Identifying a candidate for conversion to casting.

Inside This Story:

* This article shows how to work with OEMs to identify components worth redesigning into castings for possible cost and weight savings.

* Detailed within are strategies to gain insights into another side of the industry and to get OEMs thinking about conversions.

How do you spot a conversion opportunity for a casting?

For metalcasters, this should be an easy question to answer. But for many OEM purchasers and design engineers, the answer may not come as easily.

On the floor of nearly every OEM manufacturing and assembly operation lays components currently made up of several stamped, wrought or machined metal parts. Could they be redesigned into a single cast metal component for improved performance? How do you determine if the potential performance gains or cost savings would make the redesign viable?

Metalcasters possess the know-how to answer these questions. So the real dilemma for a metalcaster is getting a purchaser or design engineer to start thinking along the same lines.

The choice of whether a component is best manufactured as a welded, assembled, machined or cast component is based on the component's geometry, production costs and requirements in application.

When to Look to Change

The best way to understand another person's situation is to walk in their shoes for a while--or in this case walk along side them for a while. Bringing suppliers and purchasers together can prove to be beneficial for both sides.

Metalcasters must understand that the focus for design engineers often is time to market. Because of delays elsewhere, components often are designed to be manufactured by process(es) that will deliver them with the shortest lead times. This leads to OEMs designing and manufacturing weldments and assemblies of weldments in-house, utilizing their available capacity and labor and eliminating demands of sourcing a component (such as design changes and pattern costs).

Design engineers generally feel more comfortable designing "building blocks" of wrought shapes because this method is commonly taught in structural design. Manufacturing engineers and purchasers are comfortable with bringing weldments and assemblies of weldments into production.

Another quick-to-market option is to hog-out (machine) a shape from stock to produce a required component, simplifying the sourcing process to a one-step machining operation. Although both approaches deliver the product to the customer in a time-efficient manner, they may not deliver it in the most cost-effective manner, particularly when volume increases.

That's when OEMs must realize that weldments and assemblies of weldments create a significant volume of part numbers to produce (or buy), schedule and track. They require high levels of labor to fixture (weld, bolt, align, etc.) and typically aren't as dimensionally or structurally consistent due to inherent variances in manufacturing. Many OEMs may not know this without a metalcaster bringing it to their attention.

On the other hand, hog-outs have the advantage of known wrought stock mechanical properties but suffer from high costs because a large percentage of the bar stock they are borne from ends up on the machining room floor. Because mechanical properties of casting alloys are not as well defined or as widely available in properties literature, some structural designers feel more comfortable with wrought metal properties. As a result, the opportunity to learn more about the casting structural properties is often missed, along with the prospects of lighter, stronger and more cost-effective parts.

In prototype situations when time is of the essence, weldments, assemblies and/or machining are sometimes selected over casting to bring the product into the market quickly. Metalcasters must make it known that the key is to convert these fabricated components to the most efficient manufacturing method once volume production is ready to begin.

What to Look For in a Conversion Candidate

The first thing metalcasters must engrain in the minds of OEMs is to look for a possible conversion to casting in a component with 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 stamping and/or other wrought shapes that are welded and/or bolted together.

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

The types 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 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 ratio, which means that they are long with respect to width (rangy components).

When a fabricated component has a high number of inches weld in crucial stress areas, a greater chance exists that problems in the weld itself or from microstructural effects in 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 thing to relay to OEMs to consider 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 "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 materials are directional (stronger in the wrought direction, weaker in the transverse direction) and may be comprised 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.

Is the Conversion Feasible?

If a component or series of components have been identified for possible conversion to casting, the next step is to determine the component's castability and possible cost/weight reductions. Typically, redesigns to casting aim 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.

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.

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 absorb 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. 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 of 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, it is stiffness from shape that makes castings a break through metal product form.

The process of converting a weldment or assemblies of weldments into castings is not always a simple one. It requires purchasers and suppliers to understand each other's capabilities and requirements. By meeting with and talking to OEMs, metalcasters can gain in sight into another world and better pinpoint conversion candidates.

7 Considerations for Conversions

When examining a weldment or assembly for a possible conversion to casting, following are seven considerations to determine the feasibility of the redesign:

1 Geometry--Is the component complex enough to warrant a redesign to casting?

2 Inches of weld--The greater the inches of weld needed to make up a component from several smaller components, the more likely it is a candidate for conversion to casting.

3 Number of parts--Is the component made up of several smaller parts that have been attached together? If it is, would a single casting reduce the cost associated with part number inventory as well as free up in-house operations for other more important manufacturing tasks?

4 Dimensional consistency--How tight are the tolerances for the component? Is warping ever an issue? The inherent dimensional inconsistency with fabrications is eliminated with a single cast component.

5 Scrap rates--Are scrap rates high for the component during in-house fabrication? If so, the component may be better manufactured out-of-house via a different production method.

6 Appearance--Is the component visible? Does it require a clean, streamlined appearance with a fine surface finish?

7 Field problems--Does the component fail in use? Are the stresses exerted on it too great? Castings provide consistent properties throughout each component, eliminating much of the variability found in assemblies and weldments.

