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

摘要

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. (C) 2018 Elsevier B.V. All rights reserved.