航太用高強度鋁合金銲接熱裂性與異質銲接銲後熱處理之研究

Abstract

本文主要以研究高強度鋁合金與含鈧鋁合金之銲接熱裂性及高強度鋁合金異質銲接為研究主題，研究共分為二部份:第一部份為A2024、A2219、A7050等三種高強度鋁合金之熱裂性敏感性研究與M6、M7B、M7E與M9H4等四種鋁鈧合金之熱裂性研究；第二部份為A7050/A7075、A2024/A7050高強度鋁合金同質與異質銲接，實驗結果顯示: 在熱裂敏感性研究方面，A2024、A2219、A7050等三種鋁合金材料成份相較，以A2024與A7050合金最接近最高熱裂傾向成份，其熱裂敏感性較A2219合金高。並隨熱循環次數增加，均呈現相同趨勢，即A2024 > A7050 > A2219。實驗中亦發現，多重熱循環對銲件之熔融區熱裂敏感性並無明顯之影響。但對熱影響區卻有顯著的影響，其中又以銲接金屬熱影響區(W. M. HAZ.)最為嚴重。透過EDS成份分析結果顯示，在部份熔融區三種合金均有Cu元素偏析，且偏析情形隨熱循環次數增加而加劇，熱裂形成均以液化熱裂機制存在。 在含鈧鋁合金之熱裂性研究方面，M6、M7B、M7E與M9H4等四種鋁鈧合金相較，經多重熱循環過程後，熔融區之熱裂敏感性無太大變化；在銲道金屬熱影響區方面，M7B、M7E與M9H4合金，熱裂縫會隨著熱循環次數增加而有明顯的增長，惟M6合金則呈相反之趨勢。在不同含鈧鋁合金材料比較，隨熱循環次數增加其熱裂敏感性之裂縫總長度有增加之趨勢，再與其合金成份相較，均呈現相同趨勢，即M9H4> M7E > M7B> M6。裂縫破斷面觀察方面，在熔融區均觀察到三個明顯區域，樹枝狀區(D區)、樹枝-平滑轉換區(D-F區)與平滑區(F區)，可證實熔融區均屬於凝固型熱裂機構；在熱影響區均為沿晶脆性破壞之液化熱裂形式。並在部分熔融區處經EDS分析，發現M6與M9H4二種合金金屬在晶界附近均有相當大的Cu偏析現象，且隨著熱循環次數增加而加劇。因此，M6與M9H4二種合金在部分熔融區均為偏析熱裂。M7B、M7E兩種合金的Zn含量在熱影響區之晶界附近有提升現象，然Mg含量則無明顯地增加。此二種合金在熱影響區之部分熔融區，均顯示為純粹由晶界的液化現象。 A7050/A7075、A2024/A7050高強度鋁合金同質與異質銲接部份，本實驗採用氣體鎢極電弧銲，並以ER5183、ER5356、ER5556與ER2319等填料進行銲接，主要探討不同填料的添加與銲後熱處理製程對異質銲件之機械性質與微觀結構進行影響評估。銲件透過銲後固溶處理、人工時效與過時效等熱處理過程，以拉伸測試與微硬度等測試做為銲件之機械性質評估方式，再配合光學顯微鏡及掃描式電子顯微鏡，進行各階段熱處處理之微結構與拉伸破斷面之觀察。結果顯示，透過銲後熱處理可大幅地提升銲件銲道的機械性能，但大部分仍略低於母材。整體而言，銲件之最弱處仍為銲道。其中採用ER5356或ER556填料，並透過人工時效處理與過時效過程，將可得到銲後銲道之較佳機械性能，且其拉伸破斷面均為延性破斷形貌。

The objectives of this research are to investigate the hot cracking characteristics of weldment of high-strength aluminum alloys and aluminum-scandim alloys, and the effect of post-weld heat treatment of on the mechanical properties of high-strength aluminum alloys. This research includes two parts. The first part is on the susceptibility of hot cracking of high-strength aluminum alloys A2024, A2219 and A7050, and the hot cracking susceptibility of the aluminum-scandium alloys M6, M7B, M7E and M9H4. The second part is to study the effect of post-weld heat treatment on the mechanical properties and microstructure for dissimilar welding of high-strength aluminum alloys A7050/ A7075 and A2024/A7050. In the first two parts, the spot varestraint testing was used to evaluate the hot cracking sensitivity of those material. The influence of augment strain. and muti-thermal cycles on the hot cracking suscepitbility were investigated. The total crack length was measured and calcuated to evalute the hot cracking susceptibility. The results indicate that: the number of cracks increases with increasing augmented strain. This phenomenon occurs in both the fusion and the heat-affected zones. The number of thermal cycles also has a significant influence on the heat-affected zone. A2024 and A7050 have similar tendencies to be subjected to hot cracking, greater than A2219. With increasing numbers of thermal cycles, the hot cracks show the same tendency, A2024>A7050>A2219.The Energy Dispersive Spectrometer composition analysis results show that, the Cu segregation is increases with the number of thermal cycles. In the Al-Sc alloys, the effect of number of thermal cycles. and augment strain processes were evaluted and compared for the M6, M7B, M7E and M9H4 alloys. In the fusion zone, no significant results were obserred on the hot cracking sensitivity. In the weld metal heat affected zone, the number of cracks increases with increasing augmented strain and the number of thermal cycles with the different Al-Sc alloys by the thermal cycles. The trend of hot cracking sensitivity of the total crack length show that M9H4> M7E> M7B> M6. M6 and M9H4 have the Cu element segregation in the vicinity of grain boundaries, and are the Cu segregation increases with the number of thermal cycles. So that the M6 and M9H4 are segregation-induced liquation mechanism in the partially melted zone. In the grain boundaries of the heat-affected zone of M7B, M7E alloys, the content of Zn element increase significantly, but Mg content does not. These two kinds of alloys are purely grain boundary liquation. On cracking fracture surface, the experiment results show three distinct regions in the fusion zone, dendritic area (D area), dendritic-flat area(D-F area) and the flat area (F area), to confirmed the fusion zone belong to the solidification cracking. In the heat affected zone, the results indicate the brittleness fracture phenomenon along grain boundary on the fracture surface, and it is presented in the form of liquefied hot cracking. Finally, the second portion that of this desseration is to discuss the similarty and dissimilarty for A7050/A7075, A2024/A7050 similar and dissimilar welding for high-strength aluminum alloys. The Gas Tungsten Arc Welding(GTAW) was used to weld butt joint with different filler materials, such as ER5183, ER5356, ER5556 and ER2319. The post-weld heat treatment processes were carried out to evalute and analysis on the effect of the mechanical properties and microstructure. The experiment results showed that the yielding and ultimate tensile strength of weldment on the post-weld not-heat treatment has singificantly declinc to lower than 60% of the base metal strength, post-weld heat treatment can be improved and promoted the weldment mechanical properties, These are still slightly lower than the base metal. In conclusion, the weldment weakest place is still at the weld. And then uses ER5356 or ER5556 filler, it can get the optimal mechanical properties when the weldment of post-weld heat treatment are executed artificial aging and over-aging. In tensile fracture surface, its still has the morphology of ductile facture.