What to Look For in a Conversion Candidate

The first thing to look for in a possible conversion to casting is a component with a complex geometry. This could be a single component that was machined or forged, but more often than not the most impressive cost/weight savings is with a series of stampings and/or other wrought shapes that are welded and/or bolted together.

This 60-lb. hitch housing casting for the U.S. military was converted from a 35-piece, bolted and welded assembly to a 10-piece component at a cost and weight savings.

Components often are designed with the building-block mindset during the prototype stages due to design familiarity. However, the “building blocks” usually can be streamlined to one-piece cast components.

The type of complex geometry components that make good conversion candidates to casting often have a high surface-area-to-volume ratio. In addition, they have a high number of inches of weld, which leads to high fabrication costs due to the weld time, material and complicated fixtures. The complex geometry components often also have high aspect ratios, which means that they are long with respect to their width (rangy components). These factors typically indicate where the most cost-savings potential is in a casting conversion.

When a fabricated component has a high number of inches of weld in critical stress areas, a greater chance exists that problems in the weld itself or from microstructural effects in the neighboring heat-affected zones will cause failure in the field. High-stress welded joints are generally less capable in overload instances or cyclic fatigue than junctions formed in the metalcasting process.

Another factor to consider with weldments or assemblies of weldments is the number of separate parts necessary to form one component. Beyond the inconsistency in mechanical and dimensional properties developed throughout the multi-piece fabrication, the OEM must inventory all the part numbers and absorb the labor costs to manufacture the component. In addition, the in-house manufacturing facility is responsible for ensuring dimensional consistency from part to part as well as maintaining a low scrap rate during production. Many of these costs are found in burden rates or are “activity-based” and go unrecognized in a cursory value analysis of component cost.

The last factor to consider is final component performance. The well-known mechanical properties of wrought metals are directional (stronger in the wrought direction, weaker in the transverse direction) and may be compromised significantly by welding. A properly produced, high-integrity casting with isotropic mechanical properties (equal in all directions) enjoys uniformity of properties in continuous sections as well as junctions. In addition, visual appearance must be evaluated. While fabrications offer smooth surfaces, their welded junctions are not as pleasing to the eye as the continuity of a complex cast shape.