Nucleation mechanisms of equiaxed grains in the fusion zone of aluminum-lithium alloys by laser welding

The formation of different microstructures in aluminum-lithium alloys welding joints by laser welding has not been fully understood at the moment. In this study, the nucleation mechanisms of equiaxed grains in the fusion zone of aluminum-lithium alloys during laser welding are investigated. Two different kinds of equiaxed grains, i.e., non-dendritic equiaxed grains and dendritic equiaxed grains, are discovered. An improved method based on an overlap welding procedure is proposed to clarify the nucleation mechanisms. Two nucleation mechanisms, i.e., grain detachment and heterogeneous nucleation, are simultaneously responsible for the formation of equiaxed grains. The grain detachment, caused by the flow in the molten pool, leads to the formation of non-dendritic equiaxed grains and a part of coarse dendritic equiaxed grains. The direct evidence that the convection of laser welding molten pool causes grains to be detached from the base metal is found for the first time. Most of the dendritic equiaxed grains grow on heterogeneous nucleation particles. D0(22)-Al3Ti and D0(23)-Al3Zr are the main heterogeneous nucleation phases during laser welding. (C) 2021 The Author(s). Published by Elsevier B.V.