Casting From Design to Installation
To take advantage of the benefits metal castings can provide in terms of design, component consolidation, mechanical properties and service life, design engineers and purchasers must understand the development of a metal casting. Many of the choices made during the design, specifying and sourcing stages of a metal component influence its quality and, maybe more importantly, its total cost. From the casting supplier perspective, nothing may be more important for design engineers and purchasers to understand than the “total cost” of a component from design to installation, including prototypes, tooling, primary forming process and machining, because design and sourcing decisions must revolve around this figure.
As a means of reducing the total cost of metal component, some end-users have learned the benefits of working with their casting suppliers early in the design states to search out part consolidations and optimization of the overall design. From this point, the customer and casting supplier can proceed hand-in-hand until the design is finalized and tooling is made.
Following is a look at the path of a ductile iron automotive engine crankshaft of the Ecotec 2.21 L4 engine family from design to installation. GM produced this casting in one of its captive GM Powertrain facilities, however, its guidelines in casting design and sourcing transcend to all suppliers (captive or otherwise). Although every metal casting design situation is unique, this example can provide a foundation to understanding successful casting design, sourcing and production.
Crankshafts in Action
As automakers have reduced the overall size and weight of their cars to meet fuel economy and emission standards, their focus often has been on the engine. Bu as engine size has shrunk and their components converted to lighter weight materials, the stresses on each of the structural and moving components has increased.
One of the components most affected by increased engine stress is the crankshaft, as it serves as the torque transmitter for the entire automobile drive-train. Simply described, a fuel/air mixture is pushed into the combustion chamber in an engine where it is ignited, forcing the pistons, which are located within each of the engine block cylinders, up and down. These pistons, which are attached to the crankshaft by connecting rods, force the crankshaft to spin. This spinning then is passed to the flywheel, which transfers the energy to the transmission and drive-train to turn the wheels of the automobile. This spinning also is passed to the camshaft, which opens and closes the intake and exhaust manifolds to let the air/fuel mixture in and out of the combustion chamber.
The problem for automotive engineers is that as the engine has been forced to shrink, the combustion chamber shrinks. To compensate, engineers have decreased the diameter of the engine block cylinder and increased the distance the piston travels. The result is an increase in the stress on the crankshaft. On average, the crankshaft spins 30 times/sec. in operation. Throughout this spin, the crankshaft has pistons/connecting rods exerting pressure in different directions, depending on the engine design. The goal of automotive engineers is to ensure the integrity of the crankshaft through design, material selection and production.
Based on factors such as combustion chamber size, the number of engine cylinders and the configuration of those cylinders, engineers can determine the length of the crankshaft and how long the stroke (the distance it must move the piston up and down) must be. The key is to design this metal component with its surrounding structure in mind. The factors affecting this component (stress, strain, etc.) must be considered in the final design, and then the determination can be made to alter the component to reduce weight or improve functionality.
At this point, the solid and wireframe models of the crankshaft are developed. In the case of crankshaft design at GM, the design engineer, casting engineer and the manufacturing eingineer come together to discuss the design and manufacturability of the component. Through experience, GM has benefited from designing components from concept to delivery with all the process parameters in mind. This allows the design engineer to know up-front if given thin-to-thick section transition, radii, dimensional tolerance or fillet can be manufactured. In addition, this provides an opportunity for the casting engineer to suggest opportunities for the casting process to be optimized through component weight reduction, the elimination of machining or, for example, the consolidation of various metal components (including castings) currently being weld-assembled into one metal casting. GM has learned from the adage that “the correction cost increases 10 times as each stage of the production passes.”
At this point, the design engineer must determine the material for the crankshaft. Historically, crankshafts have either been made of gray or ductile iron (ductile iron since the 1970s) or steel, depending on the stress that it must endure. The decision on the material also relates to the decision on the manufacturing method (historically either metal casting or forging) to produce the component. In addition, as with any component being manufactured today, the largest determining factor is cost.
