Alexander Schnell

Epitaxial deposition of MCrAlY coatings on a Ni-base superalloy by laser cladding

Abstract Single-crystal oxidation resistant coatings (MCrAlY alloys) have been deposited onto a single-crystal Ni-base superalloy by laser cladding. The influence of the primary solidification phase (γ-Ni, γ ′ -Ni 3 Al or β-NiAl) on epitaxial or non-epitaxial growth is analysed. It is shown that epitaxial growth is only obtained when the primary phase to solidify is γ.

A Study of the Weldability of Gamma Prime Hardened Superalloys

Fusion repair processes such as gas tungsten arc welding (GTAW) and laser welding have been introduced for repairing turbine parts made from Ni-based superalloy materials. The weld-repair of turbine parts is well established, however, the inherent susceptibility of ’ hardened superalloys to weld cracking remains an issue and has resulted in repair limitations for highly loaded areas of turbine parts. This study presents a view of the weldability of superalloys taking both, the impact of the weld process and the weld filler selection into consideration. This comprises the interpretation of specific process parameters into physical parameters controlling the weldability and cracking sensitivity such as thermal gradient in the weld pool and solidification speed. Alloy specific parameters of the weld filler material, such as melting point and solidification interval are studied and set in correlation with the solidification parameters during welding.

Manufacturing Optimization for Bondcoat/Thermal Barrier Coating Systems

A reliable lifetime prediction rule for bondcoat/thermal barrier coating (BC/TBC) coated parts in gas turbine operation is necessary to determine remnant service life. The specimens investigated were coated with MCrAlY plus yttria partially stabilized zirconia applied by vacuum plasma spraying and atmospheric plasma spraying processes, respectively. The performances of these laboratory specimens were statistically assessed, combining long term oxidation testing with thermal cycling, thus superimposing thermomechanical loading on the laboratory specimens to more accurately represent engine conditions. A design of experiment (DOE) approach was used for manufacturing optimization of the BC/TBC system . The life of the coating system is influenced by several manufacturing parameters such as BC thickness, BC roughness, TBC thickness, TBC porosity, and TBC stiffness. Specimens with a suitable variation in these parameters were produced to ensure a balanced test matrix of fractional factorial DOE. Based on results derived from laboratory testing the specifically tailored parts, first and second order effects of manufacturing parameters on lifetime were quantified. The findings revealed that the second order effects (the interaction of manufacturing parameters) were more important on the lifetime of the BC/TBC system than the corresponding first order effect (single parameter). For instance, the variation in BC thickness or BC roughness led to a scatter of lifetimes of 10% and 60%, respectively whereas their interaction resulted in a scatter of lifetime of 150% for the same range of coating parameters. Further examples of such pairings are also demonstrated. Finally, a lifetime prediction for three quality classes (high, medium, and low qualities) has been demonstrated. The difference in achievable lifetime highlights the importance of manufacturing parameters in determining the life of the BC/TBC system.