PSI - Issue 19

Kai Schnabel et al. / Procedia Structural Integrity 19 (2019) 442–451 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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For the additive manufacturing itself, a building chamber is flooded with inert gas like nitrogen or argon. During the process, a permanent gas flow is present. At the bottom of the chamber, the building platform is coated with the selected metallic powder. One or more laser systems melt the powder layer-wise in the pre-defined geometrical shape of each layer. Different process parameters for the contour scan, which can be performed for smoother surface of the component, as well as different parameters for the filling of the component, are available from the system suppliers. All slices together represent the final manufactured component.

Nomenclature AM

Additive Manufacturing

LPBF Laser Powder Bed Fusion

CAD Computer Aided Design

N

Load cycles Notch radius

Y OUNG ’ S module

E F

r

Force

SE

Structural Element Notch opening angle Contour thickness

α

FE K t G σ

Finite Element

Stress concentration factor

d c

ν

Stress gradient

P OISSON ' S ratio

HBV Highly stressed volume

The way in which direction the CAD designed geometry is reconstructed and with this the direction in which the AM component is manufactured has a major influence on the fatigue strength of the component, because the arrangement of the part within the building chamber determines both the microstructure and the supporting structures. Mostly, aluminum and titan alloys, steel or nickel based alloys are used and in the focus for lightweight designed components with a high fatigue strength. Finally, when safety-relevant components have to be designed, a reliable fatigue assessment concept must be available. For components manufactured by classical processes, there is a variety of assessment approaches in which the influence of e.g. mechanical and geometric properties as well as size effects on the fatigue strength can be taken into account, [ (Hobbacher 2016), (Rennert, et al. 2013)]. Because of the completely different manufacturing process, AM parts compared to classical manufacturing processes, adjustments need to be applied to these concepts or new concepts have to be developed. Over 50 key process parameters in a LPBF process can be identified [(O'Regan, et al. 2016), (Spears und Gold 2016)] which have different influences on the geometry and the internal and external structure of the component. These are for example the applied laser energy, the scan pattern, the velocity of the laser scan, the layer thickness, the hatch distance or the properties of the powder. An understanding of the correlation between this process parameters and the resulting AM-structure is important, if AM-structures should be used as safety critical components. These influencing parameters lead to differences in dimensional accuracy, mechanical properties and fatigue strength. Standard process parameter sets are optimized for a high dimensional accuracy and a good-looking surface. However, a high dimensional accuracy does not necessarily lead to a high fatigue strength. A stronger dependency on the fatigue strength can be assumed from factors like surface roughness and the type and the shape of inner imperfections [(Brandao, et al. 2017), (Scurria, et al. 2019)]. Subsequently, a global dimensional accuracy is not a good benchmark for the fatigue strength performance of AM components. In addition, the density of an AM manufactured structure, which is commonly used as a process quality measure, is just a hint but no correlation criteria for the fatigue strength, because it only gives an information about the presence of pores, but not their location and distribution. This information is of eminent importance, because pores in higher stressed areas can have a high impact on fatigue strength than pores with the same size and shape in minor stressed areas. An additional factor with importance is the local dimensional accuracy, especially in the area of geometrical notches. The difference between nominal geometry and that of received ‘as - built’ additively manufactured one can lead to different notch shapes, which have a high impact on fatigue strength.