Study On The Nonuniformity And Control Of Microstructure And Properties Of Cold-sprayed Deposits

As a rapidly advancing solid-state spraying technology which based on the supersonic gas-solid two-phase flow and high speed collision dynamics,cold spray(CS)has been widely applied for fabricating a large variety of protective and functional coatings.Besides,it is also an option of repairing and additive manufacturing techniques.Because of the inhomegeneous particle impact deformation behavior during CS,anisotropies of microstructure and mechanical properties may be developed.In addition,because of the existence of various defects,a significant loss of strength/ductility and corrosion resistance of deposits follows CS.However,only a few attempts have been made on the nonuniformity and control of microstructure and properties of CSed deposits to date,which could have a very negative influence on the further application of CS.Therefore,this study focuses on the nonuniformity and control of microstructure and properties of CSed deposits.By combining the numerical simulation and experimental method,relative mechanisms were clarified.The main conclusions are as follows:Modeling shows that the particle plastic deformation is nonuniform during impacting.The maximum plastic deformation appears at the surrounding of the contact zone rather than the center point of initial impact.The maximum temperature during impacting of Cu particle is higher than its recrystallization temperature but lower than its melting point,indicating the occurrence of recrystallization in Cu particles.Compressive and tensile stresses are all found in the single Cu splat,indicating that the state of residual stress can vary significantly,which depends on the material system and particle velocity.For single pass Cu coating,the inner temperature,strain and residual stress distributions are also nonuniform.The maximum strain and temperature appear near interfaces between particles.The residual stress can vary throughout the coating and not only compressive stress but also tensile stress exists to maintain the balance of force,which is also dependent on the material system and particle velocity.Experiments show that there exist strong microstrual and mechanical anisotropies of CSed Cu deposits.Particle impact deformation and grain size distribution behaviors are different between the v-face and p-face.The anisotropic impact deformation results in larger elastic modulus and hardness in the v-face than p-face.Strong anisotropy exists in the in-plane tensile properties of CSed deposits with the highest tensile performance occuring at the angle of 20°(20°is the direction parallel to the spraying gun traverse),and the lowest at 90°.The parallel multiple passes have the main responsibility to the in-plane tensile anisotropy.The weave-spraying has a great modification effect on the tensile anisotropy of CSed deposits.The post-spray heat treatment can effectively reduce the micromechanical anisotropy(including hardness,elastic modulus and yield stress)between the cross-section and the plane parallel to the surface.The strong anisotropy of the in-plane tensile strength can be reduced with post-spray heat treatment,while the effect on elongation anisotropy is quite limited.Although the negative effect of interparticle interfaces can be effectively decreased with post-spray heat treatment,the negative contributions of particle interfaces are still very large.Taking the 700oC annealed state as an example,0°:-46MPa,15°:-65MPa,30°:-38MPa,45°:-50MPa,60°:-67MPa and 90°:-69MPa.The parallel multiple passes in CS remain the main factor which limits the further modification of the inherent in-plane tensile anisotropy.This study trys to use post-spray heat treatment and friction stir processing(FSP)to modify the strength/ductility of CSed AA2024/Al₂O₃ MMCs.It is shown that post-spray heat treatment with appropriate parameters can effectively improve the tensile properties of CSed AA2024/Al₂O₃ MMCs.There exist the optimum tensile properties at solution+aging 6h,with an increase of 13.8%in ultimate tensile strength(UTS)and 47.8%in elongation.The post-spray FSP can effectively improve the UTS and elongation of CSed MMCs,which is due to the improved interfacial bonding and dispersion strengthening.The enhancement level can be further increased at a higher rotation speed.In this study,there exist the best tensile properties at 1500rpm,with an increase of 25.9%in UTS and 27.4%in elongation.The improved interparticle bonding due to decreased interparticle spacing(150MPa),and the dispersion strengthening due to refined Al₂O₃ particles(24MPa)are the main strengthening mechanisms.The electrochemical properties of CSed AA2024/Al₂O₃ MMCs in a 3.5wt.%NaCl solution is studied with cyclic polarization and electrochemical impedance spectroscopy(EIS).Results demonstrate an improvement of the corrosion resistance with appropriate FSP parameters.In this study,for FSP passes,the corrosion resistance follows the order:2-pass FSP>1-pass FSP>CS?4-pass FSP.Low pass number FSP can effectively enhance the corrosion resistance of the CSed MMCs,with the best corrosion performance occurring at 2-pass FSP.And the improved surface condition after FSP is the main improvement mechansim.The combined effect of the improved surface condition and deteriorated interfaces of the inside coatings takes the responsibility for the unsatisfactory corrosion performance at4-pass FSP.For rotation speed,the corrosion resistance follows the order:900rpm FSP>1200rpm FSP>CS>600rpm FSP.There exists the best corrosion property at900rpm FSP,while the low rotation speed FSP has a bad effect on the corrosion performance,which is even worse than the as-sprayed state.