Near-surface structural features and directional fatigue behavior of as-built L-PBF AlSi10MgWednesday (27.05.2020) 15:10 - 15:30
Laser powder bed fusion (L-PBF) makes it possible to produce metallic parts directly from a computer-aided design file. The automotive and aerospace industries have demonstrated significant attention for the L-PBF production of AlSi10Mg parts due to lightweight, hardenability and low powder cost. In addition, localized melting of gas atomized powder by a concentrated laser beam and subsequent solidification and cooling generates refined structures achieving mechanical strength competitive with conventionally produced Al-alloys. If left untreated after fabrication, residual stresses are expected in untreated parts and residual stresses superpose on operating stresses introducing an often unpredictable influence on the part durability. As-built surface quality also affects negatively the fatigue strength of L-PBF AlSi10Mg. However, surface machining is not only costly but often impossible on many L-PBF parts considering their geometrical complexity.
This study investigates the fatigue behavior of L-PBF AlSi10Mg under the combined influence of the untreated state and the as-built (i.e. rough) surface. Therefore, four sets of miniature specimens each with a different orientation with respect to the build direction were produced with a SLM 290 system (SLM Solutions, Germany) working at 50 m layer thickness and using the recommended process parameter. Each set consisted of approx. 15 specimens. As-built specimens were tested in cyclic plane bending at load ratio R=0 and frequency of 25 Hz.
The high cycle fatigue response of L-PBF AlSi10Mg was quantified and a strong directional behavior determined with one horizontal orientation of the specimens reaching twice the fatigue strength of the vertical specimen orientation. Such a difference cannot be explained only considering surface roughness.
To investigate the origin of the directional fatigue behavior, specimens for each orientation were examined using: i) metallographic techniques to determine the near surface material structure and quality in dependence of local process parameters, ii) micro hardness mapping to quantify near-surface structural gradients due to printing sequence and iii) EBSD in the SEM to characterize crystallographic texture.