Thermal Spray 2012: Proceedings from the International Thermal Spray Conference and Exposition May 21 24, 2012, Houston, Texas, USA R.S. Lima, A. Agarwal, M.M. Hyland, Y.-C. Lau, C.-J. Li, A. McDonald, F.-L. Toma, editors Copyright 2012 ASM International All rights reserved www.asminternational.org Deposition of Amorphous Aluminium Powder Using Cold Spray P.K. Koh,* P. Cheang School of Science & Technology, SIM University, Singapore *E-mail: pkkoh@unisim.edu.sg K. Loke Singapore Technologies Kinetics Ltd, Singapore S.C.M. Yu School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore S.M. Ang Industrial Research Institute Swinburne, Swinburne University of Technology, Melbourne, Australia Abstract Deposition of amorphous aluminium powder using cold spray technology as a corrosion prevention measure was studied. Amorphous aluminium (Al-Ni-Ce) powder was successfully deposited on 7000-series aluminium substrates using cold spray parameters of 1.7 MPa under compressed air and temperature of 450 o C. The coatings were subjected to tensile bond strength measurement and comparative studies with cold sprayed pure Al6061 coatings were conducted. The results obtained showed that the amorphous aluminium coatings exhibited better adhesive strength. In addition, salt-water immersion test was conducted. The Al-Ni-Ce coating not only demonstrated better corrosion resistance but also exhibited evidence of passivation of surface imperfections such as scratches in the coatings. Introduction Aluminium and its alloys typically undergo a process of oxidation with atmospheric oxygen, which leads to the formation of a layer of aluminium oxide, typically in the order of a few nano-meters thick. This layer acts as a protective coating to protect the underlying aluminium metal from further oxidation. However, aluminium alloys have reduced corrosion resistance due to the addition of the additives that helped to make them stronger in the first place. These additives will lead to localised galvanic reactions, for example the reaction between aluminium and copper, which can cause rapid corrosion in the right environments. Furthermore, these alloying elements can increase the amount of defects found in the aluminium, such as grain boundaries, inclusions and dislocations. These inhomogeneities are points of weaknesses where pitting corrosion tends to initiate and weaken the oxide layer on the surfaces of the alloys. In typical operating environments, aluminium and its alloys tend to be exposed to harsh elements, such as thermal expansion-contraction from day-night temperatures fluctuations, corrosive liquids and surface wear. In these environments, the regenerative surface protective oxide layer diminishes with prolonged exposure. Consequently, any exposed grain boundaries and defects will be extremely susceptible to rapid pitting and intergranular corrosion. Common methods to protect aluminium and its alloys are Alclading, chromate conversion coatings and anodising. But these methods do not have high resistance to prolonged harsh environments such as high-wear applications, and also do not have the capability to provide corrosion inhibition. Amorphous metals are metals made using a combination of customised processing methods together with special alloy compositional blends to produce a unique microstructure that is characterised by a lack of defects such as triple-point grain boundaries, inclusions and dislocations. Research suggested that amorphous aluminium metals can act as sacrificial anodes and provide protection against halideinduced pitting (Ref 1). There is also previous research which has demonstrated the use of cold spray process to produce amorphous aluminium coatings (Ref 2). There are other methods which can produce amorphous aluminium coatings, such as plasma spray or HVOF (Ref 3). But these methods will require the use of thermal energy to soften or melt the powder particles prior to the formation of the coating. The rapid cooling of the particles upon deposition onto the cooler substrate surface will assist in the retention of 249
