## 3 Results and Discussion
### Microstructure  
Figure 9 shows the microstructures of thin wall samples. It can be seen that the microstructure grows from columnar zone (CLZ) to equiaxed non-dentrite zone (NEQZ) from the bottom to the middle, and NEQZ to equiaxed dentrite zone (EQZ) from the middle to the top region of the CMT-P specimen. While, the sample with the CMT-ADV process is characterized by CLZ, cellular zone (CEZ), NEQZ and EQZ from the bottom to the top area.  
Figure 8: Longitudinal sections of WAAM components with the CMT-ADV process. (**a**) TS = 0.6 m/min; (**b**) TS = 0.5 m/min.  
Figure 9: The microstructures of thin wall samples. (**a**) Sample P0; (**b**) sample A0.  
The heat mainly dissipates along the deposited parts towards the substrate in first few layers of the depositing process. As the direction of the grain growth is mainly along the largest temperature gradients, the microstructures in the bottom to the middle are mainly slender columnar grains perpendicular to the substrate. With the increase of the deposition height, the dissipation of the heat slows down as a result of the increase of the distance between the heat source and the substrate. Moreover, due to the preheating effect of the former layers on the latter ones, the heat is accumulated gradually until a thermal balance is reached. As a result, the grains grow at almost the same speed along each direction, eventually forming the equiaxed dentrite grains. These equiaxed dentrite grains will be re-melted by the deposition of the next layer, thus forming equiaxed non-dentrite grains. So, the equiaxed dentrite grains only exist in the region of the top one or two layers. Compared to the CMT-P process, the lower heat input and the AC arc mode in CMT-ADV process result in the tiny cellular structure formation during the transition from CLZ to EQZ in WAAM Al-6.3 Cu alloy.  
Figures 10 and 11 give the microstructures of block samples built with the CMT-P process and the CMT-ADV process, respectively. It is observed that the microstructure differs even in the same pass. As shown in Figure 10, the microstructure grows from CLZ to NEQZ from the surrounding (location B) to the center (location A) region of every pass in the samples created with the CMT-P process. The samples manufactured by the CMT-ADV process consist of NEQZ and coarse EQZ in sequence along the center (location A) to the surrounding (location B) region, as shown in Figure 11.  
This phenomenon can be explained by the following. For block deposits formed with the CMT-P process, the temperature gradient of liquid metal at the bottom of the molten pool is larger than that at the top, because the heat can distribute to the former pass and layer during the cooling and solidification process. The grains at the bottom grew perpendicularly to the substrate or the former pass/layer, forming CLZ, while NEQZ formed in the middle and top areas. However, the top area is re-heated or re-melt by the newly deposited layer, and the grains in this area grow to coarse columnar crystals, leaving NEQZ in the center region. Compared with the CMT-P process, the lower heat input of the CMT-ADV process leads to a larger temperature gradient, resulting in the formation of EQZ around NEQZ. For the same arc mode, the lower heat input (P1 and A1) can refine the grain size.  
Figure 10: The microstructure of block samples formed with the CMT-P process.