Vacuum induction melting is standard operating practice for investment casting foundries, because the high-value alloys they cast must adhere to strict metallurgical standards. But as in any foundry, maximizing the value of metals calls for recycling casting returns and scrap for remelting.
Last year, the Danieli Research Center in Buttrio, Italy, developed and commissioned a remelting furnace specifically for investment casting operations producing critical, high-value parts—particularly for aerospace markets. This innovation makes it possible to recycle superalloy revert materials in house.
Vacuum induction melting (VIM) and investment casting of nickel- and cobalt-based superalloys typically results in relatively low yields, generating up to 85% scrap depending on the complexity of the cast parts. Usually, this scrap is returned to alloy manufacturers to produce new alloy “barsticks,” for use as melting feedstock.
Investment casters requiring alloys that are more difficult or expensive to source frequently experience material delays, a situation worsened by price volatility, particularly for nickel alloys.
Often, well-sorted superalloy scrap can be remelted into barsticks and remain within the alloy specification without further alloying. Additionally, some scrap treatment can be conducted on-site without requiring extensive foundry infrastructure.
Given these factors, a German investment caster contracted Danieli to develop and supply the VIM B52 furnace, to recycle up to 350 kg of superalloy scrap into as many as 14 cylindrical molds.
“Thanks to excellent scrap quality, the aim of this furnace is not alloy refining, although some metallurgical procedures typically associated with large VIMs that improve alloy quality are incorporated,” explained Matthias Knabl, vice president, Danieli Special Metallurgy. “The new furnace design accommodates casting weights between 200 and 500 kg without significant adjustments, and it is easily adaptable for higher casting weights.”
One challenge was the geometry of the scrap to be fed into the induction furnace. Fine scrap is used in the initial charge, leaving uncut sprues for back-charging. The project called for up to 25 sprues to be added to the melt in the shortest possible time.
Danieli developed and patented a system for individual back-charging of the sprues into the crucible via a load-lock device atop the melt chamber. Unlike conventional mechanisms, the new device is safe, rapid, reliable, reproducible, and fully automated.
The load-lock chamber is equipped with its own vacuum pump and a sliding gate valve toward the melt chamber.
The sprues are loaded into the chamber manually from the furnace platform. In an automated cycle of less than 95 seconds, the load-lock chamber is evacuated and the scrap piece is automatically fed by a rack-and-pinion mechanism into the induction furnace, and released. Then, the mechanism is returned to the load-lock chamber for the next charging cycle.
After melting and casting into molds via a preheated refractory tundish, the furnace door is opened and the mold table is exchanged for barstick extraction. After cutting, these barsticks can immediately be used as feed material for investment casting.
The VIM B52 operates under a vacuum atmosphere of 0.2 mbar, which minimizes evaporation of volatile elements and ensures the target chemical analysis of the alloy.
In addition to substantial CO2 emission reductions (approx. 100 tons/year) from reduced material transport, the plant has significantly decreased supply chain and inventory costs and gained more production flexibility.