TU1无氧铜钎料制备铌和316L不锈钢接头微观组织和力学性能研究

Investigation of the microstructure and mechanical properties of niobium and 316L stainless steel joints by TU1 Oxygen free copper solder

摘要:

铌和不锈钢的连接可以降低超导射频(SRF)腔体的成本,因此引起超导加速器领域的极大兴趣。在本文中,我们报道了采用TU1无氧铜作为填充材料进行反应堆级铌与316L不锈钢连接的真空钎焊技术,研究了样品和过渡接头在不同温度下显微组织和力学性能。显微组织表明形成了脆的金属间化合物层并观察到结合面有扩散现象发生。虽然在结合面有脆性金属间化合物形成,但并不影响其在SRF腔中的使用。在室温(300 K)和液氮温度(77 K)下进行了拉伸测试和剪切测试。在液氮温度下,抗张强度高于其在室温下的抗张强度,而由于脆性金属间化合物的存在导致其液氦环境下剪切强度低于室温环境。由于我们严格控制焊缝厚度,使过渡接头的剪切强度高于样品的强度。最后我们对过渡接头进行真空漏率测试结果显示其真空漏率均好于1.1×10_^-11 mbar.L/s,这表明真空钎焊技术适用于低温下使用的高真空容器,例如用于SRF腔的不锈钢氦容器。

Abstract:

Joining niobium and stainless steel is important for lowering the cost of Superconducting Radio-Frequency (SRF) cavities and thus attracting great interests in the superconducting accelerator community. In this article, we reported a vacuum brazing technique between reactor grade niobium and 316L stainless steel using oxygen-free copper as filler material. Microstructure and mechanical properties at different temperature of both samples and transition joints were investigated. The microstructure demonstrated that brittle intermetallic layer was formed and the diffusion phenomenon was observed. In spite of the existence of brittle intermetallic, the brazing joints’ application in SRF cavities was not affected. Mechanical tests, including tensile tests, shear tests were conducted both at room temperature (300 K) and liquid nitrogen temperature (77 K). At liquid nitrogen temperature, the tensile strengths are higher than those at the room temperature, while the shear strengths are reduced due to the formation of brittle intermetallic. The transition joints always have higher mechanical strengths than samples when the thickness of the welding seam was strictly controlled. Leak tests were also performed on the transition. The joints’ leak rates are all lower than 1.1×10_^-11 mbar.L/s, indicating the vacuum brazing technology is applicable to high vacuum vessels used in cryogenic temperature, such as stainless steel helium vessel for SRF cavities.