Fig. 1. Shown above is the main landing gear door uplock on Boeing's
767. The reduction in total parts achieved by the one-piece uplock
support casting (r) provides a tremendous cost savings over the
weldment. The accompanying table shows the differences of the two
different design concepts.

                                  One-Piece Casting

Shims eliminated          25 immediately, 7 more gradually
Impact to inventory          27 part numbers eliminated
Impact to                  65% reduction in internal flow;
process now                  factory shortage eliminated
Impact to                 7 drawings, 19 sheets eliminated;
engineering              1 drawing changed, 1 drawing added
Impact to planning      30 detail, assembly plans eliminated;
                               revise 4 assembly plans
Impact to tooling           eliminate 2 FAJs immediately,
                                  2 more gradually
Effective                              1 year
implementation
Concerns                       possible weight growth
Savings/airplane                      baseline
Payback period                      27 airplanes

                                 Precision Assembly

Shims eliminated                   most gradually
Impact to inventory                   no change
Impact to                    <20% in assembly flow time
process now
Impact to                      many changes over time
engineering
Impact to planning       all detail, assembly plans must be
                       revised; extensive vendor coordination
Impact to tooling          eliminate 2 FAJs after process
                           control established, eliminate
                                  2 more gradually
Effective                     some improvement evident
implementation                       in 6 months
Concerns                   variation reduction of numerous
                          details involving many suppliers;
                           engineering definition of hole
                       coordinates; possible, detail redesigns
Savings/airplane                 15-20% of baseline
Payback period                            ?

RELATED ARTICLE: Conversions to castings at Oshkosh Truck Corp.

The process of converting weldments to castings at Oshkosh Truck Corp., Oshkosh. Wis., begins by identifying candidate weldments. A "walk around" is performed on a vehicle with representatives from:

* engineering--those who understand the form, fit and function of the weldment;

* purchasing--someone who has the knowledge of the weldment's quantities and cost;

* metalcaster--some one who can identify tile castability of the weldment and what (if any) geometric changes must be performed to enable casting of the part's geometric envelope.

Forming a team, these individuals work together to understand components that are candidates for conversions. Not all weldments are good conversion candidates. Poor candidates are weldments that have a low annual use (unable to accommodate the pattern costs of casting), rely on specific material characteristics that are unavailable with ductile iron and whose geometries aren't readily castable.

Therefore, the team must work together and look at all aspects of the part Io determine if the weldment is an appropriate conversion candidate. These candidates are typically complex weldments that consist of several pieces, have a castable geometry and are welded from a material whose engineering material properties can be approximated by the foundry.

The tensile properties of the weldment only are a fraction of the possible material characteristics that must be considered when evaluating conversion candidates.

For a typical 1020 steel weldment with a minimum tensile strength of 48 ksi, 27 ksi yield strength and hard ness ranging from 96-140 Bhn, the conversion to 65-45-12 or 60-40-18 grade of ductile iron increases tensile properties.

Normally, components are designed to yield strength characteristics with an added safety factor. But, because most failures occur from repeated application of loads (fatigue) that generate stresses below the material's yield strength, notch geometries and microstructural transitions inherently present in a weldment become the critical factor in the survivability of the component. In most cases, the casting offers increased durability and resistance to fatigue failure by reducing or eliminating these discontinuities.

Although lower in ductility- and toughness than steel, ductile iron offers significant toughness, a slight reduction in density and some damping characteristics. In addition, the elimination o[ weldments provides the component with improved function through more uniform loading in the casting cross section.

Optimizing Design

Examination of a weldment for potential casting conversion also provides an opportunity to optimize the component's design. As part of the casting conversion process, a finite element analysis should be performed and confirmed with actual testing. During this activity, the design can be optimized for load-carrying capability.

Time may not have been available during the prototype stage to perform this type of in-depth analysis. Instead, reliance was placed on experience and a liberal safety factor. Hence, examining it for a casting conversion provides a unique opportunity to reinvestigate the part. A redesign may improve its function or merely improve cosmetic appearance.

Casting the weldment also will result in fewer internal stresses, depending on how it is cooled, which results in less distortion during subsequent machining operations. This will aid in more accurate dimensional conformance.

Weldments inherently contain residual tensile stresses that can approach the material's yield strength. Removing material during machining operations has the tendency to relieve these residual stresses and generates distortion. Only if the weldment is stress-relieved before machining can the distortions be eliminated. These benefits lend justification to converting the weldment into a casting.

--Robert M. Hathaway, Material Process Engineer, Oshkosh Truck Corp., Oshkosh, Wis.

For More Information

"Cost Effective Casting Design: What Every Component Designer Should Know," M.A. Gwyn, Engineered Casting Solutions, Summer 2000, p. 60-69.

"Steel Casting Conversions from Fabrications," M.A. Gwyn and H.E. Birkey, Proceedings from the Steel Founders' Society of America T&O Conference, Barrington, IL, 1997.

Michael A. Gwyn is the Director of the Metals Technology at The Advanced Technology Institute. As a mechanical engineer, he has held executive posts at two foundries prior to joining ATI.