Generally, superior mechanical properties of the horizontal specimens can be attributed to three basic factors: different size and distribution of their defects, different levels of their residual stresses, and direction of their deposited layers against the loading direction [38,40,57]. The first two factors are directly related to the different thermal cycles of the horizontal specimens in comparison to the vertical ones, which could also be correlated to the aspect ratio rather than the building orientation. The negative effects of the defect size are more prominent for the high cycle fatigue behaviour since it is more sensitive to the defects due to the lower stress levels [38,57].