1

An Overview of Magnesium based Alloys for Aerospace and

Automotive Applications

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

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© Copyright 2012

by

Siobhan Fleming

All Rights Reserved

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CONTENTS

LIST OF TABLES ............................................................................................................. 4

LIST OF FIGURES ........................................................................................................... 5

ACKNOWLEDGMENT ................................................................................................... 6

ABSTRACT ...................................................................................................................... 7

1. Introduction .................................................................................................................. 8

2. Methodology .............................................................................................................. 16

2.1 Review of Magnesium Alloys .......................................................................... 16

2.2 Review of coatings for corrosion protection .................................................... 16

3. Results and Discussion .............................................................................................. 18

3.1 Magnesium Alloys ........................................................................................... 18

3.1.1 Alloys for Casting ................................................................................ 18

3.1.2 Alloys for Wrought Parts ..................................................................... 20

3.2 Coatings ........................................................................................................... 21

4. Conclusion ................................................................................................................. 30

5. References .................................................................................................................. 32

6. Appendices ................................................................................................................ 34

6.1 Appendix A: Alloying Element Effects ........................................................... 34

6.2 Appendix B: Magnesium Alloy Applications .................................................. 36

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LIST OF TABLES

Table 1: American Society for Testing Materials .............................................................. 9

Table 2: Select Magnesium Alloys and Characteristics2 ................................................. 13

Table 3: Mechanical Properties of Mg-9Al-1Zn19

........................................................... 19

Table 4: Advantages and Disadvantages to Coating Types ............................................. 28

Table 5: General effects of elements used in magnesium alloys2 ................................... 35

Table 6: Proposed alloys for specific applications .......................................................... 36

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LIST OF FIGURES

Figure 1: Example of Hexagonal Close Packed Crystalline Structure .............................. 8

Figure 2: Magnesium die cast part ................................................................................... 11

Figure 3: Schematic of growth of anodizing coating11

.................................................... 22

Figure 4: Schematic of electroplating process11

.............................................................. 23

Figure 5: Galvanic Corrosion13

........................................................................................ 26

Figure 6: Salt Spray Exposure13

...................................................................................... 26

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ACKNOWLEDGMENT

I’d like to thank Professors Marcin and Donachie for introducing me to the fascinating

world of materials science especially the use of magnesium for aerospace and

automotive applications. I would also like to thank Professor Gutierrez-Miravete for his

support and guidance in completing this project. Finally I’d like to thank my family for

their support and encouragement in completing this final step towards my master of

engineering degree.

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ABSTRACT

Magnesium is the lightest of all light metal alloys and therefore is an excellent choice for

engineering applications when weight is a critical design element. It is strong, has good

heat dissipation, good damping and is readily available. The use of pure magnesium is

rare due to its volatility at high temperatures and it is extremely corrosive in wet

environments. Therefore the use of magnesium alloys when designing aerospace and

automotive parts is critical. Specific alloys are better for certain applications and often

also need a coating to provide the longest life of the part. This paper details specific

alloys used for certain aerospace and automotive applications. Additionally there is a

review of coatings for magnesium alloys and an analysis of alloys and coatings. Finally

it recommends an option for a future coating that may prove to be the best coating for

long lasting corrosion resistant parts.

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1. Introduction

Magnesium is an excellent metal as it is readily available commercially and it is the

lightest of all the structural metals having a density of 1.7g/cm3; it also has good heat

dissipation, good damping and good electro-magnetic shield. It is most commonly

found in the earth’s ocean. At room temperature magnesium and its alloys are difficult

to deform due to the crystal structure which is hexagonal close packed (Figure 1). This

structure restricts its ability to deform because it has fewer slip systems at lower

temperatures. Magnesium has a moderately low melting temperature making it easier to

melt for casting. Additionally it is relatively unstable chemically and extremely

susceptible to corrosion in a marine environment. It is thought that the corrosion is due

more to impurities in the metal versus an inherent characteristic. Finally magnesium

powder ignites easily when heated in air and must be handled very carefully in a powder

form. The rest of this section will review the advantages and disadvantages to

magnesium use in engineering applications. In addition, alloy types and an introduction

to coating protections will be discussed.

Figure 1: Example of Hexagonal Close Packed Crystalline Structure

Pure magnesium is rarely used in the manufacturing of aerospace and automotive

parts. In order to be used in manufacturing, it is 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.

