A Study on the Compaction of Iron Nanopowder and Related Sintering Property

Abstract

For several decades, the sintering of nanoscale metal powders has been of great interest because the full-density sintered part with uniform and fine microstructure can be achieved by low temperature. However, there are still difficulties in achieving full density and suppressing grain growth, which are primarily caused by poor compactability due to the high frictional force of nanopowder. Also, the metal nanopowder generally has a strong tendency to form agglomerate caused inhomogeneous porosity in compacts, consequently leading to defects such as voids and cracks. Low compactability of nanopowder and inhomogeneous pore in the compacts hinder full densification during pressureless sintering. To solve technical problem of compaction in this study, the agglomerate shape was controlled to be spherical for enhancing flow and packing of agglomerate and also, the size distribution in the agglomerate was controlled by heat-treatment to improve the packing efficiency during compaction process. The compaction behavior of fabricated nanopowder was compared with conventional nanopowder. Moreover, the pressureless sintering behavior of Fe nanopowder was investigated and the mechanical properties of sintered Fe nanopowder were evaluated for feasibility study to apply the powder metallurgy process. The compaction behavior of bimodal Fe nanopowder agglomerate was investigated in terms of the particle packing and microstructural development during compaction process. The Fe nanopowder agglomerate was fabricated by hydrogen reduction process of spray-dried Fe2O3 nanopowder. The fabricated Fe nanopowder had a uniform and spherical shape with diameter range of 1-5m and a bimodal particle size distribution consisting of 30 and 300nm with a porous structure in agglomerate. In die compaction experiment, it was found that the bimodal Fe nanopowder agglomerate had higher green density than conventional Fe nanopower. Especially, the compaction density reached 80%TD at a compaction pressure of 1000MPa. It was revealed that smaller nanoparticles were located between and around the larger particles which were uniformly close-packed structure. To investigate the deformation behavior of agglomerate at the low pressure range, a continuous compaction experiment was conducted. It was revealed that the low agglomerate strength of bimodal Fe nanopowder agglomerate was advantageous in particle rearrangement and higher packing efficiency by agglomerate breakage at the initial state of compaction. Based on the results of the compaction behavior of bimodal Fe nanopowder, the sintering property was evaluated. The heat-up and isothermal sintering were performed by using the compacts of 80%TD. It was found that near full densification to 97.1%TD was achieved at heat-up sintering up to 1000oC with linear shrinkage of 7%. To fulfill the requirement of full density and grain refinement effect, the isothermal sintering was carried out at 900oC for 0-4h. It was found that the 900oC/2h sintered specimen showed near full density of 97.5%TD with grain size of less than 600nm. From the tensile test, the 900oC/2h sintered specimen showed a tensile strength over 500MPa with elongation of 10%. This remarkable mechanical property of the pure iron nanopowder despite without carbon or any alloying element is well construed in terms of grain refinement effect. In conclusion, these results provide a promising route for fabricating high strength pure iron nanopowder by pressureless sintering and without carbon and alloying element. By using the compacts of 60%TD, the densification kinetic was investigated during heat-up sintering process. The apparent activation energy for densification was calculated in the range from 50.7 to 90.6kJ/mol. At the initial stage of sintering, activation energy was increased up to 90.6 kJ/mol at sintered density of 70%TD, and then gradually decreased to about 55kJ/mol indicating grain boundary diffusion of pure iron. In the microstructural investigation, the homogeneously packed bimodal particles in compacts evolved in locally dense structure like agglomerates in initial sintering stage. Such a gliding motion of agglomerates seems to be responsible for increasing activation energy. During densification, most of fine particles, which are located along the surface and boundaries of coarse grains, form hierarchical grain boundary structure, leading to near full-density with restraining grain growth. To improve compactability of incompressible spherical micropowder, the commercial Fe micropowder was mixed with high compactable bimodal Fe nanopowder and then, the densification behavior of Fe trimodal type powder was investigated to apply the commercial powder metallurgy process. In the compaction experiment, it was found that the wet-milling process provided homogeneous mixing structure and successfully fabricated sound compacts. Moreover, the trimodal powder had high green density of about 74%TD at conventional die compaction pressure of 600MPa. From the results of sintering process, compact of trimodal powder can reach near-full density at 1250oC for 3h with almost the same grain size less than 8m of initial microparticle size. This finding implies that the bimodal nanopowders are enhancing densification as well as suppressing grain growth due to the nanopowder pinning effect on grain boundary migration. The mechanical property of trimodal powder showed higher tensile strength than micropowder and sinter forged pure iron. This outstanding mechanical property of the sintered Fe trimodal powder is well explained by their full densify as well as grain refinement effect. It is concluded that the bimodal Fe nanopowder agglomerate can provide a breakthrough in solving compaction and full densification problem of nanopowder and incompressible micropowder.