection coefficient is from 0.33 to 0.72. During the gasification and solidification stages, it also shows the gasification and the solidification plateau periods. Thermal cleavage damage of monocrystalline silicon precedes thermal erosion damage. Stress damage dominates under low energy density laser irradiation, while thermal damage dominates under high energy density laser irradiation. The damage depth is proportional to the energy density and increases rapidly with the increase of the number of pulses.

Mechanical properties of Invar 36 alloy additively manufactured by selective laser melting

Invar 36 alloy is widely used in aerospace engineering, owing to its extremely low coefficient of thermal expansion. This work aims to explore the underlying influence of process parameters on the microstructures and mechanical properties of Invar 36 additively manufactured by selective laser melting (SLM) with island scanning strategy. A full factorial design for a wide range of SLM process parameter sequence was established. Density and the mechanical properties including hardness, tensile strength as well as microstructures, fractography were characterized. The results confirm that the increment of exposure time and the reduction of point distance remarkably induce stable melting and achieve superior density and hardness. The laser energy density shows a pronounced effect on the microstructures, as extremely low laser energy density induces considerable lack-of fusion pores, unmelted powder particles and blurry boundaries of islands. While, very high laser energy density induces a few keyhole pores, and makes the laser scanning tracks and island boundaries transfer to be irregular. The optimal laser energy density is recommended to be 99.2 J/mm3, and the as-fabricated Invar 36 shows the relative density of 99.5%, Vickers hardness of 1.8 GPa and ultimate tensile strength of 480 MPa those are very comparable to conventionally fabricated one. The testing temperatures of 200 °C and 600 °C induce the coarsening of γ phase, and result in a significant degeneration of tensile properties. The correlations between the SLM process and mechanical properties provide experimental basis of the additive manufacturing Invar 36 alloy.

Microstructure and properties of equiatomic Ti–Ni alloy fabricated by selective laser melting

Selective Laser Melting (SLM) as one of the additive manufacturing technologies can be used to produce Ti–Ni shape memory alloys with complex shape. In this work, equiatomic Ti50Ni50 (at.%) samples were produced by SLM with different scanning speed, and near-fully dense (99.5% relative density) parts were obtained under a low input laser energy density (40 J/mm3) with the scanning speed of 1000 mm/s. The different scanning speeds had limited influence on the phase composition, transformation temperatures and Vickers hardness. Under low magnification, the typical molten pool morphology with inhomogenous microstructure was shown in the samples of SLM-produced Ti–Ni alloy, and self-accommodate martensite (B19’) twins with a few austenite (B2), nanoscale Ti2Ni and rhombohedral (R) phases were found at higher magnification. Due to the formation of nanoscale Ti2Ni phase and inhomogenous microstructure, the SLM-produced Ti–Ni alloy exhbited lower phase transformation temperatures and larger hysteresis temperatures between the start and finish point of the phase transformation compared to the Ti–Ni powder. The formation of the R phase was contributed to the special repeat heating process and stress field formed by Ti2Ni phase and dislocations in SLM equiatomic Ti–Ni alloy. The SLM-produced Ti–Ni alloy exhibits higher compressive and tensile fracture strength but lower compressive and tensile fracture strain compared to the conventional cast samples.

Fundamentals of radiation heat transfer in AlSi10Mg powder bed during selective laser melting

Purpose This paper aims to study numerically the influence of the applied laser energy density and the porosity of the powder bed on the thermal behavior of the melt and the resultant instability of the liquid track. Design/methodology/approach A three-dimensional model was proposed to predict local powder melting process. The model accounts for heat transfer, melting, solidification and evaporation in granular system at particle scale. The proposed model has been proved to be a good approach for the simulation of the laser melting process. Findings The results shows that the applied laser energy density has a significantly influence on the shape of the molten pool and the local thermal properties. The relative low or high input laser energy density has the main negative impact on the stability of the scan track. Decreasing the porosity of the powder bed lowers the heat dissipation in the downward direction, resulting in a shallower melt pool, whereas pushing results in improvement in liquid track quality. Originality/value The randomly packed powder bed is calculated using discrete element method. The powder particle information including particle size distribution and packing density is taken into account in placement of individual particles. The effect of volumetric shrinkage and evaporation is considered in numerical model.

Mechanisms driving high-cycle fatigue life of as-built Inconel 718 processed by laser powder bed fusion

This study investigates the relationships among the high-cycle fatigue life, surface roughness, and additive manufacturing processing parameters in laser powder bed fusion Inconel 718 in the as-built condition. Standardized fatigue specimens were manufactured using 25 different sets of processing parameters by varying laser power, scan speed, layer thickness, and build orientation, with three repeat specimens per parameter set. Surface roughness measurements were conducted using white light interferometry; high-cycle fatigue life was measured; and fractography analysis was performed using scanning electron microscopy. Two processing-parameter metrics were observed to dominate high-cycle fatigue life: build orientation and laser-energy density. Build orientation affected fatigue life due to the relationship between build orientation and surface roughness. Increasing surface roughness decreased the fatigue life due to increasing number of surface-crack initiation sites. For a fixed build orientation, the laser-energy density, outside of the optimal range, decreased the fatigue life due to sub-surface defects. Specifically, fractography analysis showed that sub-surface defects consisted of lack-of-fusion pores at low laser-energy densities and secondary cracking and pores (possibly related to keyholing) at high laser-energy densities. While variability in residual stresses among the specimens could also play a role, this work focuses on geometrical surface and sub-surface defects caused by different processing parameters and their corresponding impact on total fatigue life. Based on these findings, guidelines are offered to improve fatigue life of additively manufactured Inconel 718 in the as-built, non-heat-treated condition.

