熱整形溫度對船用5083-H116鋁合金銲件機械強度及抗蝕性之影響

Effects of Flame Straightening on the Mechanical and Corrosion Properties of 5083-H116 Marine Aluminium Alloy Weldments

鋁合金板材於銲接後會產生局部變形，目前造船廠整平的工法是對變形區域以火炬局部加熱後，再沖水迅速冷卻收縮定型，而此熱整形的加熱冷卻程序即可視為一種退火熱處理，將對鋁合金船體的機械強度及抗蝕性能造成不同程度的影響。本研究藉由硬度、拉伸試驗、硝酸腐蝕質量損失測試（nitric acid mass loss test, NAMLT）及微觀組織觀察，探討不同退火溫度對5083-H116鋁鎂合金的銲道組織、熱影響區（heat affected zone, HAZ）及母材之機械強度與抗蝕性能之影響，同時藉由100～600°Ｃ高溫熱處理以模擬熱影響區之溫度變化。研究結果發現，銲道熔融凝固後主要為柱狀晶及等軸晶組織結構，其機械強度已降至最低並不受銲後退火溫度影響，熱影響區因高溫主要呈現出晶界成長及再結晶組織，再結晶溫度約為250～350°Ｃ，而母材區主要為部分再結晶與回復結構，另β相較易沿熱影響區與母材區之晶界析出，其中再結晶組織的機械強度較低，且敏化後腐蝕程度較回復組織嚴重。高溫熱影響區模擬試驗中發現，在600°Ｃ熱處理後，由於部分合金高溫熔融產生嚴重晶粒粗化現象，導致大量β相（Mg2Al3）晶出於晶界上，嚴重影響其機械強度與抗蝕性能。本研究成果可供造船廠作為鋁合金艦艇建造施工及維修之參考依據。 Shipyards currently use flame straightening (accompanied by water quenching) to eliminate welding distortion of aluminum alloy structures. This straightening process will consequently impact their mechanical and anticorrosion properties of the structures. This study uses microhardness measurement, tensile testing, optical microscopy (OM), scanning electron microscopy (SEM), and nitric acid mass loss testing (NAMLT) to examine 5083-H116 aluminum alloy specimens, and discover the changes in mechanical properties on the welds, heat affected zone (HAZ) and base metal (BM). According to the results of the experiments, welds that mainly consist of columnar and equiaxed grain structures have the poorest mechanical properties and are hardly affected by post-weld heat treatment. The HAZ of the weld is completely annealed and recrystallized at about 250~350°C. For the base metal, partrecrystallization and recovery structure are dominant. The corrosion resistance of the specimens is mainly affected by the β-phase (Mg2Al3) precipitation along the grain boundary, which tends to take place in HAZ and BM. After sensitization treatment, the recrystallized structure has more mass loss than the cold deformed and recovery structure do. The simulation experiments regarding the high-temperature HAZ showed that after heat treatment at 600°C, a substantial amount of β-phase grain was deposited on the grain boundary because of severe grain coarsening caused by partial high-temperature alloy fusing. This phenomenon significantly affected mechanical strength and corrosion resistance. The findings of this study can serve as a reference for vessel or ship manufacturers regarding the construction and maintenance of aluminum alloy vessels.