Zinc die casting runs on a small family of alloys, and the two you will meet most often are Zamak 3 and Zamak 5. They look almost identical on a certificate (both around 4% aluminum on a zinc base) but one number separates their behavior: copper. This guide covers what that difference actually changes in strength, corrosion, castability and cost, and how to pick the right grade.
Composition: The Whole Story Is Copper
| Element | Zamak 3 (AG40A) | Zamak 5 (AG40B) | What it does |
|---|---|---|---|
| Aluminum | 3.9–4.3% | 3.9–4.3% | Strength, fluidity, die release |
| Copper | ≤0.25% | 0.75–1.25% | Strength, hardness, wear resistance |
| Magnesium | 0.02–0.05% | 0.02–0.06% | Reduces intergranular corrosion tendency |
| Iron | ≤0.10% | ≤0.075% | Hard spots, machinability (hardness penalty if high) |
| Lead | ≤0.005% | ≤0.005% | Fluidity, but drives zinc growth; kept minimal |
| Cadmium | ≤0.004% | ≤0.004% | Toxic; strictly limited |
| Tin | ≤0.003% | ≤0.003% | Promotes intergranular corrosion; kept minimal |
| Zinc | Balance | Balance | Base metal |
The grades are standardized under ASTM B86 (AG40A / AG40B) and EN 1774, so a certified mill certificate states every element above. When a supplier quotes "Zamak 3" or "Zamak 5," the certificate (not the label) is what you audit.
Mechanical Properties Compared
| Property (typical, as-cast) | Zamak 3 | Zamak 5 |
|---|---|---|
| Tensile strength | ~250–285 MPa | ~320–350 MPa |
| Yield strength (0.2%) | ~210–220 MPa | ~280–300 MPa |
| Elongation | ~6–10% | ~2–4% |
| Impact strength | Higher (more ductile) | Moderate |
| Hardness | ~80–90 HRB | ~90–100 HRB |
| Fluidity in thin sections | Excellent | Good |
| Machinability | Good | Better (free-cutting, but higher tool wear) |
Values are typical ranges from published alloy data; actual properties depend on section thickness, casting parameters and the specific certified batch. Always design against the mill certificate of the material you buy, not against a generic table.
The pattern to internalize: Zamak 5 buys roughly 20–30% more strength and hardness with about 1% copper, and pays for it in ductility and corrosion margin. Parts that snap under impact or thread easily favor Zamak 3; parts that bear wear, clamp load or bearing stress favor Zamak 5.
Corrosion and the "Zinc Growth" Question
Zinc alloys have one historical weakness (intergranular corrosion in humid air, colloquially "zinc growth") and it is an impurity problem, not a grade problem. Excess lead, cadmium, tin or iron forms low-melting grain-boundary phases that corrode preferentially, swelling and cracking the part. The modern fix is composition control: certified Zamak keeps Pb ≤0.005%, Cd ≤0.004% and Sn ≤0.003%, and tight Fe limits prevent hard spots that hurt machining and plating.
Two practical rules follow. First, buy certified alloy with a mill certificate, recycled or unverified zinc is where growth problems originate. Second, protect the surface: plating, e-coat, powder coating or chromate conversion converts a corrosion-sensitive casting into a durable part. Uncoated Zamak is acceptable indoors; anything in humidity, salt or outdoor exposure needs a coating system.
Zamak 3 vs ZA Alloys: Where the Family Splits
| Family | Aluminum | Character | Typical use |
|---|---|---|---|
| Zamak 3 / 5 / 7 | ~4% | Hot chamber optimized, excellent fluidity, moderate strength | Decorative hardware, general engineering, thin sections |
| ZA-8 | ~8% | Higher strength, still hot-chamber capable with care | Light structural parts |
| ZA-12 | ~11% | Strength and bearing capacity up | Bearings, bushings, structural |
| ZA-27 | ~27% | Highest strength (can exceed 400 MPa), poor fluidity | Heavier-duty bearings and wear parts |
If your search has drifted toward "ZA-8 vs Zamak" or "ZA-27 strength," the deciding question is load: decorative and general parts stay on Zamak; bearing and structural zinc parts move up the ZA ladder, accepting the casting difficulty that comes with more aluminum.
Selection Guide
| Your part | Recommended grade | Why |
|---|---|---|
| Decorative hardware, handles, fittings | Zamak 3 (or Zamak 7 for plating-grade) | Best surface reproduction and ductility; Zamak 7 for premium plating |
| Thin-section, complex parts | Zamak 3 | Superior fluidity fills thin sections |
| Fasteners, clips, latches, locks | Zamak 5 | Strength and wear resistance under assembly load |
| Automotive interior / small mechanisms | Zamak 5 | Load capacity plus good machinability |
| Bushings, bearings, wear plates | ZA-12 / ZA-27 | Strength and bearing capacity beyond Zamak |
| Any part in humid or outdoor service | Grade secondary, coating system primary | Certified composition + plating/e-coat decides survival |
How FHY Handles Zinc Alloy Selection
FHY runs hot chamber die casting on zinc alloys with certified mill material supplied per batch. When you send a part for quoting, the review covers grade recommendation against your load and environment, wall thickness feasibility on the chosen grade, and the surface treatment route (plating, e-coat or coating) that matches the service environment. If Zamak 3 is genuinely the wrong answer for your part, the quote says so before the tool is cut, not after.