High Temperature Mechanical Properties of Al–Si–Mg–(Cu) Alloys for Automotive Cylinder Heads

Abstract

To improve the fuel efficiency and reduce automobile emissions, there has been growing demand of more durable alloys for engine components with the improved thermal and fatigue resistance. This study examined the effect of alloying elements on the high mechanical behavior of Al–Si–Mg–(Cu) casting alloys for cylinder heads. Depending on the alloying elements affecting the strength of the matrix, the thermal expansion coefficient decreased with increasing Mn and Cu content at high temperatures with a concomitant increase in the elastic modulus, hardness and tensile strength. Quantatative analysis showed that the mechanical properties of the Al₂Cu precipitate hardened alloy were maintained at temperatures over 250°C, whereas the degradation of mechanical properties of the Mg containing alloy occurred at 170°C due to coarsening of the Mg₂Si precipitation phase. The LCF (low cycle fatigue) lives decreased with increasing alloy content according to the Coffin-Manson relation due to the smaller elongation. On the other hand, an analysis of the fatigue lives with the hysteresis loop energy, which consists of both strength and elongation, showed that the fatigue lives were normalized with an alloy of the same strengthening mechanisms regardless of the test temperature.