WELLCOME MAGNESIUM A favorable strengthtoweight ratio makes magnesium

WELLCOME MAGNESIUM A favorable strength-to-weight ratio makes magnesium a desirable material for automotive and aerospace parts

New interest in magnesium has been recently aroused due to the expansion of use of magnesium alloys in the 1990 s and, especially, due to an appearance of high-strength magnesium matrix composites as lightweight advanced structural materials for automotive and aerospace. Magnesium alloys are considered as possible replacements for aluminum, plastics, and steels, primarily because of their higher ductility, greater toughness, and better castability.

Production of magnesium almost tripled last decade, and the world production capacity reached 515, 000 tons per year in 2009. Both the increased production of magnesium and applications of new highperformance magnesium alloys have posed a scientific and technical challenge to the brazing engineering community.

Characterization of Base Metals • Magnesium is the eighth most abundant element and constitutes about 2% of the Earth's crust, and it is the third most plentiful element dissolved in seawater. • Although magnesium is found in over 60 minerals, only dolomite, magnesite, brucite, carnallite, and olivine are of commercial importance. • • Magnesium and other magnesium compounds are also produced from seawater, well and lake brines and bitterns.

Magnesium is the lightest and one of the cheapest structural metals. Magnesium alloys are environmentally friendly, lighter than aluminum (only 2⁄3 of aluminum and 1⁄3 of titanium specific weights), better in heat dissipation and heat transfer due to high thermal conductivity of 51 W/m·K, and exhibit excellent ability in shielding electromagnetic interruption.

Low density, ~1. 75 g/cm 3, in combination with A relatively a high tensile strength of 228– 290 MPa, heat resistance up to (450°C), and oxidation resistance up to 500°C make magnesium alloys attractive for various structures in the automotive industries.

Especially, the magnesium alloys are attractive for various aerospace industries, as well as in textile and printing machines where lightweight magnesium parts are used to minimize inertial forces at high speed. Moreover, magnesium alloys are recyclable, which minimizes their environmental impact. However, the surface of magnesium alloys should be protected because they corrode easily when exposed to atmosphere.

• Mechanical properties (especially plasticity) of magnesium Alloys depend on the fabrication parameters and the testing temperature. For example, a considerable change in mechanical properties was observed for Alloy AZ 31 fabricated by casting, extrusion, and rolling. • The strength weakening is accompanied by a remarkable increase in ductility. The elongation increased from 21. 5% to 66. 5% as the test temperature changed from RT to 250°C.

• Magnesium alloys with reduced aluminum content AM 60, AM 50, and AM 20 are suitable for applications requiring improved fracture toughness. However, the reduction in aluminum results in a slight decrease in strength for AM alloys. • Alloys AS 41, AS 21, and AE 42 are employed for applications requiring long-term exposure at temperatures above 120°C and creep resistance.

• Magnesium compounds, primarily magnesium oxide, are used mainly as refractory material in furnace linings for producing iron and steel, nonferrous metals, glass, and cement. • Magnesium oxide and other compounds also are used in agricultural, chemical, and construction industries. • Magnesium alloys also are used as structural components of machinery. • Magnesium also is used to remove sulfur from iron and steel. •

Magnesium Alloy – Wire The highly accurate magnesium alloy wire is 20 percent stronger than extruded magnesium bar, which can be bent and coiled at the room temperature. SEI' magnesium alloy wire is high specific strength. * Tensile Strength 1. 2 - 1. 6 times (vs. Extruded) 1. 3 - 1. 8 times (vs. Die Cast) * 0. 2% Proof Stress 1. 4 - 2. 0 times (vs. Extruded) 1. 6 - 2. 5 times (vs. Die Cast) Tolerance of dimensional accuracy is =<1/100 mm. Magnesium Alloy – Tube The technology of wiredrawing skill would be used to produce for magnesium alloy tube. Mechanical properties of pipe are further more improved than conventional method. * Tensile Strength 270 MPa Achieved * Dimensional Accuracy +/-0. 1 mm Achieved * Linearity Error 1 mm/m Achieved * Bending Formability 2. 8 D Almost Achieved

Let us summarize ! Magnesium is the third most commonly used structural metal, following Fe and Al. The main applications of Mg are in order: • component of aluminium alloys; • in die-casting (alloyed with Zn); • to remove S in the production of iron and steel; • the production of titanium in the Kroll process.

Microstructures and Macrostructures • Optical micrograph: Mg-Zn. Zr-(RE) alloy, cast and aged (no soln treatment). • Note coarse gb ppts. Electron micrograph: Mg-Nd-Zr alloy, cast, soln treated and aged to peak hardness. (Note PFZ at gbs) Mg-Zr-Zn-(RE) helicopter gearbox casing Bicycle frame (alloy unspecified ! “ 6% additives”)

Magnesium alloy cast parts are gaining increasing attention from the automotive sector where the aim is weight reduction. However, the casting of magnesium alloys is still plagued with problems that are difficult to solve: porosity, macrosegregation, oxide entrainment, irregularity of microstructure, corrosion, machining safety, etc. The following figure shows that SEM image of oxide films on two opposite sides of a fracture surface of a tensile test specimen taken from AZ 91 sample.

