Effects of melt-pool geometry on crystal growth and microstructure development in laser surface-melted superalloy single crystals: Mathematical modeling of single-crystal growth in a melt pool (part I)
Abstract The effects of melt-pool geometrical parameters on crystal growth and microstructure development during laser surface melting of single-crystal alloys were studied by means of mathematical modeling and experiments. A mathematical model was developed for the three-dimensional (3-D) melt-pool geometry and single-crystalline melt-pool solidification in laser surface melting. The 3-D melt-pool geometry corresponding to the solidification interface is described by four geometrical parameters ( w , l , h , α ). The model was used to study the effects of variations in the geometrical parameters on crystal growth and microstructure development in the melt pool. Laser surface melting experiments with single-crystal nickel-base superalloys were conducted to verify the computational results of microstructure development in the melt pool. Results indicate that the melt-pool geometrical parameters have profound influences on the dendrite growth velocity and growth pattern in the melt pool. For the (0 0 1)/[1 0 0] substrate orientation, variations in l / w and α can influence both the number and the relative sizes of growth regions while the variation in h / w can only influence the relative sizes of the growth regions. Unidirectional dendrite growth along the [0 0 1] crystallographic direction can be achieved for an α value of 45° or below. The maximum ratio of dendrite-growth velocity to the beam velocity in the melt pool is related to α and l / w . Experimental microstructure observations agreed well with the computational results. These findings show that the desired dendrite growth velocity and microstructure can be obtained through proper control of the 3-D melt-pool geometry.
SINGLE-CRYSTAL LASER DEPOSITION OF SUPERALLOYS: PROCESSING-MICROSTRUCTURE MAPS
Abstract In order to extend the life cycle of modern single-crystal (SX) high-pressure high-temperature gas turbine blades, repair of cracked or worn parts is of great interest. The success of the repair technique depends critically on a close process control in order to ensure SX repair. Based on solidification theory a process called epitaxial laser metal forming (E-LMF) has been developed. This paper presents the important concepts necessary for any process control for SX repair based on processing maps which relate the expected solidification microstructures and growth morphologies to the processing conditions. These maps are obtained in two steps. Firstly, the relationships between local solidification conditions and the resulting solidification microstructures, i.e. columnar or equiaxed, are formulated. Secondly, the local solidification conditions as a function of the laser processing parameters are calculated with an analytical heat flux model. By a combination of both approaches, processing–microstructure maps are obtained which define processing windows for SX generation and repair by laser deposition.
Microfabrication of three-dimensional boron structures by laser chemical processing
A method for microfabrication of three‐dimensional structures in free space is presented. Laser‐assisted chemical vapor deposition is used to grow a material at a point where a laser beam locally heats the substrate. By moving the substrate relative to the laser beam with a micropositioning system, three‐dimensional shapes can be created. Helical shapes are generated utilizing three linear translational axes as well as an additional rotational axis. Tilting the substrate to align the growth direction with the laser beam direction facilitates improved process control. The smallest structures that can be grown with this technique are about 1 μm. Amorphous boron fibers and crystalline boron springs have been manufactured as two examples of micromechanical elements. The amorphous boron fibers show excellent mechanical properties: A modulus of elasticity of 420–450 GPa, a fracture strain of 2.7%–3.7%, and a fracture stress of 12–17 GPa. The crystalline boron springs produced so far display only moderate mechanical characteristics. This is because of the irregular fiber surface with protruding grains and amorphous nodules. Nevertheless, a maximum relative resilience of about 9% for a tightly wound spring has been obtained. The crystal structure of the grains examined show a fair agreement with the β‐boron phase, even though distinct deviations are observed. Metastable arrangement of the B12 icosahedral units is suggested as a possible explanation. A high density of (100) twins and stacking faults are identified by transmission electron microscopy. By further process and system development it is believed that micromechanical details can be tailored more or less arbitrarily for different applications.