Evaluation of Cold Spray Processing as a Novel Approach to Fabricating Bulk Nanocrystalline Al Alloys
Barrera, Enrique V
Doctor of Philosophy
This dissertation presents studies directed towards the design, characterization and analysis, of bulk nanostructured (BN) and bimodal aluminum alloy 5083 (AA5083) fabricated by cold spray processing (CSP). The results of the CSP samples were compared to the microstructures and properties of BN materials obtained using conventional powder consolidation techniques. Additionally, a proprietary non-cryogenic ball milling (NCM) technique was evaluated as a more cost effective alternative to cryogenic ball milling (CM) for processing nanocrystalline powder. Motivating factors for this research include the degradation of mechanical strength as a result of long term exposure of the nanocrystalline powder to elevated temperatures during conventional hot-processing techniques, removal of residual surface oxide that can prevent full density of the final product, and a more time and cost effective means of producing BN materials. CSP is an advantageous alternative to conventional bulk powder processing techniques as it is a low-temperature, high-velocity spray deposition technique that has been found to retain, if not further refine, the microstructure of the deposited powder, is capable of breaking up surface oxide resulting in clean metal bonding, and is capable of producing near-net-shape parts that require little post processing to achieve a final product. Furthermore, the NCM process does not use liquid nitrogen during milling operations, which results in significant cost savings compared to CM. It was found that the nanocrystalline powders produced using the NCM process were comparable in size, morphology, and grain size to the nanocrystalline powders developed using CM. It was further determined that NCM AA5083 exhibited a higher average hardness and potentially higher thermal stability compared to CM AA5083. These results indicate that NCM is as effective as CM in developing nanocrystalline microstructures. Analysis of the bulk CSP samples revealed that the average grain size was smaller compared to BN materials fabricated using conventional techniques, and also exhibited a higher hardness. The tensile strength of the CSP samples was found to be low compared to the conventionally processed counterparts and is attributed to the presence of microstructural defects arising from incomplete particle bonding. Analysis of bimodal bulk CSP AA5083 revealed that the intended bimodal microstructure was not retained as the conventional grain sized powder undergoes significant grain refinement during deposition. Though areas of ductile failure were observed in the fracture surfaces, an overall lack of ductility and strength was found and attributed again to microstructural defects. These findings suggest that CSP is a viable alternative for processing BN materials.
Nanocrystalline; bulk; high strength; powder metallurgy