Study On The Forming Process And Mechanism Of Fabricating Alumina Ceramics Via Direct Selective Laser Sintering

Direct Selective Laser Sintering?dSLS?is a branch of additive manufacturing technology.Based on the layering-superposition principle,dSLS technology utilizes the thermal effect of laser to sinter ceramic powder together to form ceramic parts.It is an ideal additive manufacturing technology for ceramic fabrication because of the advantages such as no organic binder used,no post-treatment process i.e.debinding involved,short forming process and high utilization rate of ceramic materials.However,the high hardness and high melting point of the ceramic material itself have led to the current dSLS technology,which has the characteristics of difficulty in forming,high requirements on equipment,and low quality of finished products.In this study,porous and dense alumina ceramics were formed by adding hard ceramic B₄C and powder pressure treatment technology,respectively.The influencing factors and forming mechanism of the two methods to fabricate alumina ceramics were studied,and the following conclusions were obtained:?1?The addition of boron carbide greatly expands the dSLS processing window of alumina,and can increase the particle size of alumina from about 3?m?3 wt%B₄C?to about 5?m?5 wt%B₄C?.The pore size of alumina porous ceramics decreases with the addition of boron carbide increasing.The average pore size of the sample with 1 wt%B₄C content is 7.05?m,and the average pore size decreases to 3.39?m when the amount of B₄C reaches 9 wt%.?2?The optimal boron carbide addition amount of alumina porous ceramics should be5 wt%,and the dSLS sample possesses a density and compressive strength of 1.04 g/cm³and 0.74 MPa respectively,while after post-treatment the density can reach 1.38 g/cm³ with a compressive strength of 2.91 MPa.When the amount of boron carbide adds up to 9 wt%,the dimensional expansion ratio is 5.69%.?3?The pressure treatment greatly enlarges the alumina dSLS formable area,and the forming quality is optimal in the range of energy density of about 100 to 600 J/cm² at 350MPa.As the applied pressure increases,the packing density of powder increases and the grain size of obtained alumina samples decreases.When the pressure is 50 MPa,the bulk density of the powder is 1.84 g/cm³,and the bulk density is 2.39 g/cm³ at 350 MPa.When the pressure is 50 MPa,the alumina grain is the largest with the maximum thickness of the ceramic layer?135.55?m?,while the smallest grain size at 350 MPa with the minimum thickness of 61.10?m.?4?When other parameters are constant,with the laser power increasing,the size and orientation of alumina grains increase,and the edge warpage of the sample is more pronounced.When other parameters are constant,with the scanning velocity increasing,the grain size decreases,the orientation becomes disordered,and the degree of warpage decreases.Smaller hatch distance help eliminate surface defects and promote grain growth.When dSLS is forming a multilayer alumina ceramic,the micro defects will directly affect the layup,pressure treatment and laser sintering of the next-layer powder,and the flatness of each layer of ceramic must be ensured.The above conclusions show that the addition of boron carbide and pressure treatment can effectively improve the sintering performance of alumina,simplify the experimental conditions for the preparation of alumina ceramics by direct laser selective sintering,and lay a foundation for the preparation of alumina ceramics with excellent performance and complex structure via dSLS technology.