Evaluation of Coating Methods for Corrosion Protection of Magnesium
Castings
by
Siobhan Fleming
An Engineering Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING IN MECHANICAL ENGINEERING
Approved:
_________________________________________
Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, CT
August, 2012
© Copyright 2012
by
Siobhan Fleming
All Rights Reserved
ii
CONTENTS
LIST OF TABLES ............................................................................................................ iv
LIST OF FIGURES ........................................................................................................... v
ACKNOWLEDGMENT .................................................................................................. vi
ABSTRACT .................................................................................................................... vii
1. Introduction.................................................................................................................. 1
2. Methodology ................................................................................................................ 7
2.1
Review of Magnesium Alloys ............................................................................ 7
2.2
Review of coatings for corrosion protection ...................................................... 7
3. Results and Discussion ................................................................................................ 9
3.1
3.2
Magnesium Alloys ............................................................................................. 9
3.1.1
Alloys for Casting .................................................................................. 9
3.1.2
Alloys for Wrought Parts ..................................................................... 10
Coatings ........................................................................................................... 11
4. Conclusion ................................................................................................................. 14
5. References.................................................................................................................. 15
6. Appendices ................................................................................................................ 17
6.1
Appendix A: Alloying Element Effects ........................................................... 17
iii
LIST OF TABLES
Table 1: American Society for Testing Materials.............................................................. 2
Table 2: Select Magnesium Alloys and Characteristics2 ................................................... 5
Table 3: General effects of elements used in magnesium alloys2 ................................... 18
iv
LIST OF FIGURES
Figure 1: Example of Hexagonal Close Packed Crystalline Structure .............................. 2
Figure 2: Magnesium die cast part..................................................................................... 3
v
ACKNOWLEDGMENT
Type the text of your acknowledgment here.
vi
ABSTRACT
Magnesium is an excellent readily available light metal alloy for engineering
applications when weight is a critical design element. It is a strong and light material
that can be cast into thinner parts than aluminum. However, it can be volatile at high
temperatures and extremely corrosive in wet environments. Magnesium parts have
specific benefits that are required in design applications when magnesium is in alloyed
form and has a protective coating. Sometimes only a coating or alloyed magnesium is
needed in design, but both have benefits. When alloys and coatings are introduced into
the design there is more evaluation required by the designer. Coatings may not offer
enough protection depending on applications or may endanger the health and safety of
the part manufacturers or operators.
magnesium
parts
and
the
This paper evaluates different coatings for
advantages
vii
and
disadvantages
to
each
type.
1. Introduction
Magnesium is an excellent light metal alloy as it is readily available
commercially and it is the lightest of all the light metal alloys. It is readily found in the
earth’s ocean and the crystal structure is hexagonal close packed (Figure 1) which
restricts its ability to deform because it has fewer slip systems at lower temperatures.
However, it is rarely used without being alloyed with other metals. Some of the most
common alloyed elements in commercial alloys are: aluminum, zinc, cerium, silver,
thorium, yttrium and zirconium. In order to name magnesium alloys the American
Society for Testing Materials developed a method for designating the alloys show in
Table 2. The first two letters indicate the principal alloying elements according to the
code listed in Table 1. The one or two letters are followed by numbers which represent
the elements in weight % rounded to the nearest whole number. For example AZ91
indicates the alloy Mg-9Al-1Zn.
Code Letter
Alloying Element
A
Aluminum
B
Bismuth
C
Copper
D
Cadmium
E
Rare Earth
F
Iron
G
Magnesium
H
Thorium
K
Zirconium
L
Lithium
M
Manganese
N
Nickel
P
Lead
Q
Silver
R
Chromium
1
S
Silicon
T
Tin
W
Yttrium
Y
Antimony
Z
Zinc
Table 1: American Society for Testing Materials code for designating magnesium alloys
Magnesium can also be alloyed with rare earth elements, which increase the
strength of magnesium especially at high temperatures.
The key properties of
magnesium alloys are that it is light weight, low density (two thirds that of aluminum),
good high temperature mechanical properties and good to excellent corrosion resistance.
Depending on the specific application of magnesium alloys there are specific
preferred alloys. Aeropspace and automotive industries have specific preferred alloys
for different applications depending on how the part is made and how it will be used.
