Masato Tsujikawa

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.

Laser surface alloying of zinc-nickel doubly electroplated films on steel

A new method has been developed in which zinc-nickel doubly-coated films were electroplated onto steel substrates and then irradiated, using a high powered YAG laser with kaleidoscope for surface modification to alloy the zinc-nickel films.

Pearlite Stabilisation by Copper on Ductile Cast Iron

In ductile cast irons with copper, cementite stability was investigated against an annealing heat treatment used to obtain a fully ferritic matrix. Copper controls cast-iron mechanical properties, but its role in the matrix microstructure formation remains unclear. Some reports suggest the copper layer around graphite or cementite. They can be barrier to carbon diffusion at eutectoid reaction, however it is difficult to understand the mechanism of pearlite stability by copper. To confirm the existence of the barrier and effect of copper addtion, ten 9-mm-thick spheroidal graphite cast iron castings were prepared with different copper contents of 0.16 wt% – 0.69 wt%. The samples’ as-cast microstructures included spheroidal graphite, ledeburite, and pearlite. The pearlite fraction degreases to about 10% by heat treatment for ordinary ductile irons without intentional copper addition. The samples’ copper content and the pearlite fraction after heat treatment are not linearly related. The retained pearlite increased suddenly with increased copper content greater than 0.4 wt%. However, even the sample with the highest copper content showed no precipitation of a copper solid solution around graphite nodule or cementite.

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.

Hardness profile improvement of plasma nitrided high speed steel by glow discharge heating

The hardness of a plasma nitrided high speed steel surface is extremely high at the surface because of the formation of nitrides and the diffusion layer. It decreases steeply to the value of the substrate. Such a brittle surface and steep change in hardness are not favourable for tool applications because of tool chipping. This paper investigated the improvement of such a hardness profile of plasma nitrided high speed steel using glow discharge heating as a post-nitriding diffusion treatment. High speed steel was plasma nitrided at 500°C under 667 Pa for 2 h in a gas mixture of nitrogen and hydrogen. A diffusion process was then conducted in the same chamber by changing the gas to hydrogen or argon. The plasma nitrided high speed steel was also diffusion treated by furnace heating. The microstructures and the hardness profiles of these two post-nitriding treated samples were compared. Diffusion treatment by glow discharge heating lessens the hardness gradient from steep to gentle more effectively than fur...

Crystallisation by laser for Zr based bulk metallic glass

Abstract Bulk metallic glasses (BMGs) show high amorphous phase stability. The solidification process of BMG occurs with the cessation of epitaxial crystal growth. For a laser heated and melted surface of BMG, crystallisation occurs during cooling, along with crystalline growth from the heat affected zone. Such crystallisation stops at a certain location where the residual melt solidifies as amorphous. The amorphous formation can occur after crystal nucleation. Thermal treatment using a diode laser was used for surface modification of 2 mm thick Zr55Al10Ni5Cu30 (at.-%) BMG plates. When laser conditions (scanning speed, defocused irradiated spot size and output power) were changed, the maximum temperature and heating cooling rate were changed, and microstructures were changed. Cessation of crystallisation was observed.

Influence of Contineous Transversal Inclination of Tool on FSW Joints

The influence of roll angle of the FSW tool rotation axis on tensile properties of joints was examined to clarify the allowance of deviation from proper tool posture for three-dimensional friction stir welding. A5083-O plates of 3 mm in thickness were butt welded by three-dimensional controlled FSW machine. Transversal inclination of the tool (roll angle of the tool) was continuously changed from 0 degrees to 10 degrees for both sides. Butt joints ruptured at base metal within the range of roll angle of the tool plus/minus 2 degrees, and ruptured at the thin part made by inclined tool shoulder entering in the range by plus/minus 7 degrees. Over the range of plus/minus 7 degrees, butt joints ruptured at elongated cavities made by tool inclination at the border of stir zone and base material.

Microstructure of friction stir processed Mg-Y-Zn alloy

Friction stir processing (FSP) is the effective method of the grain refinement for light metals. The aim of this study is to acquire the fine grained bulk Mg-Y-Zn alloy by ingot metallurgy route much lower in cost. Such bulk alloy can be formed by the superplastic forging. The microstructure of as-cast Mg-Y-Zn alloy was dendrite. The dendrite arm spacing was 72.5 [(m], and there are the lamellar structures in it. FSP was conducted on allover the plate of Mg-Y-Zn alloy for both surfaces by the rotational tool with FSW machine. The stirring passes were shifted half of the probe diameter every execution. The dendrite structures disappeared after FSP, but the lamellar structure could be observed by TEM. The matrix became recrystallized fine grain, and interdendritic second phase particles were dispersed in the grain boundaries. By using FSP, cast Mg-Y-Zn alloy could have fine-grained. This result compared to this material produced by equal channel angular extrusion (ECAE) or rapid-solidified powder metallurgy (RS P/M). As the result, as-FSPed material has the higher hardness than materials produced by the other processes at the similar grain size.

Dynamic Recrystallization during Hot Extrusion in Mg-3Al-0.1Y Alloy

Dynamic recrystallization behavior of Mg-3.1wt.%Al-0.1wt.%Y alloy was investigated. During extrusion, dynamic recrystallization occurred and very fine microstructure was obtained. We compared the relationship between Zener-Hollomon parameter, Z, and the dynamically recrystallized grain size, drec, and between the normalized recrystallized grain size, drec/d0, where d0 is the initial grain size, and Z for Mg-3.1wt. %Al-0.1wt.%Y with commercial Mg-3Al-0.5Zn alloy. As the result, the dynamically recrystallized grain size of Mg-3Al-0.1Y was finer than that of Mg-3Al-0.5Zn alloy under the equivalent Z, while the initial grain size of Mg-3Al-0.1Y was coarser than Mg-3Al-0.5Zn alloy. The normalized grain size for Mg-3Al-0.1Y alloy was much smaller than that of Mg-3Al-0.5Zn alloy under the equivalent Z. We conclude that small addition of Y to Mg-Al alloys is effective for grain refinement by dynamic recrystallization.

Two Pass FSW Joint of Magnesium Alloy

Mechanical properties of friction stir welding (FSW) butt joint are affected by back surface defect. The back surface defect is as un-welded zone that is often remained unstirred by tool. FSW from both surfaces can eliminate the back surface defect basically. Furthermore, the residual stress after welding can be lowered than in single pass FSW by using smaller tool. Mechanical properties of magnesium alloy butt joint made by FSW from both surfaces of surface were compared with that of ordinary single pass welding. It was found that the tensile strength and fatigue life were improved and the tensile residual stress at back surface of single pass FSW changed to compressive stress.