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Zinc Die Casting Alloys: Properties, Aging, and Finishing Based on SAE J469
Zinc die casting alloys are valued for their dimensional accuracy, mechanical strength, and finishability. SAE J469 offers authoritative guidance on two commonly used alloys—Z33520 and Z35531—covering their aging behavior, corrosion protection, and surface finishing requirements. This article distills the essential engineering insights from the standard.
Aging and Dimensional Stability
Due to changes in solid solubility, both alloys undergo a slight shrinkage after casting. At room temperature, about two-thirds of this total shrinkage (≈0.0005 in/in) occurs within five weeks. To accelerate this process, a stabilizing anneal is recommended:
- 3 to 6 hours at 100 °C (212 °F)
- 5 to 10 hours at 85 °C (185 °F)
- 10 to 20 hours at 70 °C (158 °F)
After annealing, the casting experiences approximately 0.0004 in/in shrinkage, with the remaining 0.0001 in/in occurring over the next few weeks. For tight-tolerance assemblies, stabilizing before machining or assembly is critical.
🛠️ Design Insight: Stabilized castings maintain dimensions within ±0.00025 in/in up to 100 °C (212 °F) under normal service conditions, making them reliable for precision applications.
Corrosion Protection and Surface Finishing
When exposed to stagnant moisture or condensation with limited oxygen, zinc die castings can develop a bulky white corrosion product. Chromate conversion coatings are highly effective in preventing this. For organic finishes such as lacquers and enamels, a phosphate pretreatment is widely used to improve adhesion. Most zinc die castings that are painted will receive this pretreatment. ⚠️ Proper pretreatment is essential to avoid finish failure in the field.
Material Properties and Selection Guidance
Table 1 summarizes key mechanical properties for both alloys in the as-cast state. Z35531 offers higher strength and hardness but must not be used above 93.3 °C (200 °F) to prevent dimensional growth and impact strength loss. Z33520 is stable up to 100 °C and is the preferred choice when elevated temperature exposure is expected.
⚠️ Important: Alloy Z35531 must not be subjected to prolonged temperatures above 93.3 °C (200 °F) as it becomes dimensionally unstable and loses impact strength.
| Property | Z33520 | Z35531 |
|---|---|---|
| Ultimate Tensile Strength (MPa) | 283 | 328 |
| Elongation (%) | 10 | 7 |
| Hardness (Brinell 500 kg) | 82 | 91 |
| Shear Strength (MPa) | 214 | 262 |
| Compressive Strength (MPa) 0.1% Offset | 414 | 600 |
| Impact Strength (J) Unnotched | 58 | 65 |
| Fatigue Strength (MPa) 5×10⁸ cycles | 48 | 57 |
Note: Tensile properties are determined from cast test specimens—test bars machined from castings do not provide reliable results.
Frequently Asked Questions
- Why is a stabilizing anneal recommended for zinc die castings?
- It accelerates the natural dimensional shrinkage that can take weeks, allowing earlier machining and assembly while maintaining tight dimensional tolerances.
- What is the maximum service temperature for Z35531?
- Prolonged exposure above 93.3 °C (200 °F) causes dimensional growth and reduces impact strength; for higher temperatures, use Z33520.
- How can white corrosion on zinc die castings be prevented?
- Chromate conversion coatings are effective for protection against stagnant moisture. For organic finishes, a phosphate pretreatment is recommended before painting.
- How long does natural dimensional aging take at room temperature?
- Approximately two-thirds of the total shrinkage occurs within five weeks. A stabilizing anneal can achieve equivalent stability in hours.