Figure 1: Example of Hexagonal Close Packed Crystalline Structure
Magnesium alloys are good for engineering applications because they have good
strength, ductility and creep properties. Magnesium is strong and light making it an
excellent choice for aerospace applications. In particular cast magnesium alloys have
specific design and manufacturing advantages:
1. Castings can be made with thinner walls than aluminum (1-1.5mm versus 22.5mm).
2
2. Castings cool more quickly due to a reduced latent heat of fusion per unit
volume.
3. High gate pressures can be achieved using moderate pressures due to the low
density of magnesium.
4. Iron from casting dies has low solubility in magnesium alloys, which reduces any
tendency to die soldering.
Magnesium alloy components are mostly produced by high-pressure diecasting and a
disadvantage to this process is that they may contain relatively high levels of porosity.
Sand casting is also successful with magnesium alloys. Additionally permanent metal
moulds have been used to cast ingots for producing wrought products.
Figure 2: Magnesium die cast part
The disadvantage to using pure magnesium is that it is extremely susceptible to
corrosion. When alloyed the corrosion resistance is improved, but specific alloys have
been proven to be more corrosion resistant than others. For example as mentioned above
Mg-Al-Zn is the most widely used alloy for castings; however these castings originally
suffered severe corrosion in wet or moist conditions. This corrosion susceptibility was
greatly reduced with the discovery that small additions (0.2%) of manganese gave
increased resistance. Each of the different alloys has specific characteristics that are
beneficial to different uses a selection of magnesium alloys and characteristics are
described in Table 2.
Alloy
Characteristics
3
AZ63
Good room temperature strength and ductility
AZ81
Tough, leaktight castings with 0.0015 Be, used for pressure
diecasting
AZ91
General-purpose alloy used for sand and diecastings
AM50
High-pressure diecastings
AM20
Good ductility and impact strength
AS41
Good creep properties to 150ºC
AS21
Good creep properties to 150ºC
AE42
Good creep properties to 150ºC
ZK51
Sand castings, good room temperature strength and ductility
ZK61
As for ZK51
ZE41
Sand castings, good room temperature strength, improved
castability
ZC63
Pressure-tight castings, good elevated temperature strength,
weldable
EZ33
Good castability, pressure-tight, weldable, creep resistant to
250ºF
HK31
Sand castings, good castability, weldable, creep resistant to
350ºC
HZ32
As for HK31
QE22
Pressure tight and weldable, high proof stress to 250ºC
QH21
Pressure-tight, weldable, good creep resistance and proof stress
to 300ºC
WE54
High strength at room and elevated temperatures
WE43
Good corrosion resistance, weldable
M1
Low-to medium- strength alloy, weldable, corrosion resistant
AZ31
Medium-strength alloy, weldable, good formability
AZ61
High-strength alloy, weldable
AZ80
High-strength alloy
ZM21
Medium-strength alloy, good formability, good damping
4
capacity
ZK30
High-strength alloys
ZK60
Good formability
ZMC711
High-strength alloy
HK31
High creep resistance to 350ºC, weldable
HM21
High creep resistance to 350ºC, short time exposure to 425ºC,
weldable
WE43
High temperature creep resistance
WE54
High temperature creep resistance
LA141
Ultra-light weight
Table 2: Select Magnesium Alloys and Characteristics2
An additional disadvantage found in early magnesium alloy castings were that
the grain size tended to be large and variable, which often resulted in poor mechanical
properties and microporostiy.
While the alloys provide a significant improvement to corrosion resistance an
additional way to protect the surface of magnesium and its alloys is coating the
magnesium part. This is specifically beneficial in cases where the part is in contact with
other metal parts and could cause galvanic corrosion. Some examples of protective
coatings are fluoride anodizing, chemical treatments, electrolytic anodizing, sealing with
epoxy resins, standard paint finishes, vitreous enameling, electroplating and cold spray.
The military has been a long user of magnesium alloys for many different
applications. Past applications were commonly aircraft and vehicle structural platforms
and lethality applications, but not in personnel protection or armor applications. In
World War II magnesium was heavily used in aircraft components. Specifically the B36 incorporated 8,620Kg of magnesium: 5,555Kg of sheet, 700Kg of forgings and
300Kg of castings. In 1951 the Sikorsky H-19 “Chicasaw” had the highest percentage by
weight of magnesium castings and sheet of any aircraft then in service at 17%. The
M274 “Mechanical Mule” proves that while light weight magnesium is a strong metal;
the cargo carrier weighed only 870lbs and could transport up to 1000lbs for 90-150
miles. Present applications in the military are vehicle and helicopter transmission
5
housings such as the UH60 Blackhawk transmission. There is still no use in current
lethality or armor applications, but systems are being developed which could allow for
use in those applications.
