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Gases In Metal Casting

Gases During The Manufacture Of A Casting

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The molten metal used during the casting process may trap and contain gases. There are various reasons that gases are absorbed into the metal melt during manufacture. Turbulent flow of the casting material through the system may cause it to trap gas from the air. Gases may be trapped from material or the atmosphere in the crucible when the melt is being prepared. Gases may be trapped from reactions between the molten metal and the mold material.

Since liquid metal has a much higher solubility than solid metal, as the casting solidifies these gases are expelled. If they can not escape they may form vacancies in the material, causing porosity in the metal casting.

Whether a vacancy in a cast material is a result of gases or shrinkage is sometimes hard to tell. If the vacancies are spherical and smooth they are most likely a result of gases. Angular and rough vacancies are most likely a result of shrinkage. Gross absences of material within the metal casting are a result of shrinkage.

Prevention Of Gas Defects When Manufacturing A Part By Casting

  • Gases being expelled by the material during solidification can be eliminated by a proper venting system in the mold. This can be planned out during the manufacturing design phase of the metal casting process.
  • Mitigating the amount of turbulence in the fluid flow will reduce gas absorption into the metal.
  • Removal of slag will help eliminate gases and other impurities in the casting.
  • Gases may be removed by flushing a metal melt with inert gas.
  • Elimination of gases may also be accomplished by pouring the metal casting in a vacuum.

Material Selection

The selection of proper materials is important in the design of a metal casting process. Here are a few things to remember when selecting manufacturing materials.

  • Metals, when in a molten state, may react a certain way with other materials they encounter during the casting process. This should always be a consideration. For example, liquid aluminum will react readily with iron. Iron ladles and surfaces contacting the molten aluminum can be covered with a spray on ceramic coating to prevent this.
  • When selecting a type of manufacturing process, remember that some materials may be more applicable to different metal casting techniques than others.
  • Knowing the specific heat of the mold and that of the metal used for the casting will be influential in controlling the thermal gradients in the system.
  • Section of casting metal will factor heavily on the melt's fluidity.
  • A material with a high heat of fusion will take longer to solidify and may improve flow characteristics within the casting.
  • When manufacturing a casting with a metal alloy that freezes over a temperature range, problems may occur due to the solid phase interfering with the liquid phase, both of which will be present within the temperature range. To help reduce this problem, a metal alloy with a shorter solidification temperature range may be selected to manufacture the casting. Or select a mold material with a high thermal conductivity, which could reduce the time spent in the solidification range by increasing the cooling rate.