30079_07.pdf

CHAPTER 7

MAGNESIUM AND ITS ALLOYS

Robert S. Busk

Hilton Head, South Carolina

7.1 INTRODUCTION

109

7.4 FABRICATION

110

7.4.1 Machining

110

7.4.2 Joining

110

7.2 USES

109

7.4.3 Forming

112

7.2.1 Nonstructural Applications

109

7.2.2 Structural Applications

109

7.5 CORRROSION AND FINISHING 113

7.5.1 Chemical-Conversion

Coatings

7.3 ALLOYSANDPROPERTIES

110

113

7.3.1 Mechanical Properties of

Castings

7.5.2 Anodic Coatings

113

110

7.5.3 Pointing

113

7.3.2 Mechanical Properties of

Wrought Products

7.5.4 Electroplating

113

110

7.3.3 Physical Properties

110

7.1 INTRODUCTION

Magnesium, with a specific gravity of only 1.74, is the lowest-density metal available for engineering

use. It is produced either by electrolytic reduction of MgCl 2 or by chemical reduction of MgO by Si

in the form of ferrosilicon. MgCl 2 is obtained from seawater, brine deposits, or salt lakes. MgO is

obtained principally from seawater or dolomite. Because of the widespread, easy availability of

magnesium ores (e.g., from the ocean), the ore supply is, in human terms, inexhaustible.

7.2 USES

Magnesium is used both as a structural, load-bearing material and in applications that exploit its

chemical and metallurgical properties.

7.2.1 Nonstructural Applications

Because of its high place in the electromotive series, magnesium is used as a sacrificial anode to

protect steel from corrosion; some examples are the protection of buried pipelines and the prolon-

gation of the life of household hot-water tanks. Alloys used for this purpose are produced by per-

manent-mold castings and by extrusion.

Magnesium in powder form is added to gray cast iron to produce ductile, or nodular, iron, an

alloy that has many of the producibility advantages of cast iron but is ductile and strong.

A significant use for magnesium powder is its addition to the iron tapped from blast furnaces to

remove sulfur prior to converting to steel, thereby increasing the efficiency of the blast furnace and

improving the toughness of the steel.

Magnesium powder is also used to produce the Grignard reagent, an organic intermediate used

in turn to produce fine chemicals and Pharmaceuticals.

Magnesium sheet and extrusions are used to produce photoengravings.

Magnesium in ingot form is one of the principal alloying additions to aluminum, imparting

improved strength and corrosion resistance to that metal.

7.2.2 Structural Applications

Magnesium structures are made from sand, permanent-mold, investment, and die casting, and from

sheet, plate, extrusions, and forgings. The base forms produced in these ways are fabricated into

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

finished products by machining, forming, and joining. Finishing for protective or decorative purposes

is by chemical-conversion coatings, painting, or electroplating.

The most rapidly growing method of producing structural parts is die casting. This method is

frequently the most economical to produce a given part and is especially effective in producing parts

with very thin sections. A stimulus for the recent very high growth rate has been the development

of a high-purity corrosion-resistant alloy that makes unnecessary the protective finishing of many

parts. See alloy AZ91D in Table 7.1. Die castings are produced by cold chamber, by hot chamber,

and by a recently developed method analogous to the injection molding of plastic parts. The latter

technique, known as Thixomolding, 1>2>3 ' 4 uses a machine that advances the alloy in a semisolid state

by means of a screw and then injects an accumulated amount into the die. The melting step is

eliminated, production rates are at least as high as for hot-chamber die casting, and metal quality is

superior to that produced by either cold- or hot-chamber die casting. Two major fields dominate the

die-casting markets: automotive (e.g., housings, brake pedals, transmissions, instrument panels) and

computers (e.g., housings, disc readers).

Those properties mainly significant for structural applications are density (automotive and aero-

space vehicle parts; portable tools such as chain saws; containers such as for computers, cameras,

briefcases; sports equipment such as catcher's masks, archery bows); high damping capacity

(antivibration platforms for electronic equipment; walls for sound attenuation); excellent machina-

bility (jigs and fixtures for manufacturing processes); high corrosion-resistance in an alkaline envi-

ronment (cement tools).

7.3 ALLOYS AND PROPERTIES

Many alloys have been developed to provide a range of properties and characteristics to meet the

needs of a wide variety of applications. The most frequently used are given in Table 7.1. There are

two major classes—one containing aluminum as the principal alloying ingredient, the other containing

zirconium. Those containing aluminum are strong and ductile, and have excellent resistance to at-

mospheric corrosion. Since zirconium is a potent grain refiner for magnesium alloys but is incom-

patible with the presence of aluminum in magnesium, it is added to all alloys not containing

aluminum. Within this class, those alloys containing rare earth or yttrium are especially suited to

applications at temperatures ranging to as high as 30O 0 C. Those not containing rare-earth or yttrium

have zinc as a principal alloying element and are strong, ductile, and tough.