In the future new ground and air vehicle structural
applications should be created, but modern tools need to be used to address the
signigicant scientific challenges which have prevented prior use.
Some of these
challenges are similar to disadvantages of using magnesium already discussed:
1. High maintenance intervals and long product lifetime are unfavorable due to
corrosion behavior.
2. Coated or treated parts can still corrode due to wear, abrasion and mechanical
damage which can initiate corrosion.
3. Joining of dissimilar metals and exposure to moisture due to poor engineering
design.
Some of the coating solutions described by the military include: electrochemical plating,
conversion coatings, anodizing, gas phase deposition, laser surface alloying/cladding,
organics, plasma gel coating and cold spray.
Further detail on coatings will be discussed as they apply to specific industries
more than others. As previously described magnesium is an excellent light metal alloy
for automotive, aerospace, appliance and sporting good parts.
The required
improvement in all industries is improved corrosion resistance depending on specific
applications.
6
2. Methodology
Magnesium alloys are beneficial for use in light weight applications and the purpose of
this paper was to review applications focus on the disadvantage of corrosion and
determine how prolong the life of magnesium components with coatings. In completing
the review the automotive and aerospace industries were focused on because of their
need for light weight, strong parts. There are other areas that magnesium alloys are
beneficial, but due to the overwhelming use in these two industries and the amount of
detailed information that was found it was determined that reviewing and compiling data
for magnesium alloy types and coating types to prevent corrosion for the automotive and
aerospace industries would be most beneficial. In order to determine the best alloys and
coatings for automotive and aerospace applications materials textbooks and technical
journal articles were reviewed. The review was broken down into two parts to allow
detailed study and provide better conclusions to the best alloys and coatings for specific
applications.
In order to determine conclusions for best materials the results and
discussion was formed directly from the literature review. To expand further it would be
beneficial to complete some experimental studies on alloy and coating type in a salt fog
chamber or set up for galvanic corrosion to further test the theory of best alloy, best
coating conclusions.
2.1 Review of Magnesium Alloys
The review of magnesium alloys was completed by reviewing several texts specifically
Light Alloys From Traditional Alloys to Nanocrystals. This provided a detailed list of
many alloys that were later reviewed in technical journal articles for specific industry
applications. The list of alloys and characteristics reviewed can be found in Table 2,
additionally Appendix A on page 17 lists alloying elements and their effects on
Magnesium. This was specifically used in the review of magnesium alloys that are
particularly good for specific industries.
2.2 Review of coatings for corrosion protection
The Cole Library provided numerous technical articles for review of magnesium alloy
coatings. These were specifically helpful in developing conclusions for industries as
7
they provided specific test results that could not be completed with the resources for this
review. Detailed review was covered on cold spray which is a relatively new promising
coating method for magnesium castings.
8
3. Results and Discussion
The results section will be broken into two sections one discussing the most relevant
magnesium alloys for aerospace and automotive applications and one discussing the
coatings used for the same applications. This will allow separation between two critical
design components. Suggestions for best combinations will be made in the conclusion
section; therefore separating the results into the two sections will allow for further
research to be completed more easily by allowing one to pick different alloys and
coatings from the separate sections to design an experiment for testing.
3.1 Magnesium Alloys
Magnesium alloys reviewed for this paper are documented in Table 2. Each alloy has
specific advantages and disadvantages depending on the application it will be used for.
The results from this review are specific to automotive and aerospace applications and
are further broken down for the results into casting and wrought specific alloys. Some
of the parts overlap and in Appendix B on page 18 all alloys described in the below
sections are listed in table form with proposed applications.
3.1.1
Alloys for Casting
For castings AZ91 is the most widely used magnesium alloy.
From the naming
convention in Table 1 this is the alloy Mg-Al-Zn. This alloy can be used in both
automotive and aerospace applications and is used specifically for its good casting
qualities and generally satisfactory resistance to corrosion. The disadvantages to this
alloy are its susceptibility to creep at temperatures above 120ºC and that the corrosion
resistance is impacted by the presence of cathodic impurities such as iron and nickel. In
order to improve the corrosion resistance higher-purity versions of AZ91 have been
formed and are comparable with corrosion rates in testing for some aluminum casting
alloys.
For automotive applications where greater ductility and fracture toughness are
required magnesium alloys such as AM60, AM50 and AM20 are used. These are high
purity alloys with reduced aluminum contents and are used in the following automotive
applications: wheels, seat frames and steering wheels.
