**3. Discussion and results**

### **3.1 Microstructure**

**Figure 3** shows the microstructures of welded samples relating to different welding conditions, according to **Table 3**. According to **Figure 3**, different weld region zones, namely, stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ) are also observed for FSSV welded specimens, as well as FSS welded specimens. The microstructures of the stir zone for FSS and FSSV welded specimens are shown in **Figure 4**. It is obvious that the presence of vibration during welding reduces the grain size of the stir zone for both rotational speeds; additionally, grain sizes of samples welded samples with high plunge depth are lower than those welded by low plunge depth. These can be related to the effect of plastic deformation on dislocation production in metals.

High plunge depth or presence of vibration increases the plastic deformation. Studies have noted that dislocation density increases as plastic deformation increases. As dynamic recrystallization (DRX) is the main reason for grain refinement during FSW [21, 22], an increase of dislocation density leads to enhanced DRX and correspondingly, finer grains are developed.

**Figure 5** shows the stir zone grain size values for different welding conditions. It is observed that the stir zone grain size for all welded specimens is lower than the base metal grain size. Additionally, **Figure 5** shows that FSSV welded specimens have lower grain sizes for FSS welded specimens. Based on to Kaibyshev [23], the microstructure modification during severe plastic deformation includes two consecutive processes: (i) the formation of three-dimensional arrays of low angle boundaries (LABs) and (ii) the gradual transformation of LABs into high angle boundaries (HABs) (≥15°). LABs with low misorientation (~1°) are constantly formed in pure aluminum and its alloys by dynamic recovery during deformation by rearrangement of accumulating lattice dislocations (**Figure 6a**). At high strain values, mobile dislocations migrate across sub-grains and are trapped by sub-boundaries increasing their misorientation. Extensive rotation of sub-grains leads to increasing misorientation of LABs with strain within sub-grains. These processes result in the formation

#### **Figure 3.**

*Microstructures of TMAZ, HAZ, and SZ of different samples, (a, c) FSSVW and (b, d) FSSW (a and b relate to the welding situation 1 of Table 3, c and d relate to the welding situation 2 of Table 3).*

#### **Figure 4.**

*SZ microstructure of FSS (a) and FSSV (b) welded samples (welding situations 2 of Table 3).*

#### **Figure 5.**

*Stir zone grain size results for different samples welded by different welding conditions (welding factors values were based on Table 3; (−) and (+) signs indicate non-presence and presence of vibration, respectively).*

of individual segments of HABs, and this can be considered as proof for the occurrence of dynamic recrystallization (**Figure 6b**). The recrystallized grains persistently replace sub-grains evolved at small strains through the continuous transformation of their boundaries, and accordingly, grain size refinement occurs [24].

### **3.2 Mechanical characteristics**

Shear strength curves of different welded specimens are presented in **Figure 7**. According to **Figure 7**, samples welded using the FSSVW method have higher strength compared to samples welded using the FSSW method, and additionally, maximum shear load increases as plunge depth increases. It was observed (**Figure 3**) that the presence of vibration, decreases the grain size. As grain size decreases, the volume fraction of grain boundaries increases, and the movement of dislocations decreases. According to the Hall–Petch equation (*σ = σ0 + kD−1/2*) [25], strength (σ) increases as grain size (d) decreases. Additionally, as plunge depth increases, more volume fraction of material enters within the stir zone and more mixing of up and down workpieces is carried out in the weld area and this leads to more strength of the weld.

*Study on Microstructure Evolution and Mechanical Properties of Al5083 Joint Obtained… DOI: http://dx.doi.org/10.5772/intechopen.102082*

#### **Figure 6.**

*Schematic illustration of dynamic recrystallization: A dynamic recovery and formation of LABs and b grain size refinement due to gradual transformation of LABs into HABs [23].*

#### **Figure 7.** *Lap-shear strength curves of FSS and FSSV welded specimens.*

Fracture surfaces of FSS and FSSV welded specimens, after the shear test, are seen in **Figure 8**. According to **Figure 8**, fracture surfaces of all specimens show dimples. The presence of dimples is characteristic of ductile fracture surfaces [26]. It is known that during the straining of ductile materials, voids form within the microstructure, and as straining proceeds, voids coalescence and grow. These voids are responsible for the constitution of dimples [27, 28]. It is observed in **Figure 8** that dimples for FSSV welded specimens are smaller than those observed in FSS welded specimens and dimples for specimens welded under high plunge depth are smaller than those constituted in specimens welded under low plunge depth.