Laser Direct Deposition of CoCrAlSiY/YSZ Composites: Densification, Microstructure and Mechanical Properties

A comprehensive study of densification behavior, microstructural features, microhardness and wear properties of the CoCrAlSiY/YSZ composite coatings fabricated under laser direct deposition (LDD) is presented. The relationship of the laser energy density, microstructures and material properties has been built. Due to the presence of a balling phenomenon at a low laser energy density input, the relative density of the formed materials was low. As the laser energy density was increased to 200 kJ/m, a near-complete densification sample was yielded. At the same time, with the increase of the laser energy density, the microstructure of LDD-prepared CoCrAlSiY/YSZ composites went from cluster crystals to columnar crystals, to slender and uniformly distributed columnar crystals, and finally to the shape of coarsened columnar crystals. The results of the sliding wear tests indicated that the CoCrAlSiY/YSZ composites prepared by a 200-kJ/m energy density laser had the most uniform microhardness distribution with a mean value of 657 HV0.2, the smallest friction coefficient of 0.4 and the lowest wear rate of 2.83 × 10−4 mm3/Nm, which resulted from the finest microstructure of the material prepared by this laser.

Fragmentation and refinement behavior and underlying thermodynamic mechanism of WC reinforcement during selective laser melting of Ni-based composites

The WC-reinforced Inconel 718 composites were successfully fabricated through selective laser melting (SLM) additive manufacturing technology. Influences of the applied laser energy densities on the thermodynamics within the molten pool, fragmentation behavior of WC particles, and underlying fragmentation mechanism were investigated through experiments and simulations. The results revealed that the fragmentation behavior of WC particles was greatly dependent on the applied laser energy input. As a relatively low laser energy density was applied, the alloyed reaction layer formed around WC particles, and then experienced a fragmentation into a certain number of carbides in the vicinity of WC particles, due to the temperature gradient and resultant thermal tensile stress exerted at the interface of WC particles and molten Inconel 718 alloy. This fragmentation of WC particle was defined as the dissolution-diffusion-fragmentation mechanism. For an elevated laser energy density of 330 J/m was used, an increased temperature gradient and attendant thermal stress formed, and the initially incorporated WC particles experienced a severe heat damage, thereby directly fragmenting the WC particles into the refined pieces. Meanwhile, the molten liquid with more intense thermal convections favored the homogeneous distribution of the broken WC particles and the formation of the alloyed reaction layer between WC particles and molten Inconel 718 alloy liquid, which was predominated by the fragmentation-dissolution-diffusion mechanism.

Effect of Doping on the Properties of Hydrogenated Amorphous Silicon Irradiated with Femtosecond Laser Pulses

A comparative analysis of the effect of femtosecond laser irradiation on the structure and conductivity of undoped and boron-doped hydrogenated amorphous silicon (a-Si: H) is performed. It is demonstrated that the process of nanocrystal formation in the amorphous matrix under femtosecond laser irradiation is initiated at lower laser energy densities in undoped a-Si: H samples. The differences in conductivity between undoped and doped a-Si: H samples vanish almost completely after irradiation with an energy density of 150–160 mJ/cm2.

Effects of TiB2 Particle and Short Fiber Sizes on the Microstructure and Properties of TiB2-Reinforced Composite Coatings

In this study, particle and short fiber-reinforced titanium matrix composite coatings are prepared via laser in situ technique using (0.5 and 50 μm) TiB2 and Ti powder as cladding materials. The microstructure and properties of the composite coatings are studied, and the changing mechanism of the microstructure is discussed. The results reveal that particle agglomeration is prone to appear with using fine TiB2 particles. Decomposition of the particles preferentially occurs with using coarse TiB2 particles. The cracks and pores on the surface of the coating are formed at a lower laser energy density. With the increase in the laser energy density, cracking on the surface of the coating diminishes, but the coating exhibits depression behavior. The depression extent of the coating using fine TiB2 particle as the reinforcement is much less than that of the coating using coarse TiB2 particle. Moreover, the size of the aggregate and the tendency of cracking can be reduced with the increase in Ti addition. Meanwhile, short TiB fiber bundles are formed by the diffusion mechanism of rod aggregate, and randomly oriented TiB short fibers are formed mainly by the dissolution–precipitation mechanism of fine TiB2 particles. Moreover, the growth of short TiB fibers can be in an alternating manner between B27 and Bf structures. The micro-hardness and wear resistance of the coatings are evidently higher than that of the titanium alloy substrate. The wear resistance of the large size TiB2 coating is higher than that of the small size TiB2 coating under the condition of low load.