Cracking Mechanism And Suppressing Methods For Laser Melting Deposition Of Al₂O₃ Ceramic

Ceramic materials are characterized by outstanding mechanical properties and some special physical properties.Working as important engineering materials,ceramics are widely utilized in every industry of military and civil fields.Especially as the ultra high temperature structural materials,ceramics are mainly applied to hypersonic aircraft and high performance aeroengine technical fields.This is the key to realize the aero engine efficiency and high temperature turbine without cooling.However,traditional sintering manufacturing methods have quite complicated process,long preparing circle and requirements to add binders,which limits the fulfillment of ceramic's excellent properties.Ceramic Direct Additive Manufacturing uses high power beam to melt ceramic powders directly and deposite to form the shape.This achieves intergration of ceramic shaping and sintering and can be used to fabricaiton of complex high-properties ceramic structures rapidly.It has great research value and significance to overcome traditional processing defects and fulfill the demand of parts working under extreme environments.Nevertheless,the intrinsic brittleness of ceramic material may result cracks during the direct additive process,which largely restrains the development of this technique.The present paper uses Laser Melting Deposition system to practice the direct fabrication of Al₂O₃-based ceramic structure,analyzes the generating mechanism of shaping structure cracks and states the related cracking criterion.Based on the cracking mechanism and criterion,targeted process optimizations are conducted,a series of crack suppressing methods are proposed and crack-free high-properties ceramic structures are fabricated successfully.Specific research contents and conclusions are as follows:?1?Materials circular re-melting behavior of Al₂O₃ single-bead multi-layer structure during the depositing process has been analyzed,which reveals the mass conservation relationship between valid depositing material and melting powder,and the energy conservation relationship between laser energy cost and the increased internal energy of deposited material.Finite element thermodynamic coupling model which reflects re-melting characteristics as well as the process model which reflects the relationship between processing parameters and depositing geometry characteristics are set up.?2?By analyzing cracking features and fracture morphology of the Al₂O₃ ceramic single-bead multi-layer structure,growth mechanism of directional cell crystal and elements micro-segregation of the direct additive manufactured materials are discussed,reflecting the cracking mechanism of intercrystal liquid film forming the initial crystal crack and growing macroscopic expansion under tensile stress combining the shaping process constraint stress and thermo-stress evolution pattern.Further analytically expresses the stress and self-fracture strength of the shaping structure by using elastic mechanics and fracture mechanics,proposes the cracking criterion of direct additive fabrication of ceramic structure,reflects the quantitative relationship between materials properties,process parameters,other influential factors and cracking sensitivity of the shaping structure.?3?Based on the affecting pattern of process parameters on cracking sensitivity,this paper uses process model directly caculating the needed laser power and powder flow rate for fabricating structures with specific geometry dimensions,and design cracks restrain optimization experiments of scanning speed and layer increment under large process window respectively.The outcomes reflect that deposition with high layer increment is good for the decrease of laser energy cost,the reducing of shaping stress as well as the weakening of directional growth tendency of microstructure,and can realize effective suppression of the cracks.?4?Based on the affecting pattern of material physical properties on cracking sensitivity,this paper proposes three crack suppressing methods:reducing the coefficient of thermal expansion,refining crystal size and enhancing the fracture surface energy.The three suppressing methods are verified by fabrication of Al₂O₃/TiO2?Al₂O₃/ZrO2 and Al₂O₃/SiCP ceramics.Crack-free ceramic structures of hollow blade and four pytamid with cross-section length of 50 mm are successfully manufactured.For the first time,macro property test specimens are supplied.The property testing results show that the fabricated specimen can achieve nearly 100%bulk density,and maintain the chemical purity highly consistent with raw material.Micro-hardness and fracture toughness of specimens reach 18.91 GPa and 3.55 MPa·m1/2,while flexural strength and compressive strength peaking at 350 MPa and 618 MPa,which are able to meet certain practical application requirements.