PSI - Issue 77

R. Branco et al. / Procedia Structural Integrity 77 (2026) 376–381 Branco et al. / Structural Integrity Procedia 00 (2026) 000–000

377

2

Nomenclature a

crack length

B

bending moment bending-torsion ratio

B/T

R T

stress ratio

torsion moment

nominal normal stress amplitude nominal normal mean stress normal stress to shear stress ratio von Mises stress amplitude nominal shear stress amplitude nominal shear mean stress

σ a σ m σ / τ

σ a,vM

τ a τ m

strengthening and embrittlement mechanisms are predominantly governed by the cellular morphology (Huang, 2024). Post-processing heat-treatments are generally used to tailor both the microstructure and the mechanical response. However, its distinct metallurgical features have led to the use of non-standard heat treatment procedures, with changes in holding temperature and holding time necessary to achieve the desired microstructural and mechanical properties. A promising post-processing heat treatment to improve fatigue resistance is the so-called low-temperature stress relief, typically carried out at 250 °C (i.e., slightly below the first exothermic peak of L-PBF-processed AlSi10Mg aluminium alloy, which usually occurs around 260 °C. Recent studies (Fiochi, 2021; Fernandes, 2024) reported that heat treatments slightly below the first exothermic peak can lead to a balance between ductility and strength, while also contributing to effective stress relief. Regarding the fatigue performance, Fernandes et al. (2025) demonstrated that this post-processing heat treatment (250 °C for two hours) has a beneficial effect under low-cycle fatigue conditions but is relatively ineffective in the high-cycle fatigue regime. Under multiaxial fatigue loading, there is very limited information. Papuga et al. (2024) studied the phase shift effect in additively manufactured hollow geometries made from AlSi10Mg aluminium alloy subjected to a post-processing heat treatment at 240 °C for 6 hours. However, so far, the influence of heat treatment under multiaxial loading has not been investigated. This paper aims to investigate the effect of low-temperature stress relief heat treatment on notched AlSi10Mg hollow geometries produced by L-PBF and subjected to combined bending-torsion loading. Both untreated and heat treated conditions are investigated. In addition, crack initiation sites and early-stage crack growth directions are evaluated for the different material states and loading scenarios. 2. Experimental procedure The material used in this study is L-PBF-processed AlSi10Mg aluminium alloy. Both untreated and heat-treated states were investigated. The post-processing heat treatment consisted of a low-temperature stress relief at 250 °C for two hours followed by water quenching. Further details on the chemical composition, microstructure characteristics and mechanical properties of both untreated and heat-treated states can be found in previous studies by the authors (Fernandes, 2024; Fernandes, 2025). The specimen geometry used in the proportional bending-torsion fatigue tests is presented in Figure 1. It consists of a notched hollow cylindrical bar with a 5 mm-diameter through-hole machined laterally on one side of the wall. The specimens were printed vertically via L-PBF using Renishaw AM400 3D equipment. The main processing parameters were: maximum laser power of 350 W, layer thickness of 30 μm , scanning speed of 1800 mm/s, and hatch distance of 80 μm. After the manufacturing stage, the lateral hole w as machined via CNC. The external surface around the notch region of both untreated and heat-treated specimens was carefully polished using sandpaper of progressively finer grits (P#600, P#1200, and P#2500) and 3- µ m diamond paste. Multiaxial fatigue tests were conducted in-phase by applying combined bending-torsion loading scenarios. Two bending-torsion ratios (B/T) were studied: B/T=2/3 ( σ a / τ a =4/3) and B/T=1 ( σ a / τ a =2). For each B/T ratio, at least two