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

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and favorable strength-to-weight ratio, are typically subjected to heat treatments to enhance their mechanical properties. A review by Fiocchi et al. (2021) identified three primary heat treatment strategies for LPBF aluminium alloys including solution heat treatment followed by artificial aging (T6), direct aging (T5), and annealing at intermediate temperatures. These heat treatments have been extensively studied, particularly for the Al-Si-Mg system, including age-hardenable AlSi7Mg and AlSi10Mg alloys. The T6 treatment, adapted from conventional Al-Si based casting alloys, involves solution heat treatment at 510–540 °C followed by artificial aging at 150 –250 °C (Li et al. (2004)). For cast AlSi10Mg alloys, the T6 treatment enhances strength due to the precipitation of Mg₂Si phase. However, in LPBF AlSi10Mg, the T6 treatment reduces strength, as the high-temperature solution treatment induces microstructural changes. A key feature of LPBF AlSi10Mg alloys is their fine microstructure, composed of α -Al solid solution cells surrounded by a Si- and Mg-rich network, which enhances mechanical performance. However, during T6 heat treatment, this network is fully disintegrated, generally leading to a decrease in yield strength, ultimate tensile strength, and hardness, while improving elongation at break (Aboulkhair et al. (2016); Fousová et al. (2018)). To better suit the unique microstructure of LPBF Al-Si based alloys, alternative heat treatments have been explored. Direct aging (T5) takes advantage of the supersaturation of the aluminum matrix induced by rapid cooling in the LPBF process, enabling precipitation at temperatures of 140–180 °C without the need for solution treatment (Fiocchi et al. (2021)). This approach increases hardness in LPBF Al-Si-Mg alloy through precipitation while preserving the cellular structure (Fousová et al. (2018); Rao et al. (2019)). Surprisingly, both studies found that in the T5 condition, Si precipitates formed instead of the expected Mg₂Si phase. This phenomenon is attributed to the high Si supersaturation in the as-built state, which promotes Si diffusion over Mg. Additionally, Mg segregation at cell boundaries further limits its availability for precipitation, reinforcing the formation of Si precipitates (Fiocchi et al. (2021)). Annealing at intermediate temperatures (250–350 °C), on the other hand, induces significant microstructural changes compared to direct aging (T5). Around 300 °C, the network in LPBF AlSi7Mg alloy rupture s, leading to a sudden drop in strength (Kimura and Nakamoto (2016)). While LPBF Al-Si-Mg alloys have been extensively studied, the growing industrial adoption of LPBF necessitates expanding research to other aluminum alloys with enhanced performance. One such alloy is the age-hardenable AlSi9Cu3 alloy, which exhibits higher mechanical properties in the LPBF as-built state compared to its conventionally cast counterpart (Fiocchi et al (2020)). The AlSi9Cu3 alloy is widely used in engine components, particularly in thin-walled parts exposed to dynamic loading, such as cylinder heads (Zamani et al., 2015). Despite its potential, the heat treatment behavior of LPBF AlSi9Cu3 remains insufficiently explored, and the few available studies on T6 treatment have reported inconsistent findings. Fiocchi et al. (2020) reported that T6-treated LPBF AlSi9Cu3 alloy exhibited decreased strength and increased ductility, which is consistent with findings on other LPBF Al-Si based alloys. In contrast, Roudnická et al. (2020) found that T6-treated LPBF AlSi9Cu3 led to increased strength while maintaining ductility. These conflicting results highlight the need for further investigation into the T6 heat treatment of LPBF AlSi9Cu3. This is particularly important, as T6 has been shown to improve fatigue performance in LPBF Al-Si alloys compared to the as-built condition (Maskery et al., 2015). To address these discrepancies and expand the limited dataset, this study examines whether the T6 response of LPBF AlSi9Cu3 resembles that of the extensively studied AlSi10Mg alloy, in which T6 treatment typically leads to reduced mechanical performance. In addition, this work investigates whether the presence of additional alloying elements in AlSi9Cu3, particularly Cu and Fe, can enhance the thermal stability of the network structure compared to AlSi10Mg, where degradation of the Si- and Mg-based network has been observed at elevated temperatures (around 300 °C). To investigate this aspect, the microstructure of AlSi9Cu3 was analyzed follow ing exposure to temperatures of 250 °C, 300 °C, and 350 °C. 2. Materials and methods Gas atomized AlSi9Cu3 powder from SLM Solutions Group AG (DE) with the following chemical composition (wt. %): 9.2 ± 0.1 Si, 3.2 ± 0.1 Cu, 0.7 ± 0.1 Fe and Al in balance was used for the manufacturing of billets. The billets with dimensions 12×12×80 mm, (w × h × l) were built horizontally on the preheated platform (100 °C) using an SLM Solutions 280 HL machine. Process parameters were: a laser power of 350 W, scanning speed of 1400 mm/s, layer thickness of 50 µm, and hatch distance of 120 µm, building strategy “chessboard”. AlSi9Cu3 billets were removed from the base plate using wire electrical discharge machining and subsequently heat-treated. The conventional T6 heat treatment was applied according to standard DIN EN 1706:2020. It consisted