Study On Preparation Process, Characterization And Mechanical Properties Of Aluminum Foams By Gas Injection Method

The aluminum foam is a kind of advanced structural and functionalmaterial with broad application prospects. Among the methods that could beused to fabricate aluminum foams, gas injection foaming is one of the mostpromising processes. It has the possibility to produce the foams both incontinuous way and in batch type at low cost and the foams with low densityand large cell size can be achieved.Two different powder dispersion processes, i.e. the liquid phrase processand the liquid-solid process, were compared to analyze their influences onparticles aggregation and pore size distribution. It was found that the Al2O3particles tend to aggregate and flow to the surface of the melt, but that doesn’t affect thefoaming process. There are many sub-millimeter pores in the solidified structures of themelt as well as the cell wall, which reduces the relative density of aluminum foams. Thehigh viscosity of the melt in liquid-solid process contributed to the entrapment of thegas in the melt. The equipment for collecting aluminum plates was designed and thefeasible parameters for collecting aluminum plates were determined in terms of thedefects of the plates.The relative density, average wall thickness and topologicalcharacterization of aluminum foams prepared by gas injection method werestudied in this thesis. The cell structure of the samples reveals that the cells areanisotropic due to the collecting process and gravity. An improvedtetrakaidecahedron model is put forward based on the actual structure to analyzethe relationship among the relative density, average wall thickness and cellgeometry theoretically. An approximate characterizing formula is obtained fromthe model, which is verified by measured data. The model is more accurate andpromises a good characterization of cell structure with high porosity and highanisotropy ratio compared to Kelvin’s model.The uniaxial compressive stress-strain behavior of aluminum foams wasdiscussed and the compression strength, densification strain and the energyabsorption per unit volume were determined by the stress-strain curves. Fourdeformation rupture mechanisms of cell walls, i.e. the rupture of the matrix, the rupture at the particles and matrix interface locations, the rupture at particlesaggregating sites and the rupture at voids and cracks were verified in terms ofthe microstructures and fracture morphologies. The plateau stress and energyabsorption properties were determined with a modified formula ignoring the cellface effects in Gibson and Ashby’s model and further verified with experimentaldata. It was found that the formula only contains two degree term of relativedensity.Sandwich panels with foam cores were prepared and the three-pointbending properties were measured. It was found that the failure modes of asandwich beam include core failure, indentation, face wrinkle and bond failure.The moment of inertia of the panel increases violently while the weightincreases little by adding the foam core, making the panel an efficient structurefor bend loading. The three-point bending properties of the sandwich panels aredetermined mostly by the failure mode and the shear and compression strengthof aluminum foams.