Fabrication and properties of particulate reinforced aluminum matrix composites by spontaneous infiltration

Metal matrix composites has been developed over decades and gradually formed an important part in materials system. Nowadays in the popularization of metal matrix composites, in emphasis for composite materials is changing from a performance-driven to a cost-driven environment. Therefore it’s necessary and important to develop economical efficient techniques for producing metal matrix composites. Spontaneous infiltration (also called pressureless infiltration) is one of the most cost efficient liquid state techniques for manufacturing metal matrix composites, which uses adhesion forces between the matrix melt and the solid preform to realize the infiltration of matrix into porous preform. However there are several shortcomings in the existing approaches of spontaneous infiltration. For example, low infiltration rate, undesired contamination (AlN in the PREMEXTM process) and poor mechanical properties of the initial product etc. In this paper an innovative approach of spontaneous infiltration for fabricating particulate reinforced aluminum matrix composites was presented and researches were focused on the problems related with the thermodynamics and kinetics of this approach. TiB2, SiO2, sand, TiO2 etc. ceramic particulates and Al powder were selected as the starting materials, 6060 alloy was chosen as the matrix material. Mixture of TiB2-Al, SiO2-Al, sand-Al, TiO2-Al, TiB2-SiO2-Al or TiB2-sand-Al were prepared to make the preforms by cold pressing. The blending of Al powder into ceramic particulates was found essential to the spontaneous infiltration. Al powder was believed to improve the wettability of ceramics by reacting or communicating with them ahead of the infiltration. Infiltration temperature and cold pressing pressure were found to have influence on the infiltration. For TiB2-Al, higher temperature or pressure would promote the infiltration. During the infiltration, external melt did not fill all the pores along the infiltration path but preferentially infiltrated part of the pores throughout the preform in the beginning and then gradually spread to the rest of the pores. For TiO2 the infiltration kinetics was more complicated, the reactions with Al increased the volume of solid phases which could block the infiltration paths and even break the preform. Some properties of PAMCs in TiB2-SiO2-Al series fabricated by spontaneous infiltration at 900 oC for 1 hour were obtained. Both of TiB2 and Al2O3 had a positive influence on improving the defect insensitive properties such as increasing elastic modulus, Vickers hardness and controlling CTE. However the flexural strength property increased with TiB2 fraction but decreased with Al2O3 fraction. Fracture analysis shows TiB2 particles had a good bonding with matrix and Al2O3 particles had a poor bonding with matrix and even some pores were found around them