Capitalizing on Die Casting Technology

Presented by Mr. Horst M. Glaser and Dr. Tohru Arai, The TD Center, Columbus, Indiana.
— Reprinted from meeting notes with permission from the SME (Society of Manufacturing Engineers). June 5–6, 1996. Detroit, Michigan, Holiday Inn Fairlane.

Productivity Improvements in the Die Casting Industry through the use of Thermal Diffusion (TD)


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.

Figure 1

Accumulated number of Casting Components

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.

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.

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.

Cause of Die FailureMaterial Properties Required
CorrosionChemical inertness. Resistance to corrosion damage.
Erosion, Washing OutChemical inertness. Resistance to corrosion damage. Resistance to wash out.
Sticking, Soldering, GallingChemical inertness. Resistance to corrosion damage. Resistance to sticking of solidified aluminum.
Wear by Repair (Polishing) Resistance to abrasive wear
Heat Checking, Thermal FatigueResistance to thermal fatigue
Gross Cracking of Large MoldResistance to cracking. Resistance to crack propagation.
Breakage of Core PinsToughness. Fatigue strength.

Improvement of the Related Properties by TD Process

Figure 2

Comparative Resistance to Corrosion by Molten AI

Figure 3

Comparative Resistance to Erosion

Figure 4

Comparative Resistance to Heat Checking

Figure 5

Damage on Aluminum Die Casting Pins

Figure 6

Comparative Resistance to Abrasive Wear

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 Al2O3 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.

Table 1: Comparative resistance to galling evaluated by load required to galling (Kg)
Block—Counter RingVC Coated SteelNitrided SteelCr Plated SteelHardened 1.5C–12Cr Die Steel
Austenitic Stainless Steel7.
Gray Cast Iron11.84.1
Al-Si Alloy3.
VC Coated Steel>14*

* 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:

Components – core pins, inserts, sprue bushing (nozzles), sprue spreaders, squeeze pins and bushings, vacuum valves.

Casting Method – ordinary die casting, low pressure die casting, high pressure die casting.

Cast Metal – Aluminum, Zinc.

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.

TD Coated Cores Examples of TD Coated Core Pins

2) Obtainable Profit

As shown in Photos 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.

Figure 7

Comparison of Frequency of Polishing
Before and After Polishing with Emery Paper

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.

Figure 9

Increase in Number of Casts made by Two Different Dies

Figure 8

Decrease in Frequency of Die Reset

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.

Figure 11

Decrease in Time Required to Repair Core Pins

Figure 10

Decrease in Repair Works
Table 2: Comparison of pin life and repair work of salt bath nitrided and TD-VC coated core pins which were used exclusively by a Japanese automotive component maker between 1972 and 1979.
Quantity of Machines 1972 = 10, 1979 = 16Ratio
Surface Treatments on Core Pins  → →Nitriding:TD VC Coating
Number of cast products produced100:190
Life of pins100:417
Number of core pins abandoned, per machine100:28
Frequency of repair work for pins, accompanied by die reset, per machine100:41
Time required for repair work for pins, accompanied by die reset, per machine100:22
Time required for polishing of pins during casting, without die reset, per machine100:25
Time required each day for polishing of pins, per machine100:25

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.

Figure 12

Core Pin Life Increase
Table 3: Life improvement of aluminum die casting core pins
Pin Dia., in0.670.870.240.390.390.780.670.78
Pin Length, in9.251.429.4510.71.181.576.304.132.76
S/B nitrited pins, shots (000's)3–53–51.55–80.9530251010
TD-VC coated pins, shots (000's)9181242 still in use1514.48011.5183145

Figure 13

Comparison of Pin Diameter Changes

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.

Figure 14

Life Improvement of Die Casting Core Pins

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.

Table 4: Examples of life improvement of aluminum die casting cores.
CoreSalt Bath NitridedTD Coated
A180,000 shots with 25 repetition of repair by built-up welding for erosion damage350,000 shots with no repair
B70,000 shots with repeated repair by built-up welding for erosion damage120,000 shots with no repair
C180,000 shots with 25 repetition of repair by build-up welding for erosion damage350,000 shots with no repair
D180,000 shots with 30 repetition of repairs by built-up welding for erosion damage250,000 shots with no repair
E3,000 shots, needed repair work3,900 shots, still no need for repair
F20,000 shots, life40,000 shots, life

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

  • Ejector Pin and Bush in Die Casting
  • H11, H13 pins hardened normally had to be hammered and chiseled out of the die, rendering them useless after six shifts. TD-coated H11 and H13 were still usable after 14 shifts.
  • Pins in Gravity Casting
  • Hardened H13 pins needed recoating by refractory material after one shift. No need for any repair after two shifts when TD coated.
  • Cores in Low Pressure Die Casting
  • Nitrided carbon steel cores can last only one week. TD-coated H13 cores last longer than two months. Results from another caster showed five times increased life.
  • Sprue Bushing in Low Pressure Casting
  • Hardened bushings had to be replaced each week. TD coated bushings were still usable after two months.
  • Pins in High Pressure Casting
  • TD-coated H13 pins can last more than three times that of salt bath nitrided H13 pins.
  • Pins in Zinc Die Casting
  • TD-coated H13 pins showed two to over six times life improvement compared with hardened H13 pins, depending on the loading conditions of the pins. (Usefulness of the TD coating on zinc die casting pins can be recognized also by articles in “Die Casting Engineering No. 34” (5). Sept-Oct. 1990, introducing the research results made by the BNF Metals Technology Centre, at the request of the British Zinc Development Association.)

