Binder Jet 3D Printing And Densification Of 316L Stainless Steel Bimodal Powder

Binder Jet 3D Printing(BJ3DP)is considered to be one of the most promising,digital and low-cost additive manufacturing technologies for metals,with advantages of low cost,high speed and wide range of applicable materials compared with other laser-based additive manufacturing technologies.However,this technology has not been widely used due to the problems of low molding accuracy and poor strength of the parts.In this paper,orthogonal experiments were conducted to analyze the effects of printing parameters on the quality of green using 316 L stainless steel bimodal powder as raw material,and to summarize the printing parameters for higher density,lower surface roughness,smaller dimensional deviation and faster printing speed of green.Finally,the effects of bimodal powder and sintering temperature on the density,organization,shrinkage and mechanical properties of the samples were investigated.The main conclusions of this paper are as follows.The layer thickness of the powder bed is a significant factor affecting the density of the green,which increases as the layer thickness decreases.The dimensional deviation of all green shows a pattern of x>y>z,which is related to the penetration and expansion of the binder in the powder bed.The surface roughness is most affected by the layer thickness and increases with increasing layer thickness due to the "step effect" of larger layer thickness.Faster powder spreading speed and larger layer thickness are beneficial to improve the printing efficiency.The orthogonal experiment finally determined the optimal process for the layer thickness of 100 μm,binder concentration of 95%,and powder spreading speed of 15 cm/s.316L stainless steel bimodal powder can significantly improve the powder characteristics and green density.The bimodal powder was prepared by mixing coarse+ fine powders of different particle sizes,and the compaction density was nearly 72%,which was 7.88% higher than that of a single coarse powder.Using bimodal powders to print green,the density and surface finish of the green increased with increasing fines content,and the dimensional accuracy was slightly lower than that of a single coarse powder.The density and mechanical properties of the 316 L stainless steel bimodal powder were significantly higher than those of the single coarse powder under the low temperature short time sintering condition of 1400 ℃ × 3 h.The sintered density and mechanical properties are the lowest due to the low density of the green of the single coarse powder and the presence of a large number of pores in the sintered tissue.For bimodal powders,with the increase of fines content,the densities and mechanical properties have a pattern of increasing,then decreasing and then increasing.10 wt.%fines fill the coarse powder gap well,and when the fines increase to 15 wt.%,the wedging effect occurs in the powder accumulation and coarse pores are formed,resulting in a decrease of densification.When adding 20 wt.% fines,the contribution of small particles to the sintering densification is greater than the effect of wedging,the densification density reaches 92.80%,tensile strength reaches 477 MPa,the performance is close to commercial stainless steel standards.The organization of 316 L stainless steel bimodal powder after sintering at different temperatures is typical of austenite organization,which proves that the vacuum sintering process densification stainless steel is feasible.Under high temperature sintering conditions at ≥1420 ℃,the density and properties of the bimodal powder and the single coarse powder were similar,but the former showed less dimensional shrinkage.At temperatures ≥1420 ℃,the mechanical properties of each sample exceeded the performance requirements of commercial 316 L stainless steel.With the increase of temperature,the sintered density and mechanical properties of the single coarse powder increased significantly.The best mechanical properties of bimodal powder at 1420 ℃,the tensile strength of 583 MPa,yield strength of 224 MPa,elongation of 97.5%,continue to increase the temperature,due to grain coarsening and pore irregularity,mechanical properties instead slightly decreased.