In their design, engineers must weigh the cost benefits (of one manufacturing process or material) against the mechanical property benefits (of another process or material) to determine which ones they should use to manufacture the crankshaft. The three key considerations for a crankshaft in terms of mechanical properties are modulus of elasticity (the amount of stress it can take before failure), hardness (how rapidly it will wear) and noise dampening capabilities. Following is a look at the four material options for crankshafts:
Cast ductile iron—GM casts 99% of its crankshafts in ductile iron (which has a higher modulus than gray iron). The main reason for this is cost. The finished, cast ductile iron crankshaft for a 4-cylinder engine costs $25 less than a forged steel one (the only other process/material combination used to manufacture crankshafts); for a V6, it costs $35 less; and for a V8, it costs $50 less. This cost savings is due primarily to reduced machining and material costs. In general, steel is more difficult to machine than iron, but also, according to GM engineers, cast crankshafts hold tighter tolerances with less finish stock than forged ones. In the future, however, as engines are further compacted, the stresses on the crankshaft will grow beyond what can be handled by ductile iron. As a result, there may be a shift to more forged steel crankshafts.
Cast austempered ductile iron (ADI)—Although this option has only been tried in low volume applications, the austempering process (a form of heat treating) increases the mechanical properties of ductile iron to that of cast or forged steel. Although the modulus of ADI is the same as ductile iron, the increased mechanical properties may allow casting to remain a viable option for crankshafts at higher stress levels than available with just ductile iron.
Cast steel—In the case of crankshafts, cast steel isn’t an option for high volume due to the manufacturing issues with the component’s design. To ensure the directional solidification of the molten steel that results in a defect-free casting with the necessary mechanical properties, extra gating and risers (for feeding of the molten metal into the mold) are required beyond that used for cast iron. As a result, the machining cost (which is already higher for steel due to the material’s modulus) for the cast steel component will be higher than a forged steel component.
Forged steel—This is the default option for automobile designers when the stress levels are too high for the cast ductile iron crankshaft. However, this is becoming an increasing popular option for automobile engineers because the stress on crankshafts is increasing with every new design. With a modulus 20% higher than that of iron, as well as increased hardness and noise dampening, steel is often a better option for mechanical property regulations. As a high-enough level, certain iron will not pass the safety-critical tests related to engine stress, so the jump has to be made to forged steel regardless of cost concerns.
Once the material has been chosen, the solid model is ready to undergo finite element, structure, and thermal analysis. In addition, a casting and gating system model can be created (by the design engineer or foundry) for casting process modeling (mold filling solidification). Then, a pattern and tooling model is created (by the design engineer, foundry or tooling shop) to generate rapid tooling and rapid prototypes.
Each of these models and analyses are vital to the successful design of the cast component because they predict with very high confidence that the proper physical and mechanical properties will be achieved. Once production-intent tooling is produced, it is costly to return to the design board for changes.
Before GM reaches the manufacture of hard tooling stage, it often has manufactured several rapid prototypes of the crankshaft (in metal, plastic and wood-like materials) and tooling for test runs. Due to the size of its production runs, the speed and accuracy at which prototypes can be produced (within days) makes them a critical step to casting design. In addition, many rapid prototype techniques for casting can provide soft tooling for small production runs (in the hundreds) while hard tooling for high-production sand casting is made. In terms of the production of hard tooling for a cast component, GM typically builds in a 12-week lead time.
Sourcing the Casting
In the case of our crankshaft example, it is sourced as a ductile iron casting due to the lower cost that can be achieved. The first question then is, What type of foundry can cast a ductile iron crankshaft? Based on the past experience of GM, ductile iron crankshafts have been cast in three molding processes—green sand, shell and lost foam. But how does an end-user choose the process or supplier?
At this point, the decision must be based on discussions with various foundries that determine which plant can provide the most optimized cast component (including all post-casting processing) at the lowest system cost. If the sourcing decisions were based only on the cost of producing the casting itself, then the process decision would be, according to GM: 1. green sand; 2. shell; and 3. lost foam. This decision, however, would only focus on the unit cost.
The molten metal that solidifies in the mold at a foundry is a casting; however, in most cases, this alone is not what is being supplied to the customer. Many castings require heat treatment, grinding, machining, polishing, painting, assembly and other value-added services after they leave the mold. It is vital for casting designers and buyers to incorporate all of these services (and their costs) into their final decision on part design and where to source a component (whether the foundry performs all these operations or not) because this is the only accurate method to determining if one manufacturing method (green sand casting, lost foam casting, forging, welding, etc.) is more cost-effective than another.
