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Feasibility of FSW to produce defect-free welds and for microstructure refinement in a ZK60 magnesium alloy has been examined over wide ranges of rotational and welding speeds. The results showed that joined-state was affected very much by the thickness of plates to be joined: for ''thin'' 3-mm thick plates, FSW was successful in a relatively wide range of welding conditions, whereas all ''thick'' 6-mm welds were found to be defective irrespective of rotational and welding speeds used. We have demonstrated that FSW is a very effective tool for microstructure refinement in the ZK60 alloy plates: A coarse, mm-scale, original grain structure was efficiently refined down to fine-grained range. It appears that second-phase particles containing Zr play an important role in the grain refinement.

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In practical application, an appearance of low temperature superplasticity (LSTP) is one of necessaries conditions. In this paper, to estimate an appearance and deformation mechanisms of this superplasticity, the role of grain boundary sliding (GBS), intragranular deformation and the change of microstructure during superplastic deformation have been investigated for ultrafine-grained Al-Mg alloy with a grain size of less than 1 mm using Multi-Axial Alternative Forging (MAF) technique. In these materials, it shows that the elongation and strain rate sensitivity (m-value) were 340% and 0.39, respectively, at 473 K under a strain rate of 2:8 Â 10 À3 s À1 . These results show that superplastic appearance is possible at 473 K. The void formed at 473 K elongated in parallel to the tensile direction, with a length of 15 mm and a width of 5 mm. The intragranular deformation contribution was estimated from the aspect ratio of the grains after deformation and its contribution ratio was about 33.5 %. Therefore, for the appearance of lower temperature superplasticity with large elongation and m-value, the role of intragranular deformation was the most important factor together with GBS under these conditions. As described above, the MAF technique is one of the most effective methods to produce ultrafine-grained material and appearance of lower temperature superplasticity.

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