Zinc Alloys: Properties, Die Casting Process and Applications
Zinc alloys have a low melting point, excellent fluidity, easy weldability and brazing, as well as good plastic workability. They exhibit corrosion resistance in the atmosphere, and scrap material is easy to recycle and remelt. However, they have low creep strength and are prone to natural aging, which can cause dimensional changes.
They are prepared by melting methods and formed into products through die casting or pressure processing.
Zinc alloys can be classified into cast zinc alloys and wrought zinc alloys based on manufacturing processes. The primary alloying elements in zinc alloys are aluminum, copper, and magnesium. Cast zinc alloys offer good fluidity and corrosion resistance, making them suitable for die-cast instrument housings, automotive parts, shells, and similar applications.
Physical Properties
Zinc is a bluish-white, lustrous metal with diamagnetic properties. Although commercial zinc products are typically processed, diminishing these characteristics, its density is slightly lower than that of iron, and it has a hexagonal crystal structure.
At room temperature, zinc is hard and brittle, but it becomes ductile between 100°C and 150°C. Above 210°C, it becomes brittle again and can be easily crushed by hammering. Zinc has moderate electrical conductivity. Among all metals, its melting point (420°C) and boiling point (900°C) are relatively low. Except for mercury and cadmium, it has the lowest melting point among transition metals.
Characteristics
- Low melting point, melting at around 385°C, facilitating easy die casting.
- Excellent castability enables the production of complex shapes, thin-walled precision parts with smooth surface finishes.
- Good atmospheric corrosion resistance.
- High-dimensional stability and excellent precision in finished products (tolerances as tight as 0.03 mm).
- Low production costs and relatively long die life.
Zinc Die Casting Process
The traditional high-pressure die casting process consists of four main steps: die preparation, filling, injection, and trimming.
During die preparation, lubricant is sprayed into the die cavity. In addition to controlling die temperature, the lubricant aids in part ejection. The die is then closed, and molten metal is injected under high pressure, typically ranging from 10 to 175 MPa.
After filling, pressure is maintained until the casting solidifies. Ejector pins then push out the solidified parts. Since a single die may contain multiple cavities, each cycle can produce several castings.
The trimming or gate separation process removes excess material, including sprues, runners, gates, and flash. This is usually done by squeezing the casting with special fixtures. For fragile gates, the parts can simply be broken off by impact, saving labor. Excess sprue material can be remelted and reused. Typical material yield is around 67%.
The high-pressure injection fills the die extremely quickly, allowing the molten metal to fill the cavity before any section solidifies. This enables even difficult-to-fill thin-walled sections to achieve continuous surfaces without defects.
However, rapid filling can trap air, as it has little time to escape. Venting along the parting line can mitigate this issue, but even with precise processes, porosity often remains in the center of castings. Most die castings require secondary processing such as drilling, tapping, reaming, or polishing, to achieve features that cannot be formed directly by casting.
For customers seeking professional zinc die casting service, Getzshape offers reliable, high-precision solutions for complex zinc alloy components. Leveraging advanced hot-chamber die casting technology, Getzshape custom zinc die casting service delivers good surface finish, tight tolerances, and affordable production solutions for industries including electronics, automotive, consumer goods, and instrumentation.
Equipment
Die casting machines are mainly divided into two types: hot-chamber and cold-chamber die casting machines. Hot-chamber machines are generally used for zinc alloys, while cold-chamber machines are typically used for aluminum, magnesium, and copper alloys.
Hot-chamber die casting involves molten metal in a liquid state within the machine. Under pressure, the metal fills the die.
At the start of each cycle, the plunger is retracted, allowing molten metal to fill the gooseneck. Pneumatic or hydraulic pressure then forces the metal into the die. Machine clamping forces range from 10 tons to 400 tons, with the most common sizes being 88 tons, 138 tons, 168 tons, and 200 tons.
Advantages of this system include fast cycle times, ease of automation, and convenient metal melting.
Disadvantages include the inability to process metals with higher melting points, and it is unsuitable for aluminum alloys, as aluminum would dissolve and carry iron from the melting pot.
Hot-chamber die casting is typically used for zinc, tin, and lead alloys. It is also difficult to produce large castings with this method, and it is generally limited to small parts.
Considerations When Using Zinc Alloys
When impurity elements such as lead, cadmium, and tin exceed allowable limits, the castings undergo aging and deformation. This manifests as volume expansion, a significant reduction in mechanical properties, and eventual cracking over time.
Lead, tin, and cadmium have very low solubility in zinc alloys and therefore segregate at grain boundaries, acting as cathodes, while the aluminum-rich solid solution acts as the anode. In water vapor (as an electrolyte), this promotes intergranular electrochemical corrosion. Die castings age due to this intergranular corrosion.
The microstructure of zinc alloys primarily consists of zinc-rich solid solutions containing Al and Cu, along with aluminum-rich solid solutions containing Zn. Their solubility decreases with falling temperature. However, due to the extremely rapid solidification rate during die casting, the solid solutions remain highly supersaturated at room temperature. Over time, this supersaturation gradually relaxes, causing slight changes in the shape and dimensions of the castings.
Zinc alloy die castings should not be used in high-temperature or low-temperature environments. Zinc alloys exhibit good mechanical properties at room temperature. However, tensile strength decreases significantly at elevated temperatures, and impact toughness drops sharply at low temperatures.