Recently, the high-purity casting alloys, AZ91E for sand and permanent mold castings and

AZ91D, AM60B, AM50A, and AS41B for die castings, have been developed. The high-purity die

casting alloys are superior in corrosion resistance to the commonly used aluminum die casting alloy.

These alloys have been largely responsible for the large expansion in magnesium automotive

applications.

7.3.1 Mechanical Properties of Castings

Magnesium castings are produced in sand, permanent, investment, pressure die-casting molds.

Castings produced in sand molds range in size from a few pounds to a few thousand pounds and

can be very simple to extremely complex in shape. If production runs are large enough to justify

higher tooling costs, then permanent instead of sand molds are used. The use of low pressure to fill

a permanent mold is a low-cost method that is also used. Investment casting is a specialized technique

that permits the casting of very thin and intricate sections with excellent surface and high mechanical

properties. Die casting is a process for the production of castings with good dimensional tolerances,

good surface, and acceptable properties at quite low cost.

Mechanical properties of cast alloys are given in Table 7.2.

7.3.2 Mechanical Properties of Wrought Products

Wrought products are produced as forgings, extrusions, sheet, and plate. Mechanical properties are

given in Table 7.3.

7.3.3 Physical Properties

A selection of physical properties of pure magnesium is given in Table 7.4. Most of these are

insensitive to alloy addition, but melting point, density, and electrical resistivity vary enough that

these properties are listed for alloys in Table 7.5.

7.4 FABRICATION

7.4.1 Machining

Magnesium is the easiest of all metals to machine: it requires only low power and produces clean,

broken chips, resulting in good surfaces even with heavy cuts.

7.4.2 Joining

All standard methods of joining can be used, including welding, riveting, brazing, and adhesive

bonding.

Table 7.1 Magnesium Alloys in Common Use

ASTM

Designation

AM50A

AM60B

AS41B

AZ31B

AZ61A

AZ80A

AZ81A

AZ91D

AZ91E

EZ33A

KlA

MlA

QE22A

WE43A

WE54A

ZE41A

ZE63A

ZK40A

ZK60A

max

0.004

0.005

0.0035

0.005

max

0.002

0.005

Rare

Earth

4.9

6.0

4.2

6.5

8.5

7.6

0.32

0.42

0.52

0.6

0.33

0.31

0.24

0.33

0.26

0.22

0.22max

0.12

0.9

0.5

0.7

2.5

Forms

S, P, F, E

F, E

SC, PM, IC

SC. PM

SC, PM

S, PM, IC

F, E

1.0

0.005

0.002

0.0010

3.2

0.7

1.6

2.5

2.2

1.2

2.6

0.7

0.01

0.15

0.005

0.20

4.2

5.8

5.5

A = 4 Yttrium; 3 RE

B - 5.1 Yttrium; 4 R.E.

DC = die casting; E = extrusion; F = forging; IC = investment casting; P = plate; PM = permanent

mold; S = sheet; SC = sand casting

Table 7.2 Typical Mechanical Properties for Castings

Tensile Strength

Yield Strength

Elongation in 2 in.

Alloy

Temper

(MPa)

(%)

Sand and Permanent Mold Castings

AZ81A T4

AZ91E F

EZ33A T5

KlA F

QE22A T6

WE43A T6

WE54A T6

ZE63A T6

Investment Castings

AZ81A

276

165

275

160

185

275

235

270

295

195

105

205

190

195

190

275

165

275

180

275

255

175

260

100

140

110

185

AZ91E

EZ33A

KlA

QE22A

Die Castings

AM50A

200

220

210

230

110

130

140

160

1 8

AM60B

AS41B

AZ91D

Table 7.3 Typical Mechanical Properties of Wrought Products

Tensile Strength

Yield Strength (MPa)

E | ongatio n i n 2 in .

Alloy

Temper

(MPa)

Tensile

Compressive

(%)

Sheet and Plate

AZ31B

H24

255

290

150

220

110

180

Extrusions

AZ31B

AZ61A

AZ80A

260

310

340

380

255

275

340

365

200

230

250

275

180

255

250

305

130

140

240

125

140

185

250

MlA

ZK40A

ZK60A

Forgings

AZ31B

AZ61A

AZ80A

260

195

315

345

305

325

195

180

215

235

250

205

270

115

170

195

185

195

170

ZK60A

Welding is by inert-gas-shielded processes using either helium or argon, and either MIG or TIG.

Alloys containing more than 1.5% aluminum should be stress-relieved after welding in order to

prevent stress-corrosion cracking due to residual stresses associated with the weld joint. Rivets for

magnesium are of aluminum rather than magnesium. Galvanic attack is minimized or eliminated by

using aluminum rivets made of an alloy high in magnesium, such as 5056. Brazing is used, but not

extensively, since it can be done only on alloys with a high melting point, such as AZ31B or KlA.

Adhesive bonding is straightforward, and no special problems related to magnesium are encountered.