Some magnesium matrix composites exhibited impressive increases in mechanical performance in contrast with nonreinforced matrix alloys. For example, the composite consisting of Mg-14 Li-1 Al matrix and 30 vol-% of steel fibers has a tensile strength 600– 700 MPa at room temperature and 450– 480 MPa at 200°C, while the matrix alloy exhibits only 144 MPa at room temperature, and 14 MPa at 200°C.

Magnesium matrix composites

₪ Advanced Mg-based materials have great potential to improve mechanical performance. New nontraditional reinforcing systems reach strength characteristics comparable with some steels or titanium alloys. ₪ For instance, the squeeze-casting composite of the matrix AZ 91 D alloy reinforced with 10 vol-% of Al 18 B 4 O 33 particles exhibits a tensile strength 480 MPa.

• Even the low-alloyed magnesium matrix MB 15 reinforced with 30 vol-% of Al 18 B 4 O 33 whiskers demonstrates a yield strength of 230 MPa and very good rigidity characterized with Young’s modulus 76 GPa and 0. 5% elongation. An increase in volume fraction of the reinforcing component can result in drastic change of For mechanical properties. example: • The Swiss company EMPA recently reported about the super-strength composite Mg. Al 1/T 300 containing 60 vol-% of graphite fibers. This material exhibited tensile strength of 1470 MPa and Young’s modulus 155 GPa.

q Magnesium matrix composites also have potential for high -damping to reduce mechanical vibrations. For example, undirectional solidification of Mg-2 Si alloy yields Mg/Mg 2 Si composite structure with a mechanical strength as high as the industrial cast Alloy AZ 63 but with a damping capacity 100 times higher. q. A similar Mg-10 Ni alloy with Mg/Mg 2 Ni structure provides a damping capacity 40 times higher than that of AZ 63 cast. Moreover, Mg-2 Si alloy reinforced with long carbon fibers has a Young’s modulus of ~200 GPa with a damping capacity of 0. 01 for strain amplitude of 10– 5.

• Due to the low solidus limitation of the matrix, only low-temperature filler metals such as P 380 Mg and P 430 Mg can be used for joining casting composites based on ZK 51 A and QE 22 A matrix alloys, or forged composites based on ZK 60 A and ZC 71 matrix alloys. • Joining other cast or forged composites can be performed by placing filler metal GA 432 or P 380 Mg between the brazed parts and heating to 390°– 400°C thoroughly controlling temperature. Joining of wrought magnesium composites based on Mg-Zn matrixes is preferably carried out by soldering with Zn-Al solders.

• Creep-resistant alloys Mg-Al-Ca-Sn and Mg-Al. Ca-Zn were recently developed , and they showed yield strengths of 190– 203 MPa, ultimate tensile strengths of 240– 250 MPa, and elongations of 3– 5% at room temperature. • The minimum creep rate was less than 0. 9 x 10– 9 s – 1 at 200°C under loading of 55 MPa. Similar improvement of creep resistance was also measured for the Ca-added Mg-Al-Mn Alloy AM 60 B. It showed at least 10 times lower creep rate at 200°C at the load of 90 MPa than Ca-free cast alloy.

• Experiments with composite Mg-based filler metals were recently started and will be finished in the near future to respond to strength requirements of new high-strength base materials such as magnesium matrix composites. Filler metal matrix reinforced with fine ceramic particles can increase yield strength in brazed joints by at least 20% and creep strength by 50– 70%.

• The Mg-Al-Li system, which has a eutectic Mg-36. 4 Al-6. 6 Li (wt-%) composition at 418°C , looks like a possible candidate in the liquid phase to prepare and test for composite brazing filler metals. There also other low-melting Mg-based alloys that might have good plasticity in solid state.

• Another alloy of this system Mg-8 Li-5 Al-1 Zn is a filler metal with a melting point around 560°C. This alloy demonstrates an unusually high tensile strength of 220 MPa after age hardening. Supposedly, the strength can be further increased by adding a small amount of zirconium.

Typical Applications for Magnesium Alloys • The use of magnesium alloys in car design is expanding, and now includes ultralightweight matrix composites. Typical automotive applications are engine blocks, cylinder liners, pushrods, valve spring retainers, instrument panels, clutch and brake pedal support brackets, steering column lock housings, and transmission housings.

Joining of magnesium alloys! • Material-handling equipment and commercial applications include parts for magnesium dockboards, grain shovels, gravity conveyors, luggage, computer housings, digital camera housings, electrical conductors, and hand-held tools. • In the aerospace industry, lightweight and stiff magnesium alloys are employed in various units and devices, for example, aircraft transmission systems and their auxiliary components, gear housings, rotor housings, and generator housings in cold areas of engines. • In audio, video, computer, and communication equipment, plastics are being replaced by magnesium alloys that have advantages in strength, heat sink, and service life. Consequently, thin magnesium net shapes are used now in many models of cellular phones, laptop computers, and camcorders.