Material selection is an issue that every engineer must consider at the beginning of a project. There are many magnesium alloy grades available on the market, and each material has its own composition, processing window, and performance characteristics. In this article, we will give you a detailed introduction to the advantages, disadvantages, and application cases of AZ91D magnesium alloy castings to help you make decisions quickly.
What Is AZ91D?
AZ91D is the most widely used alloy in the magnesium die casting industry. In its designation, A represents aluminum (Al), Z represents zinc (Zn), and the numbers 9 and 1 correspond to approximately 9% aluminum and 1% zinc as the primary alloying elements, respectively. You can also verify this naming convention from the chemical composition table below.
Element | Mg | Al | Zn | Mn | Si | Cu | Ni | Fe |
Content (%) | Balance | 8.5–9.5 | 0.45–0.90 | 0.17–0.40 | ≤0.05 | ≤0.025 | ≤0.001 | ≤0.004 |
Advantages of AZ91D Die Castings
Lightweight with High Specific Strength
Magnesium alloys have a density of only 1.78–1.83 g/cm³, approximately two-thirds that of pure aluminum. The tensile strength can reach 240–260 MPa, and the yield strength is approximately 160 MPa. It can simultaneously meet your project’s requirements for both weight reduction and load-bearing capacity.
Excellent Damping Capacity
AZ91D has excellent energy absorption capability. Its damping coefficient reaches 25% under a stress level of 35 MPa, and can reach 53% at 100 MPa. This makes it particularly suitable for components subjected to vibration and impact loads, improving structural stability.
Excellent Casting and Machining Performance
AZ91D is highly suitable for manufacturing thin-wall die castings, with a minimum wall thickness of 0.6 mm. The castings have good dimensional stability and require only a small draft angle. If you also need additional machining, AZ91D castings are also highly suitable for machining and can be used for high-volume precision manufacturing.
Recyclability
Magnesium alloys are fully recyclable with very little loss of mechanical properties.
Disadvantages of AZ91D Die Castings
Corrosion Resistance
The natural oxide film on magnesium is not dense and provides limited protection. Therefore, AZ91D die castings must undergo reliable surface treatment to meet service requirements. Micro-Arc Oxidation (MAO) can be your preferred surface treatment process, as it is simple and efficient, producing a uniform and hard coating. Chemical conversion coating (such as the SCCT process) is also a good option, providing more than 1,000 hours of salt spray protection for general structural components. Due to environmental regulations, traditional anodizing has gradually been phased out.
High-Temperature Performance
AZ91D exhibits good mechanical properties at room temperature. However, in environments above 150°C, its strength and creep resistance decrease significantly. At 150°C, the tensile strength is only about 160 MPa (approximately 100 MPa lower than at room temperature), and the creep strength is also very limited. If your project requires long-term service under high-temperature conditions, AZ91D should be selected with caution.
Difficult to Weld
AZ91D manufactured by the HPDC process is highly susceptible to severe weld porosity in the weld zone during welding, reducing the mechanical properties and sealing performance of the welded joint. Vacuum die casting and semi-solid casting can effectively reduce internal porosity in AZ91D castings, but cannot completely eliminate hydrogen-induced porosity. Therefore, certain limitations in welding still remain.
High Safety Risk and High Burn-Off Rate
Magnesium has a strong affinity for oxygen. In the molten state, it is highly susceptible to oxidation and combustion, and may even present an explosion risk. Therefore, protective gas must be used during production. At the same time, the burn-off rate of magnesium is approximately 5%–8% during continuous production, while for small-batch or intermittent production, the burn-off rate can reach 20%–25%.
Mechanical Properties of AZ91D Die Castings
AZ91D die castings consist of an α-Mg matrix and a β-Mg₁₇Al₁₂ eutectic phase distributed along the grain boundaries. The surface of the casting usually has a dense fine-grained chill layer with a thickness of approximately 0.05–0.2 mm, and its hardness is 15%–20% higher than that of the core. You can refer to the table below for the detailed mechanical properties.
Property | Value |
Density | 1.80 g/cm³ |
Tensile Strength (20°C) | 240–260 MPa |
Yield Strength (20°C) | 160 MPa |
Elongation (50 mm Gauge Length) | 3%–6% |
Brinell Hardness | 70 |
Elastic Modulus | 45 GPa |
Impact Strength | 8 J |
Fatigue Strength | 95 MPa |
Creep Strength (150°C / 100 h / 0.2% Strain) | 15 MPa |
Applications of AZ91D Die Castings
Automotive Industry
Automotive lightweighting is the most important application of AZ91D. It is widely used in conventional fuel vehicles for powertrain components such as engine blocks, timing covers, transmission housings, and reducer housings, as well as chassis and structural components such as strut towers and flywheel housings. Among them, engine blocks, covers, and conventional housing products are usually mass-produced by the high-pressure die casting (HPDC) process to achieve one-shot precision forming of complex thin-wall geometries, which can save you from subsequent machining.
For example, the commercial vehicle flywheel housing produced by FAW Foundry & Forging using AZ91D and the HPDC process has a wall thickness of 6 mm and weighs only 16 kg. In the new energy vehicle (NEV) industry, AZ91D is also used for key components such as electric drive housings, motor housings, and electronic control housings. The world’s first mass-produced magnesium alloy electric drive housing released by SAIC Motor uses AZ91D and is manufactured by the semi-solid injection molding process. Compared with the conventional die-cast ADC12 aluminum alloy housing, this AZ91D housing reduces the weight by approximately 30%.
Consumer Electronics
You will often find AZ91D in laptop housings, smartphone middle frames, tablet brackets, and the housings of various portable electronic devices. These products are usually manufactured by the high-pressure die casting (HPDC) process. While meeting the design requirements for lightweight and thin products, AZ91D can also provide good structural rigidity and a refined surface finish.
Other Industrial Applications
In the aerospace and medical equipment industries, AZ91D is also commonly used to manufacture lightweight components, brackets, and other castings by the vacuum die casting process or SSM casting.
Final Thought
Although processes such as vacuum die casting and semi-solid die casting can effectively improve the internal quality of AZ91D castings, each process has its own trade-offs in equipment investment, mold complexity, production cycle, and yield control. If you still have any questions after reading this article, feel free to contact CZCindustry at any time. We can provide a more in-depth DFM evaluation and material selection analysis according to your actual requirements.
FAQ
Q: Is AZ91D suitable for marine applications?
A: No, unless heavily coated. Its corrosion rate in seawater exceeds 0.5 mm/year—use 316 stainless steel instead for marine environments.
Q: How does AZ91D compare with aluminum 6061?
A: Magnesium alloy AZ91D is 35% lighter but has lower tensile strength and higher corrosion rates. It excels in weight-critical applications (e.g., electronics) where 6061’s extra strength is unnecessary.
Q: Can AZ91D be heat-treated to improve its mechanical properties?
A: T6 treatment does not improve the properties of die-cast AZ91D parts with 6 mm sections due to grain growth during the solution anneal. For most die-cast applications, AZ91D is used in the as-cast condition.
