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TECH SPOTLIGHT Modern technology has overcome the traditional corrosion problems and disproved the flammability myth.

Many military helicopters such as these Sikorsky Black Hawk helicopters contain gearbox casings made of magnesium because of its low and corrosion resistance. Image courtesy Sikorsky. High-Performance Magnesium magnesium will burst into flame if in applications as diverse as jets, hel- Nicholas Jeal struck by sunshine. Fortunately, icopters, inline skates, and vascular Magnesium Elektron modern technology stents. Military and civil helicopter Manchester, England has overcome corrosion problems, gearbox casings are often made of and expanding knowledge among magnesium, and BMW’s new six t two-thirds the weight of engineers has disproved the flamma- cylinder engine has a hybrid magne- aluminum, magnesium is bility myth. In fact, magnesium must sium/aluminum block with a mag- the lightest structural metal melt before it can burn, which re- nesium bedplate and cam cover. and is almost infinitely quires temperatures of around 600°C DaimlerChrysler’s new seven speed abundant in sea water and (1112°F), similar to the melting tem- 7G-Tronic gearbox has a magnesium Ain dolomite rock. However, perature of aluminum. Clearly most casing. General Dynamics latest mil- it has not been applied as other materials in products such as itary amphibious vehicle for the U.S. widely as might be expected, because automobiles would have already ex- Marines, the Expeditionary Fighting alloys have historically been suscep- pired by the time they reached this Vehicle (EFV), includes magnesium tible to poor corrosion performance, temperature. in its complex transmission casings. as well as to the misconception that Magnesium alloys today are found The main magnesium alloy being ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2005 65 MAGNESIUM.qxp 9/12/2005 12:23 PM Page 2

Tensile strength Yield strength 300 43.5

250 36.3

200 29

150 22 Stress, ksi Stress, MPa 100 14.5

50 7.3

100 200 300 400 100 200 300 400 (212) (392) (572) (752) (212) (392) (572) (752) Temperature,°C (°F) Temperature,°C (°F)

WE43B-T6 Elektron 21-T6 ZE41A-T5 AZ92A-T6 Tensile properties of Elektron 21 are higher than those of other magnesium alloys, especially at elevated temperatures. evaluated for the EFV is Elektron 21, At ambient temperatures, typical a new sand or investment casting tensile strength of Elektron 21 is 280 alloy combining castability, corrosion MPa (40.6 ksi), compared with 250 performance and the ability to op- MPa (36 ksi) for Elektron WE43 and erate at high temperatures. around 200 MPa (29 ksi) for AZ91E. This article describes the alloy However, it is elevated-temperature chemistry, mechanical and physical performance that is most impressive. properties, castability, and corrosion Up to approximately 200°C (392°F), properties. the strength of Elektron 21 is most similar to that of Elektron WE43. Chemistry of Elektron 21 Properties are superior to Elektron In designing Elektron 21, the goal RZ5 (ZE41) and AZ92, with the dif- This DaimlerChrysler Mercedes Benz was to reduce the cost of components ference increasing as temperature S-class is one of the vehicles that is to con- while maintaining mechanical prop- rises. It is important to note that in the tain the 7G-Tronic seven-speed automatic erties for applications up to 200°C graph, AZ92 represents the magne- transmission, which has a magnesium (392°F). The alloy chemistry consists sium-aluminum alloy family. The casing. In casing components, magne- of Nd-Gd (base) – Zn-Zr. Strength and graphs show the AZ92 properties as sium allows for a weight reduction of up hardness are developed through op- a line, but a spread of results is more to 25% compared to aluminum and 50% timum additions of neodymium, usual, due to the inconsistency of compared to gray cast iron. In relation to gadolinium, and . properties in parts made of the mag- the entire engine weight, these numbers Neodymium has a higher max- nesium-aluminum sand cast alloys. are 12% and 22%, respectively. imum solid solubility in magnesium All these Elektron 21 properties are Magnesium is also being considered than do rare earths such as cerium. achieved following a full solution for the crankcase, cylinder head and the This allows full solution treatment (ho- treatment and artificial aging (T6) oil pan, as well as inner door panels. mogenization) and good precipitation heat treatment, which involves eight The decisive factor in designing drive strengthening. Benefits from the hours at 520°C (968°F) and quenching systems and reducing fuel consumption gadolinium and zinc additions include followed by 16 hours at 200°C (392°F). is the weight of the vehicle. For every 100 increased hardness, improved age kilograms of reduced weight, roughly 0.2 hardening response, delay of the onset Castability of Elektron 21 to 0.4 liters of fuel per 100 kilometers is of overaging, and changes to the mor- A commercially viable alloy must saved, depending on the type of engine. phology of the as-cast eutectic. Unifor- possess more than good mechanical Image courtesy DaimlerChrysler. mity of properties is underpinned by properties. The alloy must be castable zirconium grain refinement. enough to meet the demands of ever The alloy therefore combines good more complex sand or investment strength up to 200°C (392°F), consis- castings. The castability of Elektron tent properties, ease of casting, and 21 is affected by the neodymium and good corrosion resistance. Tensile gadolinium, total rare earth (TRE) properties are compared with other content, and the level of grain refine- magnesium alloys in the graph ment from the zirconium. shown above. As neodymium content increases 66 ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2005 MAGNESIUM.qxp 9/12/2005 12:24 PM Page 3

