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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of a solution heat treatment at 520 °C for 6 hours, followed by water quenching and artificial aging at 160 °C for 10 hours, then air cooling to room temperature. To explore the potential of T5 treatment (i.e., direct aging without prior solution treatment) , artificial aging was performed at 160 °C for various durations (2, 6, 10, and 24 hours), followed by air cooling. The temperature of 160 °C was selected to match the aging step of the T6 treatment. Annealing at intermediate temperatures was carried out at 250 °C, 300 °C, and 350 °C for 2, 6, 10, and 24 hours, also followed by air cooling. All heat treatments were performed in an open-air furnace (PP49/65, LAC, CZ). Hardness measurements were performed on both the as-built and heat-treated samples using a Vickers Qness Q10A tester (QATM GmbH, Germany). The reported hardness values represent the average of ten individual measurements. To evaluate tensile properties, the billets were machined into cylindrical test specimens with a gauge diameter of 6 mm and a gauge length of 30 mm, in accordance with DIN 50125. Tensile tests were carried out for selected conditions, with three specimens tested per condition. The tensile testing was performed using a Zwick/Roell Z250 universal testing machine (Germany), equipped with a MultiXtens extensometer. Fractographic and microstructural analyses were conducted using a scanning electron microscope (SEM, Zeiss Ultra -Plus, DE) equipped with energy dispersive spectrometer (EDS, Oxford Ultim Max, UK), Metallographic sample preparation involved hot-pressing (ATM OPAL X-Press, DE), followed by wet grinding and polishing with diamond paste down to 1 µm abrasive particles (Struers Pedemin DAP-7, DK). The microstructure was revealed by etching with Keller's reagent. 3. Results 3.1. Mechanical properties Fig. 1a illustrates the hardness evolution of LPBF AlSi9Cu3 alloy after the applied heat treatments conducted at different temperatures. The as-built condition and the T6 treatment are also included for comparison, with the T6 condition symbolically represented at time zero. In the as-built condition, the initial hardness was 140 ± 3 HV1. The T6 treatment resulted in a hardness of 136 ± 6 HV1, comparable to the as-built state. T5 treatment at 160 °C led to hardening, with the peak hardness observed after 10 hours. After 24 hours at 160 °C , slight softening occurred, but the hardness remained higher than in the as-built state. Annealing treatments at 250, 300, and 350 °C caused progressive softening over time, with hardness decreasing as temperature increased. Tensile tests were performed under three specific conditions: as-built, T5 treatment (maximum hardness 160 °C / 10 h), and T6 treatment, as its hardness is comparable to the as-built state. The tensile properties are summarized in Tab. 1. The T5 treatment (160 °C / 10 h) led to an increase in yield strength of nearly 30 % compared to the as- built state (from 270 ± 3 MPa to 347 ± 1 MPa) , without significant reduction in plasticity, as elongation (A) remained at 3.9 ± 0.2 %. T6 treatment produced only a slight increase in YS but resulted in a decrease in ultimate tensile strength and elongation. Moreover, stress-strain curves shown in Fig. 2b clearly demonstrate different strain hardening abilities, with the as-built and T5 samples exhibiting a higher UTS/YS ratio compared to the T6 co ndition.

Tab. 1: Mechanical properties of LPBF AlSi9Cu3 alloy in the as-built and heat-treated.

Yield strength (MPa)

Ultimate tensile strength (MPa)

Elongation at break ( % )

Hardness (HV1) 140 ± 3

Condition

as-built

270 ± 3 347 ± 1 315 ± 4

493 ± 4 520 ± 1 402 ± 2

5.6 ± 0.2 3.9 ± 0.2 4.5 ± 0.6

T5 (160 °C / 10 h)

155 ± 3 136 ± 6

T6 (520 °C / 6 h + 160 °C / 10 h)