Zinc alloys with low aluminum content are commonly used in die casting due to their composition being near the eutectic point. This results in a narrow solidification temperature range, lower melting temperature, and good fluidity. However, when the zinc content is high, the solidification temperature range increases, reducing fluidity. The addition of magnesium can lead to the formation of magnesium oxide on the liquid surface, further decreasing fluidity.
Under similar superheat conditions, high-aluminum zinc alloys exhibit flowability comparable to aluminum alloys. However, due to a wider solidification temperature range, there is a higher risk of segregation. As the solidification range increases, so does the tendency for segregation. Among ZA-type zinc alloys, ZA27 has the highest aluminum content, the widest solidification temperature range, and the most severe dendritic and regional segregation.
The Al-rich phase in these alloys solidifies first, leading to regional segregation. This often results in shrinkage cavities at the bottom of the casting, especially during slow cooling.
Welding performance of die-casting zinc alloys is limited due to the presence of 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 have excellent machinability, allowing cutting speeds several times higher than gray cast iron and comparable to free-machining brass, with minimal tool wear. Additionally, zinc alloy castings do not develop residual stresses from machining, which contributes to better dimensional stability.
Aging in zinc alloys can occur due to impurities such as Pb, Sn, and Cd. These elements have very low solubility in zinc and tend to accumulate at grain boundaries, forming micro-cells that accelerate corrosion under suitable temperature and humidity conditions. During chemical etching, grain boundaries become loosely bonded, reducing strength and hardness. Therefore, the levels of these impurities must be strictly controlled.
Moreover, both β and α phases in zinc die castings are in a supersaturated state. Over time, they undergo desolvation, and the volume of corrosion products at grain boundaries expands more than that of the matrix metal. This leads to a significant reduction in mechanical properties and causes the castings to expand, resulting in aging.
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