Microstructure and mechanical properties of friction stir welded AA6060-T6 tubes

Friction stir welding (FSW) has received increasing attention in recent years thanks to its advantages over traditional welding processes, reducing distortion and eliminating solidification defects. Since melting does not take place and joining occurs below the melting temperature of the material, this welding process allows to obtain a weld characterized by very high quality with low heat input, minimal distortion, no filler material, and no fumes. FSW is also highly efficient and it is characterized by improved environmental performance if compared to traditional welding methods. For instance, FSW is particularly advantageous in the pipeline industry because this innovative welding process usually entails reduction in energy usage of up to 80% if compared to conventional fusion welding processes. Moreover, also alloys normally difficult to be welded can be considered with this technique. The objective of the present study is to establish and to study the weldability of aluminum tubes by means of FSW process. The study shows preliminary results on circumferential FSW of AA6060-T6 aluminum tubes and the influence of the welding process on weld quality. The experimental campaign was performed on tubes having a thickness equal to 5 mm and an external diameter equal to 80 mm. Tubes were welded by means of a four axes CNC machine tool. Particular care was paid to the fabrication of the inner support for the tube. The mandrel was designed in order to guarantee limited bending during the welding process. Some preliminary tests were carried out by varying the welding parameters, namely feed rate (f) and rotational speed (S). A tool having conical shoulder and cylindrical pin was used. The weld quality investigation was based on tensile tests, microhardness and macrostructure analysis of the joints.