Laser-induced Microstructure Research Articles

A hybrid mesoscale-continuum approach to understand and predict melting kinetics of Al powders during laser processing

Abstract Laser interaction with metallic powders during additive manufacturing (AM) leads to fast heating and cooling rates that can affect the quality of the final products due to the formation of defects. One of the first steps towards predicting microstructures generated during AM, therefore, requires an accurate understanding of the laser energy deposition mechanisms that determine the melting kinetics at the level of individual powders. The critical challenge, however, is the availability of computational methods that can model the laser energy absorption, heat transfer, and the related microstructure evolution in individual metal powders at the length and time scales of AM. This manuscript demonstrates the capability of a novel scale-bridging methodology that combines the mesoscale quasi-coarse-grained dynamics (QCGD) simulations with a continuum two-temperature model (TTM) to account for the atomistic mechanisms of laser energy deposition and microstructure evolution and predict the kinetics of melting of individual powders at the experimental time and length scales. The scale-bridging capability of the hybrid QCGD-TTM simulations is demonstrated here by investigating the laser-induced microstructure evolution in aluminum powders with various sizes ranging from 200 nm to 20 µm. The analysis of the evolution of temperature, pressure, phase fraction, and melt fronts suggests the melting mechanism is heterogeneous due to the interaction with a laser, and the melting time is observed to decrease exponentially as the laser intensity increases. The solid-liquid interface velocity can be quantified to identify correlations with interface temperatures, and the predicted values satisfy the theoretically reported limits of crystal stability of metals against homogeneous melting. In addition, the pre-melting is found at the grain boundaries of 20 µm polycrystalline aluminum powder, while a minute contribution to melting is observed. This manuscript demonstrates the capability of the QCGD-TTM method to capture laser-powder interaction and allow the investigation of the kinetics of laser melting.

Lightweight and drift-free magnetically actuated millirobots via asymmetric laser-induced graphene

Millirobots must have low cost, efficient locomotion, and the ability to track target trajectories precisely if they are to be widely deployed. With current materials and fabrication methods, achieving all of these features in one millirobot remains difficult. We develop a series of graphene-based helical millirobots by introducing asymmetric light pattern distortion to a laser-induced polymer-to-graphene conversion process; this distortion resulted in the spontaneous twisting and peeling off of graphene sheets from the polymer substrate. The lightweight nature of graphene in combine with the laser-induced porous microstructure provides a millirobot scaffold with a low density and high surface hydrophobicity. Magnetically driven nickel-coated graphene-based helical millirobots with rapid locomotion, excellent trajectory tracking, and precise drug delivery ability were fabricated from the scaffold. Importantly, such high-performance millirobots are fabricated at a speed of 77 scaffolds per second, demonstrating their potential in high-throughput and large-scale production. By using drug delivery for gastric cancer treatment as an example, we demonstrate the advantages of the graphene-based helical millirobots in terms of their long-distance locomotion and drug transport in a physiological environment. This study demonstrates the potential of the graphene-based helical millirobots to meet performance, versatility, scalability, and cost-effectiveness requirements simultaneously.

Reconstruction of laser-induced microstructures temperature profiles

Metallic microstructures are currently widely used in micro-optoelectronics, sensorics, microfluidics, and other applications. A promising direction in the creation of such microstructures is pulsed laser processing of thin metal films. To improve laser technologies and simulate ongoing processes, data on the temperature fields that arise during laser exposure are needed. The article shows that the temperature distribution on the surface of a material during pulsed laser heating can be obtained from optical, scanning electron or atomic force micrographs. The proposed approach opens up the possibility of estimating temperature fields from archival photographs of various microstructures.

Effect of laser pre-treatment on adhesive joint performance and evaluation of interfacial strain distribution using mechanoluminescence

ABSTRACT In this study, factors that improve bonded joints structural response through laser surface pre-treatment on aluminum surfaces were investigated. Specifically, the focus was put on analyzing the laser treatment conditions, formation of the interface structure, and behavior of interface strength at the adhesive interface. The results revealed a specific range of laser treatment conditions in which tensile shear strength and fracture mode are stable. Although laser-treated adhesive joints showed cohesive failure similar to that observed in sandblasting or chemical treatment during the double cantilever beam test, the fracture toughness value was improved by a factor of 2 and 1.5, respectively. The laser-induced microstructure on the aluminum interface contributed to the improvement of fracture toughness. Furthermore, observation of the adhesive layer using a microscopic mechanoluminescence measurement system revealed that changes in the 3D structure of the interface significantly altered the stress luminescence pattern reflecting strain. As a result, direct visualization of interface strengthening via laser processing was achieved.

