熱處理型鋁合金銲接性之研究

Abstract

本論文旨在探討2024-T351、6061-T6及7075-T6三種熱處理型鋁合金之銲接性，研究共分為三部份：第一部份為熱裂性研究，探討外加應變量、熱循環次數及冷加工對6061-T6及7075-T6鋁合金熱裂敏感性之影響，研究結果發現：多重熱循環對上述鋁合金銲件熔融區熱裂敏感性並無明顯之影響，但對熱影響區有顯著之影響，隨著熱循環次數的增加，在熱影響區之熱裂縫會有明顯的成長，其中又以銲接金屬熱影響區(Weld Metal HAZ)較為嚴重。而隨著外加應變量之增加，各材料之熱裂敏感性均會增加，而且不論是熔融區或熱影響均呈現相同之趨勢。材料實施過冷加工後，能降低其熱裂敏感性，實驗結果亦顯示7075-T6鋁合金較6061-T6鋁合金具有更高之熱裂敏感性，而且7075-T6鋁合金在部份熔融區(PMZ)有明顯Cu的偏析現象產生，亦造成此區域較高之熱裂敏感性，而6061-T6鋁合金則無成份偏析之現象產生。 第二部份為角變形量之研究，將2024-T341、6061-T6及7075-T6三種鋁合金以GTAW進行單V形槽(Vee Preparation Angle)之對接銲，以比較在不同之單V形槽角度及有無拘束(Restraint)之條件下其角變形量之差異。研究結果顯示：當V形槽角度為0∘(I形槽)時，受到拘束之銲件其角變形量大於無拘束之銲件，當V形槽角度為45∘、60∘、90∘時，則受到拘束之銲件其角變形量小於無拘束之銲件。銲件接頭單V形槽之角度大小(銲道熔填金屬量)會影響銲件之角變形量，在無拘束狀況下，角變形量會隨單V形槽之角度增加而增加；而在拘束狀況下，當單V形槽之角度為0∘時，會產生較大之角變形量，隨著角度增加其角變形量會逐漸下降，在60∘時，其角變形量最小，超過60∘則會有再上升之趨勢。 第三部份為銲件之殘留應力量測，將完成角變形量測之試片，以鑽孔法量測其殘留應力，研究結果顯示：對接銲件接頭開槽角度越大，其殘留拉伸應力就越大，但過小之開槽角度，因銲道熔填金屬量與原先接頭預置之幾何形狀會有較大之差異，因此適當之開槽角度才能使殘留應力降低。銲接時受到拘束之銲件會產生較大之殘留應力。同時在靠近受拘束處亦會產生一殘留拉伸應力；銲件之最終殘留應力係由平衡溫度時材料之降伏強度所決定，平衡溫度時降伏強度越高之材料其殘留應力越高，鋁合金因其熱傳導係數較大，因此試片之溫差較小，所有材料之殘留拉伸應力相較於其本身之降伏強度均較低。而銲件之角變形與材料之殘留拉伸應力之關係，就同一材料而言，角變形量與殘留拉伸應力呈現正比之關係，就材料是否受拘束而言，未受拘束之銲件其角變形量都大於受拘束之銲件，而其殘留之拉伸應力都小於受拘束之銲件。

ABSTRACT The study aims to investigate the weldability of Heat-treatable aluminum alloys. It includes three parts. The first part is the susceptibility of hot cracking, the second part is the angular distortion, the third part is the residual stress of weldment. The first part, the spot varestraint testing was used to evaluate the hot cracking susceptibility of several aluminum alloys namely 6061-T6, 6061-T6 (H), 7075-T6, 7075-T6 (H). The effect of augment strain, the numbers of thermal cycles and cold working (rolling) on the cracking susceptibility were investigated, the total crack length was used to evaluate the hot cracking susceptibility. The results indicate that the number of thermal cycles is irrelevant to the hot cracking susceptibility in the weld fusion zone, but does affect this susceptibility in the heat-affected zone (HAZ). More thermal cycles correspond to larger hot cracks in the HAZ, especially in the weld metal HAZ. The hot cracking susceptibility of materials increased with augment strain in both the fusion zone and the HAZ. Cold working of the materials can reduce their hot cracking susceptibility. The hot cracking susceptibility of 7075-T6 aluminum alloys is higher than that of 6061-T6. The second part conducte single V-groove butt welding (GTAW) on three types of heat treatable aluminum alloys 2024-T351, 6061-T6 and 7075-T6 and compared the angular distortion levels of the three aluminum alloys at different Vee preparation angles with or without restraint. Experimental results demonstrated the single Vee preparation angle (amount of filler metal) in butt welding affected the angular distortion of the weldment. Non-restraint, the angular distortion tended to increase with the single Vee preparation angle. Meanwhile, the angular distortion tended to decrease when the single Vee preparation angle was 60°. Additionally, a restrained weldment had high angular distortion when the single Vee preparation angle was 0°. The high-temperature yield strength of a material also affected its angular distortion. Angular distortion increased with high-temperature yield strength. Experimental results also showed that, from strongest to the weakest, the high-temperature yield strength of the three types of aluminum alloys followed the order 6061-T6 and 2024-T351＞7075-T6. The third part investigates the residual stress of weldment. The residual stress increases with the Vee preparation angle above 60°. When the Vee preparation angle was below 50°, the volume of melted metal in the weld differed greatly from that in the V-groove before welding. Therefore, when the Vee preparation angle was below 50°, estimating the residual stress from the Vee preparation angle was more difficult. The weldment to restraint have a large residual tensile stress, residual tensile stress in the restrain corner is large. The final residual stress is determined by the yield strength at the equilibrium temperature of the weldment. The angular distortion is proportional to the residual stress in the same material. The non-restrained weldment has a large angular distortion but a small residual tensile stress