1. F2008SC031
MAGNESIUM: THE WEIGHT SAVING OPTION
1
Guillén Abásolo, David*,
1
University of Burgos, Spain
KEYWORDS – Electrolytic process, Thermal reduction, lightest, reactive, Surfaces
treatments, recyclability, HPDC
ABSTRACT - Magnesium is the 8th most abundant element in the earth’s crust and its
extraction is done by Electrolytic process and Thermal reduction. These processes are costly
(technically and economically). It has good properties: the lightest of all structural metals,
conserves weight without sacrificing strength and rigidity which can be an alternative material
for inner components as seats, window regulators or steering wheels in order to reduce the
weight and therefore saving fuel consumption (CO2 emissions). Another important property is
its recyclability. Mg is very reactive, that is why it can be alloyed (Al, Zn, Rare earths, Be),
and receive surface treatments as the electrochemical one. The most common manufacturing
process is High Pressure Die-Casting. In general there are three main types of HPDC alloys:
The AZ91 which exhibits good combination of strength, ductibility and castability. The
AM60 and AM50 for high energy absorption applications, and the ones used for high temp.
Applications such as the AE42 and AS31 (powertrain at temperatures up to 150ºC and
200ºC). The alloys are produced in ingot form. Finally, thanks to its attractive characteristics
some pioneering companies in Europe are starting to integrate it in its products.
TECHNICAL PAPER – History until 1945, Joseph Black (1755), Scottish chemist,
discovered that magnesia contained a new element: magnesium. In 1808, Sir Humphrey Davy
isolated the metal. Magnesium found steady interest only in Germany which was the only
producer in the world, using the metal mostly as powder or ribbon for flashlights and other
pyrotechnical purposes. The first Beetle car whose weight contained 20 Kg of this metal was
designed by Porsche engineering. Since 1945, In Europe, production of magnesium decreased
drastically. At the same time, countries like America or Asia, were improving the process to
obtain pure magnesium metal and increased the production of magnesium and alloys to use it
in other tools like Civilian user: Cars, Kitchen furniture, Aircraft, Cell batteries, etc..
PRODUCTION TECHNOLOGIES OF MAGNESIUM
Magnesium is the eighth most abundant element on Earth and constitutes about 2% of the
Earth’s crust. It is the third most plentiful element dissolved in seawater and it can be found in
over 60 minerals. There are six sources of raw materials for the production of magnesium:
1. Magnesite (28.8% weight of Mg) [ MgCO3 ]
2. Dolomite (28.8% w.o. Mg) [ MgCO3*CaCO3 ]
3. Bischofite (11.96% w.o. Mg) [ MgCl2 * 6H2O ]
4. Carnallite (8.75% w.o. Mg) [ MgCl2 * KCl * 6H2O ]
5. Serpentine (26.36% w.o. Mg) [ 3MgO * 2SiO * 2H2O ]
6. Olivine (33% w.o. Mg) [ (Mg, Fe)2SiO4 ]
7. Sea water (0.038% - 20.8% w.o. Mg) [ Mg2+(aq) ]
Magnesium always appears in nature bivalent ionic form.
Mg2+ + 2e- = Mg [E0 = -2.375 V] (1)
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2. In 2006, production of primary magnesium in the world was approximately 726.000 metric
tons. As shown in figure 1, evolution of production per year by continent.
Production of primary Magnesium in XXI century
800
700
ASIA
600 AMERICA
EUROPE
Metric Tons
500
400
300
200
100
0
2000 2001 2002 2003 2004 2005 2006
Years
Figure 1: Statistics of Magnesium production from IMA
On the one hand, all the “Electrochemical technologies” use direct current electricity form,
which passes through the electrolysis cells and discharges chlorine and magnesium ions into
gaseous chlorine and metallic magnesium. The basic raw materials for the production of
magnesium with this process are generally divided into two: salts containing chloride and raw
materials that must be transformed into salts containing chloride. Eventually, all the materials
will become either Bischofite or Carnallite prior to drying and feeding into the electrolysis
cells.
On the other hand, the “Thermal Reduction” methods are based on heating of magnesia in the
presence of various reduction materials, to a variety temperatures. The only ores used in the
production of magnesium are Dolomite and Magnesite. The ores are extracted through
traditional mining methods, mainly through open mining. The ore extracted from the mine
undergoes calcinations at temperatures of 700-1000ºC.
