汽車熱交換器用Al-Mn系合金製程中分散相演化及再結晶行為之研究; Evolution of dispersoid and behavior of recrystallization on Al-Mn alloys during producing process of automobile heat exchanger

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Title:	汽車熱交換器用Al-Mn系合金製程中分散相演化及再結晶行為之研究;Evolution of dispersoid and behavior of recrystallization on Al-Mn alloys during producing process of automobile heat exchanger
Authors:	黃信文;Hsin-wen Huang
Contributors:	機械工程研究所
Keywords:	均質化處理;分散相析出粒子;固溶度;二次再結晶;擠製成形;再結晶;硬銲處理;Extrusion;Solid solution;Precipitation;Dispersoid;Homogenization treatment;Recrystallization;Brazing;Second recrystallization
Date:	2008-12-31
Issue Date:	2009-09-21 11:52:48 (UTC+8)
Publisher:	國立中央大學圖書館
Abstract:	本論文主要是對汽車熱交換器用之3003鋁合金，在工業製造上所遭遇的問題，進行系統性的研究與分析。為了提升汽車用鰭管式熱交換器的散熱效率，其多穴tube之截面設計非常複雜，故常使用成形性極為優良的3000系鋁合金，進行多穴tube之擠製成形。而3000系鋁合金在擠製前之均質化處理，會產生分散相粒子的析出。利用均質化條件之設計，控制分散相粒子的析出結果及材料的固溶狀況，便可以掌握擠製成形的加工性及成品特性。因此，首先研究3003鋁合金分散相粒子於不同均質化條件下之析出演化行為。本研究設計出八種不同的均質化條件，以觀察3003鋁合金之分散相粒子，在析出演化時之過程與結果，及其對擠製成形性的影響。在均質化處理過程中，分散相的析出演化，取決於核生成、核成長、Ostwald ripening mechanism及異質析出的進行。均質化處理初期，分散相粒子開始成核，然後經過成長階段，在Ostwald ripening mechanism之回溶及粗化的過程後，完成了最後的析出狀態。其中，在較低的均質化溫度中(400℃, 460℃)，傾向於析出較不穩定的灰色Al6(Mn,Fe)粒子，在較高的均質化溫度中(600℃)，則傾向於析出較穩定的黑色α- Al12(Mn,Fe)3Si粒子。而在600℃x9h?460℃x3h這種最後溫度低於先前溫度的階段式均質化條件中，容易經由異質析出的行為，產生雙色共存的析出粒子。擠製成形時所產生的再結晶舉動，是決定加工成品機械特性的關鍵。承續上述研究，挑選出均質化處理後，固溶及析出狀態差異性較大的四組條件，進行均質化條件對3003鋁合金擠製成形時再結晶舉動之影響研究。鑄錠經由四種不同的均質化處理後，進行擠製成形，以調查不同的固溶量及析出狀態對擠製再結晶舉動的影響。OM、SEM的微結構觀察、導電度量測、擠製突破壓力擷取以及硬度分析為本實驗的研究方法。結果顯示，低溫均質化傾向於析出細密的分散相粒子，這些粒子會產生強烈的差排釘阻效應，使得差排不易移動，擠製再結晶亦較困難；高溫均質化則傾向於析出粗疏的分散相粒子，差排釘阻效應較薄弱，而擠製再結晶則較容易。然而，在本研究中最高的均質化溫度630℃處理後，幾乎沒有分散相粒子的析出，擠製再結晶卻較600℃均質化處理來得困難。推測是因為其超大量的固溶原子，亦會對差排的移動造成限制，而產生這個現象。另外，雖然階段式均質化條件600℃x9h?460℃x3h的固溶量，比600℃x9h均質化條件低很多，但在析出分散相粒子的數量及分布相近的情況下，再結晶的狀態亦幾乎完全相同。最後，將不同的固溶量和析出狀態，對於擠製再結晶行為之影響，區分為被拉長的再結晶粒，以及等軸的再結晶粒兩種形成模式。汽車用鰭管式熱交換器，在其多穴tube擠製成形之後，必須要與鰭片硬銲接合，才能製成熱交換器。而硬銲接合通常是在600℃x10min的環境下進行。在如此高的溫度下，擠製再結晶極有可能產生變化，而影響材料的機械特性。所以，本研究選出了加工成形性較佳的四組均質化條件，以進行汽車熱交換器用3003鋁合金擠製成形性及硬銲特性之研究。首先，施以不同均質化處理，可發現460℃x9h低溫均質化處理會析出緻密的分散相粒子；而有經過600℃x9h過程的均質化處理，分散相粒子皆較為稀疏。接著，進行擠製成形，可發現當析出粒子較為粗大稀疏時，擠製突破壓力由固溶程度所主導，且再結晶較容易；析出粒子細小緻密到一個程度時，固溶度的主導地位將被其所取代，再結晶較困難。而擠製後硬度，以完成再結晶的部分硬度較低，未完成再結晶的部位硬度較高。最後，進行硬銲處理模擬，發現460℃x9h的條件會從部份再結晶轉變為完全再結晶，強度降低；600℃x9h→460℃x3h的條件發生二次再結晶的局部區域，硬度大幅下降。 The 3003 aluminum alloy which contains Mn, Fe and Si as alloying elements is widely used in the container, packaging, and automobile industry, because of its excellent specific strength, corrosion resistance and formability. During solidification, most of the Mn atoms can be solid-dissolved in the aluminum matrix, which results in a supersaturated solid solution. This supersaturated solid solution decomposes via the precipitation of dispersed particles during the homogenization treatment prior to hot rolling or extrusion. Therefore, controlling the size, density and distribution of the precipitated particles, as well as quantity of Mn atoms in the solid solution during homogenization are very important. First, we study evolution of precipitation during different homogenization treatments in a 3003 aluminum alloy. The evolution of the precipitation of second phase particles dispersed in a DC cast 3003 aluminum alloy during different homogenization treatments was investigated. Eight kinds of homogenization conditions were designed. We conclude that the evolution of precipitated dispersed particles during homogenization is controlled by nucleation, growth, Ostwald ripening process and hetero-precipitation. Nucleation of the particles would occur