Verfahrenstechnische Grundlagen für das Laserstrahl-Umschmelzen einkristalliner Nickelbasis-Superlegierungen

In the present work procedural fundamentals are developed for the laser remelting of single crystal turbine blades made from the nickel-base superalloy PWA 1483. Such a process aims to remove cracks in the surface layer by maintaining the single crystal structure. Single crystal repair using laser remelting requires controlled solidification conditions in order to prevent nucleation and growth of spurious grains ahead of the columnar dendritic front, i.e. to ensure epitaxial growth and to prevent the columnar to equiaxed transition. The development of the procedural fundamentals is subdivided into three steps. A laser remelting process is first developed on flat sample geometries. A process window is determined by varying all relevant process parameters. By maintaining the single crystal structure, a remelted depth up to 1.2 mm is reached in the surface layer. The development of an overlapping strategy is used to remelt whole areas and to achieve a homogeneous remelted depth. In addition investigations are carried out to measure the emitted temperature radiation on the surface in the melt pool. In comparison to the experimental process development, heat conduction calculations based on the finite element method are carried out to determine the solidification conditions (temperature gradient, solidification velocity) and the temperature field within the melt pool. The experimental and calculated results are compared with results from previous literature. The first step is completed by remelting artificial cracks and maintaining the single crystal structure. In the second step, the developed process is transferred to complex samples with changing material thickness and curved surfaces. The measurement of the emitted temperature radiation of the surface of the melt pool is used to implement a temperature control. By using this control a nearly constant remelted depth of zm = 1 mm  0.1 mm is achieved along the remelted track. This means that the deviation in remelted depth is decreased in comparison to laser remelting with a constant laser power. In the third step, the process is successfully applied to remelt different areas on turbine blades (platform, blade, fillet) without cracking.