Influence of aluminium-rich intermetallics on microstructure evolution and mechanical properties of friction stir alloyed Al-Fe alloy system

In the present investigation, the Al-Fe alloy system is developed by using the solid-state friction stir alloying (FSA) technique. The objective is realized by using multiple passes (1-4) of FSA with 100% overlap at constant process parameters. The TEM analysis confirms the interfacial reaction between the Al and Fe particles leading to the formation of nano-sized hard and brittle Al13Fe4 intermetallic (IMC) phase. The SEM, EDS, and XRD analysis also indicates the existence of a very small amount of Al5Fe2 IMC layer at the Al-Fe interface. The microstructural analysis from EBSD confirms that the grain size is reduced from -27 mu m in the base material to -2 mu m in the 4 pass FSAed Al-Fe alloy through dynamic recrystallization mechanism. The microhardness in the SZ is increased by -14% in the 4 pass FSAed Al-Fe alloy as compared to that with 1 pass. The increased hardness of Al-Fe alloys as compared to the 4 passes processed Al alloy (without Fe) suggest that the Al-Fe alloys system presented here can be an alternative solution for underwater friction stir welding (UFSW) to compensate for the stir zone softening occurred by the high heat input process parameters or precipitate dissolution in the heat treatable Al alloys. The ultimate tensile strength and percentage elongation are increased by -30% and -48% in the 4 pass FSAed Al-Fe alloy as compared to the alloy fabricated at 1 pass, respectively. The increased strength and hardness of the 4 passes FSAed alloy is attributed to the increased reaction rate between Al and Fe particles owing to the higher heat input which results in the precipitation of a more number of Al13Fe4 IMC phases during 4 passes. Also, the uniform dispersion of nano-sized IMC phases in 4 passes FSAed alloy as compared to the alloy fabricated with 1 pass, contribute significantly to the dislocation blockade and dispersion strengthening mechanism. The shreds of evidence provided in the present investigation suggest that the precipitation hardening is the dominant strengthening mechanism in the Al-Fe alloy system as compared to the Hall-petch strengthening mechanism.