9
If silicon is introduced into the Mg-Al alloys creep properties can be improved.
Two such alloys used in automotive applications are AS41 and AS21, while AS21
performs better with less aluminum AS41 is easier to cast with better fluidity. An
application specific to these alloys was the use in the rear engine of the Volkswagon
Beetle. These alloys were used to replace the cast iron crank case and transmission
housing saving nearly 50Kg in weight. This weight savings was critical for the road
stability of the vehicle.
Alloys that are specifically used in aerospace industry include AZ31 which was diecast for the military Falcon GAR-1. This alloy was used for the stabilizer fins. Another
alloy found in aircraft landing wheels, gearbox housings, and helicopter rotor fittings is
QE22. This alloy has superior tensile properties over most magnesium alloys which are
maintained to 250ºC. However, this alloy is relatively expensive due to the silver and
attempts have been made to replace silver with copper with some success although no
practical alloys have been found thus far.
3.1.2
Alloys for Wrought Parts
Due to the hexagonal crystal structure of magnesium it has fewer slip systems than face
centered cubic aluminum which restricts its ability to deform; therefore wrought
magnesium alloy products are normally carried out by hot working.
Additionally
extrusion speeds are five to ten times slower than is possible with aluminum alloys.
Instead of describing the specific parts in automotive or aerospace application the best
way to detail the results of the literature review of magnesium alloys is by describing the
wrought product.
Sheet and plate alloys are most commonly AZ31 which is the most widely used
magnesium alloy for applications at or slightly above room temperature. Sheets made
from AZ31 have been used for prototype testing for automotive sheet panels, but as the
cost of these panels is very high they are not seen often in cars; however it could offer
unique opportunities in the future.
The strongest alloy for extrusion is AZ81, but the most common general purpose
extruded alloy is AZ61. Magnesium must be extruded five to ten times slower than a
typical aluminum alloy and more costly.
10
Similarly to sheet alloys if the cost of
manufacturing can be brought down in the future there may be more opportunities for
use.
Magnesium forgings can only be fabricated from alloys with fine grained
microstructures. They tend to be made from AZ80 and ZK60 for parts that will be used
as ambient temperatures and WE43 is used for elevated temperatures. Forgings are
important for manufacturing parts that have an intricate shape and mush have a strength
higher than can be achieved with castings.
3.2 Coatings
There are numerous coatings that provide corrosion protection for magnesium parts.
These will be briefly described, but the focus will be on cold spray which is a relatively
new process being developed, which has some of the best results for aerospace
applications. A typical coating procedure for an aerospace part would be fluoride
anodizing, pretreatment by chromating or anodizing, sealing with epoxy resin, followed
by chromate primer and top coat.
Flouride anodizing involves using alternating current anodizing at up to 120V in a
bath of 25% ammonium biflouride. The film is then stripped in boiling chromic acid
before further treatment as it does not alloy for adhesion to organic treatments.
Electrolytic anodizing deposits a hard ceramic-like coating which offers some abrasion
resistance, some examples include Dow 17 and HEA. These offer little protection in an
unsealed state.
In the aerospace coating applications the next step would be to seal with an epoxy
resin. This requires the part to first be heated to 200-220ºC to remove moisture and then
after cooling the part is dipped in the resin solution. In order to build up the desired
coating heat treatment can be repeated once or twice.
After the part is prepared a standard paint finish can be applied. The paint should be
a chromate-inhibited primer followed by a good quality top coat. This is the standard
procedure for aerospace parts and some or all are also used on automotive parts.
However, if any of the coating is damaged in building or in use it provides no corrosion
protection. Therefore it is desirable to have a different coating that provides better,
longer lasting protection.
11
Cold-sprayed aluminum coatings are being studied in detail as the next best
coating for aerospace applications. For a UH-60 Seahawk that spends a significant
amount of time in an extremely corrosive environment on the deck of a ship it is critical
that the transmission gearbox can stand up to the environment.
While it may
theoretically be better to build the transmission housing out of a better material such as
aluminum with better corrosion resistance the weight of aluminum inhibits this choice.
Therefore, the next best thing is to adhere an aluminum coating using cold-spray to the
entire magnesium housing so that the part now reacts to the environment the same way
as a housing made from aluminum.