Temperature Relaxation in Glass-Forming Materials under Local Fast Laser Excitations during Laser-Induced Microstructuring

The ability to control the temperature distribution T(t,r) and the rate of temperature change Rt,r inside glasses is important for their microstructuring. The lattice temperature is considered at time t, exceeding the electron–phonon thermalization time, and at a distance r from the center of the model spherical heating zone. In order to describe thermal excitations, the heat capacity of glasses must be considered as a function of time due to its long-term relaxation. A method for the analytical calculation of T(t,r) and R(t,r) for glasses with dynamic heat capacity cdyn(t) is proposed. It is shown that during laser microstructuring, the local cooling rate −R(t,r) significantly depends on the time dispersion of cdyn(t). It has been established that at the periphery of the model heating zone of the laser beam focus, the local cooling rate can reach more than 1011 K/s. Strong cooling rate gradients were found at the periphery of the heating zone, affecting the microstructure of the material. This effect is significantly enhanced by the time dispersion of cdyn(t). The effect associated with this time dispersion is significant, even well above the glass transition temperature Tg, since even short relaxation times of the dynamic heat capacity cdyn(t) are significant.

Laser-induced locally controllable craze-like microstructures for polymer white structural coloration.

As a promising candidate for the inkless coloring method, white structural color has undergone widespread investigation because of its fascinating properties. Recently, various methods have been developed to prepare disordered micro/nanostructures to produce white structural color. However, complex and high-cost processing procedures severely restrict the efficient and large-scale preparation of disordered micro/nanostructures for achieving white structural color. Herein, we report an ingenious way to realize white structural color by laser-inducing craze-like microstructures in core-shell microfiber-based polymers. A microfiber with copper nanowires (CuNWs) as the core surrounded by a polyformaldehyde (POM) shell is prepared by a simple in situ fibrillation method. The craze-like microstructures with micro/nanofibrils and micropores are locally constructed in polymers by a facile, efficient, inexpensive, controllable, and environmentally friendly laser direct writing (LDW) technique. Ascribed to the broadband visible light reflection caused by disordered microstructures, the laser-induced craze-like microstructures in polymers based on CuNWs@POM core-shell microfibers exhibit a distinct white structural color. This work paves a way for achieving white structural color and provides a novel insight for utilizing the previously considered useless crazing phenomenon.

The Shape Modulation of Laser-Induced Nanowelded Microstructures Using Two Colors

The light-based nanowelding of metallic nanoparticles is of particular interest because it provides convenient and controlled means for the conversion of nanoparticles into microstructures and the fabrication of nanodevices. In this study, we investigated the wavelength dependence of laser-induced nanowelded shapes of silver nanoparticles (AgNPs). We observed that the nanowelded microstructures illuminated with only a 405 nm laser were more branched than those formed via illumination using both the 405 nm and 532 nm lasers. We quantified this observation by two compactness descriptors and examined the dependence of the power of the 532 nm laser. More importantly, to understand the experimental observations, we formulated and tested a hypothesis by calculating the wavelength-dependent electric field enhancement due to the surface plasmon resonance of the AgNPs and nanowelded microstructures when illuminated with lights at the two wavelengths. Based on the different patterns of hot spots for welding AgNPs from these calculations, numerical simulations successfully reproduced the different shapes of nanowelded microstructures, supporting our hypothesis. This work suggests the possibility of light-based control of the shapes of laser-induced nanowelded microstructures of metallic nanoparticles. This work is expected to facilitate the development of broader applications using the nanowelding of metallic nanoparticles.

Comparative study of bubble forming sites at the laser-induced microstructure on a Ni electrode during hydrogen and oxygen evolution reactions

We study the formation of hydrogen and oxygen bubbles on the identical Ni electrode with a single laser-induced microstructure to understand the role of local surface morphology on bubble formation. All the forming sites of hydrogen bubbles are found at the microstructure, while those of oxygen bubbles are found not only at the microstructure but also on the flat area. Interestingly, not all but only a few of the sites at the microstructure produce both hydrogen and oxygen bubbles, and we find that these sites have partially confined structures. In contrast, the presence of oxygen bubble forming sites on the flat area implies that the local surface morphology is not the only decisive factor to be a bubble forming site. Through the elemental analysis of the electrode surface, we find the tendency that the sites with more nickel oxides show the less bubble forming activities.

