High rotation speed friction stir welding is a promising low-force welding technique that enables the application of friction stir welding on in situ fabrication and repair. High rotation speed friction stir welding experiments (above 3000 rpm) were conducted on an Al-Mn aluminum alloy. The effect of rotation speed on nugget structure and property was investigated in order to illuminate the process features. The results indicate that a notable increase of nugget size occurs at high rotation speeds of 5000-8000 rpm. With increasing rotation speed, the thermal effect is firstly strengthened and then achieves a steady state. The microstructure evolution is more sensitive to welding temperature as rotation speed varies, and thus, the evolution trends of nugget structure morphology (grain size and substructure distribution density) with rotation speed resemble that of welding temperature. Increasing rotation speed above 4000 rpm effectively improves the nugget hardness due to the enhancement of strain hardening.
In regard to the non-tool-tilt friction stir welding (NTT-FSW) process, the shoulder concavity has significant effects on the control of heat generation and material flow, and thus is an important geometrical feature for the tool design. In this paper, three types of shoulder concavity angles (SCAs), i.e., 0°, 5°, and 10°, were selected to explore the impact of SCA on the NTT-FSW of 5052 aluminum alloy. The results indicate that the increase of SCA lowers the tool axial force, reduces the nugget width, and weakens the band structure in the nugget. A weakening of shoulder thermal effect occurs from 0°to 5°SCA, leading to an improvement of the structure-property of the NTT-FSW joint. Further increasing the SCA to 10°causes the generation of the secondary sliding frictional heat at the interface between the shoulder-driven material and the base material, and thus the shoulder thermal effect does not show a continuous weakening trend as expected but becomes stronger instead, resulting in the deterioration of the microstructure evolution and the degradation of the tensile strength of NTT-FSW joint.
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