The results of the aging process show that there is a fast increase of the oxygen content after a short time period and that there is an approach to an oxygen saturation. Furthermore, it can be seen, that the maximum allowed oxygen content is reached after approximately four hours (0.199%). Compared to these results there is only a small increase of the nitrogen content even after a longer aging time (0.028%). In summary, it can be said that there is a meaningful aging time for the virgin powder of four hours because a high oxygen content is reached but it is still below the maximum allowed limit.
Tensile and fatigue tests
The impact on the tensile strength and the elongation of the aged material is very high for both PSDs. The comparison between both virgin conditions shows that the ultimate strength is in the same range, but the elongation to fracture for the larger PSD is lower. The aging treatment has an effect on the tensile strength which is reduced for both PSDs. The elongation to fracture is significantly reduced by the aging treatment, especially, for the higher PSD. The virgin material has a higher fatigue strength for both PSD. The larger PSD is severely influenced by the aging treatment. With a smaller PSD the aged material shows a poorer fatigue behavior.
Fatigue crack growth experiments
The results for the tests with different PSD show, that for the examined powders the crack propagation curves have almost the same curve progression, especially in the threshold area and in the middle region. Only in the fracture toughness region some slight differences are observable. In summary it can be said, that there is obviously no significant influence on the crack propagation curve or the threshold value when using different PSDs.
In case of an aged powder, the PSD has a significant influence. For the powder with the larger median value, the aging process leads to a barely noticeable change in the threshold value as compared with the virgin condition. In contrast, an aging process of the powder with smaller median value leads to a significant decrease of the threshold value. The fundamental reason for this result can be the smaller particle size. Due to the smaller particle size, there is a larger surface, which is more influenced by the aging process. Therefore, the potential oxygen uptake of the powder is higher with smaller sized particles. With regard to a fracture mechanical analysis, it can be said that in the presence of an aged powder, a powder with a larger average particle size has less influence on the fracture mechanical properties.
Further investigations could be tensile, fatigue and fracture mechanical experiments with a mean PSD from the same powder lot. In addition, the influence of different heat treatments, the building direction or the powder humidity could be investigated.