Effect of filler material and post process ageing treatment on microstructure, mechanical properties and wear behaviour of friction stir processed AA 7075 surface composites

AA 7075 based surface composites reinforced with three different filler materials viz. Multiwall carbon nano tubes (MWCNT), Copper (Cu) and Silicon carbide (SiC) were successfully manufactured via Friction Stir Processing route with homogeneous dispersion of all the filler particles. The comparative effect of different filler materials and subsequent age hardening on the microstructural evolution of friction stir processed Aluminium matrix surface composites were characterized using optical and scanning electron microscopy (OM and SEM), electron backscattered diffraction (EBSD) and X-Ray diffraction (XRD). Hardness, impact toughness and wear behaviour was studied using microhardness test, Charpy impact test and pin on disc wear test respectively. Noteworthy enhancement in the microhardness (143 HV0.1 for as received AA 7075 T6 to 253 HV0.1 for FSPed with SiC filler followed by age hardening), impact toughness (15.8 kJ to 27.5 kJ) and wear resistance (wear loss dropped from 8.57 mg to 5.49 mg) were obtained through the addition filler particles during friction stir processing followed by age hardening. The enhancement in the mechanical and wear properties was primarily ascribed to grain refinement, Zener pinning effect (attributable to incorporation of filler material) and precipitate evolution during subsequent age hardening. SiC filler was found to be the best among the three fillers for the set of FSP and age hardening parameters used in the experiment. Nearly twofold increase in microhardness, impact toughness and wear resistance was achieved in age hardened SiC filler surface composite.