Title

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A FRICTION STIR PROCESSED Al-Zn-Mg-Cu ALLOY

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A FRICTION STIR PROCESSED Al-Zn-Mg-Cu ALLOY

Authorship

Vargas, Margarita; Lathabai, Sri

Vargas, Margarita

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Lathabai, Sri

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DOI

10.5151/2594-5327-33744

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Abstract

Friction stir processing (FSP) was performed on AA 7075-T6, a heat treatable high strength Al-Zn-Mg-Cu alloy. The two main FSP parameters, the tool rotational (ω, RPM) and travel speed (v, mm/min), were varied systematically in order to understand their influence on the microstructure and mechanical properties of the processed zone. It was demonstrated that the range of FSP parameters used did not significantly influence the nugget zone grain size. However, the mechanical properties of the processed zone varied significantly. For instance, at a given rotational speed, increasing the travel speed increased the microhardness of the nugget (stir) zone; for a given travel speed there appeared to be an optimum rotational speed which resulted in the highest microhardness. For all parameters tested, the processed zone displayed lower tensile strength, but significantly higher ductility when compared to the as-received alloy. The variation in tensile strength with changes in rotational and travel speed followed the same general trend as in the case of microhardness. In all cases, the ductility was within the range 20-25% elongation. The observed mechanical properties could not be explained as a direct consequence of grain refinement, by the Hall-Petch relationship. It is suggested that these are a result of the complex interactions between the FSP thermo-mechanical effects and the processes of dissolution, coarsening and re-precipitation of the strengthening precipitates in this alloy.

Friction stir processing (FSP) was performed on AA 7075-T6, a heat treatable high strength Al-Zn-Mg-Cu alloy. The two main FSP parameters, the tool rotational (ω, RPM) and travel speed (v, mm/min), were varied systematically in order to understand their influence on the microstructure and mechanical properties of the processed zone. It was demonstrated that the range of FSP parameters used did not significantly influence the nugget zone grain size. However, the mechanical properties of the processed zone varied significantly. For instance, at a given rotational speed, increasing the travel speed increased the microhardness of the nugget (stir) zone; for a given travel speed there appeared to be an optimum rotational speed which resulted in the highest microhardness. For all parameters tested, the processed zone displayed lower tensile strength, but significantly higher ductility when compared to the as-received alloy. The variation in tensile strength with changes in rotational and travel speed followed the same general trend as in the case of microhardness. In all cases, the ductility was within the range 20-25% elongation. The observed mechanical properties could not be explained as a direct consequence of grain refinement, by the Hall-Petch relationship. It is suggested that these are a result of the complex interactions between the FSP thermo-mechanical effects and the processes of dissolution, coarsening and re-precipitation of the strengthening precipitates in this alloy.

Keywords

Friction Stir Processing, 7xxx aluminium alloy, Mechanical Properties.

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