TRIBOLOGICAL BEHAVIOURS OF MAGNESIUM ALLOY SECTOR SHAPE PAD WITH SURFACE MODIFICATION

Abstract

In the ongoing demand for light weight materials, magnesium alloys are equally comparable to aluminium alloys because of attributes such as high specific strength, low density, high damping capacity and better machinability. Magnesium-Aluminium-Silicon (Mg-Al-Si) is one of the Mg alloy series which is well recognised for elevated temperature applications as they contain the thermally stable Mg2Si intermetallic compound. However, the coarse and brittle nature of Mg2Si deteriorates the mechanical and tribological properties; therefore researchers are working on its modification and refinement to enhance the widespread use of heat resistant Mg-Al-Si alloys. The rapid emerging surface modification phenomena like Friction Stir Processing (FSP) have proved its potential in achieving significant grain refinement and morphological modification in the materials thus enhancing their mechanical and wear behaviour. In the present research, Magnesium-Aluminium-Silicon (Mg-Al-Si) based AS21A magnesium alloy was examined for microstructural, mechanical and tribological characterization in respect of the cast and Friction Stir Processed (FSPed) conditions. The Taguchi − Grey Relational Analysis (GRA) − Principal Component Analysis (PCA) hybrid methodology was applied to achieve the optimized values of FSP parameters. Microstructural features exhibit the fragmentation of coarse Mg2Si to fine particles thus creating their homogeneous dispersion and eliminate the casting defects present in the parent material. The morphological structure modification achieved through FSP resulted in the significant enhancement of mechanical properties like strength, ductility etc. Further, fractography images proved the brittle failure in cast AS21A sample while ductile failure in FSPed AS21A sample. In tribological study, FSPed AS21A samples exhibited noteworthy improvement in the wear characteristics at all assessment conditions. It was established that the morphology vi of Mg2Si precipitates had an active contribution in the wear behaviour of cast and FSPed AS21A samples. The notable mechanisms found responsible for the wear of samples were adhesion, abrasion, oxidation, delamination and plastic deformation. An attempt has also been made to replace the Babbitt coating with the investigated alloy in thrust bearing applications in the form of sector shape pad.