Exploring 6 production aspects for vigilant design.

Inside This Story:

* OEMs' outsourcing of responsibilities for individual components has made diecasters pick up the slack during the early stages of casting design.

* This article highlights six possible conditions that components may face during their lifetime and what should be considered during the design process.

As OEMs continue to outsource responsibility for individual components, suppliers, such as metalcasters, must pick up the slack. In the case of cast components, metalcasters are being asked to become part of the solution during the early stages of casting design. As a result, diecasters must understand the basics of casting design and how the components will operate in application.

To this end, diecasters and design engineers have created a list of six conditions a component could face during its lifetime. They include:

* operating temperature;

* applied loads;

* interfacing components and subassemblies;

* electrical and electronic requirements

* corrosion;

* accidental of unexpected conditions.

Diecasters must understand that if a component is not up to-par when manufactured, they are likely to get the initial blame, though it could likely be the fault of the design of alloy type, not the casting. Therefore, by working with design engineers in the CAD stages of the casting, diecasters can ensure that there will not be any defective designs that may result in poor castings.

This article addresses the six feasible conditions die castings can face and that diecasters must understand as they assist customers in component design.

Operating Temperature

Like so many things, temperature is not always what it seems to be. There fore three key issues about temperature environment are addressed.

Cyclic vs. Steady-State--In a cyclic environment, the maximum temperature often doesn't represent what temperature the part will actually reach. For example, in gasoline engines, diecast aluminum and magnesium pistons are often exposed to temperatures above 3,000F (1,648C). However, the piston is exposed to this extreme for only a brief portion of the cycle. Gas turbine wheels, on the other hand, operate in an environment of about 2,200F (1.204C), yet require high temperature alloys. Why? Because the turbine wheels are exposed to a continuous flow of hot gases--a steady-state environment.

Continuous or Transient--Transient high temperatures, or "spikes," often occur in heat-generating devices, such as furnaces or internal combustion engines, especially when the device is shut down. The temperature can rise briefly while heat dissipation begins through convection. There is the urge to omit die castings from design because of the temperature spikes. However, these spikes are almost always brief and diecast components often are designed to easily weather these extremes. Further, diecasters must be sure that their components will remain as part of the end product, because elimination of the components could lead to higher manufacturing costs for OEMs.

Internal and External--The temperature surrounding a component is frequently much different than the level reached by the component itself. For example, the temperature under a car's hood often reaches 220F (104C). Yet fuel pumps, which are frequently diecast, also are exposed to a steady stream of lower temperature fuel, The result: the pump itself runs at a temperature somewhere between the two--substantially less than the "under-hood" air temperature.

Applied Loads

Applied loads are always a concern for equipment makers. After all, these loads cause stress and deflections and need to be vigorously assessed. Diecasters need to produce components that will withstand four types of loads during strength tests:

* long-term or continuous load--with their ability to induce creep of stress corrosion cracking;

* short-term loads--that are applied relatively few times;

* cylic loads--that are repeated thousands or millions of times in a product's life and may induce fatigue failures;

* impact loads--that are applied rarely but have the potential to cause gross distortion or component fracture.

Many of today's engineered diecasting alloys can withstand the four load types described above. Pinpointing specific alloys is too broad of a task because some alloys exhibit good properties for one type of load but inadequate for another. Table 1 presents a brief overview of alloy family properties. Diecast alloys are always superior to their plastic equivalents, and often match up well to more costly copper-base and ferrous alloy options. Thorough data on diecast alloy strength, rigidity and endurance should be provided to designers so they can choose the best materials to be cast.

Interfacing Components & Subassemblies

Countless OEMs have lowered product costs, increased manufacturing flexibility and improved product performance and life expectancy by replacing complex assemblies with well-designed cast parts. Often, die castings are integrated with formerly attached components, saving time and dramatically reducing the total number of parts required. Creating components from fewer of even single parts provides an elegant solution to the handling and assembly of multi-component parts.

When casting a component that will be used as part of an interfacing or multiple-part assembly, diecasters must make sure the metal that's cast is compatible with its surrounding components. If this is overlooked, the final assembly could likely find faults after manufacturing. Three factors to be aware of are:

* dissimilar materials--materials that might have conflicting coefficients of thermal expansion and the potential for galvanic corrosion. A good example of this is if zinc and stainless steel parts were matched with the same assembled product;

* differences in material properties--creep properties that may prohibit or permit the use of tapped threats or press-fit inserts. Likewise, ductility (or the lack of it.) may limit options when considering factors, such as crimping;

* mounting surfaces--when metal-to-metal surfaces are bound together two things need to be observed: that the mating surfaces are in excellent condition and allow no unwanted porous gaps or chipped spots that could compromise the component's durability; and that the diameter of each hole where the fastener will be in place is exactly the same. If the diameters don't match, the torque on the fastener during tightening could induce long-term stresses on the assembly.

Electrical & Electronic Requirements

For makers of electronic devices, electromagnetic interference (EMI) is a constant concern. The problem has resulted in the adoption of military and ASTM standards, as well as action from the Federal Communications Commission (FCC) due to the these emissions. EMI shielding is not only a concern for devices generating EMI but also to those exposed to it.

One of the best ways to achieve EMI shielding is to enclose the device in question within a conductive cover. When EMI impinges on a conductive cover, ohmic currents are induced in it and dissipated as heat. Such use is seen in laptop computers, which use metallic covers, such as magnesium, instead of plastic. This design delivers two benefits: heavy durability from any accidental impact and the ability to withstand EMI.

