The thesis is devoted to research and development of new high-efficiency laser-arc (laser-plasma) processes for welding and treatment of materials, creating of specialised devices for practical realisation 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 modelling 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 modelling 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 thenear-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 sizesare 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 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.
