A comparison was made between the tensile properties of an Al-5 pct Si-3 pct Cu-1 pct Fe-0.3 pct Mg (319-type) alloy in the as-cast condition. The alloy was either unmodified or modified with Mn or Sr. The additions of 0.6 wt pct Mn, or 0.9 wt pct Mn, or 300 ppm Sr were made. All modifying additions increased the average and decreased the scatter in tensile strength, and increased elongation, with the Sr addition being most effective among the three. Fractographic examinations showed that the b phase, the polyhedral a phase, and porosity appear to be the main defects responsible for failure. The distribution of tensile strength has been quantitatively assessed using both two-and three-parameter Weibull distribution functions. In the twoparameter Weibull analysis, the high Weibull modulus (46) achieved by Sr modification is due to its binary effect on eutectic silicon and b platelets. Reservations about potentially misleading implications of the two-parameter approach are explored. The three-parameter Weibull analysis provides new information. In particular, minimum values of strength below which the material is extremely unlikely to fail are found. These values again show the clear benefit of Sr addition. The results are explained by the effects of modifying agents on the microstructure.

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This paper investigates the effect of different tool pin morphologies on the inter-mixing capability, microstructure, mechanical properties, corrosion, fracture, and wear behaviors of the dissimilar friction stir welded AA6061-T6 alloy--Al 20wt% Mg2Si composite. Grooved shoulder tools with varying pin profiles such as tapered cylindrical, threaded tapered cylindrical, and triangular tapered pins were used for joining the base materials. The parameter combination of 80 mm/min travel speed and 1000 r/min rotation speed (at which no visible flow-induced defect was obtained) was used for this study. Among the pin profiles, the triangular tapered pin produced significantly improved intermingling/inter-material flow, fragmentation, and dispersion of the primary Mg2Si particles in the AA6061-T6/Al-Mg2Si joint. The triangular tapered tool reduced the average grain sizes of the AA6061 alloy and the Mg2Si particles from 18.4 to 4.6 µm and from 115 to 7.5 µm, respectively. Intermetallic phases of Mg2Si, AlFe, Al3.21Si0.47, and Al0.7Fe3Si0.3 are formed in the weld nugget of all the AA6061/Al-Mg2Si joints. The tensile strengths of the joints fabricated with tapered cylindrical, threaded tapered cylindrical, and triangular tapered tools are 108, 139, and 141 MPa, respectively. Abrasive wear is promoted in the joints fabricated with triangular tapered tool due to the homogeneous dispersion and fragmentation of the inherent hard Mg2Si particles. The corrosion attack is dependent on the fragmentation level of the Mg2Si phase. Triangular tapered pin tool is recommended for dissimilar Al/composite welding due to favorable tool-induced material flow, dispersion, and fragmentation of reinforcement.