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Assessment of semisolid forming of aluminium metal matrix composites

Taking advantage of thixotropic properties, semisolid state processing of metal matrix composites offers many advantages over conventional shaping processes. These advantages include reduction of operating temperatures, porosity, shrinkage, energy consumption, tool wear, damaging of the reinforcing phases and segregation of the reinforcing phases as well as easier machining. The main objective of the current research is the assessment of semisolid state processing as a new near net shape forming technique for metal matrix composite (MMC). The rheological behaviour of semisolid MMC in the semisolid state. Microstructural and mechanical characterisation will, finally, assess the potential of semisolid forming of MMC.
Partially solidified aluminium silicon(7)magnesium(0.3) matrix alloys and composites are pseudoplastic and thixotropic, which is the result of the specific structure of semisolid slurry and the interactions of the primary particles. In addition to earlier experiments, viscosity changes were monitored as a function of waiting time during which no stirring was applied. As a function of waiting time, the viscosity increases. It is found that the viscosity increase, which is attributed to agglomeration and bonding of particles during the period of rest, is a long time effect, whereas return to the steady state value of the viscosity after recommencement of stirring, which is attributed to disruption of particle bonds, is a much shorter time effect. It was further found that the presence of silicon carbide inhibited bond formation and thus lowered the thixotropic nature of the slurry. Furthermore, the composite was partially remelted by heating in the semisolid region. Globularisation of the structure takes place more rapidly in the composites than in the matrix alloy, which is very beneficial for the forming capabilities of the material. Compression tests in the semisolid state using a standard compression machine confirm the advantage of having a composite material since the stresses needed to deform the composites are smaller than those required for the matrix alloys.

Metal matrix composites have applications in the automotive industry and in sports equipment.
For better exploitation of this work, a partnership is necessary to work in industrial conditions and develop the use of these materials.

Reported by

Technische Universiteit Delft
Rotterdamsweg 137
2628 AL Delft
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