Fig. 1. Schematic of the challenges in LPBF processed Al-Zn-Mg-Cu alloys (a) and the synergistic grain-refining strategy for solidification structure control (b-d). (a) Diagram of a typical columnar grain structure of LPBF processed Al-Zn-Mg-Cu alloys without any modification. (b) Diagram of grain refinement induced by NPs growth restriction. (c) Diagram of grain refinement controlled by powerful segregating elements. (d) Diagram of the synergistic grain-refining strategy including heterogeneous nucleation, solute-driven growth restriction and NP-induced growth restriction.

Fig. 2. SEM photographs of the Al-Zn-Mg-Cu alloy powders (a), TiC NPs (b), TiH2 particles (c) and functionalized powders (d). Schematics of the LPBF process (e), scanning strategy applied in LPBF (f) and schematic depiction of the built samples (g).

Fig. 3. OM images of Al-Zn-Mg-Cu alloy (a), A1 sample (b), A2 sample (c), B0 sample (d), B1 sample (e) and B2 sample (f). (g) Crack density and relative density of the as-built samples. OM images of AB1 sample (h) and AB2 sample (i).

Fig. 4. SEM images showing typical grain microstructures of the XOZ cross-sections of the as-built alloys: (a) Al-Zn-Mg-Cu alloy, (b) A1 sample, (c) A2 sample, (d) high magnification of A2 sample marked in (c), (e) B0 sample, (f) B1 sample, (g) B2 sample, (h) high magnification of B2 sample marked in (g), (i) AB1 sample, (j) AB2 sample and (k) high magnification of AB2 sample marked in (j).

Fig. 5. IPFs and corresponding grain size distributions (a-d) and PFs (e-h) of LPBF processed Al-Zn-Mg-Cu alloy (a, e), A2 sample (b, f), B2 sample (c, g) and AB2 sample (d, h).

Fig. 6. XRD patterns of the as-built samples: (a) Al-Zn-Mg-Cu alloy, (b) A2 sample, (c) B2 sample and (d) AB2 sample.

Fig. 7. SEM images showing typical phase morphologies of the as-built samples after etching by NaOH reagent, with embedded EDS results of corresponding cuboidal particles: (a) Al-Zn-Mg-Cu alloy, (b) A2 sample, (c) B2 sample and (d) AB2 sample.

Fig. 8. TEM results of the as-built A2 sample: (a) high-angle annular dark field (HAADF) image, (b) EDS elemental mappings, (c) EDS analysis of TiC NPs, (d) SAED pattern of TiC and (e) HRTEM image of the α-Al/TiC interface with the corresponding fast Fourier transform (FFT) pattern.

Fig. 9. TEM results of the as-built B2 sample: (a) HAADF micrograph, (b) EDS elemental mappings, (c) EDS analysis of L12-Al3Ti particles, (d) SAED patterns of L12-Al3Ti particles and (e) HRTEM image of the α-Al/L12-Al3Ti interface with the corresponding FFT patterns.

Fig. 10. TEM results of the as-built AB2 sample: (a) HAADF micrograph, (b) EDS elemental mappings, (c) HRTEM image of the α-Al/TiC interface with the corresponding FFT patterns, (d) HRTEM image of the α-Al/L12-Al3Ti interface with the corresponding FFT patterns and (e) SAED patterns of the L12-Al3Ti particles.

Fig. 11. Mechanical properties of the LPBF processed alloys: (a) stress-strain curves of LPBF processed Al-Zn-Mg-Cu alloys and AB2 samples, with the inset table showing average values of UTS, YS and EL, (b) a comparison of the mechanical properties of the AB2 samples with other AM 7xxx series aluminum alloys and wrought 7xxx alloys previously reported [3, 28, 41, [43], [44], [45], [46], [47], [48]]. Fracture morphologies of the as-built Al-Zn-Mg-Cu alloy (c), T6 Al-Zn-Mg-Cu alloy (d), as-built AB2 sample (e) and T6 AB2 sample (f).