Free oscillations of the keyhole in penetration laser beam welding are studied theoretically with regard to characteristic frequencies, damping rates and stability at large amplitudes. The normal modes form a discrete set which may be characterized by axial and azimuthal numbers. Due to viscous damping, only the lowest modes survive many oscillation periods, which yields a limited range of frequencies for the dynamic response of the keyhole to fluctuations of external welding parameters.
Variety and complex interaction of physical processes during laser cutting is a critical characteristic of the laser cutting of metals. Small spatial and temporal scales complicate significantly the experimental investigations of the multi-phase fluid flow in the conditions of laser cutting of metals. In these conditions, the surface formed during the cutting is an indicator determining the melt flow character. The quantitative parameter reflecting the peculiarities of the multi-phase fluid flow, is normally the roughness of the forming surface, and the minimal roughness is the criterion of the qualitative flow [1-2]. The purpose of this work is to perform the experimental comparative investigation of the thermophysical pattern of the multi-phase melt flow in the conditions of the laser cutting of metals with the laser wavelength of 10.6 μm and 1.07 μm. During the laser cutting, the local material melting and melt removal by an assistant gas jet take place. Cutting of low-carbon steel sheets is usually carried out in an oxygen jet (oxygen-assisted laser cutting). In this case, the exothermic reaction of iron oxidation is an additive energy source. The power balance for the oxygen-assisted laser cutting and the cutting with a chemically inert gas is written respectively as: AW + W ox = W m + W cond (1) AW = W m + W cond (2) where A is the absorption coefficient; W is the laser power; W ox is the power released at the exothermic reaction; W m is the power consumed for melting; and W cond is the power lost from the cut region due to thermal conductivity. In [2, 3] the authors calculated the energy losses from the thermal conductivity in the laser cutting conditions W cond = λ m t ∆T f(Pe), where f(Pe) is the dimensionless function of the Peclet number Pe = (V b ρ m С m) / λ m , ΔT=T*-T 0 = 2000 0 С [4], λ m is the metal thermal conductivity, С m is the metal heat capacity, ρ m is the metal density, t is the steel sheet thickness, and b is the cut width. Passing to dimensionless variables, we have the general expression for the energy balance: Q + Q ox Pe [1 + L m / (C m ΔT)] + f(Pe), where the following designations are used: dimensionless laser power Q = A W ⁄ (λ m t ΔT),
A dynamic model of melt ejection by a gas jet in laser cutting is presented. The molten material is removed due to friction forces and the pressure gradient of the gas flow. The solution of the stationary equations yields the thickness of the molten layer and its velocity of flow, dependent on cutting speed, gas jet formation and the viscosities and densities of the melt and the gas. A stability analysis of the stationary flow shows instabilities for a pressure gradient controlled melt removal. It is argued that these instabilities correlate with ripple formation on the cutting surface.
The paper presents applications of newly developed room-temperature mercury based photo-detectors (single-element and array type) in the measurement of infrared CO2 laser radiation during material processing. A specially designed measuring device to determine the intensity profile of high-power CO2 lasers in two perpendicular cross-sections with a time-resolution up to 10 kllz for a whole line-scan is presented. The data-acquisition and analyzing equipment based on a digital-signal-processor (DSP) enables to calculate parameters which characterize the temporal stability of the laser mode on-line.Experimental results illustrate the possibilities of the device. Moreover, the time-resolved measurement of laser beam parameters during material processing serves as a tool to investigate the influence of the laser beam on the dynamic process behavior. Optical signals obtained from the-laser beam welding process are correlated with laser beam power variations to determine the grade of interaction.Keywords: mercury based photo-detectors, digital-signal-processor, CO2 laser beam measurement, beam power, intensity profile, laser beam welding, process diagnostics.
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