# Investigating the Welding Parameters in Friction Stir Welding of Yellow Brass 405-20
## 4 Microscopic Examination of Friction Stir Welding Zone (FSWZ)  
Welded and fractured samples were analyzed at the friction stir welding zone (FSWZ) after the samples were welded and tested under shear loading. The friction stir welding zone is that area under investigation where all the vital thermal and mechanical gradients in terms of maximum temperature and the highest strengths are highly likely. Therefore, microscopic examination of FSWZ was thought to reveal crucial phenomena while friction stir welding of brass. Microscopic examination of this research was performed using both the coordinate measuring machine (CMM) and a scanning electron microscope (SEM). In CMM, there were available three magnification levels employed and placed as 70\(\times\), 30\(\times\), and 10\(\times\) in each microscopic image; 70\(\times\) means the features' dimensions have been magnified up to 70 times and so on using 30\(\times\) and 10\(\times\). Exciting and fascinating features were revealed in this microscopic investigation including their effects on the little errors found between numerical and empirical studies. Cracks in the weld zone are shown microscopically in Figures 15 and 16, causing few variation points of the welding strength to occur, as shown in Figure 13, when these welded samples passed through the tensile tester. Effects of frictional heating and FSW factors on microscopic features may be seen in Figures 17 and 18, including pin exit location, ripples due to pin forward movement, marks of uniform stirring of brass material, and the borderline between parent and welded brass. This validates further the causes of a few errors in results between both thermal and mechanical studies, i.e., numerical and empirical, which will be discussed in detail. In addition, the regular circles (see Figure 16) had the same diameter as that of the tool pin, as shown in Figure 8.  
As far as yellow brass 405-20 is concerned, this is an alpha brass with 20-36% zinc by weight in copper. Few alloying elements (e.g., tin and lead, as shown in Table 1) may be added in the copper-zinc (Cu-Zn) brass alloy, resulting in acquiring the desired brass characteristics, such as machinability, color, corrosion resistance, etc. Major brass behavior in terms of strength and ductility largely depends on the addition of Zn in Cu. For example,when Zn percentage is added in Cu up to 50%, tensile strength increases with a substantial reduction in ductility due to an abrupt rise in the hardness value [38]. This reduction in ductility and increment in brittleness is clearly evident from the findings of Figure 14 for weld hardness. Yellow brass 405-20 has 63% of Cu and 34.7% of Zn, which is why it has a yellow color. The yellow brass under investigation is considered to be the better percentage combination of Cu and Zn, imparting the best strength and ductility. There is another advantage of the valuable alloying addition of Zn in the copper, which basically lowers the cost of the resulting brass material. Of course, the color of brass strongly depends on the percentage blend of both Cu and Zn. For instance, if the percentage of Zn is reduced to a range between 5 and 20%; the color of brass is changed to red with substantial changes in its various properties and its processing requirements [38]. Alpha brasses have a face-centered cubic (f.c.c.) crystal structure since copper is f.c.c. As yellow alpha brass has already better strength with higher ductility, it is suited to various cold working processes. This is due to the reason that yellow alpha brasses are subjected to pitting corrosion, which is known as dezincification [38]. Friction stir welding is a cold working process due to the presence of frictional heating during the welding of materials. FSW welds the brass and other materials while stirring and heating materials at a temperature below their melting points. Zn is found to be evaporated when brasses are welded by conventional joining techniques, which melt the materials to be welded.