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containing less than 12% are hypoeutectic alloys, and those with greater than 12% are hypereutectic alloys. When combined with small amounts of phosphorus, small insoluble particles of AlP are formed that serve as nuclei to help refine the grain size. The silicon containing 3XX castings alloys account for about 80% of all sand and permanent mold castings. Copper is another alloying element that is frequently used in casting alloys because it makes them respond to precipitation hardening, but copper reduces fluidity, promotes hot shortness, and increases the susceptibility to stress corrosion cracking. For example, the copper and silver containing alloy A201.0 is often overaged to the T7 temper to improve its resistance to stress corrosion cracking. Magnesium is frequently added to improve strength, machinability, and corrosion resistance. Grain refiners, such as titanium and boron, are used that form small crystals (TiAl2 or TiB2) which serve as nucleation sites. The 2XX casting alloys, although harder to cast than the 3XX alloys, can be heat treated to the highest strength levels of the aluminum casting alloys. In order of decreasing castability, the alloy groups can be classified in the order 3XX, 4XX, 5XX, 2XX, and 7XX.

Premium quality castings provide higher quality and reliability than conventional cast products. Important attributes include high mechanical properties determined by test coupons machined from representative parts, low porosity levels as determined by radiography, dimensional accuracy, and good surface finishes. Premium casting alloys include A201.0, A206.0, 224.0, 249.0, 354.0, A356.0, A357.0, and A358.0. The requirements for premium quality castings are usually negotiated through special specifications such as SAE-AMS-A-21180.

Grain size control for castings is important because fine grain sizes result in higher strengths and greater ductility. However, the grain size of aluminum castings can be as small as 0.005 in. and as large as 0.5 in. Normally, grain sizes no larger than 0.04 in. are desired for premium quality castings. Since the size of porosity in an aluminum casting scales somewhat with grain size, finer porosity goes with finer grain sizes. In addition, shrinkage and hot cracking are more prevalent in castings with a coarse grain size. Grain size is a function of pouring temperature, solidification rate, and the presence or absence of a grain refiner. Low pouring temperatures, faster solidification rates, and grain refiners, such as titanium and boron, all produce finer grain sizes.

Aluminum ingots for casting are usually reheated and melted using one of three types of furnaces: direct fuel fired furnaces, indirect fuel fired furnace, or electric furnaces. Direct fuel fired furnaces use hydrocarbon fuels to heat the metal which places the hot combustion gases in direct contact with the charge being melted. Indirect fuel fired furnaces also use hydrocarbon fuels, but the charge is separated by a crucible from direct contact with the hot combustion gases. The advantage of the indirect method is that it helps prevent combustion products, hydrogen in particular, from being absorbed into the melt. Electric furnaces consist of low frequency induction furnaces, high frequency induction furnaces, and electric resistance furnaces. Low frequency induction is by far the most common; a typical furnace operates at 60 cycles, 20-200 kW with capacities of 700-3000 lb. The induced electromagnetic field stirs and mixes the melt, thus aiding in maintaining uniform melt temperatures.

Proper temperature control during melting and pouring is critical; many casting problems have eventually been traced to poor temperature control. The equipment must be capable of holding a temperature tolerance of ±10° F to insure satisfactory results. If the pouring or casting temperature is too low, misruns and cold shuts can occur, while if the pouring temperature is too high, coarse grains, excessive porosity, excessive shrinkage, and hot tearing are all possible.

Molten aluminum is an extremely reactive metal that readily combines with other metals, gases, and even some refractories. It experiences both a large solidification shrinkage (6%) and contraction shrinkage (10%). It also has a high surface tension that, when combined with an oxide film, makes it difficult to obtain sound castings in thin sections. Rather than attempt to alloy the melt themselves, most foundries purchase prealloyed ingot; however, alloys containing magnesium are prone to loss by oxidation and evaporation in the melt and must be replenished prior to casting. To prevent segregation in the melt, the melt must be stirred; however, excessive stirring promotes oxidation of the melt.

Molten aluminum is also subject to contamination by iron, oxides, and hydrogen; therefore, proper steps must be taken to control all three. Iron reduces the ductility and toughness of the casting and promotes the formation of sludge that accumulates in the bottom of the crucible. To prevent sludging, the iron equivalent, %Fe + 2(%Mn) + 3(%Cr), must generally be held below 1.9%. Sludge that accidentally gets poured into a casting is often discovered as hard spots during machining, or worse yet, as stress cracks in service.

Oxides of aluminum and magnesium form as a thin film on the bath surface that actually prevents further oxidation as long as the film is not disturbed. Oxidation can result from moisture introduced by the furnace charge, excessive stirring, pouring from too high a temperature, or pouring from too high a height. Surface oxides are removed by fluxing and then skimming the surface.

If hydrogen is not effectively removed from the melt, the likely result will be a casting containing excessive porosity. Hydrogen originates from moisture on the furnace charge and from hydrocarbon combustion products. At temperatures less than 1250° F, hydrogen absorption is minimal but increases rapidly at higher temperatures. Hydrogen is removed from the melt by using degassing fluxes after the oxides are removed. Degassing fluxes include chlorine gas, nitrogen-chlorine mixtures, and hexachloroethane. The removal of hydrogen is a mechanical, not chemical, process in which the hydrogen attaches itself to the fluxing gas.

Sludge formation and settling is a problem with alloys containing 5% or more of silicon. Three practices that minimize sludge formation are: keeping the iron content as low as possible; keeping the melting furnace at temperatures lower than 1350° F; and keeping the holding furnace at temperatures of 1200° F or less. If sludge does build up in the bottom of the crucible, it is necessary to scoop it out.

2.8.1 Sand Casting

Sand casting is perhaps the oldest casting process known. The molten metal is poured into a cavity shaped inside a body of sand and allowed to solidify. Advantages of sand casting are low equipment costs, design flexibility, and the ability to use a large number of aluminum casting alloys. It is often used for the economical production of small lot sizes and is capable of producing fairly intricate designs. The biggest disadvantages are that the process does not permit close tolerances, and the mechanical properties are somewhat lower due to larger grain sizes as a result of slow cooling rates. However, the mechanical properties are improving as a result of improvements in casting materials and procedures. The steps involved in sand casting are shown in Fig. 2.24 and consist of the following:

1. Fabricate a pattern, usually wood, of the desired part and split it down the centerline.

2. Place the bottom half of the pattern, called the drag, in a box called a flask.

3. Apply a release coating to the pattern, fill the flask with sand and then compact the sand by ramming.

4. Turn the drag half of the mold over and place the top half of the flask on top of it. The top half of the pattern, called the cope, is then placed over the drag half of the pattern and release coated.

5. Risers and a sprue are then installed in the cope half of the flask. The sprue is where liquid metal enters the mold. In a complex casting, the sprue is usually gated to different positions around the casting. The risers are essentially reservoirs for liquid metal that keep the casting supplied with liquid metal as the metal shrinks and contracts on freezing.

6. The cope half is then packed with sand and rammed.

7. The two halves are separated and the patterns are removed. If hollow sections are required, a sand core is placed in the drag half of the mold. A gating system is then cut into the sand on the cope half of the mold.

8. The two halves are reassembled and clamped or bolted shut for casting.

• Wood Split Pattern

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Drag Half of Pattern Pack Drag Half with Sand in

Drag Half of Flask

Drag Half of Flask

Riser

Invert Drag Half Install Cope Parts and Pack with Sand

Riser

Invert Drag Half Install Cope Parts and Pack with Sand

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