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Introduction to Magnesium Alloys

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Engineering Properties of Magnesium Alloys Copyright © 2017 ASM International® Charles Moosbrugger, editor All rights reserved www.asminternational.org CHAPTER 1 Introduction to Magnesium Alloys MAGNESIUM is the lightest common struc- application. On the other side of the ledger, the Designation Systems tural metal with a density of 1.74 g/cm3 in its strong galvanic potential of magnesium and its solid state. The data in this collection focus on weak surface oxidation make corrosion behavior mechanical and physical properties of magne- a major consideration. Fortunately, good design No designation system has universal accep- sium that are relevant to engineers in the design practices and preventive measures are available tance. Names of alloys have evolved from trade of lightweight components and structures. Other to ameliorate environmental degradation. names of the pioneering companies to chemical references (Ref 1, 2) are suggested for details on Structural applications include automotive, and numerical systems. the various manufacturing processes employed. industrial, materials handling, commercial, and The ASTM Standard Alloy Designation This collection contains physical data that are aerospace equipment. The automotive applica- System is widely used by the industry. Details of helpful for the metal processor and for process tions include clutch and brake pedal support the ASTM system are given in Table 1 (Ref 2, 4). simulation. The effect that various manufactur- brackets, steering column lock housings, and As an example of how this alphanumeric system ing processes have on the resulting magnesium manual transmission housings. In industrial ma- works, consider magnesium alloy AZ91E-T6. components is evident by comparing the data chinery, such as textile and printing machines, The first part of the designation, AZ, signifies among the various casting and wrought forms. magnesium alloys are used for parts that oper- that aluminum and zinc are the two principal al- Proper use of the data requires a clear un- ate at high speeds and must be lightweight to loying elements. The second part, 91, gives the derstanding of the material behavior the values minimize inertial forces. Materials-handling rounded-off percentages of aluminum and zinc represent. Consensus definitions of material equipment includes dockboards, grain shovels, (9 and 1, respectively). The third part, E, indi- properties are found in the Glossary of Terms in and gravity conveyors. Commercial applications cates that this is the fifth alloy standardized with this book� include handheld tools, luggage, computer hous- approximately 9% Al and 1% Zn as the principal Effort has been made to attribute the source of ings, and ladders. Magnesium alloys are valu- alloying additions. Letters are used in alphabetic the data. Multiple sources are given when avail- able for aerospace applications because they order, except for O and I, which are not used. able to give the reader an indication of the ve- are lightweight and exhibit good strength and The fourth part, T6, denotes that the alloy is so- lution treated and artificially aged. The common racity and range of the data. The comparison of stiffness at both room and elevated temperatures tempers are listed in Table 1. data found in literature is made challenging by (Ref 2). Pure magnesium (98.8% Mg or higher) is the variety of test methods and reporting formats Pyrotechnics. The first applications of mag- designated by the required minimum amount of employed by researchers, and by varied designa- nesium powder were components of fireworks, magnesium. Several grades are commercially tions given to alloys. flares, and other incendiary devices to produce available for metallurgical and chemical uses. brilliant white light. Fine magnesium wire was These are rarely used for structural engineering used for photographic flash bulbs. Magnesium is applications. The grades are designated 9880A still used in fire starters for survival kits. Representative Applications (UNS M19980) and 9880B (UNS M19981) for Metallurgical. Magnesium is used as an al- 98.80% min; 9990A (UNS M19990) for 99.90% loying element in nonferrous alloys, such as min, 9995 (UNS M19995) for 99.95% min, and Magnesium is used in a wide variety of appli- aluminum, zinc, and lead. It is used as an oxy- 9998A (M199980) for 99.98% min. The Unified cations from medical and metallurgical to chem- gen scavenger in nickel and copper alloys and Numbering System (UNS) is a complementary ical and pyrotechnic. Although the main focus as a desulfurizer in iron and steel production. designation system of ASTM and the Society of of this book is on the structural applications of Magnesium improves the toughness and