1

Separate Halves and Remove Patterns

Separate Halves and Remove Patterns

Completed Casting

Fig. 2.24. Sand Casting Process

Completed Casting

Assemble Halves, Clamp and Cast

Fig. 2.24. Sand Casting Process

Molding sands usually consist of sand grains, a binder, and water. The properties that are important are good flowability or the ability to be easily worked around the pattern, sufficient green strength, and sufficient permeability to allow gas and steam to escape during casting. Sand cores for molded-in inserts can be made using either heat cured binder systems or no-bake binder systems. No-bake binder systems are usually preferred since they provide greater dimensional accuracy, have higher strengths, are more adaptable to automation, and can be used immediately after fabrication. The no-bake systems typically consist of room temperature curing sodium silicate sands, phenolic-urethanes, or furan acids combined with sand.

Gates are used to evenly distribute the metal to the different locations in the casting. The objective is to have progressive solidification from the point most distant from the gate toward the gate, i.e. the metal in the casting should solidify before the metal in the gates solidifies. Normally, the area of the gates and runner system connecting the gates should be about four times larger than the sprue. When feeding needs to be improved, it is better to increase the number of gates rather than increase the pouring temperature.

2.8.2 Plaster and Shell Molding

Plaster mold casting is basically the same as sand casting except gypsum plasters replace the sand in this process. The advantages are very smooth surfaces, good dimensional tolerances, and uniformity due to slow uniform cooling. However, as a result of the slow solidification rates, the mechanical properties are not as good as with sand castings. In addition, since plaster can absorb significant moisture from the atmosphere, it may require slow drying prior to casting.

Shell molding can also be used in place of sand casting when a better surface finish or tighter dimensional control is required. Surfaces finishes in the range of 250-450 ^in. are typical with shell molding. Since it requires precision metal patterns and more specialized equipment, shell molding should be considered a higher volume process than sand casting. Shell molding, shown in Fig. 2.25, consists of the following:

1. A fine silica sand coated with a phenolic resin is placed in a dump box that can be rotated.

2. A metal pattern is heated to 400-500° F, mold released and placed in the dump box.

3. The pattern and sand are inverted allowing the sand to coat the heated pattern. A crust of sand fuses around the part as a result of the heat.

4. The dump box is turned right side up, the pattern with the shell crust is removed and cured in an oven at 650-750° F.

5. The same process is repeated for the other half of the mold.

6. The two mold halves are clamped together and placed in a flask supported with either sand or metal shot.

Box and Pattern Inverted

Box Turned Back Over

Mold Half Removed From Pattern

Box Turned Back Over

Completed Mold Ready For Pouring

Casting Ready for Machining

Completed Mold Ready For Pouring

Casting Ready for Machining

Fig. 2.25. Shell Molding Process

2.8.3 Permanent Mold Casting

In permanent mold casting, liquid metal is poured into a metal mold and allowed to solidify. This method is second only to die casting in the number of aluminum castings produced annually. However, due to the tooling costs, it is usually reserved for high volume applications. The castings produced are normally small compared to sand casting and rather simple in shape. The process produces fairly uniform wall thicknesses but, unless segmented dies are used, is not capable of undercuts. Compared to sand castings, permanent mold castings are more uniform and have better dimensional tolerances, superior surfaces finishes (275-500 ^in. are typical), and better mechanical properties due to the faster solidification rates. Mold materials include gray cast iron and hot work die steels such as H11 and H13. When a disposable sand or plaster core is used with this process, it is referred to as semipermanent mold casting. Another variant of the permanent mold process is low-pressure permanent mold casting. Here the casting is done inside a pressure vessel and an inert gas is used to apply 5-10 psi pressure on the liquid metal. This results in shorter cycle times and excellent mechanical properties.

