Tensile testing of wrought magnesium alloys is the fundamental mechanical evaluation used to verify the strength, ductility, and quality of rolled, extruded, and forged magnesium products such as AZ31B, AZ80A, and ZK60A. This article explains the applicable international standards (ASTM B557, ASTM E8/E8M, ISO 6892-1, GB/T 228.1), specimen preparation requirements, measured properties, test procedures, and typical industrial applications of wrought magnesium alloy tensile testing.
Technical Overview
Wrought magnesium alloys are magnesium-based materials shaped by plastic deformation processes such as rolling, extrusion, forging, and drawing, rather than by casting. Common commercial grades include AZ31B, AZ61A, AZ80A, ZK60A, M1A, ME20M, and rare-earth-containing alloys such as WE43. Compared with cast magnesium products, wrought alloys generally exhibit higher strength, better ductility, and improved fatigue performance, which makes them attractive for weight-critical structures in aerospace, automotive, and portable electronics applications.
Tensile testing is the most widely used method for characterizing the mechanical properties of wrought magnesium alloys. Because magnesium has a hexagonal close-packed (HCP) crystal structure with limited active slip systems at room temperature, wrought products develop strong crystallographic textures during processing. As a result, tensile properties can vary significantly with specimen orientation relative to the rolling or extrusion direction, and mechanical behavior may exhibit tension-compression asymmetry caused by deformation twinning. A properly executed tensile test must therefore control specimen orientation, strain rate, temperature, and axial alignment to generate reliable and comparable data.
Testing Samples
Sheet and Plate Specimens
Flat rectangular specimens are machined from magnesium alloy sheet and plate, typically with gauge widths of 12.5 mm or 6 mm and thicknesses retained from the product. Specimens are usually taken in three orientations: parallel to the rolling direction (0 degrees), at 45 degrees, and perpendicular to the rolling direction (90 degrees), because the pronounced texture of magnesium sheet produces measurable anisotropy in strength and elongation.
Extruded Bar, Rod, and Profile Specimens
Round tension specimens with standard gauge diameters of 12.5 mm, 10 mm, 8 mm, 6 mm, or 5 mm are machined from extruded bars, rods, and profiles along the extrusion direction. When product dimensions permit, transverse specimens may also be prepared to quantify the anisotropy introduced by the extrusion texture.
Tube and Pipe Specimens
Longitudinal strip specimens cut from the tube wall are commonly tested, and full-section tube specimens may be tested with appropriate plugs in the grips to prevent crushing. Testing orientation should reflect the principal loading direction expected in service.
Forging Specimens
Specimens are machined from forgings at locations and orientations specified by the applicable material specification or drawing, since grain flow and die deformation produce property gradients within the part.
In all cases, specimen machining must avoid excessive heat generation, surface burns, and residual stresses, and machined edges must be deburred and free of notches that could cause premature fracture outside the gauge length.
Testing Items
A standard tensile test on wrought magnesium alloys determines the following mechanical properties:
- Ultimate tensile strength (Rm), the maximum engineering stress sustained before fracture
- Yield strength, typically the 0.2 percent proof strength (Rp0.2) determined with an extensometer
- Elongation after fracture (A or A50mm), a measure of ductility
- Reduction of area (Z), applicable to round specimens that fracture within the gauge length
- Elastic modulus (Young's modulus, E), determined from the initial linear portion of the stress-strain curve when required
- Optional derived values such as the strain hardening exponent or uniform elongation for research and forming studies
Testing Methods
Tensile testing of wrought magnesium alloys is performed in accredited third-party laboratories or in-house material laboratories according to recognized international and national standards, including:
- ASTM B557, tension testing of wrought and cast aluminum- and magnesium-alloy products
- ASTM E8/E8M, tension testing of metallic materials at room temperature
- ISO 6892-1, metallic materials tensile testing at ambient temperature
- ISO 6892-2 and ASTM E21, tensile testing at elevated temperature for high-temperature service evaluation
- GB/T 228.1, the Chinese national standard for metallic material tensile testing at room temperature
The typical test procedure is as follows:
- Verify specimen dimensions and surface condition, and mark the original gauge length
- Calibrate and verify the universal testing machine force system in accordance with ISO 7500-1 or ASTM E4, and the extensometer in accordance with ISO 9513 or ASTM E83
- Mount the specimen with proper gripping and axial alignment to minimize bending
- Apply loading under controlled strain rate or stress rate; ISO 6892-1 Method A typically specifies a strain rate of 0.00025 per second for yield strength determination
- Determine the yield strength by the 0.2 percent offset method or extension-under-load method using an extensometer attached to the gauge length
- Continue loading to fracture, record the maximum force, then measure final gauge length and, for round specimens, final diameter to calculate elongation and reduction of area
- Issue a test report listing the material grade, temper, specimen orientation, standard applied, test temperature, and all measured results
Room-temperature tests are normally conducted between 10 and 35 degrees Celsius, with 23 plus or minus 5 degrees Celsius used as the reference condition when temperature sensitivity is a concern. Because magnesium chips and fines are flammable, specimen machining and post-test fragment handling must follow appropriate safety practices.
Applications
Tensile test data for wrought magnesium alloys support a wide range of engineering activities:
- Quality control and release testing of rolled sheet, extruded profiles, bars, and forgings against material specifications
- Aerospace structures such as brackets, housings, gearboxes, and actuator components where low density and high specific strength are essential
- Automotive lightweighting components including steering wheel armatures, cross-car beams, seat frames, and instrument panel structures
- Rail transit and portable electronics housings that benefit from excellent electromagnetic shielding and low weight
- Biomedical research on biodegradable magnesium implants, where mechanical degradation behavior must be quantified
- Research and development of new magnesium alloys and thermo-mechanical processes, failure analysis, and generation of design allowables
FAQ
Which standards should be used for wrought magnesium alloy tensile testing?
For wrought magnesium mill products, ASTM B557 is the product-specific standard, while ASTM E8/E8M, ISO 6892-1, and GB/T 228.1 define general room-temperature tensile methodology. Elevated-temperature evaluations follow ISO 6892-2 or ASTM E21. The standard selected should match the customer specification and the destination market.
Why must specimens be tested in different orientations?
Rolling and extrusion create strong crystallographic textures in magnesium alloys. Yield strength, elongation, and strain hardening behavior can differ substantially between the processing direction and the transverse direction, so multi-orientation testing is necessary to fully characterize anisotropy for design purposes.
What specimen sizes are commonly used?
Round specimens with 12.5 mm, 10 mm, 8 mm, 6 mm, or 5 mm gauge diameters are used for bars, forgings, and thick sections, while flat rectangular specimens taken at full product thickness are used for sheet and plate. Proportional and non-proportional gauge lengths are both permitted, provided the gauge length is stated in the report.
Can wrought magnesium alloys be tested at elevated temperature?
Yes. Elevated-temperature tensile testing is performed with a calibrated furnace and high-temperature extensometry to evaluate strength retention for powertrain and aerospace applications, or to study superplastic forming behavior of fine-grained magnesium sheet.
How many specimens are required for a valid test program?
Most specifications require a minimum of two or three specimens per condition and orientation. If a specimen fractures outside the gauge length or at a defect, the result is typically discarded and a retest is performed.
What safety precautions apply to magnesium tensile testing?
The tensile test itself is routine, but machining of magnesium specimens requires sharp tools, minimal heat input, and controlled chip handling, because magnesium fines are combustible. Fractured specimens and turnings should be collected and stored in accordance with fire safety regulations for reactive metals.