Localized Surface Modification of High Strength Aluminum Alloys using Cold Spraying and Friction Stir Processing

  • Author / Creator
    Jibran, Wania
  • Dimensional restoration and repair methodologies for cold-worked aluminum (Al) alloys are very limited due to strength degradation that occurs during heat application in the process. For example, friction stir processing (FSP) is a well-documented repair methodology for various alloys, but it causes recrystallization in the microstructure of cold-worked Al alloys, negatively affecting their mechanical properties. This study presents a novel approach for surface repairs of high-strength Al alloys without negatively affecting the material strength. Low-pressure cold gas dynamic spraying (150 psig) was used to fabricate aluminum-alumina (Al-Al2O3) metal matrix composite (MMC) overlays, hereafter referred to as coatings, on aluminum alloy (AA) 5052-H32, a commonly used cold worked Al grade. The coatings were fabricated from custom mechanical powder blends with varying concentrations of the Al and Al2O3 powder, up to a maximum of 75 wt.% Al2O3. The coatings (~2.5 mm thick) were then friction stir processed (FSPed) using a cylindrical tool with rotational and traverse speeds of 1200 RPM and 9 mm/s, respectively. Optical microscopy, scanning electron microscopy (SEM), and image analysis were conducted to quantify the Al2O3 content in the coatings and analyze the difference in microstructure between the as-sprayed and post-FSPed coating samples. Vickers hardness, abrasion and tensile testing were performed on both the as-sprayed and post-FSPed coatings. The Al2O3 content in the metal matrix of the coating increased as the Al2O3 content in the powder blend increased, producing a maximum of 35 wt.% Al2O3 in the coating fabricated from the powder blend containing 75 wt.% Al2O3. FSP increased the uniform distribution of Al2O3 in the coating matrix, reduced the porosity in the coating, improved matrix-reinforcement due to ceramic particles and improved interparticle bonding that favorably impacted the coating performance compared to as-sprayed coatings. The post-FSPed coating with 35 wt.% Al2O3 exhibited a 22% increase in hardness over bulk AA 5052-H32. The increase in hardness was attributed to the decrease in mean free path between the Al2O3 particles following FSP. Post-FSPed coatings also exhibited lower wear rates compared to the as-sprayed coatings. Significant improvement of the tensile properties was observed after FSP, with the 35 wt.% Al2O3 coating having the highest ultimate tensile strength at 172 MPa. The coating elongation and toughness also increased by an order of magnitude following FSP for all coating samples. The post-FSP coating with 5 wt.% Al2O3 exhibited the highest elongation of 7% and toughness of 8 J/mm3. The improved strength of the coatings following FSP was attributed to the improved distribution and dispersion of Al2O3 particles in the matrix following FSP. The enhanced ductility of the post-FSPed coatings was attributed to grain refinement that occurs due to dynamic recrystallization from FSP. The results from this study indicated that the microstructural refinement due to FSP significantly enhanced the mechanical performance of cold sprayed coatings. Therefore, it can be concluded that hybridization of low-pressure cold spray with FSP to produce Al-Al2O3 coatings can be an effective method for dimensional restoration of high-strength aluminum alloys with high potential also for functional performance.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.