MICROSTRUCTURE CONTROL DURING SINGLE CRYSTAL LASER WELDING AND DEPOSITION OF NI-BASE SUPERALLOYS

摘要

In the Epitaxial Laser Metal Forming (E-LMF) process, metal powder is injected into a molten pool formed by controlled laser heating with the aim of producing a single crystal deposit on a single crystal substrate. It is a near net-shape process for rapid prototyping or repair engineering of single crystal high pressure/high temperature gas turbines blades. Single crystal repair using E-LMF requires controlled solidification conditions in order to prevent the nucleation and growth of crystals ahead of the columnar dendritic front, i.e., to ensure epitaxial growth and to avoid the columnar to equiaxed transition. In the follow-up of previous research of the authors new strategies for microstructure control during E-LMF will be presented; (ⅰ) for off-axis dendritic growth, (ⅱ) for the case of growth competition of grains of different orientation and (ⅲ) for cellular dendritic growth where secondary branches are missing. Control of these phenomena is of major importance for an effective industrial application of the E-LMF process. For this purpose, models have been developed and used in association with extended experimental observation to predict the expected solidification morphology for a given set of laser processing parameters. New microstructure features such as oriented-to-misoriented transition and the loss of epitaxy in the critical branching zone are presented and measures for avoiding the formation of spurious grains within the repaired zone are discussed.