鋁鋰合金2090惰氣鎢極電弧銲之銲接性研究

Abstract

摘 要 鋁鋰合金目前已廣泛應用在飛機、火箭、衛星等航太工業的製造上。鋁鋰合金具有比傳統鋁合金更低的密度與更高的強度，是被發展用來取代7075、2024等的新一代高強度鋁合金。由於它還具有優良的低溫特性與銲接性，因此被NASA選擇作為太空梭液態燃料槽的主要材料。但鋁鋰合金的缺點是成本較高且韌性較低。 本研究主要是利用惰氣鎢極電弧銲來研究鋁鋰合金的銲接特性，包括鋁鋰合金在銲接後的金相組織變化、強度與微硬度等。此外，探討對銲件實施不同的熱處理後，對其顯微組織與機械性質的影響。實驗項目包括：拉伸試驗、微硬度試驗、金相組織觀察、SEM等。 實驗結果顯示：鋁鋰合金2090在銲接時易發生的缺陷包括氣孔、裂縫、熔透不全等。氣孔較易發生在融熔線上。直銲後的接合效率大約為65％，直銲後固溶強化是主要的強化機構，其強度低於200Mpa。銲件經適當熱處理後可改善銲道與熱影響區的強度與硬度，在銲後時效處理上，於160℃/32小時及190℃/16小時可得最高強度。而230℃之時效處理後銲道強度較低。將銲件施以銲後全程熱處理可以得到最高的強度（時效160℃/32小時）。

ABSTRACT Currently Lithium-containing aluminum alloys are of considerable interest in the aerospace and aircraft industries because the addition of lithium to aluminum can improve the modulus and decrease the density compared to conventional aluminum alloys. Because the lack of weldability studies, several alloys developed for aircraft applications were usually involving mechanical fastening. This research was to explore the weldability of Aluminum-Lithium Alloys 2090 under Gas Tungsten Arc Welding (GTAW) process without filler metals. The quality of the welds was evaluated through mechanical tests (hardness and tensile tests) and microscopical observations. The series of experiments included post-weld heat treatment microstructure observation, tensile testing, micro hardness testing and SEM etc. The results of the study showed that the major welding defects were porosity, heat-tearing cracking, poor penetration, etc. In the as-welded condition, solid solution strengthening is the primary strengthening mechanism present. From the mechanical testing result, the strength of the weldments are less than 200 Mpa. In the post-weld aged condition, The highest strengths of weldments can be obtained by aging at 160℃for 32 h or at 190℃for 16 h . Aging at 230℃lead to the coarsening of precipitates and the strength was decreased. The best overall weldment properties are obtained in the solution heat treated and then aged conditions. This result was associated with a homogeneous distribution of strengthening precipitates.