Magnesium and Magnesium Alloys

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Introduction to Magnesium and Its Properties

Magnesium alloys represent a cornerstone in modern engineering design, primarily due to their exceptional strength-to-weight ratios, superior machinability, and competitive cost-effectiveness on a per-piece basis. As the lightest structural metal available, magnesium's specific gravity of 1.74 translates to significant weight advantages in practical applications. Magnesium alloys typically weigh between 0.064 to 0.067 pounds per cubic inch (1.75 to 1.85 grams per cubic centimeter), comparing favorably to aluminum alloys at 0.091 to 0.108 pounds (2.5 to 3.0 grams per cubic centimeter) and steel at 0.283 pounds (7.8 grams per cubic centimeter). This remarkable weight efficiency makes magnesium alloys particularly valuable in transportation and portable equipment applications, especially where components undergo frequent and rapid position changes.

Manufacturing Forms and Processing Capabilities

Magnesium and its alloys demonstrate remarkable versatility in their available commercial forms. Commercially pure magnesium (99.8%), is supplied in multiple formats including ingots and sticks for remelting operations, as well as powder, ribbon, wire, and both extruded and rolled strip forms. The alloy variants are equally diverse, being produced as:

• Castings
• Sheet and plate materials
• Structural shapes
• Forgings
• Bars and rods

The manufacturing versatility extends to both standard structural shapes and specialized section designs through extrusion processes. Sheet magnesium demonstrates excellent formability, particularly suitable for drawing and spinning operations. A notable characteristic of magnesium alloys is their exceptional performance in hot forming applications. These materials also excel in assembly processes, readily accommodating both welding and riveting techniques. Their superior machinability allows for higher processing speeds and reduced operational costs compared to most conventional metals.

Mechanical Properties and Performance Characteristics

The mechanical properties of magnesium alloys vary significantly between cast and wrought forms. Cast magnesium alloys achieve tensile strengths up to approximately 40,000 pounds per square inch (≈280 MPa), with yield strengths reaching 23,000 pounds (≈160 MPa). Wrought variants demonstrate superior strength characteristics, with tensile strengths extending to 52,000 pounds (≈360 MPa) and yield strengths up to 44,000 pounds (≈300 MPa).

Through appropriate heat treatment of specific compositions, engineers can achieve various combinations of mechanical properties. The endurance limits are notably different between cast and wrought forms:

• Cast alloys: up to 14,000 pounds per square inch (≈100 MPa)
• Wrought alloys: up to 18,000 pounds (≈125 MPa)

The fundamental elastic properties remain consistent across most magnesium alloys:

• Modulus of elasticity: 6.5 million pounds per square inch (45 GPa)
• Modulus of rigidity: 2.4 million pounds (16 GPa)

Temperature significantly influences the performance characteristics of magnesium alloys. At elevated temperatures (around 300°F/150°C), there's a notable decrease in yield strength, tensile strength, and hardness, while elongation increases. Some specialized compositions maintain structural integrity even above 500°F (260°C). Conversely, at subzero temperatures down to minus 100°F (minus 70°C), these alloys typically exhibit slightly improved yield strength, tensile strength, and hardness, though with reduced elongation and impact resistance.

Beyond mechanical properties, magnesium alloys feature distinctive physical characteristics, including relatively high coefficients of thermal expansion, thermal conductivity, and specific heat, coupled with comparatively low electrical conductivity.

Economic Considerations and Design Implementation

Despite magnesium's higher cost per pound compared to steel, its implementation often results in significant cost savings during part production. When evaluating material costs, it's essential to consider the volume-to-weight relationship rather than simply comparing per-pound prices. For accurate cost comparison, the price per pound of magnesium should be adjusted by the specific gravity ratio:

• For steel comparison: divide by approximately 4.2
• For aluminum comparison: divide by 1.6

In casting applications, while light alloys may require somewhat thicker sections than steel to achieve equivalent tensile strength, this consideration is often secondary to foundry limitations. Frequently, the same section thickness in a magnesium-alloy casting can provide adequate strength for specific applications, matching steel's performance while maintaining weight advantages.

The economic benefits of magnesium alloys extend beyond material costs:

• Superior machinability often compensates for higher material costs
• Die-casting capabilities offer unique manufacturing advantages
• Reduced tooling wear and faster machining speeds lower production costs
• Higher cutting speeds and reduced power requirements improve manufacturing efficiency

These factors make magnesium alloys particularly attractive for applications where the total cost of production, rather than raw material cost alone, is the primary consideration.

