Study On The Performance Evolution And Its Mechanism Of Magnesium-aluminum Spinel Refractories For Casting

In this work,a composite material consisting of magnesium-aluminum（MgAl₂O₄）spinel refractory with a high melting point,low thermal conductivity,moderate thermal expansion coefficient,and good thermal shock resistance was systematically examined.Due to its enhanced properties,it is widely used in steel,cement,non-ferrous metal smelting,and other industrial applications.However,several issues arise during the employed production processes that should be addressed.More specifically,the development of MgAl₂O₄is accompanied by 5-7%volume expansion,which leads to the densification of the material need higher sintering temperature.Moreover,the thermal expansion mismatch of MgO-MgAl₂O₄materials would induce the generation of extensive micro cracks,resulting in the reduction of the material’s strength.Additionally,Al₂O₃-MgAl₂O₄materials in the case of high density would reduce the material’s thermal shock resistance,causing the matrix to crack or spalling.To effectively resolve the above-mentioned problems,MgAl₂O₄,MgO-MgAl₂O₄,and Al₂O₃-MgAl₂O₄series refractories were prepared in this work by utilizing the solid phase sintering method and using high purity oxides and natural raw materials.The phase composition,microstructure,elemental distribution,and electronic configuration of the as-prepared materials were characterized by performing XRD,SEM,EDS,and XPS measurements.The sintering behaviour and performance mechanism of magnesium-aluminum spinel refractories prepared by high purity oxides and natural raw materials were studied.In parallel,the impact of the additives and MgAl₂O₄content on the performance of MgAl₂O₄refractory and the strengthening mechanism was explored.As a result,MgAl₂O₄refractory composites meeting the performance requirements（densification,mechanical properties,and thermal shock resistance）were obtained,and magnesium-aluminum spinel artificial spherical casting sand was prepared by using strong mixing technology.The main conclusions include:（1）MgAl₂O₄refractory was prepared by high purity oxides and natural raw materials.The sintering behaviour of the material was studied,and the underlying mechanism of ion diffusion enhancing the properties of MgAl₂O₄was revealed by using pyrolusite and dolomite as additives.From our analysis,it was found that for the MgAl₂O₄composite prepared from high purity oxide,the apparent porosity was 0.44%and the flexural strength of 112.62 MPa.When the MgAl₂O₄material prepared by natural raw materials was sintered at 1400℃,the apparent porosity was 0.47%and the flexural strength was 100.28 MPa.As a result,the problem of high sintering temperature was addressed but the flexural strength of the material was reduced.By studying the impact of adding pyrolusite and dolomite on the properties of MgAl₂O₄material,it was found that Mn²⁺and Ca²⁺can form a solid solution with MgAl₂O₄.The latter improved the flexural strength and thermal shock resistance of the material.The fracture mode was changed from intergranular fracture to the coexistence of transgranular fracture and intergranular fracture,and the thermal shock resistance mechanism was explained by means of the critical stress fracture theory.（2）MgO-MgAl₂O₄series refractories were prepared by high purity oxides and natural raw materials.The impact of MgAl₂O₄content on the properties of MgO-MgAl₂O₄series refractories was elucidated by studying the sintering behaviour and revealing the mechanism of ion interdiffusion during sintering.The results showed that MgO-MgAl₂O₄refractory sintered at 1600℃exhibited a porosity of 2.31%and a flexural strength of 107.22 MPa.On the contrary,the natural raw materials can be sintered at 1550℃,with the lowest apparent porosity of 1.3%and the highest flexural strength of 113.23 MPa.Hence,the sintering temperature can be effectively reduced and the densification and mechanical properties of the material can be improved.Increasing the content of MgO in the raw materials can promote the formation of the(Mg_1-xAl_x/2)Mg_x/4O solid solution and achieve solid solution strengthening.The addition of Al₂O₃to magnesite increased also the content of the MgAl₂O₄phase,and the occurrence of intergranular fracture enhanced the thermal shock resistance of the material.The thermal shock resistance mechanism can be interpreted by using the theory of thermal shock damage resistance.When the MgAl₂O₄grain was encountered at the front of the expanding crack,the expansion path of the crack was deflected,and the residual strength retention rate of the material after thermal shock was up to 91.05%.（3）Al₂O₃-MgAl₂O₄series refractories were prepared by using high purity oxides and natural raw materials.The sintering behaviour of refractories was studied,and the mechanism of regulating spinel content to enhance the properties of the material was revealed.The results pointed out that the Al₂O₃-MgAl₂O₄refractory sintered at 1600℃by high purity oxides had25.66%of apparent porosity and 104.33 MPa of flexural strength.For the Al₂O₃-MgAl₂O₄refractory prepared by employing natural raw materials,the synergistic effect of mass transfer in the liquid phase and diffusion mass transfer in the solid phase enhanced the solid solubility of Al₂O₃in MgAl₂O₄,leading to the formation of the Mg_1-xAl_2+2x/3O₄solid solution.The apparent porosity of the material was as low as 0.64%and the flexural strength was as high as132.11 MPa.By adding magnesite to aluminum source（with a mass ratio of bauxite to alumina of 2:1）,the content of the MgAl₂O₄phase was increased.The fracture mode of the sample changed from transgranular fracture to the coexistence of transgranular fracture and intergranular fracture.The transgranular fracture enhanced the thermal shock damage resistance.At the same time,the existence of larger spinel grains can nail the expanding crack front end.Consequently,the residual strength retention rate of the material after thermal shock was increased up to 94.36%,which effectively solves the problem of low thermal shock resistance of the material.（4）Magnesium-aluminum spinel artificial spherical casting sand was prepared by a strong mixing and stirring process.The influence of sand molds made of artificial sand and traditional silica sand on casting was studied.From our analysis,it was proven that because the traditional silica sand is sheet-shaped and angular,it could be easily slip during the compaction process.Furthermore,its angular coefficient was higher than 1.63,the thermal expansion coefficient was as high as 16×10^-6℃^-1,the mud content was as high as 0.50%,the permeability of the sand was as low as 174.2,and the wet compressive strength was as low as0.0023 MPa.As a result,the creation of defects such as grooves and pores on the surface of the casting was induced.Using magnesite and bauxite as raw materials and pyrolusite and dolomite as additives,the MgAl₂O₄casting sand yielded high sphericity.In addition,an angular coefficient of less than 1.15,thermal expansion coefficients of 9.8948×10^-6℃^-1and9.3244×10^-6℃^-1,mud contents of 0.21%and 0.18%,wet compressive strengths of 0.0035MPa and 0.004 MPa,the permeabilities of 277.56 and 253.52 were recorded for pyrolusite and dolomite,respectively.The surface of the casting was smooth,and the grain development was good,which can meet the requirements of large-scale casting and precision casting.