*3.3. Hardness Distribution*

The hardness variation along the skin and the stringer of the friction stir welded T-butt joint may give a better prediction about the phenomena involved in this welding process and the causes of loss or increase in strength compared with the base material. The hardness distribution maps of the T-butt joints for all the applied rotational speeds indicate good material mixing at the nugget zone as shown in Figure 7, which is evidence of selecting a proper tool design with shoulder/pin diameter ratio (D/d). The D/d ratio in the current study was 3.20. This ratio agrees well within the value that reported by Saravanan et al. [28] who focused their work on studying the effect of the D/d ratio in the range of 1–4 on the microstructure and mechanical properties of the dissimilar butt-joint of AA2024-T6 and AA7075-T6 Al-alloys. They concluded that the butt joint friction stir welded using a D/d ratio of 3.00 exhibited higher mechanical properties when compared to the other welded joints [28].

**Figure 7.** Hardness maps of the T-butt joint AA2024-T4 and AA7075-T6 friction stir welded at different rotational speeds of (**a**) 400 rpm, (**b**) 600 rpm and (**c**) 800 rpm.

The hardness maps show higher hardness at the SZ of the mixed alloys compared to the skin base material AA2024-T4 at the rotational speeds of 400 and 600 rpm, as shown in Figure 7a,b. This increase in the hardness at the SZ can be attributed to two reasons: (i) high proportion of the AA7075-T6 (high hardness alloy) in the SZ and (ii) the finer grain size (dynamically recrystallized) at SZ compared to the base material. According to Hall-Petch relation [29], smaller grain size leads to harder material property. The increasing amount of AA7075-T6 material in the microstructure of the stir zone seems to be also a reason for the higher hardness measured within the nugget zone of the T-butt joint. The highest rotational speed of 800 rpm leads to higher heat generation and slower cooling rate and then causes the formation of relatively coarse grains. And also, leads to the formation coarser precipitates during re-precipitation in the cooling cycle, thus result in lower hardness in the SZ and TMAZ compared to other joints welded at 400 and 600 rpm. The lower hardness observed in the HAZ of both advancing side (AS) and retreating side (RS) of the joints are results of coarsening and dissolution of strengthening precipitates [1–3,13,30]. No obvious differences are seen in the mean values of hardness of HAZ at both the AS and RS of all the welded joints. For the T-joint welded at 800 rpm, a wider softened area evidenced with slightly lower hardness in the SZ as well as in the HAZ of both sides. The slight lower hardness in the SZ and the wider softened region in this joint can be attributed to the high heat input experienced which affects the precipitates coarsening and/or dissolution. The softening behavior at HAZ with a minimum hardness ranging from 98 to 105 Hv is observed. The observed best joint hardness at a rotational speed of 600 rpm which gives proper mixing with adequate generated heat and cooling rate at travel speed of 50 mm/min.

The hardness profile of friction stir welded of heat-treatable AA2024 and AA7075 aluminum alloys depends strongly on the grain size structure, and the precipitates in terms of type, amount, morphology and distribution [4,5]. The difference in grain shape and size of the two base materials is surely important. Furthermore, the amount of solute atoms (Zn, Mg and Cu) of the mixed two materials is also an essential reason of the high hardness in the nugget zone.
