State Key Laboratory of Solidification Processing;
Shaanxi Key Laboratory of Friction Travel Technologies;
Northwestern Polytechnical University;
Xi'an 710072;
China;
Capital Aerospace Machinery Company;
Beijing 100076;
China;
Department of Mechanical Engineering;
Hellenic Mediterranean University;
Heraklion;
Crete 71004;
Greece;
friction stir welding; bobbin-tool; aluminum alloy; microstructure; mechanical properties; adjustable-gap;