the amorphous phase. However, crystallisation of the amorphous phase can occur with successive spray coating passes which leads to a localised temperature increase (Ref 4). Cold Spray is a process which can produce coatings at temperatures very much lower than the melting temperatures of the raw materials. This ensures that the input powder materials do not experience any grain growth, oxidation or phase changes when formed into coatings. This ability of cold spray has already been proven by research which showed that the process can produce coatings retaining nano-sized and sub-micron particles (Ref 5). It is this property of cold spray which makes it attractive as a method to deposit amorphous metal coatings while retaining their own unique material properties. In view of the characteristics of the cold spray process as well as the attributes of the amorphous aluminium metals, the potential of cold sprayed Al-Ni-Ce as a corrosion prevention coating will be explored. The present study will focus on the mechanical properties of cold sprayed amorphous aluminium coatings, such as the hardness, tensile bond strength and microstructural features as well as their behaviour when subjected to salt-water immersion test to evaluate their corrosion resistance capability. Experimental Procedure Cold Spray Powder The Al-Ni-Ce powder was prepared by gas atomization. The scanning electron microscope (SEM) images of the Al-Ni-Ce feedstock powder are shown in Figure 1. The SEM micrographs indicate that the particles are spherical in shape and range in size from 5 µm to 50 µm. (b) Figure 1: SEM images of the Al-Ni-Ce feedstock powder at (a) 500X and (b) 1000X. Cold Spray Parameters The cold spray system was obtained from the Russian Academy of Science, Siberia and co-developed with Singapore Technologies Kinetics Limited. The spray gun with a de Laval circular cross-sectional nozzle was mounted on an ABB Robot (IRB2400). 7000-series aluminium alloy substrates were cleaned with acetone prior to the spraying. Compressed air was used as the accelerating and carrier gas operating at pressures of 17 bars in the pre-chamber. The gas was pre-heated to a temperature of 450 C. The standoff distance from the spray gun exit was 15 mm. The traverse speed of the gun relative to the substrate was 60 mm/s. Coating Evaluation Microhardness measurements were made using a Matsuzawa MXT70 tester. The samples were polished and indentation measurements were performed using a 100 g load (HV 100g ) and a dwell time of 15 seconds. The microhardness results are the average of 5 measurements. The microstructure of the amorphous aluminium coating was analysed using a field emission scanning electron microscope (JEOL JSM-6700F). The samples were sectioned and prepared following standard metallographic techniques. X-ray diffraction (XRD) measurements were carried out using the Shimadzu LabX XRD-6000 generator diffractometer with a Cu Kα operated at 40 kv, 30 ma radiation at 2-theta step increments of 0.02 deg, count rate of 2 deg/min, fixed divergence angle of 1 deg, receiving slit width of 0.3 mm. (a) Tensile bond strength testing was carried out in accordance to ASTM C 633-01 Standards (Ref 6) using the MTS Sintech 65G Universal Testing Machine. Circular test-studs measuring 25.4 mm in diameter and 38.1 mm in overall length were first grit-blasted prior to the cold spray coating. An average of coating thickness of between 200 µm to 250 µm was obtained for all the test samples. 250
Corrosion Resistance Test The salt-water exposure test was modified from the Standard Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5% Sodium Chloride Solution (Ref 7). The samples were kept in separate set-ups which were sealed with plastic top-covers shown in Figure 2. The samples were put through a cyclical immersion and drying process twice daily (12 hours/cycle). The test lasted for a total of 240 hours or 20 full cycles. ductile nature of the Al-Ni-Ce powder could act as a binder to hold the Al6061 powder together as the composite material impact onto the substrate. Microstructure Evaluation Scanning Electron Microscopy pictures of the Al-Ni-Ce coating at different magnification are shown in Fig. 3. The coating thickness was between 200 and 300 µm. The images demonstrated that there is very low porosity in the coating, consistent with typical coating obtained using a low temperature cold spray process. This result is yet another validation of the ability of the cold spray system to produce high density coating with little or no oxidation. The well defined interface between the coating and substrate also suggested good adhesion of the coating to the base material. Figure 2: The samples were put through a salt-water exposure test using a modified ASTM G44 alternate immersion test. Results and Discussion Mechanical Properties Evaluation Microhardness values of 300 ± 20 HV 100g was obtained for the Al-Ni-Ce coatings. This is in comparison to the 120 ± 30 HV 100g value obtained for conventional aluminium alloy. The increased microhardness of the coating is likely to be the result of a gradual compaction due to the constant and repeated impact of the cold sprayed impinging particles. The lack of porosity in the underlying layers observed by SEM in the deposited material is evidence of the coating coherency. The