Using PVD Coatings to Reduce Diecasting Costs

Diecasting often is the lowest-cost method to produce castings, especially when large volumes of components are required. However, the reusable steel dies used in diecasting typically are expensive, and may be a significant portion of overall production costs. Therefore, extending die life can have a significant effect on reducing production costs. Dies typically fail for one of three reasons: heat checking, excessive soldering, or erosion, and using PVD coatings to address these mechanisms can extend die life, reduce die maintenance, and so minimize overall manufacturing costs.

Diecasting involves injecting liquid metal into a reusable steel die at extremely high rates (gates speeds between 80-250 ft/sec, cavity fill times of 0.05-0.2 sec) and high pressures (6,000 to 15,000 psi.) Due to these aggressive conditions, soldering (sticking) of the castings to the die can be a problem, and to minimize soldering, casters use a water-based organic lubricant (basically a parting agent) sprayed onto the die face before each shot. The lubricant forms a barrier between the casting and the steel die to minimize soldering and sticking.

While the lubricant is required to ensure problem-free ejection of the casting from the die, it also produces a number of negative effects, especially when used in excess, including added cost to purchase the lubricants, a reduction in die life, reducing the quality of the castings by producing gasses that may be trapped in the castings as residual porosity, and any effluents produced have to be disposed.

Erosion normally occurs at regions of the die where the high-speed liquid metal stream impinges directly on the die surface, eroding the die in that region. Heat checking is caused by stresses generated in the die surface during cyclical heating and cooling when each casting is produced, and the use of excessive die spray can over-cool the die, increasing the stress level, and so speed the onset of heat checking⁽¹⁾. The mechanisms responsible for soldering of aluminum diecastings to steel dies have been examined by several researchers. Shankar and Apelian⁽²⁾ found that a reaction occurs between the molten aluminum and the steel die, producing Fe-Al and Fe-Al-Si intermetallic compounds on the die surface, and the solidified aluminum diecasting alloy then sticks to these compounds, resulting in soldering and problems with ejection of the castings. Viswanathan and Han⁽³⁾ suggested that soldering will not occur until the die surface reaches a critical temperature, around 950^oF for A380 alloy.

Diecasting dies are primarily cooled by internal water cooling channels, but soldering often occurs at regions of the die that are difficult to cool, such as long skinny core pins. To minimize soldering, diecasters often use high levels of die spray to cool these regions, thereby applying excess die lubricant and intensifying the problems described above.

An alternate method to minimize soldering and sticking of diecastings to these hotter regions of the die is by applying permanent PVD coatings. Physical Vapor Deposition (PVD) coatings are thin ceramic coatings applied to the surface of die components, and similar to organic lubricants they form a physical barrier between molten aluminum and the steel die, preventing formation of Fe-Al intermetallic compounds. PVD coatings also may be used to reduce the level of heat checking, and to address erosion.

PVD coatings — Physical Vapor Deposition involves vaporization of atoms from a solid source (a target), and the transportation and deposition of these atoms onto a substrate of interest. The most commonly used PVD coatings are metal nitrides (e.g., TiN, CrN, TiAlN, and AlCrN) produced by bleeding low pressures of nitrogen gas into the PVD vacuum deposition chamber, allowing the metallic atoms vaporized from the target to react with the nitrogen gas during deposition on the substrate. PVD coatings are normally about 3-to-6 mm in thickness (Figure 1.)

There are a number of commercial PVD processes, and Cathodic Arc Evaporation (CAE) is the most commonly used by diecasters, as it produces coatings with extremely high levels of adhesion, cohesion and density. However, one of the drawbacks of the cathodic arc process is the ejection from the target material of relatively large (about 2-10 mm diameter) macro-particles, which can become incorporated into the coating (Figure 2a.) These macro-particles form when unwanted droplets of liquid metal splashed from the arc source land on the substrate during coating growth. As these particles are similar in size to the thickness of the PVD coating, and often are poorly adhered to the substrate, they are detrimental to the coating’s integrity and may significantly reduce coating life.

