EVALUATING CHARACTERISTICS OF ALUMINUM COLD SPRAY AS A FIELD REPAIR FOR MAGNESIUM TRANSMISSION

HOUSINGS

Sid P. Raje

sraje@textron.com

Textron LDP:E (Leadership Development Program: Engineering)

Bell Helicopter Textron Inc. Fort Worth, TX, USA

Abstract

This paper investigates characteristics of aluminum cold spray on a ZE41A magnesium substrate. A corrosion study was

conducted which showed the galvanic tendency of cold spray on a magnesium substrate. This study was also used to deter-

mine the optimal coating for a cold spray repair. Chrome-manganese performed the best of the coatings tested. Tensile testing

of cold spray on a magnesium substrate was conducted and the results were compared to baseline magnesium data. Fatigue

testing was also conducted to produce an S-N curve, which showed that cold spray on magnesium substrate was comparable

to baseline magnesium. Based on the test results described in this paper, design recommendations are made, and a path to

production is outlined.

1

1. Introduction1

Cold spray is a coating deposition process used in many ap-

plications in the defense, aerospace, as well as the commer-

cial industry. It uses a carrier gas stream moving at a super-

sonic velocity in order to deposit a given material onto a

substrate. The particles impact the substrate at a high veloci-

ty, resulting in uniform bonding. This process occurs at a

temperature that is below the melting point of the coating

material, hence the name cold spray. This allows for coating

of a broad range of substrate materials such as composites as

well as thin walls (Ref. 4). The deposited coating has a high

density and low porosity properties. Ductile metallic pow-

ders, such as aluminum, zinc, titanium, nickel, carbide-

cobalt, and tungsten-copper are all examples of coating ma-

terials (Ref. 1). The particle sizes for these powders range

from 1 to 50 µm. With the use of proper spray parameters,

near theoretical densities with virtually no inclusions are

attainable with the cold spray process. The properties and

microstructure of the initial powder particles are retained.

This allows for manufacture of parts of various sizes and

shapes as well as the ability of the spray to be machine fin-

ished (Ref. 2). Additionally, because of the localized deposi-

tion properties, the need for masking is eliminated (Ref. 4).

The use of magnesium to construct transmission housings

during the Vietnam era was common in an effort to reduce

weight and increase aircraft performance. Magnesium is

35% lighter than aluminum and offers increased stiffness

and damping characteristics (Ref 3). The biggest disad-

vantage to using magnesium in such an application is its

1Presented at the American Helicopter Society 69th Annual

Forum, Phoenix, Arizona, May 21–23, 2013. Copy-

right © 2013 by the American Helicopter Society Interna-

tional, Inc. All rights reserved.

susceptibility to corrosion and pitting. Aluminum cold spray

has been proposed as a method to repair corrosion/pitting

damage on rotorcraft transmission housings made out of

magnesium.

The application of cold spray described in this paper was

conducted at Army Research Laboratories using a 6061

aluminum powder and a helium carrier gas.

2. Corrosion Testing

A corrosion study was conducted in order to evaluate gal-

vanic corrosion characteristics of 6061 aluminum cold spray

on a ZE41A magnesium substrate. A groove was machined

out of each magnesium test coupon and a base coating of

high purity aluminum was cold sprayed on half of the cou-

pons. Then, in order to simulate a gearbox, a protective coat-

ing of either brush Tagnite or chrome-manganese was ap-

plied. A test matrix of the test coupons and corresponding

coatings can be seen in Table 2.1. These test coupons were

subject to atmospheric testing in a salt spray and humidity

cycle seen in Fig. 2.1.

Figures 2.2 and 2.3 show the test coupons before and after

the atmospheric cycle, which was successful in building the

desired amount of corrosion on the test coupons. Figures 2.4,

and 2.5 show a typical cross section of the test coupons after

the atmospheric cycle. In all the cases, the magnesium sub-

strate was observed to show more pitting due to corrosion

than the aluminum cold spray, as expected. However,

chrome-manganese acted as a better corrosion protecting

coating on the magnesium substrate than the brush Tagnite.

Chrome-manganese was also observed to be a better corro-

sion protecting coating at the magnesium/aluminum inter-

face than brush Tagnite.

The chrome-manganese coating did not adhere to the alumi-

num cold spray during application. In order to keep the cou-

pons consistent, the Tagnite coating was removed from the

aluminum cold spray. During this process step, the T

coating may have been unintentionally re

aluminum/magnesium interface. Consequen

a source of error in the results observed at the alum

num/magnesium interface.

