4.2 Experimental results and discussion
To determine the influence of the gap width on melt pool movement, discharge processes under different gap widths were observed. Figs. 20 and 21 show images of the melt pool using the experimental setup shown in Figure 19 with an extremely small gap width (10 µm) and a large gap width (> 200 µm), respectively. To observe the discharge process with an extremely large gap width (> 200 µm), the tool electrode contacted the workpiece and quickly moved upward. A discharge process with an extremely large gap width can be observed as the tool electrode moves upward.
Figure 20 indicates that when the gap width was 10 µm, the melt pool was bowl-shaped throughout the discharge process. The molten materials in the melt pool were pushed out, forming a depression in the center and a bulge around the edge. However, the image of the melt pool with an extremely large gap width (> 200 µm) indicates that the melt pool remained flat during most of the discharge, as shown in Figure 21. With a larger gap width, an explosion of the melt pool is more likely. The explosive process of the melt pool is shown in Figure 22. In the first frame, the melt pool remained flat. In the second frame, the melt pool started to bulge. In the third frame, the melt pool expanded to its maximum and began to explode, as shown in frames 4-8. Eventually, the melt pool became flat.
The above phenomena can be explained as follows. Based on the fluid mechanic theory, the metal vapor jets emitted from the tool electrode and workpiece collided and interacted in the gap, which can not only generate the shear force acting on the melt pool but also a discharge reaction force acting on the melt pool, which have also been verified by the experiment. Then, it can be known that when the gap width was smaller, the interaction of metal vapor jets was stronger, generating higher external pressure acting on melt pool. Thus, the melt pool was pressed into a bowl under the action of higher external pressure, as shown in Figure 20. But, when the gap width was larger, the interaction of metal vapor jets was weaker, generating low external pressure acting on the melt pool. As a result, the melt pool kept flat. The influence mechanism of the gap width on the melt pool can be explained through Figure 23. Furthermore, in the previous study, it has been demonstrated that a pressure was generated inside the melt pool during the discharge process, which made the melt pool bulge outward and served as another material removal motivity in EDM. Thereby, when the gap width was much larger, under low external pressure acting on the melt pool, the melt pool could explode easily due to the action of internal pressure inside the melt pool, as shown in Figure 22.
The above analysis demonstrates that the gap width influences the melt pool movement during the discharge process through the action of external and internal pressures. Discharge craters with swelling in the center and a depression around the edge can be explained as follows. During the discharge process with a larger gap width, the external pressure caused by the interaction of the metal vapor jets was weaker; under the action of internal pressure, the melt pool tended to expand outward. When the discharge stopped, the external pressure caused by the interaction of the metal vapor jets disappeared immediately. As a result, the melt pool expanded outward and formed a discharge crater with swelling in the center and a depression around the edge. With a smaller gap width, the melt pool was pressed down and the molten materials were extruded under the action of greater external pressure, generating a discharge crater with a greater depth and material raised higher above the workpiece surface.
Fig. 24 shows the swelling formation process using the experimental setup shown in Fig. 5(b) with the discharge conditions shown in Table 2. Fig. 24(a) shows the progress of the discharge. Under the action of the discharge plasma, the melt pool did not swell. Figure 24(b) shows the discharge termination times. It is observed that after the discharge ended, the melt pool without the action of the discharge plasma was swollen and higher than the discharge in progress. After discharge, the melt pool swelled, as shown in Fig. 24(c). At the end of discharge process, the melt pool gradually cooled and formed a melting and recrystallization layer. During the expansion process of the melt pool, the volume of the melt pool increased, resulting a decrease in density. In the discharge process, tool electrode can also emit materials on the workpiece surface. Thus, the material removal volume was negative in discharges with a larger gap width, as shown in Figure 4(b).