first during the initial phase of homogenization. They would then undergo a process of growth, dissolution and coarsening, before reaching the final state of precipitation. Two-color particles usually appear at step-homogenization, which has a lower later temperature, 600℃x9h?460℃x3h, due to a hetero-precipitation behavior. The mechanical properties of extrusion products are mainly determined by the final result of the extrusion recrystallization. Following the priority study, we used the four conditions which had the largest difference between the precipitation and the solution quantity in the eight designed conditions to study evolutionary behavior of recrystallization during the extrusion of Al-Mn alloys. The different solution quantities and precipitation states in a homogenized Al-Mn alloy, and the effects of these on recrystallization behavior during extrusion were investigated. Homogenization at a low temperature of 460℃ resulted in a plentiful precipitation, which acted to pin down dislocations, thus making the recrystallization more difficult. At a higher homogenization temperature of 600℃, the particles were more sparsely dispersed, causing a weaker obstruction effect and making recrystallization easier. There were almost no dispersed precipitates at the highest homogenization temperature of 630℃, but dislocations were held up by abundant solution atoms, causing weaker recrystallization than that at 600℃. Although the solution quantity was much less under step-homogenization (600℃x9h?460℃x3h) than that under the 600℃x9h condition, the recrystallization situation was very similar. Finally, the recrystallization could be distinguished as elongated grains or equi-axial grains. The tubes of the automobile fin-tube heat exchangers are usually produced by extruding 3003 aluminum alloys, and are then combined with fins via brazing bonds at 600℃ for 10 minutes. In this high temperature, the extrusion recrystallization will change, and affect the final mechanical properties of the products. Therefore, the study of extrusion forming ability and brazing properties in 3003 aluminum alloys is very significant. We used the four conditions which had the better forming ability in the priority eight designed conditions to do this investigation. The effects of precipitation in homogenization treatments, recrystallization in extrusion and brazing on extrusion forming ability and final material properties are examined. At first, fine dispersoids were precipitated during the 460℃x9h homogenization treatment and coarse dispersoids were precipitated by homogenization treatments with 600℃x9h. Second, when the dispersoids were not plentiful and fine enough during extrusion, the amount of solution dominated the extrusion breakout pressure, and recrystallization was easier; on the contrary, the domination state was replaced by plentiful and fine dispersoids, and recrystallization became more difficult. Additionally, the hardness after extrusion was lower in the complete recrystallization position, and higher in the incomplete recrystallization position. Finally, in brazing, the sample under the 460℃x9h condition underwent full recrystallization with a reduction in strength; the local position of the edge of the sample under the 600℃x9h?460℃x3h condition exhibited a second recrystallization and a significant drop in hardness.
Appears in Collections:	[Graduate Institute of Mechanical Engineering] Electronic Thesis & Dissertation

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