Investigation of Al-Al\u2082O\u2083 cold spray coatings formation and properties

Coating build-up mechanisms and properties of cold sprayed aluminum-alumina cermets were investigated. Two spherical aluminum powders having average diameters of 36 and 81 microns were compared. Those powders were blended with alumina at several concentrations. Coatings were produced using a commercial low pressure cold spray system. Powders and coatings were characterized by electronic microscopy and microhardness measurements. In-flight particle velocities were monitored for all powders. The deposition efficiency was measured for all experimental conditions. Coating performance and properties were investigated by performing bond strength test, abrasion test and corrosion tests, namely, salt spray and alternated immersion in salt water tests. These coating properties were correlated to the alumina fraction either in the starting powder or in the coating.Peer reviewed: YesNRC publication: Ye

Different cold spray deposition strategies : single and multi layers to repair aluminium alloy components

Cold spraying is increasingly being used for reconstruction or repair of damaged aluminium alloy components, especially in the aviation industry. Both thin (<0.5 mm) and thick (up to 1 cm) coatings are necessary to achieve dimensional recovery of such components. Thin and above all thick coatings can be deposited in a single pass (single layer) or in several passes (multi-pass), resulting in different thermal and stress effects in the component and the coating itself. The thermal input, the amount and type of residual stresses and the porosity affect various characteristics such as adhesion, crack propagation and mechanical properties of the coating. In this study, two sets (single- and multi-pass) of aluminium alloy (AA6061) coatings with different thicknesses (0.5 mm to 2 mm) were deposited onto AA6061 substrates and compared using metallographic and fractographic analyses, four-point bending testing, residual stress analysis and Vickers microhardness indentation. Finally, the coating adhesion and cohesion were measured using the standard ASTM-C633 adhesion test and tubular coating tensile test. This study demonstrates that the single-layer strategy results in greater adhesion and lower porosity, while multilayer coatings have higher elastic modulus. Independent of the strategy, the compressive residual stress decreases as a function of coating thickness.Peer reviewed: YesNRC publication: Ye

Mechanical Behavior of Ti Cold Spray Coatings Determined by a Multi-Scale Indentation Method

The effect of the indentation load on the hardness measurements on cold spray Ti coatings was evaluated. Varied load indentation was carried out with nanoindentation at loads between 1 and 20mN and microindentation at loads between 0.1 and 5N. Nanoindentation measurements showed an indentation size effect and were fit to the Nix\u2013Gao model for strain gradient plasticity. The microindentation measurements depended on the indentation size in a manner different from the Nix\u2013Gao model and were strongly affected by the material porosity and cohesive strength between cold sprayed particles. By comparing results from the two indentation techniques, a hardness loss parameter was formulated that helped explain the mechanical behavior at all length scales and its relation to the various defects within the coating. Comparisons of the hardness loss parameter between cold spray coatings and a bulk Ti plate showed that this new parameter provided a good measure of when the mechanical behavior of the cold spray coatings approached that of a traditionally manufactured material. Evaluation of the hardness loss parameter of coatings and the bulk material, combined with microstructural characterization before and after indentation, was used to examine the relationships between processing conditions, coating microstructure and mechanical behavior for coatings fabricated with spherical and non-spherical Ti powders.Peer reviewed: YesNRC publication: Ye

Cold-spray processing of titanium and titanium alloys

The knowledge base for cold spray has grown significantly in recent years and advances in the technology have shown viability for the process for titanium-based materials. For titanium and its alloys, the capability to rapidly produce thick, unoxidized deposits in open air makes cold spray a particularly attractive process. Here, selected results from the available literature are discussed to highlight key practical and fundamental aspects in cold-spray processing of titanium-base powders. An overview is presented for the general process and its underlying particle deformation and bonding mechanisms. The properties of cold-sprayed titanium material are characterized and process\u2013microstructure\u2013property relationships reviewed. Coating and additive manufacturing applications are then briefly outlined with a discussion of practical considerations. In the concluding remarks, views are offered on the current status and possible future directions of development for the technology.Peer reviewed: YesNRC publication: Ye