The final mechanical properties of a casting are determined by many factors other than just the choice of the alloy and its chemistry. Many processing factors interact with the material to produce the final result, such as heat flow and cooling rates, mold materials and methods, voids and irregularities, and post-casting treatments.

Heat flows from hot to cold regions. The greater the temperature difference, the faster the rate of heat flow. Cooling rates—how fast liquid metal solidifies in the mold to become a casting—are related to heat flow and can affect the grain size of an alloy. In turn, the grain size can affect the mechanical properties of the alloy. It is a general metallurgical principle that a finer microstructure produces higher strength properties. The faster a casting solidifies, the finer the structure.

In addition to insulating materials, exothermic materials, and chill materials, casting geometry plays a significant role in the local cooling rate of a metal.

Wall Thickness

Thinner walls exhibit a faster cooling rate than thicker walls in the casting. For sand castings, smaller fillets risk creating a hot spot in the mold because the sharp corner of sand cannot remove the heat fast enough. This can produce a local shrinkage defect near the surface or a coarse structure.

In parts with thicker walls and inside corners, the cast alloy materials’ thermophysical properties control the heat flow in those wall sections during solidification. Inside corners of the casting radiate heat into the same area of the mold and can be saturated with heat, quickly resulting in slow cooling rates.

Outside corners of the casting have greater surface areas and are surrounded by a larger volume of mold materials; they will cool quicker.

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Casting Irregularities

Three categories of casting irregularities can have a negative effect on the mechanical properties of a casting: voids, nonmetallic inclusions, and bi-films.

Voids can include shrinkage and gas porosity. Voids reduce the effective cross-section of the material, which increases the resulting stress with the same applied force.

Additionally, the surface of shrink porosity can create stress raisers on jagged surfaces (where the surface area approaches zero).

The gates and risers are designed to provide the correct amount of liquid metal at the right time and would be the last volumes in the system to solidify. Large, long, and/or isolated volumes in the casting increase the need for feeding and risk for shrinkage porosity.

If the last areas to solidify in the casting are not adequately fed, shrinkage porosity occurs. Some areas are more prone to shrinkage porosity than others and largely depend on the solidification rates; faster solidification rates reduce the risk for shrinkage-porosity formation.

As the microstructure becomes finer, microporosity becomes smaller and more widely dispersed, reducing its harmful effects. The nature of the casting alloy and its type of solidification structure greatly affect whether it is prone to macro- or microporosity.

Gas porosity generally has a smooth surface resulting from the gas bubble that created it. It can be large or small. The gas may be present in the molten metal (such as hydrogen in aluminum) or be the result of mold-metal interactions (as in green sand molding or near sand cores).

Nonmetallic inclusions in the form of oxides, silicates, sulfides, nitrides, foreign material, etc. have much higher strength. Inclusions interrupt the integrity of the metallic structure, often with sharp edges that act as stress raisers. They are essentially an initiation point for cracks. The sources for nonmetallic inclusions can be refractory materials in the furnace, ladle, and/or mold. They may also be created in the liquid melt from tramp (undesirable) elements such as but not limited to manganese sulfides or aluminum nitrides.

Oxides and inclusions are nonmetallic materials generated in the furnace, in the ladle, and in the gating system during the filling of the mold. All efforts should be taken to prevent them from forming and eliminate those that do with clean melt practices and handling, filtration, and good gating design.

Bi-films are a characteristic of metals that form oxides which are highly stable and create films—most notably in aluminum. While oxides will always form, care is needed to keep turbulence during filling to a minimum to prevent folding and entrapment of these oxides.

Post-Casting Treatments

While most castings are used in the as-cast state (plus machining), many post-casting processes are available to improve the mechanical properties and structure of the part. These include heat treatment, local mechanical working, various surface treatments such as carburizing and nitriding, and hot isostatic pressing (HIP).

Heat treatment is any cyclical heating/cooling process that changes the final mechanical properties of a casting. Figure 1 shows one generic type of heat treatment that can be applied to several alloy families. The times and temperatures will vary with the alloy and casting configuration. In ferrous alloys, this cycle is called a quench and temper (Q&T) treatment and is intended to affect the fundamental nature of the microstructure. In nonferrous alloys, a similar profile is called solution treatment and aging. It is intended to closely control the formation of very small strengthening particles.

Note this is just one of many heat treatment processes. The typical outcomes of heat treatment are:

• Increased material hardness.
• Increased material tensile strength.
• Softening of the material.

These result in a reduction of elongation. Heat treatment can have a wide variety of objectives.

Certain processes can also redistribute segregated elements producing a more uniform composition resulting in more uniform mechanical properties.

The success of the heat treatment process to reach the desired casting mechanical properties is dependent upon the properties achieved during the casting process. The number of grain boundaries and the amount of segregation determine the processing time required to achieve the desired property of a casting.

Note: There are many different methods for heat treatment for both nonferrous and ferrous alloys. Detailed discussions on the topics are provided in the specific alloy courses/e-learning modules taught at the AFS Institute. Detailed discussions on the topics are provided in the specific alloy instructor-led course/e-learning module taught by the AFS Institute. Detailed examples will not be given here.
Quenching involves rapid cooling from an elevated temperature for hardening. It is normally achieved when an object is immersed in water, oil, or some other heat extraction media to harden.
Figure 2 is a TTT (Time Temperature Transformation) diagram for low carbon, low alloyed steel. The phases that are achieved with each of the five different cooling rates is shown at the bottom.

As with cooling rates during the casting solidification, the same principles apply during the quenching process. A faster quench will produce a harder, strong, and more brittle material. Slower quench will result in a material of less strength, but greater ductility.

HIP is a method to collapse voids within a casting to create fully-dense materials by applying pressure to a heated casting to close voids inside the microstructure. These voids inside the walls would otherwise have caused leak paths (for pressure vessels) or low strength/early failures due to a reduced cross section. The word “isostatic” is important too. The pressure is applied equally over the entire casting, so it doesn’t deform the part. For optimum effect, the HIP process must not only close the voids, but also allow fusion of the material to eliminate discontinuities. It is typically applied only to light metal alloys.  CS

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