Formation of solute band in aluminum alloy welds

Basic studies on solidification of welds (4th report)

Abstract

This paper discusses the morphology of solute bands (or layer lines) observed in MIG or TIG welds of a commercially pure aluminum and Al-Mg, Al-Zn-Mg and Al-Mg-Si alloys, and their effects on several properties of welds.
The solute bands were noted as macrostructures having light etched lines or bands or as microstructures corresponding to the region of coarse dendrite cells in 99.5%Al and -4.5%Mg alloy welds; or as microstructures corresponding to the grain-refined region in Al-4%Zn-2%Mg alloy welds containing some grain-refined elements. They were observed to be formed penetrating the grains in cycles perpendicularly to the direction of solidification or heat flow, independent of the presence of feathery grains.
The effects of solute bands on the properties of welds were confirmed by the variation of hardness or indirectly by the marks on fractures of impact test specimens. Some of the effects were also observed by the distribution of porosity in parallel bands and the cyclic propagation of weld cracks corresponding to the solute bands.
The formation of solute bands depended upon the welding speed and heat inputs. They had comparatively similar characteristics to those of the wavy surface ripples on the weld bead surface, but seemed not to be identified with the ripples because of the differences in number and region of their occurrence.
The formation of solute bands was considered to be directly related to the periodic fluctuations in growth rate of weld solidification interface, resulting from the effects inherent in weld solidification such as the release of the latent heat of fusion on the liquid-solid interface. Moreover, these fluctuations seemed to be affected also by the external factors associated with the welding process.