Cold spray is also known as cold gas dynamic spraying, high-velocity particle
consolidations and supersonic particle deposition. Coatings are applied in the solid state
at a much lower temperature than plasma spray, which avoids the common problems
associates with traditional thermal-spray methods such as oxidation, evaporation,
melting, crystallization, residual stresses, debonding and gas release problems. In the
cold spray process a carrier gas (N2 or He) is expanded to supersonic speed and send
through a converging/diverging nozzle. Particles are introduced to the gas flow at the
nozzle inlet and accelerated through the nozzle. Once the particles from the nozzle
impact the part being cold sprayed the particles undergo plastic deformation at very high
strain rates.
Cold spray can be used for numerous different metals, but the most
experimented and best use for magnesium alloys is using aluminum. Testing has been
completed on commercially pure Al, high purity Al, AA5356 and AA4047. In cases of
galvanic corrosion high purity Al performed the best with no galvanic corrosion when it
was cold sprayed onto test pieces of ZE41. Al 5356, Al 4047 and commercial pure Al
suffered galvanic corrosion when cold sprayed onto magnesium test pieces the values
tested were roughly 50 times greater than the current Mg-Mg couple. Commercially
pure Al and high purity Al were also tested in a salt fog chamber for 28 days and
reviewed every 7 days. Again the high purity Al performed the best with less than 5%
weight loss versus nearly 50% weight loss.
Cold sprayed aluminum will greatly reduce any other magnesium corrosion issues,
but more work needs to be completed on what is the best aluminum coating to use.
12
Using a non compatible coating could introduce new corrosion issues on a magnesium
part.
13
4. Conclusion
Magnesium is a critically important metal in design of aircraft and automotive parts
because of its desirable mechanical properties and low density. In order to determine
best alloys for specific applications specific experiments need to be completed to ensure
that the combination of alloy and coating will succeed in the environment. There are
numerous combinations to review when choosing an alloy and determining how the
product will be made as many alloys are best used in castings and various others are
better in a wrought product. Once the alloy has been chosen and the part made one must
determine if the environment of use requires specific coatings. Based on the corrosion
properties and use of most magnesium alloys in corrosive environments some form of
coating is required. A list of potential alloy combinations, coating types and applications
is listed in Appendix B. This list provides a useful baseline for alloy choice based on an
in-depth literature review. In order to further this study an experiment could be designed
to test the combinations in similar environments and prove that they are the best
combinations for specific use. Additionally there is constant development on new alloys
and new coatings it is possible in the future that a better coating or better alloy will be
designed to eliminate any of the previous issues seen with the alloys and coatings
reviewed in this paper.
14
5. References
1. Made-in-China.com
http://www.made-in-china.com/showroom/yuanlongjason/productdetailIblESqjdSMRB/China-Magnesium-Alloy-Die-Casting.html
2. Polmear, I. Light Alloys from Traditional Alloys to Nanocrystals. Amsterdam:
Elsevier, 2006.
3. Magnesium Alloys – An Introduction,
http://www.azom.com/article.aspx?ArticleID=355
4. ASM Handbook. Volume 15 Casting. Materials Park: ASM International, 2008.
5. Shackelford, James. Introduction to Materials Science for Engineers. Upper
Saddle River: Pearson Prentice Hall, 2005.
6. Hexagonal Close Packed Structure.
http://www.miniphysics.com/2010/12/hexagonal-close-packed-structure.html
7. Ying-Liang, Cheng. Comparison of corrosion behaviors of AZ31, AZ91, AM60
and ZK60 magnesium alloys. Transactions of Nonferrous Metals Society of
China: v. 19, pg 517-524. 2009.
8. Li, Juanguo; Xia, Canjuan; Zhang, Yijie; Wang, Mingliang; Wang, Howei.
Effects of TiO2 coating on microstructure and mechanical properties of
magnesium matrix composite reinforced with Mg2B2O5w. Materials and Design,
v. 39, pg 334-337. 2012.
9. Bu, Hengyong; Yandouzi, Mohammed; Lu, Chen; Jodin, Bertrand. Effect of heat
treatment on the intermetallic layer of cold sprayed aluminum coatings on
magnesium alloy. Surface and Coatings Technology, v. 205, pg 4665-4671.
2011.
10. Wu, Chao-yun; Zhang, Jin. State-of-art on corrosion and protection of
magnesium alloys based on patent literatures. Transactions of Nonferrous Metals
Society of China, v. 21, pg 892-902. 2011.
