*2.1. Measurement of Weld Pool Surface Deformation*

A5052P aluminum alloy is adopted as the work piece and the chemical composition is in Table 1. The size of aluminum plate is 150 mm × 100 mm × 5 mm.

**Table 1.** Chemical composition of A5052P aluminum alloy (wt. %).


The setup for observing weld pool free surface is set as shown in Figure 1. The plasma arc torch includes tungsten electrode, plasma gas nozzle and shielding gas nozzle. The parameters are listed in Table 2. The plasma gas and shielding gas both are argon gas.

**Figure 1.** Measurement system of weld pool free surface in digging process of VPPA welding.


**Table 2.** Parameters of VPPA welding in digging process.

In order to clearly observe the weld pool free surface, a telephoto micro lens (AF Micro Nikkor 200 mm, Nikon, Tokyo, Japan) with a 640 nm band-pass filter is used to filter out the arc light, a high-frequency pulsed diode laser light source system (Cavilux HF system, Cavitar, Tampere, Finland) to illuminate the weld pool, a high speed video camera (HSVC) (Memrecam-Q1V, Nac Image Technology, Tokyo, Japan) to record the images. The frame rate of HSVC is 2000 fps. The HSVC sends out an electrical pulse signal to the laser light generator at the start moment of shutter opening, ensuring each picture is clear and reducing the effect of laser heat on the weld pool.

Figure 2 shows the rectangular wave current of VPPA. It is in the EN phase when the current is positive. The duration of the EN phase is longer. The tungsten electrode is connected to the negative pole of power supply while the workpiece is connected to the positive pole. Correspondingly, the current of the EP phase is negative. The duration is shorter. The current flows through the following path: tungsten electrode-workpiece-power supply-tungsten electrode.

**Figure 2.** Current waveform of VPPA.
