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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The microstructural evolution of LPBF AlSi9Cu3 alloy at elevated temperatures is very similar to Al-Si-Mg alloys, as reported by Fousová et al. (2018) and Kimura and Nakamoto (2016). Alloying elements initially dissolved in the supersaturated solid solution α -Al begin to precipitate both inside the cells and along the original cell boundaries within the network region. As the temperature increases, continued precipitation leads to a gradual breakdown of the network. In this study, network fragmentation was observed at 300 °C (see Fig. 3e), which was accompanied by a significant decrease in hardness from 140 ± 3 HV1 (as - built) to 114 ± 6 HV1 (300 °C / 2 h). Compared to the significant hardness loss at 300 °C, annealing at 250 °C for 2 hours caused only a slight reduction to 133 ± 3 HV1. According to Fiocchi et al. (2021), the temperature of 250 °C is suitable for the reduction of residual stresses, with the advantage of preserving the cellular structure compared to treatments at higher temperatures. In contrast to annealing at intermediate temperatures (250– 350 °C), heat treatment at 160 °C led to a n improvement in both hardness and strength o f the alloy. The T5 condition used in this study (160 °C / 10 h) resulted in YS = 347 MPa and UTS = 520 MPa, which is considerably higher than the values reported by Roudnická et al. (2020) for a T5 condition at 140 °C / 26 h (YS = 300 MPa, UTS = 395 MPa). This indicates that the heat treatment parameters applied in this work were more effective in enhancing the mechanical performance of the LPBF AlSi9Cu3 alloy. Interestingly, not only the aging temperature of 160 °C but also the optimal aging time of 10 ho urs closely resemble the conventional artificial aging conditions used for HPDC AlSi9Cu3 alloys. This similarity is notable given the fundamentally different nature of the manufacturing processes (high-pressure die casting vs. laser powder bed fusion technology). Hardening of the LPBF AlSi9Cu3 alloy in the T5 condition is attributed to precipitation inside the α -Al cells (Fig. 3b), which contributes to the strengthening of the aluminum matrix. According to Roudnická et al. (2020), precipitation of Si parti cles and the strengthening θ’ phase was observed in LPBF AlSi9Cu3 after T5 treatment at 140 °C for 26 hours. It is therefore assumed that similar types of precipi tates are forming in the present study as well. Regarding T6 treatment, the hardness was comparable to the as-built state, despite the complete breakdown of the cellular network (Fig. 3f). The key difference between annealing at temperatures ≥ 300 ° C and T6 treatment is the different effect of temperature on the microstructure. At temperatures ≥300 °C, the original network composed of Si- and Cu-rich phases coarsens which leads to its gradual breakdown. Additionally, precipitates formed inside the cells at lower temperatures also coarsen, thereby losing their strengthening effect. Concurrently, the strengthening associated with the supersaturation of the solid solution is diminished. All these phenomena reduce the hardness. In the case of T6 treatment, the solution treatment (520 °C / 6 h) also causes the complete breakdown of the cellular network. However, subsequent water quenching results in a supersaturated solid solution by trapping a significant amount of alloying elements (Si and Cu) in the aluminum matrix. This supersaturation enables the alloy to fully utilize its precipitation hardening potential during the following artificial ageing step. In the present study, the AlSi9Cu3 alloy subjected to T6 treatment exhibited YS, UTS, and A of 315 MPa, 402 MPa, and 4.5%, respectively. These values are compar able to those reported by Roudnická et al. (2020), who applied the same T6 conditions (520 °C / 6 h, 160 °C / 12 h), and achieved YS of 326 MPa, UTS of 380 MPa, and A of 2.6 %. In the study by Fiocchi et al. (2020), the AlSi9Cu3 alloy subjected to a modifi ed T6 heat treatment (470 °C / 6 h, 160 °C / 24 h) exhibited a YS of 206 MPa, which is approximately 100 MPa lower compared to the present study. The significantly reduced YS may be attributed to two main factors: (1) insufficient dissolution of the Al₂Cu phase during solution treatment at 470 °C, which is near the lower limit of its dissolution range (460 – 530 °C according to Lombardi et al. (2019) , making 520 °C a more effective temperature for homogenisation; and (2) potential overageing of the alloy due to the prolonged artificial ageing time (24 h), likely resulting in a coarsening of precipitates and a consequent reduction in precipitation strengthening efficiency. Although the T6 treatment utilizes the precipitation potential of the alloy, both YS and UTS were lower compared to the T5 condition in this study. This indicates that the original cellular microstructure with an interconnected network is a dominant contributor to strength and cannot be fully compensated by conventional T6 heat treatment. Therefore, direct ageing (T5 treatment) applied to the as-built state appears to be a particularly promising heat treatment strategy for enhancing the strength of LPBF AlSi9Cu3 alloy.

5. Conclusions In this study, LPBF AlSi9Cu3 alloy was manufactured to investigate the changes in microstructure and mechanical properties after different heat treatments. The main conclusions can be drawn as follows: