The main purposes of this project are: combining of the advantages of the known methods of laser and plasma transferred arc treatment of metals and widening of their possibilities within new hybrid processes of laser + plasma arc welding (LPAW), cutting (LPAC) and surfacing (LPAS).

The project provides: theoretical investigations of the physical phenomena occurring at hybrid processes of metal treatment, development of integrated laser-arc plasma torches for LPAW, LPAC and LPAS, as well as their technological examinations.

The expected results are: improving of efficiency and productivity of the laser methods of metal treatment, as well as widening of metal thickness range for the laser welding, cutting and surfacing without considerable degrading of their quality and increasing of the laser power used.

Objectives:

• To investigate interaction of the focused laser beam with the plasma transferred arc and their combined effect on different metals as applied to LPAW, LPAC and LPAS of various materials, including aluminium alloys.
• To develop the main principles of the hybrid processes and concept of the integrated laser-arc plasma torches for LPAW, LPAC and LPAS.
• To create the prototypes of integrated plasma torches for laser + plasma welding and surfacing as applied to the special technological processes.
• To carry out the experimental and technological examinations of LPAW, LPAC and LPAS.

Brief background:

Alongside with the conventional methods of metal joining and treatment that use arc (plasma transferred arc), laser or electron beam heating of metal, the hybrid technological processes are increasingly developing. The essence of these processes is in the simultaneous using of two different heat sources, for example, the laser beam and the electric arc (see, e.g., V.S. Gvozdetsky, I.V. Krivtsun et al., Laser-arc discharge: Theory and applications, Welding and Surfacing Rev., Vol. 3, Harwood Academic Publishers, 1995).
In the realisation of the hybrid laser-arc processes for welding and cutting the application of additional arc plasma heating of metal increases the efficiency of the corresponding laser process. This is especially important when low-power lasers are used. On the other hand, the usage of the electric arc together with the laser beam improves the spatial stabilisation of the arc root on the metal surface at low current and high travel speed. It allows to increase greatly the stability and productivity of the corresponding arc process.
The most rational scheme of the hybrid welding and surfacing processes is the scheme, using plasma transferred arc with the introducing of the laser beam along the axis of plasma-shaping channel of the plasma torch. In contrast to the known methods of laser-arc welding and cutting, this scheme ensures the required coaxial nature of the effect of the laser beam and the electric arc on a workpiece. Moreover, this scheme is characterised by new possibilities of mutual control of the characteristics of the plasma arc and the laser beam, as sources of thermal and dynamic effects on the surface of metal being treated.
The coaxial combination of the laser beam with the plasma arc requires special devices to be developed - the integrated laser-arc plasma torches (see, e.g., I.S. Dykhno, I.V. Krivtsun and G.N. Ignatchenko, Combined laser and plasma arc welding torch. Pat. No.5700989, USA, Int. Cl. B23K 26/00, 10/00, published 23.12.97), the special feature of which is the electrode unit design, that enables the laser beam to be introduced along the plasma-shaping channel. The design peculiarities of such devices are determined by the parameters of the laser beam used (CO₂ or Nd-YAG laser, power range, mode and focusing of the initial beam); the kind and flow rate of plasma gas (Ar, He, or their mixtures, air, O₂, or N₂); the current range and polarity of the plasma arc, as well as the kind and thickness of the metal being treated.
Our team has great experience in the field of gas discharge physics (arc, optical and combined laser-arc discharge), engineering of plasma and integrated equipment for metal treatment, as well as its industrial applications. To our mind, the hybrid laser + plasma technologies (LPAW, LPAC, LPAS) present a new step in the development of material treatment methods and their industrial applications are very important at this time.

