Fabrication and Properties of Aluminium Matrix Composite Reinforced with High Volume Fraction Silicon Carbide Particles

Abstract

This thesis details the investigation relating to the production method and mechanical properties of two types of aluminium/silicon carbide composites. These materials are manufactured using specialised powder metallurgy (P/M) techniques, which includes mixing/milling, powder compaction, sintering and hot extrusion. The first type of composite utilises pure aluminium as the matrix, whilst the other type uses matrix consist of aluminium and in-situ oxide particles, namely sintered aluminium powder (SAP). The second type of composite combines silicon carbon particles with very small in-situ oxide particulates inside the aluminium matrix, creating a dual reinforcement system. These composites are not often characterised in literature as the fabrication process is very challenging, particularly using P/M methods. Therefore, this work offers rare insight into aluminium composites with a combination of different size reinforcement particles, which is important in establishing more novel aluminium based composites.

Composites with nearly full density (> 99% theoretical density) were successfully produced via this method. The unreinforced aluminium specimens were relatively easy to extrude and defect-free extrusions can be produced in a range of parameters. The composite material not only required a higher extrusion load, but also produced a small amount of surface defects resulting sub-optimal surface finishing. A suitable remedy is to use aluminium foil or an aluminium disc in front of the extrusion for the composite material. The foil or disc acts a solid lubricant for the hard composite material, which significantly improves the surface finishing. Using this method, crack free extrusions can be obtained at reinforcement contents up to 30 volume percent silicon carbide.

Microstructural analysis using optical and electron microscopies shows the particle dispersion in the material is relative homogenous. However, particle banding and alignment are observed along the extrusion direction of the specimens. Transmission electron microscope analysis shows the particle/matrix interface is free of any major reaction, but a small layer of inter-diffusion occurred between silicon and aluminium. The sub-structure of the material adjacent to the silicon carbide particle is observed to be sub-micron. This can be related to particle –assisted nucleation during extrusion and possibly the effect of high dislocation density near the particle-matrix interface, which produces a very fine structure.

Mechanical testing showed the composite reinforced with silicon carbide (1200 mesh, 13 micron) possessed a significant improvement in mechanical strength and stiffness as compared to the unreinforced material. The improvement did not diminish with increasing in reinforcement volume fraction, but follows a linear trend. This increase is related to the load transfer from the matrix to the reinforcement, the generation of thermal dislocations (which is related to the difference in co-efficient of thermal expansion between the matrix and reinforcement particles) and the small substructure near the particle-matrix interface. However, composites reinforced with the larger 65 micron SiC particle did not show any significant improvement in mechanical strength, but the ductility of the sample was reduced significantly. This is because the large reinforcement particles are easy to fracture during tensile loading because of high intrinsic flaw density. Once particles are broken, the load cannot be effectively transferred from the matrix to the reinforcement, therefore the improvement in mechanical strength is reduced.

Short-term corrosion testing in 3.5 wt. % NaCl solution (24-hour immersion) of the composite material showed that they did not suffer accelerated corrosion as compare to the unreinforced material. The short term immersion tests were performed because they offer a quick analysis of the corrosion performance of the composite material. The corrosion rate and the polarization resistance of the composite material are similar to the unreinforced material. Although pitting corrosion near the particle-matrix interface are observed in the composite material. However, it is likely that the volume expansion of the corrosion product has filled up any voids near the interface, which can inhibit further ingress of the corrosive media into the material. The overall electrochemical behaviour of the aluminium was not significantly affected by the reinforcement addition during the short-term corrosion test.

Pin on disc wear testing was also conducted on the aluminium composite, using a ruby pin and with the aluminium material as the disc material. The unreinforced aluminium showed high wear rate with continuous delamination. The wear tracks of the specimens are measured to be very deep and wide. However, the composite material with 10 vol. % SiC reinforcement showed substantial improvement in wear resistance with about 4 times lower wear rate. When the volume fraction is increased to 20 %, an order of magnitude reduction in wear rate was obtained and the wear co-efficient is about 7 times lower than as compare to the unreinforced material. However, the corrosion resistance of the composite, especially at high volume fraction is significantly reduced as compare to the unreinforced material.