Analisi microstrutturale di giunzioni friction stir welding in alluminio AA6060-T6: Correlazione tra parametri di processo e dimensione dei grani

The present paper reports the results of an experimental campaign aimed at the study of the microstructure of Friction Stir Welded butt joints. In this solid-state welding technology, a rotating tool moves into the material and translates along the interface of two or more parts. During this process friction heats the material which is extruded around the tool and forged by the large pressure produced by the tool shoulder. The experimental campaign was performed by means of a CNC machine tool. Butt joints were obtained on sheets having a thickness equal to 8 mm. An AA6060 aluminum alloy in the T6 artificially aged condition was used for this purpose. Sheets were welded by using a standard tool with smooth plane shoulder and cylindrical pin. The tool was fabricated using AISI 1040 steel, shoulder and pin diameters are respectively equal to 20 and 8 mm. The tilt angle, was fixed at 2.5° and the pin was inserted into the workpiece for 7.8 mm. The joints were executed by varying the process parameters, namely rotational speed (S [rev/min]) and feed rate (f [mm/min]). The selection of the parameter values was based on a DOE (Design of Experiments) approach. The metallurgical analysis was carried out to evaluate the microstructure of the joints transverse section as a function of the process parameters and to evaluate the grain size in the different regions of the joints: Nugget, HAZ - Heat Affected Zone, TMAZ - Thermo Mechanical Affected Zone. The grain count was executed according to the standard ASTM E 112-96(2004). The ratio between feed rate and rotational speed resulted to be a significant parameter for the control of the joints grain size. The Nugget showed a microstructure with very small grains with an average dimension equal to one third with respect to the grains of the base material. The grain size linearly decreases for increasing values of the f/S ratio. The TMAZ is characterized by strongly elongated grains, oriented in a different way with respect to the ones of the other regions. Also in this case, an almost linear correlation between grain size and f/S ratio was observed. On the opposite, in the HAZ, the grain size decreases for increasing values of the f/S ratio. A tensile test campaign was also carried out to evaluate the mechanical properties of the joints. Specimens having a width and a thickness equal respectively to 18 mm and 8 mm were tested orthogonally with respect to the welding direction by means of a Galdabini machine. Except for the joints containing defects, the rupture always occurred in the HAZ. The increase of UTS was observed for increasing values of the ratio between feed rate and rotational speed (f/S). An inverse behaviour was observed for the strain at rupture. Finally, the quality of the joins, in terms of tensile strength, was compared with the metallurgical properties observed on the joints. The tensile strength resulted to increase for decreasing values of the grain size