# Wire Arc Additive Manufacturing of Stainless Steels: A Review
## 2 Macroscopic Characteristics of Samples  
WAAM process parameters determine the heat input and thermal accumulation, thereby impact the macro morphology [92; 93]. [PERSON] et al. [52] indicated that the forming efficiency and precision could be improved by starting and ending the arc of each layer through a back-and-forth scanning strategy. In the study of GMAW forming characteristics, [PERSON] et al. [94; 95] found that higher wire feeding rate reduces the stability of the molten pool and larger scanning speed makes the arc less stable. Both can lead to an increase in surface roughness. However, a lower interlay temperature could decrease surface roughness. The research of [PERSON] et al. [92], [PERSON] et al. [93], [PERSON] et al. [52], and [PERSON] et al. [94; 95] showed that the macroscopic morphology of WAAM varies with different forming parameters. The process parameters and their influence on macroscopic morphology of WAAM stainless steel parts will be introduced in detail later. [PERSON] et al. [96] studied the influence of the bottom current mode, scanning speed, and interlayer cooling time on the macro morphologies of 316L stainless steel samples using MIG (metal inert gas)  
Figure 2: Arc power and scanning speed of WAAM (also called DED-GMA), DED-L, and PBF-L processes for 316 stainless steel [26]. welding additive manufacturing. The process parameters of different samples are listed in Table 4. As presented in Figure 3, there were no significant defects between the layers of the samples. A thin and tall sample, G3010 (gradual reduction of bottom current, scanning speed was 30 cm/min, cooling time was 10 s), had uniform layer height, as demonstrated in Figure 3a. Through the samples in Figure 3b, [PERSON] et al. [96] found that gradual reduction of current improves bottom formation. As shown in Figure 3a,c (samples G3010 and G3510), the increase in scanning speed leads to a decrease in the deposition rate, resulting in the unevenness on the outer surface. Compared with sample G3000 whose cooling time is 0 s, the morphology of sample G3005 was improved; however, both ends of the layers collapsed due to the extremely high temperature. However, formation of the ends got better if cooling time was increased to 10 or 15 s [96]. Compared to GMAW and GTAW, plasma arc welding based additive manufacturing technology has a more concentrated arc, relatively higher energy density (as listed in Table 5), and shaping precision. [PERSON] et al. [100] investigated the forming quality of SWF-PAM (single-wire feed and plasma additive manufacturing) and DWF-PAM (double-wire feed and plasma additive manufacturing) processes. When using the same deposition current 130 A to prepare Cr-Ni (H00 Cr21 Ni10) stainless steel samples, an increase in scanning speed decreases the layer height of the deposited walls but \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Scanning Speed/cm.min-1** & **Cooling Time/s** & **Bottom Current Mode** \\ \hline G3010 & 30 & 10 & Gradual reduction of bottom current \\ \hline T3010 & 30 & 10 & Transient reduction of bottom current \\ \hline G3510 & 35 & 10 & \\ G3000 & 30 & 0 & Gradual reduction of bottom current \\ G3005 & 30 & 5 & \\ G3015 & 30 & 15 & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Process parameters of different samples [96]. Figure 3: Morphologies of different parts in samples prepared using different parameters [96]: (**a**) profile of sample G3010; (**b**) the bottom parts of different deposited samples; (**c**) the upper-right parts of different deposited samples (the parts in (b) correspond to the bottom region of (a), and the parts in (c) correspond to the upper-right region of (a)). improves the surface-quality and reduces waviness. The macrographs of the deposited samples are presented in Figure 4. Although with the same deposition current and scanning speed, the surface waviness of SWF-PAM samples was slightly better than that of DWF-PAM samples. However, the detailed surface condition of SWF-PAM and DWF-PAM samples was not studied.