Advantages and disadvantages of the different processes
Thermal
Comparative parameter Electrolytic technology reduction
technology
Magnesite, Dolomite,
Magnesite,
Raw materials Bischofite, Carnallite,
Dolomite
Serpentine, Sea water
Energy Sources Hydro power, Gas, Fuel Coal, Gas
Energy consumption per ton of Mg [ Mw / h ] 18-28 45-80
Capital investment per ton of Mg [ € ] 7000-12000 Up to 1300
Operational conditions Continuous process Batch Process
Operational Man Power X Up to 5X
Table 1: Comparison between electrolytic and thermal reduction processes
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3. PROPERTIES PURE OF MAGNESIUM
Property Conditions Measure Comment
Classified Alkaline earth metal
Crystal Structure hcp
Density [ gr/cm3 ]
20ºC 1,738 ++
( Lightest structural material)
Coif. of linear thermal expansion 20ºC-100ºC 26,1 x 10(-6)
++
“αT” of polycrystalline ( /ºC) 20ºC-500ºC 29,9 x 10(-6)
Young's modulus of elasticity (Gpa) 20ºC 44 =
Module of stiffness (Gpa) 20ºC 17 -
Melting Point (ºC) 650 +
Boiling Point (ºC) 1107 +
Coefficient thermal conductivity 0ºC 155,3
++
(W/mK) 423ºC 154,1
Reactive High --
Castability
Manufacturing Good ++
Ductibility
Table 2: Properties of Magnesium; (++) = Very good, (+) = Good; (=) =equal; (-) = poor; (--) = Very poor
Another property is damping capacity, the ability to dissipate elastic energy. There are two
main types of damping: anelastic and hysteretic. High damping capacity will only reduce
vibrations that cause anelastic strains in the part body. In general, all Mg-alloys can be tuned
to those critical frequencies where noise, vibration and harshness (NVH) are reduced.
Magnesium parts have excellent damping capacity in spite of damping depends on many
factors, e.g. purity, grain size, alloy composition, amplitude, temperature and frequency. The
Mg-alloys have a damping average value, measured with “The loss factor” of 0.004. It can be
introduced another subject “Squeaks & rattles”, because the possibility of magnesium like
“package bodies” reduces the assembly parts.
DESIGNATION OF MAGNESIUM ALLOYS
Magnesium, like other structural metals, must be alloyed with other metals to be employed for
engineer applications. The ASTM employs the following system:
Ex: AM60a
A (alpha): Primary alloying element designation
M (alpha): Secondary alloying element designation
6 (numeric): Primary alloying element nominal wt% (from 5, 5% to 6,5%)
0 (numeric): Secondary alloying element nominal wt% (from 0% to 0, 5%)
a (alpha): Revision
Alloying elements designations:
A- Aluminium; E- rare Earths; J- Strontium; K- Zirconium; M-Manganese; S-Silicon;
X- Calcium; Z-Zinc.
Main types of Magnesium alloys:
1. Conventional: AZ91D, AM60B, AM50, AM20
2. Creep resistant: AS31, RE-alloys, Ca-alloys, Sr-alloys
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4. 3. New alloys: AM-lite, HyMag 1, HyMag 2, MRI 153M, MRI 230D
4. Other: AS21, AS21X, AS41
MANUFACTURING
Mg-alloy component can be manufactured by all the conventional techniques including
casting, forging, extruding and rolling.
Figure 2: Type of manufacturing of Mg-alloys
According to the figure 2, there are so many types of manufacturing Mg, but the most
common Mg-alloy components are produced via High Pressure Die Casting (HPDC) whose
process has two ways, either “Hot chamber” and “Cold chamber”.
Hot chamber [H.C.] Cold Chamber [C.C]
The injection system is dipped in molten Speeds and pressures are higher can be obtained
metal majors.
Shorter Cycles More compact piece
Necessity to fuse fine thicknesses Majors investments
High limitations in the design of the pieces. Major Probably oxidation of the material
Process limited a few magnesium alloys There are only alloys that can be injected in C.C.
Greater possibilities in designing pieces
Table 3: Properties between different chambers in HPDC
Figure 3a: Hot Chamber Figure 3b: Cold Chamber
HPDC is by far the most common production method accounting for 70% of the magnesium
used and it is growing in excess of 15% per year thanks to automotive applications.
