製程參數對鑄造鋁合金品質影響之研究; Effect of processing parameters on the quality of Al alloys

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题名:	製程參數對鑄造鋁合金品質影響之研究;Effect of processing parameters on the quality of Al alloys
作者:	黃立伍;Li-Wu Huang
贡献者:	機械工程研究所
关键词:	氧化膜;超音波震盪;尖晶石;莫來石;紅柱石;反光剝落區;界面反應;稀土金屬;除氣;介在物;相對孔洞率;foggy area;ultrasonic-vibration treatment;spinel;interfacial reaction;degassing;inclusion and relative porosity;rare earth metal;mullite or andalusite;oxide film
日期:	2003-06-25
上传时间:	2009-09-21 11:32:41 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	本論文分三部份；第一，鋁中的氧化膜分析；第二，除氣對鋁中氧化膜與介在物的影響；第三，氧化膜與介在物對鋁合金的相對孔洞率影響。第一部份鋁中的氧化膜分析鋁合金中的氧化膜(oxide film)在基地中極不容易觀察或辨識，本篇利用光學顯微鏡(optical microscopy, OM)結合超音波震盪技術(ultrasonic vibration treatment)，來診斷或顯現鋁合金鑄件中的氧化膜位置，藉此分析鋁鑄件中氧化膜。以超音波洗淨器震盪後的鋁合金試片拋光表面，顯示出不同外形的反光記號(reflective areas、shining areas or foggy areas)，即為氧化膜的剝落處。純鋁鑄件拋光面上的氧化膜型態為尺寸較大的長條羽毛狀(feather like)，鋁矽合金中的矽為減少氧化膜霧化區的寬度及長度，而在Al-XSi-YMg (X:6-7%, Y:0-1%)中鎂含量的增加，會增加點狀的霧化區數目外，鎂亦會與鋁中的氧化膜產生化學反應形成尖晶石(spinel)產物。這尖晶石會改善鋁基地與膜之間的鍵結能力。而在上述超音波震盪的同時蒐集剝落的氧化物顆粒，並以電子微探儀(EPMA)與SEM附加能量散射光譜儀(EDS)進行其微區組成的分析(microanalysis of constituents)。視矽含量或氧化矽含量的多寡，氧化物微粒會轉變為次圓形或圓形。增加鎂含量或氧化鎂(或尖晶石, spinel)含量，且需考慮氧化鋁、氧化矽及氧化鎂等三者含量的多寡，氧化物微粒會變為不規則狀或片狀。若其中的氧化鎂佔大部份且取代氧化鋁則變成節瘤狀。第二部份除氣對鋁中氧化膜與介在物的影響除氣是生產鋁合金鑄品不可缺少的製程，但在高速攝影機(10-3 s-1)與專業軟體Fluent5.0的模擬雙重驗証下，得知除氣會增加鋁液中的氧化膜與介在物(inclusion)的含量。因當除氣氣泡上升至鋁液表面時，在Young’s equation中束縛力、表面張力與氣泡浮力的不平衡，使氣泡在鋁湯液面(鋁液與大氣的界面)產生爆破後，飛散的鋁液珠瞬間形成氧化物，而此同時鋁液中形成的對流場，將氧化鋁液珠捲入鋁湯中增加了鋁中的氧化膜或介在物來源，使除氣後的Al-7wt%Si鋁湯中氧化膜與介在物的數目(count/mm2)因而增加。但A356鋁合金除氣後因其氧化物在鋁湯中聚集、結合，使其除氣後冷激片的氧化膜面積率(area ratio of foggy area, %)下降外，介在物數目亦降低。第三部份氧化膜與介在物對鋁合金的相對孔洞率影響鋁合金鑄件中通常存在氣孔，而密度或孔洞率的量化常是評估其品質的指標，本研究利用相對孔洞率[(dc – dr)/ dc；dc為鑄件冷激片密度，dr為鑄件減壓片密度]來評估並建立純鋁、Al-6Si、Al-13Si與A356合金的相對孔洞率，對其鋁液中所測氫含量的關係。純鋁中的氧化物大部份是氧化鋁且其與基地界面完整，此界面幾乎沒有發現有任何裂縫存在，因此純鋁中的相對孔洞率在本實驗中最低，但增加純鋁中的矽含量(即Al-6Si合金)，則會增加鋁液中的介在物數目，且大部份的氧化物皆含有矽的莫來石成份，因此與純鋁比較氫的擴散時是顯得較困難的，此外其合金的凝固粥狀區也較大，造成鋁矽鑄件凝固時成核氣孔的機會大增，這也就是造成Al-6%Si合金在高含氫量時，其相對孔洞率指數式增加的原因，雖然Al-13%Si合金在的相對孔洞率對氫含量的趨勢與Al-6%Si合金很一致，但它在凝固時大量潛熱釋放，與本身有較低的介在物數目，使其相對孔洞率較Al-6%Si合金低。 Al-Si-Mg合金中鎂的含量增加，造成氧化膜含量的增加，加速氣泡的預先成核機會增加，使Al-7%Si-0.51%Mg合金在本實驗中的相對孔洞率最高。而A356高鍶合金中，鍶的添加增加矽的表面積且礙氫的向外擴散，且降低鋁合金液的表面張力，此幫助凝固時氣泡的成核成長更容易，使試片的孔洞形成數目多、分佈均勻但尺寸小，但其試片最後的相對孔洞仍比Al-6%Si高。A356低鍶合金中的介在物大部份為尖晶石，其與鋁液間界面能高使其孔洞成核速率減緩，最後獲得比Al-6%Si平均更低的相對孔洞率。 This dissertation mainly contains three related issues as following; (1) Analysis of the oxide film in Al-XSi-YMg alloys, (2) Effect of degassing on the oxide film and inclusion of Al-Si alloys, (3) Effect of oxide film and inclusion on the relative porosity of Al-(0-13%)Si and A356 alloys. The details of each part were addressed behind: (1) It’s experimentally difficult to directly observe and differentiate the entrapped oxide films in aluminum matrix of a casting. A few researchers have focused on the morphology of entrapped oxide films in aluminum castings and fully discussed there. Their morphology and size of oxide film was analyzed mostly based upon scanning electron microscopy (SEM) equipped with EDAX analysis. Using the ultrasonic-vibration treatment (UVT) to reveal the exact position of an entrapped oxide film on the polished surface of a specimen is the main technique used before diagnosis and analysis. Therefore, the entrapped oxide film of a specimen in the morphology, the size and its distribution were investigated and measured here. The polished samples were placed on the bottom of the container in ultrasonic vibrator. Foggy marks (as reflective areas) would gradually show on the polished surface of chilled sample during ultrasonic vibration treatment. These marks were distinguishable and had been identified as oxide film. The shining marks shown on the chilled samples of pure Al are usually lengthy and feather-like strips. Increasing silicon content in the Al-Si alloys decreases the extent of shining cloud and/ or reduces the length of shining strips. But the counts of shining spots are increased. Function of Mg in the Al-XSi-YMg (X:6-7%, Y: 0-1%) alloy tended to reduces the size but greatly increased the counts of shining spots. Mg in the Al-Si-Mg melt causes a reaction to form a spinel on the interface of the oxide film and the melt. This improved the bonding between the matrix of the aluminum alloy casting and the entrapped oxide film. The oxide film broke into particles during ultrasonic vibration treatment. These particles were collected by filtration from the water in the container of the ultrasonic vibrator. The constituents of oxide particles were also analyzed by EDAX and/or EPMA mappings. Summarily, the oxide film entrapped in the aluminum casting may be in the form of cloud- or