The first two letters indicate the principal alloying elements according to the code listed

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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

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 they are light weight, with low density (two thirds that of

aluminum), and have good high temperature mechanical properties with good to

excellent corrosion resistance.

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Magnesium alloys are good for engineering applications because they have good

strength, ductility and creep properties. Magnesium alloys have replaced engineering

plastics in many applications because they have a comparable density but are stiffer,

more recyclable and less costly to produce. Magnesium is strong and light, making it an

excellent choice for aerospace applications. Magnesium is also used in a number of

other products such as hand-held devices (chain saws, power tools, hedge clippers), in

automobiles (steering wheels and columns, seat frames, transmission cases, crank case,

camshaft sprocket, gearbox housings), and in audio-video-computer-communications

equipment (laptop computers, camcorders, TV sets, cellular telephones). 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 2-

2.5mm).

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 can be successfully produced with nearly all of the

conventional casting methods. These methods are sand, permanent and semi-permanent

mold and shell, investment and die casting. In early castings it was found that the grain

size tended to be large and variable, which often resulted in more mechanical properties

and microporosity. Not all alloys are suitable for production by all casting methods.

Sand castings are generally used in aerospace applications because they have a clear

weight advantage over aluminum and other metals. Permanent mold casting can also be

used with similar alloys used for sand casting. The advantage over sand casting is the

mold or die can be used repeatedly, but the initial cost of the die is expensive so the

number of parts to be made must be high. Die-casting is ideally suited for high-volume

production parts and typically uses the Mg-Al-Zn type alloys.

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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. Magnesium is susceptible to

different types of corrosion one type, galvanic corrosion, can sometimes be designed out

of the part. Two ways to protect from galvanic corrosion are: (1) to minimize the

chemical potential difference between the magnesium/magnesium alloys and the

dissimilar materials and (2) maximize the circuit resistance. This corrosion

susceptibility was greatly reduced with the discovery that small additions (0.2%) of

manganese gave increased resistance. There are also metallurgical factors that affect the

corrosion performance of a magnesium part which are composition and its

corresponding microstructure and the alloy temper/heat treatment. Each of the different

alloys has specific characteristics that are beneficial to different uses. Some alloys such

as AZ91E, WE43B and Elektron 21 are corrosion resistant alloys. Incorporating these

into the design is beneficial for having a part with a longer life. A selection of

magnesium alloys and characteristics are described in

Table 2.

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Alloy Characteristics

AZ63 Good room temperature strength and ductility

AZ81 Tough, leaktight castings with 0.0015 Be, used for pressure die-

casting

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

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AZ80 High-strength alloy

ZM21 Medium-strength alloy, good formability, good damping

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

While the alloys provide a significant improvement to corrosion resistance, an

additional method to protect the surface of magnesium and its alloys is to coat 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.

While there are a number of advantages and disadvantages to using magnesium

alloys the military has continued to use them for many different applications. Past

applications were commonly aircraft and vehicle structural platforms and lethality

applications. In World War II magnesium was heavily used in aircraft components.

Specifically the B-36 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” proved that magnesium is a strong metal even

though it is light weight; 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 housings such as the UH60 Blackhawk transmission. There is still no use

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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

significant 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. Another coating used specifically in the

aerospace industry is rockhard two pack cold cure and single pack stoving epoxy. These

provide maximum corrosion protection to magnesium. They are also formulated free

from heavy anti-corrosive pigments which results in a lower film density than most

protective paints. Finally, due to their high corrosion resistance fewer coats are required,

a single coat instead of two or two instead of three which allows the weight to be

minimized; a critical design component in the aerospace industry. The biggest

disadvantage to current coatings is they are hazardous to the environment. Common

coatings that fall into the wet methods (conversion film, electrochemical plating,

anodizing, painting and sol-gel) are less expensive but often contain chromates and

cyanide among other toxic carcinogens. Other common coatings that fall into the dry

methods (thermal spray, laser surface alloy, physical or chemical deposition, and solid

diffusion) have less environmental impact but often require special apparatuses that are

expensive. Therefore, while there are numerous coating types and application processes

available, there are significant disadvantages to all existing coatings and no current

coating allows for 100% corrosion protection. Through literature reviews and new

testing there maybe new coatings available that will provide this required corrosion

protection.

