Magnesium Alloys
The American Foundry Society Technical Dept., Schaumburg, Illiniois
C
ast magnesium alloys have gained
more popularity in recent years due
to their ability to maintain high
strengths at light weights. Magnesium possesses unique
properties that can open the
door to important markets
for structural applications
and has gained widespread
use in automotive components. Further, non-automotive applications, spurred on
by the computer, electronics
and power tool industries, continue to expand.
Magnesium has a density twothirds that of aluminum and only
slightly higher than that of fiber-reinforced
plastics and possesses excellent mechanical
and physical properties (Tables 1-2). When
coupled with the inherent advantages
of the metalcasting process, magnesium
alloys yield cost-effective solutions to
product needs by allowing for part consolidation and weight savings over other
materials and manufacturing methods.
Advantages of Magnesium
Magnesium alloy properties can provide a casting designer with several advantages as an engineering material over
other lightweight alloys.
Weight—The lightest of all structural
metals, magnesium preserves the light
weight of a design without sacrificing
strength and rigidity (Fig. 1). This benefit
is important when portability is a key
element of the product design, such as
with chainsaws, pneumatic nailers, circular saws, luggage, laptop computers and
cellular phones. Automobiles and other
transportation equipment continue to take
advantage of magnesium’s low density in
expanding application areas ranging from
under-hood and driveline uses found
in engine brackets and transfer cases to
numerous interior parts, such as steering
column components, pedal brackets, instrument panel supports and seating.
Damping Capacity—Magnesium
is unique among metals because of
its ability to absorb energy. Increased
vibration absorption capacity provides
for quieter operation of equipment
when magnesium castings are used for
housings and enclosures.
Dimensional Stability—Annealing, artificial-aging or stress-relieving treatments
normally are not necessary to achieve
2006 CASTING SOURCE DIRECTORY
The conversion of this military helicopter
transmission housing from a fabrication to
a magnesium casting allowed for a 30%
cost reduction and lowered scrap
rate by 40%.
case with magnesium alloys. As a result, there have
been few problems associated with the dimensional
change of castings in assemblies. Magnesium shrinkage
rates are more consistent and
predictable in comparison
to other nonferrous metals.
Components release from the
die with minimal force and
distortion, hence they have
minimal residual casting stress.
Impact & Dent Resistance—
The elastic energy absorption
characteristics of magnesium result
in good impact and dent resistance
and energy management, which is
one reason magnesium castings can
be used for automotive safety-related
stable final dimensions.
Metallurgical changes in the
structure of some metals can affect
dimensions after prolonged exposure to
elevated temperatures, but this is not the
Table 1. Typical Mechanical Properties of Magnesium at Room Temperature
Property
Unit
AZ91
AM60
AM50
AM20
AS41
AS21
AE42
Ultimate Tensile Strength
MPa
240
(250)
225
(240)
210
(230)
190
(210)
215
(240)
175
(220)
230
(230)
Tensile Yield Strength
(0.2% offset )
MPa
160
(160)
130
(130)
125
(125)
90
(90)
140
(140)
110
(120)
145
(145)
Compressive Yield Strength
MPa
160
130
125
90
140
110
145
%
3
(7)
8
(13)
10
(15)
12
(20)
6
(15)
9
(13)
10
(11)
Elastic Modulus, tension
GPa
45
45
45
45
45
45
45
Elastic Modulus, shear
GPa
Fracture Elongation
Brinell Hardness
Impact Strength
Charpy un-notched test bars
J
17
17
17
17
17
17
17
70
65
60
45
60
55
60
6
(9)
17
(18)
18
(18)
18
(18)
4
(16)
5
(12)
5
(12)
Note: Values in parentheses show mean property values obtained from separately diecast test bars.
Table 2. Typical Physical Properties of Magnesium
Property
Density
Unit
g/cu cm
Liquidus Temperature
F
Incipient Melting
Temperature
F
Linear Thermal
Expansion Coefficient
µm/m
Temp (F) AZ91
68
AM60
AM50
AM20
AS41
1.81
1.8
1.77
1.75
1.77
1,110
1,139
1,148
1,182
1,144
AS21 AE42
1.76
1.79
1,169 1,157
788-815 788-815 788-815 788-815 788-815 788-815 1094
68-212
Specific Heat of Fusion kJ/kg
26
26
26
26
26.1
26.1
26.1
370
370
370
370
370
370
370
Specific Heat
kJ/kg*K
68
1.02
1.02
1.02
1.02
1.02
1.02
1.02
Thermal Conductivity
W/K*m
68
51
61
65
94
68
84
84
Electrical Conductivity
MS/m
68
6.6
nm
9.1
13.1
nm
10.8
11.7
ENGINEERED CASTING SOLUTIONS
41
applications, such as air
bag systems. Portable tools
and handheld electronics also benefit from this
combination of properties, offering mechanical
shock resistance.
Anti-Galling—Magnesium alloys possess a low
galling tendency and can
be used as a bearing surface in conjunction with
a shaft hardness above 400
Brinell measurement.
with higher levels of iron,
nickel and copper. Sand
casting alloys often are
produced with a fine grain
structure due to small additions of zirconium.
