Laser-optic Measurements of Velocity of Particles in the Powder Stream at Coaxial Laser Cladding

The problems of particle velocity and temperature measurement can be solved with commonly-known methods of registration based on spectrometry and a complex of laser and optical means. The diagnostic technique combines two independent methods of particle velocity measurement, namely the passive way which is based on the intrinsic radiation of the heated particles in a gas flow, and the active one which utilizes the effect of the laser beam scattering. It is demonstrated that the laser radiation can affect significantly the particles velocity at the laser cladding. Presented bar charts of statistical distributions of the particles velocities illustrate two modes of the coaxial nozzle performance, with and without СО2-laser radiation. Different types of powders (Al2O3, Mo, Ni, Al) were used in tests, the particle size distributions were typical for the laser cladding; air, nitrogen, argon were used as working gases, continuous radiation of the СО2 laser reached 3 kW. It is shown that in the laser-radiation field, the powder particles undergo extra acceleration due to the laser evaporation and reactive force occurrence resulting from the recoil pressure vapors from the beamed part of particles’ surfaces. The observed effect of particles acceleration depends on the particles concentration in the powder flow. Due to the laser acceleration, the velocities of individual particles may reach the values of about 80 – 100 m/s. The trichromatic pyrometry method was utilized to measure the particles temperature in the powder flow.