For most buyers specifying a magnesium die casting for the first time, the alloy question arrives as a choice between two designations. In practice, AZ91D vs AM60B is the most common fork in magnesium die casting specification, from housings and brackets to covers and tool bodies. On a datasheet the two grades look interchangeable until you read the numbers carefully. One is a strength-and-castability alloy with meaningful zinc. The other trades a little of that away for roughly double the elongation.
That trade shapes everything downstream. It decides which wall thicknesses you can risk, how the part behaves in a crash or a drop test, and what happens when a bracket is over-torqued during assembly. This comparison walks through the designation logic, the certified numbers that differ, and how to specify the choice on a drawing instead of inheriting it from a supplier’s stock list. Where the data below needs a mill certificate or a foundry record, it says so.
What AZ91D and AM60B Names Mean
Both names follow the ASTM magnesium alloy naming convention. The two letters are the principal alloying elements, and the numbers give approximate weight percentages. AZ91D is magnesium with about 9 percent aluminum and 1 percent zinc, and the trailing D marks tightened limits on iron and nickel. AM60B is magnesium with about 6 percent aluminum and manganese as the secondary addition, with no meaningful zinc.
Those few percentage points drive the personality difference. Aluminum is the primary strength contributor in Mg-Al alloys. Zinc in AZ91D improves fluidity and supports the age-hardening response, while manganese in AM60B buys corrosion tolerance by controlling iron. The lower aluminum content in AM60B leaves less of the beta-phase network that gives AZ91D its strength edge. In one sentence: AZ91D is the stronger, better-casting alloy, and AM60B is the more ductile, more impact-tolerant one.
Composition Ranges Behind the Difference
The AZ91D vs AM60B split starts at composition. The table below gives nominal ranges as specified in ASTM B94, the standard specification for magnesium-alloy die castings. These are the ranges a mill certification references, and a foundry verifies incoming metal against them with its own spectrographic checks. Ask for the certification that ships with each melt lot rather than trusting the datasheet average.
| Element | AZ91D (wt%) | AM60B (wt%) | Why it matters |
|---|---|---|---|
| Aluminum | 8.5 – 9.5 | 5.6 – 6.5 | Primary strength element in both grades |
| Zinc | 0.45 – 0.90 | ≤ 0.20 | Supports hardening and fluidity in AZ91D |
| Manganese | 0.17 – 0.40 | 0.25 – 0.50 | Controls iron content and supports corrosion tolerance |
| Iron | ≤ 0.004 | ≤ 0.004 | The most damaging impurity for corrosion performance |
| Nickel + Copper | ≤ 0.001 + ≤ 0.025 | ≤ 0.001 + ≤ 0.008 | Trace elements capped for corrosion control |
| Magnesium | Balance | Balance | Same base metal and about 1.8 g/cm³ density |
One shared line is worth expanding. Both alloys freeze over a range rather than at a single point. Per International Magnesium Association data, AZ91D solidifies between about 470 °C and 595 °C, and AM60B between about 540 °C and 615 °C. The width of each window shapes feeding, hot spots and heat checking, so the two grades need different thermal handling on the same machine.
Mechanical Properties Trade Strength for Ductility
Die-cast AZ91D typically delivers ultimate tensile strength in the 230 – 280 MPa region, with yield around 160 MPa. Its elongation lands in the low single digits, commonly 3 percent on standard test bars and less on thin sections. AM60B gives back some strength, with UTS generally in the 220 – 260 MPa band and yield in the 140 – 160 MPa region. It roughly doubles the elongation, commonly 6 – 10 percent in the die-cast condition.
Both alloys sit at about 45 GPa modulus and 1.8 g/cm³ density, so stiffness-limited and mass-limited designs behave similarly per geometry. The divergence is how they reach failure. AZ91D carries more load and then stops; AM60B carries less load, bends, and absorbs energy before it breaks. When two datasheets look close enough on strength, the elongation column is where the specification decision lives. Treat every figure above as typical: certified values depend on wall thickness, gate location and test-bar orientation, so require the supplier’s certificate for numbers that enter your calculations. For the background on the metal itself, see the profile of magnesium as the lightest structural metal.
How Each Alloy Fills the Die
AZ91D is generally the easier metal to cast. Its higher aluminum-plus-zinc content gives better fluidity, which supports thinner achievable walls, more complete fill on long-flow geometries, and fewer misruns and cold shuts. AM60B casts well and has anchored ductile automotive and tool families for decades, but its lower aluminum content reduces fluidity. The same thin-wall, long-flow part will sit closer to the process limit.
The solidification windows above also differ in width. AZ91D freezes across roughly 125 degrees, and AM60B across roughly 75, which shifts feeding behavior and hot-spot location between the two. Treat hot-tearing tendency as a geometry question rather than a ranking: constrained cores and abrupt section changes should go through the foundry’s fill analysis for the specific alloy. For a buyer this shows up in DFM conversations. A part pushed toward AM60B may need local wall thickening, coring changes, or gate repositioning that AZ91D would not require.
Both alloys run on the same cold chamber hardware under the same melt protection, so switching grades on the molding side is modest. It is the die and the part geometry that encode the choice. Die steel selection, cooling layout, and runner sizing should therefore be reviewed against the chosen alloy before tooling is quoted, not corrected after the first shots arrive.
