The thermal modeling of underwater friction stir welding （FSW） was conddcted with a three-dimensional heat transfer model. The vaporizing characteristics of water were analyzed to illuminate the boundary conditions of underwater FSW. Temperature dependent properties of the material were considered for the modeling. FSW experiments were carried out to validate the calculated results, and the calculated results showed good agreement with the experimental results. The results indicate that the maximum peak temperature of underwater joint is significantly lower than that of normal joint, although the surface heat flux of shoulder during the underwater FSW is higher than that during normal FSW. For underwater joint, the high-temperature distributing area is dramatically narrowed and the welding thermal cycles in different zones are effectively controlled in contrast to the normal joint.
The reverse dual-rotation friction stir welding(RDR-FSW) has the capability to adjust the heat generation because of the separately designed tool shoulder and tool pin.The welding torque exerted on the workpiece by the reversely rotating shoulder is opposite to that exerted by the rotating tool pin,so the total welding torque is reduced,which is beneficial to reducing the clamping requirement of workpieces.In the present paper,a RDR-FSW joint was welded in a condition similar to the optimal welding condition of conventional FSW,and microstructures in various zones were investigated by comparison,aiming to highlight effects of the reversely rotating assisted shoulder.Due to the heat conduction of the middle cylinder and the bottom end cover on which the assisted shoulder was machined,the thermal effect of RDR-FSW was smaller than that of the conventional FSW.Moreover,the effect of assisted shoulder on the plastic flow or deformation of material or was constrained in a thin layer near the weld top surface,and thus the flow of material especially along the thickness direction was clearly decreased in the RDR-FSW.In the heat-affected zone(HAZ),the precipitate coarsening was the main evolution and was completed through the dissolution of small precipitates and the continuous growth of large precipitates.By contrast,the dissolution degree of precipitates increased significantly in the thermomechanically affected zone(TMAZ),and a small amount of original meta-stable precipitates transformed to block-shaped stable precipitates.Precipitate evolutions in the shoulder affected zone(SAZ)and the weld nugget zone were similar,i.e.the majority of original meta-stable precipitates dissolved into the matrix and the remainder transformed to stable precipitates,though the dissolution degree was greater in the SAZ.Compared with the conventional FSW joint,the coarsening degrees of precipitates in the HAZ and TMAZ of RDR-FSW joint were much smaller,as well as the dissolution degrees of precipitates in all four specified zones.
AA6005 A-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-aff ected zone(HAZ)retained the similar grain structure as the base material except some grain coarsening,and the density of dislocations andβ′precipitates were almost unchanged,indicating that the high welding speed inhibited the coarsening and dissolution ofβ″precipitates via fast cooling rate.The thermo-mechanically aff ected zone(TMAZ)was characterized by elongated and rotated grains,in which a low density ofβ′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 theβ″precipitates and most of theβ′precipitates dissolved into the matrix,and a fewβ′precipitates were transformed intoβprecipitates.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.
The AA6005A-T6 aluminum hollow extrusions were friction stir welded at a high welding speed of 2000mm/min and various axial forces. The results show that the nugget zone （NZ） is characterized by fine equiaxed grains, in which a low density of equilibrium phase β is observed. The grains in the thermo-mechanically affected zone （TMAZ） are elongated, and the highest density of dislocations and a low density of β precipitates can be found in grains. The heat affected zone （HAZ） only experiences a low thermal cycle, and a high density of β precipitates and a low density of β precipitates remain in the coarsened grains. The microhardness evolutions in the NZ, TMAZ and HAZ are governed by the grain refinement and dislocation strengthening, the dislocation and precipitation strengthening, and the precipitation and solid solution strengthening, respectively. When increasing the axial force, the changing trend of one strengthening mechanism is contrary to the other in each zone, and the microhardness increases in different zones. As a result, the tensile strength roughly increases with raising the axial force, and all joints show good tensile properties as the high welding speed inhibits the coarsening and dissolution of strengthening precipitates significantly.
Xiangqian LiuHuijie LiuTianhao WangXiangguo WangSi Yang
