ピストン用Al-Si系合金の熱疲労強度

アルミニウム合金の熱疲労強度に関する研究(第2報)

抄録

Failures due to thermal fatigue often occur on the pistons made of aluminum alloys, especially on those of Diesel-engines. This paper reports the thermal fatigue resistance of Al-Si casting alloys used for pistons. The resistance to thermal fatigue was evaluated by measuring the number of cycles to failure versus total amount of strain, constraint stress or plastic strain. An apparatus for thermal fatigue testing was developed, where a high frequency induction coil was employed as a heating device. The results obtained were summarized as follows:
(1) The thermal fatigue resistance of Al-Si alloy castings was in the following order:AC8B>AC8A> 15% hypersilumin> 18% hypersilumin> 23% hypersilumin. This indicates that increase in Si content lowers the resistance to thermal fatigue.
(2) The resistance to thermal fatigue was related to elongation of specimens at tensile tests. Namely, materials with higher ductility showed higher resistance.
(3) The hypoeutectic and eutectic Al-Si alloy castings failed owing to stress concentration by an necking effect produced by plastic deformation.
(4) The hypereutectic Al-Si alloy castings, especially containing more than 18%Si, failed by preferential cracking in primary Si crystals. The cracking was caused by the difference in thhe amount of thermal expansion between Si crystals and the Al matrix.