PSI - Issue 43

Tibor Varmus et al. / Procedia Structural Integrity 43 (2023) 184–189 Tibor Varmus / Structural Integrity Procedia 00 (2022) 000 – 000

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Table 2. Chemical composition of Al2024-RAM2 alloy according to the SPECTRO analysis. Element Si Fe Cu Mn Mg Zn Ti Al Wt. % 0.12 0.01 3.68 0.57 1.47 0.02 2.43 balance

Fig. 1. (a) view of miniature specimen orientations with respect to build direction; (b) scheme of individual specimen test method.

Each set consisted of 16 specimens that were tested in their as-built state (i.e. no surface post-processing) to determine anisotropy of the fatigue behavior. One additional set was hand polished for reference. Microstructure of the flat surface where the fatigue cracks initiate was observed on longitudinal section after classic metallographic preparation (grinding and polishing) using an OM Zeiss Axio Observer Z1M. The specimens for structure analysis were etched with Dix-Keller and then Weck-Al (W-Al) reagent; both for 10 s. Details of microstructure were analyzed by means of a scanning transmission electron microscope TALOS (Thermo Fischer Scientific) using high-resolution STEM and TEM imaging. The surface roughness of the flat surface where cracks initiate was characterized with a SA6220 profilometer (SAMA tools Italy). Rockwell B hardness (HRB) tests were performed on as-built specimens using Agroup 206 EX machine.

3. Results and discussions 3.1. Microstructural analysis

The characteristic microstructure of the Al2024-RAM2 alloy after fabrication by L-PBF and after T6 post heat treatment is shown in a 3D cube representation in Fig. 2a. No hot cracks were identified. Distinct melt pools characteristic for L-PBF technology were not observed due to the applied heat treatment. Large light and dark coarse grains (CG) of solid solution α -Al phase characterize the microstructure where different color of grains is related to different orientations of individual grains. The areas that are formed by the very fine grains (FG) of the α -Al phase have the same shade, Fig. 2b, and can be well recognized from coarse grains. Very fine nanoparticle clusters based on Ti and La are present similarly in both CG and FG regions of microstructure, Figs. 2d and 2e. STEM mapping distribution of boron, Fig. 2e, in the matrix copies the distribution of Ti and La ceramic nanoparticles. Based on our present observations and those of Mair et al. (2021), TiB 2 ceramic nanoparticles are most probably the grain-refining agent, which was added to the base Al2024 alloy. The presence of Cu-rich phases such as Al 2 Cu and Al 2 CuMg was confirmed on the fine grain boundaries. Average size of CG is in the range of 100 to 150 μm compared to an average size of the FG, w hich is of 1.5 to 2 μm, Figs. 2b and 2c . Reinforcing particles based on Cu and Mn were also observed, Fig. 2c.