Future power systems for spacecraft and lunar surface systems will likely have a strong dependence on nuclear power. The design of a space nuclear powerplant involves integrating the major subsystems of the reactor, the power-conversion system, and the heat-rejection system. Optimum material choices for subsystems and between subsystems can vary significantly depending on the design. A heat-rejection system made of titanium could be integrated to a power-conversion system made of superalloys or stainless steels. The transition and stability of the joined dissimilar materials are critical points in the overall system and key factors in determining life and performance. This work describes preliminary investigations by researchers from the NASA Glenn Research Center and the University of Toledo to join a commercial, wrought superalloy, HASTELLOY X (HX), to a widely used titanium alloy, Ti-6Al-4V (titanium-6 wt% aluminum-4 wt% vanadium).
Titanium alloys, a lightweight alternative to stainless steels, are used extensively in aeronautic applications. In space power applications, replacing some stainless steel components with titanium alloys could reduce the overall mass of the spacecraft. However, the use of titanium alloys in a space power system requires understanding the limitations of joint formation and temperature stability. Fusion welding can be problematic because of the reactive nature of titanium and the increased potential formation of brittle intermetallic compounds. A few previous attempts to join Ti or Ti-alloys to superalloys through explosive bonding, brazing, and diffusion welding (with or without thin interlayers of other metals to minimize the formation of deleterious brittle intermetallic phases) have been reported in the literature (refs. 1 to 3).
Interdiffusion reaction zone widths for HX/Ti-6Al-4V diffusion couples with and without a V interlayer after annealing at 1150 K.
In this investigation, joints between the superalloy and titanium alloy were produced by hot pressing small coupons of each material for 4 hr at 1150 K (877 °C). An exploratory 70-μm-thick V foil interlayer was inserted between the two dissimilar alloys in one of the couples prior to hot pressing. After the initial bond was formed, the “diffusion couple” was annealed in an ultra-high-purity argon atmosphere for 100 and 300 hr at 1150 K to accelerate the diffusion process. The table shows the diffusion couples that were formed as well as the temperatures and anneal times. The diffusion-affected region of the bond was subsequently examined by scanning electron microscopy, and the couples were also examined for compositional variations.
| Diffusion couple | Annealing time at 1150 K, hr |
||
|---|---|---|---|
| HX/Ti-6Al-4V | 4 | 100 | 300 |
| HX/V/Ti-6Al-4V | 4 | 100 | 300 |
For the HX/V/Ti-6Al-4V system, the reaction zone was narrower than for the couple without the V interlayer.
A single diffusion layer formed at the HX/V interface in the reaction zone of the HX/V/Ti-6V-4Al couple. Conversely, multiple phase layers developed in the reaction zone of the HX/Ti-6V-4Al couple. The following elemental maps (below) show the distribution of Ti and Ni in the two diffusion couples after annealing 300 hr at 1150 K. At this accelerated exposure temperature, minimal elemental inter-mixing occurred in the HX/V/Ti-6Al-4V system, but extensive diffusion of Ti and Ni occurred in the HX/Ti-6Al-4V system.
Electron microscopy images and elemental maps for Ni and Ti for HX/V/Ti-6Al-4V and HX/Ti-6Al-4V diffusion couples after hot pressing for 300 hr at 1150 K, showing different depths of Ni and Ti diffusion with or without the V interlayer.
In summary, although traditional welding of nickel to titanium alloys is problematic, nickel-based alloys can be joined to Ti alloys by nonfusion techniques, and the addition of a V interlayer may minimize the formation of deleterious, brittle intermetallic phases in a superalloy/Ti-alloy dissimilar metal joint.
Find out more about the research of Glenn’s Structures and Material Division: http://www.grc.nasa.gov/WWW/5000/MaterialsStructures/
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