# Wire Arc Additive Manufacturing of Stainless Steels: A Review
## 5 Defects  
Common defects and their formation reasons in WAAM metallic parts are listed in Table 8. Porosity [116], cracks, and lack of fusion [101] are the typical defects found in stainless steel parts produced by WAAM. Process parameters, such as deposition paths and heat input, cause defects during deposition. A complex deposition path is more likely to produce the spatter ejection or insufficient fusion, creating voids or gaps in the affected areas [33]. Compared to GTAW- and PAW-based WAAM, GMAW have more problems of excessive heating, spattering, or porosity, because the electric current acts directly on the feedstock [88]. Some porosities and lack of fusion appears on the cross section of GMAW duplex stainless steel samples, especially between the beads, as shown in Figure 11 [101]. [PERSON] et al. [116] investigated porosity data for three different locations in the WAAM stainless steel 304 build. Thermal history causes different porosity for top-right, top-left, and bottom-right locations. The data for the three locations show obvious multimodality (see Figure 4 in [116]) because of gas generation or the insufficient energy for complete melting the layers during the process.  
Figure 11: Typical cross-section of WAAM type-2205 duplex stainless steel samples. The brighter area contains more austenitic and the darker more ferritic. A pattern may be seen in the deposited sample, where lines 1 and 2 represent the brightest and darkest etched regions, respectively [101].  
\begin{table}
\begin{tabular}{c c c c c} \hline
**WAAM** & **Material** & **Defects** & **Reasons** & **Reported by** \\ \hline WAAM & \begin{tabular}{c} 304 stainless \\ steel \\ \end{tabular} & Porosity & \begin{tabular}{c} Insufficient energy for complete melting the \\ layers; gas generation. \\ \end{tabular} & \begin{tabular}{c} [PERSON] et al. \\ [116] \\ \end{tabular} \\ \hline WAAM & \begin{tabular}{c} Metals (Steel, \\ Al, Ti, etc.) \\ \end{tabular} & Porosity & \begin{tabular}{c} Raw \\ material-induced \\ \end{tabular} & \begin{tabular}{c} Contaminants of wire and \\ substrate \\ \end{tabular} & [PERSON] et al. [33] \\ \cline{3-5}  & & & Process-induced & \begin{tabular}{c} Insufficient fusion or \\ spatter ejection \\ \end{tabular} \\ \cline{3-5}  & & Delamination & \begin{tabular}{c} Incomplete melting or insufficient re-melting \\ of the underlying solid between layers \\ \end{tabular} \\ \hline GTAW & Inconel 625 & \begin{tabular}{c} Solidification \\ cracking \\ \end{tabular} & \begin{tabular}{c} Existence of liquid film at terminal \\ solidification \\ \end{tabular} & [PERSON] et al. [117] \\ \hline GTAW & \begin{tabular}{c} Intermetallic \\ Al/Cu \\ \end{tabular} & \
\begin{tabular}{c} Grain \\ boundary crack \\ \end{tabular} &
\begin{tabular}{c} Intermetallic phase-equilibrium is freely \\ broken \\ \end{tabular} & [PERSON] et al. [118] \\ \hline \end{tabular}
\end{table}
Table 8: Defects in WAAM components and the formation reasonsThe raw material factor, such as contamination of the substrate and filler metal, is another cause of defects. Dirt, moisture, and grease on the surface of substrate and filler metal can be easily absorbed into the molten pool, thereby producing porosity after solidification [33].  
In summary, defects--such as lack of fusion, cracks, and porosity--need to be controlled to a minimum level to achieve sound mechanical properties. Precise control of heat input and thermal history, proper shielding gas and tight gas seals, high quality feedstock, and clean substrate surfaces are helpful to reduce defects during stainless steel WAAM process.