7.4.3 Forming

Magnesium alloys are formed by all the usual techniques, such as deep drawing, bending, spinning,

rubber forming, stretch forming, and dimpling.

In general, it is preferable to form magnesium in the temperature range of 150-30O 0 C. While this

requires more elaborate tooling, there is some compensation in the ability to produce deeper draws

(thus fewer tools) and in the elimination or minimizing of springback. Hydraulic rather than me-

chanical presses are preferred.

Table 7.4 Physical Properties of Pure Magnesium

Density

Melting point

Boiling point

Thermal expansion

Specific heat

Latent heat of fusion

Latent heat of sublimation

Latent heat of vaporization

Heat of combustion

Electrical resistivity

Crystal structure

1.718 g/cm 3 (Ref. 5)

65O 0 C (Ref. 6)

1107 0 C (Ref. 6)

25.2 X 10- 6 /K (Ref. 7)

1.025 kJ/kg-K at 2O 0 C (Ref. 8)

360-377 kJ/kg (Ref. 8)

61 13-6238 kJ/kg Ref. 6)

5 150-5400 kJ/kg (Ref. 6)

25,020 kJ/kg (Ref. 10)

4.45 ohm meter X 10~ 8

Close-packed hexagonal: a + 0.32087 nm; c = 0.5209 nm;

da = 1.6236 (Ref. 9)

45 Gpa

16.5 Gpa

0.35

Young's modulus

Modulus of rigidity

Poisson's ratio

Table 7.5 Physical Properties of Alloys 1 0

Melting Point ( 0 C)

Electrical Resistivity

(ohm-metres x

10- 8 )

Density

(g/cm 3 )

1.79

1.77

1.8

1.80

1.81

1.83

1.74

1.76

1.81

1.83

Alloy

AM60B

AS41B

AZ31B

AZ61A

AZ80A

AZ81A

AZ91D

EZ33A

KlA

MlA

QE22A

ZK60A

Liquidus

Solidus

615

540

620

565

13.0

9.2

12.5

15.6

13.0

17.0

7.0

5.7

5.4

6.8

5.7

632

605

620

525

610

490

610

490

595

470

645

545

649

648

649

648

645

545

635

520

7.5 CORROSION AND FINISHING

Magnesium is highly resistant to alkalies and to chromic and hydrofluoric acids. In these environ-

ments, no protection is usually necessary. On the other hand, magnesium is less resistant to other

acidic or salt-laden environments. While most magnesium alloys can be exposed without protection

to dry atmosphere, it is generally desirable to provide a protective finish.

Magnesium is anodic to any other structural metal and will be preferentially attacked in the

presence of an electrolyte. Therefore, galvanic contact must be avoided by separating magnesium

from other metals by the use of films and tapes. These precautions do not apply in the case of 5056

aluminum alloy, since the galvanic attack in this case is minimal.

Because magnesium is not resistant to acid attack, standing water (which will become acidic by

absorption of CO 2 from the atmosphere) must be avoided by providing drain holes.

7.5.1 Chemical-Conversion Coatings

There are a large number of chemical-conversion processes based on chromates, fluorides, or phos-

phates. These are simple to apply and provide good protection themselves, in addition to being a

good paint base.

7.5.2 Anodic Coatings

There are a number of good anodic coatings that offer excellent corrosion protection and also provide

a good paint base.

7.5.3 Painting

If a good chemical-conversion or anodic coating is present, any paint will provide protection. Best

protection results from the use of baked, alkaline-resistant paints.

7.5.4 Electroplating

Once a zinc coating is deposited chemically, followed by a copper strike, standard electroplating

procedures can be applied to magnesium to give decorative and protective finishes.

REFERENCES

1. M. C. Flemings, "A History of the Development of Rheocasting," in Proceedings of the Work

Shop on Rheocasting, Army Materials and Mechanics Research Center, Feb. 3-4, 1977, pp. 3-10.

2. S. C. Erickson, "A Process for the Thixotropic Casting of Magnesium Alloy Parts," in Proceed-

ings of the International Magnesium Association, May 17-20, 1987, p. 39.

3. R. D. Carnahan, R. Kilbert and L. Pasternak, "Advances in Thixomolding," in Proceedings of

the International Magnesium Association, May 17—18, 1994, p. 21.

4. K. Saito, "Thixomolding of Magnesium Alloys," in Proceedings of the International Magnesium

Association, June 2-4, 1996.

5. R. S. Busk, Trans. AIME 194, 207 (1952).

6. D. R. Stull and G. C. Sinke, Thermodynamic Properties of the Elements, Vol. 18, Advances in

Chemistry, American Chemical Society, Washington, DC, 1956.

7. P. Hidnert and W. T. Sweeney, J. Res. Nat. Bur. St. 1, 111 (1955).

8. R. A. McDonald and D. R. Stull, J. Am. Chem. Soc. 77, 529 (1955).

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