Stage I. Development of 1D model (one-dimensional, local model) of the physical processes occurring in a system "laser radiation - near-surface plasma - metal surface", as applied to laser and hybrid methods of metal treatment:

1. Investigation of metal evaporation into near-surface plasma (shielding or plasma gas at the atmospheric pressure). Calculation of particle (mass) and energy flows from the metal, pressure at its surface and temperature of heavy particles in the plasma (with taking into account the reversed particle flow) as functions of local values of the metal surface temperature and Mach number outside Knudsen layer.
2. Determination of the near-surface plasma composition (ionisation-recombination processes with taking into account one-dimensional mutual diffusion of shielding gas and metal vapor particles), depending on the metal surface and plasma electron temperatures. Calculation of thermodynamic characteristics (mass density, specific heat), transport coefficients (viscosity, thermal and electrical conductivity) and optical properties (plasma radiation losses, refractive index and absorptivity of laser radiation) for this multi-component plasma.
3. Investigation of laser radiation (CO2, Nd-YAG - lasers) absorption and reflection by the metal surface (with taking into account refraction and absorption of the laser radiation in near-surface plasma). Calculation of laser heat flow into the metal as a function of local values of the metal surface and plasma electron temperatures, intensity, polarisation and angle of incidence of the laser radiation.
4. Investigation of energy balance of the near-surface plasma (laser or combined plasma). Calculation of plasma electron temperature (with taking into account one-dimensional plasma expansion).
5. Calculation of electric potential distribution in the near-surface region and flows of particles and energy from plasma to the metal surface as functions of local values of electric current density in plasma and metal surface temperature.
6. Determination of total energy balance and pressure on the metal surface as functions of local values of arc current density, incident laser radiation characteristics and metal surface temperature.

Deliverables:

Local dependencies of the heat flow into the metal and pressure at its surface on this surface temperature, laser radiation characteristics, electric current density in the near-surface plasma and its expansion velocity, as applied to laser and hybrid processes of metal treatment.

1. Development of computer model of the near-surface laser plasma with axisymmetrical distributions of the plasma and laser beam parameters (with taking into account laser plasma expansion).
2. Investigation of non-stationary thermal processes inside the metal with preassigned boundary conditions on its surface (with taking into account movement of the phase transition boundaries).
3. Investigation of the metal surface and liquid metal dynamics with preassigned boundary conditions for erosion and pressure on its surface (for given melting boundary geometry).
4. Development of 2D computer model of the cathode phenomena at hybrid processes (with taking into account electron emission peculiarities, cathode spot mode and near-cathode plasma expansion).
5. Development of 2D computer model of the anode phenomena at hybrid processes of metal treatment (with taking into account arc current distribution inside the anode region and near-anode plasma expansion).
6. Complex simulation of non-stationary thermal and dynamical processes inside the metal with self-consistent boundary conditions on its surface at laser and hybrid methods of metal treatment (for different arc polarity and motionless heat source).

Deliverables:

Self-consisted 2D computer models of non-stationary metal penetration processes at laser and hybrid methods of metal treatment with the motionless heat source. This models can be used for computer estimation of the metal penetration dynamics, depending on the metal thickness, initial laser beam characteristics (wave length, power, mode and focusing conditions), arc current and its polarity, as well as the time interval of their affecting on the metal.

1. Investigation of quasi-stationary thermal processes inside the metal with preassigned boundary conditions on its surface (with taking into account movement of axisymmetrical heat source).
2. Investigation of quasi-stationary dynamics of the weld pool surface and liquid metal dynamics with preassigned boundary conditions (axisymmetrical) for erosion and pressure on its surface (for given melting boundary geometry).
3. Determination of self-consisted boundary conditions (axisymmetrical) on the weld pool surface as applied to laser and hybrid processes.
4. Complex computer simulation of metal penetration at laser and hybrid welding of different metals.

Deliverables:

Self-consisted 2D, 3D computer models of quasi-stationary metal penetration processes at laser and hybrid welding. These models can be supplied by Windows friendly interface for future commercial realisation of the developed software. This software can be used for computer simulation of laser and hybrid welding of metals, depending on the metal thickness, initial laser beam characteristics (wave length, power, mode and focusing conditions), arc current and its polarity, as well as the welding speed.