Corrosion Behavior and Coating Response
Neither alloy is corrosion-proof as-cast. Both depend on coating systems for outdoor or aggressive duty, and both owe much of their baseline tolerance to the iron caps in the composition table. The conventional ranking puts AM60B slightly ahead of AZ91D in bare and conversion-coated salt-spray behavior, because AZ91D’s beta-phase network distributes cathodic microchemistry. In practice the gap should rarely decide anything. A properly applied paint, powder coat, or plating system dominates the difference.
Specify the finish by service environment, then verify with salt-spray testing to a stated standard and duration. That test, not the alloy datasheet, is the corrosion contract. Adhesion preparation matters as much as the top coat. Both grades need the conversion coating or primer stage recommended by the coating supplier for magnesium before the decorative layer goes on. A zinc-containing AZ91D part in a coastal application needs the same designed coating life as any magnesium part, with isolation from less noble fasteners designed in from the start.
Where Each Alloy Wins on the Part
Choose AZ91D when the part is a thin-wall, flow-long, dimensionally demanding housing or frame and the loads are static. That covers the classic volume applications: electrical enclosures, lamp bodies, gearbox and motor end-cases, and handheld tool bodies where stiffness and fit dominate. Interior automotive carriers that hold trim and modules rather than crash loads belong here too.
Choose AM60B when the part sees impact, shock, or assembly overload, or when ductility is a safety requirement. Seat structures, steering components, and drop-loaded tool housings are its traditional ground because the metal yields and absorbs energy instead of cracking at a corner. AM60B is also the calmer choice for designs with stress-concentrating bosses, snap-fit features, or thin attachment tabs that assembly lines will stress.
The pattern worth internalizing: AZ91D is specified around the casting process, AM60B around the service event. If neither driver is real for your part, AZ91D is the conventional default on castability, and the choice deserves a revisit only when a test fails. Bring the load case and the planned acceptance tests to that conversation rather than the datasheet alone.
Specification, Machining, and Joining Notes
Whichever alloy you choose, put the requirements on the drawing rather than in an email. Name the alloy to ASTM B94 and state the mechanical properties on the test-bar orientation that matters to your load path. Define corrosion testing to a named salt-spray standard with hours and acceptance criteria, and give any X-ray or leak requirement its severity class.
On the shop-floor side, both alloys machine cleanly. Magnesium is among the easiest engineering metals to cut, and its chips need a fire-safe handling program of their own. Both accept the standard fastening and joining vocabulary: threaded bosses cast in place, inserts, and self-tapping screws with proper edge distances.
Joining to steel or aluminum should carry galvanic isolation from the first CAD layout, the same rule that governs every magnesium design review. The foundry-side checks behind these lines are worth verifying before award, and the magnesium safety guide on this site covers the chip-handling and melt-protection side in detail.
Switching Alloys Is an Engineering Change
A surprising number of programs arrive with the alloy choice already made, inherited from a previous supplier or copied from a competitor’s teardown. A straight swap in either direction is a real engineering change, not a material substitution on a purchase order. Wall thicknesses sized for AM60B ductility can be over-conservative mass in AZ91D. Features relying on AZ91D fluidity may sit at the fill limit when moved to AM60B.
Run the swap through DFM and mold-flow review before the die is cut. Prototype on the production tooling when the choice is genuinely marginal, and let drop, torque, and salt-spray results close the decision. Ask the foundry which alloy it recommends for your geometry and why, in writing. A supplier who cannot argue the choice against your load case is telling you something.
The alloy decision also interacts with the aluminum-versus-magnesium question upstream. The guide to die casting aluminum alloys covers the equivalent trade-offs on the other side of the table, which is useful when a supplier proposes switching material families entirely.
Questions About AZ91D vs AM60B
What is the main difference in AZ91D vs AM60B?
AZ91D carries about 9 percent aluminum plus 1 percent zinc, which buys higher strength, roughly 230 – 280 MPa tensile, and better fluidity, at around 3 percent elongation. AM60B has about 6 percent aluminum and no zinc, slightly lower strength, and roughly double the ductility. It absorbs impact instead of cracking at a corner.
Is AM60B more corrosion resistant than AZ91D?
Slightly, in bare and conversion-coated conditions. Both are high-purity grades capping iron at 0.004 percent or less per ASTM B94, and AZ91D’s more cathodic beta-phase network costs it a little in salt-spray behavior. The difference is smaller than proper coating design, so choose the finish by service environment and verify with a named salt-spray test.
Which magnesium alloy is better for thin-wall die castings?
AZ91D is the conventional choice. Its higher aluminum and zinc content gives better fluidity, so it fills long thin flows and fine ribs more reliably and cold shuts less often. AM60B casts thin walls too, but the same part sits closer to its fill limit. Confirm the achievable wall with the foundry’s flow analysis for your geometry.
Can I switch my part from AZ91D to AM60B or back?
Treat it as an engineering change, not a substitution. Walls sized for AM60B ductility may be excess mass in AZ91D, and features relying on AZ91D fluidity can approach the fill limit in AM60B. Run the swap through DFM and mold-flow review before tooling, and validate with drop, torque, and salt-spray results against the drawing.
Final Thoughts
AZ91D vs AM60B is a choice between castability and ductility, decided by the part’s service event rather than by datasheet averages. Put the alloy, the certified properties, and the test requirements on the drawing, and make the foundry argue its recommendation against your load case. When you are ready to price a part, send a drawing through the contact page for a quote and a free DFM analysis report, the services this site advertises.