Elektron 21 was designed to improve on the collective properties of these magnesium sand casting alloys • AZ91E: This alloy is a member of the oldest magnesium sand casting alloy family, the magnesium-aluminum alloys. Although inexpensive and offering good corrosion resistance because the E version is a high-purity form, AZ91E is temperature-limited and produces parts with inconsistent properties. • Elektron RZ5 (ZE41): In the 1950s, zirconium was added as a grain re- fining agent, which resulted in sand casting alloys without the problems of grain growth from the slow cooling rates involved. Elektron RZ5 (ASTM designation ZE41), the first of the zirconium-refined alloys, offered in- creased properties and consistency. It provides only average corrosion re- sistance, and is limited to a maximum operating temperature of around 150°C (302°F). • Elektron QE22 / Elektron MSR-B: These -containing zirconium- refined alloys, developed in the 1960s/1970s, could function at much higher The U.S. Marine Corps Systems Com- temperatures. However, as modern corrosion requirements became ever mand in Quantico, Va., has awarded more stringent, they began to fall out of favor. General Dynamics a $43 million contract • Elektron WE alloys: These were developed in the 1980s, and achieve to continue the systems development and the best properties of all the magnesium sand casting alloys. They can func- demonstration phase and full-up system tion up to the highest temperatures, around 250 to 300°C (482 to 572°F). live-fire testing of the Expeditionary Furthermore, at these temperatures, they maintain their properties to a Fighting Vehicle, which includes magne- much greater extent than aluminum alloys, meaning that they are both sium in it complex transmission casings. lighter and stronger than aluminum. The EFV is an amphibious vehicle that In combination with excellent corrosion resistance, the Elektron WE43 allows Marines to implement operational alloy becomes an ideal choice for applications in aerospace. However, the maneuvers from the sea . It can launch Elektron WE alloys are more expensive than aluminum, and because they forces from 20 to 25 nautical miles at sea, contain , they require a high degree of foundry experience to en- and transport its crew of three and 17 sure that a good part is cast. combat-ready Marines to shore at speeds in excess of 20 knots, three times faster than the current platform’s speed. bility over the Elektron WE alloys to impact corrosion performance. The EFV is capable of speeds up to 45 means fewer mistakes, faster devel- From corrosion tests to the ASTM B117 mph on land and is replacing the 30-year- opment time, and fewer rejected cast- Salt Fog Tests, Elektron 21 records a old, less capable amphibious assault ve- ings, all resulting in a lower compo- corrosion rate of 10 to 30 mils per year hicles. Image courtesy General Dynamics nent cost. Elektron 21 castings have on machined specimens. Amphibious Systems. now been made in foundries around in Mg - Nd - Zr alloys, a peak in ulti- the world, and in addition to the EFV Aerospace applications mate tensile strength is reached, after program, Elektron 21 components are Elektron 21 has already achieved which strength begins to decline. This found in top-ranked racing cars, and an Aerospace Material Specification reduction approximately coincides are being investigated for several (AMS 4429) and has a confirmed with the solid solubility of aerospace projects. entry in the new edition of the neodymium in magnesium, leading MMPDS (formerly MIL-HNDBK-5) to a brittle excess grain boundary Corrosion resistance design handbook, due to be reprinted phase after heat treatment. The corrosion performance of the at the beginning of 2006. However, when the gadolinium modern zirconium-refined magne- In addition to Elektron 21, Mag- component of the TRE is increased, sium alloys is close to that of alu- nesium Elektron is in the final stages properties are not reduced. This ef- minum alloys. Maintaining very low of developing a high strength, high fect can be explained by the higher levels of impurities is the key to good temperature wrought alloy called solid solubility (than neodymium) of corrosion resistance, and Elektron 21 Elektron 675. At 200°C (392°F), gadolinium, which allows dissolu- is a high-purity alloy. The presence of Elektron 675 has twice the strength tion of some of the gadolinium-rich iron above 0.005% can be particularly of aluminum and half the weight grain boundary phase into the matrix harmful, but zirconium effectively of titanium, a highly attractive during heat treatment. As a result, the controls this by forming Fe-Zr parti- proposition. TRE level may be increased without cles that sink to the bottom of the cru- detriment to tensile properties. (Re- cible during processing. For more information: Nicholas Jeal, search studies indicate a reduction in Also, Elektron 21 does not contain Magnesium Elektron, P.O. Box 23, Swin- microshrinkage). Unfortunately, there any major strengthening elements at ton, Manchester, M27 8DD, England; tel: is a limit to which the TRE can be in- levels that detract from its ability to 44-161-911-1055; fax: 44-161-911-1020; creased, beyond which fluidity be- withstand corrosion. In this regard it e-mail: nick.jeal@magnesium-elektron. gins to diminish. differs from Elektron RZ5 (ZE41), com; Web site: www.magnesium-elektron. For foundries, the enhanced casta- which includes about 1% zinc, enough com. ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2005 67