PSI - Issue 57

Jan Papuga et al. / Procedia Structural Integrity 57 (2024) 79–86 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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definite universal answer. One of these questions is how the materials and the estimation fatigue criteria respond to the out-of-phase loading induced by a phase shift between concurrently acting load channels, see Papuga et al. (2019, 2021). Number of reliable experiments in the HCF region remains too low to formulate a decisive verdict which would help to eliminate the abundant number of contradicting theories. This paper joins both aspects and deals with the multiaxial fatigue response to in-phase and out-of-phase loading on AM hollow specimens from AlSi10Mg alloy.

Nomenclature a, B, C, 

parameters of the Kohout-V ěchet model, Kohout and Věchet (2001) shear stress amplitude, mean shear stress, respectively

C a , C m

number of cycles

N

N a , N m

normal stress amplitude, mean normal stress, respectively

equivalent stress amplitude of the specific multiaxial fatigue strength criterion fatigue strengths in fully reversed axial loading or in fully reversed torsion, respectively

 eq,a

 FS ,  FS

 H √ 2

hydrostatic stress

second invariant of the stress tensor deviator

2. Setup of the experiments 2.1. Specimens

Specimens were fabricated from AlSi10Mg powder within the L-PBF additive manufacturing process using the Concept Laser M2 printing machine by GE. All here discussed specimens were manufactured on one platform, the setup of which is shown in Fig. 1, right. The platform was used to build two sets of specimens: a batch of common dog-bone hollow specimens, located at the left half., while the right side is populated by the cavity specimens (Fig. 1, left) discussed hereafter. All specimens were built in the upright position, which usually results in a weaker fatigue performance than the horizontal orientation if the platform is not heated – see Brandl et al (2012). The manufacturing process follows a specific machine default setup for the aluminum alloy. It manufactures a skin at each 25  m high layer with lower energy input first (see Table 1) to ensure optimum roughness, which could govern the fatigue damaging process. Then the contour is created with a higher energy input. The contour corresponds to the inner space into the depth of 2 mm below the specimen surface. This part is also made with a layer height of 25  m. Then the platform moves downward, and new skin and contour are created. Every second layer, the high energy input from the laser is used to melt the powder in the core area in the chess-board strategy.

Table 1. Parameters of the AM process. Power [W]

Speed [mm/s] Trace spacing [mm] Spot size [µm]

Layer height [mm]

Skin

200 370 370

800

0.112 0.112 0.112

140 190 190

0.025 0.025 0.050

Contour

1400 1400

Core

The specific specimen design, shown in Fig. 1 left, was chosen intentionally for three reasons. Firstly, the internal cavity would not be easily achievable within the subtractive manufacturing. Secondly, these experiments were a part of a broader experimental campaign, within which the thermal response of cyclically loaded specimens is observed. Larger perimeter of the active area results in a larger area with only a small deviation from the perpendicular direction to the thermal camera. Thirdly, specimens should comply with the ASTM request (Kalluri and Bonacuse (2000)) of the least possible shear stress decrease across the specimen wall. Apart from thickness, the other option to decrease the shear stress gradient is to increase the external diameter of the specimens. It is worth noting that a similar geometry was also proposed by Fatemi and Molaei (2020). The specimens were left with the surface as built, without any subsequent machining even on gripping heads. All of them were heat treated at 240 °C for 6 hours in the furnace, and this phase was followed by cooling them on air.