| Hypoeutectic Al-Si alloys is one of the most promising light metal alloys for railway vehicles nowadays for its low specific density, high specific strength, good castabilky and machinability, easy reclamation, energy save property, environmental protection etc., and so has great potentials in modern industry. Based on detail reviews of the progress of hypoeutectic Al-Si alloys, the present study investigated some hypoeutectic Al-Si alloys with moderate mechanical strength for non-heavy-stressed components in railway vehicles. Traditionally smelting processes like grain refining and modification, and mathematically orthogonal heat-treatment methods, were carried out together. The effects of Al-Ti-C refiner, Sr and Ti elements on the microstructure and mechanical properties of testing hypoeutectic Al-Si alloys have been investigated by means of mechanical property measurement, optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), respectively. The refinement mechanism of TiC and the interactivity between TiC and Sr have been discussed according to the experimental results. Influence on the heat-treated microstructure and properties of testing hypoeutectic Al-Si alloys caused by different melt processes, that is, refined by Al-Ti-C mater alloy and/or modified by the combination of Al-Ti-C mater alloy and Sr element, has been analyzed. Similarly, the influence of heat treatment parameters such as solution temperature, solution time, ageing temperature and ageing time on the microstructure and performance of testing hypoeutectic Al-Si alloys has also been studied.The main results are as follows:(1) The a -Al phases of hypoeutectic Al-Si alloys were greatly refined by Al-Ti-C addition and the mechanical properties were increased to some extent. Furthermore, the eutectic Si phases were modified to some extent too, though the acicular/flake structure could not be thoroughly changed by Al-Ti-C mater alloy. In the core of α-Al crystal, particles which are rich in Ti and C were found by meansof electron probe microanalysis (EPMA).(2) The eutectic Si phases were evidently modified by Sr modifier. But, the size of a -Al was relatively larger when compared with that of a -Al refined by Al-Ti-C refiner. When the addition of Sr exceeded 0.2%(mass percent), there appeared over-modified microstructure which led to poor mechanical properties. Combined with 0.05%Sr, much lower Al-Ti-C could lead to finer crystals of a -Al and eutectic Si than that added by either of Sr or Al-Ti-C alone. The performance of the hypoeutectic Al-Si alloys was enhanced owing to the finer crystals caused by the coexistence of Al-Ti-C and Sr. Furthermore, there was no "poisoning effect" during this experiment.(3) When Ti addition was 0.75%, the refinement effect of Al-Ti-C on a -Al phase disappeared. Using a combination of metallographic and micro-analytical techniques, coarse acicular/flake phases were found mainly constituted of Al, Ti, C and O. Within the coarse phases, it was found that the atom fraction of Al to Ti was approximately 3:1. This experimental result was beneficial to new type of hypoeutectic Al-Si alloys.(4) Various processing parameters of orthogonal heat-treatment result in different effects on microstructure and performance of testing hypoeutectic Al-Si alloys refined by either Al-Ti-C or "Al-Ti-C + Sr". It was found that hypoeutectic Al-Si alloys refined only by Al-Ti-C required longer solution time than that of alloys refined by "Al-Ti-C + Sr". The tensile strength and elongation of the heat-treated alloys were improved as a result of the conbination of Sr and Al-Ti-C. The results indicated that the best heat-treatment processing parameters for hypoeutectic Al-Si alloys refined by Al-Ti-C alone should be: 520°C X 18h +170°C X 8h. While the best heat-treatment processing parameters for hypoeutectic Al-Si alloys refined by "Al-Ti-C + Sr" should be: 520°C X 16h+170°C X 8h.The results of this investigation could provide references for further studies in high strength and high toughness hypoeutectic Al-Si alloys used in railway vehicles.
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