- 學位論文
摩擦攪拌Mg-Li-Al-Zn合金之改質組織特性及拉伸機械性質應變速率效應探討
Microstructural Feature and the Effect of Strain Rate on Tensile Mechanical Properties of Friction Stir Processed Mg-Li-Al-Zn Alloy
摘要
鎂合金具有低密度、高比強度、散熱性佳、制震性佳及電磁波遮蔽效應等優點，廣泛應於電子、交通等多種產業的應用。鎂合金添加鋰元素可形成HCP結構的富鎂α相及BCC結構的富鋰β相，鎂鋰合金的密度約1.35~1.65g/cm3，密度較一般的鎂合金更低，更可達到輕量化的目的。此外，添加鋰元素可使合金系統的延性增加，並提升合金的低溫塑性；添加鋁元素有助於提升合金最大抗拉強度；添加鋅元素能去雜質的功能，且增加抗蝕性。本實驗採用Mg-9Li-3Al-1Zn(LAZ931)合金擠型材，並利用摩擦攪拌製程(Friction Stir Process,FSP)來進行改質。經摩擦攪拌製程後，由XRD的結果顯示，FSP材織構較微弱而不明顯，其硬度略微上升，而抗拉強度及降伏強度都有下降之趨勢，但延伸率卻明顯提高，這亦表示經摩擦攪拌後的試片具有較好的加工性；此外，本研究也對不同應變速率及拉伸方向做實驗，從實驗結果可發現，在低的應力應變速率和沿ED/PD方向拉伸，也能得到較好的加工性。
並列摘要
Magnesium (Mg) and its alloys have several advantages such as low density, high specific strength, excellent heat and vibration resistances, and radiation absorption of electromagnetic waves. They are now widely used in consumer electronic industries, automobiles and aerospace of nowadays for the purpose of reducing vehicle weight. Mg-Li alloys show lower density (1.35-1.65 g/cm3) than commonly used Mg alloys. Adding Li to Mg alloys can transform the hcp structure to a body-centered cubic (bcc) structure, substantially increasing the ductility of the Mg-Li alloys and further reducing its density. The addition of Al provides solid solution strengthening. When Zn is added to the Al-containing Mg alloys, it can help to reduce the impurity content and improve the corrosion resistance. In the present study, the extruded Mg-9Li-3Al-1Zn (LAZ931) alloy was selected as the raw material. The modification effects of friction stir process (FSP) on the microstructural features and tensile mechanical properties of LAZ931 are investigated in this study. The XRD results show that the peaks intensity of the hcp α-phase is significantly reduced and the texture of extruded LAZ931 alloy is changed after the FSP. The microhardness within SZ is increased and higher than the the base metal region. The yield strength and ultimate tensile strength are decreased, however, the elongation of LAZ931 is significantly increased after the FSP. It indicates that the formability of LAZ931 is improved after the FSP process. In addition, the effect of strain rate on the tensile mechanical properties of LAZ931 is examined in this study. The elongation of friction stirred LAZ931 is increased while applying a tensile direction parallel to the extursion and stir processing direction at lower strain rate.