The Preparation Of 3D Printing Nano-copper And Molecular Dynamics Simulation Of Powder Sintering

Copper is an important strategic material.The wide application of 3D printing technology in this field will greatly promote the development of copper and copper alloy industry.Because of the high thermal conductivity and high reflectivity of copper,3D printing of copper products is difficult and high power consumption.Reducing the print melting point of copper by using the size effect of nanomaterials is an effective way to solve the problem of laser sintering of copper.Therefore,the experimental preparation of nano-copper powder for 3D printing and the simulation of sintering evolution were carried out in this paper,which was of great practical significance for accelerating the technological development of 3D printing of copper.Firstly,the preparation of 10 nm spherical nano-copper powder based on the liquid phase reduction method was studied.The choice of reductant and protective agent in the preparation process played an important role on the size,shape and dispersion of nano-copper powder.Spherical nano-copper powders could be prepared by using hydrazine hydrate as reducing agent,and the composite protective agent of stearic acid and PEG was the key factor to control the size,morphology uniformity and dispersion of copper powders.In this paper,spherical nano-copper powders with good dispersion were prepared by using ethanol as solvent,hydrazine hydrate as reducing agent,CuCl2 as oxidant,stearic acid and PEG as protective agent(mass ratio less than 1).Ultrafine nano-copper powder of 10 nm level has high surface activity.It is difficult and costly to study its 3D printing laser sintering by experimental means.In this paper,it was continued to simulate the sintering evolution process of nano-copper powder by molecular dynamics,focusing on the effects of particle size,sintering temperature,sintering time and other factors on the sintering evolution of nano-copper powder.The results showed that the theoretical melting points of 2 nm,4 nm and 8 nm nano-copper powders are 985 K,1233 K and 1341 K respectively,which verified the size effect of nano-copper powders.Sintering temperature was the most important factor affecting the sintering evolution of nano-copper powder,followed by particle size,while sintering time had little effect on the structure evolution of sintered body.The structural evolution of the surface and sintering neck of copper powder particles was the most obvious.The sintering process of nano-copper powder had undergone three stages:sintering neck formation,growth and structural stability.When the structure of sintered body reached dynamic stability,there would be alternate evolution of non-FCC structure and FCC structure in the sintered body.It was inferred that the structure evolution process of nano-copper powder sintering melting was FCC?BCC?HCP?disordered structure.Finally,based on molecular dynamics simulation,theoretical characterization and analysis of the densification of nano-copper powder sintered were carried out.The results showed that,the densification of sintered body increased rapidly in the first 20 ps of constant temperature sintering stage,and tended to be stable after 20 ps.In the cooling sintering stage,the density of the sintered body increased further,and the densification changed linearly with the sintering time.Sintering temperature and particle size were the key factors affecting the densification of sintered body.The smaller the particle size of copper powder was,the smaller the influence of sintering temperature on the densification of sintered body.For sintered copper powders with particle size over 2 nm(excluding 2 nm),the higher sintering temperature,the smaller particle size,the lower shrinkage rate and the higher densification.The densifications of sintered copper powders at 2,4 and 8nm were 97%,95%and 82.5%respectively after sintering 200 ps at a temperature lower than its theoretical melting point of about 200 K and then sintering 300 K at a rate of 1 K/ps.