X

Cup, Tip, ^^WfM Furnace Nozzle H Hi

Cyclone Separator

Atomization Chamber

Cyclone Separator

Collector

Fig. 6.10. Gas Atomization System for Superalloy Powder Production17

Collector

Fig. 6.10. Gas Atomization System for Superalloy Powder Production17

of the gas stream. Oxygen contents are on the order of 100 ppm, depending on particle size. Finer particle sizes are obtained by screening out oversize particles. Other powder production processes include soluble gas atomization, centrifugal atomization and the plasma rotating electrode process.

Powder consolidation is usually conducted by HIP, or by extrusion, followed by isothermal forging. For HIP, the powder is packed in a sheet metal container, evacuated at either room or elevated temperature, sealed and placed in the HIP chamber and consolidated at 15 ksi argon pressure and 2000-2200° F. HIP consolidated billets can then be processed by isothermal forging or hot die forging. In isothermal forging, the workpiece and the die are heated to the same temperature, while in hot die forging, the die temperature is higher than that for conventional forging but lower than that for isothermal forging. The use of hot tooling and slow strain rates allows more precise shapes. Compared to conventional forging, these processes offer the following advantages: reduced flow stresses, enhanced workability, greater microstructural control and greater dimensional accuracy. The low strain rates, in combination with fine grain sizes, allows production of near net shapes which reduces material and machining cost. HIP followed by forging can be more economical than conventional cast or wrought products, due to improved material utilization, fewer forging steps and reduced final machining. A comparison between conventional ingot metallurgy and forging with the powder metallurgy approach is shown in Fig. 6.11. The biggest disadvantages are high die costs (e.g., TZM molybdenum) and the requirement for forging in vacuum or in an inert atmosphere.

Superalloy powders can also be consolidated by extrusion. Powder, normally containerized, is hot extruded at a reduction ratio in the range of 13-1 to achieve a fully consolidated billet. The extruded billet is then hot worked by conventional press forging, isothermal forging or hot die forging. More recently, as-HIP PM products (no forging), which require only minimal machining, are emerging as an even lower cost route.

6.4.2 Mechanical Alloying1

Mechanical alloying (MA) is a process used to make ODS alloys such as MA6000. Some of these alloys combine y' strengthening along with oxide dispersion so that their stress rupture properties at 1830° F (and higher) are superior to y' strengthening alone.

Mechanical alloying1 is a high energy dry ball milling process used to make composite metallic powders with a controlled fine microstructure. As shown in Fig. 6.12, powders of the desired composition are blended and then placed in a high energy ball mill. The intensive milling process repeatedly fractures and then rewelds the powder particles. During each collision with the grinding balls, the particles are plastically deformed to the extent that the surface oxides are broken, exposing clean metal surfaces.

Ingot

Pancake Forge

Blocker Forge

Finish Forge

Machined for Heat Treating

Machined Sonic Envelope

Conventional Forging

210 lb I

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