High temperature low cycle fatigue and creep-fatigue behavior of a casting Al-9Si-CuMg alloy used for cylinder heads

摘要

The low-cycle fatigue test and the compression creep-fatigue test of the cast Al-9Si-CuMg alloy for cylinder heads were carried out at high temperature. The cyclic mechanical response characteristics of the material under two kinds of test loads were analyzed. The microstructural evolution process and fracture mechanism of the material in the process of failure were studied, and the damage modes of the material under two kinds of loads were summarized. The results showed that, the stress-strain response (hysteresis loop) of the cast Al-9Si-CuMg alloy exhibited cyclic stability under high temperature low-cycle fatigue loading at 200 ℃, and the crack occurred at the holes and defects. When the temperature was raised to 350 ℃, the voids and defects in the material would no longer be the crack source, and the cyclic stress-strain response of the material would also show the characteristics of continuous softening. When coupled with the effect of compressive creep, the yielding radius and elastic modulus of the cyclic stress-strain response were almost unchanged, but the yield center moved upward significantly. Microscopic observation indicated that, to some extent, the effect of compressive creep at high temperature could prevent the initiation and propagation of the large-sized crack within the material, and prevent the rapid development of damage under low-cycle fatigue loading. The eutectic silicon particles inside the material firstly cracked under the compressive creep-fatigue load and formed many tiny cracks or cavities. These tiny cracks extended in the aluminum matrix and connected with the neighboring cracks until they broke.