Structure and texture of weld banded interfaces in aluminum alloys

Abstract

In this paper, a 14 μm core diameter laser was adopted to deep-penetration weld 2A12 aluminum alloy. The high energy density and severe laser-material interactions produced new-found banded interfaces ranging from 12 μm to 28 μm. Overall, there was a laminar feature originating from the fusion line, especially at the bottom. Far away from the lower middle position, the laminar structure presented an alternative appearance of narrow coarse equiaxed bands between stable cellular and dendritic growth. Adjacent to keyhole, an additional planar growth was found to evolve from the already solidified zone or even nonmelted base metal. Under this condition, continuous epitaxial growth (pole density: 7.0) was blocked and resultantly transformed into extremely fine grains (pole density: 3.5) in the weld bottom. Metallurgically, local heat and mechanical fluctuations transmitted from wall surface of keyhole decreased the temperature gradient and elevated the supercooling at the solidification front. In the weld bottom, extremely high temperature gradient and solidification rate left merely instant time for solidification, therefore promoting the planar growth. In the subsequent growth, an alternant rise-and-fall supercooling produced the recurrent columnar-to-equiaxed structural transformation.