# Correlation of Microstructure and Mechanical Properties of Metal Big Area Additive Manufacturing
## 4 Discussion
### Correlation of Charpy Impact Toughness to the Microstructure  
The Charpy impact toughness tests did show some scatter. At each testing temperature there was a range of impact energies, with the \(-20\)\({}^{\circ}\)C test temperature having the largest scatter [37]. As discussed in Section 3.2, the scatter in the data tends to show lower toughness with increases in build height. The microstructure can also be used here to rationalize the impact test results. At the bottom of the build there is a quick transition from a mixed microstructure of bainite/acicular ferrite and from allotriomorphic ferrite to a fine grain polygonal ferrite region. This transition is represented by the samples from Y1 and Z1. These microstructural features are shown to improve the toughness in C-Mn steel weldments [17; 20; 27; 33; 38]. Therefore, it should be expected that these samples maintain a higher toughness. Furthermore, by increasing build height, the toughness decreases. The polygonal ferrite grain size increases with build height, and, therefore, the toughness should decrease as illustrated in Figure 6. Although it should be noted that even though the \(-20\)\({}^{\circ}\)C test results highlight this trend, all the other testing temperature results were closer with regards to scatter (less than 80 J separation). Therefore, it should be concluded that the \(-20\)\({}^{\circ}\)C data scatter was due to sampling. Next, comparing the amount of scatter observed to literature values, the MBAAM built wall had a lower degree of scatter compared to a range of over 250 J [17; 22], even though the \(-20\)\({}^{\circ}\)C test temperature resulted in the largest scatter. This scatter is expected because traditional multipass weldments contain several different HAZ's including coarse grain, fine grain, inter-critically reheated coarse and fine grain HAZ. Since the builds are thin walls the thermal cycles continually reheat the same material in a vertical progression as the wall is built, i.e., each location in the middle of the build height generally receives the same thermal cycles that can make microstructural changes. This repeated cyclical behavior produces a more homogeneous microstructure of polygonal ferrite, leading to a relatively low scatter in toughness compared to the literature [17; 22]. A previous study by [PERSON] et.al on C-Mn steel fabricated using GTAW based additive manufacturing processes showed a strong dependence of build orientation on scatter [24]. Significant scatter in the toughness was found to be due to the presence MA constituents in certain build geometries (thick walls). However, the previous study has shown that for thin walls, the cooling rate is adequate to suppress the formation of MA constituents [24]. Therefore, the nature of the thermal cycles should eliminate the formation of local brittle zones of martensite/austenite (MA) constituents, which lead to low toughness and high scatter, observed in multipass weldments [39; 40]. The microstructural characterization did not show the presence of MA constituents. Reheating to the inter-critical phase field caused the nucleation of small austenite regions on grain boundaries and high carbon rich regions, such as pearlite. Upon rapid cooling, these austenite regions transformed into MA constituents, but the slow cooling rates observed here do not allow the MA to form instead ferrite and pearlite [41].  
Figure 11: Continuous Cooling Transformation (CCT) diagrams for 0.1 wt% C. The CCT diagram is used to understand the evolution of the microstructure on cooling from face centered cubic (FCC) austenite. Drawn using the thermodynamic software JMatPro®.  
[PERSON] and [PERSON], investigated the microstructure and toughness of C-Mn weldments [20]. The resultant values of toughness of the MBAAM build compare well with the impact toughness found in the literature for comparable chemical compositions, even though the microstructure does not consist of the desired acicular ferrite typically observed in weldments [18; 32; 33].