鈹中和劑與鍶、銻改良劑對A357鋁合金微結構及性質之影響

Effects of Be neutralizer and Sr / Sb modifiers on the microstructure and properties of A357 alloys

楊進義(Ching-Yi Yang)

指導教授 ： 李勝隆

國立中央大學/工學院/機械工程學系/博士(2006年)

摘要

A357鋁合金(Al-7Si-0.7Mg)具有優良的鑄造性、焊接性、耐熱脆裂性及高比強度等優點，因此廣泛使用於航太與汽車工業的零件製造上。鐵為Al-Si-Mg合金中最常見的一種雜質元素，其極容易於鑄造時與Al、Si及Mg形成各種金屬間化合物，對材料的機械性質造成危害，添加中和劑可改變富鐵相的外貌至較無害的形態，故本論文探討A357鋁合金中添加不同比率的鐵與鈹，研究其含量對富鐵相之形態，以及合金機械與腐蝕性質之影響。此外，矽粒子在鋁合金中為硬脆的第二相，其大小與形狀會影響合金的耐磨耗性，而添加改良劑可改變共晶矽的形態，本論文將研究添加不同改良劑(鍶與銻)對A357鋁合金經T6熱處理後之滑動磨耗性質的影響。
本研究藉由光學顯微鏡、電子顯微鏡、電子微探儀、影像分析、導電度量測及熱差掃描分析等微結構之觀察與分析，結合拉伸、腐蝕與磨耗等性質的試驗，獲得以下結果：
A357鋁合金中存在的富鐵相有粗針狀(β-FeSiAl5)及中文字形(π-FeMg3Si6Al8)，鈹的添加可使兩種富鐵相轉變為球形不含鎂的Fe-Si-Al富鐵相，使基地中所固溶的鎂含量增加，並增加強化相(Mg2Si)的析出動力及析出量，進而提升合金的拉伸性質，同時，鈹的添加可讓矽粒子球化與細化，並減少富鐵相的數量，可增加合金的機械性質與耐腐蝕性，然而，鐵含量的增加，可使富鐵相的數量增加，導致機械與耐腐蝕性質的下降。
在磨耗試驗方面，A357鋁合金的磨耗行為會受到矽粒子形態與機械混合層(MML)的影響，對於添加改良劑的合金有較好的耐磨耗性，且鍶改良劑比銻改良劑的效果好，此乃因添加鍶改良劑的合金比添加銻改良劑與未添加改良劑的合金有較細小且球形的共晶矽，在磨耗過程中合金內有較少裂縫產生，另一方面，含鍶改良劑的合金在磨面上有一層較穩定的機械混合層，可保護基材免於磨損。

關鍵字

中和劑 ； 富鐵相 ； A357鋁合金 ； 耐腐蝕性 ； 矽形態 ； 機械混合層 ； 改良劑

並列摘要

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.

並列關鍵字

A357 alloy ； Neutralizer ； Iron-bearing phase ； Corrosion resistance ； Silicon morphology ； Modifiers ； Mechanically mixed layer

參考文獻

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