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Magnesium is an important metal for light weight applications. However, due to

some of the disadvantages previously mentioned it cannot be used without being alloyed,

and alloys still do not eliminate all of the concerns. This paper documents some of the

developments that have been made in making alloys better for aerospace and automotive

applications. As mentioned above the military finds magnesium alloys to be very

important to new product development especially if coatings can eliminate the concern

of corrosion and lead to longer life. Many technical papers have proven one alloy or one

coating to be the best for a specific application. This review illustrates why these alloys

have been chosen, and the advantages and disadvantages of each alloy. The results are

summarized in a table in Appendix B. An additional table (Table 4) summarizes the

advantages and disadvantages to coatings used to protect magnesium parts.

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2. Methodology

Magnesium alloys are beneficial for use in light weight applications. The purpose of this

paper is to review aerospace and automotive applications for magnesium alloys; focus

on the disadvantage of corrosion and determine how to prolong the life of magnesium

components with coatings. In completing the review, this paper only focuses on the

automotive and aerospace industries 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

Light Alloys from Traditional Alloys to Nanocrystals. This provided a detailed list of many alloys

many alloys that were later reviewed in technical journal articles for specific industry applications.

applications. The list of alloys and characteristics reviewed can be found in

Table 2. Additionally, Appendix A on page 34 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

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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.

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3. Results and Discussion

The results section is broken into two sections one discussing the most relevant

magnesium alloys for aerospace and automotive applications and the other discussing

the coatings used for the same applications. This allows separation between two critical

design components. Suggestions for best combinations are made in the conclusion

section and documented in table form in Appendix B; therefore separating the results

into the two sections allows 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

Some of the magnesium alloys reviewed for this paper are documented in

Table 2. The specific alloys reviewed are detailed in section 3.1.1. 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 into alloys for casting and alloys for wrought applications.

Some of the parts overlap and in Appendix B on page 36 some alloys described in the

below sections are listed in table form with the best applications for those alloys.

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. Additionally it is less costly

in comparison to other magnesium alloys available on the market. The aluminum in the

alloy causes an increase in the tensile strength and hardness of the alloy to a temperature

of 120ºC and improves castability. 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.

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Alloy YS

(MPa)

20ºC

YS

(MPa)

180ºC

UTS

(MPa)

20ºC

UTS

(MPa)

180ºC

Elongation

(%) 20ºC

Elongation

(%) 180ºC

Mg-9Al-

1Zn (as-

cast)

94 72 157 138 4 14

Mg-9Al-

1Zn (T6)

150 121 250 212 5 11

Table 3: Mechanical Properties of Mg-9Al-1Zn19

In order to improve the corrosion resistance higher-purity versions of AZ91 have been

formed and have comparable corrosion rates in testing to 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.

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 Volkswagen

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 die-

cast for the military Falcon GAR-1 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 used to make it; attempts

have been made to replace silver with copper with some success although no practical

alloys have been found thus far.

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Another alloy widely used in aircraft and automotive industries is Mg-5Y-4RE-Zr.

Some parts cast from this alloy are gear housings of helicopters and parts of engines for

racing cars. Similarly to QE22 it is one of the most expensive magnesium alloys due to

the presence of expensive materials in the alloy, which in this case is yttrium versus the

silver in QE22.

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 formed.

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 is therefore, more costly. 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; WE43 is used for forging parts for use at elevated

temperatures. Forgings are important for manufacturing parts that have an intricate

shape and must have strength higher than can be achieved with castings.

It is important to know the capabilities of wrought alloys because future

development could make these parts very important to automotive and aerospace

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applications. As noted above both sheet and extruded alloys could very easily be made

into automotive parts but the cost inhibits the ability to be used.

3.2 Coatings

There are numerous coatings that provide corrosion protection for magnesium parts.

Some that were reviewed and detailed in this section are electrochemical plating,

conversion films, anodizing, gas-phase deposition, laser surface alloying/cladding and

organic coatings. Additionally a typical aerospace coatings process is also discussed.

Table 4 documents all coatings discussed in with advantages and disadvantages to each

coating type. For many applications a single coating process is successful enough to

protect the magnesium alloy from corrosion, but for aerospace applications a

combination of coatings is often required. A new coating technique that is still in

development is cold spray using aluminum particles which is proving to be very

successful for protection of magnesium parts and therefore testing results are discussed

in this section. This new coating technique may be the best new coating for magnesium

alloys that is environmentally friendly and long lasting.

