Study On The Characteristics Of Brazing Between Tungsten And Steel Using Various Interlayers

Abstract:With the rapid development of science and engineering technology, more severe reliability of the W/steel joints is required in the next generation of high-performance helium cooled high performance divertors. Brazing is one of the main technologies to realize the joint of W/steel components for its simple process and strong adaptability of product size. However, large residual stress is always concentrated in the joint interface during the cooling process of brazing due to remarkable difference of coefficients of thermal expansion between W and steel. The residual stress is always considered to affect the strength of W/steel joints and leads to the failure of the joints. Inserting an appropriate interlayer material between W and steel is proved to be one of the most efficient methods to solve the challenge of residual stress induced failure.In the present study, V, Nb, Ni-Cu and Ni-Fe alloys are used as the stress relaxing interlayers in the brazing process of W/steel. The microstructure characterization and mechanical properties of the joints with different interlayers were systematically investigated. The conclusions drawn from our present work are as follows:(1) Brazing of W and steel was successfully realized with a rapidly solidified Ni-based foil-type filler. The micro hardness of the W/Ni-based filler brazed interface was as high as23GPa due to the formation of the B-rich brittle phase. The average tensile strength was about160MPa and most of the fractured sites occured in the W substrate near the joining seam, where the residual stress was concentrated.(2) A reliable bonding between tungsten and steel could be obtained by using vanadium interlayer. But, a large number of cracks were found perpendicular to the interface. The micro hardness of the W/Ni-based filler brazed seams increased to25GPa due to the presented of the brittle compounds, produced by interaction between V atoms of vanadium interlayer and Ni, B atoms in the Ni-based filler. The average tensile strength of the joint was about143MPa with a brittle fracture mode.(3) The bonding interfaces of W/Nb/steel joint were sound, and only sporadic cracks were observed in the W/Nb interface. The tensile strength of the joint was improved, approximate to284Mpa because some of the residual stress was relaxed through the Nb interlayer. Most of the bonded samples fractured in the W matrix and W/Ni-based filler brazed seams; while other fractures took place in the Nb/Ni-based filler brazed seams, where existed a large number of Ni-Nb intermetallics. Similarly, the high micro hardness of23GPa was caused by the formation of the B-rich phase.(4) Excellent metallurgy bonding interfaces were obtained by using multi-interlayer Ni-Cu and Ni-Fe alloy, which induced the relaxation of residual stress effectively. The average tensile strength was improved to be as high as300MPa and310MPa, respectively. All specimens fractured in the joining seam near W substrate with a brittle fracture mode. The micro hardness of those two joints at the W/Ni-based filler brazed seams were measured to be30GPa and23.9GPa, resulting from the formation of the B-rich and Cr-rich phase, respectively.