Test coupons where the magnesium substrate was coated in

high purity aluminum showed similar corrosion patterns

with coupons where the substrate was not coated.

it was concluded that base coating the substrate with high

purity aluminum does not have a significant impact on the

corrosion properties.

Table 2.1

Coupon Number

1

2

3

4

5

6

pons consistent, the Tagnite coating was removed from the

aluminum cold spray. During this process step, the Tagnite

emoved from the

quently, this may be

a source of error in the results observed at the alumi-

Test coupons where the magnesium substrate was coated in

high purity aluminum showed similar corrosion patterns

with coupons where the substrate was not coated. Therefore,

it was concluded that base coating the substrate with high

purity aluminum does not have a significant impact on the

Further corrosion studies must be conducted in order to d

termine the best coating to prevent atmospheric and galvanic

corrosion. The primary requirement for this coating must be

that it needs to adhere to both magnesium and aluminum so

that experimental error due to adhesion, masking, or removal

of coating is reduced. This requirement will also ensure that

galvanic corrosion will not occur at the magnesium

aluminum interface due to exposure to the atmosphere.

Table 2.1 – Test Coupon Matrix

Base Coating Protective Coating

None Brush Tagnite

None Chrome Manganese

None Brush Tagnite

None Chrome Manganese

HP Aluminum Brush Tagnite

HP Aluminum Chrome Manganese

HP Aluminum Brush Tagnite

HP Aluminum Chrome Manganese

HP Aluminum Brush Tagnite

HP Aluminum Chrome Manganese

None Brush Tagnite

None Chrome Manganese

Figure 2.1 – Humidity Cycle

Further corrosion studies must be conducted in order to de-

coating to prevent atmospheric and galvanic

corrosion. The primary requirement for this coating must be

that it needs to adhere to both magnesium and aluminum so

that experimental error due to adhesion, masking, or removal

irement will also ensure that

galvanic corrosion will not occur at the magnesium-

aluminum interface due to exposure to the atmosphere.

Figure 2.2 – Corrosion Coupons before Atmospheric Testing

Figure 2.3 – Corrosion Coupons after Atmospheric Testing

Figure 2.4 – Typical Cross Section (HP Aluminum Base Coating) (Coupon #4)

Figure 2.5 – Typical Cross Section (No Base Coating) (Coupon # 1)

Typical Cross Section (HP Aluminum Base Coating) (Coupon #4)

Typical Cross Section (No Base Coating) (Coupon # 1)

Typical Cross Section (HP Aluminum Base Coating) (Coupon #4)

Typical Cross Section (No Base Coating) (Coupon # 1)

3. Tensile Testing

Tensile testing was conducted per ASTM-E-8. The test spec-

imen used for tensile testing can be seen in Fig. 3.1. A

groove was machined out of the specimen and cold sprayed.

The specimen was then machined down to its final dimen-

sions, Fig. 3.2. The maximum ratio of cold spray to magne-

sium was 1:1 at the center cross section of the test specimen.

Non-cold sprayed tensile specimens were also machined

using ZE41A magnesium in order to collect baseline data.

A summary of the results of tensile testing can be seen in

Table 3.1. The results show the mean, min, and max yield

stresses in the cold sprayed specimens were 6%, 5%, and

11% higher respectively than the baseline magnesium data.

This exhibits the integrity of the bond strength between cold

spray and magnesium. However, the mean, min, and max

ultimate stresses in the cold sprayed specimens were 7.5%,

32%, and 2% lower respectively than those in the baseline

magnesium specimens. The difference between mean yield

Figure 3.2 – Post-Cold Spray Tensile Specimen

Figure 3.1 – Pre-cold spray Tensile Specimen

Table 3.1 – Summary of Results

Cold Spray Samples (T16-T30) Yield Stress @ 0.02% (KSI) Ultimate Stress (KSI) TE Manual %

Mean 20.9 29.2 2.6

Standard Deviation 0.7 3.1 2.0

Min 20.0 20.1 0.6

Max 22.7 32.0 8.8

Median 20.9 29.8 2.4

Magnesium ZE-41A Baseline

(T1-T12) Yield Stress @ 0.02% (KSI) Ultimate Stress (KSI) TE Manual %

Mean 19.7 31.6 4.3

Standard Deviation 0.4 0.9 0.8

Min 19.0 29.6 3.2

Max 20.4 32.7 5.8

Median 19.75 31.5 4.07

and ultimate failure points was 8.3 ksi in the cold sprayed

specimens and 11.9 ksi in the baseline magnesium speci-

mens. A contributing factor to this may be the inherent non-

uniformity in materials in the cold sprayed specimens. Fail-

ure modes observed in the cold sprayed specimens consisted

of a variation between a straight fracture plane as well as a

concave/convex fracture plane. This is shown in Fig. 3.3.

