Cross-Section Bead Image Prediction in Laser Keyhole Welding of AISI 1020 Steel Using Deep Learning Architectures

A deep learning model was applied for predicting a cross-sectional bead image from laser welding process parameters. The proposed model consists of two successive generators. The first generator produces a weld bead segmentation map from laser intensity and interaction time, which is subsequently translated into an optical microscopic (OM) image by the second generator. Both generators exhibit an encoder & x2013;decoder structure based on a convolutional neural network (CNN). In the second generator, a conditional generative adversarial network (cGAN) was additionally employed with multiscale discriminators and residual blocks, considering the size of the OM image. For a training dataset, laser welding experiments with AISI 1020 steel were conducted on a large process window using a 2 KW fiber laser, and a total of 39 process conditions were used for the training. High-resolution OM images were successfully generated, and the predicted bead shapes were reasonably accurate (R-Squared: 89.0 & x0025; for penetration depth, 93.6 & x0025; for weld bead area)

On vaporization in laser material interaction

In this article, vaporization processes in the laser interaction with materials are studied theoretically and computationally, focusing on evaporation and homogeneous bubble nucleation. Simulations are carried out using the Redlich-Kwong equation of state and temperature-dependent material property models that can be used up to the critical point. From theoretical considerations, four important temperatures are identified in the understanding of laser material interaction. This study also shows that there are upper limits to the amount of energy that can be consumed by vaporization, which takes place at a temperature that is lower than the material's critical point. This study also discusses the transition from the thermal mode of ablation to the nonthermal mode in terms of the energy capacity of homogeneous boiling.open6

Lattice-Boltzmann simulation of laser interaction with weakly ionized helium plasmas

This paper presents a lattice Boltzmann method for laser interaction with weakly ionized plasmas considering electron impact ionization and three-body recombination. To simulate with physical properties of plasmas, the authors' previous work on the rescaling of variables is employed and the electromagnetic fields are calculated from the Maxwell equations by using the finite-difference time-domain method. To calculate temperature fields, energy equations are derived separately from the Boltzmann equations. In this way, we attempt to solve the full governing equations for plasma dynamics. With the developed model, the continuous-wave CO(2) laser interaction with helium is simulated successfully.open4

FDTD method for laser absorption in metals for large scale problems

The FDTD method has been successfully used for many electromagnetic problems, but its application to laser material processing has been limited because even a several-millimeter domain requires a prohibitively large number of grids. In this article, we present a novel FDTD method for simulating large-scale laser beam absorption problems, especially for metals, by enlarging laser wavelength while maintaining the material???s reflection characteristics. For validation purposes, the proposed method has been tested with in-house FDTD codes to simulate p-, s-, and circularly polarized 1.06 ??m irradiation on Fe and Sn targets, and the simulation results are in good agreement with theoretical predictions.open1

CO2 and Er:YAG laser interaction with grass tissues

Plant leaves are multi-component optical materials consisting of water, pigments, and dry matter, among which water is the predominant constituent. In this article, we investigate laser interaction with grass using CO2 and Er:YAG lasers theoretically and experimentally, especially targeting water in grass tissues. We have first studied the optical properties of light absorbing constituents of grass theoretically, and then have identified interaction regimes and constructed interaction maps through a systematic experiment. Using the interaction maps, we have studied how interaction regimes change as process parameters are varied. This study reveals some interesting findings concerning carbonization and ablation mechanisms, the effect of laser beam diameter, and the ablation efficiency and quality of CO2 and Er: YAG lasers.open0

Fabricating functionally graded films with designed gradient profiles using pulsed laser deposition