Temporal-spatial dynamics of electronic plasma in a femtosecond laser-induced sapphire microstructure.

In this study, the time-spatial evolution of single-pulse femtosecond laser-induced plasma in sapphire is studied by using femtosecond time-resolved pump-probe shadowgraphy. Laser-induced sapphire damage occurred when the pump light energy was increased to 20µJ. Based on its shadowgraphy image, the threshold electron density can be estimated to be about 2.48×1020 c m -3. The evolution law of the transient peak electron density and its spatial position as femtosecond laser propagation in sapphire were researched. The transitions from single-focus to multi-focus as the laser focus shifted from the surface to a deeper part were observed from the transient shadowgraphy images. The focal point distance in multi-focus increased as the focal depth increased. The distributions of femtosecond laser-induced free electron plasma and the final microstructure were consistent with each other.

Light nurtures plants: The picosecond laser-induced lithops-like microstructures on titanium alloy surface with broad-band ultra-low reflectivity

Micro-nanostructures on metal surfaces have received significant attention due to their widespread applications. Although ultrafast laser processing was a feasible method to fabricate micro-nanostructures, some issues involving the formation and application of novel micro-nanostructures still exist. Here, we proposed the unique lithops-like microstructures on titanium alloy (Ti-6Al-4V) by ultraviolet picosecond (UV-ps) laser treatment. These structures were either separate tomb-like bumps or bump clusters consisting of several interconnected bumps, which were similar in appearance to lithops. Due to nanoparticle deposition, these bumps were also covered with nanostructures. Titanium alloy was oxidized to form Ti2O3 and TiO2 during treatment. The formation of lithops-like microstructures involved the laser-induced physical/chemical processes of melting, flowing, vaporization, oxidation, solidification, and deposition of material. As laser energy increased, the smooth surface gradually evolved into lithops-like microstructures under the synergistic effects of electromagnetic interference, the Marangoni effect, preferential valley ablation, and thermal effects. By combining geometric light trapping within microstructures with surface plasmon resonance (SPR) absorption within nanostructures, these structures exhibited ultra-low reflectivity (0.02%∼2.31%) in the ultraviolet–visible-near-infrared band (200 ∼ 2500 nm). They also showed ultra-low reflectance (<3%) over a wide range of incidence angles (5°∼70°). The anti-reflection properties of lithops-like microstructures were superior to those of some typical micro-nanostructures.

Dynamics of hydrogen bubbles formed at a laser-induced microstructure on a Ni electrode during hydrogen evolution reaction

We perform the optical detection of hydrogen bubbles formed at a laser-induced microstructure on a Ni electrode during hydrogen evolution reaction, and study the dynamics of individual bubbles. Using the side-illumination technique to take both images of the bubbles and microstructure on a polished electrode we successfully correlate the bubble forming sites and surface morphology with an accuracy of ∼μm, and find that there are more than a few bubble forming sites at the periphery of the single microstructure. Interestingly, the responses of those sites to the current density are different, and as the current density increases more sites become active to form bubbles. Under the constant current operation the growth rates and formation periods of the bubbles are different at different sites within the single microstructure. These findings imply that a subtle local structure within a single microstructure greatly matters to the bubble forming activity of the sites.

Fabrication of micro-nano multi-scale hierarchical porous structure on the surface of Inconel718 nickel-base superalloy by one-step method