Diecasters should acknowledge that with laptop computers in constant demand, the production of diecast covers offers the customer the most sufficient product. This is because the inherent electrical conductivity of diecasting alloys usually meets of exceeds EMI standards and provides added impact strength and benefits. Diecast components for such applications also are more cost efficient than those made of plastic, which tend to add the shielding through foil or mixed metal particles in the plastic itself.

Corrosion

The diecasting industry is familiar with corrosion issues and has developed a number of low cost solutions. These include selecting alloys that are galvanically compatible, de signing out features that trap and retain moisture, and providing moisture-proof barriers at the casting interface. Such barriers include epoxies, fillers and plating applications that would prevent any moisture from surface contact. This allows alloys to work in environments that were previously unacceptable. After production, components can experience two types of corrosion: atmospheric corrosion and galvanic corrosion.

Atmospheric corrosion is caused by water, pollutants and other chemicals in the atmosphere. It's typically not an issue for diecast parts, in fact, the initial process usually forms a protective coating on the castings, effectively blocking progressive damage. This coating is typically an oxide that develops with time while casting properties remain the same. The most obvious example of this is copper roofs or statues, though not diecast, which have turned green due to an oxidation process.

Ferrous alloys, on the contrary, gradually decay over time, causing the casting to become porous if continuously exposed to such atmospheric conditions while not being treated.

Galvanic corrosion results from electrochemical reactions that occur at the interface of dissimilar metals exposed to liquids capable of conducting electricity. In the galvanic series of metals, opposites could likely form an electric current if they come in contact. An example would be if steel bolts were used to attach a zinc component. If there's any moisture in the vicinity, the current could start decaying the structure. The popular diecasting family of alloys--aluminum, magnesium and zinc--are all "active" metals and potentially subject to problems, which is why galvanic corrosion cannot be overlooked.

Accidental or Unexpected Conditions

If a customer reports a component has failed in unexpected conditions, the diecaster must deliver this information to the engineers to let them try to foresee as many unusual conditions as possible for each component. Some conditions, such as stone(impact)damage and environmental (acid rain) pollution simply require a margin of safety established and designs created with these potential conditions in mind and the reasonable levels of protection factored in.

Helping engineers better understand the diecasting process as well as knowing what alloys would work best in particular conditions are keys to casting components that can endure anything. When it comes to standing up to the unexpected, the superior strength of die castings often do provide protection--even against the unknown.

Help Your Industry--Assist Customers

By working with design engineers in the design stage of production, diecasters can learn of a component's applications and give feedback of what design would be most beneficial. The diecaster must understand that particular components might be set next to an electical unit in its final assembly of will experience weathering strain if placed outside. The design engineer must understand whether or not the particular alloy can be readily cast in the proposed design and if the design will allow for a strong die casting. Once this is established, diecasters will experience fewer problems during casting and lessen the chance of errors on an end product. Altogether, fewer customers will criticize a component's durability, and the entire production process from design engineer to customer would become smoother.

Table 1. Guide to Diecast Alloy Family Properties

                      Aluminum               Magnesium

Cost                  Lowest cost per        Can compete with aluminum
                      unit volume.           if thinner wall sections
                                             are used. Faster hot-
                                             chamber process possible
                                             on smaller parts.

Weight                Second lowest in       Lowest density.
                      density next
                      to magnesium.

Structural            Highest Modulus        Highest strength-to-
Properties            of Elasticity.         weight ratio, best
                                             vibration dampening
                                             characteristics.

Surface Finish        Good choice for        Good as-cast surface
& Coatings            coating processes      finishes can be achieved.
                      that require high
                      temperatures.

Wear                  *                      *
Resistance

Corrosion             *                      *
Resistance

Machinability         Good.                  Best machinability in
                                             terms of tool-life,
                                             achievable finish, low
                                             cutting forces and energy
                                             consumption.

Thermal Properties,   Best choice for heat   Electro-magnetic
Conductive, &         transfer. Good         shielding.
Electromagnetic       electrical
Shielding             conductivity.

                      Zinc                        Zinc-Aluminum

Cost                  Effective production of     *
                      small parts. Long-term
                      tooling cost savings
                      (tooling lasts 3-5 times
                      longer than aluminum).

Weight                Heaviest of die cast        Weight reduction as
                      alloys, but castable with   compared with the
                      thinner walls than          Zinc family of
                      aluminum, which can         alloys.
                      offset the weight
                      disadvantage.

Structural            Highest ductility and       Highest tensile and
Properties            impact strength.            yield strength.

Surface Finish        Best as-cast surface        *
& Coatings            finish readily accepts
                      electro-coatings and
                      decorative finishes.

Wear                  *                           Best as-cast wear
Resistance                                        resist.

Corrosion                                         *
Resistance

Machinability         Good.                       Good.

Thermal Properties,   Best electrical             *
Conductive, &         conductor. Good heat
Electromagnetic       transfer.
Shielding

* Wear and corrosion resistance can be improved in all alloys
through surface treatments and coatings.

This article was adapted from, "Product Design for Die Casting "North American Die Casting Assn., 1998.

For More Information

"High Integrity Diecasting Process Variations," J.L. Jorstad, Proceedings from the AFS International Conference on Structural Aluminum Casting, November 2003, p. 137-148.

"Computer Model Diecasting Shot Sleeves to Predict Distortion," Jerald Brevick, Kaushik Narayan, Prashant Shastri, Anand Jain, MODERN CASTING, March 2002, p. 34-37.

Visit the North American Die Casting Association's web site at http://www.diecasting.org/ddc.

Leo J. Baran is the director of membership and marketing on the North American Die Casting Association (NADCA), Rosemont, III. NADCA provides technical/design literature to assist design engineers, specifiers and buyers.