Analysis of track formation during laser metal deposition

Abstract

A fundamental understanding of the physical phenomena associated with the coaxial laser metal deposition (LMD) process is essential to enhance the science base and thereby improve the process itself. The interaction between the laser beam and the powder particles leads to an attenuation of the beam intensity and to a temperature rise of the powder particles before reaching the melt pool. To understand this mutual influence and their influence on the process result better, the powder gas stream was analyzed with an image-based particle density measurement system. This system illuminates the powder gas stream from the side with a line laser light source and monitors the particles plane by plane with a coaxially aligned camera through the powder nozzle. A high image rate allows to record the number and position of individual powder particles. The particle density distributions in the planes are determined by averaging over the images. With this, a statistical model can be derived with regard to the particles trajectories. Finally, with these results it is possible to analyze the shadowing of the laser beam by the powder particles and the change of the particle temperature in detail. The transmitted laser intensity distribution and mean particle temperature are used as boundary conditions for the governing equations of the mathematical model for the real LMD process. LMD represents mathematically a free boundary value problem. This means that the track geometry is part of the solution. The mathematical model is based on an integration of the time dependent heat equation and the computation of the track geometry with the nonlinear Young-Laplace equation, whereby a mass balance with regard to the particles which reach the melt pool surface is taken into account. The governing equations are solved with the Finite Element Method and the moving mesh approach is used in order to implement a track adapted meshing. The results concerning the track geometry show excellent agreement with the experimental ones which are determined by cross sections of laser material deposited tracks and images from high speed videography. The model does not use any geometrical approximation of the melt pool surface but determines the melt pool surface shape by the balance equation for capillary forces and mass balance.