*3.1. The Evolution of Keyhole and Weld Pool in Digging Process of VPPA Welding*

In the digging process of VPPA welding, the variation of free surface of weld pool in one current period is analyzed, as shown in Figure 7. The time interval for each picture is 1 ms. The TSM means the time to starting moment. The start moment is the fifth second after arc ignition. One period is separated into four stages: EN phase, EN to EP phase (the polarity switches from EN to EP), EP phase and EP to EN phase (the polarity switches from EP to EN). The disturbance of the weld pool free surface can be obtained from the above four stages. As Figure 7a shows, the weld pool surface in center region concaves while the weld pool edge protrudes up in the EN phase. It does not change when the polarity switches from EN to EP as shown in Figure 7b. After the polarity changes to the EP phase, the weld pool surface in the center rises to the upside. The edge region descends as shown from Figure 7c to Figure 7e. The surface continue to rise up in the EP to EN phase and the start segment of the EN phase is shown from Figure 7f to Figure 7h. Subsequently, the free surface begins to deform rapidly from the center extending outward. The state remains basically stable after TSM-9 ms. Then it moves on to the next cycle and repeats the above phenomenon. The free surface moves up within the red line while it moves down outside the red line.

**Figure 7.** The evolution of weld pool free surface in one current cycle. (**a**) Starting moment (EN); (**b**) TSM-1 ms (EN to EP); (**c**) TSM-2 ms (EP); (**d**) TSM-3 ms (EP); (**e**) TSM-4 ms (EP); (**f**) TSM-5 ms (EP to EN); (**g**) TSM-61 ms (EN); (**h**) TSM-7 ms (EN); (**i**) TSM-8 ms (EN); (**j**) TSM-9 ms (EN); (**k**) TSM-10 ms (EN); (**l**) TSM-11 ms (EN).

Figure 8 shows the keyhole dynamics measured by the X-ray image system, from which the keyhole boundary inside the weld pool can be obtained. Results in one current cycle (about 8 s from arc ignition) are selected to analyze the keyhole status in different polarity. The schematic illustration is drawn based on the X-ray image. The keyhole size including depth and width obviously reduces from the start of the EP phase to the end. The keyhole size gradually increases from the start of the EN phase as shown from 0 to 3 ms. It remains basically stable when the keyhole increases to a certain size as shown from 4 ms to 7 ms. Through the above observation of the weld pool free surface and keyhole boundary, it is found that there are periodic fluctuations in the state of weld pool during the digging process in VPPA welding for aluminum alloy, which contrasts with the previous study [27,28].

**Figure 8.** Dynamic characteristics of keyhole during the digging process shown by X-ray imaging technology.

The keyhole boundary is obtained by image edge extracting technology based on the X-ray results, as shown in Figure 9. The variation of keyhole depth with time can be quantitatively analyzed by the keyhole boundary results.

The keyhole depth with time in the whole digging process is shown in Figure 10. It includes the results of the EP and EN phases, measured depending on the above keyhole boundary. The keyhole digging process can be divided into three distinct regimes of behavior, which are the stages of RPF, VF and BP. The RPF means regular periodic fluctuation, VF means violent fluctuation and BP is blasting penetration, respectively. In the RPF stage, the keyhole depth in the EN phase gradually increases. The keyhole of the EP phase in the RPF stage is in the blind vision region. Combing with the observation results of weld pool free surface, it is considered that the keyhole depth of the EP phase in the RPF stage is close to zero. In the VF stage, the plasma arc in EN phase continually digs the keyhole and the depth increases by about 2.5 mm. Then it gradually increases with a little fluctuation. In this stage, the keyhole of the EP phase appears in the view region and the depth also increases with fluctuation. At the end of the VF stage, the depth of the EN phase is 4.15 mm and that of the EP phase is 2.78 mm. After that it goes into the BP stage and the keyhole is quickly fully established.

**Figure 9.** The keyhole boundary indicated by image edge extraction.

**Figure 10.** The keyhole depth of a different polarity in the whole digging process.

The diagram of keyhole and weld pool during digging process is shown in Figure 11. In this figure, EP-S means the start moment of the EP phase. The EP-E means the end of the EP phase. EN-S and EN-E have equivalent meanings for the EN phase. In the stage of RPF, the fusion depth is smaller than the unmelt height, as shown in Figure 11b. The heat of the weld pool is easily transferred out because the unmelt region is large, resulting in a relatively slow melting speed and small size weld pool. Moreover, the plasma arc keeps a strong action on the weld pool if the intensity does not reduce much when the keyhole depth is small. Therefore, in this stage, the response of the keyhole weld pool to the arc status is rapid. Then the plasma arc intensity attenuates seriously with the keyhole depth increase, causing the keyhole weld pool fluctuation, as shown in the VF stage.

**Figure 11.** Diagram of keyhole and weld pool in the three stages during digging process. (**a**) start of EP in RPF stage; (**b**) end of EP in RPF stage; (**c**) start of EN in RPF stage; (**d**) end of EN in RPF stage; (**e**) start of EP in VF stage; (**f**) end of EP in VF stage; (**g**) start of EN in VF stage; (**h**) end of EN in VF stage; (**i**) start of EP in BP stage; (**j**) end of EP in BP stage; (**k**) start of EN in BP stage; (**l**) end of EN in BP stage.

In addition, the unmelt height becomes small with the increase of the melting depth, leading to heat loss at the weld pool bottom becoming more difficult. Heat gradually accumulates in the unmelt region. The melting speed gets faster. The keyhole digging speed also increases. Predictably, with the keyhole depth increase, the thermal accumulation also increases. The keyhole digging process and thermal accumulation are mutually reinforcing, resulting in the blasting type penetration in the last stage. Through the above analysis, the keyhole instability during digging process mainly occurs in the first two stages. Adjusting the output of plasma arc pressure to stabilize the keyhole state of EN and EP phases is crucial to realizing stable penetration.