Aluminum is the principal alloying element for
many magnesium alloys as
it can improve the mechanical strength, corrosion
properties and castability
of magnesium castings.
The most widely used general purpose sand casting
Alloy Families
Fig. 1. Magnesium’s light weight has allowed it to become the alloy of choice alloy is AZ91. In the alloy
Magnesium alloys can for a number of new markets and applications, such as the automotive, power nomenclature, the letters
be used in multiple ap- tool, computer and electronics industries.
A and Z denote the maplications, but they easily
jor alloy. However, not all
can be divided into two
properties improve with
groups: sand casting alloys and diecastaluminum and zinc additions. Ductilalloys. Most magnesium alloys are proing alloys. Alloys also can be classified as
ity and fracture toughness are gradually
duced as high-purity versions
general purpose, high-ducreduced when more aluminum is added.
to reduce potential corrotility and high-temperature
This effect led to the introduction of a
sion problems associated
series of alloys with reduced aluminum
contents (the AM series), which is used
extensively for automotive safety-related
components. These include manganese,
which is added to control the iron content
of the alloys. Several alloys, such as AM60
(6% aluminum, 0.05% manganese), have
found widespread applications in parts,
including instrument panel supports, steering wheel armatures and seat parts.
Some applications expose the casting to higher operating temperatures or
continuous stresses that lead to concerns
about long-term deformation and creep.
Castings for use in higher temperature
service conditions can be produced in alloys, such as the AS and AE series, based
on the addition of either silicon or rare
earth elements (E), which promote the
formation of finely dispersed particles at
the grain boundaries.
Recent property and castability improvements have been shown with new
magnesium creep-resistant alloys that
use specialized rare earth elements, such
as calcium or strontium, as the significant
alloying elements. These new alloys can
produce cast components with superior
This engine cradle for a high-performance sports
car was redesigned from a low-pressure aluminum permanent mold casting to two different
processes: a magnesium vacuum die casting
(top), which helped lower porosity defects; and
a low-pressure permanent mold magnesium
casting (bottom), which allowed for a sand
core to form a hollow cross support beam
rather than the ribbed support design of
the die casting.
42
ENGINEERED CASTING SOLUTIONS
2006 CASTING SOURCE DIRECTORY
This 2.3-lb AM60B magnesium shift tower
assembly is a one-piece diecast component.
Converted from a multiple steel fabrication,
the magnesium casting design resulted in a
75% weight savings.
mechanical properties at in-service higher
temperature ranges.
Casting Processes
Along with magnesium’s multiple alloys, the material can be cast by a variety
of methods, including high-pressure
diecasting, permanent mold casting, sand
casting, semi-solid and squeeze casting.
Different alloys may be specified for these
different processes, but in cases where the
same alloy is used with different casting
processes, the properties of the finished
castings will depend on the method.
The most prevalent casting method for
magnesium is diecasting. In this process,
complex, thin-walled parts are produced
at high production rates aided by the lowheat content per volume of molten metal.
Both hot chamber and cold chamber machines currently are used for magnesium.
For optimum performance, it is recommended that higher shot speeds are used
for magnesium compared to aluminum,
especially for thin-walled parts. Diecasting process variants (such as vacuum
diecasting) can produce components with
lower porosity and better properties than
standard diecasting.
Magnesium also is conducive to using
semi-solid casting methods, typically with
magnesium alloy granules or partially
solidified alloys rather than liquid magnesium. Semi-solid molding commonly is
used for smaller parts, such as those used
in the electronics industry.
Design Considerations
aluminum, molten magnesium does
not react with tool steels, resulting in
longer die life and increased productivity. Because of low erosion and reduced
heat input, which reduce the propensity
for thermal fatigue (heat checking of the
die), casting magnesium can warrant
three to four times the die life than if
aluminum were used.
Machining—Magnesium is recognized
as the easiest of structural metals to
machine and is the “standard” of the
cutting tool industry when comparing
machinability of metals. The low power
requirements for machining magnesium
alloys permit the use of deeper cuts and
higher feed rates, thus permitting fast
and efficient machining when compared
to other metals. Magnesium alloys also
normally produce well-broken chips,
ECS
which are easy to handle.
This article was adapted with permission
from materials prepared by the International Magnesium Assn., McLean, Va.,
and the North American Die Casting
Assn., Wheeling, Ill.
When evaluating the various alloys
and processes for a magnesium casting,
there are a number of characteristics to
consider to obtain a quality, low-cost
component. This includes the
end-use application, the postcasting operations and how
casting magnesium will factor
into tooling costs.
High Stiffness-to-Weight Ratio—This characteristic is important
where resistance to deflection is desired
in a lightweight component.
Improved Die Life—Unlike molten
Cast through the rubber plaster molding process in AZ91D
magnesium alloy, this projector frame and interface offered the
customer improved durability and rigidity, high heat tolerance
and eliminated EMI plating requirements when compared to its
previous plastic injection mold design.
2006 CASTING SOURCE DIRECTORY
ENGINEERED CASTING SOLUTIONS
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