Microstructure and Corrosion Behaviour of Friction Stir Processed AZ91 Mg Alloy

Magnesium (Mg) and its alloys are gaining wide popularity as promising candidates for automobile, aerospace and structural applications. On the other hand, friction stir processing (FSP) is a solid state technique used to alter the microstructural characteristics of a metal. In the present study, AZ91 Mg alloy was processed by FSP at optimized process parameters. The microstructural changes were analyzed and found that the presence of secondary phase (Mg₁₇Al₁₂) was reduced. The network like structure of secondary phase that was appeared at the grain boundary before FSP has disappeared after the processing. Grain size was also found to be reduced from (66.5 ° 8.7 μm) to (4.1° 1.6 μm). The corrosion behavior was assessed by immersion test in 3.5%NaCl solution and found that the processed AZ91 has lower corrosion rated during the early hours of corrosion attack. However, after 72 h of immersion the corrosion resistance of the processed AZ91 was found to be reduced. Hence from the present study it can be understood that the presence of secondary phase has a great influence on the corrosion behavior of AZ91 mg alloy and that can be altered by grain refinement.