Spatial Distribution Of Laser Energy Research Articles

Effect of transmission characteristics of femtosecond laser focusing on micro-/nanostructure morphology of titanium alloy

Various types of micro-/nanostructures can be fabricated by femtosecond laser irradiation on metal surfaces. It has received extensive attention and research because of its potential application value. In this paper, the effect of laser focusing position on the micro-/nanostructure formation on the surface of a titanium alloy (TC4) was systematically researched. The experimental results show that, in the process of femtosecond laser focusing, a spatial distribution of laser energy with a narrower beam center and a ring around the beam is formed in front of the geometric focus and the distribution of laser energy changes along the laser transmission direction. This property can affect the morphology of the micro-/nanostructure on the surface of irradiated materials. It is believed that the new spatial distribution of laser fluence caused by focusing is due to the nonlinear process of plasma defocus and laser self-focusing caused by multi-photon or tunnel ionization in front of the target by focusing the femtosecond laser. The discovery of the nonlinear characteristics of the focused femtosecond laser and the application of the unique energy spatial distribution to the surface morphology of the fabricated surface put forward a new idea for the direct fabrication of micro-/nanostructures via femtosecond laser, which has certain significance for the development of micro-/nanofabrication technology of femtosecond lasers.

Influence of the free surface reconstruction on the spatial laser energy distribution in high power laser beam welding modeling

An accurate and efficient description of the spatial distribution of laser energy is a crucial factor for the modeling of laser material processing, e.g., laser welding, laser cutting, or laser-based additive manufacturing. In this study, a 3D heat transfer and fluid flow model coupled with the volume-of-fluid algorithm for free surface tracking is developed for the simulation of molten pool dynamics in high-power laser beam welding. The underlying laser-material interactions, i.e., the multiple reflections and Fresnel absorption, are considered by a ray-tracing method. Two strategies of free surface reconstruction used in the ray-tracing method are investigated: a typical piecewise linear interface calculation (PLIC)-based method and a novel localized level-set method. The PLIC-based method is discrete, resulting in non-continuous free surface reconstruction. In the localized level-set method, a continuous free surface is reconstructed, and, thus, the exact reflection points can be determined. The calculated spatial laser energy distribution and the corresponding molten pool dynamics from the two methods are analyzed and compared. The obtained numerical results are evaluated with experimental measurements to assure the validity of the proposed model. It is found that distinct patterns of the beam multiple reflections are obtained with the different free surface reconstructions, which shows significant influence not only on the molten pool behaviors but also on the localized keyhole dynamics.

Spatial distribution of laser energy and its influence on the stability of extreme narrow keyholes during ultra-high power laser welding

Spatial distribution of laser energy and its influence on the stability of extreme narrow keyholes during ultra-high power laser welding

Formation of linked nanostructure-textured mound-shaped microstructures on stainless steel surface via femtosecond laser ablation

Linked nanostructure-textured mound-shaped microstructures (termed N-mounds) were induced by femtosecond laser on AISI 443 stainless steel surface in air via multiple raster scans. The formation mechanism of the linked N-mounds is discussed in this paper. With increasing number of raster scans, four stages of the linked N-mounds’ formation are identified. First, femtosecond laser-induced periodic surface structures (FLIPSSs) are produced on the stainless steel surface. Then precursor protrusions are generated on the FLIPSSs because of unhomogeneous spatial distribution of laser energy caused by modifications of surface morphology. Further increasing the number of raster scans, precursor protrusions will grow into randomly distributed N-mounds owing to preferential ablation resulted from geometric effect. Finally, linked N-mounds gradually form as N-mounds’ growth reduces the width of the valleys between N-mounds. Surface morphology will not change significantly with increasing number of raster scans once the linked N-mounds are formed. Besides, the N-mounds are covered with concentric ripples and irregular nanostructures as a result of redeposition of ablated materials which is also the reason why the linked N-mounds rise above the surface formed in the third stage.