The benefits of TD coating are summarized below:

  • Reduced die maintenance
  • Decreased downtime of casting machines
  • Improved labor conditions due to less repair work
  • Increased life of die components, such as pins, cores, etc.
  • Improved casting quality—surface quality and dimension
  • Reduced cost of tool for post processing
  • Reduced use of lubricant (parting compound)

Process Consideration

The TD Process, like other processes, cannot provide successful results unless the TD processing and proper substrates are treated under well-considered conditions. The major points to be considered are:

  1. Substrate Material Selection
  2. Distortion Control
  3. Surface Finishing
  4. Edge Finishing
  5. Defects on Coating Surface

1) Substrate Material Selection

Figure 15

Comparison of life of Pins Used in Aluminum Gravity Casting

Chromium hot working die steels such as H11, H13, premium H13, etc., used in aluminum die casting worldwide, are good materials for TD coating. Tungsten- and molybdenum-hot working steels and high speed steels, being used in the most severe casting conditions, can also be employed successfully. For example, TD-coated cores, 70mm dia., 70mm long, made of YXRS (Japanese low carbon high speed steel), exhibited 100,000 shots life to make automobile components with ADC12 alloy at 600°C. TD-coated H13 cores lasted only 40,000 shots.

Less expensive steels can be used for less severe conditions. As shown in Figure 15, TD-coated carbon steel (1045) pins were successfully used in gravity casting, which requires less mechanical loading than in die casting, providing lower tool cost through decreased machining time. The TD Process cannot produce a coating on carbon-deficient materials such as Maraging steels within a reasonable production time unless carbon is increased by carburizing before treatment.

2) Distortion Problem

The TD Process is a high temperature process. Therefore, both deformation (shape change) and size movement (dimensional change) should be taken into consideration. Long and slender pins are susceptible to bending mainly during quenching. Pronounced core pin bending causes larger mechanical loading on one side of pin surface during the die casting operation, resulting in more remarkable galling.

The warpage of pins can be minimized by proper fixturing and quenching conditions in TD processing and by post-processing for warpage correction. However, it is desired that pin makers make efforts to leave the least possible residual stress.

Shape distortion problems may be a concern for cores with complicated shapes. However, distortion problems may not be serious unless air hardening steels, like H13 and many other hot working die steels, are employed for substrate, judging from the fact that the TD Process has been successfully applied to some cores, as previously shown in Photo 2. If necessary, cores that have already reached the end of their useful lives can be utilized to determine the optimum coating condition for new cores.

Size movement problems can be more serious than shape distortion problems for large cores, while less serious for pins. Size movement is usually caused by size change of substrates because coating by the TD Process are thin enough to not usually be of concern. Preliminary hardening of the substrate should be done for cores and pins with tight dimensional tolerances. Good matching of TD coating condition and substrate hardening condition is indispensable to minimize size change of substrates. The standard hardening condition recommended for the used substrate steel should be employed.

However, preliminary hardening can be omitted for loosely toleranced parts such as core pins. The TD Process is applied to the finished core pins in their annealed state and substrate hardening is carried out during cooling in the TD processing.

Some parts need extremely close tolerance (± 10 µm) on non-working surface. In this case, it is recommended to leave stock on those places which may be finish ground after TD coating.

3) Surface Finishing

Figure 16

Better Finishing of Pins Provides Better Results in Aluminum Die Casting

It has been confirmed that die casting core pins with the smoothest working surface exhibited less soldering and longer life compared with those with rough surfaces, as shown in Figure 16.

It has also been confirmed that polishing parallel to a pin's length gives better results than polishing perpendicular to the pin length. If mirror-like finishing is applied, time consuming polishing parallel to pin length may be unnecessary. Usually, successful application is achieved with 2–3µm finishing, which can be obtained by finishing the pins to this value before treatment and no polishing after TD coating. For further improvement, it is recommended to finish the pin to a mirror-like finish and polish with diamond paste after TD coating. However, there may be an optimum roughness value for each application by application. “Too smooth” surface repelled lubricant and tended to stick more. This may be true in some cases.

4) Edge Finishing

Damages are prone to occur at sharp edges; this is common with most surface treated parts. It is therefore recommended to put a radius, even very small if possible, to the edges in contact with aluminum.

5) Defects on Coating Surface

If TD-coated components have defects reached to the steel substrates, corrosion by molten aluminum will start at this point and increase with the increased number of shots. If dents exist on the TD coated surface, whether reached at the substrates or not, molten aluminum can get into the dents and trigger a large scale sticking of aluminum. Embedded polishing compound before TD processing, deep scratches and flaws by poor handling conditions before and after TD coating should be eliminated.


Carbide coatings by the TD Process have extremely good resistance to the damages caused by molten aluminum and have already provided great benefits to aluminum casting industries in Japan and elsewhere for the past 20 years. We hope American industry will enjoy the process too.

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