Study On Sintering Kinetics Of Nano-copper Powder And Atomic Scale Simulation

Powder metallurgy is a near-net-shape process for manufacturing high-tech materials.It can effectively control the structure and composition of materials,and can obtain products with specific microstructures and good performance.Sintering is the last step in the entire powder metallurgy process,which plays a decisive role in the microstructure and mechanical properties of the entire sintered product.So far,most of the development of sintering process has been carried out based on experimental methods,which is time consuming and laborious.It is difficult to dynamically characterize the evolution of the microstructure of the sintered compact by experimental methods.For this reason,the design of the powder sintering process and the quantitative prediction of the microstructure urgently require the establishment of computer modeling and multi-scale computer simulation of powder sintering.This has important guiding significance for the preparation and microstructure control of metal nanostructured materials.In order to understand the sintering mechanism and sintering kinetics of nano-scale copper powder,the solid phase sintering of copper powder with an average particle diameter of 100 nm was first studied.The linear shrinkage of nano-copper powder samples at different temperatures was measured.Surface morphology of nano-copper powder sintered samples was observed by scanning electron microscopy.The results show that the nano-copper powder has been partially formed after incubation at 400? for 2 hours,and a sintered neck has been formed between some of the particles observed by scanning electron microscopy.After holding at 900? for 2 hours,the shrinkage curve was converted into a density curve.The density of the finished sample reached about 93%.Due to the size effect of the nanoparticles,copper nanoparticles have large specific surface area and high surface energy.so that powder particles can be bonded at low temperatures.When the temperature reaches 400?,a sintering neck is formed between some particles.When the temperature reaches 900?,all grains in the nano-copper sample have grown,and individual grains have coarsened and eventually reached the micron size.In the sintering process,in addition to surface diffusion and grain boundary diffusion,there is also a viscous flow of the substance.In order to characterize the evolution of the microstructure of the nano-scale copper powder during sintering,this thesis simulates the sintering behavior of a single nano-copper particle on the copper plate substrate,the influence of the grain boundary between the sphere and the plate,the sintering temperature and the particle size were considered.In order to obtain the melting point of nano-copper,the melting process of nano-copper particles was simulated by continuous heating process.The results show that when the diameters of the nano-copper particles are 4.329 nm,6.588 nm,and 8.055 nm,the simulated melting points are respectively 1277 K,1318K,1343 K,which are all lower than the melting point of bulk copper at 1357.77 K.The simulation results of the solid-phase sintering of the nano-sphere-plate model show that the sintering of sphere and the plate samples with large-angle grain boundaries will cause the grain boundaries between the Sphere-plate to migrate into the spherical grains,leading to rearrangement of atoms in the sphere.Pre-existing grain boundaries accelerate the growth of the sintering neck.After sintering,the sinter neck of the spherical plate sample with large angle grain boundary is larger than the sample without grain boundary.The effect of sintering temperature was also studied.As the sintering temperature increased,the sintering neck became longer.It is found that when the temperature reaches 1073 K,the grain boundaries between the sphere and the plate begin to migrate into the sphere;on a microscopic scale,sintering consumes surface energy to establish the bonding between the particles.Among them,small particles have a larger surface energy than large particles,so they have faster sintering speed.At the same time,the grain boundary migration rate in the sample will gradually increase as the particle size decreases.In order to study the solid-phase sintering behavior of nano-copper powder compacts,the discrete element method was used to simulate the packing process of copper powder particles.Based on this,an atom-scale nano-copper powder model was established and its solid-phase sintering was simulated by molecular dynamics.The results show that the relative density of the simulated sample increases linearly with the sintering time during the sintering process and slowly increases during the later stage of sintering.The simulation results have a similar trend to the sintering kinetic curve obtained during the sintering experiment of nano-copper powder.It is shown that the sintering kinetics of the nano-copper particles in the simulation process is basically consistent with the experiment.After the sintering is completed,a small amount of pores in the microstructure of the sample are rounded,and some of the particles are rotated to cause the grain boundaries to disappear.The driving force of solid-phase sintering of nanocopper powder is the grain boundary energy between particles and the particles have high surface energy.During the sintering process,the nanocrystals grow gradually,with a small amount of crystal rotation.As the formation of sintering necks or grain boundaries between the particles,a large number of dislocations are found on the grain boundaries.Dislocations,as a fast channel for diffusion,also contribute to sintering densification to a certain extent.