**3. Results and Discussion**

*3.1. Joint Appearance and Macro-Examinations*

Figure 4 shows the appearance of the T-butt joints friction stir welded at different rotational speeds of 400, 600 and 800 rpm. It can be observed that the top surfaces are defect free and also the exit holes are complete with no indications of any tunnel defects.

**Figure 4.** T-butt joint appearance of friction stir welded AA2024 and AA7075 at 50 mm/min welding travel speed and different rotational speeds (**a**) 400 rpm, (**b**) 600 rpm, and (**c**) 800 rpm.

The examination of the cross-section macrostructure of welding zone may be used to clarify the quality of friction stir welded joints. It has been reported that four different zones, namely stir zone (SZ) or nugget zone, thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and base material (BM) can be identified in the macrostructure of welds [4]. Several variables like types of BM, thickness of work piece, welding parameters, and tool design play a significant role in the formation of friction stir welded joint features. Actually, the amount of heat input has a great effect on material flow, SZ shape, microstructure grain size and FSW defects. Hence, the quality of joints can be enhanced if the rotation and welding speeds of the FSW process can be carefully controlled. Typical transverse cross-section macrographs of the friction stir welded T-butt-joints of AA2024-T4 and AA7075-T6 are shown in Figure 5. It can be observed that the transverse cross sections showed sound T-butt joints at all the applied FSW parameters. However, the mixing process between the two alloys is significantly enhanced by increasing the rotation speed of the FSW tool such that at the low rotation speed is entirely pertains the whole stringer material (AA7075) within the SZ zone, by increasing the rotation speed the stringer material starts to get dispersed and mixed with the skin material (AA2024). This implies that increasing the rotation speed has resulted in a complex flow pattern that enhanced the two materials mixing and joining.

It is worth mentioning here that the dimensions of the SZ are controlled by the dimensions of the tool shoulder and pin. SZ is usually diffused to be little bit larger than the pin dimensions and has a like-conical form [10]. The widest region is beneath the shoulder diameter, which is ~32 mm and become narrower with the depth to be determined by the pin diameters (12 to 10 mm) and length (*h* = 3.8 mm) as it has been defined in the Figure 5 by the dashed lines.

**Figure 5.** Macrographs of T-butt-joints of friction stir weldedAA2024 and AA7075 at different rotational speeds of (**a**) 400 rpm, (**b**) 600 rpm, and (**c**) 800 rpm.
