Strain path and microstructure evolution during severe deformation processing of an as-cast hypoeutectic Al–Si alloy

Microstructure evolution in an as-cast Na modified Al–7%Si (wt. pct.) alloy was examined during redundant and monotonic straining by repetitive equi-channel angular pressing (ECAP) under ambient temperature conditions, and during friction stir processing (FSP). Redundant straining during repetitive ECAP was accomplished by processing following route BC while monotonic straining employed route A. Single- and multi-pass FSP was conducted on this same as-cast material using an FSP tool having a threaded pin. The as-cast microstructure comprises equiaxed primary α dendrite cells embedded in the Al–Si eutectic constituent. The evolution of this microstructure during repetitive ECAP can be described by idealized models of this process. The primary and eutectic constituents can still be discerned and the Si particle distribution is not homogenized even during ambient temperature processing involving von Mises strains >9.0. In contrast, the primary and eutectic constituents cannot be distinguished in the stir zone after even a single FSP pass. Strain estimates based on the shape change of the primary α constituent indicate that the Si particle distribution has become homogeneous at local von Mises strains of 2.5–3.0 during the FSP thermomechanical cycle. Mechanical property data are consistent with strain path during SPD processing by repetitive ECAP and FSP.