| In binder jetting(BJ)process,the printhead(often with a width of several centimetres)selectively jets billions of liquid binder droplets per second on the spreading powder bed to glue the particles and form the 2D geometry,which is then built up layer by layer to fabricate green metal parts.The high-performance metal parts could be obtained by post-processing,such as infiltration and sintering.BJ is expected to solve the current problems of high-cost and low-speed faced by additive-manufactured metal parts.However,the metal BJ technique faces problems such as low green strength,less sintering densification and inferior control of dimensional accuracy.Meanwhile,the influence of printing parameters on the parts quality and the micro structural evolution during sintering is ambiguous.Therefore,we prepared a binder with low binder content and high strength.The effect of printing parameters on the quality of BJ printed 316L stainless steel green parts,the effect of sintering temperature on the sintered density,porosity and mechanical properties,and the effect of hot isostatic pressing(HIP)and cold isostatic pressing(CIP)treatments on the enhancement of BJ 316L parts were investigated.The research is concluded as follows.(1)A binder with low binder content and high strength(42 wt.%A-stage phenolic resin,28 wt.%ethanol,30 wt.%ethylene glycol)were prepared to address the problem of low green strength.The effects of layer thickness,binder saturation and powder particle size distribution on the density,dimensional accuracy,surface quality and mechanical properties of the BJ printed 316L green parts were investigated.The results showed that the selfdeveloped binder’s viscosity,surface tension and density were 9.8 mPa·S,33.05 mN/m and 1.09 g/mL,respectively,which meet the piezoelectric printhead requirements.The binder saturation was reduced from 60～150%to 7.5～30.2%by using the self-developed binder.The binder provides strength by forming a solid C-stage phenolic resin bond neck between the powders.With the increase of layer thickness,decrease of binder saturation,and the wider powder particle size,the green strength decreased.This was due to the uneven binder distribution along the Z direction in green parts,resulting in poor interlayer bonding.When using a layer thickness of 100 μm,a binder saturation of 30%and a powder particle size of 0～30 μm,the compressive strength of green parts can reach 25 MPa.(2)A densification process consisting of pyrolytic debinding,solid-state sintering and HIP treatment was proposed to solve the problem of low sintered density.The effects of sintering temperature and HIP treatment on the density,shrinkage,microstructure and mechanical properties of sintered BJ 316L parts were studied.When the sintering temperature was increased from 1300℃ to 1380℃,the sintered density increased significantly,the pores became small and the shape changed from irregular to round.The tensile strength and elongation of the sintered samples increased rapidly,reaching 473 MPa and 40%,respectively,comparable to the metal injection molding(MIM)316L.There were three types of pores within the sintered BJ 316L part.The type Ⅰ and type Ⅲ pores impeded densification,while the type Ⅱ pores helped decrease the grain size and could be eliminated by press-less sintering.The numerous type Ⅲ pores distributed in the BJ 316L may originate from the poor packing of powders within the green parts,resulting in larger Z-shrinkage and difficulty in densification.Grain size and mechanical properties at different sintering temperatures correlate significantly with porosity:the ratio of pore diameter to grain size and porosity were linearly related,and the ultimate tensile strength was also linearly related to porosity.The HIP treatment reduced the porosity of the sintered part and improved mechanical properties by reducing the volume of individual closed pores.The tensile strength of sintering under a nitrogen atmosphere and then subjected to 1150℃/130 MPa/4 h HIP treatment reached 678.5 MPa,which exceeded the standard of forged 316L by 32%.This was due to the interstitial solid solution strengthening of the nitrogen element and the reduction in porosity.(3)A new process route comprising BJ printing,pre-sintering,CIP treatment and high temperature sintering was proposed to overcome the difficulty in dimensional control during sintering.The pre-sintering temperature of 1050℃ resulted in a sintered neck with an appropriate strength and plasticity to maintain the part shape,while the lower yield strength(35.96 MPa)facilitates particle rearrangement in the CIP treatment.As the CIP pressure increased,the pre-sintered part density increased,linear shrinkage became larger,porosity decreased and microstructure became more homogeneous.The relative density of the presintered parts increased from 62%to 80%when 350 MPa pressure was applied.The densification behaviour of CIP pre-sintered parts at different pressures was investigated.The higher the CIP pressure,the lower the sintering temperature required to achieve the same sintered density.After CIP treatment at 350 MPa pressure,the sintering temperature required to achieve 94%density was reduced from 1400℃ to 1300℃,while the sintering line shrinkage was reduced from 11.5%to 4.9%.This will reduce the risk of distortion during the sintering of large,complex-shaped B J metal parts,providing insight to improve dimensional accuracy control.The increase in density of the pre-sintered parts after CIP treatment was mainly due to the rearrangement of the powder particles and slight plastic deformation.The increased sinterability after CIP treatment comes from improved powder packing density,higher particle coordination numbers and shorter distances between the powders,resulting in shorter atomic movement distances during the high temperature sintering. |
PDF Full Text Request