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Friction stir welding is a solid state, environment friendly and energy efficient welding process for joining many engineering alloys including magnesium alloys. In this study, the effect of different cooling rates on microstructural and mechanical properties of friction stir welded AZ91C magnesium alloy was investigated. AZ91C magnesium samples were friction stir welded in air as well as in water. The effects of submerging environment on tensile and hardness properties of samples were discussed. The results showed that water, as a welding environment, has a considerable impact on the mechanical properties of the weld.

The intention of authoring this paper was giving the report of reviewing task of the previous researches done on the strengthening mechanisms and heat treatment of 7xxx aluminum alloys. The overall conclusion is that, the most powerful and so the most common strengthening mechanism in 7xxx alloys is the precipitation hardening (T6 heat treatment). On the other hand, stress corrosion is a problem in T6 treated 7xxx alloys, to address which, researchers have turned to special heat treatment called RRA (retrogression and re-aging) which picks the strength from T6, and stress corrosion resistance from T7x heat treatments.

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Auxetic materials and structures are a novel class of materials which have negative Poisson's ratio. They have improved mechanical properties such as fracture toughness, indentation resistance, etc. They have several potential applications in medical, sports, automobile, defense, etc. Design and modeling of auxetic materials is still in progress. In this article we briefly introduce these materials and their different features and properties.