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 AlSiMg(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 Al 2 Cu 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 Mg 2 Si 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.

Effect of Microstructure on High Temperature Mechanical Properties of A319 Casting Alloy for Automotive Cylinder Heads

“…In addition, the tan δ value changed at 349 C, which is coincident with the results showing that the reliability of the thermal expansion coef cient is limited over 350 C due to the collapse of the solid. On the other hand, for the A356 alloy 17) , as shown in Fig. 6(b), a change in tan δ was observed near 168 C, which represents the precipitation and growth temperature of Mg 2 Si, which coincides with the articial aging of the Mg-hardened aluminum alloys generally conducted at 160-180 C. This is in accordance with the results shown in Figs.…”

“…4(a). In particular, the addition of Cu reduced the thermal expansion coef cient signi cantly at identical temperatures, and a stable state was obtained up to 350 C for A319, whereas for A356 alloys stability was achieved at 250 C 17) . Alternatively, the elastic modulus decreased almost linearly with increasing temperature, and the elastic modulus decreased by as much as 7 GPa with increasing DAS to 70 μm.…”

“…The hardness of the eutectic phase increased with increasing DAS due to the enlarged size of the eutectic particles, whereas the hardness of the primary phase was similar regardless of the DAS, due to the precipitates that formed during heat treatment. As the temperature was increased, the hardness decreased and was maintained at temperatures greater than 200 C. Alternatively, without the addition of Cu, such as in sample A356, it was reported that the hardness decreased signi cantly and rapidly at temperatures greater than 150 C 17) . The decrease in hardness was attributed to the stability of the precipitate with chemical compounds.…”

“…The thermal expansion coef cient increased with increasing temperature to 250 C, became saturated up to 350 C, and then decreased with further increases in temperature. This can be explained by the softening of the solid with increasing temperature, at which the general properties of the solid cannot be maintained 16,17) . Therefore, the reliability of the thermal expansion coef cient over 350 C is limited due to the collapse of the solid.…”

Improvement of the Mechanical Properties of Al-7Si-0.35Mg Cast Alloys by the Optimised Combination of Alloying Elements and Heat Treatment

“…특히, Cu계 석출상은 β(Mg 2 Si)상 보다 열적 안 정성이 우수하여 알루미늄 주조재의 합금설계에 있어 고온강 도 향상을 위한 목적으로 첨가되고 있다 [10,11]. 본 연구에서는 A356합금의 조성을 기본으로 하면서 상온 인장강도 및 약 150 o C 이상의 고온강도를 향상시키기 위하 여 미량의 Mg, Cu를 첨가하되, 주조성을 저하시키지 않도록 그 첨가량을 각각 0.5 wt.% 및 0.7 wt.% 수준으로 제한하였 다.…”