The main in die casters in Europe are: Grupo Antolin Magnesio, Dynacast, HDO, George
Fisher Mossner, ECKA Granulate GmbH, Magna Steyr, Dead Sea Magnesium, Magnesium
Elektron, Gjutal, Tonsberg, Meridian, Zitzmann Druckgrass,…
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5. Traditionally the Workhorse alloy has been Mg-Al and most of the alloys used are types
AZ91 because it shows superior castability and good mechanical properties, combined with
good corrosion resistance for the high purity versions of the alloy. Introduction of the high-
ductility, energy absorbing alloys in the AM-series is a major driving force behind the
expansion of the automotive use of magnesium die-casting. These last alloys are used for
components like steering wheels, seats, doors, body parts… that are subjected to deformation
during a crash. Table 4 shows the main factors which can to be considered to alloy elements:
Room temperature mechanical properties of die casting alloys
Elements Yield Tensile
Elongation Hardness
Al Mn Zn Other Strength Strength
Types [ N/mm2 ] [ N/mm2 ] [%] [ HB ]
AZ91 9,00% 0,13% 0,70% 160 250,00 7,00 70,00
AM20 2,10% 0,10% 90 210,00 20,00 45,00
AM50 4,90% 0,26% 125 230,00 15,00 60,00
AM60 6,00% 0,13% 130 240,00 13,00 65,00
AE42 4,00% 0,10% 2,5% RE 145 230,00 11,00 60,00
AS41 4,20% 0,20% 1% Si 140 240,00 15,00 60,00
Table 4: Comparison between different mechanical properties of Mg-alloys
MANUFACTURING PRODUCT DESIGN
The main demands on potential Magnesium components affect four vehicle modules: drive
train, interior, body and chassis. Magnesium can improve vehicle design and add unique
customer feature. One of the reasons why Mg could be an alternative in body design is as
“element housing”. In addition, Components consolidation eliminates expensive assembly
operations.
The following Basic design guidelines should give a first help in designing magnesium High
pressure die cast structure [HPDCs]:
1. Specify thin sections which can easily be die cast and still provide adequate strength
and stiffness (wall thickness from 0.5 to 10mm).
2. Keep sections as uniform as possible
3. Keep shapes simple and avoid stress concentration
4. A slight crown is more desirable than a large flat surface.
5. Specify coring for holes or recesses where we can save in metal and overall costs
outweigh tooling costs.
6. Avoid small cores (cored holes diameter should be more than 2mm).
7. Provide sufficient draft on side walls and cores to allow easy removal of the die
casting (average 2º).
8. Appropriate corner design (Radii should be more than thickness of fillets).
9. Cast-in inserts should be designed to be held firmly in place with proper anchorage
provided to retain them in the die casting.
10. Design parts to minimise flash removal costs
11. Never specify dimensional tolerance closer than essential (increases costs)
12. Where machining is specified, allow sufficient metal (nominal size + tolerance +
0.2mm) for required cuts and consider the draft angles.
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6. Magnesium applications
Mg-alloys are very attractive in applications for the automotive, railway and aerospace
industries as shown figure 4 and figure 5.
Figure 4: Seat, safety test by Grupo Antolin, Spain
Figure 5: Seat of Mg-alloy [AM60] casting by Grupo Antolin magnesio, Spain
POST-MANUFACTURING (SURFACE FINISHING)
Commonly what is done with the Post-manufacturing in Mg-alloys is to improve corrosion
resistance, maintaining a good surface appearance (low roughness), at least in parts of current
series. According to the experience in automotive area, terms of a good coating could be:
Measure thickness (30-45 microns), good adhesion, good finish roughness, Ra <1 micron,
resistance to atmospheric corrosion, Galvanic resistance, Filiform corrosion resistance
(scratch resistance), Coatings free of chromium and Cheap (the automotive sector working
with small margins).
Corrosion
Magnesium alloys have had a poor reputation for corrosion lower resistance for many
decades, particularly in salt-water environments. There are so many factors involved to
corrosion, in essential, we can found two types of cause of corrosion: external factors (which
include the humidity, content and type of salt ions in water, temperature and assembly
practices”Galvanic attack”) and internal factors (which included the alloys composition,
purity of Mg, contaminants, the fabricated form: die-casting, microstructure and surface
quality).
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7. Figure 6: Factors influencing the corrosion properties of Mg-Alloys
Galvanic Corrosion (GC)
GC occurs when magnesium is connected to other metallic materials in the presence of an
electrolyte. Figure 7 shows the basis for galvanic corrosion for the case of bolting two
magnesium castings together. The anode (corroding metal) and cathode (noble metal)
reactions must balance each other, which mean that by reducing the consumption of electrons
in the cathode reaction, the production of electrons in the anode reaction is reduced
accordingly.
Figure 7: Basis for galvanic corrosion
The key factor is the selection of the cathode material. Such materials are much more
compatible with magnesium. Table 5 gives an overview of compatible and non-compatible
materials.