Parametric assessment of acoustic output from laser-irradiated nanoparticle volumes

A photoacoustic technique is being investigated for application to intracellular drug delivery. Previous work [Chakravarty et al., Nat. Nanotechnol. 5, 607–611 (2010)] has shown that cells immersed in nanoparticle-laden fluid underwent transient permeabilization when exposed to pulsed laser light. It was hypothesized that the stresses leading to cell membrane permeabilization were generated by impulsive pressures resulting from rapid nanoparticle thermal expansion. To assist in the study of the drug delivery technique, for which high uptake and viability rates have been demonstrated, an experimental method was developed for parametric assessment of photoacoustic output in the absence of field-perturbing elastic boundaries. This paper presents calibrated acoustic pressures from laser-irradiated streams, showing the impact of parameters including particle type, host liquid, and spatial distribution of laser energy.

Characteristics of laser-induced incandescence from soot in studies of a time-dependent heat- and mass-transfer model

The temporal behavior of the laser-induced incandescence (LII) signal is often used for soot-particle sizing, which is possible because the cooling behavior of a laser-heated particle is dependent on the particle size. The heat- and mass-transfer model describing the temporal LII-signal behavior has in this work been extended to include the influence of the primary particle-size distribution and the spatial distribution of laser energy. When evaluating primary particle size, a monodisperse size distribution is often assumed, although it is well known that a polydisperse distribution is a better description of the real situation. In this work the impact of this assumption is investigated for Gaussian and lognormal size distributions of different widths, and the result is a significant bias towards larger particle sizes because of the higher influence of larger particles on the LII signal. Moreover, the dependence of the LII signal on the laser fluence is studied for different spatial distributions of the laser energy. The top-hat, Gaussian sheet and Gaussian beam distributions were tested and it is established that the LII signal is strongly dependent on the choice of distribution. However, in this case the influence of particle size is minor.

The effects of process variations on residual stress in laser peened 7049 T73 aluminum alloy

This paper reports measurements of the distribution of residual stress with depth from the surface in laser peened coupons made of a high-strength aluminum alloy. Residual stresses were measured using slitting (also known as the crack compliance method). Measurements were made on several coupons to: compare laser peening (LP) and shot peening (SP) residual stresses; ascertain the influence of LP parameters on residual stress; determine whether tensile residual stress existed outside the peened area; assess the variation of residual stress with in-plane position relative to the layout of the laser spots used for peening; and, determine the importance of a uniform spatial distribution of laser energy within the spot. Residual stress 0.1 mm from the surface due to LP and SP were comparable and the depth of the compressive stress for LP was far greater than for SP. Variations of most LP parameters did not significantly alter residual stress at shallow depths, but greater laser energy and larger layer overlap increased residual stress at depths between 0.2 and 0.6 mm from the surface. Residual stresses adjacent to the peened area were found to be compressive. Decreased levels of surface residual stress were found when laser spots had a non-uniform distribution of laser intensity.

The Effects of Process Variations on Residual Stress Induced by Laser Peening

Measurements of the distribution of residual stress with depth from the surface in laser peened coupons were made in a high-strength aluminum alloy. Residual stresses were measured using slitting (also known as the crack compliance method). Measurements were made on several coupons to: compare laser peening (LP) and shot peening residual stresses; ascertain the influence of LP parameters on residual stress; determine whether tensile residual stress existed outside the peened area; assess the variation of residual stress with in-plane position relative to the layout of the laser spots used for peening; and, determine the importance of a uniform spatial distribution of laser energy within the spot. Residual stress 0.1 mm from the surface due to LP and shot peening were comparable and the depth of the compressive stress for LP was far greater than for shot peening. Variations of most LP parameters did not significantly alter residual stress at shallow depths. Residual stresses adjacent to the peened area were found to be compressive. Decreased levels of surface residual stress were found when laser spots had a non-uniform distribution of laser intensity.