Daria Zhemchuzhnikova

This work was undertaken in an attempt to ascertain the generic characteristics of fatigue behavior of friction-stir welded aluminum alloys. To this end, different alloy grades belonging to both the heat-treatable and non-heat-treatable types in both the cast and wrought conditions were studied. The analysis was based on the premise that the fatigue endurance of sound welds (in which internal flaws and surface quality are not the major issues) is governed by residual stress and microstructure. Considering the relatively low magnitude of the residual stresses but drastic grain refinement attributable to friction-stir welding, the fatigue performance at relatively low cyclic stress was deduced to be dictated by the microstructural factor. Accordingly, the fatigue crack typically nucleated in relatively coarse-grained base material zone; thus, the fatigue strength of the welded joints was comparable to that of the parent metal. At relatively high fatigue stress, the summary (i.e., the cyclic-plus residual-) stress may exceed the material yield strength; thus, the fatigue cracking should result from the preceding macro-scale plastic deformation. Accordingly, the fatigue failure should occur in the softest microstructural region; thus; the fatigue strength of the welded joint may be inferior to that of the original material.

In this work, a two-step approach was applied to commercial Al–Mg–Mn alloy to produce fine-grained sheets with improved properties. To this end, the material was initially subjected to equal-channel angular pressing at 300°C to an accumulated true strain of ∼12. This resulted in a formation of a fully recrystallized equiaxed structure with average size of ∼1 µm. The relatively high thermal stability of the deformation-induced microstructure was provided by the constituent Al₆Mn dispersoids of ∼25 nm in size. Then, the severely-deformed material was undergone a conventional plane rolling at 300°C to a total thickness reduction of 80%. The final microstructure and mechanical properties of the produced material were studied.

The mechanical properties of an Al–Mg–Mn–Sc alloy with a grain size of ≈0.7 μm were examined at temperatures ranging from 77 to 293 K. An ultrafine grain structure was produced by equal‐channel angular pressing at 593 K up to a total strain of ≈12. It was demonstrated that extensive grain refinement enhances strength and ductility at ambient and cryogenic temperatures and highly enhances fracture toughness at room temperature. The alloy exhibits a well‐defined ductile–brittle transition attributed to the influence of temperature on nucleation and the growth of microvoids.

Effect of initial grain size on fatigue behavior of an Al-6%Mg-0.35%Mn-0.2%Sc- 0.08%Zr-0.07%Cr alloy was examined. The initial CG microstructure with an average grain size of ~ 22 μm was manufactured by casting followed by solution treatment at 360 °C for 12 h. To produce the UFG condition, the alloy was subjected to equal-channel angular pressing (ECAP) at 320 °C up to a total strain of ~ 14. Extensive grain refinement provided the formation of fully recrystallized structure with an average grain size of 700 nm. It was shown that the formation of UFG structure provided +60% increases in yield stress and +25% increases in fatigue strength. Fundamentals of this effect of microstructure on the static strength, fatigue resistance and fracture modes are discussed.