Compatible material Non-compatible material
Aluminium 5xxx and 6xxx series Steel and stainless steel
Tin Copper
Zinc Nickel
Plastics and polymers Titanium
Selected aluminium alloys ( 2xxx and A380)
Table 5: Compatible and non-compatible materials with magnesium die casting
Principles and prerequisites for Optimum Surface Protection
In order to achieve the optimum surface protection for the Mg-based material, the following
principles, prerequisites and recommendations should be observed in table 6:
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8. Smoothing by polishing, honing, grinding or
Methods of pre- Mechanical
blasting
treatment of the metal
Use of organic solvents and/or alkaline
surface Cleaning
cleaning agents
Electroless Electrochemical
Physical methods
treatment treatment
Anodisation ( DOW 17,
Chromating PVD
HAE, ANOMAG)
Methods of applying Anodical plasmachemical
Chromiumfree Flame or plasma
inorganic coatings treatment (ASD)
systems spraying
on the Mg surface (MAGOXID, TAGNITE)
Electroless nickel Electroplating (Zn, Cu, Laser or electro
Ni, Cr, etc..) beam treatment
Painting
Water paints
Methods of applying organic Powder paints / EPS
coatings on the Mg surface or on Structural paints
the inorganic undercoating Immersion paints
Anti friction paints
Table 6: Surface treatment of magnesium based materials
RECYCLING
Environmentally respectful and cost effective use of Mg-alloys in automotive applications
assumes efficient closed loop recycling of die casting returns and post-consumer scrap. These
two recycling processes are issues capable of regaining the original chemical composition and
cleanliness of the Mg-alloys. The energy requirement for melting and recycling Mg is only
about 5% of the energy to produce the same quantity of primary material. Traditionally,
recycling by melting with flux has been used, but over the last decade, flux-free solutions
have emerged, partly linked to in-house recycling in die casting shops. While the industry
today effectively recycles the clean process scrap (class 1) in closed loop, the challenge in the
short term is to close the loop for the lower grade scrap such as droop and chips.
The classification system applied by “Hydro Magnesium” divides the scrap into the following
classes:
Sorted clean returns (trimming casting defects) [Class 1], Sorted clean returns with insert
(Other returns) [C2], Sorted oily/painted returns (Other returns) [C3], Sorted dry chips
(Machining) [C4], Sorted oily/wet chips (Machining) [C5], Dross – salt –free (Melt loss) [C
6], Sludge- with salt [C7], Mixed and off-grade returns (Other returns) [C8].
Regardless of class, except for ELV scrap, recycling of Mg will follow a general route:
Figure 8: General process steps for Mg-alloy recycling
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9. CONCLUSION, AUTOMOTIVE APPLICATION
Environmental conservation is one of the principal reasons for the focus of attention on
magnesium to provide vehicle weight reduction, CO2 emission and fuel economy.
Improvements in Mg alloying and processing techniques will make it possible for the
automotive industry to manufacture lighter, more environmentally friendly, safer and cheaper
cars. The dramatic benefit that reduced automobile weight can give in terms of improved fuel
consumption is showed in figure 9.
FUEL ECONOMY Vs AUTOMOBILE WEIGHT
30,00
FUEL ECONOMY [ Km / litre ]
25,00
20,00
15,00
10,00
5,00
-
54,00 450 908,00 1100 1.362,00 1.816,00 2.300,00 2.770,00
VEHICLE WEIGHT [ Kg ]
Figure 9: Fuel economy versus vehicle weight
ACKNOWLEDGEMENT - The author would like to express his acknowledgment to Grupo
Antolin company: Mr. Diego Val Andrés (Magnesium Technique), Ms. Rosalia Arribas
Fernandez (Marketing), Mr. Abel Dionisio Rodriguez Tejido (Simulation & test), Mr. Oscar
Calvo Herrera (Simulation & test), Mr. Mariano Cabrerizo Juarez (Acoustic & Vibration),
Mr. Jose Luis Pascual García (Acoustic & Vibration), Mr. Rafael García García (Advanced
System Engineer) and Mr. Francisco Javier Martinez Moral (Chief R&D) for theirs support
and suggestions. Finally, this work was supported by Grupo Antolin Company.
REFERENCES
(1) Horst E. Friedrich, Barry L. Mordike, “Magnesium Technology: Metallurgy, Design
data, Applications”, Ed. Springer, ISBN-10 3-540-20599-3, 2006.
(2) Grupo Antolin Company, “White book of magnesium”, 1999.
(3) International Magnesium Association (IMA), www.intlmag.org
(4) D.Eliezer, E.Aghion, F.H.( SAM) Froes, “Magnesium Science, Technology and
Applications”, 1998.
(5) Mustafa Kemal Kulekci, “Magnesium and its alloys applications in automotive
industry”, DOI 10.1007/s00170-007-1279-2, 2007
(6) Hydro Magnesium, Technical Brochure.
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