ductil- Automotive Engineers (SAE). It is not a specifi- magnesium, other uses of magnesium alloys are ity of cast iron by making the graphite particles cation because it does not establish requirements also addressed. nodular. This is the greatest use of magnesium such as mechanical properties or heat treatment, Structural. The high strength-to-weight ratio by weight. but it provides identifying numbers that are use- of magnesium alloys is usually a prime reason Electrochemical Applications. Magnesium ful for searching literature. All magnesium met- for considering these materials in engineering is highest on the electromotive series among als and alloys have UNS numbers starting with designs. High stiffness-to-weight, castability, metals in salt water, making it desirable as a M, but the M category is defined as “miscella- machinability, and excellent damping are desir- sacrificial anode for cathodic protection. Con- neous nonferrous metals and alloys,” so several able properties of magnesium alloys that factor structive uses of this mechanism are employed UNS M alloy numbers are not magnesium- into the material selection process. The unique- in batteries. based. ness of the magnesium alloys is illustrated in Medical. Magnesium alloys are used in por- Using the ASTM alphanumeric designation an Ashby diagram of Young’s modulus against table medical equipment where light weight is system encourages grouping magnesium alloys density among engineering materials (Fig. 1, advantageous. It is also employed for wheel- by principal alloy composition: Ref 3). The position at a corner of the triangu- chairs used in sporting activities (where every lar shape representing all engineering alloys and ounce is critical). Because of magnesium’s bio- • Magnesium-manganese (M) its position shared by engineering composites compatibility and bioabsorbability, alloys with • Magnesium-aluminum-manganese (AM) highlight the special qualities of magnesium al- other biocompatible elements (such as calcium) • Magnesium-aluminum-zinc-manganese loys. The thermal properties of magnesium fac- are being evaluated for cardiovascular stents and (AZ) tor into the castability of the alloys and serve in orthopedic devices for internal bone fixation. • Magnesium-zirconium (K) 2 / Engineering Properties of Magnesium Alloys Fig. 1 Ashby diagram of Young’s modulus, E, plotted against density, ρ, for various engineered materials. The heavy envelopes enclose data for a given class of material. The diagonal contours show the longitudinal wave velocity. The guide lines of constant E/ρ, E1/2/ρ, and E1/3/ρ allow selection of materials for minimum weight, deflection-limited, design. Source: Ref 3 • Magnesium-zinc-zirconium (ZK), with rare The Physical Properties of the Alloys Are melting, casting, and welding alloys. It is suc- earth (ZE) Influenced by Their Chemical Composition. cessfully used in die-cast and wrought products • Magnesium–rare earth metal–zirconium In general the constituent elements have the fol- but must be used judiciously in sand-casting be- (EZ) lowing effects. cause it coarsens the grain. • Magnesium-silver–rare earth metal–zirco- Aluminum has a favorable effect on magne- Calcium is added in small amounts to help nium (QE) sium. It is used up to 10 wt%, with optimum metallurgical control because it increases grain refinement. It is added just prior to pouring to • Magnesium–yttrium rare earth metal–zirco- strength and ductility at approximately 6%. Alu- reduce oxidation. It improves rolling of sheet nium (WE) minum improves strength and hardness. It wid- ens the melting range, which makes the alloy products, where it is used below 0.3 wt% so • Magnesium-zinc-copper-manganese (ZC) easier to cast. With aluminum content higher the product can be welded without cracking. It • Magnesium-aluminum-silicon-manganese than 6%, the alloy is heat treatable. improves thermal and mechanical properties of (AS) Beryllium is used in small amounts (up to the alloy, including creep resistance. There is • Magnesium-aluminum-strontium (AJ) 0.001 wt%) to decrease surface oxidation when interest in magnesium-zinc-calcium alloys for Introduction to Magnesium Alloys / 3 Table 1 Standard four-part ASTM system of alloy and temper designations for magnesium alloys See text for discussion. (Example AZ91E-T6 in parentheses) First part (AZ) Second part (91) Third part (E) Fourth part (T6) Indicates the two principal alloying elements Indicates the amount of the two principal Distinguishes between different alloys with the Indicates condition (temper) alloying elements same percentages of the two principal alloying elements Consists of two code letters representing the Consists of two numbers corresponding to Consists of a letter of the alphabet assigned in Consists of a letter followed by a number two main alloying
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