稀土釹與旋鍛製程對AZ71鎂合金之影響

The effect of Rare Earth Nd and Rotary Forging on AZ71 Mg Alloy

陳宇辰(Yu-Chen Chen)

指導教授 ： 陳貞光

國立臺北科技大學/工程學院/材料及資源工程系研究所/碩士(2013年)

https://doi.org/10.6841/NTUT.2013.00718

摘要

本實驗在AZ71鎂合金中添加0.5~2.0 wt.%的稀土釹，發現釹能有效抑制鑄態之β-Mg17Al12生成、細化鑄態之晶粒尺寸；當釹含量超過1.0wt%時，其細化效果減緩，在1.0wt% Nd有最佳化效果，配合旋轉鍛造及退火熱處理，更能使鑄態晶粒尺寸下降至30μm以下。本實驗針對這些含釹AZ71合金，以32%及42%兩個不同應變量進行旋鍛，採用42%高應變量之試片其開始及完成再結晶溫度較低，但完成再結晶後之晶粒與32%應變量試片並無太大差異，其維氏硬度值差異亦不大。由拉伸試驗結果顯示，其抗拉強度及延性會隨稀土釹之含量上升而提高，但是當稀土釹含量超過1.0wt%時，強度與韌性均有下降之趨勢。稀土釹除了能提供鎂合金固溶強化、生成AlxNdy化合物提供析出強化等強化機制外，稀土釹的添加提高了鎂合金的c/a值、疊差能以及臨界分解剪應力，使延性大幅提升。但過量的稀土釹添加，導致桿狀析出物(Al11Nd3)的大量生成以及團聚之現象，使抗拉強度及延性下降。稀土釹添加量在1.0wt%時，析出物散佈均勻，常溫下最高抗拉強度達到253 MPa、最大延性為10.7%，擁有最佳韌性。
本研究亦利用170°C高溫拉伸試驗確認，抑制β-Mg17Al12共晶相之生成為鎂合金高溫抗拉強度提升之主要原因，而以高溫相的析出提升鎂合金之高溫抗拉強度則為次要原因。AZ71合金在添加1.0wt%釹時，無論在鑄態或在加工退火後，在常溫與高溫下均有最佳之韌性表現。

關鍵字

AZ71鎂合金 ； 旋轉鍛造 ； 晶粒細化 ； 再結晶 ； 機械性質 ； 退火

並列摘要

This study investigates the addition of 0.5~2.0wt% of rare earth element Nd, in AZ71 magnesium alloy. Nd additions are found to suppress the formation of β-Mg17Al12 and refine the as-cast grain size. 1.0 wt.% addition of Nd appears to demonstrate most effective grain refinement effect. The grain size can be further reduced to below 30μm after rotary swaging and annealing. 32% and 42% swaging strains are employed to observe its effect on annealing and mechanical property. The start and finish recrystallization temperatures for 42% swaged samples are lower than those swaged by 32%. However, their grain sizes after recrystallization are similar, and their Vickers hardness do not vary much, either. Both tensile strength and ductility increase with Nd concentration up to 1.0 wt.%. Both solid solution and precipitation strengthening by forming AlxNdy compound give rise to the increase of strength. In addition, Nd increases c/a value, stacking fault energy, and critical resolved shearing stress of the magnesium alloys, which all lead to the increase of ductility. When over 1.0 wt.% of Nd is added, large amount of Al11Nd3 rod precipitates are formed in aggregation, which results in reduction of tensile strength and ductility. In the 1.0 wt.% Nd added AZ71, the precipitates are uniformly distributed, 253 MPa tensile strength and 10.7% elongation could be achieved, showing the optimum toughness combination.
High temperature tensile tests further confirm that Nd suppresses the forma -tion of β-Mg17Al12 eutectic phase formation, which is believed to be the prime cause for improved high temperature tensile strength. Precipitation of high temperature intermetallic compounds is the secondary cause. In summary, addition of 1.0 wt% Nd to AZ71 alloy generates the best toughness in either as-cast or annealed states tested at both room temperature and 170°C.

並列關鍵字

AZ71 magnesium alloys ； rotary swaging ； grain refinement ； recrystallization ； mechanical property ； annealing

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