Coating can address heat checking, excessive soldering, and erosion, to extend die life, reduce die maintenance, and minimize overall manufacturing costs.
Excerpt from the Foundry Management & Technology February 2020 article by David Bell, Viktor Khominich, and Steve Midson.
Die casting often is the lowest-cost method to produce castings, especially when large volumes of components are required. However, the reusable steel dies used in die casting 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. Using PVD coatings to address these mechanisms can extend die life, reduce die maintenance, and so minimize overall manufacturing costs.
Die casting 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. Some negative effects include 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. The mechanisms responsible for soldering of aluminum die castings to steel dies have been examined by several researchers. Shankar and Apelian found that a reaction occurs between the molten aluminum and the steel die, producing Fe-Al and Fe-Al-Si inter-metallic compounds on the die surface, and the solidified aluminum die casting alloy then sticks to these compounds, resulting in soldering and problems with ejection of the castings. Viswanathan and Han suggested that soldering will not occur until the die surface reaches a critical temperature, around 950oF for A380 alloy.
Die casting 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, die casters 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 die castings 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 inter-metallic 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 die casters, 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.
