1. Aluminum Coatings Applied by the Cold Spray Process for the Repair of Marine Grade Aluminum
Sponsor: Dr. Airan Perez, ONR 333
Primary Investigator: Ben Bouffard, NSWCCD Code 614
Distribution A – Approved for Public Release

2. Outline Project Purpose Sensitization Powder Down-Selection
Al-Al2O3 Coating Analysis
Mechanical Properties of Coating
Summary

3. Project Purpose AA5456 in fleet is prone to sensitization and cracking
Talks with NAVSEA and TWH’s reveal that ships often crack, but immediate permanent repair is not possible
Need temporary solution that will ‘keep the water out’ until next scheduled availability
Currently use composite patch, which is not ideal:
Needs to be implemented by highly skilled workforce
Experiences adhesion problems in extreme UV environments
Fire spread issues
Cold Spray has well documented success in the DOD for dimensional repair of aluminum and magnesium components
This project aims to repair cracks through material

4. Sensitization AA5xxx is supersaturated with Mg
At temperatures above 65ºC Mg migrates out of solution and forms beta phase (Al2Mg3) along grain boundaries
Beta phase is anodic to substrate, can lead to IGSCC

5. Powder Down-Selection
Initially 3 powders procured for testing:
Commercially pure Al
Aluminum-Alumina mixture
AA5456
Low pressure cold spray system being used has maximum temperature of 400 ˚C and pressure of 175 psi.
Commercially pure Al and AA5456 have not deposited well with these parameters
Aluminum-Alumina coating deposits extremely well-will be focus of this presentation

6. Al-Al2O3 Coating Analysis

7. Sensitized vs Unsensitized Substrate
Pressure: psi
Temperature: ˚C
Standoff Distance: mm
Gun Transverse Speed: mm/sec
Gas Feed Rate: % of max
Thickness: inches
Al-Al2O3 Coating
Al2O3 Particle
Both interfaces show:
Large Al2O3 particles within coating and embedded into substrate
Coating particles result in a large deformation front (~20 um) within the substrate.
Unsensitized Substrate
Al-Al2O3 Coating
Both coatings show:
CP-Al particles seem to deform around Al2O3 particles
Even distribution of Al and Al2O3
Low porosity observed within coating
Al2O3 Particle
Sensitized Substrate

8. EDS Results Majority of coating is comprised of aluminum & oxygen
Substrate
Al, O, Ti
Coating
Majority of coating is comprised of aluminum & oxygen
Argon is present (used to atomize powder)
Titanium is present in coating
-Separate EDS analysis showed titanium present in original powder

9. Hardness
sensitized
unsensitized
Sprayed Material
Base Material

10. EBSD Analysis
Alumina shows little deformation or change in general powder morphology
Trend of large grains in center of particle and smaller grains along outside of particle
Smaller interparticle boundaries are created during the severe strain localization resultant of spray process, which suggests this results in localized dynamic recrystallization
CP-Al Particles
Al2O3 Particle
Kang, K.; Wang, J; Bae, G; Ha, S; Lee, C. Interfacial bonding and microstructural evolution of Al in kinetic spraying. Journal of materials Science 47 (2012)
CP-Al Particles
Al2O3 Particle

11. TEM of Single Particle Splat
Image taken using selected area aperture to highlight dislocations
Triple point intersection of grains
Many dislocations visible, expect to see larger dislocation density along edge of particle

12. Potentiodynamic Curves
High Epitt Potential
Large Passivation Region
Large difference in OCP
Shows need for further galvanic study

13. Salt Fog Testing
1500 hours of B117 salt fog testing complete on Al-Al2O3 and AA5456 coatings
AA5456 is showing pitting in coating, no damage around scribe
Al-Al2O3 shows no pitting or corrosion in coating and no damage around scribe
Al-Al2O3 Coating over unsensitized Al
Al-Al2O3 Coating over Sensitized Al
AA5456 Coating over Sensitized Al
Representative original photos of select B117 panels

14. Salt Fog Testing
Scribe panels show overall corrosion resistance, but in practice cold spray repair will be small area compared to substrate
Made mock crack repair plates to test in ASTM B117
Use 0.25” and 0.5” width mock crack repairs in 4”x6” plates
11/16/2018

15. Salt Fog Testing Initial 50 hour 350 hour Al2O3 Coating Al2O3-AA5456
Couple
U-50-A-half-1 for fill, S-50-A-H-1 for just cold spray
11/16/2018

16. Salt Fog Testing
Minor pitting in Cold Spray material initiated at 350 hours (Pit diameter of about 10 microns)
Pit density of cold spray is more severe than that observed in AA5xxx
Pitting in 5xxx coalesces as intergranular corrosion begins
No accelerated pitting in Cold Spray due to couple with AA5456
600 hour
950 hour
Al2O3 Coating
Al2O3-AA5456
Couple
11/16/2018

17. Al-Al2O3 Mechanical Properties
11/16/2018

18. Al2O3 Adhesion Properties
ASTM C633 Adhesion CPAl-Al2O3
Specimen
Avg Adhesion Value (ksi)
Std Dev
Majority of Failure Modes
Sensitized Handheld
8.06
0.98
Adhesive
Sensitized Robot
7.89
2.28
Unsensitized Handheld
7.83
n/a
Unsensitized Robot
4.60
3.24
Coating
11/16/2018

19. Tensile Tests-All Cold Spray
Tensile specimens machined out of bulk build
Initial gauge diameters ~0.25 in
Specimens tested in both transverse and longitudinal directions
Transverse
Longitudinal
11/16/2018

20. Tensile Tests-All Cold Spray
Specimen Test Direction
Average UTS (ksi)
st dev
Transverse
10.37
1.28
Longitudinal
7.21
1.45
Specimens tested in transverse spray direction show higher UTS than those tested in longitudinal direction
Speed of robot important to consider, specimens tested with traversing gun speed of 20 mm/s showed UTS half the value of specimens tested with gun speed of 10 mm/s
Important consideration for handheld gun application
11/16/2018

21. Fractography of Tensile Specimens-All Cold Spray
Combination of intergranular and transgranular cracking
Primarily cleavage fracture, representative of brittle failure
No preferential fracture at alumina particles
No failures at interpass regions
Cleavage
Alumina
Flake point
11/16/2018

22. Tensile Tests– Cold Spray Repair
Repaired different mock crack widths varying from 0.1” to 0.5”
Crack widths similar to nozzle width could not be filled due to jet stream of process building over cavity (Takana 2010)
11/16/2018

23. Tensile Tests– Cold Spray Repair
Fracture consists of both transgranular and intergranular cracking
No preferential fracture at root or AA5456-Al2O3 coating interface
Fracture cohesive through cold spray repair
11/16/2018

24. Summary
Successful deposition of Al-Al2O3 using low pressure and air to deposit onto sensitized and unsensitized AA5456
Metallurgical analysis of Al-Al2O3 shows dense coating with low porosity and oxide formation, and similar deposition and interface between coating and sensitized and non-sensitized samples.
Samples show an increased hardness at the interface and severe particle deformation within the coating
Satisfactorily high (~8-10 ksi) adhesion and UTS values
Tensile fracture surfaces show combination of intergranular and transgranular fracture representative of brittle failure

25. Acknowledgements Dr. Airan Perez Dr. Jennifer Wolk Caroline Scheck
NISE 219 Funding under Dr. Jack Templeton
Fred Lancaster
Joseph Christophersen
Wesley Monillas

26. Back Up Slides
File: NAVAIR Brief