Melting for casting non-ferrous alloys
After selecting and preparing the alloying ingredients from the charges, melting begins to produce the desired alloy. This is accomplished using one of several common methods.
Electric arc furnace melting is accomplished by transferring the charges from the holding crucible to the melting crucible with a ladle crane, or pouring from ladles into the melting crucible. One or two carbon electrodes, depending on the size of the melter, are lowered into the crucible, contacting the charges. Voltages of 400 - 1200 volts and currents of 1200 - 6000 amperes are used to heat the crucible and the charges in it. Melting is usually complete within 1 to 5 minutes.
Induction melting is accomplished with power applied by induction [1]. The power is generated by an induction generator and is induced into thecharge material through a coil. The charge is melted in a short time period of usually less than one minute.
Resistance melting is accomplished by passing an electrical current through the charge material. One technique is direct resistance melting in a single crucible with electrodes in the form of bars or rods in the crucible or melted material [2]. This technique is normally used for smaller quantities and for low-melting alloys. The application of the electric current depends on the type and quality of the charge material [3].
Crucible melting is normal for melting the alloys, especially aluminum alloy. The crucible is of silica sand or cast iron and is heated by means of an external heating unit. For aluminum alloys, the fuel often used is gas, oil, or coal. For aluminum alloys, the charge must be heated for an extended period of time until a depth of around 3 inches is achieved.
Vacuum melting is utilized to reduce the amount of oxidation and gas inclusions in the alloy. This process uses a vacuum of up to 30 torr. The process is used to melt small amounts of charge (usually 10 pounds or less) under vacuum usually in a mild steel retort [4].
These various melting processes can be used to produce alloy with desired properties. Melting time and alloy purity are greatly affected by the type and quality of the charge materials. Care must be taken to select the charge materials so that it will meet the desired alloy requirements. In addition, the operational parameters of the melting process must be adjusted to produce the desired cast alloy result.
References
[1] W. J. Dieterich, Heat Treaters Guide, 4th Edition, ASM International, 2012.
[2] J. M. Bessone and A. D. McGaw, Fundamentals of Applied Metallurgy, John Wiley & Sons, Inc., 1987.
[3] B. Schaefer, “Modern Melting Procedures for High Quality Metal Castings,” Engineering Controls, March 1985.
[4] S. S. Mollan, E. L. Cawley, and H. J. Hoerner, Fundamentals of Vacuum Metallurgy, McGraw Hill, Inc., 1963.