As mentioned above there are numerous coatings that have been used to protect

magnesium alloys. These include conversion films, electrochemical plating, surface

coatings and multiple surface treatments. The first to be discussed is chemical

conversion film. These are superficial films of substrate metal oxides, chromates,

phosphates or other compounds. These are produced by chemical or electrochemical

treating on a metal surface. The films are then chemically bonded to the metal surfaces.

Chromate conversion coat is the most effective and mature process and is used most

commonly due to its excellent adhesion and corrosion resistance. The downside is that

the Cr6+

in chromate bath is a highly toxic carcinogen and is gradually facing restrictions

preventing its use. An alternative is phosphate treatments. A chromate-free phosphate-

fluoride conversion film was invented to improve the corrosion rate and compactness of

phosphate films. The challenges with phosphate film are that the grains are coarse and

cracks can occur due to the high activity of magnesium alloy and heavy metal ions in

phosphate solution can cause environmental pollution.

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Anodizing is another successful technology for corrosion protection of magnesium

alloys. This is an electrolytic process producing a thick and stable oxide film on the

part. This can also be used to improve paint adhesion to metals or as a passivity

treatment. The two types of anodizing are oxygen precipitation and film forming. The

anodized coat is formed in three stages show in Figure 3. The first is the forming of the

compact layer followed by the porous layer and finally the growth of the porous layer.

The properties of the coating depend on various parameters such as electrolyte

composition, voltage and time. One type of anodize treatment is Dow 17 which was

invented by Dow Chemicals. However this contains toxic chromate and therefore the

application has been limits. One disadvantage to anodic coatings on magnesium is the

electrochemical inhomogeneity due to the phase separation in alloys. Another

disadvantage is that the coatings are brittle and are prone to cracking or shedding after

collision.

Figure 3: Schematic of growth of anodizing coating11

Another successful coating method is plating of magnesium alloys. Plating can

be divided into two categories: electroplating (Figure 4) and electroless plating. The

process is that a metal salt in solution is reduced to its metallic form and deposited on a

part. The difference between electroplating and electroless are that in electroplating the

electrons are supplied by an external power source versus a chemical reducing agent in

the solution. The process for electroplating is:

1. Cations are gathered at the cathode surface by concentration diffusion.

2. Displacement reaction occurs and the cathode and the cations are consumed.

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3. Film is formed by the deposition of metal crystal from the displacement reaction

on the substrate surface.

Figure 4: Schematic of electroplating process11

The downside to plating is that due to the high chemical activity of magnesium the

plating films have a weak adhesion to magnesium alloys. In order to improve the

adhesion a pretreatment of Cr plating was invented. This was successful, but the Cr

plating solution contains toxic substances. Another difficulty that occurs with plating

that magnesium is very prone to galvanic corrosion. In particular Ni as an impurity in

Mg alloys reduces corrosion resistance severely and is a disastrous element to corrosion

resistance of Mg alloys yet most coatings contain Ni, which must be carefully removed

when parts are recycled.

Sol-gel is another coating method. It is often used instead of electroless plating as

electroless plating can only achieve a relatively uniform metallic layer where sol-gel is

an advanced technique that synthesizes high quality oxide thin films and powders. In one

test TiO2 was applied using sol-gel on a magnesium alloy. This film was chosen because

it has good physical properties, chemical stability, low toxicity and low cost. After

testing it was shown that it could be applied successfully using the sol-gel method and

that the coating can improve the interfacial bonding strength between the matrix and

magnesium alloy which provides a higher efficiency of load transfer from the matrix and

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higher mechanical properties of the composite. Finally the flexural strength and flexural

modulus were improved.

The problem with a number of coatings as previously mentioned is their

environmental impact. The chromates found in most of the common chemical methods

are environmentally hazardous. One coating that is not hazardous is TAGNITE, which

is a chromate-free, anodic electrodeposition surface treatment. It has been proven useful

in touch up areas on helicopters and in testing comparison with HAE and Dow 17 it has

been shown to have significantly better abrasion resistance, wet paint adhesion, impact

resistance and fatigue properties. In the testing it was also shown that there is a

significant advantage in salt spray corrosion. While it may be possible to use TAGNITE

as a complete protection it has been proven most useful in fine tolerance areas such as

liner bores and faying surfaces, where organic finishes are prohibited or must be very

thin. Using TAGNITE improves corrosion resistance and eliminates the use of

environmentally hazardous chromates.

