# Correlation of Microstructure and Mechanical Properties of Metal Big Area Additive Manufacturing
## 3 Results  
### Tensile Testing  
For deployment of AM, it is necessary to make sure that a build meets or exceeds the material performance necessary for the intended application. Here, SuperArc(r) L-59(r), equivalent to ER70S-6, is utilized and conforms to AWS A5.18/A5.18M specifications [31]. However, due to differences in cooling rates and geometrical challenges, traditional comparisons to welding and standards may not yet apply to MBAAM and large-scale additive manufacturing. Figure 5 shows the tensile stress-strain curves for three different orientations with reference to the build direction: perpendicular (90\({}^{\circ}\)), 45\({}^{\circ}\), and parallel (0\({}^{\circ}\)) to the build direction. The samples from the interface region of the build have the highest yield point (approximately 420 MPa, with red and orange curves). This region (bottom 50 mm) is generally removed and discarded and is not typically associated with a structural component. The yield point drops to an average of 360 \(\pm\) 7 MPa (black curves) in the remainder of the build for all orientations.  
The heat transfer in the walls manufactured by MBAAM differs from standard multipass welding butt joints (i.e., all weld metal testing) and can result in lower yield strengths when compared to traditional welding. The heat transfer in traditional multipass welding consists of 3D conduction through the work piece and radiation, generally resulting in fast cooling rates. The heat transfer in this thin wall build is dominantly just radiative, which can result in relatively slower cooling rates. This difference highlights the variation in MBAAM versus traditional multipass welding and shows that alloy development and new standards may be required for MBAAM construction methods.  
Most of the tensile curves show the well-known yield point phenomenon that is usually observed in annealed low carbon steel. Previous studies, where samples were fabricated using TIG welding, also showed similar discontinuous yielding [24]. In these samples, uniform plastic deformation began roughly at 5% strain. Near the build plate, however, the yield point phenomenon was reduced in magnitude, i.e., uniform plastic deformation began at approximately 0.5% to 1% strain. The arrows denote the change in uniform plastic deformation in the tensile specimens as we move in the Z direction. This difference can be rationalized with the microstructure characterization.  
Figure 5: Stress versus strain curves (**a**) perpendicular to build direction, (**b**) 45\({}^{\circ}\) to build direction, (**c**) parallel to build direction, and (**d**) graphical representation of yield and ultimate tensile stress and elongation averages with the standard deviation of results (the elongation average does not account for the red/orange curves from the interface).  
The ultimate tensile strength (UTS) is the largest in the samples extracted from the interface region (red and orange curves) of the build (525 MPa) compared to other samples. The UTS dropped to an average of 475 \(\pm\) 4 MPa (black curves) in the samples from rest of the build. The elongation at fracture of all the orientations, except one sample that contained a discontinuity (i.e., a lack of fusion), ranged from 25% to 42%. Similar trends are seen with increasing build height. The lowest elongations to fracture are at the interface region of the build. However, for most of the build, the elongation to failure ranged from 34% to 42%. For the main body of the build, the curves exhibited fairly isotropic yield and tensile strength properties (black curves) with respect to the build direction. There was some scatter in the elongation at failure. This semi-isotropic behavior shows that the MBAAM method can generate reliable and consistent mechanical properties once a quasi-steady state thermal condition (a migrating thermal transient up the build) is achieved.