Microstructure evolution and strength-ductility synergy in friction stir welded ZrB2/7085Al-Er nanocomposite joints

Abstract

Dispersed nanoparticles are expected to improve the weldability of 7085Al alloy for load-bearing structure of aerospace vehicles. In this work, Er elements were added to enable ZrB₂ nanoparticle dispersion through the modified interface and then the well-designed in-situ ZrB₂/7085Al-Er nanocomposites were joined by friction stir welding (FSW). The results showed that the coherent (10 $\bar{1}$ 0)_ZrB2//(111)_Al3Er//(111)_Al interface constructed by primary Al₃Er improved the interfacial wettability between ZrB₂ and Al, enabling the dispersion of ZrB₂ nanoparticle during solidification. After FSW, finer equiaxed recrystallized grains were formed in the nugget zone (NZ) thanks to the particle stimulated nucleation (PSN) mechanism and Zener pinning effect from ZrB₂ and Al₃(Er, Zr), which was in favor of alleviate welding hot cracking. The coarsen process of precipitates in the heat-affected zone (HAZ) was weaken. The coarse grain boundary precipitates were modified into fine precipitates with discrete distribution by ZrB₂ and Al₃(Er, Zr). The ultimate tensile strength and elongation of ZrB₂/7085Al-Er joint were 484 MPa and 14.8 %, which were enhanced by 36.3 % and 45.1 % compared with 7085Al joint. The joint efficiency reached up to 75.2 %. The strength-ductility synergy of ZrB₂/7085Al-Er joint came from grain refinement, avoidance of strain localization at grain boundary and activation of high density of intragranular dislocations brought by dispersive ZrB₂ and Al₃(Er, Zr).