#### **Figure 8.**

*SEM fracture surface of a) FSV welded sample with PD: 2 mm, b) FSVS welded sample with PD: 2 mm, c) FSS welded sample with PD: 2.5 mm and d) FSV welded sample with PD: 2.5 mm.*

Generally, less ductile metals show dimples with larger sizes and fracture occurs at lower values of strain [26]. Correspondingly, more ductility is predicted for FSSV welded specimens compared to FSS welded specimens. Additionally, more ductility is anticipated for specimens welded with higher plunge depth compared to those welded with lower plunge depth. These predictions are in agreement with the results presented in **Figure 7**. It is obvious in **Figure 7** that displacement at maximum load for FSSV welded specimens are higher than that for FSS welded specimens and this variable increases as plunge depth increases.

**Figure 9** shows the hardness values of different weld zones of FSS and FSSV welded specimens. Although, the average grain size in the SZ is smaller than the BM, the microhardness values of the SZ are lower than the BM. It can be explained by the existence of two competing phenomena. First, the reduction in the average grain size induced by DRX results from severe plastic deformation which contributes to the increase in the microhardness. Second, the dissolution of the iron-rich phases and the precipitates resulting from intense mixing under severe plastic deformation and the high temperature contributes to the softening of the material. These two competing mechanisms have a strong influence on the final mechanical properties of the different zones and the entire weld. Based on **Figure 9**, hardness values of SZ and TMAZ regions for FSSV welded specimens are higher than those relating to FSS welded specimens. Additionally, **Figure 9** shows that the hardness value increases as plunge depth increases. These can be related to the effect of grain size refinement as vibration is applied and plunge depth increases. It was observed (**Figure 3**) that the presence of vibration and increase of plunge depth both result in more grain refinement. As grain size decreases, the impediment to dislocations movement enhances, and strength and hardness increase. Grain size refinement is known as a strengthening mechanism [29].

*Study on Microstructure Evolution and Mechanical Properties of Al5083 Joint Obtained… DOI: http://dx.doi.org/10.5772/intechopen.102082*

#### **Figure 9.**

*Micro-hardness values of various zones of FSSW and FSSVW samples: a) SZ, b) TMAZ, and c) HAZ (− and + sign indicate without and with vibration, respectively).*

#### **3.3 Effect of vibration frequency**

**Figure 10** shows the shear strength curves of various FSSV welded specimens and **Figure 11** shows the SZ hardness values of these specimens. For all of these specimens, the welding conditions are the same but the vibration frequency is different. According to **Figure 10**, the maximum shear strength increases as vibration frequency increases. It should be mentioned that DRX is the main mechanism for grain refinement during FSSW [30]. As vibration frequency increases, more strain is applied to the material within the stir zone. It has been known that dislocation density increases as straining increases [31–33]. Higher dislocation density leads to more DRX and correspondingly, finer grains are developed and higher strength and hardness are obtained.

The fracture surfaces of the base metal and FSSV welded specimens with various vibration frequencies are presented in **Figure 12**. Dimples, which are characteristics of ductile fracture surfaces, are seen in fracture surfaces of all specimens. **Figure 12** shows that dimples for base material are the largest and dimple size decreases as vibration frequency increases. This can be related to the effect of vibration frequency on grain size refinement. Barooni et al. [34] found that SZ grain size decreases as vibration frequency increases. Voids, which are responsible for the constitution of dimples in the fracture surface of ductile materials, form around the second phase particles and inclusions as well as dislocation locks. As deformation proceeds, the voids grow and coalescence of them form large voids. Grain boundaries act as barriers to the growth of voids. As grain size decreases, more nuclei for void formation are constituted and on the other hand, the voids cannot grow large and the voids are

#### **Figure 10.**

*Shear strength curves of FSSV welded samples with various vibration frequencies (welding factor values were based on Table 3).*

#### **Figure 11.**

*Stir zone micro-hardness values of FSSV welded samples with various vibration frequencies (welding factor values were based on Table 3).*

*Study on Microstructure Evolution and Mechanical Properties of Al5083 Joint Obtained… DOI: http://dx.doi.org/10.5772/intechopen.102082*

**Figure 12.** *SEM fracture surface of FSSV welded specimens with different vibration frequencies a) 55 Hz, b) 42 Hz, c) 25 Hz, and d) BM.*

smaller. In **Figure 12**, the smallest voids are seen for specimens welded with the highest vibration frequency.
