Melting units
Investment foundries melt comparatively small heats, typically from a few kilograms to a few hundred kilograms, so that each heat can be poured into hot shells without delay.
- Coreless induction furnaces are standard for steels, stainless steels, nickel and cobalt alloys and copper alloys. They melt quickly, stir the bath electromagnetically and allow frequent alloy changes. Linings are selected for compatibility with the alloy.
- Resistance or gas-fired crucible furnaces are common for aluminium.
- Vacuum induction melting and specialised cold-crucible or arc systems are used for reactive and high-performance alloys; see below.
Charge materials and melt control
Charges consist of certified ingot or bar stock, pre-alloyed material and controlled foundry returns. The proportion of returns is limited for demanding alloys because repeated remelting changes composition and increases gas and inclusion content. During melting, attention is paid to:
- Composition. A spectrometer sample is taken before pouring; elements lost by oxidation are adjusted.
- Deoxidation and gas content. Steels are deoxidised shortly before tapping; aluminium melts are degassed to remove hydrogen.
- Slag and inclusions. The bath is skimmed, and filters or bottom-pour arrangements keep slag out of the mould.
- Time at temperature. Long holding increases gas pick-up, lining reaction and loss of reactive elements.
Vacuum melting
Alloys containing reactive elements such as aluminium and titanium, including most precipitation-hardened nickel superalloys, are melted and poured under vacuum to prevent oxidation and nitrogen pick-up; titanium itself requires vacuum or inert-gas systems throughout. Vacuum casting is a distinct capability with its own equipment and qualification. Corrosion-resistant nickel alloys and stainless steels are routinely melted in air, sometimes under a protective gas cover.
Pouring temperature
Pouring temperature is defined as a superheat above the liquidus of the alloy. It is chosen together with shell temperature:
- higher superheat improves fluidity and filling of thin walls;
- lower superheat reduces shrinkage, gas solubility, grain size and metal-mould reaction.
As an orientation, carbon and stainless steels are typically poured in the region of 1550 to 1650 °C and aluminium alloys around 700 to 750 °C; the correct value depends on alloy, section thickness and gating. Temperature is measured immediately before pouring with immersion thermocouples or calibrated pyrometers and recorded for each heat.
Pouring methods
- Gravity pouring from the furnace or a hand ladle is the normal method.
- Vacuum-assisted or counter-gravity filling draws metal upwards into the shell and helps to fill very thin sections with little turbulence.
- Centrifugal casting uses rotation to increase metal pressure, mainly for small parts, jewellery, dental and some titanium castings.
- Pressure-assisted solidification and controlled directional solidification are specialised techniques for particular alloys.
After pouring
Poured moulds cool in still air or under covers. Exothermic or insulating material may be placed on the pouring cup to keep the feed metal liquid. For carbon steels, reducing conditions around the cooling shell limit scale and decarburisation. Each mould is identified with its heat number so that every casting can be traced to its melt analysis.
Related: gating and feeding, gas porosity, material testing.