Spray Forming - Melting

Melting

The earliest spray forming work was based on a resistively heated electric holding furnace. The melt then passed through a 3 mm diameter Al₂O₃ nozzle. However the low flow rate made a high superheat necessary to prevent solidification in the nozzle. The next generation melting procedures in spray forming applications were bottom pour induction units, which offer many benefits. In this system, the melting crucible is directly above the atomiser head with a ceramic nozzle feeding directly from the furnace to the atomiser. A stopper rod runs through the melt to the top of the pouring nozzle, the rod is withdrawn when the melt reaches the designated temperature for spraying, typically 50 to 150 °C (122 to 302 °F) above the alloy's liquidus. Alternatively a pre-prepared plug of alloy to block the nozzle is used, and at a specified superheat this plug melts allowing the contents of the furnace to drain through the nozzle. Another problem associated with bottom pour furnaces is the change in flow rate associated with the reducing metalo-static head in the crucible. In some cases, introducing an inert gas overpressure during pouring can compensate for this effect.

An alternative approach is the tilt-pour furnace whereby an induction furnace is tilted to pour the melt into a conical tundish that in turn delivers the molten metal to the melt delivery nozzle. The tilt pour system provides the advantage that melting is decoupled from the spraying procedure so that melting problems and remedial solutions do not affect or disturb the critical set-up of the melt delivery nozzle.

In the most complex melting arrangement, used only for production of nickel superalloy turbine forging blanks by spray forming, vacuum induction melting, electroslag re-melting and cold hearth crucibles have been combined by GE to control alloy impurity levels and the presence of refractory inclusions in the molten metal supply. Clean metal spray forming (CMSF) combines the electroslag refining process, cold walled induction guide and gas atomised spray forming. This approach has led to a reduction in the number of melt related defects (pores, inclusions, etc.), a finer average grain size, the ability to produce larger ingots and the ability to process a wider range of alloys.