tensile bond strength of the Al-Ni-Ce coatings was evaluated using the ASTM C633-01 test. The cold sprayed coatings failed at 24 ± 5 MPa, partly in the coating-substrate interface and partly in glue-pull off bar interface. Comparatively, the bond strength results for Al6061 cold sprayed coatings performed under identical standards yielded a lower average adhesion strength of 16 MPa, indicating a significantly stronger tensile bond strength for the Al-Ni-Ce coatings. A third set of coatings, comprising 50% Al-Ni-Ce and 50% Al6061 feedstock material, yielded an average of 21 MPa when subjected to the same test, suggesting that the more (a) (b) Figure 3: SEM mircrographs of a cross-section of the Al-Ni- Ce coating at (a) 200x magnification and (b) 1000x magnification. Figure 4 shows the XRD patterns for the Al-Ni-Ce powders together with 2 separate coatings obtained using both helium and compressed air as the carrier gas respectively. The coatings deposited were about 0.5 mm in thickness. The 251
results show minimal differences between the starting powder and the coatings, implying that no microstructural changes took place during the spraying process. The ability of the cold spray process to retain the amorphous nature of the coating was verified with the results obtained. Figure 5b: Salt-water exposure test of Al-Ni-Ce coated aluminium alloy. Figure 4: XRD patterns for Al-Ni-Ce powder and coatings produced using Helium gas and compressed air as carrier gases. Salt-Water Exposure Test The uncoated 7000-series aluminium alloy was found to be extensively corroded with severe pitting detected on almost all the exposed surfaces (Figure 5a) whereas the Al-Ni-Ce coating was found to be free of corrosion pitting (Figure 5b). A cross sectional view of the surface of the Al-Ni-Ce did not reveal pitting or significant corrosion in the exposed base material. This suggests that the coatings may have passivating properties that promote active corrosion prevention of scratched or cut surfaces. This finding is significant as coatings used in industry applications are constantly subjected to environmental and mechanical damages. More tests will be conducted to confirm the anodic protection ability of the Al- Ni-Ce coating as well as its effectiveness in protecting against stress corrosion cracking. Figure 5a:. Salt-water exposure test of 7000-series aluminium alloy. Figure 5c: Comparative salt-water exposure test results of uncoated and Al-Ni-Ce cold sprayed coated aluminium alloy. Conclusion Al-Ni-Ce amorphous aluminium coatings were successfully coated on 7000-series aluminium alloy using cold spray parameters of 17 bars under compressed air and temperature of 450 o C. XRD analysis verified that the cold sprayed coatings were similar to the feedstock powder, validating the ability of the cold spray process to retain the coating s amorphous nature. The coatings obtained were also found to be of higher microhardness than conventional aluminium alloy and demonstrated relatively better adhesion strength compared to pure Al6061 cold sprayed coatings. Preliminary corrosion resistance evaluation in the form of a modified salt-water immersion test also suggested the ability of the Al-Ni-Ce coating to actively prevent corrosion of exposed scratched or cut surfaces. 252
Acknowledgement The authors would like to acknowledge the funding support from SIM University for this project (RF11SST01) as well as the joint research collaboration with ST Kinetics Limited (Singapore). References 1. M.E. Goldman, N. Unlu, G.J. Shifflet, J.R. Scully, Selected Corrosion Properties of a Novel Amorphous Al- Co-Ce Alloy System, Electrochem. Solid State Lett., 2005, 8(2), pb1-b5 2. E. Sansoucy, G.E. Kim, A.L. Moran, B. Jodoin, Mechanical Characteristics of Al-Co-Ce Coatings produced by the Cold Spray Process, J. Thermal Spray Tech, 2007, 6(5-6), p 651-660 3. B. Gauthier, N. Tailleart, S. Eidelman, D. Book, J.R. Scully, Spray Applied Amorphous/Nnaocrystalline Aluminium Alloy Coatings as a replacement for Aluminium Cladding, The Electrochemical Society, 2008, 11(15), p59-74 4. Y. Wu, P. Lin, G. Xie, J. Hu, M. Cao, Formation of Amorphous and Nanocrystalline Phases in High Velocity Oxy-Fuel Thermally-Sprayed afe-cr-si-b-mn Alloy, Materials Science Engineering A, 2006, 430, p34-39 5. A.S.M. Ang, C. Berndt, P. Cheang, Deposition Effects of WC Particle Size on Cold Sprayed WC-Co Coatings, Surface & Coatings Tech, 2011, 205(10), p3260-3267 6. Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings, C 633-01, Annual Book of ASTM Standards, Vol. 02.05 7. Standard Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5% Sodium Chloride Solution, G 44-99, Annual Book of ASTM Standards, Vol. 03.02 253