PSI - Issue 51

Lucia Pastierovičová et al. / Procedia Structural Integrity 51 (2023) 135 – 140 L. Pastierovi č ová et al. / Structural Integrity Procedia 00 (2022) 000–000

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energy consumption, environmental problems, etc., which are related to the casting of primary (ore) aluminum alloys as reported Kuchariková (2016). Secondary (recycled scrap) aluminium is a solution to many of the social and economic challenges that Europe is facing in its efforts to build a more and more sustainable and resource-efficient global economy. With the increasing amount of recycled scrap, the mixing and densification of impurities in the recycling of aluminum alloy components represents a serious problem, as the cast components are expected to exhibit higher working performance as reported by Závodská (2019). Secondary Al-Si cast alloys are typically used to produce castings for the automotive industry, especially for engine parts such as cylinder heads, engine blocks, etc., owing to their high specific strength, excellent castability, and good corrosion resistance as reported by Ceschini (2020) and Závodská (2019). Especially A357.0-T6 cast alloys used in the automotive industry are subjected to cyclic loading also fatigue failure. The fatigue failure prediction is complicated due to the various physical-mechanical processes. These processes, which prevail in the gradual accumulation of fatigue damage during the fatigue life of a metal component are complex and interconnected according to Islam (2018). The cyclic plasticity of such alloys depends on many parameters including size, shape, and distribution of Si and intermetallic phase precipitations, coherence to the matrix, heat treatment, casting defects, etc., as reported by Islam (2018) and Tupaj (2015). As these casting alloys are made by recycling, they contain a higher amount of undesirable elements. One of the most harmful elements is iron, precipitating in the form of long sharp needles, which by their shape act as a stress concentrator and crack initiator thus promoting casting defects such as shrinkage and porosity. According to Kuchariková (2018), one of the possibilities to minimize the negative Fe impact is heat treatment. Several research works have been extensively focused on the heat treatment of alloys that can improve the mechanical and fatigue properties of the alloys. The authors Islam (2018) and Kuchariková (2018) agree that heat treatment of Al-Si-Mg alloys improved the fatigue properties by refining the silicon particles and transforming them into globular shape, not only those but also refining the shape of the needle-like Fe particles. However, this did not affect the casting defects which, even after heat treatment, still act as initiation crack sites. All these modifications affect not only the structure and fatigue properties but also the corrosion behaviour of the alloy. The automobile components made of A357 cast alloy are susceptible to several corrosive environments, such as defrosting salts, windshield washer fluids, etc., combined with fatigue from the engine motor vibration. The majority of literature results about the interactions between fatigue and corrosion have found that the reduction of fatigue life in NaCl solutions might be attributed to premature crack initiation from surface corrosion defects. Dolley et al. (2000) proved for AA 2024-T3 alloy that the fatigue life was significantly reduced by more than one order of magnitude after 384 hours of precorrosion in a 0.5 M NaCl solution than uncorroded specimens. The decrease in fatigue life was related to the time exposure to the corrosive environment and the corrosion pits were evaluated as crack initiation sites. Mutombo and du Toit (2011) have characterized the fatigue–corrosion behavior of a welded 6061-T651 aluminum alloy in a 3.5% (by weight) NaCl solution. The fatigue life reduction in the corrosive solution compared to the fatigue life in air was strongly related to the presence of pits, which acted as preferential fatigue crack initiation sites. These results might be helpful, however, until now no one has done research on the simultaneous fatigue and corrosion properties of secondary Al-Si cast alloys. Therefore, the aim of this study is to investigate chemical mechanical interactions during the simultaneous application of cyclic stress and exposure to a corrosive environment of a progressive secondary A357.0-T6 cast alloy. 2. Materials and methodology In the present work, a secondary A357.0-T6 hypoeutectic cast alloy was used as an experimental material. The alloy was supplied by UNEKO, Ltd (CZ) company and delivered in the form of bars ø 20 x 300 mm. Three melts with different Fe content were cast into sand moulds by the gravity die casting method. The melt was refined with ECOSAL AL 113S refining salt at 740 – 745 °C. Heat treatment T6 was applied to the experimental bars consisting of solution heat treatment at 530 °C ± 5 °C for 7 hours then rapid cooling to the temperature of 50 °C and artificial ageing at 160 °C for 6 hours. The chemical composition of supplied A357.0-T6 according to the delivery list is shown in tab.1.