DESIGNING WITH ALUMINUM
When determining how a component will function, the first question to ask is: what purpose will the component serve? Choosing the alloy, casting process and thermal treatment requires knowledge of the service conditions of the proposed part, so defining the end-use functions and requirements is always the starting place in the procurement of an aluminum casting. If high-strength, safety-critical components are required, the number of potential casting processes is narrowed, and a high-integrity casting process, such as permanent mold, premium sand casting or a semisolid casting process, will be chosen. The alloy selection also cannot be determined until the component’s application and end-use requirements are defined. The range of possible mechanical properties varies widely because there are many alloy and thermal treatment combinations. For example, many commercial castings often do not have critical service requirements; therefore, a more economical alloy and production method can be utilized.
Once the function of the desired component is determined, engineers and purchasers must ask questions relating to design issues, such as size, weight and part complexity. The size and design features of the casting and available alloys can drive the choice of casting process and cost of the component. Sand casting often is used to produce parts with hollow cavities and a complex arrangement of ribs and pockets that make them less suitable for casting in permanent molds.
In some cases, the finished component cost can be reduced by including features in the design that will produce a near-net-shape casing and eliminate or minimize additional post-casting processing costs, such as machining. However, features like complexity and surface finish or special properties can increase casting cost.
Regardless of cost, the process choice might be limited because of a component’s size. For example, sand casting may be the only process option for large or heavy castings. Although this process typically requires lower tooling cost, the unit price of the castings and finished part can be high. Permanent mold casting has higher tooling cost, but the unit price is lower, particularly for higher quantities. Diecasting has the highest tooling cost but also the lowest piece price on large quantities.
Changes from an initial design can improve design efficiency and/or decrease production costs. Still, even if a casting has a sound design for a specific process, it may have a shape that is conducive to distortion during heat treating. Such errors can be minimized through design changes.
Another critical factor in determining the appropriate casting process and cost is the volume of parts to be purchased. The permanent mold, diecasting or automated sand casting processes can be used to produce high quantities if the size and design features of the casting and available alloys are suitable. However, the tooling for permanent mold and diecasting can be costly, so a large quantity would be required to justify the tooling costs. If low-quantity parts and ultra-large castings are required, the best option may be sand casting, which offers the lowest tooling cost with the capability to cast large components multiple times.
In general, cast aluminum should be selected for applications such as gear housings, intake manifolds and ornamental castings, which require lower weight and structural soundness without necessarily needing high strength at elevated temperatures. Premium high strength aluminum alloys are available that can be used for pump impellers, rotor hubs and low temperature turbine applications.
Many different casting methods and alloys can be used to produce cast aluminum components. The choice of alloy and casting process plays a major role in the procurement process, affecting both component properties and cost. The procurement process for cast aluminum parts should begin with the design engineer defining the three major factors that drive the quality and cost of a cast aluminum component:
- Functionality (service requirements).
- Design (shape and size).
- Production quantity.
Each of these factors will have significant bearing on the choice of casting method, alloy selection and cost, as well as final component quality.
Choosing the alloy, casting process and thermal treatment requires knowledge of the service conditions of the part. More than 60 common standardized aluminum casting alloys are in use today, with up to five different thermal treatment options. This results in a large number of alternatives to choose from to satisfy individual requirements. Because of these many alloy and thermal treatment combinations, the possible range of typical mechanical properties varies widely. Since commercial castings often do not have critical service requirements, the metalcasting facility should be consulted as to the most economical alloy and production method for the job.
Designers, with their knowledge of the service requirements for their castings, must confine the alloy choice to those that provide the necessary properties and then be guided by the metalcasting facility for the final choice.
Sometimes, the alloy that shows the best properties on paper may have production characteristics that make it less desirable on an overall basis than other eligible alloys. The metalcaster is in the best position to advise on factors such as availability, relative ingot costs, production costs and reproducibility of results. When this is coordinated with the designer’s knowledge of service requirements, such as strength, hardness, corrosion resistance, impact strength and machinability, the best possible selection will result.
Because of this coordination, changes from the initial design may be indicated to improve design efficiency and/or lower production costs. For instance, a casting with sound design after casting may have a size or shape conducive to distortion in heat treating that could be minimized through design changes.
Production and service requirements have significant bearing on the choice of casting method, as do the size and shape of the part. For example, castings required in large quantities often are most economically made by the permanent mold, diecasting or automated sand casting processes, provided the size and design features of the casting and available alloys are suitable.
Sand casting often is used to produce parts with hollow cavities and a complex arrangement of ribs, pockets, etc., and for parts unsuitable for casting in metal molds. In some cases, it is advantageous to redesign a casting for either permanent mold or diecasting methods. Sand casting usually requires a minimum tooling charge, but the unit price of the castings and the finished part can be high. Permanent mold casting requires a higher tooling charge, but the unit price is lower, particularly for longer runs. Diecasting usually requires the highest tooling charge but also the lowest piece price on large quantities.
Once the casting method is determined, the alloy choice is narrowed, because not all alloys can be used with all casting methods. The next considerations are service requirements. If high strength is required, heat-treatable alloys must be used. Alloy choice can be narrowed further when remaining requirements, such as pressure tightness, corrosion resistance and machinability, are considered.
In some instances, it may be required to maximize one property—for example, the highest possible yield strength. This limits the alloy and heat treatment choices, as well as the casting method, to one or two choices. In addition, compromises will have to be made on other requirements, such as ductility.
Source: Casting Source
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