Is the Conversion Feasible?

If a component or series of components have been identified for possible conversion to casting, the next step is to bring in a metalcaster to determine the component’s castability and possible cost/weight reductions. Typically, redesigns to casting aims for a 40% cost reduction from the weldment, assembly or hog-out to overcome the labor and lead-time benefits associated with the other manufacturing methods.

The first step in identifying a casting partner is to determine the material in which the redesigned component could be made. Weldments and assemblies of weldments often are fabricated from wrought steel shapes because carbon and low-alloy steel is the most easily weldable of metals and provides high toughness, strength, ductility, yield stress and stiffness. The problem is that steel often is chosen solely on its weldability. If the component is going to be a casting instead of a weldment, then weldability may not be as important. Therefore, other materials should be considered.

Potential questions to ask to determine the correct material include: What material is required for the application from a physical and mechanical property standpoint? Will the component be used in a severe service application with high fatigue and impact resistance or is it a cosmetic part? Does it need to adsorb vibration? Does it need to control sparks? Does it need to resist corrosion? Does it need to be lightweight?

Steel is the material of choice when high impact resistance and fatigue life are required for severe service applications. In addition, if the cast component is going to be welded to another component, then steel is a logical choice. Generally, however, the opportunity exists to replace a steel weldment with a casting from another alloy family, such as iron or aluminum.
If the component is going to be used in a fatigue application, but isn’t a severe service application, then iron may be the logical choice because of its castability (ease of casting). Ductile iron in particular has made a name for itself in the conversion of steel fabrications to castings because it combines excellent castability with reasonable toughness.

When weight is an issue without severe service application, aluminum often is the casting material choice. With new technology in alloying and molding, aluminum is increasingly becoming a material of choice in important fatigue applications, such as replacing stamped steel fabrications with squeeze-cast aluminum in automotive suspension components.

Regardless of choice of alloy family, the most significant factor in the success of castings in structural applications is the use of geometry to control stress and stiffness. Only the casting process offers so much variety in shape at low cost. Even though some alloys are stiffer than others (for example, steel is 3 times stiffer than aluminum), it is stiffness from shape that makes castings a breakthrough metal product form.

Once a material is selected, design engineers must then go to their purchasing colleagues to begin the search for a casting partner to assist in the redesign of the component to casting. Ask your purchasing colleagues to find casting sources who appreciate the importance of structural geometry in casting design. Such a casting producer will offer the most overall capability in cost reduction and component improvement when converting stamped steel fabrications, weldments and weldment assemblies to metal castings.