2.8.4 Die Casting

Die casting is a permanent mold casting process in which the liquid metal is injected into a metal die under high pressure. It is a very high rate production process using expensive equipment and precision matched metal dies. Since the solidification rate is high, this process is amendable to high volume production. It is used to produce very intricate shapes in the small to intermediate part size range. Characteristics of the process include extremely good surface finishes and the ability to hold tight dimensions; however, die castings should not be specified where high mechanical properties are important because of the inherently high porosity level. The high pressure injection creates a lot of turbulence that traps air resulting in high porosity levels. In fact, die cast parts are not usually heat treated because the high porosity levels can cause surface blistering. To reduce the porosity level, the process can be done in vacuum (vacuum die casting) or the die can be purged with oxygen just prior to metal injection. In the latter process, the oxygen reacts with the aluminum to form an oxide dispersion in the casting.

2.8.5 Investment Casting

Investment casting is used where tighter tolerances, better surface finishes, and thinner walls are required than can be obtained with sand casting. Although the investment casting process is covered in greater detail in Chapter 4 on Titanium, a brief description of the process is that investment castings are made by surrounding, or investing, an expendable pattern, usually wax, with a refractory slurry that sets at room temperature. The wax pattern is then melted out and the refractory mold is fired at high temperatures. The molten metal is cast into the mold and the mold is broken away after solidification and cooling.

2.8.6 Evaporative Pattern Casting

A process that is used quite extensively in the automotive industry is evaporative pattern casting. Part patterns of expandable polystyrene are produced in metal dies. The patterns may consist of the entire part or several patterns may be assembled together. Gating patterns are attached and the completed pattern is coated with a thin layer of refractory slurry which is allowed to dry. The slurry must still be permeable enough to allow mold gases to escape during casting. The slurry coated pattern is then placed in a flask supported by sand. When the molten metal is poured, it evaporates the polystyrene pattern. This process is capable of producing very intricate castings with close tolerances, but the mechanical properties are low due to the large amounts of entrapped porosity.

2.8.7 Casting Heat Treatment

The major differences between heat treating cast aluminum alloys,18 as compared with wrought alloys, are the longer soak times during solution heat treating and the use of hot water quenches. Longer soak times are needed because of the relatively coarse microconstituents present in castings that do not have the benefit of the homogenization treatments given to wrought products before hot working. Boiling water is a common quenchant to reduce distortion for castings that normally contain more complex configurations than wrought products. Aluminum-copper castings are usually solution treated at 950-960° F and then quenched in hot water maintained at 150-212° F to minimize quenching stresses and distortion. Cast aluminum alloys are usually supplied in either the T6, T7, or T5 tempers. The T6 temper is used where maximum strength is required. If low internal stresses, dimensional stability, and resistance to stress corrosion cracking are important, then the casting can be overaged to a T7 temper. The T5 temper is produced by aging the as-cast part without solution heat treating and quenching. This treatment is possible because most of the hardening elements are retained in solid solution during casting; however, the strengths obtained with the T5 temper will be lower than those with the T6 heat treatment.

2.8.8 Casting Properties

Since the mechanical properties of castings are not as consistent as wrought products, it is normal practice to use a casting factor (CF) for aluminum castings. The CF usually ranges from 1.0 to 2.0 depending on the end usage of the casting. For example, if the casting factor is 1.25 and the material has a yield strength of 30ksi, the maximum design strength would be 30ksi/1.25 = 24ksi. In addition, sampling is used during production in which a casting is periodically selected from the production lot and cut-up for tensile testing. The sampling plan depends on the criticality of the casting. All premium castings are subjected to both radiographic and penetrant inspection. Premium castings can also be hot isostatic pressed (HIP) to help reduce internal porosity. HIP is usually conducted using argon pressure at 15 ksi and temperatures in the range of 900-980° F. HIP usually results in improved mechanical properties, especially fatigue strength but, of course, it adds to the cost and cycle time. The improvement in fatigue life for A201.0-T7 as a result of HIP is shown in Fig. 2.26. Surface defects in o HIP • No HIP

Cycles to Failure

Fig. 2.26. Effect of HIP on Fatigue Life ofA201.0-T7 Casting19

Cycles to Failure

Fig. 2.26. Effect of HIP on Fatigue Life ofA201.0-T7 Casting19

casting are normally repaired by gas tungsten arc welding (GTAW) using filler wire cast from the same alloy as the casting.

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