"Combined
laser-arc processes for materials treatment and devices for their realization"
Manuscript.
Dissertation for
a Doctor of Sciences degree on speciality 05.09.10 - Electrothermal Processes
and Installations, E.O. Paton Electric Welding
Institute of the National Academy of Sciences of Ukraine, Kyiv, 2002
Summary
The dissertation is devoted to research and development of new high-efficiency
laser-arc (laser-plasma) processes for welding and treatment of materials,
creating of specialized devices for practical realization of the combined
laser-plasma processes, and investigation of the technological capabilities
of such devices.
Theoretical studies were conducted, a mathematical model was developed,
and detailed numerical modeling of physical phenomena occurring in interaction
of the focused laser beam and the electric arc plasma, which are joined
in combined process, was performed. It was established that a special
type of the gas discharge may be formed under certain conditions in the
"electric arc - CO2-laser beam" system. This discharge is a
combined laser-arc discharge, which differs both from the conventional
arc and from the optical discharge maintained by laser radiation. It is
shown that by affecting plasma of the arc column in the gas flow with
the focused CO2-laser beam, it is possible to control effectively the
characteristics of this plasma through varying power and degree of focusing
of the initial laser beam. It is also shown that the plasma lens, the
focusing properties of which depend upon the arc current, composition
and flow rate of the plasma gas, may be formed in the system under consideration.
This allows self-focusing of the laser beam in the combined discharge
plasma to be controlled through varying the above arc burning conditions.
It is scientifically justified that the laser-arc discharge, being a
heat source with wide capabilities for controlling concentration of thermal
and electromagnetic energies, can be used as a basis for making a new
class of plasma devices, i.e. integrated laser-arc torches for combined
welding, cutting and surface modification. Principles of design and methods
for calculation of laser - transferred and non-transferred arc torches
operating in laminar or turbulent modes of the plasma gas flow were developed.
A special design of the tubular thermionic cathode, the working end of
which is additionally heated by laser radiation being passed through the
cathode, is suggested. The mathematical model of the cathodic phenomena
for the above design of the thermionic cathode is described, and the results
of numerical modeling of the combined discharge in the plasma torch with
such a cathode is presented. Prototypes of the integrated plasma torches
for laser-plasma powder deposition (using CO2-laser beam) and laser-microplasma
welding (using YAG-laser beam) were developed. Experimental and technological
studies of the developed plasma torches were carried out. The high efficiency
of practical application of such devices for welding, surfacing and other
combined processes is demonstrated.
The theory of interaction of laser radiation and arc plasma with metals
was elaborated. It allows processes of the arc, laser and combined effect
on the metal surface to be described from the unified positions with a
self-consistent account for the entire set of physical phenomena which
occur in the near-surface plasma, on the surface and in the bulk of a
workpiece. Characteristics of the thermal and dynamic effect on the metal
surface by the combined heat source components during laser-arc welding
and heat treatment were investigated. It was established that the use
of an external ionizer (electric arc) in laser beam welding, without formation
of the plasma plume, allows a substantial decrease in the melt surface
temperature, at which the transition from the thermal conductivity mode
of penetration to a more efficient mode of deep penetration begins. In
addition to extra arc heating of metal, this is one of the main causes
of the higher efficiency of the combined welding process, compared with
laser beam welding.
Theoretical investigations of processes of interaction of laser radiation
and arc plasma with fine-dispersed materials were carried out, and mathematical
models of these processes were developed. It is shown that in calculation
of the dynamics of laser heating of ceramic particles, whose sizes are
commensurable with the laser radiation wavelength, it is necessary to
take into account an interference structure of the electromagnetic field
excited in the bulk of a particle and the associated spatial non-uniformity
of distribution of the radiation power absorbed by the particle. It is
scientifically grounded that the required (if necessary uniform) distribution
of temperature in the bulk of particles can be achieved by combining plasma
(surface) and laser (volumetric) methods of heating fine-dispersed ceramic
materials. Owing to this fact, the use of the combined laser-plasma spraying
method offers wide possibilities for deposition of new ceramic coatings,
the SiO2 coatings in particular.
Software package for computer aided simulation of plasma spraying process
(CASPSP) was developed. It enables quantitative
estimation of thermal and gas-dynamic characteristics of the turbulent
plasma jets and simulation of processes of heating and motion of the spray
material particles in accordance with the technological parameters of
a spraying process to be made quickly and with a sufficient accuracy for
practical purposes.
Key words:
electric arc, plasma, laser beam, welding, materials treatment, integrated
plasma torch
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