Hidetoshi Somekawa

Low temperature diffusion bonding in a superplastic AZ31 magnesium alloy

Abstract Diffusion bonding behavior of fine-grained magnesium alloy with the average grain size of 8.5 μm, that was processed by hot rolling, was examined. The material behaved in a superplastic manner at temperatures of 523 and 573 K, and successfully diffusion bonded at these temperatures.

Diffusion bonding in superplastic magnesium alloys

The superplastic characteristic and diffusion bonding behavior were investigated in a commercial AZ31 magnesium alloy sheet having equiaxed grain size with an average size of 16.8 μm. The alloy behaved in a superplastic manner from 623 to 723 K, and, the present material was successfully diffusion bonded at the superplastic temperature by selecting appropriate pressure and time. The maximum of lap shear strength was 0.85 at a bonding pressure of 3 MPa and a bonding temperature of 673 K, with a bonding time of 3 h. In the specimen with high ratio of lap shear strength, the bond line was not identified by optical microscopy.

Fine-grain processing by equal channel angular extrusion of rapidly quenched bulk Mg-Y-Zn alloy

The fine-grain processing of ingot metallurgy (IM) Mg–Y–Zn alloy, WZ73, was investigated. The alloy was initially produced by casting into a copper mold at a cooling rate of ∼50 K/s. The rapidly quenched bulk material was processed by means of equal channel angular extrusion (ECAE). The ECAE-processed material had equiaxed grains of 5.1 μm in size, and fine second-phase particles of Mg 12 YZn were dispersed in the grain boundaries. The Vickers hardness of the ECAE-processed material was 78. The dispersion of the second-phase particles, solid solution strengthening, and grain refinement contributed to the material’s hardness. The structure remained virtually unchanged, at least up to 673 K because the Mg 12 YZn phase served to pin the grain boundaries. The microstructure of IM WZ73 alloy, which is a suitable starting material for ECAE, was also considered.

Fracture Toughness in Ultra Fine-Grained Magnesium Alloy

The fracture toughness was investigated using in an extruded AZ31 magnesium alloy with an initial grain size of 1.0 μm. Since the small scale yielding condition was not satisfied with the present thin thickness, the value of plane-strain fracture toughness, KIC = 27.9 MPam1/2, was measured from Stretched Zone analysis. The values of KIC in AZ31 magnesium alloys were dependent on the grain size. The grain refinement was found to be one of the improvement methods for fracture toughness in magnesium alloy.

Influence of Second Phase Particles on Fracture Toughness in AZ31 Magnesium Alloys

Three kinds of thin AZ31 wrought magnesium alloys sheets were used in order to investigate the influence of the second phase particles on fracture toughness. From the theoretical model, the ratio of λp/dp would be estimated 5~ 6. On the other hand, from the microstructural observation, average particle spacing on each material was sample A: 13.1µm, sample B: 14.1, and sample C: 12 µ. In addition, average particle size on each sample was sample A: 2.1, sample B: 1.9, and sample C: 2.3 µm. Therefore, the ratio of λp/dp calculated from fracture surface observation would be predicted 6 ~ 7. In comparison with the result of the prediction by theoretical analysis was in good agreement with the result of fracture toughness observation. It was found that the variation in plane-strain fracture toughness on AZ31 were affected by both of particle spacing and particle size.

Superplastic Behavior in Magnesium Alloy with Dispersion of Quasicrystal Phase Particle

Superplastic behavior was investigated using an extruded Mg-Zn-Y alloy with the dispersion of the quasicrystal phase particle in fine-grained matrix. Tensile tests showed that the low temperature superplasticity was behaved at a temperature of 473 K and maximum elongation was 462 % at 573 K in 1 uf0b4 10-5 s-1. The deformed microstructure observation showed that the dominant deformation process was grain boundary sliding. The present alloy also demonstrated a high possibility for secondary forming, such as superplastic forge forming. Furthermore, the forged alloy had a homogeneous microstructures, no mechanical anisotropy and uniform micro-hardness properties in any portion of a forged product.

Diffusion Bonding on Superplastic-Aluminium and -Magnesium Alloys

The superplastic characteristics and diffusion bonding behaviors were investigated in commercial 7475 aluminum alloy and AZ31 magnesium alloy sheets. In this study, the presently used materials behaved in a superplastic manner at 773 K (7475Al) and 523 K (AZ31). Then, by the theoretical relationship between pressure and time, these materials were successfully diffusion bonded at the superplastic temperatures. The bonding strength was more than 65 MPa. The experimental bonding conditions in high quality joining, times and pressures, were good agreed with prediction analysis.