Pure Magnesium and Commercial Applications

Pure magnesium, with a minimum purity of 99.8%, occupies a unique position in the materials spectrum. While rarely used in structural or stress-bearing applications, commercially pure magnesium serves essential roles in various industrial processes and specialized applications. Its mechanical properties in cast form are characterized by:

• Yield strength: approximately 3,000 psi (20 MPa)
• Tensile strength: 12,000 psi (20 MPa)
• Elongation: 6%
• Brinell hardness: 30

The primary non-structural applications of pure magnesium include:

• Pyrotechnic applications
• Alloying agent in non-ferrous metallurgy
• Cathodic protection systems for other metals
• Chemical and metallurgical processes

When alloyed with other elements, magnesium's properties can be significantly enhanced, creating materials suitable for a wide range of engineering applications. This versatility in alloying makes magnesium-based materials adaptable to various industrial requirements while maintaining their fundamental lightweight characteristics.

Casting Alloys and Their Compositions

Magnesium casting alloys represent a sophisticated blend of elements carefully formulated to enhance specific properties. The primary alloying elements include:

• Aluminum (not exceeding 10%)
• Manganese
• Zinc

Recent developments in magnesium casting alloys have introduced advanced compositions incorporating:

• Rare-earth metals
• Zirconium additions
• Calcium (approximately 0.10% for specialized applications)

These modern formulations, particularly those containing rare-earth metals and zirconium, demonstrate superior performance characteristics, including:

• Enhanced creep resistance
• Improved stress-rupture properties at elevated temperatures
• Better overall structural stability

A notable characteristic of magnesium casting alloys is their response to heat treatment. Nearly all compositions can be heat-treated to enhance mechanical properties, with alloy M1A being a significant exception. This heat-treatment capability allows manufacturers to optimize material properties for specific applications.

These casting alloys serve as the foundation for components in various industries, including:

• Aerospace applications
• Automotive components
• Consumer electronics
• Industrial machinery

Wrought Alloys and Manufacturing Forms

The development of wrought magnesium alloys has resulted in several specialized compositions specifically engineered for manufactured products. Among these, five alloys have emerged as particularly significant in industrial applications: M1A, AZ31B, AZ61A, AZ80A, and ZK60A. Following the pattern of casting alloys, these wrought compositions primarily incorporate aluminum, manganese, and zinc as their principal alloying elements.

Each alloy demonstrates unique characteristics and manufacturing capabilities. M1A stands out for its manufacturing versatility, finding applications in sheet metal, forgings, and extrusions, though it typically exhibits lower strength properties compared to other alloys. AZ31B serves primarily in sheet and extrusion applications, offering balanced mechanical properties that make it suitable for a wide range of uses.

The AZ61A and AZ80A alloys are predominantly utilized in extrusion applications, with AZ80A having the notable limitation of being unsuitable for hollow shapes or tubing production. For specialized applications, TA54A is exclusively employed in forging operations, while ZK60A is dedicated to extrusion processes.

These wrought alloys are classified into two main categories:

• Non-heat treatable alloys
• Heat treatable alloys (including AZ80A and ZK60A)

In the heat-treatable category, both AZ80A and ZK60A undergo strength enhancement through aging processes, though this improvement in strength comes at the cost of reduced elongation properties. For sheet applications, M1A and AZ31B are available in various conditions:

• Hot rolled
• Cold rolled
• Annealed

The commercial market offers these alloys in numerous forms, including sheet and flat-rolled products, various extrusion profiles, forgings, screw-machine stock, and impact extrusions. This versatility in available forms allows manufacturers to select the most appropriate material and form for specific applications, considering both mechanical requirements and manufacturing constraints.

ASTM Nomenclature and Classification System

The ASTM system for designating magnesium and its alloys follows a systematic approach that provides clear identification of material composition and grades. For pure magnesium, the designation consists of numerals indicating the minimum specified purity, with all digits included but the decimal point omitted. An arbitrarily chosen letter follows to differentiate between grades of identical purity that may have varying requirements. For example, 99.80% minimum magnesium is designated as 9980A.

The designation system for magnesium alloys employs a more complex methodology. Similar to aluminum alloys, the system uses no more than two letters to represent the primary alloying elements, arranged either by decreasing percentage or alphabetically when percentages are equal. These letters are followed by whole numbers representing the approximate percentage of each element. A final letter, assigned alphabetically, distinguishes between alloys that would otherwise share identical designations.

The letters used to represent the alloying elements are given below.

Table 1. The letters used to represent the alloying elements

A - Aluminum	N - Nickel
B - Bismuth	P - Lead
C - Copper	Q - Silver
D - Cadmium	R - Chromium
E - Cerium	S - Silicon
F - Iron	T - Tin
G - Magnesium	V - Arsenic
J - Phosphorus	W - Sulfur
K - Zirconium	Y - Antimony
L - Beryllium	Z - Zinc
M - Manganese