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.
The microstructure of rapidly solidified AlFe alloys subjected to laser surface treatment
Abstract Using traverse rates of between 0.1 and 6.0 m/s, laser-melted tracks were produced on AlFe alloy samples containing between 0.25 and 8.0 wt% Fe. The local solidification rates were measured by taking a longitudinal section through the centre of the laser trace and the corresponding microstructures were studied quantitatively using transmission electron microscopy. Three types of microstructures were observed. At low growth rates, cellular/dendritic structures were obtained. At high growth rates, a banded structure was formed which consisted of a succession of light and dark bands which lay approximately parallel to the solid-liquid interface. At the lower concentrations, precipitate-free structures were obtained at very high solidification rates. A recent model, for dendritic growth under rapid solidification conditions, was compared with the experimental results and a good correlation was found. It was shown that, at rates close to the limit of absolute stability, steady-state planar-front growth was not the preferred growth morphology; but rather a banded structure. It was only at much higher rates that a fully precipitation-free structure, probably involving plane-front growth, developed.
Suitability and water quality criteria of an open drainage municipal sewage water at Coimbatore, used for irrigation.
Sanganur canal is the major open drainage system which has intricate linkage with storm water supply, domestic sewage and industrial effluent disposal. Water samples from various stations were collected and analysed for physicochemical parameters to assess the water quality of the Sanganur canal system. The study revealed that physicochemical parameters like pH, EC, TDS, DO, BOD, COD exceeded the permissible limit, clearly indicating the need of proper treatment of waste water before discharge into the Noyyal river.