Thermal expansion behavior of aluminium matrix composites reinforced with Al₂O₃ fibres and graphite

Krzysztof Naplocha, Kazimierz Granat, Andrzej Janus Politechnika Wrocławska, Instytut Technologii Maszyn i Automatyzacji, ul. Łukasiewicza 3/5, 50-371 Wrocław, Poland

Quarterly No. 2, 2007 pages 76-82

DOI:

keywords: composite, fibre, graphite, thermal expansion

article version pdf (0.82MB)

abstract Thermal expansion behavior of aluminium matrix composite reinforced with alumina fibres and graphite have been reported. Preforms were infiltrated using direct squeeze casting method to produce composite with about 6.515.0 vol. % Al2O3 fibres (Saffil) and 1.512.0 vol. % graphite in form of flakes or fibers. Porous preforms with good permeability and appropriate strength reveal semi-oriented arrangement of fibres and graphite flakes. Binder o the base of silica was used to join alumina fibres and harden preform. The composite microstructures exhibit regular arrangement of alumina fibres with graphite flakes. Some problems with stirring of both types of fibres needed to incorporate extra operation during preform production. Observation of composite fracture revels that graphite-matrix bonds are rather weak and interface was free of any visible chemical reaction products. Tests of thermal expansion were carried out using direct dilatometric apparatus in 20300°C temperature range. For all composite samples first heating-cooling cycle curve runs with hysteresis loop leaving residual strain. It was especially evident for composites with low graphite content. The mismatch in thermal expansion coefficient CTE could introduce large stresses during manufacturing and correspondingly increase the dislocation density. This mismatch leads to compressive and tensile stresses during heating, respectively in matrix and reinforcement. After next cycles hysteresis slowly reduce due to relaxation of residual stresses and plastic deformation of the matrix. Comparison of strain-temperature curve for monolithic Al-Si7 alloy and its composites reveals that alumina fibres considerably constrain thermal expansion and addition of graphite slightly intensifies this effect. On the basis of strain results thermal expansion coefficients were calculate for all three cycles. The CTE increases with the temperature increasing, reaches a maximum and than decrease again. Thorough analyse of curve shape allows to ascertain that after each cycle this point of maximum moves towards the high temperature. It marks the moment when the stresses overcome the yield strength and plastic deformation of matrix occurs. Reinforcing of matrix with Saffil fibres results in decreasing of CTE especially in higher temperature range. Probably fibres which form rigid preforms constrain expansion of the matrix that becomes more plastic at higher temperature. Addition of graphite slightly decreases values of CTE over entire temperature range irrespectively is it fibres or flakes. Differences between composites reinforced with various amount of alumina and graphite show that the reduction effect on CTE is enhanced when alumina fibres content increases and graphite decreases.