Effects of in-process cryocooling on metallurgical and mechanical properties of friction stir processed Al7075 alloy

Friction stir processing (FSP) of heat treatable aluminum alloys is reported to deteriorate strength and hardness properties in spite of huge grain size refinement. The improvement in strength due to the grain size refinement after FSP could not be able to offset the reduction in strength due to coarsening or dissolution of the strengthening precipitate(s). To overcome this, the present work investigates an effective way of heat rejection from the FSP zone to control the precipitate evolution besides the grain size refinement during FSP of Al7075 alloy. The FSP was carried out with standardized process parameters (i.e. 720 rpm, 65 mm/min traverse speed) under two different cooling environments: (i) at normal air cooling and (ii) by rushing a chilled mixture (30 degrees C) of liquid nitrogen and methanol through the bottom of the backing plate. Microstructural evolution was examined through optical, scanning and transmission electron microscopy and electron backscatter diffraction to confirm the grain size and morphology of precipitate(s) formed. The in-process cryocooling during the FSP led to 65 and 72% improvement in the Vickers hardness and tensile strength, respectively, in comparison to the solutionized base metal. The results have been analyzed in terms of precipitate interface characteristics and its strengthening effect, Hall-Petch strengthening due to grain size reduction and tradeoff between them.