## 4 Results and Discussion
### Traverse Stage and Model Validation
#### 4.2.2 Weld Appearance and Defects  
Figure 14a-e shows the surface appearance and flash formed for all joints from FSW experiments and modeling results using EVF-VOID. EVF-VOID is the volume fraction of the empty regions in a Eulerian domain. An EVF = 1 indicates that the element is empty, while EVF = zero indicates that the element is completely filled with the material instance. No surface defects, i.e., grooves and/or lack of fill, appear on the weld surface of all samples, except the sample welded at 650 rpm. Samples welded at 250 and 350 rpm have a good surface appearance and a uniform surface ripple, as can be seen in Figure 14a,b, respectively. The surface roughness increases with an increase in the tool rotation rate, and the ripples become deeper and discontinuous, as exhibited in Figure 14c,d. The black ellipse in Figure 14b-d refers to the rough surface at the high tool rotation rates. [PERSON] et al. [50] reported that a smooth weld appearance with fine ripples was produced at 800 rpm, while increasing the tool rotation to 1500 rpm resulted in a rough surface and non-uniform ripples [50]. In addition, the weld surface color of the sample welded at 550 rpm shows that the weld surface is overheated. This agrees with the temperature distribution for the same sample shown in Figure 10a. The sample surface temperature exceeds 1300 \({}^{\circ}\)C. This temperature is more than 90% of the workpiece material melting point. As the tool rotation increases (650 rpm), i.e., the welding temperature increases, the steel may begin to show a tendency toward decohesion at the weld surface. At this point, the tool shoulder is insufficient to constrain and form the plasticized weld metal. Some loss of material will occur, either in the form of weld flash or in excessive positive reinforcement of the weld surface. The result is a lack of material to flow completely behind the tool from the retreating side to the advancing side of the weld, as shown in Figure 14e.  
A little flash is formed at the lowest tool rotation, i.e., 250 rpm, in both experimental and computed results. The flash size increases with increasing the tool rotation rate. The increase in the tool rotation rate enhances the heat generated during the FSW process. As explained earlier, the increased heat input leads to more plasticized material. The softened material splits under the tool pressure out in the form of flash [52]. Furthermore, for all welding conditions, most of the flash is formed in the RS indicated by the white ellipse numbered 1 in Figure 14a-d. On the other hand, no or little flash is formed in the AS indicated by the white ellipse numbered 2 in Figure 14a-d. During the traverse stage, the material from the leading edge is continuously softened and streams to the trailing edge through the RS. If the material under the tool shoulder is dramatically plasticized, the material spews out from the weld cavity forming the flash in the RS [53]. The traverse stage of the numerical work is shorter than that of the experimental work in order to reduce the computational cost; there was no remarkable change when using a longer time for one sample.  
Figure 15a-e shows the computed weld side view at different tool rotation rates. No volumetric defects occur with increasing the tool rotation up to 550 rpm. However, a small void initiation can be observed at 550 rpm (Figure 15d). Such a defect did not appear in the experiment due to the fixed coefficient of friction and featureless tool geometry used in the model. By increasing the tool rotation to 650 rpm, a lack-of-fill defect is produced, as shown in Figure 15e. Figures 14 and 15 indicate that the weld surface appearance and generated defects of the computed results agreed well with the experimental ones.  
Figure 14: Visualization of the generated flash and weld surface appearance experimentally and numerically using EVF-VOID for samples welded at (**a**) 250 rpm, (**b**) 350 rpm, (**c**) 450 rpm, (**d**) 550 rpm, and (**e**) 650 rpm.