An appraisal of Characteristic Mechanical Properties of Aluminium 6061 alloy – Silicon Carbide (SiCp) Metal Matrix Composite (MMC) exposed to different thermal conditions

Abstract

Aluminium 6061 alloys have been proposed for extensive use in automotive engine applications and there have been discrete cases of experimental implementation. In order to enhance the usability of this material, it has been investigated in composite forms with various ceramic
reinforcements. Viability of the different constituents depends on the compatibility of their physical and chemical properties. The service conditions are characterized by extreme stress and temperature conditions very close to failure. Hence thermal stresses play an important role in success of these materials.
The difference in the CTE of the alloy matrix and the ceramic reinforcement results in residual thermal stress build up. It may so cause plastic deformation of the matrix in the vicinity of the reinforcement in order to reduce the residual stresses. However, mismatch in thermal strain
values may lead to cracking of the matrix in this process.
In comparison with the matrix and reinforcement, the interface is rather a porous, non- crystalline portion. Therefore residual stresses are released at these sites with relative ease. When the particle fraction is high, interface availability is more; hence, failure of the MMC is due to formation and propagation of cracks at the interfaces. On the contrary, when particle fraction is less,
interface availability is poor; failure is predominantly due to particle cracking. In the present work, cylindrical Al 6061-SiCp MMCs are fabricated in the solid state
processing route. The sintering temperature and time of holding at the sintering temperature are varied. The samples are subjected to no thermal shock and thermal shocks at +80 ºC and -80 ºC in different batches. The compressive strengths are determined using an Instron (1195 ) adopting the ASTM E9 standard for hard metals. Fractured specimen are extensively analyzed with an SEM for
failure modes.
Assessment and evaluation on the basis of mechanical properties reveal that at relatively higher sintering temperature and for short term use, the thermal shock is not much damaging. For short- term use, the thermal shock at an elevated temperature is more damaging for samples sintered at lower temperature. For long-term use, the thermal shock due to a sub-ambient temperature is
more damaging when test specimen is sintered at relatively a lower temperature. The micrograph studies reveal that in general when the thermal shock is due to exposure to
an elevated temperature, inter-diffusion is high, resulting in strong bonding; hence, the dominating failure mode is cavity generation due to generation of discontinuities at the interface. And when the thermal shock is due to exposure to a sub-ambient temperature, the dominant failure mode is interfacial failure and/or matrix damage.