分流擠型製程對鋁合金與鋅合金微結構與機械性質之影響

The effects of cross channel extrusion process on microstructures and mechanical properties of the Al and Zn-Al alloys

周政宇(Cheng-Yu Chou)

指導教授 ： 李勝隆

國立中央大學/工學院/機械工程學系/博士(2008年)

摘要

細晶材料已被證實具有優越的機械或物理性質。製造細晶材料的方法眾多，但大多無法達到兼具塊狀、高角度晶界與次微米晶粒等優點。劇烈塑性變形乃藉由導入並累積大量塑性變形於材料而使之形成次微米結構之方法。數種劇烈塑性變形方法於近年來已被廣泛且深入的研究。分流擠形製程為一新穎的劇烈塑性變形製程，乃由李勝隆博士、本論文作者與其研究團隊所提出。該製程設計要點著重於改善製作細晶塊材之流程，以提升工業應用之效益。本論文之主要目的即為利用各種不同合金，深入探討細晶材料之變形特徵、擠製條件對擠製件微結構與機械性質之影響。
藉由觀察A356鋁合金擠製件之巨觀與微觀結構可得知，一道次擠製件之外側部位受到剪應力的影響而產生變形，中心部位則因擠製時材料由相對方向互相擠壓而形成一帶狀結構。經偶數擠製道次時，外側變形結構會回復至原本狀態，但帶狀結構之面積會因道次的累積而變的較複雜且逐漸成長。經量測十道次試片之微硬度可知，試片各處之微硬度具有相近數值，代表試片經多道次擠製後可得加工大致均勻之結構。
AA6061鋁合金經不同擠製條件加工後，可得0.2至3 μm大小的晶粒結構。當降低擠製溫度時，除了可得較小晶粒結構與較高角度晶界外，硬度與拉伸強度亦因回復作用效果降低而有較高的數值。當提升擠製道次時，因材料於模具內發生回復作用，導致晶粒結構較為等軸且粗化，硬度與拉伸強度些許下降。於擠製前進行固溶處理之試片，經於448 K溫度下擠製8道次再時效處理後，可得的最大拉伸強度為364 MPa，與傳統T6熱處理合金相比，擠製前固溶處理之試片因具有細晶結構與細小析出相，強度提升13%。
Zn-22%Al合金隨著擠製道次的增加，原本呈現層狀且分布不均的富鋁相轉變成顆粒狀且逐漸分布均勻，而較低的擠製溫度有助於形成較細小的富鋁相。細小且分佈均勻的富鋁相有助於提升其超塑性。當合金於373 K下擠製10道次後，於473 K 下以初始拉伸速度為5.0×10-3 s-1進行高溫拉伸測試時，可得最大的延伸率為1092％。

關鍵字

機械性質 ； 分流擠型 ； 劇烈塑性變形 ； 細晶塊材

並列摘要

Fine-grained materials have attracted consideration interesting among researches, because the present of a large amount of grain boundary area in the materials results in unusual and extraordinary changes in both mechanical and physical properties. Numerous severe plastic deformation methods had been invented to produce fine-grained materials by introducing large plastic strains into bulk materials. Among them, the Cross-Channel Extrusion Process invented by Dr. Lee, the author of this dissertation and their group is a newly disclosed SPD method, which was currently designed to improve the procedure of fabricating bulk UFG materials.
In this dissertation, the main object is to establish a series of experiments to confirm the efficiency of the CCE process on achieving fine-grained materials with enhanced mechanical properties. The second objective is to study the metal flow of the extruded sample, which is much different from the other well-known methods. The further objective is to investigate the relationship among extrusion conditions, microstructures and mechanical properties of the series of experiments.
The metal flow and its characteristics are demonstrated by pure tin and A356 alloy. The 1-pass A356 sample shows a distorted structure except the by-punch area and a belt structure is also formed at the center of the sample. The deformation at the center portion of sample is caused by the material pressed in the opposite direction; the outer distorted portion is caused by shear stress. When the number of extrusion pass is even, the distorted structure recovers to initial state; however, the area of belt structure becomes larger and more complex with increasing extrusion passes. Further more, the whole area of extruding sample can be well deformed when the number of extrusion passes is increased to 10.
The AA6061 aluminum is applied to CCE process under various extrusion conditions. The fine-grained structures with the size between 0.2 and 3 μm are obtained after extruded for up to 8 passes at temperatures of 473 K~573 K. The finer grain structure with high angle boundaries is easier obtained when decreasing the extrusion temperature and increasing the number of extrusion passes. Hardness and tensile strength decrease with increasing extrusion temperature because working hardening rate is lowered, and they also decrease with increasing the number of extrusion passes because the grain structure is coarsened. When the Post-CCE ageing treatment is applied to the AA6061 alloy, the processed sample shows a tensile strength of 364 MPa which is 13% higher than commercial T6 treated sample since it consists of fine-grained matrix and precipitation hardening effect.
Superplasticity of a Zn-22%Al alloy is also discussed in this dissertation because it can be enhanced by finer grains. The finer and well-distributed Al-rich phase is obtained when decreasing the extrusion temperature and increasing the number of extrusion passes. At initial strain rates higher than 5.0×10-3 s-1, the finer and well-distributed Al-rich phase results in better superplasticity when tested at 473 K. The 10-pass sample extruded at 373 K shows a best superplasticity of 1092％ in elongation when tested with a suitable initial strain rate of 5.0×10-3 s-1.

並列關鍵字

fine-grained material ； severe plastic deformatio

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