Cold-Sprayed Aluminum Alloy-Quasicrystal Composite Coatings : Correlative Structural Characterization and Functional Performance

Abstract

The demand for sustainable usage of lightweight aluminum (Al) alloys with superior properties necessitates developing new strategies in material design and processing. Al and Al alloy matrix composites (AMCs), with enhanced strength, stiffness, and mechanical and wear damage resistance compared to standard Al alloys, can be effectively fabricated as coatings or additively manufactured parts using solid-state cold spray (CS) process. The performance of these composites depends on both the reinforcement type and nature, as well as the microstructural changes during spraying. Therefore, a fundamental understanding of the process–microstructure–property is essential for the successful industrial adoption of resulting products.

This study focuses on cold spraying of AMCs reinforced with quasicrystals (QC), a class of solid materials that have ordered atomic structure with rotational symmetry without showing translation periodicity, a forbidden order in classic definition of crystalline materials. The composite coatings were deposited by spraying feedstock blends of Al alloy (AA6061) particles mixed with varying ratios of a commercially available Al-based QC particles using a high-pressure CS system operating with compressed nitrogen as process gas. The outcome was considerably dense and well-adhered Al alloy-QC composite coatings on Al-based substrates that are challenging, if not impossible, to achieve by separately spraying either of the QC and Al alloys particles.

The cornerstone of this dissertation is a comprehensive microstructural analysis at each stage, from feedstock selection to fabrication and functionality assessment. Using a correlative advanced microscopy scheme (high-resolution scanning electron microscopy, electron backscattered diffraction, and (scanning) transmission electron microscopy), the impact-induced mechanisms and associated microstructural changes that govern the co-deposition of these materials, dissimilar in physical and mechanical properties, and the resulting performance were identified. Upon high velocity impact of particle during CS (average particle velocity within ~ 590–630 m/s) on Al alloy substrate, in general, Al alloy particles and substrate undergo plastic deformation and jetting, whereas QC particles primarily penetrate the substrate with noticeable particle fracturing. Impacting on relatively harder stainless-steel substrate, caused less plastic deformation to substrate, intensified flattening and jetting of Al alloy particles and severe brittle fracture, shattering and fragmentation of QC particles. Evidently, QC-QC impact does not lead to bonding due to high probability of particle shattering and rebounding. In contrast, Al alloy-QC particles interactions favor the dissimilar bonding and retention of QC within the plastically deforming and flowing Al alloy. Evidently, such dissimilar particle bonding is the consequence of synergy of multiple contributing mechanisms such as intimate metallurgical contact at Al alloys and QC interface, robust mechanical interlocking particularly enhanced along jagged QC fracture surfaces with Al-based matrix, with vortex-like intermixing and structural defects elimination promoted by in-situ hammering or micro-forging the structure, by repeated hard particle impacts.

Coating formation was accompanied by pronounced microstructural refinement in the Al alloy matrix via impact-induced recovery and recrystallization. The resulting structure comprises moderately deformed particle interior transiting to an ultrafine-grained (~100–500 nm) structure toward the particle periphery, and a nanocrystalline (~30–100 nm) region adjacent to the hard QC phase. Strengthening is potentially linked to collective contributions from effective densification, Hall–Petch strengthening due to grain refinement, dislocation-based mechanisms (e.g., Taylor and hetero-deformation-induced strengthening), and efficient load transfer across cohesive Al alloy-QC interfaces.

The Vickers microhardness increased with QC fraction, reaching ~150 HV2 at high QC retention, a hardness gain of ~60% relative to the non-reinforced coating. Particularly, the nanohardness of Al alloy matrix rises by approximately 25% (from ~1.41 to 1.76 GPa) which demonstrates that the improvements are not simply the effect of secondary phase embedding. Under cavitation erosion by vibratory apparatus, which was repurposed to assess the interparticle adhesion/cohesion of the coatings, the damage mode shifts from the detachment of large material chunk along weak inter-particle boundaries (observed in the non-reinforced Al alloy) to a uniform removal of fine debris, indicating enhanced interfacial adhesion/cohesion and improved strength of the structure in presence of QC particles. In sliding wear tests, the QC-reinforced coatings exhibit wear rates seven-fold lower than those of their non-reinforced counterparts, with an associated transition from adhesive/adhesive wear with extensive ploughing and large subsurface cracks to milder abrasive and oxidative wear.

Overall, this dissertation presents the first effort in fabrication and systematic investigation of aluminum alloy-quasicrystal composite coatings by cold spraying, revealing AA and QC particles deformation/co-deformation behavior and bonding mechanisms under high-velocity impacting condition. Beyond adapting this industrial fabrication process for this material combination, a majority of novelty falls within the multiscale characterization strategy (from macro- to nanoscale) employed in this work, which facilitated direct visualization of impact-induced microstructural evolution and correlation with improved mechanical and functional properties (hardness, wear resistance and integrity) of final composite coatings. These findings established a practical pathway for fabricating durable AMC coatings and repair overlays with potential to extend component service life in industrial applications.