PSI - Issue 71

R. Anil Kumar et al. / Procedia Structural Integrity 71 (2025) 302–308

305

fine melt pool zone and there exists region in the previously solidified layers, where the temperature is above the solidus and below liquidus temperatures and α -Al converts to coarse globular morphology and silicon network is also coarsened as evident in Fig. 2c. In the adjacent regions of previously solidified layers, where temperature is below solidus temperature, silicon rich network is broken down into globular structures constituting HAZ.

Fig. 3. Secondary electron SEM micrographs of directly aged AlSi10Mg at 245 °C for (a) 30 mins, (b) 60 mins, (c) 90 mins and (d) 120 mins. (e) SEM micrograph of AlSi10Mg underwent T5 treatment.

Fig. 3 (a-d) shows microstructures of LPBF AlSi10Mg which were heat treated at 245 °C for 30, 60, 90 and 120 minutes. It is observed that the silicon rich network remained interconnected in sample 41 and started breaking down in samples 42 and 43 which were aged for 60 and 90 minutes respectively. In sample 44, which was aged for 120 minutes, silicon rich network is breaking down and started to form globules more evidently. During T5 stress relieving treatment at 300 °C, silicon-rich network is observed to be significantly broken down to silicon globules as widely reported in the literature (Fig. 3e). It is reported in literature that nano-sized silicon particles which were initially precipitated during the solidification grows in size during the T5 treatment. It is observed in sample SR (T5 treatment, Fig. 3e) in line with literature and is also evident in sample 44 (Fig. 3d).

3.2 Mechanical properties

Micro Vickers hardness of the as-built sample is higher (121 HV) than heat treated samples due to solid solution strengthening and residual stress accumulated in the sample (Fig. 4a). Higher cooling rates of 10 6 – 10 8 °C/s encountered during laser powder bed fusion, apart from accumulation of higher residual stresses, a highly super saturated solid solution of aluminium with silicon and magnesium is formed. T5 treated sample showed lower hardness (85 HV) due to the combined effect of stress relieving, breaking down of the Si network and precipitation of excess solute atoms. Direct aged samples showed a slight decrease of 8 – 12 % hardness when compared to that of as built sample which is attributed to stress relieving. Sample aged for 2 hrs at 245 C has shown slightly higher hardness (114 HV) compared to others aged for lower times and it may be due to the presence of more globular silicon particles in the microstructure. From commercialisation point of view, lower time leads to better cost economics and hardness