# Review on Quality Control Methods in Metal Additive Manufacturing
## 3 Numerical Quality Monitoring Methods in Metal AM and Welding
### Discussion on Numerical Quality Monitoring Studies in Metal AM and Welding  
Table 1 summarizes the numerical quality monitoring methods for metal AM and welding. Based on the literature review, the numerical quality monitoring methods in metal AM focused on analyzing the formation of the cladding track and identifying the causes of residual stress. Multiple cladding tracks form a single 2D layer, and multiple 2D layers are stacked to produce the final 3D-shaped product. Thus, the dimensional errors of a single-cladding track may significantly affect the dimensions of the final product [46].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Process** & **Input Process Parameters** & & **Outcome** & **References** \\ \hline \multirow{4}{*}{Metal AM} & - & Laser power & - & Identified the cross-section, width, and height of a \\  & - & Nozzle speed & & single-cladding track \\  & - & Energy density & - & Identified the width and height formation of multiple \\  & - & Cladding temperature & & stacked cladding tracks \\ \cline{2-5}  & - & Thermal flow & - & Increasing the track layers can cause residual stress \\  & - & Laser power & - & Identified the residual stress generation under \\  & - & Number of track layers & & multilayer stacking situations \\ \hline \multirow{4}{*}{Welding} & & & - & Identified the specific section of welded area \\  & & & & experiencing maximum residual stress and \\  & - & Welding speed & & deformation \\  & - & Cooling time & - & Identified the magnitude difference between the \\  & - & Welding direction & & longitudinal and transverse residual stresses \\  & & & - & Identified the influence of welding speed on the \\  & & & residual stress generation \\ \hline \multirow{4}{*}{Welding} & & & - & Reduced residual stress by concentrating the energy \\  & - & Cooling time & & beam and substrate preheating \\ \cline{1-1}  & - & Substrate preheating & - & Reduced residual stress with faster welding speeds \\ \cline{1-1}  & - & temperature & - & Optimized the welding power, welding speed, and \\ \cline{1-1}  & - & Welding temperature & & martensitic transformation temperature to reduce \\ \cline{1-1}  & - & Laser spot size & & residual stress \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of numerical quality control studies of metal AM and welding.  
Numerical studies in welding have focused on identifying and further reducing the residual stress generation in welded areas. According to Table 1, the main purpose of conducting numerical studies in welding is to ensure the durability and reliability of the welded area, in that welding is generally used to join and repair metal parts.  
It was found that the phenomenon of residual stress generation during the processes was actively studied in both metal AM and welding. Numerical studies of residual stresses in metal AM still focus on identifying the causes of residual stress generation due to the complex design of the products. Due to the nature of metal AM, which deposits a number of layers to realize complex 3D shapes, temperature profile during the process and residual stress generation are affected by various process parameters. Several studies reported that residual stress in metal AM products can be reduced through preheating the substrate, as well as determining proper process parameters [32; 33; 47; 48; 49]. Numerical studies aiming to find methods for reducing residual stress generation in welded areas are expected to be promising for improving product quality in metal AM.  
In welding, it is necessary to examine more numerical studies on the formation of cladding tracks. Similar to the metal AM process, the dimensional error of cladding tracks in welding affects the dimensions of the welded area. This can interfere with the formation of smooth and precise welded surfaces, which are required in aerospace products to reduce weight and drag [50], thus, introducing numerical studies on cladding track formation in metal AM processes may be promising for welding.