*2.2. Measurement of the Keyhole Boundary during Digging Process of VPPA Welding*

Figure 3 is the X-ray transmission system used to observe the keyhole boundary in digging process [26]. It consists of one set of X-ray power sources, HSVC and image intensifiers, welding power source and control personal computer (PC). The X-ray power source maximum outputs a tube current of 1.0 mA and a tube voltage of 230.0 kV. In this measurement, X-ray power source is used at 700 μA tube current and180 kV tube voltage. The aluminum plate and parameters are same with that of Section 2.1. As a result, it is difficult for the X-ray to penetrate the entire base metal horizontally. In order to get the relative clear image, the axis line between X-ray source and HSVC is set at 30◦ with the flat direction to make it easy for X-ray transaction. The X-ray transmission images are recorded by the HSVC at the frame rate of 2000 fps. The camera has an image resolution setting of 800 × 600 pixels to capture a real dimension of 22 mm × 20 mm, as shown in Figure 4. It should be pointed out that due to the angle setting, there is a blind vision region in the observed image. The blind vision region is ignored because of its small size is related to the whole thickness of base metal.

**Figure 3.** X-ray observation system of the keyhole boundary in digging process of VPPA welding.
