Capitalizing on Die Casting Technology

Introduction

Die casting is an economical way of producing large quantities of complicated-shaped products with high precision. However, for many years the aluminum die casting industry has been troubled with problems in product quality and in production efficiency related to insufficient performances of die materials. Die casting dies are subjected to severe damage: soldering, corrosion, erosion, and heat checking due to thermal and mechanical cyclic loadings and reaction with aggressive aluminum.

To overcome these problems, improvements in die materials and application of nitriding onto die steel have been made over the years, resulting in improvement in efficiency but far from those desired by die casters. Development of the TD Process in Japan in 1971 should be considered a landmark both in surface coating technology and aluminum casting technology. Carbide coating by this process has provided a tremendous benefit to aluminum casting industries in Japan and elsewhere.

Figure 1 illustrates the increased use of TD in Japan. Through 1995, more than 600,000 core pins have been TD coated in Japan, demonstrating the usefulness of the process in aluminum die casting industry. The TD coated pins, cores, and other parts are sent to U.S. Japanese transplants from Japan. This fact eloquently speaks that the TD Process is indispensable in the Japanese aluminum casting industry.

The TD Process

The TD Process is performed by immersing parts into a fused salt bath kept at temperatures of 871 to 1037°C (1600-1900°F) for one to eight hours. This temperature range is suitable for quench hardening many grades of low alloy steels and tool steels. Carbide constituents dispersed in the salt bath combine with carbon atoms contained in the tooling substrate, which must contain a carbon content of .2% or greater. A carbide layer is formed into and onto the surface of the substrate by diffusion of carbon and nitrogen from the substrate. The carbide layer produced has a fine, non-porous composition and is metallurgically bonded into the surface through diffusion rather than by coating.

Parts to be processed are pre-heated to minimize distortion. They are then TD processed at the austenitizing temperature (up to 1900°F) recommended for the grade of steel being treated. After processing, the parts are quenched in air or salt to produce the hardened substrate. The parts then receive the proper tempering cycle.

Steels that have austenitizing temperatures greater than 1900°F may be post-heat treated in vacuum or protective gas to achieve full substrate hardness after TD treatment.

The TD Process produces layers of Vanadium Carbide, Niobium Carbide, and Chromium Carbide, depending on the carbide-forming elements used in the salt bath. Tantalum, Titanium, Tungsten, and Molybdenum can also be used. Vanadium and Niobium exhibit superior peel strength and resistance to wear, corrosion, and oxidation when compared to other processes. Chromium carbide has lower wear resistance; however, it has higher resistance to oxidation.

Since most tooling applications require the hardest surface possible, the TD Center uses the Vanadium Carbide application. TD-treated substrates with Vanadium Carbide will show surface hardness in the range of 3200 to 3800 on the Vickers hardness scale. For comparison, most cemented carbide used in tooling applications will register only in the range of 1800 on the Vickers scale.

Causes of Die Failure in Die Casting and Related Die Material Properties

Die Failures

The most typical types of major damages to die casting components that are encountered in aluminum die castings are listed below. Some of this damage may occur in injection components of die casting, die components of low pressure die casting, squeeze die casting, permanent mold casting, and zinc die casting.

Improvement of the Related Properties by TD Process

The high coating temperature used in the TD Process develops a strong adhesion of the coating to the substrate, creating a metallurgical bond between substrate and coating. Consequently, peeling-off of the coatings caused by the thermal cycles encountered in aluminum die casting is not a problem. Toughness and fatigue strength of steel substrates are also not so affected by the coating process which eliminates anxiety about gross cracking and pin breakage problems.

The TD Process converts the part's surface to carbides which are chemically inert against aluminum. Therefore, TD-coated steels have excellent resistance to corrosion as exemplified in Figure 2, which was published by Ohio State University (OSU). TD-coated steel specimens showed the smallest diameter decrease when compared to specimens that were coated with nitrides and carbides by CVD and PVD, etc. The much superior resistance of the TD coating to nitriding and nitro-sulphurizing has been confirmed by a number of the similar tests in Japan with and without specimen rotation.

