Abstract:

A novel ultra high temperature gradient directional solidification apparatus for metal slice has been set up, in which a high energy laser beam was used as the heating source. The microstructure of a Ni base superalloy under the condition of ultra high temperature gradient (1.0×10 4K/cm) with a wide range of G/R ratio was comprehensively investigated. Both superfine dendrite with average primary spacing of 10.6μm and a novel "superfine cellular crystal", which is the extremely fine needle like cell without sidebranches, have been obtained in the laser directionally solidified microstructures. The cause for the formation of the superfine cellular crystal has been analyzed, namely, under the condition of high temperature gradient and high solidification rate, the sidebranch loses its space and time to grow and develop. The result on microsegregation shows that there exists severe microsegregation in coarse dendrite, however, the microsegregation in the laser directionally solidified microstructures is significantly improved,or even completely eliminated.

Key words: Ni base superalloys, laser directional solidification, superfine cellular crystal