Objective As the production of powder metallurgy parts develops in the direction of miniaturization and precision, the processing technology for powder metallurgy parts also has higher requirements. Powder compaction is an important part of powder metallurgy, with dynamic compaction technology currently being a primary area of research focus. However, existing dynamic compaction methods face several limitations, including challenges in controllability, achieving uniform density distribution, and applicability in the preparation of microparticle compression blanks. Laser impact is used to conduct an experimental study on the dynamic compaction of Cu-Ni composite nanopowder, and a numerical simulation is combined with the molecular dynamics (MD) method to provide a new method for preparing microparticle blanks by dynamic compaction. Methods A neodymium-doped yttrium aluminum garnet (Nd∶YAG) laser is used to complete this dynamic compaction study on a self-designed experimental platform. An optical microscope, scanning electron microscope, and Vickers hardness tester are used to study the relative density of the compaction billet, surface morphology, microstructure, and mechanical properties, and to explore the effect of laser energy and Cu-Ni mass fraction ratio on the relative density of the compaction billet. Changes in particle shape, lattice evolution, and dislocation slip behavior during dynamic compaction by laser impact are investigated using the MD method. Results and Discussions The relative density of the compacted copper-nickel hybrid powder increases with increasing laser energy. This is because, with an increase in laser energy, the compaction impact force gradually increases, forcing the particles to undergo displacement and plastic deformation, thus filling the pores. In addition, the relative densities of the mixed powders after compaction increase with increasing Cu content. This is attributed to the lower hardness of Cu, which makes it more susceptible to deformation during compaction [Fig. 5(b)]. As the copper content increases with laser energy, the interparticle pores become smaller, and several of the particle boundaries are blurred, with nearly no pores, forming tight junctions. This is due to the low hardness of copper which is prone to plastic deformation. As the laser energy increases, the main energy is absorbed by the particle boundaries and then dissipates in the form of plastic deformation, inter-particle friction, kinetic energy, and defects, which leads to internal heating, localized melting, or solid-state diffusion bonding (Fig. 6). The microhardness of the central region of the billet is significantly higher than that of the edge region. This is because the distribution of the powder is still uneven, as well as the nonuniform propagation of the shock wave leads to an uneven density distribution of the billet, causing uneven microhardness distribution, which ultimately affects the mechanical properties of the billet. The overall hardness value of the billet of the hybrid powder demonstrates a decreasing trend with increasing Cu content. This may be due to the lower hardness of Cu compared to that of Ni, and more Cu dominates the hardness of the final billet (Fig. 7). At the particle junction, the hexagonal close-packed (HCP) structure and dislocations are generated. This suggests that the pressure applied during the relaxation phase causes certain physical or chemical reactions of the Cu/Ni particles near the contact point, inducing plastic deformation, generating high local stresses and microstructural changes, and facilitating impact compaction. This corroborates the fact that pre-pressing can change the initial morphology of the powder, thus favoring the relative density improvement of the powder-compacted billet (Fig. 11). Once the compaction pressure exceeds 2.5 GPa, the relative density of the Cu-Ni composite nanoparticle compacted blanks increases slowly. This results in strain hardening, and the number of crystal structures tends to balance, indicating that the plastic deformation of the particles is blocked, necessitating larger force compaction (Fig. 13). At 109/s strain rate, when the compaction time is 300 ps, the grains produce an obvious cross-slip phenomenon, and compression rod dislocations are formed at the junction of cross-slip, and the compression rod dislocations are fixed inside the grains, which hinders the dislocations from further slipping and produces macroscopic strain hardening during the compaction process, leading to the slow growth of relative density of the compacted billet (Fig. 14). An adiabatic temperature rise is generated during compaction; however, the powder temperature does not exceed the melting points of Cu and Ni, and a relatively stable crystal structure is formed between the particles, suggesting that the lower diffusion activation energy and higher atomic activity contribute to the cold welding behavior of the Cu and Ni nanoparticles (Figs. 13 and 15). Conclusions The relative density of the final Cu-Ni nanoparticle blanks gradually increases with increasing laser energy and copper content. At 1.8 J laser energy, the relative density of compacted blanks mixed with 75%Cu-25%Ni particles reaches 78%. The microhardness of the billet is high inside and low at the edges, and the overall hardness of the billet decreases as the Cu content increases. Relaxation produces preliminary plastic deformation and dislocation between the composite particles, which verifies the importance of pre-compression for dynamic compaction. Compaction rod dislocations are generated between the nanoparticles, which curtails further slippage of the dislocations, resulting in strain hardening and a slow increase in the relative density of the compacted billets. The main joining mechanism of the Cu-Ni nanoparticle-compacted blanks is the formation of a strong connection via cold welding. © 2024 Science Press. All rights reserved.
