02-08-2014, 04:23 PM
Effect of reinforcements and heat treatment on tensile strength of
Al-Si-Mg based hybrid composites
Effect of reinforcements.pdf (Size: 378.48 KB / Downloads: 39)
Abstract:
Metal Matrix Composites (MMCs) constitute an important class of design and weight-efficient
structural materials that are encouraging every sphere of engineering applications and mainly in aerospace
applications. The present investigation is focused on formation of a hybrid composite by using industrial waste
fly ash and E glass short fibers by dispersing them into AA7075 alloy by Stir casting method. The MMC is
obtained for the different compositions of E-glass and Fly ash particulates. The test specimens are prepared to
the standard size by turning and facing operations to conduct tensile test and the specimens are subjected to heat
treatment. The specimens were tested for tensile test at different loads by using Universal Test Machine. The
results are plotted and it is concluded that the MMC obtained has got better tensile strength compared to
Aluminum alloy (7075) alone. Further, tensile strength slightly increased with I hour aging heat treatment.
. Introduction
Conventional monolithic materials have limitations with respect to achievable combinations of strength,
stiffness, and density. In order to overcome these shortcomings and to meet the ever-increasing engineering
demands of modern technology, metal matrix composites are gaining importance. In recent years,
discontinuously reinforced Aluminium based metal matrix composites have attracted worldwide attention as a
result of their potential to replace their monolithic counterparts primarily in automobile and energy sector. The
basic idea is that continuous fiber reinforced composite has better strength but the processing methods is
highly expensive which hinders their adoption. The continuous fiber reinforced composites do not allow
secondary forming such as rolling, forging and extrusion. As results of these limitations new efforts on the
research of discontinuous reinforcements is to be use. At early stages of development of metal matrix
composite emphasis was given on the preparation of fiber reinforced composite only. But due to the high cost
associated with the process of production, anisotropic properties of the resultant composite and difficulties
associated with the fabrication process, production of this type of composites has been limited. Now a days the
particulate reinforced Al matrix composite are gaining importance because of their low cost with advantage
like isotropic properties. The strengthening of Aluminium alloys with dispersion of fine ceramic particulate
composite materials were developed as an alternative of unreinforced alloy, for obtaining materials with high
stiffness (high strength/modulus and low density) with special interest for the wear resistant and structural
applications. The dispersion strengthened alloys can be classified, based on the size and volume % of particles
uniformly dispersed in the matrix.
Aluminium alloys reinforced with ceramic particles exhibit superior mechanical properties to
unreinforced Al alloys and hence are for engineering applications. The aluminum metal matrix composites are
produced by casting route. The former has the advantages of producing the composites as lower cost of
production and possibility of producing larger components. However, the inherent difficulties of casting route
are non-wettability of ceramic particles by liquid aluminium segregation of particles, higher porosity level and
extensive inter-facial reaction due to higher processing temperature. Wettability of the particles can be
Stir Casting
This involves incorporation of ceramic particulate into liquid aluminium melt and allowing the mixture to
solidify. Here, the crucial thing is to create good wetting between the particulate reinforcement and the liquid
aluminium alloy melt. The simplest and most commercially used technique is known as vortex technique or
stir-casting technique. The vortex technique involves the introduction of pretreated ceramic particles into the
vortex of molten alloy created by the rotating impeller. Lloyd (1999) has reports that vortex-mixing technique
for the preparation of ceramic particle dispersed aluminium matrix composites was originally developed by
Surappa& Rohatgi (1981) at the Indian Institute of Science, Bangalore. Subsequently several aluminium
companies further refined and modified the process which is currently employed to manufacture a variety of
aluminium metal matrix composites on commercial scale.
The vortex method is one of the better known approaches used to create and maintain a good distribution
of the reinforcement material in the matrix alloy. In this method, after the matrix material is melted, it is stirred
vigorously to form a vortex at the surface of the melt, and the reinforcement material is then introduced at the
side of the vortex. The stirring is continued for a few minutes before the slurry is cast. There are different
designs of mechanical stirrers. Among them, the turbine stirrer is quite popular. During stir casting for the
synthesis of composites, stirring helps in two ways: (a) transferring particles into the liquid metal, and (b)
maintaining the particles in a state of suspension. Micro structural in homogeneities can cause notably particle
agglomeration and sedimentation in the melt and subsequently during solidification. Non-homogeneity in
reinforcement distribution in these cast composites could also be a problem as a result of interaction between
suspended ceramic particles and moving solid-liquid interface during solidification. Generally it is possible to
incorporate up to 30% ceramic particles in the size range 5 to 100 µm in a variety of molten aluminium alloys
.The process is not suitable for the incorporation of sub-micron size ceramic particles or whiskers. Another
variant of stir casting process is compo casting. Here, ceramic particles are incorporated into the alloy in the
semi solid state.
Conclusion
MMC of Al (7075) reinforced with E-glass and fly ash particulates are found to have the improved tensile
strength Compared to Al (7075) alloy alone. Hence, the MMC formed is superior to Al (7075), with almost
same density as that of the individual. Further, The specimen C8 (with 3%fly ash &2% E-glass fiber
composition) of solution heat treatment and 1 hour aging exhibit excellent tensile strength. i.e 254.09N/mm2 ,
3 and 5 hours of aging has not incresed the tensile strength of the composite