Surface Modification of a Cold Gas Dynamic Spray-Deposited Titanium Coating on Aluminum Alloy by using Friction-Stir Processing

In this research, the parameters of the cold spray process were initially assessed for deposition of a pure titanium coating layer with the thickness in the range of 800-850 mu m on an AA5083 alloy substrate. Thereafter, to enhance the structural integrity of Ti-coating layer and decrease the coating porosity, friction-stir processing was employed as a post-modification technique by using a flat cylindrical tungsten carbide tool. The plunge depth of the friction-stir tool (in the range of 0.3-0.5 mm) was found to significantly affect the densification of the porous titanium coating layer. Optical microscopy, field emission-scanning electron microscopy, electron backscattering diffraction, transmission electron microscopy analysis and indentation Vickers micro-hardness testing were conducted on the thickness cross-sections of cold-sprayed coatings to characterize the microstructural features and mechanical properties before and after friction-stir modification performed using two different plunge depths. Furthermore, residual stress profiles on the surface were determined by using x-ray diffraction analysis technique. Significant grain refinement, from an initial cold-sprayed coating grain size of less than 25 mu m to grain sizes < 1 mu m, was observed across the thickness section of modified samples with a gradient profile from the coating surface toward the interface depending on the plunge depth. After friction-stir processing, the hardness of a thin layer close to the surface of coating increased up to seven times higher as compared to the cold-sprayed material.