The concave/convex fracture plane is observed when the

failure occurs at the bond line. Some failures were observed

at the magnesium substrate. Bond integrity of cold spray is

further reinforced by these results because the cold spray-

magnesium bond strength was comparable to the magnesi-

um-magnesium bond strength. The tensile test results for all

the specimens are included in Appendix A.

Future studies involving tensile testing at elevated tempera-

tures are recommended. The effect of temperature on tensile

strength are required in order to prevent the cracking in the

transmission case in when using cold spray repairs on a

magnesium gearbox, especially during a loss of lubrication

event (temperatures up to 400°F) because the propagation of

cracks in the transmission housing may result in separation

of gear meshes.

4. Fatigue Testing

Fatigue testing was performed on cold sprayed ZE41A mag-

nesium coupons per ASTM E-466. The pre cold spray and

post cold spray test specimen geometry is shown in Fig. 4.1

and Fig. 4.2 respectively. Testing was conducted at a mean

stress of 0 ksi and 5 ksi. For both mean stresses, testing was

conducted at room temperature (75°F) and elevated tempera-

ture (250°F). The results of fatigue testing were used to cre-

ate an S-N curve, shown in Fig. 4.3.

As expected, the specimens that were tested under a 5 ksi

mean stress had a lower fatigue life than those tested at 0 ksi.

Similarly, specimens tested at elevated temperatures had a

lower fatigue life than those tested at room temperature. The

SN curve generated from this test data was compared to leg-

acy ZE41A magnesium data. Cold sprayed coupons were

observed to have a similar fatigue life trend to pure magne-

sium coupons. However, the limited set of legacy data points

from baseline magnesium showed that magnesium was able

to withstand higher fatigue stresses than the tested cold spray

specimens. Additional testing and data points are required

for ZE41A magnesium to reinforce this trend. The inherent

non-homogeneity in the material of the cold spray specimen

may be a contributing factor to the difference observed be-

tween baseline magnesium data and cold spray data. The

failure modes that were observed consisted of: fracture at the

shank area outside of the cold spray section (pure magnesi-

um inside the plane of failure, Fig. 4.4), fracture at the shank

area inside the cold spray section (Fig. 4.5), and fracture at

the grip (area where fatigue testing apparatus was clamped

to specimen). The full test results are shown in Appendix B.

Of the specimens tested, 1 specimen (F-8) failed in the cold

sprayed area, shown in Fig. 4.4.

Figure 3.3 – Failure Modes of Tensile Specimen

Figure 4.1 – Pre Cold Spray Fatigue Specimen (Shank)

Figure 4.2 – Post Cold Spray Fatigue Specimen (Shank)

Figure 4.3 – S-N Curve, Cold Spray

Figure 4.5 – Fatigue fracture in shank (cold sprayed ar-

ea) (Specimen F-8)

Figure 4.4 – Fatigue fracture in shank (magnesium)

(Specimen F-15)

Figure 4.6 – Disbond fracture plane

Figure 4.5 – Failure Origin (Specimen F

Figure 4.7 –

Specimen F-8 was tested at a mean stress of 0 ksi at room

temperature (75 °F). Figure 4.5 shows the origin of the fra

ture. The fracture originated at a subsurface location in the

magnesium substrate. The aluminum cold spray sho

more favorable characteristics during testing than the ZE41A

magnesium. The location of the fracture origin may be due

to the geometry of the fatigue specimen. A bond line failure

(concave/convex fracture plane) was only observed in f

tigue specimen F-8 where the failure occurred within the

cold sprayed area, Fig. 4.6.

A next logical step in evaluating fatigue characteristics is to

conduct flexure fatigue testing. The intent of this testing is to

simulate an asymmetric transmission housing repair. The

shank specimen is axially symmetric and therefore the su

strate/cold spray interface is loaded uniformly. An asymme

ric repair area in the flexure specimen will induce non

uniform stresses on the substrate/cold spray interface and

provide fatigue life predictions for a configuration that is

more representative of a field repair. The post cold spray

flexure fatigue test specimen can be seen in

5. Path to Production

By using probability, material properties, and geometry, the

fatigue life of a part can be determined analytically (Ref. 5).

