Preparation Of Lamellar Al-Si（-Mg）/Al₂O₃Composites Using Freeze Casting And Melt Infiltration Techniques

Metal matrix composites are widely used in the aerospace, automotive, electronics andother fields, due to their high specific strength, high specific modulus, wear resistance, heatresistance, electrical conductivity, thermal conductivity, anti–radiation, low thermalexpansion coefficient and other excellent performance. The properties of the metal matrixcomposite are not only related to the base metal, but also to the properties and the number ofthe second phase, and the interface between the matrix and the reinforcing phase. The shelland other natural organism with a hierarchical structure from the micro to the macro, hashigh strength and good toughness. Therefore, the preparation of high–performancecomposites with lamellar structures similar to the shell is a hot research of materialscommunity.In view of the above reasons, in this study we used micron–sized powders to preparelamellar porous alumina scaffolds by freeze casting. The strength of the sintered scaffoldswas improved by adding a small amount of MgO–Al2O3–SiO2nanopowders in a eutecticcomposition into water–based slurries as sintering aid. Finally, the biomimetic compositeswith lamellar structures were fabricated by infiltrating Al alloy into the porous ceramicscaffolds. The experimental procedure and equipment are very simple and the raw materialsare cost–effective. Moreover, the composites exhibited high compressive strength. Thefactors of the process were investigated to optimize the composite manufacturing process inthis study. The main results of this study are as follows:(1) The high-strength Al–Si–Mg/Al2O3composites with lamellar structures were firstprepared using freeze casting and pressureless infiltration by infiltrating Al–12Si–10Mg intothe porous Al2O3scaffolds with the addition of3wt.%MgO–Al2O3–SiO2nanopowders assintering aid. The composites inherited the lamellar structure from the ceramic performs. Themaximum compressive strength of the composites with the30%initial solid loading reached1190±50MPa, which was about3.5times as large as that of the matrix alloy. While for thecorresponding strain it reached4.6±0.5%, which was about1/2as large as that of the matrixalloy.(2) The Al–Si/Al2O3composites were prepared using freeze casting and vacuumpressure infiltration by infiltrating Al–12Si into the porous Al2O3scaffolds without addingsintering aid. Though the Al–Si/Al2O3composites inherited the lamellar structure from the ceramic performs, the compressive strength of the composites was low due to thelow-strength Al2O3scaffolds, not due to the preparation method, which was even lower thanthe strength of the matrix alloy.(3) The properties of composites prepared using freeze casting and infiltration wereprimarily related to the integrity of the lamellar structure of the Al2O3scaffolds and thestrength of the scaffolds, which could be changed by adjusting the initial solid loading andfreezing temperature. Meanwhile the infiltration process also had a significant impact on theperformance of the composites.(4) In this study we used5μm Al2O3powders to prepare porous alumina scaffolds withobvious lamellar structures by freeze casting with being sintered in air at1500oC for2h.The strength of the sintered scaffolds was improved by adding a mixture (3wt.%of all thepowders) of MgO–Al2O3–SiO2nano–powders in a eutectic composition of25:21:54inweight ratio into water–based slurries as sintering aid. The viscosities increased while theporosities decreased with increasing initial solid loading. However, The lamellar structures inthe sintered preforms are not obvious with the initial solid loading of35vol.%due to the toohigh viscosity.(5) Both the wavelength, λ, and wall thickness, δ, of the alumina scaffolds prepared byfreeze casting with being added sintering aids and sintered in air at1500oC for2h increasedwith the distance away from the bottom rod along the freezing direction. The wavelengthdecreased while the wall thickness increased with increasing initial solid loading at the samedistance away from the cold finger. The maximum compressive strength of the sinteredscaffolds with different initial solid loadings with obvious lamellar structures reached64±2MPa, which were freezed at–10oC. Both the wavelength and the wall thickness decreasedwith lowering the freezing temperature at the same distance away from the cold finger. Themaximum compressive strength of the20vol.%sintered scaffolds with different freezingtemperature reached29.5MPa, which were freezed at–20oC.