11. Bierwagen, Gordon; Brown, Roger; Battocchi, Dante; Hayes, Scott. Active
metal-based corrosion protective coating systems for aircraft requiring. Progress
in Organic Coatings v. 68, pg 48-61. 2010.
12. DeForce, Brian. Materials Performance: Cold Sprayed Aluminum Coatings for
magnesium aircraft components. Materials Performance, v. 48, pg 40-44. 2009.
13. DeForce, Brian. Cold Spray Al-5%Mg Coatings for the Corrosion Protection of
Magnesium Alloys. Journal of Thermal Spray Technology, v. 20, pg 1352-1358.
2011.
14. Norton, Brian. Transactions of the Institute of Metal Finishing: Aerospace
coatings –A specialist field. Transactions of the Institute of Metal Finishing, v.
84, pg 277-278. 2006.
15. Arruebarrena, G. Materials Science & Technology Conference proceedings:
Weight reduction in aircraft by means of new magnesium castings. Materials
Science and Technology, v. 3, pg 13-20. 2005.
16. Duffy, Laurence. Magnesium Alloys: The Light choice for Aerospace. Materials
World, v.4, pg 127-130. 1996.
15
17. Mathaudhu, Suveen. Magnesium technology: Magnesium alloys in U.S. military
applications: Past, Current and future solutions. Magnesium Technology, pg 2730. 2010.
18. Kiebus, Andrzej. Microstructure and properties of sand casting magnesium alloys
for elevated temperature applications. Diffusion and defect data, solid state data.
Part B, Solid state phenomena, v. 176, pg 63-74. 2011.
19. Wendt, Achim. Magnesium castings in aeronautics applications – Special
requirements. Magnesium technology, pg 269-273. 2005.
16
6. Appendices
6.1 Appendix A: Alloying Element Effects
Alloying
Element
Melting and Casting
Behavior
Ag
Al
Improves castability,
tendency to microporosity
Be
Significantly reduces
oxidation of melt surface
at very low concentrations,
leads to coarse grains.
Effective grain refining
effect, slight suppression
of oxidation of the molten
metal.
System with easily
forming metallic glasses,
improves castability.
Ca
Cu
Fe
Magnesium hardly reacts
with mild steel crucibles
Li
Increases evaporation and
burning behavior, melting
only in protected and
sealed furnaces.
Mn
Control of Fe content by
precipitating Fe-Mn
compound, refinement of
precipitates.
System with easily
forming metallic glasses.
Ni
Mechanical and
technological
properties
Improves elevated
temp. tensile and
creep props. In the
presence of rare
earths.
Solid solution
hardener,
precipitation
hardening at low
temps.
Improves creep
properties.
Solid solution
hardener at ambient
temperatures,
reduces density,
enhances ductility.
Increase creep
resistance.
Corrosion behavior
I/M produced
Detrimental
influence on
corrosion behavior
Minor influence
Detrimental
influence on
corrosion behavior
Detrimental
influence on
corrosion behavior,
limitation
necessary.
Detrimental
influence on
corrosion behavior,
limitation
necessary.
Decreases corrosion
properties strongly,
coating to protect
from humidity is
necessary.
Improves corrosion
behavior due to iron
control effect.
Detrimental
influence on
17
Rare Earth
Si
Th
Y
Zn
Zr
Improve castability, reduce Solid solution and
microporosity.
precipitation
hardening at
ambient and
elevated temps;
improve elevated
temp. tensile and
creep properties.
Decreases castability,
Improves creep
forms stable silicide
properties.
compounds with many
other alloying elements,
compatibile with Al, Zn,
and Ag, weak grain
refiner.
Supresses microporosity.
Improves elevated
temp tensile and
creep properties,
improves ductility,
most efficient
alloying element.
Grain refining element
Improves elevated
temp. tensile and
creep properties.
Increases fluidity of the
Precipitation
melt, weak grain refiner,
hardening, improves
tendency to microscopy.
strength at ambient
temps, tendency to
brittleness and hot
shortness unless Zr
refined.
Most effective grain
Improves ambient
refiner, incompatible with temperature tensile
Si, Al, and Mn, removes
properties slightly.
Fe, Al, and Si from the
melt.
Table 3: General effects of elements used in magnesium alloys2
6.2 Appendix B: Magnesium Alloy Applications
18
corrosion behavior,
limitation
necessary.
Improve corrosion
behavior.
Detrimental
Influence.
Improves corrosion
behavior.
Minor influence,
sufficient Zn
content
compensates for the
detrimental effect of
Cu.