## 6 Verification on a Grain-Scale FE Model Based on the EBSD  
This final section assesses the impact of including mechanical deformation in the solidification prediction on ultrasonic images once more--this time, simulating ultrasonic acquisition using a grain-scale FE model based on an EBSD measurement of the specimen. The EBSD measurement was performed at the University of Stuttgart.  
The dataset recorded by the microscope was processed and reconstructed into a numerical microstructure using Dream.3D, a free software for polycrystal synthesis and reconstruction. The grains with their associated full 3D orientations were modelled in the FE model, solved by Pogo, with the same setup as described at the beginning of this paper. A schematic diagram of the FE model showing the grain structure is shown in Figure 33. For simplicity, the areas not covered by the weld were set to isotropic as in the model with macroscopic weld description. This time, we did not subtract the \"background\"  
Figure 31: Local grain orientation for the coupled thermo-mechanical and solely thermal weld formation simulation.  
Figure 32: The effect of including mechanical deformation in weld formation simulation on the TFM imaging.  
dataset from the signals with defects, which is expected to be a better representation of a realistic acquisition.  
The TFM images for the defects along the left and the right chamfer are shown in Figures 34 and 35, respectively. We compare the effect of including the mechanical deformation in solidification simulations, a factor which was hypothesised to have less effect on ultrasonic imaging than it has on residual stress prediction at the beginning of this study. As in the previous examples, the images were scaled with respect to the magnitude at the middle SDH. To retain legibility, given the high level of grain noise, the magnitude range was limited to \(-12\) dB from the maximum.  
The images based on the EBSD data show a significant level of noise, and not subtracting the \"intact\" datasets resulted in a strong influence of the array nearfield. Nevertheless, both the coupled thermo-mechanical simulations and the simplified approach presented a similar performance for the EBSD-based grain-scale numerical \"experiment\". For the case with defects on the left chamfer, defect SNRs are similar, but the one closest to the backwall could not be distinguished from the background noise.  
Figure 34: The effect of including mechanical deformation in weld formation simulation on the TFM images from time signals calculated using a grain-scale FE model based on the EBSD data (left chamfer defects).  
Figure 33: Schematic diagram of the EBSD-based FE model; only the left-chamfer defects are illustrated.  
For the imaging defects on the right chamfer, the level of noise is elevated, but all defects can be distinguished. A plausible explanation of the poorer performance for left-chamfer defects is that the EBSD scan was notably noisier over that area yielding results with a low confidence index. Consequently, the microstructure underpinning the FE ultrasound simulations could be not well represented by the solidification modelling. We should also mention that there are numerous factors at play in this example, which are impossible to account for, even for a numerical exercise. However, given that in reality many parameters are not known, this case is a good representation of a practical scenario.