From structural repairs to additive manufacturing; exploring new challenges for cold spray

Cold spray, a member of thermal spray family, uses converging-diverging nozzle to generate supersonic gas stream, which accelerates powder particles towards high velocities ranging between 200 - 1200 m.s-1. At such high velocities, the particles impact substrate and adhere to through plastic deformation. Hence, as oppose to the other thermal spray methods, during which the deposited material melts, the main feature of the cold spray is the solid state nature of particles’ deposition. Thanks to this, cold spray possess many advantages mainly related to the avoidance of deleterious effects due to high temperatures. It is also "environmentally friendly" technology as no chemicals or toxic gases are used (only nitrogen or helium gas) and power consumption is lower with respect to the other methods. Thanks to this, cold spray has developed into a reliable industrial technology with many application, amongst which repair of aerospace components is particularly distinguishable thanks to enormous savings brought. Cold spray is used for repairing non-structural parts made of Al, Mg and Ti alloys, for which repair requirements are simpler and less strict than for components carrying load. This kind of applications (non-structural) are already well established in the industrial environment and cold spray is frequently used for geometric restoration. As the cold spray technology evolves, coating properties are enhanced thanks to continuous improvement of process equipment and extension of its application to parts transmitting loads does not sound unrealistic anymore. With cold spray application for structural parts restoration, new testing procedures need to be developed to prove that repaired components fulfill high safety requirements in aerospace. In particular, some publications indicated that 50-90% of failures is due to cyclic load, thus, it is of primary importance to assess whether cold spray repaired parts are able to guarantee the same fatigue properties as those of the base material or whether the fatigue strength of repaired part is too low for given application. In this thesis, potential of cold spray for the structural repair is demonstrated using coatings made of F357 aluminum alloy, a casting alloy widely used in aerospace. The coating underwent set of standard tests as well as newly developed fatigue test to gain an information about potential of cold spray for repair and additive manufacturing of loaded parts. With optimal spray parameters, coating deposition on substrate with smooth surface resulted in relatively good bonding, which can be further improved by application of grit blasting on substrate’s surface. However, no enhancement of adhesion was obtained for shot peened surface. Process temperature, which was set either to 450 C or 550 C, was shown to have an effect on adhesion and cohesion strength but it does not influence residual stress in the coating. To assess cold spray perspectives for additive manufacturing, flat tensile specimens were machined from coating and tested in as sprayed and heat treated (solution treatment and aging) condition. Tensile properties of the coating after the treatment correspond to properties of the cast F357-T61 aluminum alloy. Finally, two novel specimens to assess behavior of the coating under cyclic load were proposed and successfully tested. Both specimens are designed for axial fatigue test, which is the most conservative and straightforward method. In the case of the first specimen, relatively thick layer of the coating is deposited circumferentially, which allows to generally assess performance of the coating with respect to standardized specimen produced from bulk material. The coated fatigue specimen reached 94% of the fatigue limit of the base material. Second fatigue specimen contained a spherical cavity inducing stress concentration factor of 1.5, which is filled using the cold spray and tested. Calculated fatigue limit yielded 94% of the fatigue limit of the base material. Neither of the fractured fatigue specimen did not reveal crack initiation in the coating or at the interface. Based on the experimental results, it can be concluded that the high pressure cold spray technology can be successfully applied also to the parts subjected to cyclic loading. Another problem represents oxidation of cold spray powders made from aluminum and aluminum alloys so frequently used in aerospace. Aluminum and aluminum alloys react with oxygen under atmospheric conditions and oxide film forms easily on the powder particles during storage and handling. Multiple experimental studies demonstrated that oxide surrounding the particle hinders plastic deformation and breakage of the oxide film consumes significant part of the impact energy. results of parametric numerical study to determine effect of the thickness of oxide layer surrounding alumina particle on its deposition are presented in this work. Comparison of three particle impact models, no oxide and oxide layers with thicknesses of 0.2 m and 200 m, shows that the even the thin oxide layer prevents jetting of the particle and, depending on its thickness, also the flattening of the particle. While, no oxide cleaning was observed in the case of the thick layer, some cluster of oxide accumulated at the edge of the contact zone. Thus, the oxide cleaning mechanism, which is believed to be responsible for intimate contact of the substrate and the particle allowing the metallurgical bonding, was not confirmed by the presented numerical simulations. However, it was shown that bonding is obstructed by thick and rather continuous oxide layer in the middle of the contact zone. Finally, with increasing demands on structural integrity of the coatings, fracture properties and prediction of crack initiation and propagation grow in importance. This is addressed by development of numerical model to predict damage in the cold sprayed coatings. The model was based on cohesive zone approach, which combines experiment and numerical simulation to study crack initiation and propagation. The numerical work is based on the cohesive elements governed by a bilinear traction-separation law, which is suitable to simulate brittle fracture of cold spray coatings, the experiment approach determines the material parameters (so-called cohesive) needed as the input of the model. In the presented work, experimental procedure to determine cohesive parameters includes micro tensile test and compact tension test specimen made of thick 7050 aluminum alloy coating and adhesion test specimen and modified compact tension test specimen to determine cohesive properties at the interface between F357 aluminum alloy coating and the same material substrate. Required cohesive parameters are estimated based on the testing and trial-error fitting procedure. Good agreement between numerical and experimental results was obtained demonstrating that the presented cohesive law is able to simulate the damage in the cold sprayed coatings with reasonable accuracy. In addition, a supplementary three point bending test was used to validate numerical model. At the end drawbacks of this approach including a complicated preparation of some of the specimens and some numerical issues are pointed out.