鈮合金與鈦合金之真空硬銲接合及其接合介面之研究

Abstract

本研究以真空爐為加熱源，以Ti-15Cu-15Ni(wt.%)及Ti-20Cu-20Ni-20Zr(wt.%)兩種不同之片狀Ti基填料硬銲接合C103鈮合金【Nb-10Hf-1Ti(wt.%)】與Ti-6Al-4V合金。顯微結構觀察發現所有接合介面皆具有明顯的層狀組織，這些具有不同顯微結構及化學組成的層狀組織，主要是由硬銲過程中基材與填料間組成原子交互擴散所引起的等溫凝固及之後固態交互擴散所生成的，這些於高溫形成的組織再經由之後的冷卻降溫反應演變成接合介面中所觀察到的常溫顯微組織。 實驗結果顯示，在C103/Ti-15Cu-15Ni/Ti-6Al-4V整個接合介面中，共會出現七種不同之特性組織，此七種特性組織分別為C103鈮合金基材(Zone Ⅰ)、鈮合金反應區(Zone Ⅱ)、連續介金屬層(Zone Ⅲ)、亞共晶組織(Zone Ⅳ)、過共析組織(Zone Ⅴ)、針狀Widmanstätten組織(Zone Ⅵ)及Ti-6Al-4V基材區(Zone Ⅶ)。接合介面中Cu、Ni元素濃度會因為硬銲過程中母材溶解及等溫擴散效應而發生改變，各區域組織中的Cu、Ni元素濃度高低，將是決定接合介面形成何種組織的關鍵因素。由剪力強度試驗發現，接合介面的組織種類是攸關接合剪力強度高低的關鍵因素。接合介面中的連續介金屬層(Zone Ⅲ)及粗大的針狀Widmanstätten組織(Zone Ⅵ)皆會造成剪力強度的下降，藉由適當硬銲參數的控制將可避免接合介面形成上述二種組織而達到良好的剪力強度。以試片之剪力強度觀點而言，接合介面若由微細的亞共晶或過共析組織構成，且無連續介金屬層及粗大的針狀Widmanstätten組織，將可達到將近360MPa之高剪力強度。此外，藉由適當的銲後熱處理能夠進一步使Cu、Ni元素充分擴散而使接合介面形成更微細的過共析組織，提高接合介面之剪力強度，在此實驗中亦發現，因銲後熱處理所形成之細長Widmanstätten組織有別於因硬銲溫度過高所形成之粗大Widmanstätten組織，此種細長之Widmanstätten組織對接合介面之剪力強度並無明顯之影響。由高溫剪力測試可知，此接合試件在600℃時仍保有接近300MPa之剪力強度，當溫度提升至700℃時，其剪力強度雖會下降至150MPa，但仍符合液態火箭噴注及燃燒管接合之需求。由搭接梯數試驗可知，C103/Ti-15Cu-15Ni/Ti-6Al-4V接合試片所能承受的最大荷重(maximum load)會隨著搭接面積的增加而提高，但其剪力強度卻會隨著搭接梯數的增加而下降。隨著接合間隙增加，接合介面中之連續介金屬層亦隨之增多，因而造成剪力強度的下降，由實驗可知，對C103/Ti-15Cu-15Ni/Ti-6Al-4V而言，最理想之接合間隙為60μm。此外，在C103/Ti-15Cu-15Ni/Ti-6Al-4V接合介面之研究中，我們嘗試利用成分分析及顯微結構觀察結果，搭配硬銲過程中所發生的母材溶解、等溫凝固及降溫冷卻等過程和各種相圖，提出一C103/Ti-15Cu-15Ni/Ti-6Al-4V於1000℃進行硬銲時接合介面組織之成長機構，來加以描述說明C103/Ti-15Cu-15Ni/Ti-6Al-4V接合介面中心之不同層狀組織是如何形成的。此一成長機構共含了五個階段，其結果不但符合接合介面組織之實際顯微結構，且與一般熟知之硬銲過程及理論相吻合。 由Ti-20Cu-20Ni-20Zr(wt.%)片狀填料硬銲接合C103鈮合金與Ti-6Al-4V合金之顯微結構觀察結果顯示，此種填料需較長之硬銲時間才能完全擴散，達到最佳之硬銲效果，其接合介面雖具良好之接合強度(300MPa)，但與Ti-15Cu-15Ni(wt.%)片狀填料相比，其最佳剪力強度仍約低60MPa左右，此或許與其Cu、Ni含量偏高而會形成較多之介金屬相原因有關。此外，由於此種填料之熔點較Ti-15Cu-15Ni(wt.%)填料要低，因此若以高溫使用之觀點而言，Ti-15Cu-15Ni(wt.%)片狀填料將為較合適之Ti基硬銲填料。 對C103/Ti-15Cu-15Ni/Ti-6Al-4V及C103/Ti-20Cu-20Ni-20Zr /Ti-6Al-4V之接合而言，過低的硬銲溫度或過短的持溫時間將造成接合介面中出現連續介金屬層；過高的硬銲溫度或過長的持溫時間將使得Ti-6Al-4V合金晶粒粗大及造成粗大的針狀Widmanstätten組織，這些組織都會造成接合強度的下降，唯有恰當的硬銲溫度及持溫時間才可達到最理想的硬銲品質。