Green sand—The majority of GM’s ductile iron crankshafts are cast in green sand molds. In this process, sand, clay, water and other stabilizing materials are compacted around two halves of a pattern to form a mold for pouring. Due to the high-production nature of the process (GM casts 5.6 million crankshafts/year in North America alone), it is the most economical casting process to produce the component. However, according to GM engineers, in comparison to the identified two competitive techniques of shell and lost foam, green sand process capability may require more finish stock on the casting for machining later in production. Because no as-cast internal cavities are required in most crankshafts, the production and insertion of sand cores in the mold is generally avoided on each of the processes described.
Shell—Shell mold casting holds tighter tolerances and tooling draft angles than green sand by allowing the production of a mold that is narrower with deeper pockets, reducing the machining cost and increasing dimensional accuracy. The reason is that this process cures the sand around the pattern with heat to “glue” the grains together. In addition, its surface finish is superior to green sand molding. However, its unit cost is higher because it uses resin-coated sand that is then heated to form the molds.
Lost foam—The third process is lost foam casting. Although it has the highest unit cost of the three processes, lost foam’s advantages are recognized after the component has been cast. This loose-sand process, which replaces polystyrene patterns with molten metal, has the best dimensional repeatability (in terms of tolerances) of the three processes, reducing machining time. It also alloys designers to cast-in holes and passageways for improved functionality or the reduction of mass that otherwise would require machining or additional cores. In the case of the lost foam crankshaft, holes can be cast-in at the bearings to reduce mass.
With all of these processes, metal solidification time is another factor to consider. A foundry that is able to control the solidification and cooling times of its castings through its molding and shakeout (separation of the solidified casting from the mold) processes can aid in the development of the specified component’s mechanical properties and eliminate the need for subsequent heat treatment. Once shaken out, the cast components undergo rough finishing and grinding in anticipation for any value-added services or operations.
If a cast component is to undergo heat treatment, machining, painting, etc., this should be specified to the foundry up front. The old days of foundries being the producers of just castings have disappeared as producers of cast components offer many of these value-added services in-house or through sub-contracts. In regard to the crankshafts, some undergo heat treatment to improve mechanical properties or ensure property consistency (especially hardness and noise and vibration dampening) throughout the component. Every crankshaft undergoes machining to achieve the required tolerances and features that are cost prohibitive to achieve in casting.
Although the tolerances that customers supply to foundries differ from component to component, it is important that end-users do not over-specify the tolerances required. On crankshafts, GM’s general profile tolerance for the casting is measured to millimeters. For machining crankshafts, it will specify tolerances one-tenth of the casting tolerances. For polishing it will specify tolerances at a thousandth of the casting tolerances. Components must be designed and toleranced for the process in which they will be manufactured. As a result, engineers must know what a process can achieve. Too strict a tolerance at any step of the manufacturing process will increase the overall costs of the component. GM establishes a standard bill of design and bill of process for its components at the beginning of production as part of its Six Sigma standards to ensure that all crankshafts will be specified to suppliers with the same overall guidelines, whether it be tolerances, hardness values, microstructures, etc. This same bill of process exists for basic manufacturing architecture to ensure the component has the correct amount of draft, finish stock, etc., to be compatible with manufacturing capabilities., While this bill of process is an in-house mandate for GM, it ensures its directions to suppliers are also consistent.
Once the first production run of crankshafts is machined for assembly, the delivery is made to the GM Powertrain manufacturing lines, and the crankshaft is set into the machined block. At this point, however, Gm’s discussions with the foundry aren’t over. Throughout the production of the crankshafts, the foundry must ensure monthly that the components are being manufactured consistently across the board. For crankshafts, the key is the balance of the crankshaft as it spins to provide the power necessary to run the automobile. METAL
Authors: Fred Durek and Michael Oddi, GM Powertrain, and Alfred Spada, Metal Casting Design & Purchasing magazine.