Study on Properties of Thermal Shock and Ablation Resistance for Gr/Al-Mg Composites
|School||Harbin Institute of Technology|
|Keywords||light metal impregnated graphite composite transpiration cooling thermal shock resistance ablation resistance|
In the thesis, the transpiration coolant were determined according to the material design. Gr/Al、Gr/75Al-25Mg、Gr/50Al-50Mg、Gr/25Al-75Mg and Gr/Mg were prepared by mean of squeeze casting technology. By using optical microscope, scanning electron microscope （SEM）, dilatometer, thermal conductivity tester, three-point bending tests, the microstructure, mechanical and thermo physical properties were examined and their influencing factors were discussed. The microstructure after ablation were investigated by X-ray diffraction （XRD） and scanning electron microscopy （SEM）. Mechanisms of thermal shock and ablation resistance of the composites had also been studied.Composites which chosen pure metal as coolant have highest bending stress and thermal conductivity, and it dropped with mixing scales of metal coolant. The elastic modulus and mean CTEs （20℃～100℃） of composites both declined with the increases of volume Mg in coolant.According to GJB323A-96, we conducted oxygen-ethane torch heating test. The results showed that composite does not burst apart when the material is instantly heated and when the ablation is just over. Its thermal shock resistance is good. The transient temperature field and thermal stress fields affected by heat transfer in high temperature were simulated and analyzed based on the thermal physical data with ANSYS commercial software. The result reveal that in the first 10s , temperature changes sharply, thermal stress reached maximum at 2s; after 15s temperature fields gradually became stable, thermal stress fields dropped with it. Thermal stress fields were highest in Gr/50Al-50Mg, and it have low strength; Thermal stress fields were lowest and strength were high in composites whose transpiration coolant is pure metal. So the thermal shock resistance of pure metal impregnated graphite was best.The surface of the samples after ablation were coated by Al2O3, MgO or their mixer. Coolant presented obviously a gradual distribution in the cross section. The mass loss ratio of composites were in the range of 6.6×10-4～71.8×10-4g/s, which showed good ablation resistance properties. The mass loss ratio of composites decreased with prolonging of ablation time and increasing in mixing scales of metal coolant. The ablation resistance for 30s of Gr/50Al-50Mg was best in the five composites. The ablation-resistance mechanisms mainly included transpiration cooling, thermal chemical ablation of coolants and mechanical denudation of ceramics. It means that the coolant really played a role of transpiration cooling during elevated temperature. Theoretical derivation showed that the mass of transpiration decreased with increasing of melting temperature; the thermal ablation were controlled by oxidation rates at first, and then controlled by diffusion; mass of denudation decreased with increasing of viscosith coefficient. Theoretical discussion prove the test results.