添加Nb於鎳基合金Alloy 52銲材，能增強銲道抵抗DDC與IGSCC之能力，但卻會同時導致銲道熱裂敏感性上升。本文研究Alloy 52不同Nb含量之熱裂敏感性，透過對銲道表面形貌、凝固裂紋觀察與顯微結構分析，探討雷射覆銲製程下最佳的Nb添加量。
雷射覆銲之銲道表面結構受到Nb降低整體銲道熔點的效果影響，銲道寬度隨著Nb含量上升而有所增加。雖然銲道寬度隨Power ratio與Nb含量增加而上升，但受到雷射銲接高冷卻速率的影響，銲道邊緣仍保持平直的幾何形貌。
裂紋的生成順序主要受到局部冷卻速率高低的影響，凝固裂紋易於冷卻速率較高之區域生成，與Nb添加量無明顯關聯性。銲道受到Nb含量上升後增加熱裂敏感性之影響，最大裂紋長度有隨Nb含量上升而增加的趨勢。當Nb含量超過1.82 wt.%時，斷裂面有偏析物的存在，導致結構強度大幅下降，使得凝固裂紋提前生成。
熔池深度與合金稀釋率亦受到Nb降低銲道熔點的影響，隨Nb含量上升而熔池深度增加、合金稀釋率上升的趨勢，使得基材中雜質更容易進入覆銲層，提升銲道熱裂敏感性。
顯微組織分析結果發現當Nb含量達0.82 wt.%時，可以在晶界與枝晶間觀察到粒狀NbC碳化鈮析出，提升銲道抵抗DDC與IGSCC之能力。當Nb含量超過1.82 wt.%時，晶界與枝晶間有偏析物存在，導致晶界結構強度下降，造成銲道熱裂敏感性上升。綜合本研究結果，認為Nb添加量於0.82wt.%以下不會過度增加合金稀釋率、產生偏析物，對銲道熱裂敏感性影響較低，亦可以確保NbC存在，保持原先添加Nb之目的。

Addition of Niobium to nickel-based alloy 52 is able to increase the resistance of ductility-dip cracking and intergranular stress corrosion cracking on the welding zone, but at the same time, the solidification cracking susceptibility is also increased. In this thesis, the solidification cracking susceptibility of different Niobium addition (0.02, 0.82, 1.82, 2.53 wt.%) was discussed by means of surface structure, solidification cracking structure and microstructure of the welding zone.
Due to the effect of Nb addition, the expansion of the mushy zone has affected the surface structure of the welding zone. As the addition of Nb in the welding zone increases, the width of the welding zone also increases. Each welding edges affected by the high cooling rate of the laser beam welding process has flat topography despite higher welding power ratio causes wider surface welding width.
The solidification cracking formation on the surface of welding zone is mainly affected by the cooling rate, having no obvious correlation with the amount of Nb added. The cross-section solidification cracking is also generated at the bottom of the welding pool where it has higher cooling rate. When the amount of Nb added reaches 1.82 wt.%, there are segregates in grain boundaries and interdendrites. The structural strength of grain boundary is greatly reduced due to the segregates, inducing an increased solidification cracking susceptibility. Besides that, the maximum cracking length also tends to increase with the increased amount of Nb addition.
Both the welding depth and the dilution are affected by the Nb addition due to a decrease in the melting point of welding zone. The amount of low melting point phase increases with the higher amount of Nb. Impurities, for example, Phosphor, Sulphur and Silicon in the substrate can easily enter the welding zone, causing the solidification cracking susceptibility to increase.
In the microstructure analysis, the precipitation of particle-like NbC in grain boundaries and interdendrites in the welding zone with 0.82 wt.% of Nb addition is observed. The precipitation of NbC has improved the resistance to DDC and intergranular stress corrosion cracking. Excessive Nb addition result in a decrease the amount of NbC and causing segregation and increased weld hot crack sensitivity. Based on the results of this study, it is considered that the amount of Nb addition is 0.82 wt.% or lower. By this amount, the dilution drastically increased, no obvious segregates are formed and has low influence on the solidification cracking susceptibility. And the most of all, it can also ensure the presence of NbC and retains the purpose of Niobium addition.

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