# Correlation of Microstructure and Mechanical Properties of Metal Big Area Additive Manufacturing
## 4 Discussion
### Correlation of Tensile Properties to the Microstructure  
The tensile properties of the build manufactured by MBAAM shows fairly uniform and isotropic properties with respect to build height. The isotropy implies that the microstructure should also be fairly homogeneous in these tensile samples. The homogeneity is found to indeed be the case for most  
Figure 10: High magnification micrographs from different locations in the build height. (**a**) The last layer, top. (**b**) The middle of the build. (**c**) Bottom of the build. (**d**) Interface region between base plate and beginning of the build. Figure 9: Microstructure from different locations in the build height. (**a**) The last layer, top. (**b**) The middle of the build. (**c**) Bottom of the build. (**d**) Interface between base plate and beginning of the build. of the build. The microstructure shown in Figure 9b,c represents the majority of the build volume, not including the interface region and the last deposition layers. Otherwise, the microstructure is consistent. The main microstructural constituent is polygonal ferrite with some pearlite (5% to 7%). This suggests complete reheating of the deposits by the subsequent passes and may also indicate that there is annealing of the reheated regions. The difference in the thermal profile of the MBAAM builds from traditional welding is most likely the reason for differences in the MBAAM results. The fraction of as-welded and reheated regions in traditional butt welds should be significantly different than MBAAM. This difference can be observed from the as-deposited region in the top passes, as the region has not gone through reheating cycles. The microstructural homogeneity observed throughout the build provides the uniform tensile properties. However, there are some outliers in the tensile curves. There is a single curve from the perpendicular and 45\({}^{\circ}\) orientations with a higher yield stress and lower ductility. These tensile bars were intentionally cut in such a way that the gauge section contained the interface region between the base plate to regions that were built in the quasi-steady state conditions. From the parallel build orientation, there are two sets of samples, 1 and 2, which were cut from this same interface region. The higher yield stress and lower ductility from this sample can be rationalized by the presence of the mixed microstructure shown in Figures 9d and 10d. The literature has shown that a mixture of acicular ferrite and bainite can raise the yield stress and decrease ductility of a weldment by the Hall Petch relationship [27; 33; 34; 35]. The reason for the mixed microstructure in this region is mostly likely due to carbon uptake from the base metal from the AISI 1018 base plate. Furthermore, other factors, such as differences in thermal cycles and localized changes in heat input at this location, may also lead to the formation of the mixed microstructure. In traditional GMA welding, under weaving conditions, mixed microstructures are often observed close to the interface locations [36]. In most of the build, the lower yield strength can be rationalized by the evolution of the microstructure, which is in turn relates to the heat transfer characteristics of the wall. The build-substrate interface region has a higher thermal mass, due to the presence of a substrate, and the initial passes occur on a room temperature substrate. Here, the post-pass cooling rates are more typical of traditional butt welds (>10 \({}^{\circ}\)C/s). The mixed microstructure from the base plate interface region, transitions to polygonal ferrite at larger heights because of the repeated thermal cycles that occur during the build [18]. Due to the thermal characteristics of the build leading to slow cooling rates, the grain size of the polygonal ferrite is larger than the fine-grained acicular ferrite observed in traditional weldments [18; 20; 21].