Tomotake Hirata

Grain boundary character distribution control to inhibit cavitation in superplastic P/M7475

Abstract The influence of the grain boundary character distribution (GBCD) on cavitation behavior was investigated using P/M7475. The cavity volume increased by increasing the percentage of random boundaries because the number of cavities increased when the fraction of random boundaries increased. However, the GBCD does not influence the cavity growth rate.

Experimental prediction of deformation mechanism after continuous dynamic recrystallization in superplastic P/M7475

The deformation mechanism in high-strain-rate superplastic P/M7475 before and after continuous dynamic recrystallization (CDRX) was investigated. The recrystallization process in P/M7475 differed from that in conventional superplastic material, I/M7475. In I/M7475, the fine-grained microstructure was obtained by static recrystallization before deformation. On the other hand, the substructure in P/M7475 evolved into fine grains during deformation by CDRX. The percentage of high-angle and random boundaries was low at an initial stage of deformation. However, it increased with strain in P/M7475. The microstructural change in P/M7475 influenced a deformation mechanism and affected grain boundary sliding (GBS). The ratio of contribution of GBS to total elongation was low at an early stage of deformation in P/M7475. However, it increased with deformation progressed. It is suggested that the deformation behavior in P/M7475 changed from dislocation creep to superplasticity as the dominant deformation mechanism changed to GBS. The activation energy for superplastic flow in P/M7475 was close to that for lattice self-diffusion in pure aluminum. It is therefore concluded that the dominant deformation mechanism after CDRX in P/M7475 is GBS accommodated by dislocation movement controlled by lattice self-diffusion, similar to that in I/M7475.

Formability of Friction Stir Welded and Arc Welded 5083 Aluminum Alloy Sheets

The formability in friction stir and arc welded 5083 Al alloys was investigated. The elongation in friction stir welded material under uniaxial tensile test increased with the decrease in friction heat flow during friction stir welding (FSW) and the value of dome height under biaxial bulge test increased with friction heat flow. The ductility improved according to changes in FSW condition and FS-welded alloy sheet exhibited excellent formability.

The investigation on microcavitation behavior during dynamic recrystallization in P/M7475 aluminum alloy

Abstract The influence of microstructural change on cavitation behavior was investigated in high-strain-rate superplastic P/M7475 aluminum alloy. The percentage of random boundaries was low before deformation and increased gradually with strain by dynamic recrystallization during deformation. The rate of increase of cavity volume was lower than that of conventional superplastic material and cavitation was remarkably resisted in P/M7475.

Microstructural dynamics during high-strain-rate superplastic flow in PM 7475 alloy

A 7475 aluminum alloy processed by powder metallurgically (P/M7475) has shown superplastic elongation more than 1000 % at a temperature of 788 K. The maximum elongation of P/M 7475 was recorded near a strain rate of 1x10 -1 s -1 , which is 3 orders magnitude faster than that of ingot metallurgically processed 7475 (I/M7475). This excellent superplastic performance at a higher strain rate is owing to a fine grain size of about 1μm developed by dynamic recrystallization at an early stage of superplastic flow. Morphology of grain boundary sliding with straining was inspected by the scanning electron microscopy. The contribution of grain boundary sliding was found to be low at an early stage of deformation, but was increasing with deformation. It was found that this trend was corresponding to an increase of misorientation angle by dynamic recrystallization with straining.

Microstructural modification of cast Mg alloys by friction stir processing

Abstract Cast Mg alloys were processed using friction stir processing (FSP) to acquire a fine grained structure and high strength. Actually, FSP is a novel grain refinement method for light metal alloys. Using FSP, a cast microstructure with coarse grain size was refined to equiaxial fine grain through dynamic recrystallisation; second phase particles were finely dispersed by FSP. Moreover, FSP is effective to eliminate cast defects such as microshrinkages or porosities. Commercial die cast Mg alloy (AZ91D) and high strength Mg–Y–Zn alloy plates were prepared for FSP. Heat input using a rotational tool during FSP closely affected the microstructure in the stirred zone. Actually, FSP with lower heat input produced a finer grain size and higher hardness. Changes in the friction stir processed microstructures affecting mechanical properties were not only grain refinement, but also second phase particle distributions. Results show that alloys with high hardness by FSP have finely dispersed second phase particles without dissolution during FSP.

Superplastic Properties and Microstructure of Friction Stir Welded Joints of Zn-22wt.%Al Alloy

Deformation mechanism of room-temperature superplasticity in Zn-22wt%Al alloy was investigated by the direct observation during deformation. It was revealed that the dominant deformation mechanism of room-temperature superplasticity was grain boundary sliding. Also, superplastic properties and microstructure of friction stir welded Zn-22wt.%Al alloy were investigated, where Friction Stir Welding (FSW) has received a great deal of attention as a new solid-state welding technique. A sound jointing material was obtained successfully, and extremely fine and equiaxed grains were created in the stir zone. In addition, it was indicated that superplastic forming of the FSWed Zn-22wt.%Al alloy could be feasible. However, the tensile strength and elongation of the joint at room temperature were lower than that of the base material.