KEYWORDS: Aerospace grade Al-Alloys (Al2024, Al6061, and Al7075), Deposition thickness, Adhesion strength, FEM simulations, gas atomized powder heat treatment, cold spray, and Microstructural studies.
Aluminium alloys possess high specific strength, corrosion resistance, and finishability. Age-hardenable wrought Al-alloy components are used in the automobile and aerospace sectors. These components undergo wear and tear during service, and their replacement involves cost. Cold spray deposition of aerospace grade as a repair technique for Al-alloy components (Al2024, Al6061, and Al7075) could lead to cost saving in the industry.
Among the various thermal spray techniques, the cold spray coating deposition technique is involves feedstock powders in solid state avoiding oxidation and phase transformation. Cold spray deposition involves the acceleration of feedstock powder particles using a convergent-divergent (De-Laval type) nozzle. In general, the deposition of millimeter-thick coatings using feedstock powders was easily attained at high stagnation temperatures (400, 500, and 600 °C) by cold spray. A novel high density graphite nozzle was employed to deposit pure aluminium (Cp-Al) and age-hardenable aluminium alloy (Al2024, Al6061, and Al7075) powders with stagnation temperatures of 400, 500, and 600 °C and pressure of 20 bar without any clogging issues. Deposition of the alloys at various process gas temperatures was studied using Finite Element Modeling (ANSYS® and ABAQUS®) and correlated with the thickness of the coatings that were deposited experimentally. Based on FEM simulations, one can state that Cp-Al and Al6061 powders are relatively easy to deposit while deposition of Al2024 and Al7075 is relatively difficult irrespective of process gas temperatures. The deposition behavior is also studied based on lattice misfit and solid solution strengthening calculations. In the present study, Al-aerospace alloys were experimentally deposited by cold spray and the ease of deposition is determined to be Al6061 > Cp-Al > Al7075 > Al2024. Also, an increase in the stagnation temperature has resulted in an increase in coating thickness for Al-alloys.
The deposition of Al6061 alloy powders was found to be superior among aerospace Al-alloys. In this work, Al6061 alloy powders are deposited at a stagnation temperature and pressure of 400 °C and 20 bar respectively using air as a main process gas. The precipitation behavior of the cold sprayed Al6061 coatings is studied upon aging of the atomized-coating (as-deposited), and atomized-coating-solutionized conditions. The fine precipitates (needle-shaped β” (Mg5Si6)) result in the high hardness of atomized-coating-solutionized samples. There is an improvement in bonding for the direct aging of coatings (atomized-coating) and a significant improvement in bonding upon aging of atomized-coating-solutionized samples.
In addition, gas-atomized Al7075 alloy powders are heat treated (solutionized and aged condition) to improve strengthening from precipitation along with plastic deformability during cold spray deposition. Precipitation kinetics of heat treated powders (solutionized, and aged) and gas-atomized Al7075 alloy coatings are studied using SAXS and compared with TEM. The studies revealed that gas-atomized powders contain dendritic cell structures with solutes segregated at the dendritic cell boundaries. Such segregation of solute will hinder precipitation during direct aging. Solutionizing of Al-alloy powders removes the dendritic cell structure and continuous solute segregation. SAXS results confirm that an increase in atomized-powder solutionization time (0.5 h, 1 h, 2 h, and 4 h) did not affect the quenched precipitates (GP-zone) size. Also, aged-powder exhibits poly-dispersity in size distribution with a lower volume fraction of precipitates.
The deposition of gas-atomized (as-received), solutionized, and aged powders was performed at 20 bar and 500 °C. The aging behavior was studied for about 100 hours. The aging behavior of Al7075 alloy deposits (atomized-coating, solutionized-coating, aged-coating, atomized-coating-solutionized) is studied using hardness testing, FE-SEM, and TEM. Also, improved deposition thickness (110%) of the coatings is attained by employing solutionized Al7075 alloy powder as a feedstock. Further upon direct aging of solutionized-coating (solutionized powder + cold spray) has resulted in accelerated precipitation in eight hours (fine needle precipitates: 29 ± 8 in length, and 2 ± 1 nm in diameter).