Dynamic compaction (DC) technology generates a shock wave in the powder, allowing the final compact to achieve theoretical density in a very short time . The compacts formed by DC technogy have smaller porosity, more even density, and better mechanical properties than those formed by conventional powder metallurgy (PM) process. However, the study on dynamic compaction of micro-scale parts is relatively absent, and the current dynamic compaction technologies are unsuitable for the dynamic compaction on a micro-scale. The laser shock wave has the loading characteristics of high strain rate (106-107 s-1), controlled energy, and a small affected area. In this study, a novel approach for compacting powder on a micro-scale by laser shock wave is presented, which integrates the unique advantages of pulse laser with a high strain rate and is appropriate for micro-forming.
In the experiment, Nd∶YAG laser with the Gaussian distribution beam was employed to accomplish the dynamic compaction research on the self-designed experimental platform. The density, surface morphology, microstructure, and mechanical properties of the finished compact were investigated using an optical microscope, scanning electron microscope, and Vickers hardness tester. The effect of laser energy, powder morphology, and impact frequency on the copper compact’s density was examined.
Copper powder is compacted under different laser energies and the copper powder compact samples are ?2.5 mm×(0.2-0.15) mm in size.Figure 3 demonstrates that all blanks are complete and regular in shape with no evident crack defects and there is no flaking at the blank’s edge. With the laser energy increase, this phenomenon gradually reduces, and a compact with the best forming quality is generated at 1800 mJ laser energy. Figure 4 indicates that the pore in the compact also increases with the increase of laser energy, and it can be discovered that the pore size and distribution uniformity in the compact’s center and edge areas are accordant. Figure 5 reveals that with the increase of laser energy from 360 mJ to 1800 mJ, the compact’s relative density increases from 76.1% to 91.3%, and the trend of relative density increase slows down. The effect of three kinds of copper powder morphologies on the density of copper compacts is explained. Irregular copper compacts have good surface quality under 1800 mJ laser energy impact, whereas spherical powder compacts show an evident flaking phenomenon. According to the microstructure investigation of the cross-section of three various morphologies of copper compacts in Figure 7, there is an obvious plastic deformation of the copper particles and solid bridging between the particles in the spherical powder compacts. In addition to the pressure welding mechanism, there is an additional mechanical interlocking force between irregular copper particles. The maximum relative density of the irregular copper powder (15-20 μm) compact, spherical copper powder (0.5-2.0 μm)compact, and spherical copper powder (10-20 μm) compact is 91.35%, 94.35%, and 93.12%, respectively. Comparing the relative density and forming quality of copper compact, irregular copper powder is suitable for fabrication on a micro-scale part. The compact’s mechanical properties were examined. The finding shows that the average microhardness of the final copper compact increases to 67.0 HV, 82.8 HV, and 91.7 HV, respectively, with the increase of laser energy. This is because the powder material undergoes plastic deformation during shock wave propagation, which fills the inter-particle pore and causes strain hardening in the particles’ deformation area. The shock wave pressure increases with the increase of laser energy, increasing the compact’s strain hardening. The impact frequency also influences the final copper compacts’ relative density. Figure 10 reveals that the relative density of compacts increases with the impact frequency increases. The contribution of the first and the second impact to the relative density increase under the five laser energies are 3.03%, 4.77%, 3.23%, 1.75%, and 1.29% while the total contribution of the third and fourth impact is only 1.00%, 0.93%, 0.56%, 0.55%, and 0.75%, respectively. The best performance of the final compact can be generated under the double impact. Double impact of 360 mJ+ 1800 mJ laser energy was adopted to reduce the negative influence of stain hardening effect on the performance improvement of compact, and the relative density of the compact reaches 96.5%.
This study systematically examines the influence of process parameters, including laser energy, powder morphology, and impact frequency on relative density and mechanical properties of final compacts dynamically compacted using laser shock waves. With the increase of laser energy, the forming ability of copper compact gradually enhanced. The increase speed of relative density slows down due to the deformation resistance’s production. The compact’s microstructure investigation shows that the connection mechanism between irregular particles is solid pressure welding and mechanical internal locking, while the connection mechanism between spherical particles is only solid-state pressure welding. Thus, the irregular copper powder compact has relatively high connection strength and is not easy to generate defects, including crack and peeling, which is appropriate for the production of micro-scale parts. Multiple impacts (two or more impact times) can efficiently improve the densification of compacts, and the increase in densification is primarily contributed by the first two impacts. Double impact with first low laser energy and then high laser energy can reduce the influence of strain hardening on the improvement of compact properties.
