알루미늄 스크랩 첨가 비율 변화에 따른 주조용 Al-Si-Mg계 합금의 기계적 특성에 관한 연구

Abstract

Demand for aluminum alloys in the automobile and aviation industries has increased due to environmental regulation, which has led to an increase in raw material prices and attracted interest in the use of aluminum scrap in the aluminum casting industry on economic grounds. Although the use of aluminum scrap is a useful method in economical focus, there are many considerations on the reliability when using the recycled alloy as structural materials. In order to define the proper scrap addition ratio and the properties of recycled AC4A alloy, the evaluation of AC4A primary ingot with the scrap addition ratio of 20wt.%, 35wt.%, and 50wt.% was conducted. Afterwhich the non-addition case was also evaluated and compared with recycled alloy. The scrap used in this study were a gating system and riser part collected from the casting process. The solidification cracking strength of AC4A alloys was evaluated according to a scrap addition ratio and the tensile property of both as-cast and T6 heat-treated states was evaluated as well. After that, the effect of scrap addition ratio was studied through observation of fracture surface. As the scrap addition ratio increased, the solidification crack strength, tensile strength and elongation decreased. Moreover, the oxide film distribution increased in the fracture surface of the solidification cracking test specimen and the tensile test specimen, which it deteriorates the solidification cracking and tensile properties of the alloy. According to the test result, the scrap addition ratio of 35wt.% is that satisfies the KS standard requirement. In the AC4A primary ingot with 35wt.% of scrap(hereinafter refer to as the recycled alloy), the tensile, impact, and fatigue properties in both as-cast state and T6 heat-treated states were evaluated in three conditions (No addition, Sr-modified, the grain refined and Sr-modified). The addition of the eutectic Si modifier and the grain refiner changes the size and distribution of the oxide films in the process of modifying the morphology of the eutectic Si and refining the α-Al dendrite. Therefore, the tensile properties of the recycled alloy were improved both in as-cast and T6 heat-treated states, and the KS standard requirements were satisfied. The crack initiation and propagation energy on the as-cast and T6 heat-treated state of the recycled alloy increased through the modification of eutectic Si and grain refinement in the impact property evaluation. In the observation of the vertical section and fracture surface of the impact test specimen, the crack deflection and propagation proceeded along the oxide film and the acicular eutectic Si, and the crack deflection length was similar to the length of the eutectic Si. It can be seen that the impact property depends on the size and distribution of the oxide film and the shape of the eutectic Si. The recycled alloy improved the fatigue limit in the as-cast and T6 heat-treated state through the improvement of the eutectic Si shape and grain refinement in the evaluation of fatigue property, Crack initiation occurred at the location where the oxide film and inclusions exist and observed from the fracture surface of the fatigue test specimen. The crack deflection length observed on the fracture was similar to the size of α-Al dendrite. Fatigue crack growth is affected by the size of α-Al dendrite rather than the morphology of eutectic Si, and crack growth is delayed as the size of α-Al dendrite gets smaller. It can be seen that the fatigue property depends on the size and distribution of the oxide film and inclusions, and the size of α-Al dendrite. When using aluminum scrap, sludge formation and oxide film entrainment problems are more likely to be affected by melt treatment method and casting design. Therefore, when using aluminum scrap, systematic management of the melt cleanliness is required according to the manufacturing process.