Buyers Guide: Selecting Magnesium Alloy Grades for Buyers

Why Alloy Grade Selection Drives Part Performance

Magnesium die casting parts vary widely in service. A gearbox housing and a handheld tool case do not share the same material demands. Buyers often specify “magnesium” in the first draft of a request for quotation, then face unexpected cost or performance issues later. The correct alloy grade determines strength, corrosion behavior, machinability, and finish quality.

This guide walks through the alloy selection process for magnesium die casting. It covers the major grades used in production, the criteria that matter for structural and finish requirements, and a checklist to verify specifications before releasing RFQs to suppliers.

Which Magnesium Alloy Grades Are Used in Production

Production die casting commonly uses a small set of magnesium alloys. Each grade has a distinct balance of mechanical properties, corrosion resistance, and casting behavior.

The most common grades in structural die casting are:

1. AZ91 (sometimes written as AZ91A or AZ92A). This is the standard structural grade. It offers good strength and castability. Most industrial housings, brackets, and structural frames use AZ91 as the default when strength and formability are the priority.
2. AZ31. This grade has lower zinc and iron content than AZ91. It casts more cleanly and produces a finer grain structure. It is favored when surface finish is a primary concern, such as visible housings or parts that will not receive heavy machining.
3. ZM8. This alloy contains less zinc than AZ91 but higher iron and manganese. It is often selected for corrosion resistance and good low-temperature toughness. It is common in automotive and consumer electronics parts that sit in humid or coastal environments.
4. A380. This is a high-silicon alloy that offers excellent corrosion resistance and good castability. It is used where long-term exposure to moisture or chemicals is a concern. It is also a common choice for parts that need good machinability after casting.

Some suppliers also offer AZ93, which is a low-manganese version of AZ91. It can improve weldability and corrosion resistance. It is not as widely stocked as the grades above, so availability should be confirmed before specifying it.

How to Match Strength Requirements to Alloy Grade

Structural parts demand specific mechanical properties. The key properties to evaluate are yield strength, tensile strength, elongation, and hardness. These values depend on the alloy composition, the die casting process, and the heat treatment applied.

Most magnesium die castings are supplied in the as-cast or T5 condition. T5 treatment improves strength and dimensional stability. If your part experiences high stress, vibration, or cyclic loading, specify the heat treatment condition explicitly. Do not assume the supplier will apply it by default.

For lightweight structural parts, AZ91 is usually the starting point. It balances strength and cost. If the part is thin-walled or has complex geometry, casting defects can reduce the effective strength. In those cases, consider AZ31 or ZM8, which may cast more cleanly in tight tolerances.

How Corrosion Resistance Changes Your Specification

Magnesium corrodes faster than aluminum and is sensitive to moisture, chlorides, and temperature. A part that works fine in a dry indoor environment may fail in a coastal or automotive application.

Corrosion resistance depends on alloy grade, surface protection, and service conditions. AZ31 and AZ91 are susceptible to corrosion without proper protection. ZM8 and A380 offer better inherent resistance, but they still require surface treatment in most service environments.

When specifying corrosion requirements, state the exposure conditions. Examples include indoor dry storage, outdoor wet environments, contact with de-icing salts, or immersion in tap water. Also state the required surface finish. A powder coat, anodizing, or a zinc plating system can change the corrosion resistance of the part significantly.

The article “Why Magnesium Die Castings Corrode and How to Stop It” on this site covers the common failure modes and protective systems. Refer to it when defining surface protection in your RFQ.

How Surface Finish Requirements Drive Grade Choice

Finish requirements can influence alloy selection more than buyers expect. Some magnesium alloys cast with a smoother surface than others. AZ31 is known for producing a finer grain and a cleaner surface out of the die. This reduces the need for secondary machining or cosmetic finishing.

If your part has visible surfaces and will receive a light powder coat or anodizing, AZ31 or ZM8 may be better than AZ91. AZ91 casts with a coarser grain and may show more surface texture after sandblasting or bead blasting. That texture can be acceptable for functional parts but less desirable for consumer-facing products.

Specify the target surface quality. Examples include “smooth finish suitable for direct powder coating,” “machined surfaces to a specified Ra value,” or “anodized finish with a specified color.” Vague finish language leads to back-and-forth between the buyer and the foundry.

How Cost and Lead Time Influence Grade Selection

Alloy availability affects cost and lead time. AZ91 and AZ31 are widely available and are standard in most magnesium die casting plants. ZM8 and A380 are also common but may have longer lead times in some regions. AZ93 is less common and may require sourcing from specific suppliers.

If a grade is not in stock, the foundry may need to import the billet. That adds cost and extends lead time. Before specifying a specialty grade, confirm availability with at least two suppliers. Ask for the expected lead time for the raw material, not just the part.

The article “Cost and Lead Time: High-Speed Zinc vs Standard Die Casting” on this site covers how process speed and material choice interact. While it focuses on zinc, the same principles apply to magnesium: material availability and process speed are intertwined.

How to Write a Magnesium Alloy RFQ

A clear RFQ for magnesium die casting should specify the alloy grade, heat treatment, surface finish, and mechanical test requirements. Use the following structure:

1. Part number and description.
2. Alloy grade, with composition reference if available.
3. Heat treatment condition, such as as-cast or T5.
4. Surface finish and protective system.
5. Tolerances and critical dimensions.
6. Mechanical test requirements, such as tensile test or hardness check.
7. Surface quality requirements, such as porosity limits or defect acceptance criteria.
8. Quantity and delivery schedule.

Include a sample or a detailed drawing with material callouts. If the part is a redesign, note any previous material issues. If the part has failed in service, describe the failure mode. Suppliers can use that information to recommend an alternative grade or process change.

Criteria Table for Alloy Selection

Use this table to evaluate candidate alloys against your part requirements.

Criterion	What to look for	Why it matters
Mechanical strength	Yield and tensile strength values, heat treatment condition	Determines whether the part will hold load without deformation or fracture.
Corrosion resistance	Alloy grade, surface protection, service environment	Magnesium corrodes in moisture and chlorides. Protection must match the exposure.
Surface finish	Grain size, casting cleanliness, secondary process compatibility	Affects cosmetic appearance and the cost of finishing operations.
Castability	Complex geometry, thin walls, hot spots	Poor castability increases defect risk and rejects.
Availability and cost	Raw material lead time, supplier stock, region	Specialty grades can extend lead time and increase cost.
Regulatory or industry standards	Applicable standards or customer requirements	Ensures compliance and simplifies supplier qualification.

Decision Checklist

Before issuing an RFQ, confirm the following:

• The alloy grade matches the structural load and service environment.
• The heat treatment condition is specified.
• The surface finish and protective system are defined.
• The corrosion exposure conditions are documented.
• The mechanical test requirements are stated.
• At least two suppliers have confirmed alloy availability.
• The drawing includes material callouts and tolerance notes.
• The part design is reviewed for casting defects and wall thickness.

Selecting the correct magnesium alloy grade is a design and specification decision, not a supplier default. When the grade, heat treatment, and finish are defined in the RFQ, quotes are more comparable and production risk is lower.