Yoshiki Mizutani

Fatigue behaviour of dissimilar Al alloy/galvanised steel friction stir spot welds fabricated by scroll grooved tool without probe

Abstract Al alloy sheet was friction stir spot welded (FSSW) to galvanised steel sheet by a scroll grooved tool without probe. Tensile and fatigue tests had been conducted using tensile shear specimens, and the results were discussed in comparison with Al/steel dissimilar FSSW joint, in which steel sheet was not galvanised. Energy dispersive X-ray analyses revealed that Zn coating was melted or softened and circumferentially resolidified around the nugget. Intermetallic compound between Al and Fe was formed along the interface resulting in the joining of Al to steel. Tensile shear strength of Al/galvanised steel weld was lower than that of Al/steel one without galvanising. However, Al/galvanised steel welds exhibited higher fatigue strengths. Finite element method analyses around the nugget revealed that circumferentially resolidified Zn brought about the stress relaxation at the edge of the nugget, resulting in the better fatigue performance of Al/galvanised steel welds.

Fatigue behaviour of dissimilar friction stir welds between wrought and cast aluminium alloys

ABSTRACT Wrought aluminium alloy, A6016-T4, was welded to cast one ADC12 by a friction stir welding technique. A6016 was set on the advancing side, while ADC12 was on the retreating side based on the pre-tensile test results. The dissimilar welds exhibited lower fatigue strengths than A6016 but higher than that of ADC12. Fatigue strengths increased when the tool was offset into ADC12, which could be explained based on the risk volume concept. When ADC12 was set on the advancing side, the fatigue strengths decreased. Microstructural observation revealed that the macroscopic structure in the stir zone drastically changed by putting ADC12 on the advancing side, and insufficient mixing of dissimilar alloys resulted in the lower fatigue strengths.

Microstructural Modification of AZ91 Magnesium Alloy Using Friction Stir Processing and Carbon Fibers

The microstructure of cast magnesium (Mg) alloy, AZ91, was modified by a friction stir process (FSP) technique. FSP was applied to AZ91 plate with a narrow slit, in which carbon fibers (CFs) were filled. The rotating FSP tool consisting of probe and shoulder could modify the microstructure of the material by sever plastic deformation (SPD) and simultaneously disperse CFs into the matrix. Microstructural observation revealed that sever stirring of material by a tool resulted in the grain refinement due to the dynamic recrystallization and distribution of CFs into the stir zone (SZ). The distribution was not uniform depending on the plastic flow in the SZ, but any worm-hole defects were not formed. Hardness of FSPed AZ91 in the SZ was higher than the as-casted AZ91 because of the grain refinement and distribution of hard CFs.

New Application of Electromagnetic Vibration Process for Creation of High Potential Metallic Materials

We have developed the new process for refinement of metallic materials during solidification without addition of refiners or without rapid cooling. This process uses electromagnetic body force based on the vibrations caused by simultaneous imposition of direct magnetic field and alternative electric current on the alloy melt during solidification. The vibrations create cavitation in the melt and it breaks out during growth of it. Then explosive force is released toward the surroundings such as the primary solid particles and they are fractured finely. Finally fractured solid particles solidified as very fine grains. This process was applied to hyper- and hypo-eutectic Al-Si alloys, and AZ91D Mg alloys. Primary silicon crystals in Al-17mass%Si alloy decreased the size remarkably by the optimum electromagnetic vibration condition. Primary aluminum dendrites particles in Al-7mass%Si and primary magnesium dendrite particles in AZ91D are also decreased its size markedly. This process has also been applied to create non-equilibrium state metallic materials such as metallic glasses. We have succeeded to obtain metallic glasses in Mg-Y-Cu alloy system and Fe-Co-Si-B-Nb alloy system. These alloys solidified as fully crystals in this cooling rate without electromagnetic vibrations. These glasses are bulky and are used as structural material.