Preparation Of Coated Aluminum Powder With Low Laser Reflectivity And Its Study On 3D Printing

SLM is a metal 3D printing technology.It can fabricate parts through melting the powders in the form of point-by-point and layer-by-layer scanning mode and the rapid solidification of the melt.Compared with the traditional processing technology,SLM has the advantages of high degree of freedom design,fine and precise forming and high material utilization rate.At present,SLM has been applied in stainless steel,titanium alloy,cobalt-chromium alloy and other metal materials.Their printed components are gradually used in aerospace,biomedical and other fields.However,in the case of aluminum and its alloys,metallurgical defects,such as unmelted powder and pores,are formed during the printing process because of the very high laser reflectivity of aluminum powder.These lead to low forming accuracy and poor mechanical properties of Al parts fabricated by SLM,which seriously limits the development and application of SLM in the field of aluminum alloy manufacturing.Aiming to address the above issue,this thesis proposed a new idea to strengthen the interaction between aluminum powder and laser through the powder surface modification technique.Namely coating high-laser-absorptivity phase on the surface of aluminum powder to reduce its laser reflectivity,eliminate unmelt powder and pores during the printing process.This is likely to improve the density,forming precision and mechanical property of the printed samples.This study focused on that the phase composition and distribution regulation mechanism of the coating layer on aluminum powder,the interaction behavior between the coated powder and laser,the microstructure evolution and regulation mechanism of non-equilibrium solidification process,and the internal relationship between microstructure and properties.Through the above deep research,the innovative findings were achieved are as follows:(1)The homogeneous nucleation and growth mechanism of electroless high-laser-absorptivity nickel and cobalt plating on the surface of aluminum powder was studied.The composition ratio of complexing agent and metal salt is the key factor that affects the distribution of nickel and cobalt coated phases.When the molar ratios of the complexing agent to salt were controlled at 0.75 and 1.0,the coated phase had a good uniformity of distribution.The laser absorptivity of aluminum powder was therefore significantly enhanced after coating the nickel/cobalt phase.Compared with cobalt,nickel-coated phase has a more significant laser absorption enhancement effect.When the nickel-content reached 2.5 wt.%,the reflectivity of aluminum powder at 1070 nm laser wavelength decreased from 60%to 35%.Further study showed that the fluidity of the coated aluminum powder was not significantly affected,indicating that the coating phase does not affect the normal process of powder laying.(2)Fluidized bed chemical vapor deposition was proposed to uniformly coat carbon nanotubes and amorphous carbon on the surface of aluminum powder through nano-nickel particle catalysis and carbon source pyrolytic deposition,respectively.Firstly,the fluidization behavior of aluminum powder during coating was studied.It was found that the aluminum powder could be fluidized stably at the temperature of 550℃ and the gas velocity of 900 mL/min.The introduction of the two carbon coating phases significantly improved the laser absorption capacity of aluminum powder,and the effect of improving the laser absorptivity of aluminum powder is positively correlated with the carbon-coated content.In the fluidized bed reactor,carbon nanotubes catalyzed by nano-nickel particles were closely attached to the powder surface and exhibited a bended and encircled morphology,while the amorphous carbon formed by pyrolytic deposition presented a film-like structure on the powder surface.These two kinds of carbon coated phases have no serious damage to the sphericity and fluidity of aluminum powder,and the coated aluminum powders still maintain the initial printable shape characteristics.(3)The printability,forming quality,printed microstructure and mechanical properties of the nickel or cobalt coated aluminum powder and the carbon nanotube or amorphous carbon coated aluminum powder were evaluated.It was found that the interaction between laser and powder was significantly improved by nickel,cobalt or carbon materials coating phase.This promoted the melting of aluminum powder and improved the density and forming accuracy of the printed products.However,too high a coating content led to the evaporation and over-burning of aluminum powder,which seriously damaged the printing quality and forming accuracy.Further study showed that fine precipitates were formed in the printed microstructure of the above coated powders,which strengthened the aluminum matrix through dispersion strengthening effect or load transfer mechanism.Compared with pure aluminum,the samples printed from coated powders have better tensile properties.When the nickel-content was 0.5 wt.%,the tensile strength of the printed sample reached 182 MPa(152%higher than that of pure aluminum printed sample).When the content of carbon nanotubes was 0.96 wt.%,the tensile strength of the printed samples reached 152 MPa(85%higher than that of pure aluminum printed sample).Moreover,the above printed samples maintained the high ductility of pure aluminum.(4)The comparison of nickel,cobalt,carbon nanotubes or amorphous carbon coated aluminum powder showed that carbon materials coated aluminum powder has better laser reflectivity reduction effect.And metal coated aluminum powder had less influence on fluidity and sphericity of powder.In terms of the densification of powder printing and the improvement of forming quality:nickel>amorphous carbon>carbon nanotubes>cobalt;In terms of comprehensive mechanical properties improvement:carbon nanotubes>nickel>cobalt>amorphous carbon.In conclusion,the surface modification technology can uniformly introduce metal nickel and carbon nanotube coating phases on the surface of aluminum powder,which improves the printing quality and mechanical properties of aluminum powder.As a consequence,it is proved to be an effective way to solve the SLM issue of aluminum powder with high laser reflectivity.