Low Cost Preparation And Mechanical Properties Of Magnesia Alumina Spinel For Casting

Magnesia alumina spinel materials are widely used in high temperature industry.However,at present,the synthesis of this kind of materials requires high sintering temperature,and at present,the industry mostly uses light burned magnesium oxide as magnesium source to synthesize this kind of high temperature resistant materials,which makes the cost high.In order to reduce the cost,it is necessary to improve the synthesis process of this kind of materials as much as possible,or reduce the cost of raw materials,or seek suitable additives.In order to reduce the production cost of magnesia alumina spinel materials,this paper uses cheaper magnesite and bauxite powder as raw materials.The formula was reasonably designed and the Mg Al₂O₄ceramic material with Mg Al₂O₄as the main phase and Mg O-Mg Al₂O₄multiphase ceramic material with Mg O as the main phase were prepared.Finally,in order to reduce the sintering temperature,Fe₂O₃was selected as an additive to Mg O-Mg Al₂O₄multiphase ceramic material.The effects of raw material ratio,sintering temperature and additive content on the phase composition,microstructure,porosity,bulk density and flexural strength of the samples were investigated.The main research contents of this paper are as follows:（1）Using magnesite and bauxite as raw materials,based on the theoretical ratio of Mg and Al in Mg Al₂O₄,Mg Al₂O₄based ceramic materials were prepared by solid-state sintering method according to the proportioning of B（bauxite）/M（magnesite）of 1.07,1.31,1.61,2.00and 2.53.It is found that with the increase of bauxite content,the characteristic peak of Mg Al₂O₄shifts to a high angle.The ion vacancy generated by the solid solution reaction can promote the sintering of the sample slightly rich in magnesium（B/M=1.31）/rich in aluminum（B/M=2.00）,making the density of the sample more dense than that when proportioned according to the theoretical ratio（B/M=1.61）.Moreover,when there is too much bauxite,the increase of liquid phase makes the microstructure blurred.At the same time,too much liquid phase also makes the grain grow too fast,resulting in closed pores,resulting in the decline of physical properties.When B/M=1.31,the maximum flexural strength obtained after sintering at 1500℃is 93.53 MPa,the bulk density is 3.42g/cm³,and the lowest apparent porosity is 7.2%.（2）Using magnesite as the main raw material and adding bauxite,the content of bauxite in the whole sample is 5 wt%,10 wt%and 15 wt%.Taking the sample without bauxite as the control group,Mg O-Mg Al₂O₄multiphase ceramic material was prepared by solid-state sintering method.It is found that with the increasing proportion of bauxite,the peak strength of Mg Al₂O₄increases gradually,the number of Mg Al₂O₄distributed at the Mg O grain boundary also increases gradually,the flexural strength and bulk density increase significantly,and the porosity decreases,reaching a maximum when the proportion of bauxite is 10 wt%.Due to the excessive amount of bauxite and the increase of liquid phase,closed pores are also generated in the material,and the physical properties are reduced.According to the microstructure of material fracture,it can be seen that Mg Al₂O₄pinning at the grain boundary of Mg O hinders the extension of intergranular cracks of Mg O,so the flexural strength of the sample is improved.At 1500℃,when the content of bauxite is 10 wt%,after sintering at1500℃,the flexural strength and bulk density reach the maximum value of 92.56 MPa and3.35g/cm³respectively,and the apparent porosity reaches the minimum value of 7.5%.（3）In the best formula with bauxite content of 10 wt%,1 wt%,3 wt%and 5 wt%Fe₂O₃were added as additives and those without Fe₂O₃as control.It was found that with the increase of Fe₂O₃,the characteristic peak of Mg Al₂O₄shifted to the low angle direction due to the increase of solid solution of Fe³⁺,and the flexural strength and bulk density of the sample increased significantly with the increase of Fe₂O₃content at 1400℃,And the physical properties of the sample are equivalent to those of the sample without Fe₂O₃at 1500℃,indicating that the optimal sintering temperature of the sample is reduced by about 100℃ when 5 wt% Fe₂O₃ is added.