# Investigating the Welding Parameters in Friction Stir Welding of Yellow Brass 405-20
## 4 Microscopic Examination of Friction Stir Welding Zone (FSWZ)
The melting point of Zn is 419.5 \({}^{\circ}\)C and the boiling point of Zn is 907 \({}^{\circ}\)C. Critical and microscopic examinations including SEM are usually required after FSW of brass alloy to look closely at the heat-affected zone for any dezincification and uneven joint fracture patterns. Moreover, two other phases of brass alloys may also be encountered, which are known as beta and alpha-plus-beta phases, when the Zn percentage in copper exceeds 36%. Beta phases have body-centered cubic (b.c.c.) crystal structures [38]. Additionally, others phases of brass are also possible based on increasing the percentage weight of Zn in Cu, including beta dashed, gamma, epsilon, and hetta brasses [39]. Since the yellow brass under investigation has less than 36% Zn in copper, these other phases are not discussed further due to their absence in the joint. As described earlier, the minimum weld temperature found while FSW of brass was 430.39 \({}^{\circ}\)C, which is greater than the melting point of Zn, though it is lower than the boiling point of Zn. Dezincification is associated with the evaporation of Zn during FSW, which is likely to occur at a weld temperature greater than the boiling point of Zn. The maximum temperature found was only 598 \({}^{\circ}\)C during the numerical and empirical studies, implying that dezincification was not possible in the current research during FSW of brass even in a single DOE and especially in DOE4. However, the melting of Zn occurs in all the DOEs, suggesting that little dislocations of Zn or few micro-level pits/cracks initiators may be found during FSW of brass, as can be seen in Figure 15. Likewise, more continuous cracks may be found due to melting of Zn at the weld zone, as shown in Figure 16.  
Another problematic phenomenon occurred during the cold processing of brass is called season cracking/stress corrosion. This stress corrosion is also called intergranular corrosion and happens due to the residual stresses after the cold working of brass. Cold processed brass is usually annealed to remove these residual stresses, ultimately removing the likelihood of intergranular corrosion. This phenomenon is also found to occur at a faster rate in ammonia atmospheres [38]. Since FSW involves preparing the samples in cold conditions, the cracks in Figures 15 and 16 are thought to be the result of these residual stresses during FSW of brass in the absence of necessary annealing required for prepared samples. Moreover, variation in the joint strengths in the DOEs may also be caused by these residual stresses as a result of cold working during brass sample preparation. Moreover, micrographs from the scanning electron microscope (SEM) revealed the presence of negligible pores, the uneven granular structure of brass, FSW tool marks, and crack marks after tensile testing, as shown in Figure 19 for DOE4. Pores indicate slight evidence of evaporation of zinc whereas uneven granular structures indicate poor dynamic recrystallization. Together, all these revelations strongly contribute towards a slight reduction in the joint strength, which is why the overall strength was found to be lower than that of the parent brass material. These are also the cause of little errors between numerical and empirical results.