Epitaxial laser treatment of single crystal nickel-base superalloys

The continued drive for increased efficiency, performance and reduced costs for aircraft and industrial gas turbines demands extended use of high temperature materials, such as single crystal nickel based superalloys. The cost for hot section components used in those applications is a primary factor driving a growing need for advanced blade repair procedures, which address the problem of recuparating damaged parts. In previous research at EPFL, it has been shown that single crystal deposition for turbine blade repair is possible by an Epitaxial Laser Metal Forming (E-LMF) technique. In this 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 turbine 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. The feasability of the E-LMF process has been demonstrated in the laboratory on simple geometry substrates and platforms of turbine blades. Significant efforts are still required to up-scale this process for the repair of real, complex shaped parts on an industrial scale as solidification defects are often encountered when non-ideal processing conditions are used. Epitaxial laser treatment of single crystal nickel-base superalloys The aim of the present research is to study the microstucture development during the laser assisted deposition of Ni-based superalloys on single crystal substrates with the same composition and to gain an understanding of defect formation mechanisms, this in view of a complete control of the E-LMF process. As a follow-up to previous research, particular emphasis is placed on non ideal conditions by taking into account important aspects of dendritic solidification. In particular, the major defects encountered during single crystal laser deposition, i.e. loss of the crystal orientation of the substrate, are described taking into account the following aspects : off-axis heat flux dendritic growth, fragmentation, grain growth competition, and loss of epitaxy due to branching difficulties of cellular-dendritic structures. In this work, critical parameters, which affect epitaxy, the nucleation of spurious grains and grain structure, will be discussed and new results, which are fundamental to the full process control of E-LMF, will be presented. By contributing to the understanding of the phenomena which are responsible for loss of single crystallinity and through the definition of proper processing windows for single crystal generation and repair by laser deposition, this work will constitute a sound basis to guide future industrial practice.