Influence of internal heat transfer on reactive force applied to powder particles in laser cladding

Abstract

Formation of two-phase powder-gas flows is the key factor determining the quality of materials synthesized by laser cladding and similar laser processes. Currently, the influence of the processing laser beam on such flows is insufficiently studied. Recent experiments on high-speed imaging show that the laser beam can considerably accelerate and deviate the exposed powder particles. This phenomenon is commonly explained by the reactive force arising at laser evaporation of particles. It seems that the existing theoretical approaches considerably overestimate the reactive force because they ignore internal heat transfer in the particle. The aim of the present work is to analyze theoretically the combined conductive-convective heat transfer in the particle exposed to laser radiation and the related evaporation and formation of the resultant reactive force. Computational Fluid Dynamic (CFD) simulation has shown that the internal heat transfer can reduce the temperature gradient in the particle, thus considerably reducing the resultant reactive force in the typical conditions of laser cladding. The CFD indicates that in such conditions, the influence of convection on the reactive force is low. An analytic model of conductive heat transfer and evaporation of a spherical particle is developed. The CFD validates this model and indicates that the confidence interval of the estimated resultant reactive force is within 1%. This model can be useful for analyzing experimental data and simulation of gas-powder flows exposed to laser radiation.