A novel picosecond-laser pulsed laser deposition method has been developed for fabricating functionally graded films with pre-designed gradient profiles. Theoretically, the developed method is capable of precisely fabricating films with any thicknesses and any gradient profiles by controlling the laser beam powers for the two different targets based on the film composition profiles. As an implementation example, we have successfully constructed functionally graded diamond-like carbon films with six different gradient profiles: linear, quadratic, cubic, square root, cubic root, and sinusoidal. Energy dispersive X-ray spectroscopy is employed for investigating the chemical composition along the thickness of the film, and the deposition profile and thickness errors are found to be less than 3% and 1.04%, respectively. To the best of the authors' knowledge, this is the first method for fabricating films with designed gradient profiles and has huge potential in many areas of coatings and films, including multifunctional optical films. We believe that this method is not only limited to the example considered in this study, but also can be applied to all material combinations as long as they can be deposited using the pulsed laser deposition technique.open0

Finite-difference time-domain simulation of laser beam absorption in fully penetrated keyholes

Accurate predictions of laser beam absorptance and how laser beam energy is distributed on a keyhole surface are arguably the most important but challenging tasks in the study of laser keyhole welding. In this article, laser interaction with fully penetrated keyholes has been studied by solving the Maxwell equations for electrodynamics using the finite-difference time-domain method with the Drude model for metals. Based on the experimental observations of Fabbro [J. Phys. D: Appl. Phys. 38, 1881-1887 (2005)], the keyhole is simplified as a tilted cylinder, and we have extensively investigated laser absorption phenomena considering three materials (Fe, Sn, and Al), three beam polarizations (two linear and circular), two laser beam wavelengths (1.06 ??m and 10.6 ??m), and six keyhole tilting angles (0??, 10??, 20??, 30??, 40??, and 45??). To the best of the authors' knowledge, this is the first electrodynamic simulation of a laser manufacturing process and reveals some interesting findings concerning laser beam absorption characteristics that can be only obtained by full electrodynamic simulations.open0

Modelling of high-density laser-material interaction using fast level set method

A high-energy-density laser beam-material interaction process has been simulated considering a self-evolving liquid-vapour interface profile. A mathematical scheme called the level-set technique has been adopted to capture the transient liquid-vapour interface. Inherent to this technique are: the ability to simulate merger and splitting of the liquid-vapour interface and the simultaneous updating of the surface normal and the curvature. Unsteady heat transfer and fluid flow phenomena are modelled, considering the thermo-capillary effect and the recoil pressure. A kinetic Knudsen layer has been considered to simulate evaporation phenomena at the liquid-vapour interface. Also, the homogeneous boiling phenomenon near the critical point is implemented. Energy distribution inside the vapour cavity is computed considering multiple reflection phenomena. The effect of laser power on the material removal mode, liquid layer thickness, surface temperature and the evaporation speed are presented and discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48909/2/d10320.pd

Mass removal modes in the laser ablation of silicon by a Q-switched diode-pumped solid-state laser (DPSSL)

A fundamental study on the Q-switched diode-pumped solid-state laser interaction with silicon was performed both experimentally and numerically. Single pulse drilling experiments were conducted on N-type silicon wafers by varying the laser intensity from 108–109 W cm−2 to investigate how the mass removal mechanism changes depending on the laser intensity. Hole width and depth were measured and surface morphology was studied using scanning electron microscopy. For the numerical model study, Ki et al's self-consistent continuous-wave laser drilling model (2001 J. Phys. D: Appl. Phys. 34 364–72) was modified to treat the solidification phenomenon between successive laser pulses. The model has the capabilities of simulating major interaction physics, such as melt flow, heat transfer, evaporation, homogeneous boiling, multiple reflections and surface evolution. This study presents some interesting results on how the mass removal mode changes as the laser intensity increases.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48912/2/d6_12_023.pd

355, 532, and 1064 nm picosecond laser interaction with grass tissues

In this article, we investigate how 355, 532, and 1064 nm picosecond lasers interact with grass tissues. We have identified five interaction regimes, and based on this classification, interaction maps have been constructed from a systematic experiment. The optical properties of light absorbing grass constituents are studied theoretically in order to understand how and how much light is absorbed by grass tissues. Scanning electron microscopy and optical microscopy are employed for observing morphological and structural changes of grass tissues. To the best of the authors' knowledge, this is the first investigation into laser interaction with plant leaves and reveals some fundamental findings regarding how a laser interacts with grass tissues and how plant leaves can be processed using lasers.open1