Effect of rotation speed on microstructure evolution and mechanical properties of nugget zone in 2195-T8 Al-Li alloy friction stir welding joints

Highlights: • Systematic study about the effects of rotation speed on microstructure evolution and mechanical properties of different regions in nugget zone of 2195 aluminum alloy friction stir welding joints was conducted. • The precipitation mechanism of the T₁ phase in the nugget zone was analyzed. • Strengthening mechanism of the NZ was discussed in detail. Complicated plastic deformation and welding temperature were generated in the nugget zone (NZ) of friction stir welding (FSW) joints leading to different regions being developed, such as shoulder influence zone (SIZ), pin influence zone (PIZ) and swirl zone (SZ). To find the effect of rotation speed on microstructure evolution and mechanical properties of these regions during friction stir welding, serials of AA2195-T8 plates with 7.5 mm thickness were fabricated using different rotation speeds (500, 700 and 900 rpm) and a constant welding speed (50 mm/min). Microstructural characterization was conducted out using electron back scattered diffraction (EBSD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and X-ray diffraction (XRD) technologies. Mechanical properties were measured by micro-hardness and tensile test. The results show that grains in the SIZ were mainly shown in wedge-shaped structure and had the high density of low angle boundaries (LABs) showing dominant (100) orientation. However, in the PIZ and SZ, fine and equiaxed grains with dominant (111,101) orientation were observed. With rotation speed increasing, the fraction of LABs increased in the SIZ but showed little changes in the PIZ and SZ. After welding, a large number of T₁ and θ′ precipitates almost be completely dissolved in the NZ with many fine δ′/β′ being re-precipitated. It is notable that a few T₁ precipitates were observed in the SIZ at a higher rotation speed (900 rpm), which were re-precipitated from the Al matrix after dissolution. In the SIZ, the dominant contribution to yield strength was dislocation strengthening which enhanced with rotation speed increasing. However, in the PIZ and SZ, grain refinement strengthening was the primary strength mechanism, which decreased with rotation speed increasing.