*3.3. Mechanical Properties*

3.3.1. Macro-Hardness Distribution

Figure 9 shows the hardness profile measured at the midsection of the transverse cross sections of the FSWed joints AA5083/AA5754 at 400 rpm with the different welding speeds in Figure 9a and at 600 rpm with the different welding speeds in Figure 9b. It can be observed that the hardness is reduced in the weld zone with more reduction by increasing the rotation rate from 400 rpm to 600 rpm at each welding speed, and this is mainly due to the increase in the heat input. Moreover, the reduction in the NG hardness is also affected by the increase in the welding speed at the constant rotation rate. It can be noted that the NG hardness is reduced more by decreasing the welding speed from 60 mm/min up to 20 mm/min at the constant rotation rate. In terms of the width of the heat affected zone at each rotation rate, it is reduced by increasing the welding speed. Generally, this hardness reduction in the weld zone is mainly because of the thermal cycle on the strain hardened alloys. The thermal cycle leads to softening of the strain hardened material through the recovery and recrystallization processes that take place during FSW of the aluminum alloys [40].

Figure 10 shows the hardness profile measured at the midsection of the transverse cross sections of the FSWed joints AA7020/AA5083 at 500 rpm with AA5083 at the AS and different welding speeds in Figure 10a and with the AA7020 at the AS and different welding speeds in Figure 10b. It can be observed that both profiles show almost no reduction in the base materials hardness in all the heat affected zone of the two alloys; however, a slight increase in the hardness of the AA7020 alloy can be observed towards the NG center regardless of its position at the AS or RS. This can be attributed to the solid solution strengthening that can occur due to the stirring of the dissimilar alloys [41]. On the other hand, there is a slight decrease in the hardness of the AA5083 towards the NG center regardless of its position at the RS or the AS. This is mainly because of thermal cycle on the softening of the FSWed alloy.
