Research Of Oxidation Resistance Of Selective Laser Sintered Graphite Parts And Application

Selective Laser Sintering(SLS)is a near net forming technology developed rapidly in recent years,which provides the possibility for the fabrication of complex structural parts.Compared with synthetic graphite,the graphite part has loose porosity,poor density and more surface active points.It is very easy to react with oxidation under the condition of high temperature and oxygen,which changes the size of graphite material and destroys structure,and leads to its rapid burning loss,so it is difficult to be directly used as a high temperature resistant material such as graphite mold.In a word,it is necessary to find a new process to protect the selective laser sintering graphite parts from oxidation,so as to improve the high temperature oxidation resistance and compressive strength,and ensure its safe use under the condition of high temperature.In this paper,the oxidation resistance and compressive strength of graphite parts are ensured by changing the formula composition of 3D printing materials and proper post processing,and through the regulation of thermal conductivity and compressive strength,the preparation of graphite parts with low thermal conductivity(<2W/(m·K))and high compressive strength(>10MPa)is realized.The main work of this paper is summarized as follows:Firstly,the composition of graphite/phenolic resin mixed powder is changed,and the design,particle size analysis and fluidity analysis of the mixed powder are carried out to meet the requirements of selective laser sintering.It is showed that silicon carbide and borosilicate glass are formed in the matrix after adding silicon powder and boron carbide powder to graphite and phenolic resin at high temperature,which improve the oxidation resistance and compressive strength of graphite parts.The oxidation resistance of graphite parts at different temperatures is closely related to temperature and self-composition.Secondly,changing the post processing.After secondary curing,the density of graphite blank prepared by selective laser sintering is only 0.50g/cm~3,there are still many active points,high porosity,poor mechanical properties,poor bending strength and compressive strength.By changing the post processing,it is showed that the pores can be filled by vacuum pressure infiltration of phenolic resin solution,and the phenolic resin can be transformed into high temperature resistant and high strength glass carbon after carbonization,which improves the oxidation resistance and compressive strength,and the effect of infiltration for two times is the best.Then,the pore distribution and size are analyzed,and the nanometer silica sol solution is infiltrated under vacuum pressure again,and the reinforced phase of silicon carbide is formed after sintering at 1500℃,thus the oxidation resistance and compressive strength of graphite parts are improved.Infiltrating aluminum dihydrogen phosphate solution can reduce the oxidation weight loss rate of graphite parts,but it is not suitable for the high temperature environment above 900℃.Finally,the fabrication of graphite part with low thermal conductivity and high strength by Selective Laser Sintering is realized.The properties of the samples after the best post processing are analyzed,by adding expanded graphite to the mixed powder and making use of the expanded graphite to expand at high temperature,it is showed that the thermal conductivity can be reduced by forming a certain closed pore in the graphite part,it has little effect on the compressive strength,but cracks occurred in the preparation of thick green body.Changing the secondary curing method to hot pressing curing method can better solve the cracking problem of thick and large green graphite parts,but can not meet the requirements of low thermal conductivity and high strength at the same time.Finally,with the increase of the mass ratio of phenolic resin powder,and hot pressing curing method,the graphite parts with low thermal conductivity and high strength can be fabricated.Applied to molten steel casting experiments to verify its thermal conductivity,oxidation resistance and thermal shock resistance(repeated use times reached 36),achieved good results,and has a certain practical significance.