Studies On Fabrication And Properties Of Ceramic/ZA22 Composite Foams

Metallic foams have received extensive interest as new structural and functional materials recently with a combination of excellent mechanical performances and outstanding functional properties such as high specific stiffness, better energy absorption and vibration reduction performances, fine sound absorption capacity, excellent heat-resistant and electromagnetism-shield properties. Therefore, they can be widely used in many fields, including aerospace, transportation, metallurgy, and construction industry. Zinc-aluminium eutectoid alloy (ZA22 alloy) foams are new kind of metallic foams which have outstanding mechanical and damping properties. By adding ceramic particles or ceramic fibers into molten ZA22, the foamability of ZA22 molten foams can be improved. And also, it can enhance the properties of the ZA22 foams that the properties of the metal foams and metal matrix composites are banded together. The research on the effect of ceramic on the properties of ZA22 foams was few to date. Therefore, the deep and systemic research on ceramic/ZA22 foams has a promotive effect on the fabrication and application of the metallic foams. The following researches were carried out in this dissertation:Firstly, SiC particle reinforced ZA22 composite foams (SiC_p/ZA22 composite foams) and Al₂O₃ fiber reinforced ZA22 composite foams (Al₂O_3f/ZA22 composite foams) were fabricated by the melt foaming route using CaCO3 as foaming agent. The effects of the parameters such as the foaming temperature, the amount of the foaming agent, and the foaming time on the fabrication of ZA22 foams were roundly studied. It was found that the foamability of ZA22 foams was improved by adding SiC particles or Al₂O₃ fibers into ZA22 liquid foams.Secondly, the quasi-static compressive properties of SiC_p/ZA22 and Al₂O_3f/ZA22 composite foams were studied. The effects of the relative density and the reinforcement on the compressive properties of the composite foams were systemically investigated. The deformation mechanisms of the composite foams were observed. The results are as follows:1. The stress-strain curves of the composite foams include three stages: the linearly elastic deformation region, the collapse plateau region, and the densification region. The deformation modes of SiC_p/ZA22 and Al₂O_3f/ZA22 composite foams were the strain localized and the failure occurred layer by layer under the quasi-static loading. SiC_p/ZA22 composite foams show some brittle characteristic, but Al₂O_3f/ZA22 composite foams were charactered by ductibility;2. The yield strengths of SiC_p/ZA22 and Al₂O_3f/ZA22 composite foams increase with increasing relative density under otherwise equal conditions;3. The yield strength of SiC_p/ZA22 composite foams increases with increasing volume fraction and decreasing size of SiC particles when other acting factors are identical;4. The yield strength of Al₂O_3f/ZA22 composite foams increases with increasing volume fraction of Al₂O₃ fiber when the relative densities of the foams are identical;5. The compressive yield strengths of SiC_p/ZA22 composite foams under the quasi-static loading were analyzed by using composite sphere model. The strength formulas which can explain the effects of the relative density and the reinforcement on the compressive properties were educed.Thirdly, the dynamic compressive properties of SiC_p/ZA22 and Al₂O_3f/ZA22 composite foams were investigated by using Split Hopkinson Pressure Bar (SHPB). The effects of the strain rate, the relative density and reinforcement on the dynamic compressive properties of SiC_p/ZA22 and Al₂O_3f/ZA22 composite foams were systemically investigated. In addition, the strain rate sensitivity of the composite foams wasstudied. The acting factors on the dynamic compressive strength were investigated. The results are as follows:1. The compressive yield strengths of SiC_p/ZA22 and Al₂O_3f/ZA22 composite foams increase with increasing strain rate when other acting factors are identical. ZA22 foams, SiC_p/ZA22 composite foams and Al₂O_3f/ZA22 composite foams are sensitive to the strain rate. This strain rate sensitivity is root in the contribution of the gas pressure in pores and the intrinsic strain rate sensitivity of ZA22 matrix;2. The dynamic compressive yield strengths of SiC_p/ZA22 composite foams increase with increasing relative density, volume fraction of SiC particles, and size of SiC particles under otherwise equal conditions;3. The compressive dynamic yield strengths of Al₂O_3f/ZA22 composite foams increase with increasing relative density and volume fraction of Al₂O₃ fibers when other acting factors are identical.Fourthly, the energy absorption characteristics of SiC_p/ZA22 and Al₂O_3f/ZA22 composite foams were investigated for the first time. The effects of the relative density, strain rate, and reinforcement on the energy absorption ability and energy absorption efficiency were systemically investigated. The results are as follows:1. The energy absorption abilities of SiC_p/ZA22 and Al₂O_3f/ZA22 composite foams mainly lie on the yield strength, the length of the collapse plateau region and the strain hard exponent. The energy absorption efficiencies are dependent on the ratio of upper yield strength to lower yield strength, the ratio of the yield strain to the densification strain, and the ratio of the elastic module to the strain hard exponent; 2. The energy absorption efficiencies of SiC_p/ZA22 composite foams are independent of the relative density,while dependent on the strain rate and the volume fraction of the reinforcement. The energy absorption efficiencies of Al₂O_3f/ZA22 composite foams are independent of the relative density and the volume fraction of the reinforcement, while dependent on the strain rate.Fifthly, the damping properties of SiC_p/ZA22 and Al₂O_3f/ZA22 composite foams were investigated. The effects of the relative density and the reinforcement on the damping properties of the composite foams were investigated. The damping mechanisms of composite foams were studied. The results are as follows:1. The damping loss factor of SiC_p/ZA22 and Al₂O_3f/ZA22 composite foams increases with decreasing relative density (the increase in porosity). The loss factor of SiC_p/ZA22 composite foams increases with increasing volume fraction of SiC particles and decreasing size of SiC particles. The loss factor of Al₂O_3f/ZA22 composite foams increases with increasing volume fraction of Al₂O₃ fibers;2. The high damping properties of the composite foams are root in the contribution of the intrinsic damping of ZA22 matrix, the conversion of the stress mode around the pores, the high dislocation density damping and many kinds of interface damping;3. The Artificial Neural Network technique was used for analyzing the damping properties of the composite foams. The BP Neural Network method has good precision and perfect fault tolerance ability.Finally, the sandwiches with ZA22 foams, SiC_p/ZA22 composite foams and Al₂O_3f/ZA22 composite foams cores were prepared for the first time. The bending properties of the sandwiches with the foams cores under three-point bending experiment were studied. The results are as follows:1. P-δcurves of the sandwiches with the foams cores under three-point bending condition include three stages: the linear region, the non-linear region and the unsteady region. The failure modes of the sandwiches include the core-yield and the bonding-damage;2. The limit load of the sandwiches with ZA22 foams cores increases with increasing relative density and the thickness of the surface panel. The limit load of the sandwiches with SiC_p/ZA22 composite foams core increases with increasing volume fraction of SiC particles and decreasing of the size of SiC particles. The limit load of the sandwiches with Al₂O_3f/ZA22 composite foams core increases with increasing volume fraction of Al₂O₃ fibers.