PSI - Issue 74

Kristýna Vašáková et al. / Procedia Structural Integrity 74 (2025) 99–105 Kristýna Vašáková et al. / Structural Integrity Procedia 00 (2025) 000–000

102

4

Fig. 1: Mechanical properties of LPBF AlSi9Cu3 alloy (a) Hardness evolution at different temperatures; (b) Engineering stress-strain curves for different tested conditions.

3.2. Fractographic analyses Fig. 2 displays the fracture surfaces of LPBF AlSi9Cu3 samples after tensile testing performed at different alloy conditions. All fracture surfaces show dimple morphology (Fig. d-f), indicating a ductile fracture mechanism. Numerous gas pores (30–100 µm in diameter) originating from the LPBF process are also evident on the fracture surfaces, as marked with red arrows in Fig. 2a,b. These defects act as local stress concentrators and reduce the load bearing capacity, thereby likely limiting ductility. Although the AlSi9Cu3 alloy exhibits a similar type of failure in all conditions, notable differences can still be observed. In the as-built sample (Fig. 2a), the fracture predominantly propagates through individual meltpools, with fewer traces of the meltpool boundaries. On the other hand, the T5 treatment resulted in a more uneven surface, suggesting that the fracture primarily occurs along the meltpool boundaries, as shown by orange arrows in Fig. 2b. It can be assumed that formation of hardening precipitates during direct aging is more pronounced in the meltpool interiors, which forces a change in the crack trajectory and thus alters the fracture morphology. Finally, the T6 sample reveals predominant propagation through the meltpools, with the presence of larger and deeper dimples on the fracture surface (Fig. 2c), compared to the as-built and T5 treatment.

Fig. 2: Fracture surfaces of LPBF AlSi9Cu3 alloy: (a) as-built ; (b) T5 treatment; (c) T6 treatment.

3.3. Microstructural analysis Fig. 3 presents the LPBF AlSi9Cu3 alloy in the as-built and heat-treated conditions. The observed cross-section corresponds to the yz-plane, with the z-axis indicating the building direction. In the as-built condition (Fig. 3a), the microstructure consists of a cellular structure composed of α -Al cells surrounded by continuous network rich in Si, Cu, and Fe (see Fig. 4). In the T5 treatment (160 °C/ 10 h), the overall network structure is visible with fine precipitates already formed inside the α -Al cells, as indicated by red arrows in Fig. 3b. In the T6 condition, the original network is fully disintegrated, and significant particle coarsening has occurred in the microstructure. EDS analysis provided in T6 condition, presented in Tab. 2, reveals both large Si particles and smaller bright particles, which, based on their chemical composition, probably correspond to the Al₇Cu₂Fe phase.