Influence of titanium particulate reinforcement on microstructural evolution and mechanical performance of AZ91 magnesium matrix surface composite developed through friction stir processing

AZ91 Magnesium alloy is widely used in automobile components, bio-implants, mobile and computer parts, sporting goods, etc., owing to its low density, high strength to weight ratio, good machinability and biodegradable characteristics. However, it has moderate strength and poor ductility that hinders its engineering application. The incorporation of ceramic reinforcement in Mg alloys to form high-strength composites shows a major loss in its ductility. In this research work, friction stir processing (FSP) was used with two innovative tool designs to enhance mechanical and microstructural properties by employing titanium powders as reinforcement particles - metallic reinforcement possessing higher melting point, offering a solution to the issue. The tools were designed with stepped square profile, where in one design, the square steps were aligned with each other (S-type) and in the other design, the square edges were mis-aligned by 45 (D-type). Microstructural analysis was done using optical microscopy, scanning electron microscopy and X-ray diffraction. Mechanical properties, micro-indentation hardness test and compression test, were studied in detail.Results revealed that the FSP process performed with D-type tool yielded better results with more uniform distribution of Ti particles in the stir zone and grain refinement with refined recrystallised grains down to about 8 & mu;m due to severe plastic deformation and dynamic recrystallisation. The compressive strength of the material processed using D-tool was found to be increased to 456 MPa. Ti particles established a proper interface with the matrix delaying the failure of the material, thereby improving the ductility of the material as well. The mechanisms leading to microstructural refinement and enhancement in mechanical performance are presented in detail.