C103 and Ti-6Al-4V alloys joined by vacuum-furnace brazing using Ti-15Cu-15Ni (wt.%) and Ti-20Cu-20Ni-20Zr (wt.%) commercial filler-metals were investigated. This study examines how brazing conditions affect the microstructural evolution and shear strength of the C103/Ti-15Cu-15Ni /Ti-6Al-4V and the C103/Ti-20Cu-20Ni-20Zr /Ti-6Al-4V joints. The microstructural observations reveal the complex microstructural transition from the parent-metal throughout the brazed joints. The characteristic microstructures are formed by atomic diffusion during the brazing, including diluting effect, isothermal solidification and solid-state diffusion occurring between each zone and the parent-metals. According to the microstructural observations, all the characteristic structures of the C103/Ti-15Cu-15Ni/Ti-6Al-4V joint interface can be classified into seven categories, based on their morphology and chemical coposition. The seven characteristic zones existing in the C103/Ti-15Cu-15Ni /Ti-6Al-4V joint are the C103 parent-metal area (Zone Ⅰ), the C103 reaction area (Zone Ⅱ), the continuous intermetallic-layer (Zone Ⅲ), the hypoeutectic structure area (Zone Ⅳ), the hypereutectoid structure area (Zone Ⅴ), the Widmanstätten structure area (Zone Ⅵ), and the Ti-6Al-4V parent-metal area (Zone Ⅶ), respectively. During brazing, the diffusion of Cu and Ni atoms from the molten liquid filler-metal into the parent-metals, and the dilution of parent-metals are the main factors that control the microstructral morphology of the joint interface. The C103/Ti-15Cu-15Ni/Ti-6Al-4V joint brazed at 960℃ for 15 min was found to have joint strength of approximately 360MPa. Further prolonging the brazing time causes the formation of the acicular Widmanstätten structure, which could decrease the shear strength to a low value below 300MPa. The continuous intermetallic-layer and the acicular Widmanstätten structure existing in the joint interface would deteriorate the shear strength of the joint. Additionally, through the post-brazing treatment, the hypoeutectic structure would be changed into the fine hypereutecoid structure, increasing the shear strength of the brazed joint. After post-brazing treatment, the Widmanstätten structure still forms in the joint interface, but such a Widmanstätten structure is finer than that caused by the over-high brazing temperature. The fine, non coarse, Widmanstätten structure in the joint does not seem to affect detrimentally the joint shear strength. Moreover, the shear strength of the C103/Ti-15Cu-15Ni/Ti-6Al-4V joint can be maintained around 300MPa at a temperature below 600℃. As the temperature exceeds 600℃, the shear strength of the joint markedly declines. The results of the overlap-length investigation show the fracture load of the C103/Ti-15Cu-15Ni/Ti-6Al-4V joint raises with the increasing of the overlap-length. However, the joint shear strength decreases with the increasing of the overlap-length because of the non-uniform stress distribution during shear test. The amount of continuous intermetallic-layer in the joint interface increases with the increasing of joint clearance, deteriorating the shears strength of the joint. For the C103/Ti-15Cu-15Ni/Ti-6Al-4V joint, a joint clearance of 60μm can achieve the maximum shear strength. Moreover, a high temperature microstructural evolution mechanism is proposed with phase diagrams and multiphase diffusion theories, discussing how the lamella structures form in the joint interface. This microstructural evolution mechanism involving five steps consists with not only the microstructural observation of the joint interface, but also the brazing process and diffusion theory. The experimental results of brazing the C103 and T-6Al-4V alloys using Ti-20Cu-20Ni-20Zr (wt.%) foil reveal this filler-metal need longer brazing time to diffuse sufficiently, achieving the optimum brazing result. The shear strength of the C103/Ti-20Cu-20Ni-20Zr/Ti-6Al-4V brazed joints can approach 300MPa. However, the best shear strength of the C103/Ti-20Cu -20Ni-20Zr/Ti-6Al-4V brazed joints is less 60MPa than that of the C103/Ti-15Cu-15Ni/Ti-6Al-4V brazed joints. That may be attributed that the Cu and Ni containing of Ti-20Cu-20Ni-20Zr filler metal are higher than that of Ti-15Cu-15Ni filler metal, forming more intermetallic in the brazed joint. From the prospect of using on high temperature, the Ti-15Cu-15Ni filler metal is more suitable for brazing C103 and Ti-6Al-4V than Ti-20Cu-20Ni-20Zr filler metal because of the higher melting point of Ti-15Cu-15Ni filler metal. For the C103/Ti-15Cu-15Ni/Ti-6Al-4V and C103/Ti-20Cu- 20Ni-20Zr/Ti-6Al-4V brazed joints, excessive increasing the brazing time and the brazing temperature form the coarse acicular Widmanstätten structure in front of Ti-6Al-4V parent-metal and cause grain growth of Ti-6Al-4V alloy. However, if the brazing time is too short or the brazing temperature is too low, the continuous intermetallic-layer consisting of intermetallic compounds will remain in the brazed joints after brazing. Both the continuous intermetallic-layer and coarse acicular Widmanstätten structure would deteriorate the joint strength. The experimental results reveal that, for the C103/Ti-15Cu-15Ni/Ti-6Al-4V and C103/Ti-20Cu-20Ni-20Zr/Ti-6Al-4V brazed joints, brazing at appropriate temperature and for appropriate time could achieve the optimum brazing result.