Diffusion bonding of tantalum and stainless steel

Abstract

Summary This paper describes an investigation of diffusion bonding of commercially pure tantalum and austenitic stainless steel SUS304 in a vacuum of 8 mPa at temperatures ranging between 600–1200°C. Direct bonding of tantalum and stainless steel results in extremely low joint strength and formation of intermetallic compounds, such as e.g. FeTa, Fe7Ta3, NiTa2, NiTa, Ni2Ta, Ni3Ta, Ni8Ta, and Cr2Ta in the joint bond zone. To examine the effects of stainless steel alloying elements such as e.g. Cr, C, Fe and Ni on the joint strength, diffusion bonding was performed between tantalum and pure chromium, tantalum and carbon steel (0.42 mass%C), tantalum and pure iron, and tantalum and pure nickel. The effects of Fe, Ni and Cu foil inserts on the strength of tantalum and stainless steel joints were also studied. The results obtained may be summarised as follows: 1. When tantalum and chromium are bonded, Cr2Ta is formed in the bond zone, and the bond strength then found is so low that the bonding materials cannot be machined into tensile specimens. 2. When tantalum and pure iron are bonded, the bond strength increases with the disappearance of unbonded zones, the peak value obtained being around 155 MPa at a bonding temperature of 900°C. At any higher bonding temperature, however, voids are formed in the diffusion layers containing FeTa and Fe7Ta3 intermetallic compounds, and the bond strength is impaired. 3. When tantalum and nickel are bonded, diffusion layers containing NiTa2, NiTa, Ni2Ta, Ni3Ta, and Ni8Ta intermetallic compounds are formed, but the bond strength increases with rising bonding temperature. The highest bond strength of 210 MPa is obtained at a bonding temperature of 900°C. This value slightly decreases with rising bonding temperature. 4. When tantalum and SUS304 are bonded, FeTa, Fe7Ta3, NiTa2, NiTa, Ni2Ta, Ni3Ta, Ni8Ta and Cr2Ta intermetallic compounds are detected. Under any bonding conditions whatever, the joints produced fracture from the bond zone when the bonding materials are being machined into tensile specimens. The low bond strength then obtained is thus due to Cr2Ta being formed in the bond zone. 5. When tantalum and SUS304 are bonded using an insert, bond strengths of max. 170 MPa and 220 MPa can be obtained by bonding being performed with pure iron and nickel foil inserts. During bonding using copper foil inserts, a bond strength of 265 MPa equivalent to that of the tantalum base metal can be obtained, provided that bonding is performed under conditions leading to the disappearance of unbonded zones.