Relationship Between Microstructures and Microhardness in High-Speed Friction Stir Welding of AA6005A-T6 Aluminum Hollow Extrusions

Abstract

AA6005A-T6 aluminum hollow extrusions were friction stir welded at a fixed high welding speed of 2000 mm/min and various rotation speeds. The results showed that the heat-affected zone (HAZ) retained the similar grain structure as the base material except some grain coarsening, and the density of dislocations and β′ precipitates were almost unchanged, indicating that the high welding speed inhibited the coarsening and dissolution of β″ precipitates via fast cooling rate. The thermo-mechanically affected zone (TMAZ) was characterized by elongated and rotated grains, in which a low density of β′ precipitates and the highest density of dislocations were observed. The highest heat input and severest plastic deformation occurring in the nugget zone (NZ) resulted in the occurrence of dynamic recrystallization and a high density of dislocations. Hence, all the β″ precipitates and most of the β′ precipitates dissolved into the matrix, and a few β′ precipitates were transformed into β precipitates. The microhardness was controlled by the precipitation and solution strengthening in the HAZ, by the dislocation and precipitation strengthening in the TMAZ, and by the fine-grain and dislocation strengthening in the NZ. With the increase in rotation speed, the peak and the lowest microhardness value increased monotonously.