## 3 Results and Discussion  
### Pore Distribution  
The metallographic observations of pore distribution in the central regions for these six samples are shown in Figure 6. Figure 7 illustrates the pore characteristics, such as diameter, number and area percentage. This pore counting was performed on a section with an area of 15 mm \(\times\) 15 mm. The pore area percentage was calculated by dividing the total inspection area by the pore area in the inspection section. Limited by the resolution of the optical microscope, only pores larger than 10 \(\upmu\)m in diameter were counted. Figure 5: Micro hardness measurements of **(a)** thin wall samples and **(b)** block samples. Figure 6: Optically observed porosity for the WAAAM 2319 aluminum alloys: P0–P2 were produced by CMT-P; A0–A2 were produced by CMT-ADV. There are various sized pores distributed in WAAM 2319 alloy, as presented in Figures 6 and 7. As shown in Figure 7a, the total number and area percentage of pores in the block specimens (P1, P2, A1 and A2) distinctly decreases, compared with the thin-wall ones (P0 and A0). Moreover, no pores larger than 50 \(\upmu\)m in diameter exist in the block samples. By contrast, there is a certain number of pores larger than 50 \(\upmu\)m in the thin wall samples. In addition, the CMT-ADV process (A0, A1 and A2) can further reduce the number of pores and area percentage compared with the CMT-P process (P0, P1 and P2). For all the samples, most pores are smaller than 50 \(\upmu\)m in diameter, as shown in Figure 7b. There is a trend that the number of pores decreases with increased pore size. Furthermore, no pore larger than 110 \(\upmu\)m existed in any of the samples. Hydrogen has been widely recognized as the dominant cause of porosity in aluminum alloy welds and the WAAM process. The hydrogen pores are mainly formed through heterogeneous nucleation, and grow in the form of self-diffusion or combination in the solidification of the aluminum alloy. The nucleation rate of hydrogen pores is closely related to the grain size with certain hydrogen content [14; 17; 18; 19]. Meanwhile, the temperature gradient, cooling rate and heat input are the key factors affecting the grain size. With the increase of temperature gradient and cooling rate, and the decrease of heat input, the grain can be refined and pores can be reduced effectively. Compared with thin wall deposits (P0 and A0) where the heat from the weld pool is conducted though the underlying layers in a 2D planer manner, the temperature gradient and cooling rate of the molten pool of the block samples (P1, P2 and A1, A2) are higher as the heat from the weld pool is conducted radially though the surrounded material in a 3D manner. Therefore, the grain size is refined, and the nucleation rate of hydrogen pores is reduced, which results in the decrease of the gas pores. Compared with the CMT-P process, the heat input of the CMT-ADV process significantly decreases in the block building process, as shown in Table 2. The microstructure of the depositions with the CMT-ADV process is also refined, as described in Section 3.2. Furthermore, the oxidation film on the end of the filling wire, which can be the nucleating particle of hydrogen pores, is cleaned effectively due to the alternating current arc in the CMT-ADV process. Therefore, the number of pores in the CMT-ADV block samples are comparatively less. However, it can be found in Figures 6 and 7 that with 0.5 m/min travel speed (A2), the number of pores is less than that in the sample built with 0.6 m/min travel speed (A1), the heat input of which is lower. The reason for this phenomenon is that the weld beads generated by the CMT-ADV process are rough, and the oxidation film in the bottom of the depositing layer surface cannot be eliminated radically and thus turns into the nucleating particles of hydrogen pores.