PSI - Issue 68

J. Köckritz et al. / Procedia Structural Integrity 68 (2025) 962–968

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J. Köckritz et al. / Structural Integrity Procedia 00 (2025) 000–000

1. Introduction Additive manufacturing (AM) allows manufacturing of geometries which are impossible or difficult to manufacture by conventional manufacturing methods. Particularly powder bed fusion by a laser (PBF-LB/M) can be utilized to fabricate lightweight, topology optimized components. Apart from complex geometries with undercuts, hollow sections or grid filling, AM is usually applied for small series manufacturing or customized components. Especially PBF-LB/M is mainly applied in aerospace, automotive and medical implants industry (Pourrahimi and Hof (2024)).

Nomenclature A*

elongation at fracture load amplitude Interquartile range

mean roughness roughness depth

Ultimate tensile strength

R a R z S a

UTS

yield strength fatigue strength

F a

YS σ D

mean area roughness log. standard deviation deepest surf. notch

IQR

k1 , k2 fatigue exponents

maximum tension stress

s log

σ t,max

transition life

transition stress

N tr

S v

σ tr

PBF-LB/M has many benefits, namely the high freedom of design and the fast manufacturing of complex parts. However, the layer-wise process is accompanied by disadvantages, for example poor surface quality, low dimensional accuracy, long process times for a single part and a different fatigue behavior from bulk material. Influences on the fatigue behavior of PBF-LB/M parts, all highly dependent on printing parameters, are microstructure, anisotropy, residual stresses (Strauß et al. (2024), Diller et al. (2024)), porosity and the surface roughness (Beretta et al. (2022), Diller et al. (2024), du Plessis and Beretta (2020), Kahlin et al. (2020), Strauß et al. (2024)). Surface or subsurface defects in particular have been shown to govern AM (Murakami (2005)). Fatigue life of AM parts depends on the location, size and shape of a possible defect (Sanaei and Fatemi, (2021)) and can thus scatter widely. Consequently, surface treatments suited to the complex geometries usually manufactured by AM are of high interest to increase the expectable life of components and thus increase applicability for lightweight designs. Possible options are grinding or laser polishing for more accessible surfaces or powder blasting, chemical and electrochemical polishing (Pourrahimi and Hof (2024)), plasma-electrolytic polishing (Navickaite et al. (2024)) or abrasive flow methods such as trowalising (alternatively known as drum finishing) for components with complex geometries. A number of models have been proposed to include the surface quality in the fatigue assessment of AM parts based on fatigue tests on simple specimen, for example Kahlin et al. (2020) based on the deepest surface notch S V or Strauß et al. (2024) combining residual stress and roughness depth R z . Often, the Murakami defect size (Murakami (2005)) is employed in variations for fatigue assessment (Beretta et al. (2022), Strauß et al. (2024)). These approaches led to good fits with experimental data. There is some discussion about whether line roughness R a and R z or rather area roughness S a and S z or S V are suited for evaluating an AM surface (Kahlin et al. (2020), Diller et al. (2024)). Experimental fatigue investigations of AM parts and their roughness dependence are almost exclusively performed on simple specimen geometry. Only few investigations cover the fatigue of complex components, for example Beretta et al. (2022), Leuders et al. (2017) and Gupta et al. (2022). It is undisputed, that even though fatigue behavior of AM specimen is already complex and process dependent, the fatigue behavior of components is even less predictable, due to differences in cross section throughout the print causing widely different microstructures throughout the part, varying overhang angles at different positions and the necessity of support structures. For aluminium AM, age hardenable alloys are frequently applied (Elambasseril et al. 2022), most often AlSi10Mg. The less frequently used Al-Cu-Mg-Ag alloy Al2139 displays a high strength and low creep at elevated temperatures. In PBF-LB/M, it is susceptible to hot tearing, which can be decreased by alloying with AlTiB (Elambasseril et al. (2022)) or decreasing thermal stress by high print bed temperatures and volumetric energy density (VED) (Zhang et al. (2016)). Elambasseril et al. (2022) have employed VED of up to 1721 J/mm³ highlighting its effect on residual stress. Although significant advances in the processability of Al2139-AM have been made and applications arising, research on its fatigue behavior, roughness influence or behavior in complex components are uncommon. Hence, the aim of this study is to investigate topology optimized components made of Al2139-AM with regards to their fatigue life in a realistic, multiaxial load case as well as the effect of an industrial surface treatment on the fatigue behavior.