A new high-efficiency composite process consisting of laser machining and abrasive belt grinding was proposed to prepare multi-scale graded microstructures on the surface of Inconel718 nickel-base superalloy for aviation. The scheme is based on the machining characteristics of abrasive belt grinding, low damage molding and high-efficiency grinding and polishing, the effective combination of ultrashort pulse laser-induced surface microstructure growth, and the thermal effect in the process of high-energy beam machining, so as to realize the efficient preparation of hierarchical microstructure. In the one-step forming process, the accuracy of the material surface is ensured by abrasive belt grinding. The porous microstructure on the surface is catalyzed by the thermal effect produced by the high-energy beam laser. The scale of the porous structure is 3 µm–6 µm, the pore surface will generate nanoparticles, the size is below 100 nm, and dense nanoparticles will be generated on the surface. The porous structure can be super hydrophilic and adsorbed to form a liquid membrane. The formation of rib-like microstructures on the surface induced by ultrashort pulse laser makes the liquid film have directional spreading properties. Rib-like microstructure height is 50 µm–150 µm and it can be adjusted by setting the scanning line width. The one-step method for preparing hierarchical microstructures proposed in this study can efficiently process large-area microstructure surfaces with different wetting properties. • The porous structure, nanoparticles, and rib-like bulges can be efficiently fabricated. • The microstructure can be easily regulated.

Moore–Gibson–Thompson Thermoelastic Model Effect of Laser-Induced Microstructures of a Microbeam Sitting on Visco-Pasternak Foundations

Due to the intricacy of this topic, the thermal study of microstructures on triple-parameter foundations subjected to ultrafast laser pulses has not received much attention. It is necessary to determine the thermal performance of a structure to examine the thermoelastic properties that are caused by a heat source that is generated by a laser pulse. In this paper, the framework of a microscale beam is presented; it was exposed to harmonically fluctuating heat and rested on a visco-Pasternak base under the impact of axial stress. The Euler-Bernoulli beam model was used for this objective, and a very short laser pulse heated the medium. In addition, the Moore–Gibson–Thompson (MGT) non-Fourier thermoelastic theory was used to attempt to explain the thermal variables of the system, and the equations regulating the vibration of thermo-elastic microbeams were then constructed. A semi-analytical strategy is described to examine the properties of the studied field variables. This methodology uses the Laplace transform as well as an approximate computational method for inverse transformations. The influences of the operative parameters on the thermal deflection, axial thermal stress, displacement fields, and temperature change are presented. These effects include damping constants, laser pulses, and the stiffness of viscoelastic and elastic foundations. In addition, the results that were found were compared with previous literature in order to validate the derived model. Finally, more computational outcomes are presented to study the properties of different temperature factors including in the MGT thermoelastic model.

Femtosecond Laser-Induced Evolution of Surface Micro-Structure in Depth Direction of Nickel-Based Alloy

The surface coating properties of turbine blades are highly dependent on the material’s surface roughness, and the femtosecond laser-induced micro-structure can provide a wide range of roughness with periodicity. However, precise control of femtosecond laser-induced micro-structure is difficult. In this paper, we extend the application of the two-temperature model and combine it with experiments to accurately reveal the evolution law of micro-structure depth at different single pulse energies, as well as the influence of two processing parameters on micro-structure, namely, defocusing amount and scanning speed. The findings of this study provide reliable theoretical guidance for fast and accurate control of material surface roughness and open new possibilities for coating properties.

Optimizing the Surface Structural and Morphological Properties of Silk Thin Films via Ultra-Short Laser Texturing for Creation of Muscle Cell Matrix Model.

Temporary scaffolds that mimic the extracellular matrix’s structure and provide a stable substratum for the natural growth of cells are an innovative trend in the field of tissue engineering. The aim of this study is to obtain and design porous 2D fibroin-based cell matrices by femtosecond laser-induced microstructuring for future applications in muscle tissue engineering. Ultra-fast laser treatment is a non-contact method, which generates controlled porosity—the creation of micro/nanostructures on the surface of the biopolymer that can strongly affect cell behavior, while the control over its surface characteristics has the potential of directing the growth of future muscle tissue in the desired direction. The laser structured 2D thin film matrices from silk were characterized by means of SEM, EDX, AFM, FTIR, Micro-Raman, XRD, and 3D-roughness analyses. A WCA evaluation and initial experiments with murine C2C12 myoblasts cells were also performed. The results show that by varying the laser parameters, a different structuring degree can be achieved through the initial lifting and ejection of the material around the area of laser interaction to generate porous channels with varying widths and depths. The proper optimization of the applied laser parameters can significantly improve the bioactive properties of the investigated 2D model of a muscle cell matrix.

