Nano-indentation investigation on the local softening of equiaxed zone in 2060-T8/2099-T83 aluminum-lithium alloys T-joints welded by double-sided laser beam welding

Highlights: • Local softening is found in equiaxed zone's enrichment region by nano-indentation. • Local softening is induced by microcrack's formation and thick Al₂Cu's absence. • Al₃Zr particle can be heteromorphic core of AlCuFeMn quasicrystal in equiaxed zone. • Al₃Zr particle has smaller impact on equiaxed zone's softening than Al₂Cu particle. Double-sided laser beam welding of 2-mm-thick 2060-T8 and 2099-T83 aluminum-lithium alloys T-joint was performed with AlSi12 wire. Microstructure and nano-indentation hardness characteristics were studied especially in nondendritic equiaxed zone (EQZ). In comparison with the precipitates of LiAlSi, Mg₂Si and Al₂Cu in weld zone except for the EQZ, an AlCuFeMn icosahedral quasicrystal was detected in the EQZ and Al₃Zr particle could be the heteromorphic core of this quasicrystal phase. Local softening problem was confirmed in the EQZ's localized enrichment regions by nano-indentation tests. The cross sections of different indents in and out of the EQZ's local softening area prepared by focused ion beam (FIB) were examined by using transmission electron microscopy (TEM). For the first time, it was directly observed that an intergranular tiny hot crack was formed right underneath the indent tip in the EQZ's local softening area, and mainly thin Al₂Cu precipitates were generated and sparsely distributed in equiaxed grains and their boundaries. However, thick and continuous Al₂Cu intergranular precipitates with no microcrack defect were observed below the indent in the other area in the EQZ without softening. Obviously, local softening in the EQZ was most likely to be induced by a combined action of the formation of microcrack and the absence of abundant Al₂Cu precipitates in equiaxed grain boundaries. Since the quantity and size of Al₃Zr particle were both much smaller than those of Al₂Cu, it had more limited impact on the EQZ's local softening than Al₂Cu particle in equiaxed grain.