PSI - Issue 34

2

Benjamin Möller et al. / Procedia Structural Integrity 34 (2021) 160–165 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

161

Nomenclature d gap

gap width

Young’s modulus force, force amplitude

E

F , F a

N, N r , N k

number of cycles, number of cycles to rupture, number of cycles to failure at the knee point

survival probability

P S

R , R σ

stress ratio

scatter band based on the stress

T 

slope of the Wöhler (S-N) line (before the knee point), slope of the S-N line after the knee point

k, k * r , r ref

(reference) radius at the notch toe/root

 vM,a ;  1,a

notch stress amplitude (based on von Mises ‘vM’ or maximum principal ‘1’)

Poission’s ratio



1. Introduction Additively manufactured (AM) components gain in attractiveness in industrial applications – not only due to the flexibility in design resulting from this process, but also potentials of time and cost savings in the product development process, customization of products or spare parts manufacturing. However, safety-relevant component design sets high standards on quality and strength. In this context, limitations are the notch effect of the AM base material and (cyclic) material behavior, which comprehends the effect of process parameters on anisotropic microstructures, gradients of the local properties and imperfections, and finally imperfections resulting from the joining of AM materials. Joining, such as welding, will be an essential step to the assembly of AM components, e.g. joining of AM components to conventionally manufactured structures. Welding interferes with the material and limits the fatigue strength in general. The additional heat input of the welding process affects the surface topology and the microstructure, as well as existing imperfections. Effects of welding of AM structures is still a fairly unknown topic, especially if it comes to laser beam welding (LBW) of dissimilar aluminium materials. Aluminium welds tend to show porosity, in particular for laser beam welded dissimilar conventional aluminium alloys, as shown by Chen et al. (2020) and, if AlSi10Mg base material is used, as shown for additively manufactured AlSi10Mg laser welds by Mäkikangas et al. (2019) and for GD-AlSi10Mg T6 butt welds, as shown by Kaufmann et al. (2000). The work reported in this paper is based on previously published investigations from Möller et al. (2019) and Möller et al. (2020), where the experimental campaign is described in detail. Based on the discussion of the numerical work, a starting point for the notch stress assessment of AM dissimilar welds is addressed. Main conclusions are: • Fatigue assessment by the nominal stress approach of laser beam welded (LBW) butt joints and lap joints with fillet welds between AM and conventionally manufactured aluminum confirm FAT 12 for aluminum welds. • LBW lap joints with I-shape seam welds show fatigue failure from the notches in between the two sheets ( categorized as “root notch ” failure ) and a lower fatigue strength under shear/peel loading compared with LBW lap joints with filled welds on both sides (primary weld toe failure), for example. • Fatigue assessment of LBW lap joints with I-shape seam weld (‘ root notch ’ failure) by the notch stress approach using r ref = 0.05 mm shows that existing notch stress FAT classes of recommendations are only applicable at lower lifetimes of N f ≤ 1·10 5 and very high lifetimes of N f  2·10 6 . • A slope of k = 5 for the assessment of thin and flexible structures by FAT classes is confirmed, while k = 3 leads to a non-conservative assessment. The recent investigation extends this work to the local assessment of LBW lap joints with fillet welds by notch stresses, where cracks mainly initiate from the weld toe. Furthermore, the fatigue life estimation of a battery carrier regarding fatigue failure of laser beam welds between AM connection nodes and extruded aluminium sections is discussed.