The fatigue life of the shank specimen was determined an

lytically and plotted on an S-N curve. In order to validate

this data, the analytical and actual S

for comparison. For further validation,

flexure fatigue testing, analytical and actual fatigue S

curves will be overlaid for comparison.

Disbond fracture plane

Failure Origin (Specimen F-8)

– Post Cold Spray Fatigue Specimen (Flexure)

8 was tested at a mean stress of 0 ksi at room

temperature (75 °F). Figure 4.5 shows the origin of the frac-

ture. The fracture originated at a subsurface location in the

magnesium substrate. The aluminum cold spray showed

more favorable characteristics during testing than the ZE41A

magnesium. The location of the fracture origin may be due

to the geometry of the fatigue specimen. A bond line failure

(concave/convex fracture plane) was only observed in fa-

8 where the failure occurred within the

A next logical step in evaluating fatigue characteristics is to

conduct flexure fatigue testing. The intent of this testing is to

simulate an asymmetric transmission housing repair. The

shank specimen is axially symmetric and therefore the sub-

strate/cold spray interface is loaded uniformly. An asymmet-

ric repair area in the flexure specimen will induce non-

uniform stresses on the substrate/cold spray interface and

ictions for a configuration that is

more representative of a field repair. The post cold spray

flexure fatigue test specimen can be seen in Fig. 4.7.

By using probability, material properties, and geometry, the

be determined analytically (Ref. 5).

The fatigue life of the shank specimen was determined ana-

N curve. In order to validate

this data, the analytical and actual S-N curves were overlaid

for comparison. For further validation, upon completion of

flexure fatigue testing, analytical and actual fatigue S-N

curves will be overlaid for comparison.

A cold spray repair process, if relied on for structural integri-

ty and not just dimensional restoration, requires a successful

bench test before it can be incorporated into the field. With

the uniqueness of each potential cold spray application, and

the high cost of bench testing, the analytical fatigue life of a

cold sprayed magnesium gearbox is highly desirable in order

to assess different failure modes and durability. Contingent

on validation of analytical results based on bench test data

and qualification, feasibility of cold spray as a magnesium

repair will be determined.

In parallel to fatigue testing, corrosion studies will be run in

order to determine the optimal corrosion preventing coating

for aluminum cold spray on a magnesium substrate.

Acknowledgements

This project was funded by the Vertical Lift Consortium,

formerly the Center for Rotorcraft Innovation and the Na-

tional Rotorcraft Technology Center (NRTC), U.S. Army

Aviation and Missile Research, Development and Engineer-

ing Center (AMRDEC) under Technology Investment

Agreement W911W6-06-2-0002, entitled National Ro-

torcraft Technology Center Research Program. The author

would like to acknowledge that this research and develop-

ment was accomplished with the support and guidance of the

NRTC and VLC. The views and conclusions contained in

this document are those of the authors and should not be

interpreted as representing the official policies, either ex-

pressed or implied, of the AMRDEC or the U.S. Govern-

ment. The U.S. Government is authorized to reproduce and

distribute re-prints for Government purposes notwithstand-

ing any copyright notation thereon.

The author would like to thank Eric Sinusas, Walt Riley,

Ron Gill, Bob Lee, Cory Posvic, and the staff at Bell Heli-

copter’s Metallic Materials Laboratory, and Field Investiga-

tions Laboratory for their continued support and input. Grati-

tude is also extended to Victor Champagne (ARL), Brian

Gabriel (ARL), and VEXTEC Corporation for supporting

this effort.

References

1. Wilmot, Damien, Howe, Christopher D., Todorovic,

Ranko, Hoiland, Benjamin, “Low Pressure Cold Spray Con-

ductive Coating – A Case Study,” AMMTIAC Quarterly

Vol. 5 No. 1.

2. Karthikeyan, J. “Cold Spray Technology: International

Status and USA Efforts,” ASB Industries, 2004

3. Champagne, Victor, “The Repair of Magnesium Ro-

torcraft Components by Cold Spray,” Society for Machinery

Failure Prevention Technology, 2007.

4. Gabriel, Brian M., Champagne, Victor K., Leyman Phil-

lip F., Helfritch, Dennis J., “Cold Spray for Repair of Mag-

nesium Components,” ARL, July 2011.

5. VEXTEC, “Virtual Twin Explained,” <

http://www.vextec.com/our-products/virtual-twin-

explained>, 2011

Appendix A – Tensile Test Results

Appendix B – Fatigue Test Results

Appendix B – Fatigue Test Results cont’d