Zinc alloys are widely used in die casting due to their excellent casting performance. These alloys typically have a low aluminum content and are near the eutectic composition, which results in a narrow solidification temperature range, lower melting temperature, and good fluidity. However, when the zinc content increases, the solidification range widens, reducing fluidity. The addition of magnesium can also negatively affect fluidity by forming magnesium oxide on the surface of the molten metal.
Under similar superheat conditions, high-aluminum zinc alloys exhibit flowability comparable to that of aluminum alloys. However, due to their wide solidification temperature range, these alloys are prone to segregation. The tendency for segregation increases as the solidification range expands. Among the ZA series of zinc alloys, ZA27 has the highest aluminum content, the broadest solidification range, and the most severe dendritic and regional segregation.
Because the aluminum-rich phase solidifies first, this leads to regional segregation, making the alloy more susceptible to shrinkage cavities at the bottom of the casting, especially during slow cooling.
In terms of welding performance, die-casting zinc alloys contain aluminum, which forms a layer of Al₂O₃ during welding, making the process difficult. The only practical repair method is spot welding. For nickel-plated surfaces, a Cd-Zn eutectic alloy (Cd 82.5%, Zn 17.5%) with a melting point of 265°C can be used for brazing.
Zinc alloys also offer excellent machinability, allowing cutting speeds that are several times higher than those of gray cast iron and comparable to free-machining brass. Additionally, tool wear is minimal, and no residual stresses are generated during machining.
Another important aspect of zinc alloys is their aging behavior. Elements such as lead (Pb), tin (Sn), and cadmium (Cd) are considered harmful impurities because they have very low solubility in zinc. These elements tend to accumulate at grain boundaries, forming micro-cells that accelerate corrosion under suitable temperature and humidity conditions. During chemical etching, the grain boundaries become loosely bonded, leading to reduced strength and hardness. Therefore, the levels of these impurities must be strictly controlled.
Moreover, the β and α phases in zinc die castings are often in a supersaturated state. Over time, desolvation occurs, and the volume of corrosion products at the grain boundaries exceeds that of the matrix material. This leads to a significant reduction in mechanical properties and an increase in the size of the casting, ultimately causing aging effects.
Pvc Piled Up Door,High Speed Pvc Stacking Door,Pvc Folding Up Door,Pvc Fabric Piled Up High Speed Door
Dongguan HengTaichang Doors Co, Ltd. , https://www.dghtcdoor.com