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.
Direct laser deposition of a single-crystal Ni3Al-based IC221W alloy
Single-crystal (SX) nickel aluminide alloys have potential for structural applications where high-temperature strength and oxidation resistance are required. In this work, SX deposits of the Ni3Al-based IC221W alloy were produced on a SX Ni-base superalloy substrate by means of the laser-engineered net shaping (LENS) process. The microstructure of the deposits was characterized. The effects of processing parameters on the SX solidification in the melt pool and on the fabricability by LENS were investigated. A simple relationship between the ratio of the temperature gradient to the growth velocity and the processing parameters was derived, which can be used qualitatively to guide the proper selection of processing conditions to maintain the columnar dendritic growth during the laser deposition. On the basis of analyses and experiments, the effects of processing parameters on the susceptibility to stray grain formation and solidification cracking are discussed.
A metallographic study of the effect of more rapid freezing on the cast structure of aluminum-iron alloys
Abstract The structures present in taper-section chill castings of hypo- and hypereutectic Al-Fe alloys containing between 1 and 11 wt. % Fe have been studied by optical, x-ray, and electron metallography, in an attempt to relate slowly frozen and splat-cooled microstructures. The several microstructural transitions are described and explained, involving changes from intermetallic to α Al primary phases, from irregular to regular eutectic with FeAl3 or FeAl6 second phases, and from fine to coarse dendritic α Al. Some differences between the latter transition and the zone A to B transition in splat cooling are discussed.