The lightest structural metal
1.- Introduction
Since a few years ago magnesium alloys is thought as a lighter
alternative to existing structural metals. However, for one
reason or another, despite the advantages attributed to him, the
metal has not finished breaking the force in the design of new
products.
One of the sectors that has promoted the use of magnesium has
been the automotive sector, in fact, in 1957 a Corvette SS,
designed for racing, was built with panels of magnesium. A few
years before Mercedes-Benz had done the same in 300SLR
model, a model that had a great success until it unfortunately
became the protagonist of the most tragic accident in the
history of the 24 Hours of Le Mans.
In 1971 Porsche broke the distance record with a model that
had reduced their weight by using a magnesium structure.
However, manufacturers preferred to focus its efforts on
reducing weight to the engine block and Volkswagen and
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Porsche will be the first ones to use magnesium alloys. Recently
there have been BMW, Chevrolet and Mitsubishi who have used
new magnesium alloys in order to reduce engine weight.
Another field of application of magnesium has been electronic
devices. Because of its low weight magnesium has been used in
the manufacture of casings of mobile phones, computers,
cameras and other electronic components.
In addition, magnesium has been one of the materials used in
aerospace construction, having been incorporated into the
German military planes in the first and Second World War. As a
result of the flammability of dust the use of other structural
metals was preferred in the evolution of aviation technology.
Recently there has been a new resurgence of magnesium, the
need to reduce the energy consumption of vehicles has revived
the
interest
of
companies
and
research
centres
in
the
possibilities of magnesium. They hope that this time the work
will be fruitful and magnesium may consolidate as a real
alternative to other structural metals.
A comprehensive source on current uses of magnesium as a
structural material can be found on page www.magnesium.com
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2.- Advantages and disadvantages
The advantages of using magnesium alloys are:
2.1.- Low density (1.74 Kg/dm3). 35% lighter than aluminium.
2.2.- Easily cast able. Its high fluidity allows designing thin
metal layers (0,8 mm), which allows designs not possible in
other materials such as thermoplastics.
2.3.- Good mechanical properties with respect to plastic
materials.
2.4.- It is not influenced by temperature between -20 º C and 120
º C. In contrast to plastic which suffer aging phenomena.
2.5.- Great potential for integration. Allowing the realization of
multifunctional parts and therefore reducing the number of parts
required for the manufacture of a set.
2.6.- Low price. It has an advantageous price with respect to
engineering plastics.
2.7.- High availability in nature. This is a very abundant metal in
nature, being able to extract from seawater.
With regard to the limitations we find:
2.8.- Industrial processes are not robust. Due to the limited
knowledge that exists on magnesium technology equipment
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manufacturers and suppliers, generally, do not have sufficient
knowledge to ensure the stability of industrial processes.
2.9.- The material has a low corrosion resistance as a
consequence of their growing tendency to rust. Hence it is
absolutely necessary to protect it.
2.10.- There are erroneous preconceptions about the material.
In the minds of many designers and engineers is the idea that
the material ignites spontaneously and is very fragile. They are
erroneous ideas that prevent their use in different sectors.
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3.- Properties
The following table shows a comparison of the properties of
magnesium with the most usual structural metals.
Property
Fe
Al
Cu
Ti
Mg
Density (Kg/dm ) 7.87
2.7
8.96 7.14
4.51
1.74
Mohs Hardness
2.5
3
6
2.5
Melting Point (ºC) 1,500
660
1,085 420
1,668
650
Electric C. (MS/m) 9.93
37.7
58.11 16.6
2.38
22.6
Thermal C.
(W/mK)
80.2
237
400
116
21.9
156
Abundance
Position
4º
3º
25º
23º
9º
7º
Cast ability
No
Si
No
Yes
No
Yes
Mechanized
Yes
Yes
Yes
No
Yes
No
Welding
Yes
Yes
Yes
No
Yes
No
Place of mining
China
Australia Chile EEUU
Australia
Seawater
Brazil
Guinea
Australia
South
Africa
Russia
Jamaica
China
3
4
Zn
2.5
Canada
Australia
Kazakhstan
India
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4 .- Magnesium Alloys
The following table shows the most commonly used magnesium
alloys with nominal composition.
Alloy
Al %
Zn % Mn %
AM60
6
AZ31
3
1
0.2
AZ61
6
1
0.2
AZ63
6
3
0.2
AZ80
8
0.5
0.2
AZ91
9
1
0.2
Ag %
Zr %
Th %
Re %
0.2
EZ33
2,5
0.5
2.5
ZM21
2
HK31
0.1
0.5
3
HZ32
2
0.5
3
1
QE22
2.5
0.5
QH21
2.5
0.5
2
1
1
ZE41
4.5
0.5
1.5
ZE63
5.5
0.5
2.5
ZK40
4.0
0.5
ZK60
6.0
0.5
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5.- Corrosion of magnesium
One of the main problems in the use of magnesium is the low
corrosion resistance. This circumstance requires paying special
attention to the problem of galvanic corrosion, which is the
corrosion produced as a result of the contact of magnesium
with another metal. Although a good surface protection of
magnesium can help prevent corrosion, the best solution is
avoiding a direct contact between magnesium and certain
metals.
Group 1
Group 2
Group 3
Group 4
Group 5
Al 5052
Al 6063
Al 2024
Galvanized steel
Steel
Al 5056
Al 7075
Al 2017
Cadmiate steel
Stainless steel
Al 6061
Al 3003
Zn
Titanium
Lead
Copper
Brass
The table below shows the tendency to produce galvanic
corrosion of different metals in contact with magnesium. The
materials are sorted from lowest to highest tendency to cause
corrosion.
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6.- Magnesium Treatments
Magnesium surface treatments could be:
6.1.- Painting
It may be in liquid or powder, the basic difference is in
thickness,
which
usually
determines
the
appearance
and
properties of the coating. The high reactivity of magnesium
required to work always with primers to get a good adherence
and so an acceptable corrosion resistance.
6.2.- Anodizing
The anodizing is usually made in sulphuric acid; however, as a
result of the high reactivity of magnesium, it is difficult to
anodize in an acid medium. Hence alkaline solutions are used,
and then the anodic layer formed is so insulated that plasma
formation
occurs during
the
passage
of
electric
current,
resulting in what is known as plasmachemical anodizing.
6.3.- Plating
Another possibility for treating magnesium is given by plating,
in which a multilayered system of copper, nickel and chromium
is applied to obtain magnesium polished chrome finish or matt
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chrome depending on the type of nickel. The difficulty of this
process is to achieve good adhesion between the plate and
magnesium, since the formation of insulating oxides on the
surface of magnesium is very fast. These oxides provoke
adherence problems between deposit and substrate. To avoid
this behaviour an electroless metal interface is applied prior to
the application of an electrolytic deposit of copper, nickel and
chromium. As interfaces are commonly used zinc and nickel.
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