Study on the microstructure in a friction stir welded 2519-T87 Al alloy

The microhardness, grain size and distribution of the precipitates in various zones of a 2519-T87 aluminum alloy welded joint welded by friction stir welded were investigated. The dynamic recrystallization occurred in the weld nugget zone and the size of recrystallized grains was smaller than that in the thermo-mechanically affected zone, besides, θ′ and θ phase precipitated. The thermo-mechanically affected zone was divided into three ones: the zone close to the thermo-mechanically affected zone, where coarse θ′ precipitates were observed; the middle zone, where the mainly θ precipitates appeared; and the zone close to the heat-affected zone, where both thin θ′ and θ precipitates were observed. In the heat-affected zone, the size of θ′ precipitates was less than those in the base metal zone. On the transverse cross-section of welded joint the microhardness curve shows a W-shape.

► The relationship between grains size, precipitates and microhardness is obtained.
► The grain size increases from the WNZ to TMAZ, the precipitates are θ′ and θ phases.
► The TMAZ is divided into three regions, with different types of precipitates.
► In the HAZ, the θ′ precipitates increases in size and decrease in density.