Sometimes more than one surface treatment technology is needed for successful

corrosion protection. One example is to apply an oxide film by anodic oxidation

followed by a thermosetting resin film and finally a metallic conductive film is formed

using vapor deposition. This enhances surface characteristics such as corrosion

resistance and conductivity and is similar to what is done in aerospace to provide the

longest lasting life possible with the best corrosion resistance.

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. Fluoride 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 allow for

adhesion to organic treatments. Electrolytic anodizing deposits a hard ceramic-like

coating which offers some abrasion resistance; examples include Dow 17 and HEA.

These offer little protection in an unsealed state and thus 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

25

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.

Cold-sprayed aluminum coatings are being studied in detail as the next best

coating for aerospace applications. For a MH-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 housing 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

associated 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 sent

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 commercially pure Al suffered galvanic corrosion when cold sprayed onto

26

magnesium test pieces the results were roughly 50 times greater than the current Mg-Mg

couple (See Figure 5).

Figure 5: Galvanic Corrosion13

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 (See Figure 6).

Figure 6: Salt Spray Exposure13

Cold spray has performed well in testing. The hardness values have been

comparable to the commercial aluminum alloys and much greater than commercially

pure and high purity aluminum. The coating adhesion has been shown to be better when

27

applied with helium as the gas versus nitrogen. The coating- substrate interface appears

without defects which leads to the conclusion that the coating is expected to perform as

an effective corrosion barrier. Salt fog testing has proved to be successful in one test

with an Al-5%Mg coating applied to 0.380mm thickness a 1000hr test was completed

without failure. The only test that has been shown to not be completely successful is

galvanic corrosion. There has been some improvement depending on the coating

applied, but this appears to be the major concern holding back cold spray from being

widely used.

Cold sprayed aluminum will greatly reduce any other magnesium corrosion issues,

but more work needs to be completed to determine the best aluminum coating to use.

Using a non compatible coating could introduce new corrosion issues on a magnesium

part.

This section detailed many different coatings and table 4 below provides a summary

of the advantages and disadvantages to each coating type. Each of the coatings

discussed have specific benefits to use and all provide some element of corrosion

protection and therefore it can be difficult to pick the best one for a specific application.

The beginning of the table have the most used forms of coatings, which have been tried

and tested and found to provide good to excellent corrosion resistance. The issue with

these coatings that has required the research of new coatings listed at the bottom of the

table is that many have toxic carcinogens such as chromate and the government is

creating rules and regulations against using these products. Table 4 provides a good

reference for common coating advantages and disadvantages.

28

Coating Type Advantages Disadvantages

Chemical

Conversion

Most effective and mature

process and provides good

corrosion resistance.

Cr6+

in chromate bath is a highly toxic

carcinogen facing restrictions with use.

Phosphate

Treatments

Similar to chemical conversion

in protection, but it is chromate

Free

Coarse grains can cause cracks; heavy

metal ions in solution can cause

environmental pollution.

Anodizing Can improve paint adhesion to

metals,

Coatings are brittle and prone to cracking

or shedding after collision.

Plating (electro

and electroless)

Can improve corrosion

resistance depending on what

material is plated on surface.

Plating films have a weak adhesion to

magnesium alloys. Issues with galvanic

corrosion depending on type of metal

used.

Sol-Gel Can achieve better layering than

electroless plating. Some testing

has proved very successful using

TiO2

Potential issues with galvanic corrosion

depending on material used.

TAGNITE Chromate free, better abrasion

resistance, wet paint adhesion,

impact resistance and fatigue

properties.

Has not been proven as a complete

protection. Useful more in touch up

conditions where fine tolerances are

required.

Paint Provides a final protective

coating when used in

combination of other coating

techniques.

Only provides additional protection when

built up correctly with other coatings.

Does not provide protection when chipped

or cracked.

Cold Spray Adheres well with fewer issues

than plasma spray. Testing has

proved improved corrosion

resistance when high purity

aluminum is used as sprayed

material.

Still a new technology, not widely tested.

Concerns with galvanic corrosion

depending on coating material used.

