Thermal–mechanical fatigue behaviour of cast aluminium alloys for cylinder heads reinforced with 15 vol.% discontinous Al 2O 3 (Saffil) fibers

Abstract

Thermal–mechanical fatigue (TMF) tests were done on the aluminium alloys AlSi10Mg0.3 and AlSi10Mg0.6, unreinforced and reinforced with 15 vol.% discontinous Al 2O 3 (Saffil) fibers. The tests were carried out with total strain constraint, triangular shaped temperature cycles, a minimum temperature of 50 °C and maximum temperatures from 200 to 400 °C. Both unreinforced alloys show nearly the same lifetime- and cyclic-deformation-behaviour. At all T max, the fiber reinforcement distinctly increases the lifetimes of AlSi10Mg0.3, whereas for AlSi10Mg0.6 this is only the case at T max≥300 °C. In the first cycle, higher peak stresses occur in the unreinforced alloys and the compound based on AlSi10Mg0.6 than in AlSi10Mg0.3+15% Saffil. All materials soften cyclically due to overageing of the peak aged matrix alloys. The rate of cyclic softening increases with T max and is higher in the unreinforced alloys and in AlSi10Mg0.6+15% Saffil than in AlSi10Mg0.3+15% Saffil. At T max≥300 °C, the initial difference between the peak stresses of the compounds vanishes within a tenth of the lifetime. So, nearly identical numbers of cycles to fracture occur at both materials. At T max≤250 °C, in AlSi10Mg0.6+15% Saffil higher peak stresses are acting until the fracture and lead to the lower lifetimes of this material. The age hardenability ofAlSi10Mg0.3 is reduced by the fiber reinforcement, whereas the fibers do not affect significantly the strength of AlSi10Mg0.6 in the T6 state. This was proven by microhardness measurements HV0.025 in the α-Al crystals of the fiber reinforced and the unreinforced alloys. In AlSi10Mg0.3+15% Saffil, an enrichment of magnesium in the fiber/matrix interfaces reduces the Mg content in the rest of the α-Al matrix below the necessary value for a full hardenability. In AlSi10Mg0.6+15% Saffil with its higher Mg-content, enough Mg for age hardening remains in the α-Al Matrix in spite of the Mg-enrichment near the fibers which also occurs in this material.