Resolidification behaviour of laser surface-melted metals and alloys

Abstract

The solidification behaviour is described of two pure metals (Bi and Ni) and two eutectic alloys (Al-Ge and Al-Cu) under non-equilibrium conditions, in particular the microsecond pulsed laser surface melting. The resolidification behaviour of bismuth shows that epitaxial regrowth is the dominant mechanism. For mixed grain size, regrowth of larger grains dominates the microstructure and can result in the development of texture. In the case of nickel, epitaxial growth has been noted. For lower energy pulse-melted pool, grain refinement takes place, indicating nucleation of fresh nickel grains. The Al-Ge eutectic alloy indicates the nucleation and columnar growth of a metastable monoclinic phase from the melt-substrate interface at a high power density laser irradiation. An equiaxed microstructure containing the same monoclinic phase is obtained at a lower power density laser irradiation. It is shown that the requirement of solution partition acts as a barrier to eutectic regrowth from the substrate. The laser-melted pool of Al-Cu eutectic alloy includes columnar growth of α-Al and θ-Al₂Cu phase followed by the dendritic growth of Al₂Cu phase with α-Al forming at the interdendritic space. In addition, a banded microstructure was observed in the resolidified laser-melted pool.