Table 4: Advantages and Disadvantages to Coating Types

29

There are numerous somewhat successful technologies for improving corrosion

resistance of magnesium alloys, but there is currently no single technique that can meet

the industry requirements for Mg alloys in different service conditions. There are two

specific types of coating types: dry and wet methods. Dry methods include thermal

spray, laser surface alloy or cladding, physical or chemical deposition, and solid

diffusion. The wet methods are conversion film, electrochemical plating, anodizing or

plasma oxidizing, painting or organic/polymer coating, and sol-gel. Typically dry

methods are environmentally friendly and are suitable for treating precision. However,

they often require specific special apparatuses that are often very expensive. Wet

methods are less expensive and suitable for complex and large components used in the

automotive and aerospace industries, but require great effort for waste disposal as many

of the elements are toxic (chromium and cyanide). The future of coatings may be in

more research for cold spray if galvanic corrosion can be avoided and it can be produced

at a low cost. It is critical that the next best coating be low cost, pollution-free, and easy

to recycle.

30

4. Conclusion

Magnesium is a critically important metal in design of aerospace and automotive parts

because of its desirable mechanical properties. The low density, good heat dissipation,

good damping and good electro-magnetic shield all make it a top choice for design of

aerospace and automotive parts. However, the varying operational environments require

a material that is more corrosion resistant. Therefore, magnesium is alloyed with other

materials (metals and rare earth elements) to provide the best material for aerospace and

automotive parts.

This paper provided an overview of the numerous magnesium alloys available.

There are still new alloys being tested to provide the best combinations of properties for

specific applications. The selection of an alloy type depends on how the part will be

made (cast or wrought), the strength required, and the operational environment. There

are other considerations made in designing each specific part to help select between

several very similar alloys. This paper documents a number of different alloys that can

be used for aerospace and automotive applications and provides some specific proven

alloys for certain uses in Appendix B. There are many other alloys available as shown in

Table 2; however the most commonly proven good alloys for specific aerospace and

automotive applications are available in Appendix B.

In addition to choosing an alloy that has the best properties for a specific application

and can improve the life of a magnesium part, coatings are also critical to extending the

life. Numerous different coatings are explored in this paper. All provide good corrosion

resistance, but have varying advantages and disadvantages which are documented in

Table 4. The biggest disadvantage for most coatings is that they are not environmentally

friendly because they contain chromates. Not only are they difficult to dispose of and

hazardous to the health of employees working with them, but there is an increase in

restrictions for use of these materials by the government. Therefore, it is critical that a

new coating be tested and proven successful. This coating should improve the corrosion

resistance of magnesium alloyed parts and be inexpensive to apply.

This paper explores the possibility of using cold sprayed aluminum alloys as the

new coating for magnesium parts. However, there is inadequate research in cold sprayed

aluminum alloys. In order to use cold sprayed aluminum on flight critical parts it needs

31

to be well tested. Tests have shown that the corrosion resistance improves significantly

with high purity aluminum. However, depending on the type of material used for

coating, galvanic corrosion remains an issue. The significant advantage to using cold-

sprayed aluminum on a magnesium alloy part is that the low density properties of

magnesium are retained and the corrosion resistance of aluminum is gained. This

combination could be extremely successful for transmission housings for helicopters. If

the galvanic corrosion issues can be eliminated by using a more common aluminum

alloy than high purity aluminum, this will be the most successful combination for

aerospace applications.

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, further testing of cold sprayed aluminum alloys on different

magnesium alloys to demonstrate galvanic corrosion resistance of those alloys with the

magnesium part and would allow for added trust in using cold sprayed aluminum alloys

on critical parts resulting in parts with ultimate longer life. Continuing development of

new alloys and new coatings will serve to enhance the ability to use magnesium for more

applications allowing designers the choice of an excellent long lasting light weight metal

for automotive and aerospace applications.

While the future looks bright for alloys and coatings for now designers need to

review in detail their alloy choices assisted by research and review of specific alloys for

the application they need. Appendix B provides that assistance for aerospace and

automotive parts. Designers should also pay close attention to the types of coatings

chosen using the advantages and disadvantages laid out in table 4. Using research

reviewed in this paper will aid a designer in designing a successful part for use in

aerospace or automotive industries.

32

5. References

1. Made-in-China.com

http://www.made-in-china.com/showroom/yuanlongjason/product-

detailIblESqjdSMRB/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. Callister Jr., William D. Materials Science and Engineering An Introduction.

New York: John Wiley & Sons Inc. 2007.

