Modellierung diffusiver und konvektiver Wärme- und Stofftransportprozessee bei der Laser-Schmelzbearbeitung

During the laser remelting of cast iron the graphite inclusions are dissolved to an extent depending on temperature and time. The surface tension gradient driven convection leads to an almost uniform carbon concentration in the melt which drops within a boundary layer at the liquidlsolid interface to a temperature dependent boundary value determined by the local thermodynamical equilibrium. Therefore, carbon diffusion into the solid de- creases the melting temperature and facilitates melting. The dissolution kinetics of spheri- cal graphite inclusions in the melt pool subjected to body forces (buoyancy or centrifugal forces) is modelled using a semi-analytical approach to the moving boundary problem. Diffusion controlled melting and solidification at a planar front as well as at dissolving gra- phite particles in a solid environment above 1150 øC were studied numerically. At high feed rates andlor large graphite inclusion spacings a straight sharp boundary of the re- melted zone is fou nd since melting at the planar front dominates. At lower feed rates the melt zone boundary structure is determined by the coalescence of initially isolated molten shells originating at graphite inclusions. The result is a transition zone with mutually more or less interconnected ledeburitic regions at partly or completely dissolved graphite parti- cles. The computed dissolution kinetics as well as the proposed relationship between the melt zone boundary structure on the one hand and heating time and particle size on the other hand are corroborated by the structure observed in laser remelted GGG 40.