Laser Heating of Metals

A. M. Prokhorov

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351090841
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

In order to ensure efficient use of lasers, and for any large-scale implementation, a thorough knowledge of the fundamental laws governing the interaction of radiation with matter is required. Laser Heating of Metals provides a systematic and comprehensive presentation of the fundamental principles underlying the physical and chemical mechanisms governing the interaction of laser radiation with solid targets, and in particular metals in gaseous environments, for a wide range of beam parameters. The authors have been active in the field of interactions between lasers and materials for many years, and this book summarises the results of their work, in particular concerning the action of CO2 lasers on metals. These results are then discussed at some length. Laser Heating of Materials will be of interest to scientists at all levels with an interest in the interaction of radiation with condensed matter, and in particular to those involved in laser cutting and welding etc, and metal-working.

Chapter 1

Optical Properties of Metals

The laser heating of a material is mainly determined by that material’s absorptivity at the respective laser wavelength. This is why the metal absorptivity—or, alternatively, the reflectivity of the metal at given wavelengths—stands as the chief criterion guiding any approach to an efficient processing of metallic parts. This chapter links the key quantities of absorptivity/reflectivity to the optical properties of metals, the general condition of their surfaces and the temperature, i.e. the laser heating rates.

The heating rate of a metal sample is mainly determined by the sample absorptivity for a given wavelength—a quantity which, in turn, is determined by the optical properties of the metal itself and of the sample surface, as well as by the temperature range, heating rate, etc. That is why the metal absorptivity, A, or alternatively the metal reflectivity, R, stand as chief criteria guiding the choice of the most appropriate laser system for processing metallic parts—by surface hardening, welding, size adjustments, etc [1].

Quantitatively, the absorptivity A is the ratio of the intensity absorbed by the sample, I_a, to the incident intensity, I, at a certain moment in the process of laser heating. Accordingly, the reflectivity, R = 1 – A, is the ratio between the reflected (specularly and/or diffused) intensity, I_r, and the incident intensity, I.

The absorptivity of metals shows a general trend to increase when the incident radiation wavelength decreases from the infrared to the ultraviolet spectral range [2, 3]. Generally, for radiation with a wavelength λ ≃ 10.6 μm the absorptivity of metals is very low—of the order of a few per cent, and sometimes even of a few fractions of one per cent. As a consequence, in practice metals are largely used as the preferred raw material for manufacturing mirrors to be used in conjunction with CO₂ lasers and other laser systems generating in the mid- and near-infrared spectral ranges. At wavelengths below 1 μm, metals of sufficiently high reflectivity are no longer available—perhaps with the exception of aluminium—and consequently mirrors are usually manufactured by coating a base material with appropriate dielectric films.

At a first glance the low absorptivity values make the use of power CO₂ lasers in metal processing very inefficient. However, it has been demonstrated that the absorptivity can reach significantly higher levels—in some cases it can increase by more than one order of magnitude—during the laser action, due to the increase...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Series Editors’ Preface
• Preface
• List of Symbols and Abbreviations
• 1 Optical Properties of Metals
• 2 Basic Regimes of the Heating of Metal Targets by Laser Irradiation
• 3 Thermoelastic Deformation of Metal Surfaces
• 4 Light Stability of Metal Mirrors Exposed to Pulsed Irradiation in a Vacuum
• 5 Surface Periodic Structures
• 6 Laser-induced Oxidation and Burning of Metals
• 7 Optical Effects and Diagnosis of Thermochemical Interaction Processes
• 8 The Choice of Optical Irradiation Conditions
• References
• Index