2024鋁合金熱裂性及異質銲接機械性質之研究

Study on the Characteristics of Hot Cracking and Dissimilar Metal Welding on 2024 Aluminum Alloys

溫晉源(Jin-Yuan Wun)

指導教授 ： 鄭慶民

國立臺灣師範大學/科技學院/機電科技研究所/碩士(2010年)

摘要

本論文以2024鋁合金為研究對象，分兩部分進行研究：
一部分為2024鋁合金熱裂性研究，以惰氣鎢極電弧銲(TIG)，在不使用填料情況下，利用點可調式應變試驗(Spot Varestraint Test)，探討外加應變量及熱循環次數對2024-T351及2024-T6熱裂敏感性之影響。經由光學顯微鏡與掃瞄電子顯微鏡觀察，探討微觀組織。研究結果顯示：2024-T351、2024-T6經多重熱循環後熔融區熱裂敏感性並無明顯之影響，但對熱影響區的金屬熱影響區(W. M. HAZ)兩種鋁合金卻都有顯著之影響，隨著熱循環次數的增加，在金屬熱影響區之熱裂縫會有明顯的成長，另母材熱影響區(B. M. HAZ)之熱裂縫2024-T351也隨著熱循環次數的增加而增加，而2024-T6則無此現象。另外隨著外加應變量之增加，兩種材料之裂縫總長度(Total Crack Length, TCL)並無增加，但在最大裂縫長度(Maximum Crack Length, MCL)上，2024-T351鋁合金熔融區之最大裂縫長度會隨著熱循環次數之增加而加大，而2024-T6鋁合金並無明顯現象。由金相顯微組織圖可知，熱裂縫發生的位置在熔融區與熱影響區，很明顯的的裂縫均沿著晶粒邊界發生。再經由SEM觀察分析，兩種鋁合金材料皆呈現熔融區為凝固熱裂及熱影響區為液化熱裂的形式。
第二部分為2024鋁合金與7050鋁合金做異質銲接後，探討其銲後之微觀結構與機械性質。銲接採惰氣鎢極電弧銲並分別添加A2319與A5356填料進行對接銲。銲後再施以自然時效(T1)與固溶時效(T4)熱處理，實驗結果顯示: (1) 2024、7050異質接合後經固溶處理(T4)，熔融區之微硬度值較自然時效(T1)大幅地提升約20-30Hv (2) 2024、7050異質接合後，採ER2319較ER5356填料在T1處理後有較高的極限抗拉強度(UTS)、降伏強度(YS)與伸長率(El)，但經固溶處理(T4)後，其極限抗拉強度(UTS)、降伏強度(YS)則差異不大。

關鍵字

鋁合金 ； 銲接 ； 熱裂

並列摘要

The main purpose of this study focuses on 2024 aluminum alloys. it includes two parts:
The first part aims to investigate the hot cracking susceptibility of 2024 aluminum alloys. The specimens were experimented with tungsten Inert gas welding (TIG) at first under the condition of no filler metal used. Then, the spot varestraint testing was used to evaluate the effect resulting from the times of thermal cycles and augment strain over hot cracking susceptibility of 2024-T351 and 2024-T6 aluminum alloys. Meanwhile, the causes and patterns of hot cracking were observed and analyzed with Metallographic test and scanning electron microscope (SEM). The experimental result shows that the times of thermal cycles is irrelevant to hot cracking susceptibility in the weld fusion zone of 2024-T351 and 2024-T6. But the times of thermal cycles does affect hot cracking susceptibility in weld metal heat affected zone (W. M. HAZ) of both 2024-T351 and 2024-T6. As the times of thermal cycles increases, the length of hot cracks grows. Besides, the length of hot cracks in base metal heat affected zone (B. M. HAZ) of 2024-T351. However, this phenomenon does not appear in base metal heat affected zone (B. M. HAZ) of 2024-T6.on the other hand, as augment strain given increases, the total crack length (TCL) of these two materials both remains constant. With Metallographic test, the hot cracking occurrence was seen lying along the crystalline grain boundary in fusion zone and HAZ. Hot cracks yielded in fusion zone and in HAZ are categorized to solidification cracking and liquation one respectively. The types of hot cracking can be further identified with SEM.
The second part focuses on the research over mechanical property of dissimilar metal welding with 2024-T351 and 7050-T7451 through metallography and tensile test. TIG was adopted. ER2319 and ER5356 serve as individual filler during welding. After welding, specimens were conducted with respectively different heat treatment, natural aging treatment (T1) and solution heat treatment (T4). The experimental result indicates: (1)The micro-hardness in fusion zone of dissimilar metal welding enormously after T4 is enormously bigger than that after T1. The difference can achieve 20 to 30 Hv. (2) The specimens using ER2319 as filler have higher ultimate tensile strength (UTS), yield strength (YS) and elongation (El) than those using ER5356 after T1. But the above-mentioned parameters of specimens with these two fillers show no difference after T4.

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