臺灣博碩士論文加值系統

　　今日的交通工業致力於產品輕量化和環保訴求，鎂合金具備低密度、高比強度、回收性佳等優點，因而逐漸受到重視；鎂因具備HCP結構，使得鎂合金的加工往往要在高於室溫的環境進行，為了節省成本及便利加工，降低鎂合金的加工溫度甚至室溫加工是必要的。摩擦攪拌製程(Friction Stir Process, FSP)可藉簡易製程達到晶粒細化的目的，進而改善材料的加工性，故本研究利用不同溫度拉伸試驗比較FSP前後的差異，並探討兩者的變形機制，了解FSP對鎂合金加工性的改善能力。
　　本研究採用AZ31鎂合金之完全退火材，先以不同轉速進行FSP，並將改質後試片以固定初始應變速率為8×10-4 sec-1的條件進行室溫拉伸試驗。接著選擇1500rpm下FSP後的試料進行相同初始應變速率的100℃~500℃之拉伸試驗，觀察其變形組織特徵，了解其變形組織及拉伸特性與溫度的關係，並與退火材進行相同實驗比對。
　　拉伸結果顯示，試料的降伏強度及抗拉強度隨溫度上升而減少，總延伸率隨溫度上升而增加，而均勻延伸率呈現先下降再持平而後上升的趨勢。
　　持溫試片及破斷次表面的觀察顯示，材料在200℃以下藉晶粒伸長造成應變；300℃時晶粒成長效應逐漸顯著，在拉伸過程中主要以動態再結晶和晶粒成長兩種現象交替發生；350℃以上晶粒成長快速，拉伸性質與FSP前相近，以FSP改質的目的失去效用。
　　低轉速FSP前後的拉伸試驗結果顯示，發現FSP後試料在150℃以下的降伏強度和抗拉強度較O材小，均勻延伸率則較大；根據破斷面遠端組織觀察結果，FSP後材料的晶粒變形較O材明顯，顯示FSP後材料在平行進給方向上有較佳的均勻變形。

　　Product lightening and environmental protecting are required in trans- port industry nowadays. Magnesium alloys are of great interest for use in lightweight structuredue to its low density, high specific strength, well re- cycled. Unfortunately, the disadvantage of Mg alloys is that they exhibit limit ductility due to their HCP structure. Therfore, significant develop- ment efforts are needed, which can improve their workability. Friction stir process (FSP) can get finer grains to improve the workability by dynamic recrystallization (DRX). On the basis of that, Mg alloy’s deformed pheno- menons and mechanical properties before and after FSP by tensile tests in several temperatures were discussed to comprehend FSP’s effect of work- ability improvement on Mg alloy.
　　A full-annealed magnesium alloy AZ31 was used to go through FSP in several rotation speed initially. The tensile tests of FSPed AZ31 were per- formed at constant initial strain rate of 8×10-4 sec-1 in room temperature. FSPed AZ31 with processed in 1500 rpm was chosen for the next step of the study.
　　The tensile test of FSPed AZ31 in 1500 rpm was performed from room temperature to 500℃ at the same initial strain rate. Then the temperature dependance of deformed microstructure and tensile properties was inve-
stigated in observations of deformed microstrusture. Furthermore, full-
annealed AZ31 was done the same experiments as comparisons.
　　The experimental results indicate that yield strength (Y.S.) and ultimate strength (U.T.S.) decreased with increasing temperature. Total elongation (T.E.) increased with increasing temperature. Uniform elongation (T.E.) decreased as the temperature increased from room temperature to 200℃, broadly maintained from 200℃ to 400℃, and increased as the tempera- ture was above 400℃.
　　According to the deformed and temperature-held microstructure, defor- mation was made of grain extension below 200℃. Grain growth started above 300℃, and it took place by turn with DRX during tensile process. Grain grew rapidly above 350℃. It would make the efficacy of FSP in vain.
　　Comparing with full-annealed AZ31, FSPed AZ31 in 1500 rpm has smaller Y.S., U.T.S., and bigger U.E. . It indicated that the latter got lower deformed impedance during cold work.

目錄

中文摘要………………………………………………………………I
英文摘要………………………………………………………………III
目錄……………………………………………………………………IV
表目錄…………………………………………………………………VII
圖目錄…………………………………………………………………VIII

第一章 前言…………………………………………………………1
第二章 文獻回顧……………………………………………………3
2-1 鎂合金AZ31….………………………………………..……3
2-1-1 鎂合金之分類規範及意義…………….……………….3
2-1-2 AZ31之合金元素添加效應……………………….……3
2-2 摩擦攪拌銲接……………………………………………….4
2-3 摩擦攪拌製程……………………………………………….5
2-3-1 動態再結晶.….…....…………………………………5
2-3-2晶粒細化效應…………………….………………………6
2-3-3與其他製程之比較……………….………………………7
第三章 實驗方法……………………………………………………12
3-1 材料製備與摩擦攪拌製程………………………………….12
3-2 微觀組織觀察……………………………………………….13
3-3 X光繞射分析…………………………………………………13
3-4 拉伸試驗…………………………………………………….13
3-5 硬度測量…………………………………………………….14
第四章 實驗結果……………………………………………………25
4-1 不同轉速FSP試片之轉速改變效應…………………………25
4-1-1 不同轉速改質之微觀組織分析……………………… 25
4-1-2 常溫拉伸性質………………………………………….25
4-1-3 FSP前後之X-ray分析…………………………….……26
4-2 FSP之高溫拉伸性質…………………………………………26
4-2-1 拉伸試片外觀比較…………………………………….26
4-2-2 高溫拉伸性質………………………………………….27
4-2-3 拉伸破斷面SEM觀察..…………………………………28
4-2-4 破斷次表面觀察……………………………………….28
4-2-5 破斷位置遠端觀察…………………………………….29
4-2-6 不同溫度持溫後之微觀組織比較…………………….29
第五章 討論…………………………………………………………65
5-1 FSP轉速對常溫機械性質影響………………………………65
5-1-1 晶粒均勻性…………………………………………….65
5-1-2 改變轉速對X-ray繞射的影響…………………………65
5-2 FSP1500之高溫拉伸性質探討………………………………65
5-2-1 動態再結晶…………………………………………….65
5-2-2 晶粒成長……………………………………………….66
5-2-3 擴張頸縮……………………………………………….66
5-2-4 n值與拉伸溫度之關係………………….…………….67
5-3 低轉速FSP前後試片之高溫拉伸性質比較..………………68
5-4 轉速改變對高溫拉伸性質之影響.…………….………….69
第六章 結論…………………………………………………………84
第七章 參考文獻……………………………………………………86

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