鈮合金與鈦合金之真空硬銲接合及其接合介面之研究
Date Issued
2005
Date
2005
Author(s)
洪胤庭
DOI
zh-TW
Abstract
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.
Subjects
鈦合金
鈮合金
真空硬銲
鈦基填料
剪力強度
介金屬
C103
Ti-6Al-4V
brazing
Ti-based filler metal
shear strength
intermetallic compound
Type
thesis