58 Materials And Processes

A new process was invented to assure that the aluminum was safely contained during the manufacture of Ni3Al. In this Exomelt™ process, the exothermic heat from the reaction of aluminum with nickel melts the alloying elements that give this Ni3Al its desirable ductility and strength (Fig. 5.13).

The crucible loading sequence is critical. Nickel is placed on the crucible floor. It is topped with alloying elements, then another layer of nickel sandwiched between two layers of aluminum. The crucible is heated to 800°C (1472°F) to melt the aluminum. It reacts with the nickel to form NiAl. The resulting heat release melts the NiAl (>1639°C) (2980°F) and forms droplets that drip onto the remaining aluminum. Maintaining the heat results in exposure of the bottom nickel layer and the formation of Ni3Al.

The important feature of the Exomelt™ process (Fig. 5.14) is the layering described above. It uses the exothermic heat to drive the process, reducing energy input and resulting in an economical process, saving one-third to one-half of the energy consumed by the conventional process. The rapid heating minimizes

FIGURE 5.13 Furnace schematic of the Exomelt process. (Courtesy of John Wiley & Sons, Inc.)
FIGURE 5.14 Exomelt process loading sequence.

oxidation of the alloying elements, increases furnace life by minimizing time at high-temperature, and safely protects heating elements from attack by aluminum.

Vacuum melting in the Exomelt process is helped because all alloying elements can be loaded at the start of the process. Ni3Al is produced in heats of 150-3000 lb.

The Exomelt™ process reduces process time by 50% (Fig. 5.15). It is reproducible and extends furnace crucible life. Temperature is controllable. It is inheri-tantly safe to operate, and on-stream commercially, producing more than 100,000 lb in 1998. It utilizes vacuum melting by loading aluminum at the start of

FIGURE 5.15 Exomelt process decision tree.

the process. Remelting can be used. Totally, savings add up to 50% over earlier processes.

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