Metals for Casting

Most commercial castings are made of alloys rather than pure metals. Alloys are easier to cast and properties of the resulting product are better compared to pure metals.

Casting alloys can be classified as

  1. Ferrous or
  2. Nonferrous.

1. Ferrous Casting alloys:

The ferrous casting alloys is subdivided into two categories

  1. Cast iron and
  2. Cast steel.
1.1 Ferrous Casting Alloys: Cast iron

Cast iron is the most important of all casting alloys. The tonnage of cast iron castings is several times that of all other metals combined.

There several types of cast iron are listed under:

  1. Gray cast iron,
  2. Nodular iron,
  3. White cast iron,
  4. Malleable iron, and
  5. Alloy cast irons.

Typical pouring temperatures for cast iron is around 1400°C, depending on composition.

1.2 Ferrous Casting Alloys: Steel

The mechanical properties of steel make it an attractive engineering material and the capability to create complex geometries makes casting an appealing process. However, great difficulties are faced by the foundry specializing in steel. First, the melting point of steel is considerably higher than for most other metals that are commonly cast. The solidification range for low carbon steels begins at just under 1540°C (2804°F). This means that the pouring temperature required for steel is very high about 1650°C (3002°F). At these high temperatures, steel is chemically very reactive. It readily oxidizes, so special procedures must be used during melting and pouring to isolate the molten metal from the air. Also, molten steel has relatively poor fluidity, and this limits the design of thin sections in components cast out of steel. Several characteristics of steel castings make it worth the effort to solve these problems. Tensile strength is higher than for most other casting metals, ranging upward from about 410 Mpa (59,465 lb/in2). Steel castings have better toughness than most other casting alloys. The properties of steel castings are isotropic; strength is virtually the same in all directions.By contrast, mechanically formed parts (e.g., rolling, forging) exhibit directionality in their properties. Depending on the requirements of the product, isotropic behavior of the material may be desirable. Another advantage of steel castings is ease of welding. They can be readily welded without significant loss of strength, to repair the casting or to fabricate structures with other steel components.

2. Nonferrous Casting Alloys

Nonferrous casting metals include alloys of aluminum, magnesium, copper, tin, zinc, nickel, and titanium.

Aluminum alloys are generally considered to be very castable. The melting point of pure aluminum is 660°C (1112°F), so pouring temperatures for aluminum casting alloys are low compared to cast iron and steel. Their properties make them attractive for castings: light weight, a wide range of strength properties attainable through heat treatment, and ease of machining. Magnesium alloys are the lightest of all casting metals. Other properties include corrosion resistance, as well as high strength-to-weight and stiffness-to-weight ratios.

Copper alloys include bronze, brass, and aluminum bronze. Properties that make them attractive include corrosion resistance, attractive appearance, and good bearing qualities. The high cost of copper is a limitation on the use of its alloys. Applications include pipe fittings, marine propeller blades, pump components, and ornamental jewelry.

Tin has the lowest melting point of the casting metals. Tin-based alloys are easy to cast. They have good corrosion resistant but poor mechanical strength, which limits their applications to pewter mugs and similar products not requiring high strength.

Zinc alloys are commonly used in die casting. Zinc has a low melting point and good fluidity, making it highly castable. Its major weakness is low creep strength, so its castings cannot be subjected to prolonged high stresses.

Nickel alloys have the good hot strength and corrosion resistance, which make them suited to high-temperature applications such as jet engine and rocket components, heat shields, and similar components. Nickel alloys also have a high melting point and are not easy to cast.

Titanium alloys for casting are corrosion resistant and possess high strength-to-weight ratios. However, titanium has a high melting point, low fluidity, and a propensity to oxidize at high temperatures. These properties make it and its alloys difficult to cast.

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