On Energy Efficiency Characteristics of Laser Erosion on Oxidic Surfaces of Carbon Steels, Cast Iron and Low-alloy Non-ferrous Alloys During Deoxidizing Cleaning. Part 1

A comparison of operating characteristics has been carried out for laser erosion cleaning (LC) processes studied in recent years and prospective for metalworking manufacturing of products/pieces from a number of carbon steels, cast iron and low-alloy non-ferrous metal alloys from oxidized layers formed as products of gas or other corrosion, often having inhomogeneous structure and porosity. To analyze the efficiency of various (in terms of layer composition) laser processes, it is advisable to use a group of parameters that affect the energy efficiency of LC-processing during the deoxidizing of surfaces. This group includes: a) the time-integrated energy criterion ( K en1s ) of heating up to the melting point and/or evaporation temperatures of the layer and, sometimes, a metal substrate located underneath it (or the thermochemical efficiency of the heating, which is derived from the K en1s ), determined from energy consumption; b) irradiation power per surface unit ( N 0 ), or the ratio of N 0 to the thermal conductivity of the layer; c) the pressure amplitude of the shock wave (SW) front in the laser plasma near the surface ( P sw-p ) or the dimensionless parameter that includes it, equal to the ratio of P sw-p to the shear stress for the oxidized layer/metal substrate interface. The dimensionless K en1s criterion (or similar ones) will be more convenient in some cases for modeling and scaling of LC-processes than dimensional complexes, including thermal criteria such as DMF (“difficulty of melting factor”), which were tested in calculation of plasma spraying of ceramic materials. In this group of efficiency parameters, such a characteristic as the normalized (for example, with K en1s ) Peclet number, which characterizes the rate of propagation of the melting (or evaporation) boundary along the surface when scanning the beam, is also applicable. The considered characteristics, based on preliminary data, make it possible to evaluate the contribution of the mechanisms of the layer removal during pulsed LC, i.e.: 1) thermal effect (“ablation”) with “slow” heating to the melting point of the oxide (or to its evaporation temperature) in thermodynamically quasi-equilibrium regimes; 2) initiation of thermoelastic stresses in the crystal lattice of oxide phases under the impact of high power pulse, resulting in the formation of a network of cracks in the oxide film and its exfoliation from the metal substrate (“spallation”, it is approximately characterized by the maximum stress achieved during LC at the film/substrate interface); 3) plasmadynamic mechanism of the action of pressure on the surface due to the generation of near-surface plasma with a shock wave in it (with a pressure amplitude of up to ≥10 MPa). When assessing LC-processes taking into account efficiency characteristics, it is advisable to use a special set of verified data selected according to the thermophysical properties of layers of an analyzed type.