Debinding-sintering Fused Filament Fabrication Of 316L Stainless Steel And Copper Admixing Modification

Fused Filament Fabrication(FFF)is widely used in 3D printing of polymer materials,and has been gradually used to manufacture metal and ceramic parts in recent years,as a debinding-sintering additive manufacturing technology.The required equipment is simple,and can be operated flexibly,being suitable for printing a variety of materials.However,there are few studies on the corresponding binder formulation.After mixing metal or ceramic powder with binder polymer,the extruded filament is easy to fracture,so that the continuity of printing with FFF printer can not be guaranteed.The sintering process after debinding is affected by many factors,and it is difficult to control the final density and dimensional accuracy of parts.Under the condition of no external pressure,density of sintered parts is difficult to be increased.The porous structure of metal sintered parts often leads to poor corrosion resistance,which further limits the application of this technology.In this paper,focusing on the defects of metal materials during processing,a suitable binder formulation was developed,aiming to manufacture 316L stainless steel parts.Based on this formulation,the influence of powder particle size and copper powder admixing was studied,the effective debinding processes were designed.The effects of sintering atmosphere,sintering temperature,sintering time and heating rate on the microstructure and properties of sintered parts were studied.Local and macroscopic densification models were established by Monte Carlo method and finite element method respectively.The effects of copper admixing on densification,phase transformation,mechanics,antibacteria and corrosion resistance were studied.The main research work is as follows:(1)The orthogonal test method was used to screen and optimize the appropriate binder formulation.The volume ratio of each component was set as VPP:VSEBS:VPW=5:2:2.The suitable mixing,filament extrusion,printing and debinding processes have been developed.It is found that the strength and toughness of fine-powder filament is higher than that of coarse-powder filament.Copper admixing will reduce the filament strength,and the increase of powder loading will lead to filament toughness decreasing.Large particle size and high powder loading are favorable for solvent debinding and thermal debinding.(2)The effects of pure argon,pure hydrogen and argon-hydrogen mixture atmosphere on sintered parts were compared.The oxide can be reduced better and the original shape of green parts can be kept well in pure hydrogen.The effects of sintering temperature,sintering time,heating rate,powder particle size and powder loading were studied.The optimized processing parameters obtained by intelligent algorithm were in accordance with that obtained by control-variate analysis.(3)Monte Carlo sintering model was used to display the effects of diffusion on densification and grain growth during sintering.The process of large grains devouring small grains and gradually discharging pores was simulated,which explained the phenomenon of higher density in the edge area of sintered parts.The finite element model calculation based on continuum mechanics and rheology theory shows that two sintered parts with different powder loading may obtain similar density through creep due to the viscosity difference.When the powder particle size is small enough,the sintering process will be greatly accelerated.Both the local and macroscopic models explain well the rapid densification process.(4)The influence of copper powder content on the final properties of sintered parts was studied.The maximum tensile strength and elongation can be achieved by admixing no more than 2.182wt.%copper.From diffusion point of view,copper admixing can improve sintering activity of 316L stainless steel powder and promote sintering process.Adding copper can make sintered parts obtain excellent antibacterial performance,but the influence on corrosion resistance is complicated.Solid solution copper can improve the corrosion resistance,but excessive copper will damage the passivation film of stainless steel.It was found that the copper content should not exceed 2.182wt.%.In this paper,the appropriate binder formulation was screened and optimized through systematic evaluation methods.Metal parts with porous structure or relative density up to 0.98 were successfully fabricated by using modified desktop 3D printer and post-processing technology.The mechanical strength and corrosion resistance of the compact metal parts was improved by admixing copper,which also makes the parts obtained excellent antibacterial properties,so that the parts may meet the requirement of personalized application in biomedical field.The research methods and conclusions in this paper will provide development ideas for binder formulation designing,and provide theoretical support for optimizing sintering process and enhancing the comprehensive performance of the parts manufactured by the debinding-sintering fused filament fabrication.