7. Hexagonal Close Packed Structure.

http://www.miniphysics.com/2010/12/hexagonal-close-packed-structure.html

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. DeForce, Brian. Materials Performance: Cold Sprayed Aluminum Coatings for

magnesium aircraft components. Materials Performance, v. 48, pg 40-44. 2009.

14. 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.

15. 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.

16. 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.

33

17. Duffy, Laurence. Magnesium Alloys: The Light choice for Aerospace. Materials

World, v.4, pg 127-130. 1996.

18. Mathaudhu, Suveen. Magnesium technology: Magnesium alloys in U.S. military

applications: Past, Current and future solutions. Magnesium Technology, pg 27-

30. 2010.

19. 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.

20. Wendt, Achim. Magnesium castings in aeronautics applications – Special

requirements. Magnesium technology, pg 269-273. 2005.

21. Guo, Kelvii Wei. A Review of Magnesium/Magnesium Alloys Corrosion and its

Protection. Recent Patents on Corrosion Science, pg 13-21. 2010

22. Hawkins, James H. Assessment of Protective Finishing Systems for Magnesium.

International Magnesium Association, Pg 1-13. 1993.

34

6. Appendices

6.1 Appendix A: Alloying Element Effects

Alloying

Element

Melting and Casting

Behavior

Mechanical and

technological

properties

Corrosion

behavior I/M

produced

Ag Improves elevated

temp tensile and

creep props. In the

presence of rare

earths.

Detrimental

influence on

corrosion behavior

Al Improves castability,

tendency to microporosity

Solid solution

hardener,

precipitation

hardening at low

temps.

Minor influence

Be Significantly reduces

oxidation of melt surface

at very low concentrations,

leads to coarse grains.

Ca Effective grain refining

effect, slight suppression

of oxidation of the molten

metal.

Improves creep

properties.

Detrimental

influence on

corrosion behavior

Cu System with easily

forming metallic glasses,

improves castability.

Detrimental

influence on

corrosion behavior,

limitation

necessary.

Fe Magnesium hardly reacts

with mild steel crucibles

Detrimental

influence on

corrosion behavior,

limitation

necessary.

Li Increases evaporation and

burning behavior, melting

only in protected and

sealed furnaces.

Solid solution

hardener at ambient

temperatures,

reduces density,

enhances ductility.

Decreases corrosion

properties strongly,

coating to protect

from humidity is

necessary.

Mn Control of Fe content by

precipitating Fe-Mn

compound, refinement of

precipitates.

Increase creep

resistance.

Improves corrosion

behavior due to iron

control effect.

Ni System with easily

forming metallic glasses.

Detrimental

influence on

35

corrosion behavior,

limitation

necessary.

Rare Earth Improve castability, reduce

microporosity.

Solid solution and

precipitation

hardening at

ambient and

elevated temps;

improve elevated

temp tensile and

creep properties.

Improve corrosion

behavior.

Si Decreases castability,

forms stable silicide

compounds with many

other alloying elements,

compatible with Al, Zn,

and Ag, weak grain

refiner.

Improves creep

properties.

Detrimental

Influence.

Th Suppresses microporosity. Improves elevated

temp tensile and

creep properties,

improves ductility,

most efficient

alloying element.

Y Grain refining element Improves elevated

temp tensile and

creep properties.

Improves corrosion

behavior.

Zn Increases fluidity of the

melt, weak grain refiner,

tendency to microscopy.

Precipitation

hardening, improves

strength at ambient

temps, tendency to

brittleness and hot

shortness unless Zr

refined.

Minor influence,

sufficient Zn

content

compensates for the

detrimental effect of

Cu.

Zr Most effective grain

refiner, incompatible with

Si, Al, and Mn, removes

Fe, Al, and Si from the

melt.

Improves ambient

temperature tensile

properties slightly.

Table 5: General effects of elements used in magnesium alloys2

36

6.2 Appendix B: Magnesium Alloy Applications

Alloy Application Notes

AZ91 Cast Helicopter Transmission

Housings

Less costly, good tensile

strength, very susceptible

to creep above 120ºC

AM60/AM50/AM20 Automotive (wheels, seat

frames, steering wheels)

Greater ductility and

fracture toughness.

AS41 Automotive (crank case,

transmission housing)

Easier than AS21 to cast

with better fluidity

QE22 Aerospace (landing wheels,

gearbox housings, helicopter

rotor fittings)

Superior tensile

properties, Expensive due

to silver

Table 6: Proposed alloys for specific applications