Marangoni Convection and Fragmentation in Laser Heat Treatment

Epitaxial Laser Metal Forming (E-LMF) consists in impinging a jet of metallic powder onto a molten pool formed by controlled laser heating and thereby, generating epitaxially a single crystal deposit onto 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. A major limitation to the process lies in the formation of stray grains which can originate either from heterogeneous nucleation ahead of the solidification front or from remelting of dendrite arms due to local solute enriched liquid flow, .i.e fragmentation. To study this last aspect, heat and fluid flow modelling is required to establish the relationship between process parameters such as laser power, beam diameter and scanning speed, and the local solidification conditions plus the fluid flow in the vicinity of the mushy zone. Surface tension driven convection known as the Marangoni effect needs to be included in the model owing to its large influence on the development of eddies and on the shape of the liquid pool. The 3D model implemented in the FE software calcosoft® is used to compute the fluid convection within the liquid pool and to assess the risk of fragmentation using a criterion based on the local velocity field and thermal gradient. The computed results are compared with EBSD maps of laser traces carried out at EPF-Lausanne in re-melting experiments

Modelling Marangoni convection in laser heat treatment

Epitaxial Laser Metal Forming (E-LMF) consists in impinging a jet of metallic powder onto a molten pool formed by controlled laser heating and thereby, generating epitaxially a single crystal deposit onto the damaged component. This new technique aims to be used for the repair and reshape single crystal gas turbine components. Because of the very localised melting pool, the high temperature gradients produced during the process must be carefully controlled in order to avoid both the columnar-to-equiaxed transition (CET) and the appearance of hot tears. To this end, heat flow modelling is required to establish the relationship between process parameters such as laser power, beam diameter and scanning speed, and the local solidification conditions. When modelling the heat transfer within the sample, it is necessary to include the liquid flow pattern generated by the surface tension driven convection known as the Marangoni effect. Indeed, the fluid flow in the liquid pool dictates the shape of the traces as shown by the measurements carried out at EPF-Lausanne in re-melting experiments. A three dimensional (3D) model is implemented in the finite element software calcosoft in order to model the development of the fluid convection within the liquid pool. It is shown that the velocities due to natural convection are of the order of 1 mm/sec whereas Marangoni convection produces velocities of the order of 1 m/sec. Moreover, at low scanning speeds, the liquid pool becomes larger than the beam diameter and the development of Marangoni eddies leads to a widening and deepening of the pool. The local solidification conditions such as the thermal gradient and the solidification speed can be extracted at both the solidus and liquidus temperatures to assess the risk of CET and hot cracking

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