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.
Fabrication of functionally graded TiC/Ti composites by Laser Engineered Net Shaping
Abstract Crack-free functionally graded TiC/Ti composite materials were fabricated by laser engineered net shaping (LENS), with compositions changing from pure Ti to approximately 95 vol% TiC. By delivering the constituent materials from different powder feeders and through process control, the LENS process can be used for the fabrication of functionally graded materials.
Synthesis of metastable phases via pulsed-laser-induced reactive quenching at liquid-solid interfaces.
High-power pulsed-laser-induced transformations at liquid-solid interfaces are examined with a view to synthesize new metastable phases of materials. Specifically, two types of problems are studied: (1) laser-induced synthesis of compound films at liquid-solid interfaces (called "reactive quenching" because the synthesized compound derives its atomic constituents from the participating liquid and solid systems), and (2) laser-induced alloying of layered structures under a liquid medium whose chemical participation in the process is minimal. The reactive-quenching process has been investigated for three different systems, viz., Fe:H 2 O, Fe:NH 3 (liquid), and W:C 6 H 6 (benzene), and the results clearly demonstrate that this process can lead to interesting possibilities of synthesis of metastable phases of materials. The identification of metastable compound phases and their microstructural transformations upon subsequent thermal annealing are investigated by using the techniques of conversion-electron Mossbauer spectroscopy, glancing-angle x-ray-diffraction measurements, Rutherford-backscattering spectrometry, x-ray-photoelectron spectroscopy, and transmission-electron microscopy. In the Fe:H 2 O and Fe:NH 3 cases the as-irradiated state shows the presence of FeO and γ-Fe-N austenite, respectively, while in the W:C 6 H 6 case a multiphase composite comprised of W 3 C, β-W 2 C, and WC 1-x is observed. Laser-induced alloying of layered structures in the liquid ambient has been studied in the case of the Fe-Al system, and it has been established that processing under liquid nitrogen leads to distinctly different results as compared to laser alloying in air or at liquid-nitrogen temperature in an inert-gas ambient. Time-resolved reflectivity measurements are carried out at the liquid-solid interface to obtain information about the possible mechanisms which could be responsible for the observed effects.