Alexandre Velhinho

Functionally graded materials (FGM’s), particularly in the form of Al-Si metal matrix composites (MMC’s) selectively reinforced at the surface with SiC particles, are advanced materials, combining high wear resistance with high bulk toughness or even a thermal barrier at the surface. Centrifugal casting is one of the most effective methods for processing this type of MMC, but accurate control of the ceramic particles distribution/gradient in the metallic matrix has not yet been completely attained. In this work, precursor Al/SiC composites were prepared by rheocasting, using SiC particles and an Al-10Si–2.2 Mg alloy. Morphology of the SiC particles was previously characterized by laser interpherometry and SEM. Differing grain sizes were selected as reinforcing elements. The MMC’s were then molten and centrifugally cast in order to produce the FGM composites, whose structure and properties were investigated by XRD, quantitative image analysis of optical micrographs and longitudinal hardness profiles. Therefore, it was possible to evaluate the influence of the particle grain size on the structure and properties of the FGM. Apart from the evaluation of the effects of particle grain size per se, its influence when combined with differing casting conditions are reported as well.

The concept of functionally graded material (FGM), may be considered as a model particularly interesting to be applied in components for the automotive industry, if reliability and cost can be controlled in an advantageous way. In fact, coupling superior superficial wear resistance with a significant bulk toughness, without compromising important weight savings, by using Al-Si metal matrix composites (MMC’s) selectively reinforced at the surface with SiC particles, is likely to be considered as a innovative advance to that industrial field, if adequate production techniques are developed. Casting under a centrifugal force may well be considered as one of the most effective methods for processing Al-based FGM´s. A primary problem to be faced when producing MMC´s reinforced with ceramic particles is related with the imperfect wetting of the ceramic particles by the molten matrix alloy. A first consequence of defective wetting may be the formation of ceramic-ceramic, ceramic-gas and/or metal-gas interfaces, instead of the desired metal-ceramic interface. Secondly, wetting phenomena play an essential role regarding the physical, chemical and mechanical characteristics of the metal/ceramic interface. A general consequence of these aspects may be related with the degradation of the material properties, be it mechanical, chemical, or thermal in nature. The present work refers to an X-ray microtomography experiment aiming at the elucidation of some aspects regarding particle distribution in SiCp-reinforced functionally graded aluminium composites. Precursor composites were produced by rheocasting. These were then molten and centrifugally cast in order to produce the FGM composites. From these, small cylindrical samples were extracted and observed by X-ray microtomography at the European Synchrotron Radiation Facility (ESRF). The 3D tomographic images were obtained in edge-detection mode (phase-contrast mode), and an adequate segmentation procedure was employed to isolate the pores and SiC particles from the Al matrix. This has allowed a study of the relations between the matrix, the SiC particles, and locally intervening porosities of varying shapes, aiming at a better understanding of the mechanisms involved.

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The present work refers to an X-ray microtomography experiment aiming at the elucidation of some aspects regarding particle distribution in SiC-particle-reinforced functionally graded aluminium composites.
Precursor composites were produced by rheocasting. These were then molten and centrifugally cast to obtain the
functionally graded composites. From these, cylindrical samples, around 1 mm in diameter, were extracted, which were then irradiated with a X-ray beam produced at the European Synchrotron Radiation Facility.
The 3-D images were obtained in edge-detection mode. A segmentation procedure has been adapted in order to
separate the pores and SiC particles from the Al matrix. Preliminary results on the particle and pore distributions are presented.