Properties–microstructure relationship in Al–Fe in situ composite produced by friction stir processing

Effects of Fe particle volume percent (from 0 to 5%) and changing the tool rotation direction in each pass were studied on the microstructure evolution and mechanical properties of the in situ Al/intermetallic composites produced by three passes friction stir processing (FSP) with ∼10 µm Fe particles. Optical and scanning electron microscopes were used for the investigation of the composite homogeneity, formation of Al–Fe intermetallic compounds, and fracture surfaces. Tensile and microhardness tests were also carried out to evaluate the mechanical properties of the composites. Solid-state reactions between the aluminum matrix and Fe particles led to in situ formation of Al₃Fe and Al₅Fe₂ intermetallics in the composite. Changing the tool rotation direction in each pass reduced asymmetrical distribution of the reinforcing particles and resulted in better dispersion of Fe particles. Increase in the particle volume percent refined the composite average grain size from ∼35 μm in the sample processed without particles to ∼6 μm in the composite included 5 vol.% of the Fe particles. Zener limiting grain size relationship was used to estimate the volume fraction of the sub-micron particles in the composites. Enhancement of the reinforcing particle volume percent (to 5%) increased ultimate tensile strength and yield strength of the composite up to ∼60% and ∼95%, respectively, due to finer grain structure as well as the higher load-bearing capacity of the Fe and intermetallic reinforcing particles compared to the aluminum matrix. Contributions of the Hall–Petch effect and the other strengthening mechanisms in the yield strength of the composites were also investigated.