|Abstract: ||鈦合金基本上分為α、near α、α-β及β四大類，其中α-β及β型鈦合金可經由熱處理來改變其顯微組織，因而其機械性能隨著產生變化。Ti-6Al-4V為α-β型代表性鈦合金，但為了克服鈦合金成型性或更高強度需求，而發展其他鈦合金如SP-700、Ti-6Al-6V-2Sn及Ti-15V-3Al-3Mo-3Sn來取代原始Ti-6-4市場。SP-700合金有較Ti-6Al-4V高之疲勞強度及冷加工成型性，而β 型鈦合金未經時效處理前有良好之成型性，而時效後其強度則可大幅提升，其中以Ti-15V-3Cr-3Al-3Sn (Ti-15-3) 為典型代表合金。為了節約材料使用，或達成構件特定功能表現，工業應用多以銲接製程來接合異種材料，針對上述高強度鈦合金異種銲接少有深入之研究，因此本研究規劃以3年時間進行較深入異種鈦合金銲接研究，實驗並測定銲道不同區域成份變化，並藉以估算Mo及Al當量不同區域變化情形，並與母材Mo及Al當量進行比較，以期建立銲件Mo及Al當量與顯微組織、硬度及韌性關連性。第一年本研究為使用二氧化碳雷射，進行T-6-4對Ti-6Al-6V-2Sn鈦合金異種金屬雷射銲接，量測銲道不同區域成份分佈，銲後銲件施以銲後熱處理，評估銲後熱處理對銲道之缺口拉伸及衝擊性能之影響。為改善可能之銲道脆化，部分銲件並施以300℃銲前預熱，以瞭解銲前預熱對降低銲道脆性之功效。實驗並進行破裂韌性量測，瞭解異種銲件韌性變化。第二年本研究為針對T-6-4對SP-700高強度鈦合金異種金屬，進行板材不加填料雷射銲接，觀察異種金屬銲件顯微組織，部分銲件並施以300℃預熱，以瞭解銲前預熱對顯微組織影響，實驗並測定銲道不同區域成份變化，測定鈦合金Mo及Al當量對應其顯微組織及硬度。研究並評估銲後處理對降低銲道脆性之功效。本研究並探討缺口拉伸試驗及破裂韌性之關連性。第三年本研究為針對T-6-4及Ti-15-3高強度鈦合金，進行T-6-4對Ti-15-3異種鈦合金雷射銲接，銲後銲件及板材施行時效處理，實驗進行試件顯微組織觀察、硬度量測、缺口拉伸試驗及試件破裂韌性量測，並將板材及銲件獲致之結果相互比較，並以掃描式電子顯微鏡觀察各不同試件之破壞特微。|
Titanium alloys can be mainly divided into four groups, i.e., , near , - , and titanium alloys. In case of and - type of alloys, the microstructures can be altered through heat treatment process hence the mechanical properties will behave great changes depending on the inherent microstructures. High strength titanium alloys are often used as structural materials. Ti-6Al-4V is one of the most widely used - titanium alloys. For the demands of higher strength during service, or greater plastic deformability during fabrication, alternative alloys are developed to replace Ti-6-4 alloy, such as Ti-6-6-2, SP-700 and Ti-15-3 alloys. The SP -700 alloy possesses excellent ductility. Besides, the high forming ability together with high strength after aging treatment make titanium alloys increasing its usage in different fields. Ti-15V-3Cr-3Al-3Sn (Ti-15-3), the typical titanium alloy, is one of the most widely used titanium alloys. The demand of the Ti-15-3, in aircraft applications, is even over Ti-6Al-4V. It is known the assembly of dissimilar metals can be made by welding. Based on the open data bank, dissimilar welding of titanium alloys are less studied previously. In this work, dissimilar welding of Ti-6-4 to Ti-6-6-2, SP-700 and Ti-15-3 alloys are carried out by CO2 laser. The chemical composition in distinct regions of the weld will be determined and correlated with the Mo and Al equivalent. The Mo and Al equivalent of the welds will be used as an index to relate with the microstructures, hardness and fracture toughness (J0.2) of the welds. The first year of the project will focus on evaluating the properties of Ti-6-4 to Ti-6-6-2 dissimilar laser weld. The notched tensile strength and fracture toughness of postweld-heat-treated welds will be determined and accounted by the chemical composition and microstructures of the weld. Preheating the samples at 300 oC before laser welding is performed to realize the reduced cooling rate after welding on the improved ductility or decreased notch brittleness of the welds. With the presence of sharp notch, notched tensile strength of the weld might be possible to relate with its fracture toughness. The change in fracture features of the specimens will be examined carefully by SEM. Similar methodology will be carried out during the second and third year’s studies. In the second year of the investigation, dissimilar welding of Ti-6-4 to SP-700 alloys will be performed, whereas, dissimilar welding of Ti-6-4 to Ti-15-3 alloys will be conducted. After welding, the Mo equivalent of the fusion zone will decrease obviously, in contrast, the Al equivalent will increase according to the great difference in composition between Ti-6-4 and Ti-15-3 alloys. It is interested to realized the variation in composition on the microstructures and properties of such dissimilar titanium welds.