Fundamental study in FSW processing and FSW process-structure-property relationship for AA2024 and AA2524

Friction stir welding (FSW) is a relatively new joining technique, copious data have been generated on the properties of FSWs in many alloys, but the factors governing the microstructure and properties of the nugget zone in precipitation hardenable aluminum alloy FSWs are still not fully understood. A series of FSW experiments and analyses were performed on precipitate hardening aluminum alloys AA2024-T351 and AA2524-T351 with an attempt to develop the process-structure-property correlation and characterize the processing and banding in these process and resultant joints.; Results indicate that among the three control variables, rotation speed has the dominant effect on the nugget microstructure and property in AA2524 FSWs compared with welding speed and Fz force. For a given welding speed and z-axis force, there is an optimum rotation speed to produce a good combination of nugget ultimate tensile strength and elongation, higher rotation speed will cause overheating that results in grain boundary melting resulting in a big drop in elongation and ultimate tensile strength while having little inverse effect on the hardness and yield strength. The welding torque has a strong and good inverse proportional relationship with grain size, nugget hardness, yield strength and ultimate tensile strength. The effect of rotation speed, welding speed and z-axis force on the grain size, hardness and tensile properties can be rationalized based on their contribution to the welding torque.; Results from processing and banding analysis show that there are well-defined periodic variations in the processing forces Fx and Fz during welding and periodic, semi-circular bands on the horizontal transverse cross-section for all the welds over a wide range of welding conditions. The measured band spacing is closely equal to the oscillation period of the Fx and Fz force of the welding tool, both are in good agreement with the FSW processing pitch. Different pin form tool may result in the difference between these banded structures, but could not change the relationship between processing and banding. The periodic banded structure in the nugget consists of alternating variation of grain size, particle distribution and micro-hardness variation, and would result in periodic variations of the measured strain response under loading. The direct and consistent relationship observed among periodicity in variation of Fx and Fz and the metallurgical and mechanical behavior indicate that the formation of banded structures in FSW joints is related to the periodic variation of welding parameters. (Abstract shortened by UMI.)...