| dc.description.abstract | A357 aluminum alloy(Al-7Si-0.7Mg) is extensively used in the aerospace and automotive industries, due to its excellent properties that include castability, weldability, hot-cracking resistance and specific strength. Iron is the most deleterious impurity in the Al-Si-Mg cast alloy. Fe combines with Al, Si, and Mg to form various intermetallic compounds during solidification. The intermetallic phase is considered to have the worst effect, as it significantly decreases the ductility of material. Adding neutralizer to an alloy can change the shape of iron-bearing phase to the harmless shape. This work investigated how Be and Fe affect the morphologies of iron-bearing phase in addition to the mechanical and corrosion behaviors of A357 alloys. Furthermore, silicon is added to aluminum alloy as a second phase. The wear properties of aluminum alloys are significantly affected by the silicon morphology. The addition of modifier to A357 alloy can alter the morphology of the silicon particles. Therefore, this study examines the sliding wear characteristics of an unmodified A357 alloy and the alloy modified with Sr / Sb in the T6 heat-treated condition.
Microstructural features were elucidated by optical microscopy, scanning electron microscopy, electron probe X-ray microanalysis, image analysis, measurement of electrical conductivity and differential scanning calorimetry. The microstructure was correlated with tensile, corrosion and wear testing.
The results of present works revealed that many platelet-like (β-FeSiAl5) and Chinese-script (π-FeMg3Si6Al8) iron-bearing phases were found in A357 alloys. These structures are replaced by a nodular shape Mg-free structure of iron-bearing constituents when Be is added. Adding Be to the alloy can increase the level of solid Mg solution, change the morphology of silicon particles to a small and globular shape, subsequently reducing the amount of iron-bearing phases. Be can also enhance the precipitation kinetics and increase the quantity of Mg2Si precipitates to improve the tensile properties of A357 alloys. The corrosion behavior of A357 alloy was affected by the morphology of the silicon particles and the amount of iron-bearing phases. The corrosion resistance improved when the amount of iron-bearing phases was reduced and the silicon particles were spheroidized and refined. Analysis of the A357 alloys containing different quantities of Fe indicates that the amount of iron-bearing phases increases with increasing Fe content, decreasing the alloy’s corrosion resistance and worsening its mechanical properties.
The wear behavior of A357 alloys was influenced by the morphology of silicon particles and the stability of the mechanically mixed layer (MML). Sliding wear tests revealed that the addition of both Sr and Sb modifiers improves the wear resistance of the alloy. This beneficial effect of enhanced wear resistance was more apparent in the Sr-modified alloy than the Sb-modified one. This was attributed to the lower cracking tendency of the Sr-modified alloy owing to the near-spherical nature of silicon particles. Furthermore, the silicon particles in the Sr-modified alloy were finer and more spherical than those in the Sb-modified and unmodified alloys. Additionally, the decreased wear rate was also substantiated through the formation of a stable MML on the worn surface. Observations of worn surfaces showed more stable MML on the worn surface of Sr-modified alloy than for the unmodified and Sb-modified alloys. | en_US |