Due to high hardness, strong bond, and high resistance to corrosion, TD-coated steel specimens again exhibited the minimum erosion damage in a simulation test by OSU using a production die casting machine with specially designed core pins (see Figure 3).

The effect of heat checking behavior of steel still remains unclear because of a lack a of good testing methods which will simulate the thermal loading condition in die casting. However, simple cyclic heating and cooling tests have already shown improvement of heat checking behavior of TD-coated H13 steels, as shown in Figure 4.

Each die component used in production is subjected to different working conditions related to temperature and impingement velocity of molten aluminum, volume of aluminum which creates thermal loading to each component, lubrication condition, etc. Therefore, the degree and appearance of damage occurred on each component may be different, although the kinds of damage listed above are common to all die components. It is extremely difficult to make clear the damage on each component and to conclude how widely the specified coating method can effectively improve the problem.

The die casting tests were carried by using a specially designed die with multi-cavities and multi-pins to approach this solution. Part of the results are summarized in Figure 5. In this test, the No. 1 pin, located in the path of injected aluminum, is subjected to the highest aluminum temperature and the highest impingement velocity. No. 4 pin is the highest thermal loading (the highest pin temperature was confirmed on this pin). No. 7 pin had less severe conditions than Nos. 1 and 4 as far as temperature and velocity: and Nos. 2 and 3 the least severe. The final damages into the pins were confirmed after 5000 shots by observing pin surface and cross-section. Growth of the damage was evaluated at each number of shots by weight changes and only appearance of the pins was judged after removal of stuck aluminum. The weight change of the pins consists of the sum of weight loss caused by removal of pin material from the pin surface due to corrosion and erosion, and the weight increase, caused by aluminum adhering to the pin's surface. Both nitrided and nitro-sulphurized pins, under severe conditions, (Pins 1,4, and 7), were remarkably damaged. Each was subjected to different behavior relating to the weight changes and failures of pins (failures of nitrided and nitro-sulphurized layers) between pin numbers. However, no failures were found on any TD-coated pins. meaning TD-coated pins can last longer under a wide variety of casting conditions.

Wear by repair work (polishing) may give rise to a serious problem in production, although it is not a damage which is directly related to aluminum. Resistance of components to this kind of abrasive wear is influenced by the component material's hardness and by the hardness of the abrasive materials as shown in Figure 6. VC-coated steel shows a much smaller amount of wear in comparison with nitrided steel against hard abrasives like A1₂O₃ and SiC, which are widely used as major industrial abrasives.

In some cases, typically in squeeze pins and bushings, sliding friction between die components produces severe wear and galling problems. Carbide coatings have excellent properties to improve the problem, as is obvious in the fact that the TD Process is being widely applied to metal forming tooling and various machine components. As shown in Table 1, the VC-coated steel - VC-coated steel combination shows the maximum load to galling. However. VC coating on only one of the parts in contact is enough to improve the galling problem unless other materials, such as aluminum and dust in the working environment, are involved in the friction system.

* over the limit value measureable by the test machine.

As mentioned before, TD-coated steels are superior to nitrided and nitro-sulphurized steels in the properties related to damages by the aggressive behavior of molten aluminum and abrasive wear. Only CVD- and PVD-coated steels can be listed as coatings comparable to TD-coated steels in these properties. However, PVD coatings are less resistant to corrosion and erosion due to their thinner thickness and poor adhesion strength, which may be a serious problem. The thicker coating by thermal spraying cannot resist molten aluminum because their weak bonding strength, as a result of mechanical bonding, is quite insufficient to the thermal shock in casting operations as previously shown in Figures 2 and 3, even though the coated materials have good resistance to molten aluminum. A shot preening process, as Metalife, never improves resistance to molten aluminum attack, because of no change in chemical composition of materials.

Application and Obtainable Profit by the TD Process

1) Application

A large number of Japanese die casters are enjoying improved productivity through the TD Process. About 8,000 pins are TD coated in Japan every month. TD application has extended to other components such as cores, sprue spreaders, etc., although the largest application is still for core pins.