strip-type of clustering particles. The imposed ultrasonic energy breaks the oxide film into pieces. The morphology of oxide particle may take various shapes depending upon its constituents. If the oxide film is rich in alumina, the shape of the particle is angular. Increasing silicon content or the amounts of silica, oxide particle turns out to be sub-rounded or rounded depending on the amount of silica existed in the oxide particle. Increasing the magnesium content increases the amount of magnesia or spinel. The oxide particle becomes irregular in shape or flake-like, depending on the relative contents of alumina, silica and magnesia. If alumina has been significantly replaced by magnesia, the particle becomes lump. The area from where the oxide particles detached, appeared as an eroded or flat zone under SEM observation, but was identified as a shining mark by optical observation. The oxide film entrapped in different aluminum alloy castings varies and depends on its constituents. (2) Degassing treatment introduced in producing an aluminum casting dominantly eliminates the inclusions and then upgrades the quality of the melts. Effect of degassing treatment on inclusion particles of aluminum alloy chilled samples were then fully assessed and discussed here. Experimental observations showed that inclusion particles entrapped in chilled samples were mostly below 10 μm. A thin slice cut from chilled block (50 mm in diameter, 10 mm in thickness) was cooled by liquid nitrogen and then fractured into two pieces revealing the inclusion particles existed on fractured surface. In melting process, inclusion particles suspended in Al-melts can be effectively reduced by the floatation and/or sedimentation. However, experiment results indicated that degassing treatment by diffuser resulted in increasing inclusion particle counts in the Al-7Si melts. Such increase results from breakage of free surface of melt and bubbles collapsed on free surface. The particles entrapped into melt following the movement of convection loop near surface layer of melt during degassing treatment. (3) The oxide, mostly alumina, in pure aluminum indicates a sound interface. This interface between matrix and the oxide film hardly shows any visible cleavage or gap. Reasonably, relative porosity for pure aluminum demonstrates the lowest value in this study. Increasing silicon content in the Al melt increases the particle counts and its oxide films are mostly containing silicon; or mullite. Silicon in the Al-melt reduces the solubility of hydrogen but it displays a great affinity to attract hydrogen. Hydrogen is much more difficult to diffuse out of solidified Al-6%Si melt than that of pure aluminum. Additionally, the greater extent of mushy zone in Al-6%Si alloy results in increasing the possibility for pore nucleation during solidification. Therefore, Al-6%Si alloy exponentially increases the relative porosity at high level of hydrogen. Although, Al-13%Si alloy suggests the same trend following the Al-6%Si alloy in relative porosity, it obtains the lower value due mainly to the light reduction of inclusion content and higher latent heat release of silicon. Increasing magnesium (>0.5 wt%) in Al-7%Si-0.51%Mg alloy increases the amount of oxide films suspended in the melt. The oxide film have potentially provided the more pre-existing nucleation sites and probably accumulated the gas from the creeping flow of the melt for bubble formation. With the high level of hydrogen, the bubble initially accelerated to grow into a big pore resulting in the high relative porosity obtained in reduced pressure test sample. Adding Sr in A356 melt modified the eutectic silicon and increased the total amount of surface area of Si to retard the diffusion of hydrogen out of the melt during condensation. With the assistance of Sr addition, bubble favorably nucleated and grew in A356 melt. This resulted in pores uniformly distributed but smaller in size than that of Al-6%Si alloy over the specimen. However, A356 alloy still obtained the higher relative porosity (>5%) than that of Al-6%Si owing to its uniform distribution of pores. In A356 without Sr, the high fraction of spinel of oxides (including inclusion and oxide film) existed in matrix resulted from the interaction of oxides and melt during smelting. The high interfacial energy between spinel and melt alleviated the nucleation rate of embryonic bubble. The relative porosity, therefore, is totally lower in A356 without Sr comparing with that of Al-6%Si and A356 alloys at any levels of hydrogen measured in the melt.
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