A Proposed Mechanism for Equiaxed Grain Formation along the Fusion Boundary in Aluminum-Copper-Lithium Alloys

摘要

The effects of welding conditions, composition and solidification substrate have been systematically studied in an effort to determine the nature of nondendritic equiaxed grain formation in Al-Cu-Li welds. The equiaxed zone (EQZ) in these alloys forms preferentially at the fusion boundary of arc welds and may have important implications with respect to both weld cracking susceptibility and structural integrity. Over a wide range of weld heat input, achieved by varying weld travel speed and current during autogenous gas tungsten arc welding, it was not possible to eliminate this zone, although its width and the grain size varied considerably. The solidification substrate had a profound effect on EQZ formation. The EQZ did not form when solidification occurred from an "as-cast" weld metal substrate. Under other substrate conditions, including the as-cast/solution heat treated condition, an EQZ was always present. Both lithium and zirconium influence EQZ formation, with zirconium exhibiting a more dominant effect. At low levels of lithium and zirconium (0.5Li, 0.03Zr), an EQZ was not observed. A unified heterogeneous nucleation mechanism is proposed to describe EQZ formation in Al-Cu-Li welds. According to this mechanism, the EQZ forms in a narrow molten region adjacent to the fusion boundary that may correspond to the unmixed zone between the HAZ/PMZ and the weld metal. Within this region, heterogeneous nuclei that are remnant from the base metal promote the nucleation and rapid growth of nonden-dritic, equiaxed grains. It is proposed that these nuclei, identified as Al_3Zr and Al_3(Li_x, Zr_(1-x)) precipitates, are stable within a narrow temperature field at the fusion boundary where the EQZ occurs. Eliminating these nuclei, such as by creating an as-cast solidification substrate, prevents the formation of an EQZ. No evidence was found to support a HAZ/PMZ recrystallization mechanism that had been previously proposed for EQZ formation in these alloys. It is likely that the mechanism proposed here for EQZ formation can also be applied to other Li-bearing aluminum alloys that contain zirconium.