A modified CAE process called the Arc Plasma Acceleration (APA) has been patented by Phygen that significantly reduces the number of macro-particles and other defects within PVD coatings (Figure 2b), thereby significantly improving the overall integrity of the coating. The APA process uses a magnetic field generator that creates a magnetic field with a distinctive cusp shape, which provides enhanced trapping of the plasma particles generated from the cathodic source.

The contoured field creates an electron trap having an aperture through which the plasma ions are directed at the substrate, so that the plasma deposition rate is higher per unit of magnetic field strength than can be obtained with conventional designs. The APA process permits control over the growth of the coating both via the intensity of ion bombardment (through the plasma density control) and the energy of arriving particles (through the substrate bias potential.) A key to the process is to ensure that a large number of ions bombard the surface with a velocity in a specific range, and by tuning that range crystalline configurations with weaker bonding can be minimized while preserving the strongest bonds. This phenomenon results in growth of a dense and highly textured coating, having a low defect content and an excellent metallurgical bond to the substrate.

Commonly used PVD coatings applied to diecasting dies include CrN and AlCrN, and properties of these coatings produced by Phygen’s APA process are summarized in Table 1. These coatings are extremely hard and wear-resistant, much harder than the heat-treated die steel. For example, the steel used for diecasting dies is normally heat-treated to a hardness of about 45 Rockwell C, which converts to a hardness of 4.3 GPa in SI units, about 6-8 times lower in hardness than the PVD coatings listed in Table 1. The “plus” versions of the coatings listed in Table 1 incorporate an intermediate ion-nitrided layer applied to the surface of the steel, which provides a harder, stable base layer to support the PVD coating, and so is recommended for diecasting applications.

Coating solutions — The first application involves Phygen’s FortiPhy Plus (CrN) coating applied to core pins (Figure 3) used by Mercury Castings in a die to produce a bearing carrier insert for an outboard motor.

The core is shown before cleaning and after the production of about 10,000 castings. In this die, the liquid aluminum injected through the gate impinges directly on the surface of the core, and the line visible in Figure 3 shows where metal flowing through the gate hits the core.

The casting is produced from a low-iron aluminum structural alloy, which makes soldering and erosion at this location even more of a problem. Mercury Castings reported that core pins used without a PVD coating do not last in this application, and that an un-coated insert would have significant soldering build-up and eroded surface long before this shot count, causing bad dragging and possible part distortion during ejection.

In the second case, also from the Mercury Castings, the long cores shown in Figure 4 are used to produce a driveshaft housing. Again the castings are produced from a low iron diecasting alloy (alloy 362) that intensifies the soldering problem. Mercury Castings initially ran the die without coating the cores (see the lower core in Figure 4), and experienced major problems with the aluminum alloy soldering to the cores, often resulting in bending of the entire casting during ejection from the die.

Mercury Castings solved the soldering problem by covering the cores with Phygen’s CertiPhy Plus AlCrN coating. Coating the core eliminated the soldering problem, as shown by the upper core in Figure 4, preventing bending of the casting during core pull, and so Mercury was able to eliminate the need for 100% inspection of the castings.

The third case story involves an entire diecasting die covered with a PVD coating, part of a research project with the goal of using PVD coatings to significantly reduce (or eliminate) the need to apply conventional organic lubricants to diecasting dies. Initial research was performed at the Colorado School of Mines⁽⁴⁾, where a lab test evaluated a wide range of PVD coatings, and demonstrated that an AlCrN PVD coating eliminated soldering between diecast alloy A380 and H13 steel die.

So, a plant trial was scheduled at Mercury Castings and all surfaces of a balance shaft housing (Figure 5) were covered with Phygen’s CertiPhy Plus AlCrN PVD coating (runner, cavity, overflow and vent block for both moving and fixed sides of the die.) Testing revealed the following: •  The amount of conventional lubricant spray was reduced by about 85%. An un-coated balance-shaft housing die was sprayed for a total of 12 seconds, while the spray time for the PVD coated die was about 1.5 seconds. •  As the die was sprayed for a shorter time, the cycle rate of the diecasting machine was increased by 12%, producing more castings per hour. •  As noted, excessive die spray can produce high levels of residual porosity in the castings, and testing by Mercury appeared to show that the residual porosity levels in the castings produced in the PVD coated die were lower. •  Research has shown that die spray is a major factor in the onset of heat checking⁽¹⁾, and so reduced spray has produced a significant extension in die life. The previous un-coated balance-shaft housing die used by Mercury was retired after a total of 97,238 shots, due to excessive heat checking, and during the production of those castings significant die maintenance (including re-welding) had to be performed on at least three occasions.