Major applications in Japanese die casting industry:

These applications intend to reduce the damage by corrosion, erosion, soldering, and wear. There were no applications intended to directly reduce heat checking and other breakage problems. However, there may be cases where TD coating decreased the surface damages as a trigger of such a failure and resulted in improvement of the failure problems.

Improvement of resistance to corrosion by the TD Process is still insufficient to completely eliminate the corrosion problem of steels in molten aluminum. Hence the TD Process has not been used for components such as thermo-couple protective tubes, molten aluminum holding retorts, etc. which are kept in contact with molten aluminum for long periods of time.

Photos 1 shows various types of TD-coated core pins, ranging from approximately 2 inches to 20 inches long (1/4 inch to 2 inch diameter) with a variety of shapes.

Photos 2 shows examples of cores, about 4" to 10" high, successfully used in the automobile industry. The large cores for low pressure die casting, 10-30 kg/piece, have been TD coated. Car wheel makers in Japan started their application of the TD Process to cores and sprue bushings, etc. for low pressure die casting production.

2) Obtainable Profit

As shown in Photo 3(a), aluminum can stick on TD-coated core pins after a large number of die casting shots. However, sticking usually takes longer and the area having aluminum stuck is much smaller than that on nitrided and ordinarily hardened pins. Consequently, TD processed pins require less frequent repair work to remove stuck aluminum, as illustrated in Figure 7.

A Japanese automobile components maker evaluated the benefits obtained by substituting the TD Process for salt bath nitriding. Most pins were salt bath nitrided as of 1972. The die caster suffered the cost of very frequent repairs and long machine downtime. The company introduced a small number of TD-coated pins in March of 1973 and gradually increased the number used. A decrease of the frequency of die reset was recognized from both severe soldering on pins and pin failures such as bending and breakage within a few months (see Figure 8). The increased number of casts due to fewer repairs and downtime was also obvious, as shown in Figure 9.

Profits from the TD Process were figured during daily production over several years. The usefulness of TD coating was again clearly confirmed by the decrease of the frequency of repair and total time consumed for repair, including regular maintenance according to the production plan (see Figure 10). It should be noted that the decrease in repair was achieved at the same time the number of casting machines and total casting production increased. In the early stage of usage, aluminum stuck on TD-coated pins without damage to the pins, as shown in Photo 3(b). Furthermore, due to the extremely high hardness of the TD-coated layers, the mechanical polishing under severe conditions, such as grinding, could be easily applied to TD-coated pins with little fear of damage to the pins themselves. This fact decreased the time consumed for repair work, as shown in Figure 11. The final benefits to the company were evaluated as shown in Table 2. The number of casting machines and number of casts increased by 60% and 90% respectively between 1972 and 1979. Repair work in 1979 was about one-quarter of 1972's.

Table 2 shows that the average life of TD-coated pins is about four times that of nitrided pins. According to other information, life improvement in aluminum die casting core pins ranges from about two to 15 times, as shown in Table 3 and Figure 12.

Very high life improvement was obtained for the pins which were used to make pre-cast holes for internal threads that need precise diameter control. The aggressive mechanical polishing to remove stuck aluminum is proven to damage pin surfaces. The TD layers are harder so that an abrasive, like alumina, does less damage to the pin's coated surface, thus suppressing pin diameter decrease. In addition to tremendous life improvement (as shown in Figure 13), hole diameter deviation was reduced significantly. The improved diameter control of thread holes resulted in increased life of the thread taps.

Pin life improvement by the TD Process is larger than many other coatings, including PVD and nitro-sulphurizing, as exemplified in Figure 14. Fewer repairs and prolonged life are realized also in aluminum die casting cores, as shown in Table 4. Tremendous benefits are obtained by the elimination of bothersome built-up welding procedures and finishing operations by grinding or EDM to get the original size after welding.

Examples of life improvement in other applications reported by users are summarized below: