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

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 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 nano- particle 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).