High-Temperature Brazing of Superalloys and Stainless Steels with Novel Ductile Ni-Hf-Based Filler Metals
✍ Scribed by E. Lugscheider; S. Humm
- Publisher
- John Wiley and Sons
- Year
- 2002
- Tongue
- English
- Weight
- 351 KB
- Volume
- 4
- Category
- Article
- ISSN
- 1438-1656
No coin nor oath required. For personal study only.
✦ Synopsis
Careful investigation of the superplastic flow behavior for the differently processed materials is now underway. Accordingly, strain-rate sensitivity experiments will be performed on tensile test specimens in order to provide the strain-rate sensitivity exponent, m, as a function of test temperature. However, as a preliminary exercise, small cylinders 23 mm high and 14 mm in diameter have been machined from bars EP2 and EP3 for subsequent forging operations at 900 C using a 150 tons hydraulic press. Isothermal forging was conducted with an increasing strain rate in the range 4.5 10 ±4 s ±1 to 2.5 10 ±2 s ±1 for a first specimen and in the range 1.3 10 ±3 s ±1 to 1.2 10 ±2 s ±1 for a second one. The final shapes of the two specimens, which exhibit 80 and 93 % reduction in height, without any cracking, is illustrated in Figure 6. It appears from the stress±strain curves that work softening occurs rapidly, presumably due to dynamic recrystallization that enhances superplastic flow behavior and makes easier subsequent forging at higher strain rates. Therefore, it can be stated with some confidence that hot forging of REP consolidated material could be a viable route for the production of nearnet-shape components. Machining worksteps can be omitted, leading to a more attractive and economical processing route.
In conclusion, a whole manufacturing and processing route has been developed at ONERA to produce and consolidate pre-alloyed TiAl powders. Different processing conditions were applied in order to assess the potential of the PM route with respect to the casting route. The latter leads to better tensile properties whereas the former offers the possibility of lowering the extrusion temperature. However, it is worth pointing out that a higher extrusion temperature tends to enhance significantly room-temperature tensile elongations. Additionally, high-temperature treatments are recommended to ensure better homogenization, which in turn guarantees improved mechanical properties of PM specimens. Therefore, more development work on alloy chemistry and REP processing would still be required to achieve a good compromise between process properties and subsequent mechanical properties.
With regard to secondary processing, ingot and powder extrusion routes lead similarly to fine recrystallized microstructures, which are regarded as potentially attractive for subsequent hot-forging operations at relatively low temperatures. Accordingly, specimens prepared from extruded PM material were successfully forged at 900 C with a high reduction ratio. Once optimized, this processing route is expected to be commercially viable for the production of gas turbine components.