To date, more than 90,000 castings have been produced in the PVD-coated die, and the high quality PVD coating produced using the Phygen APA process appears to have survived to this shot count. As shown in Figure 6, only extremely minor heat checking occurred after this high level of shots, and the die has not yet required re-welding during maintenance. It is expected that that this coated die will last significantly longer than the 97,238 total shots experienced with the previous un-coated die, as the use of the PVD coating and lower level of spray has significantly extended die life.

When the coated balance shaft housing die had reached 70,000 shots, Mercury personnel performed a cost analysis of the savings and additional costs associated with coating the entire die with the CertiPhy Plus AlCrN (Table 2.) •  Based on data collected from previous un-coated dies used to produce the balance shaft housing, by after 70,000 castings dies without PVD coatings typically required significant maintenance (such as solder removal, and re-welding small pieces in critical areas) on at least three separate occasions. The low level of heat checking with the current, coated die means that this type of maintenance has not yet been required, and so these maintenance costs have been avoided.  Mercury Castings estimated the value of the avoided maintenance to be worth 10% of the purchase cost of the tool. •  Obviously, reducing lubricant spray time by 85% reduces the amount (and thus, cost) of die lubricant applied per shot. Based on the cost of the lubricant, Mercury estimated this represents a saving of 5% of the tool capital cost over the 70,000 shots. •  A 12% improvement in cycle rate means that more castings can be produced per hour. In this case, the reduced cycle and downtime enabled redeployment of another 700-ton diecasting machine. Mercury conservatively valued this as 5% of the cost of the tool for the production of the 70,000 castings. •  The die is still producing castings, and so the data does not currently exist to demonstrate total die life. However, based on information generated to-date, it is estimated that the reduced spray could extend die life by at least 25%.

These savings are summarized in Table 2, as a percentage of the initial purchase price of the cavity inserts, for the 70,000 castings produced at that time. For the four items listed in Table 2, total cost savings represent 45% of the value of the tool. Subtracting the cost addition for applying the coating (see Table 2), this shows that the PVD coating cost can be readily recouped, and can produce an overall reduction in the costs associated with making the diecasting.

This paper has described case stories where the use of PVD coatings applied to the diecasting die has addressed manufacturing problems, making the producing of the diecasting less problematic.

In addition to making diecasting more productive and less prone to failure, PVD technology can become even more effective as diecasters incorporate new technologies to fabricate dies, cores, and inserts, such as 3D printing. Recent experience has shown that PVD coatings can be readily applied to 3D-printed inserts to reduce solder and erosion.

David Bell is the president/CEO and Viktor Khominich is the chief of technology development, and both are with Phygen Coatings Inc.  Steve Midson is the principal of The Midson Group, a metallurgical consulting agency.

REFERENCES

1. Y. Zhu, D. Schwam, X. Zhu & J.F. Wallace, “Factors that Affect the Diecasting Die Life”, Transactions of 2007 NADCA Congress, paper no. T07-053.

2. Sumanth Shankar & Diran Apelian, “Soldering Tendencies of Alternate Non-Ferrous Die Materials”, Transactions of 2000 NADCA Congress, paper no. T00-045.

3. Srinath Viswanathan & Qingyou Han, “Mechanism of Die Soldering During Aluminum Diecasting”, Transactions of 2002 NADCA Congress, paper no. T02-034.

4. B. Wang, J. Song, A. Monroe, A.L. Korenyi-Both, S.P. Midson and M.J. Kaufman, “Results from a Series of Plant Trials to Evaluate the Impact of PVD Processed AlCrN Thin-Film Die Coatings to Minimize Die Lubrication”, NADCA 2017 Congress, Paper number T17-083.