Fig. 1. Feedstock powder and CSAM process: (a) surface morphology and (b) size distribution of pure Cu powders used for CSAM; (c) schematic sketch of CSAM process and the photos of standard tensile test samples.

Fig. 2. Multi-scale microstructure observation of the CSAM Cu. (a) OM of the etched sample showing the severely deformed particles; (b) 3D EBSD-IPF image illustrating the heterogeneous grain structure; (c) TEM thin-foil showing the mixture of nano and micro-scale grains; (d-g) magnified views of the EBSD-IPF image (d), grain boundary maps of the LAGB, HAGB and TB (e), KAM image indicating the dislocation density (f), grain size distribution (g); (h-j) TKD analysis of the IPF (k) and KAM (l) maps.

Fig. 3. XRD results of the CSAM Cu sample for the dislocation density analysis.

Fig. 4. Tensile property of CSAM pure Cu. (a) strain-stress curve of the CSAM Cu obtained in this work, with the inserted photo showing the tensile smaple before and after fracture. (b) A comparison of the mechanical properties with the previously reported CSAM Cu by various researchers [27], [28], [29], [30], [31] (CS-AS), CSAM Cu after annealing [32] (CS-annealed), bulk Cu materials with nanocrystalline structures produced from the processes like SMAT [33], ECAP [34], electroplating [35], laser and electron beam additive manufacturing [36], [37], [38], [39].

Fig. 5. Fracture morphology observation of the CSAM Cu. (a, b) Overview surface morphology of the fractured sample. (c) Etched cross-section morphology of the fractured sample observed by OM. (d) EBSD-IPF map revealing the cross-sectional grain structure of the fractured sample, which is acquired at the step size of 100 nm. (e-g) Bright-field TEM images of the cross-section regions of the fractured sample after 45% strain.

Fig. 6. XCT reconstruction of the fractured tensile sample exhibiting the fracture morphology and high densification rate.

Fig. 7. Multi-scale characterizations of the deformed single particle. (a) Surface observation of the deposited particle in the substrate. (b) SEM of the cross-section region after FIB milling prepared for the following characterizations. (c) EBSD IPF map of the overview grain structures at particle cross-section, which is acquired with a step size of 100 nm. (d, e) EBSD IPF maps of the magnified views for the grain structures at the bottom and peripheral regions, respectively, which are acquired with a step size of 30 nm. (f-i) Bright-field TEM images of the thin foil sectioned from FIB milled sample from figure b. (f) Bright-field TEM revealing the elongated grains with shear bands. (g) Pile-up of high-density dislocation. (h and i) elongated ultrafine grains at the bottom and peripheral regions, respectively.

Fig. 8. Formation mechanism of the CASM deposit with unique infinite loops of extremely refined nano-grained structure: (a) a raw particle with uniform micro-grained structure, (b) the deformed particle during CSAM with gradient nano-grain structure upon impact, (c) the bonding of many deformed particles, (d) the final heterogeneous microstructure in the CSAM deposit.