Mechanisms in surface coloring of titanium alloy induced by picosecond laser

Various colors can be obtained on the surface of Ti-6Al-4V (TC4) alloy through picosecond pulsed laser scanning under different laser parameters, which can be expressed by the standard CMYK (cyan-magenta-yellow-black) color mode. In this paper, the microstructure and chemical components of the colored parts were investigated by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Combined with the observation of the optical microscope images, it is found that the color is produced by the oxide layer formed on the surface of TC4. In addition, the color rendering results are affected by the resolution of human eyes, laser-induced surface microstructure and proportion of different color patches. Furthermore, the relationships between laser energy, machining parameter and coloring results were qualitatively investigated, and the limiting effect of hue on saturation was clearly stated. Under the premise of appropriate laser energy, complex paintings have been successfully produced on TC4 alloy surfaces by change scanning speed and line spacing. This study offers a new approach to produce diverse colors on TC4 alloy, which may find a wide range of applications in the laser color marking.

Laser Structured Dental Zirconium for Soft Tissue Cell Occupation-Importance of Wettability Modulation.

Various approaches are being pursued to physico-chemically modify the zirconia neck region of dental implants to improve the integration into the surrounding soft tissue. In this study, polished zirconia discs were laser microstructured with periodic cavities and convex waves. These zirconia samples were additionally activated by argon plasma using the kINPen®09. The surface topography was characterized by scanning electron microscopy and the surface wettability by water contact angle. The in vitro study with human gingival fibroblasts (HGF-1) was focused on cell spreading, morphology, and actin cytoskeleton organization within the first 24 h. The laser-induced microstructures were originally hydrophobic (e.g., 60 µm cavities 138.4°), but after argon plasma activation, the surfaces switched to the hydrophilic state (60 µm cavities 13.7°). HGF-1 cells adhered flatly on the polished zirconia. Spreading is hampered on cavity structures, and cells avoid the holes. However, cells on laser-induced waves spread well. Interestingly, argon plasma activation for only 1 min promoted adhesion and spreading of HGF-1 cells even after 2 h cultivation. The cells crawl and grow into the depth of the cavities. Thus, a combination of both laser microstructuring and argon plasma activation of zirconia seems to be optimal for a strong gingival cell attachment.

Laser-Induced Microstructuring of Silicon Under Varying Fluence Fiber-Based Nir Femtosecond Pulses

Laser-Induced Microstructuring of Silicon Under Varying Fluence Fiber-Based Nir Femtosecond Pulses

Laser-Induced Microstructuring of Polymers in Gaseous, Liquid and Supercritical Media.

New results are presented for laser formation—in particular, the “drawing” of microstructures in polymer films using continuous-wave (CW) laser radiation λ = 405 nm with an intensity of 0.8–3.7 kW/cm2. The laser drawing was carried out in the polymer system poly-2,2′-p-oxydiphenylene-5,5′-bis-benzimidazole (OPBI), which consists of two phases: a solid polymer matrix with formic acid (HCOOH) dissolved in it. The formation of microstructures, including the stage of foaming, was carried out in three media: air, water and a supercritical carbon dioxide medium containing dissolved molecules of the silver precursor Ag(hfac)COD. The morphological features of foam-like track structures formed in the near-surface layer of the polymer films by laser “drawing” are considered. A model of processes is presented that explains the appearance of periodic structures. The key point of this model is that it considers the participation of the photoinduced mechanism of explosive boiling of formic acid molecules dissolved in the polymer matrix. Using Raman spectroscopy, spectra were obtained and interpreted, which relate to different stages in the formation of microstructures in OPBI films. The effects associated with the peculiarities of luminescent microstructures on the surfaces of glasses in close contact with polymer films during laser “painting” in the air have been studied.

Formation mechanism of nanosecond-laser-induced microstructures on amorphous silicon film surfaces.

Laser-induced microstructures have attracted significant research interest owing to their wide application potential for anti-reflective surfaces and optoelectronic devices. To elucidate the characteristics of laser-patterned microstructures, nanosecond-laser-induced micro-protrusions on amorphous silicon film surfaces were investigated via single-and multi-line irradiation experiments. For the former, the results reveal that the number of periodic micro-protrusions depends on the peak power intensity. In addition, the height and the base diameter of the micro-protrusions can be tailored by adjusting the peak power intensity and scanning distance of the laser, while increasing the peak power intensity also increases surface roughness. X-ray spectroscopy confirmed that the microstructures were mainly composed of silicon. The relationship between the formation mechanism and the size of the micro-protrusions is also discussed, with the results of this study providing valuable insights into the laser-induced microstructure formation.