MATERIALS AND EXPERIMENT The materials used were stainless steel plates (SUS304) having a diameter of 40 mm and a thickness 5 mm as the target and pure aluminum rods (A1050) having an outer diameter of 11 mm, inner diameter of 6 mm and length 20 mm as the projectile. The aluminum was annealed at 623 K for 3.6 ks. The impact face of target was polished using polishing paper having a mesh of 800 after grinding. An aluminum projectile collided with the stainless steel target at an impact velocity of 200 m/s or more using compressed nitrogen gas [1]. The impact welding was carried out in a vacuum chamber because the air compressed between the target and impact face of the projectile prevented welding. The impact velocity of the projectile was evaluated by a laser beam system. The bonding area was observed and measured using a scanning acoustic tomograph. The microstructure and element distribution in joining interface of the bonded specimen sliced to a thickness of about 3 mm were analyzed by means of SEM and energy dispersive X-ray spectroscopy (EDX). A tension test for measuring the bonding strength of the sliced specimen described above was carried out [1]. BONDED AREA AND DEFORMATION OF PROJECTILE Two ultrasonic images of the bonded area are shown in Fig. 1 (a) and (b). The impact velocities are 274 m/s and 339 m/s, respectively. The central ring areas in Fig. 1 (a) and (b) are called the inner ring area. Since appearance of the outer ring area shown in Fig.1 (a) depends on the impact velocity as described later, the inner ring area is defined as the bonded area hereafter. The superimposition of a bonded area and a cross-sectional profile of the deformed projectile are given in Fig. 2. Figure 2 shows that the bonded area was not formed at the center of the impact face of the tube wall, but more to the inside. It was clarified that the outer ring area was formed by the folding of the outer wall of the tube because the inner diameters of the outer ring area were larger than the initial outer diameters of the tube. It is observed that the metal of the inner wall flowed in and piled up in the tube. An opening like a crack is observed along the deformed inner wall of the tube. (a) v = 274 m/s (b) v = 339 m/s Fig. 2 Deformation profile of projectile Fig. 1 Acoustic images of bonded area and bonded area ( v = 254 m/s)
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