Search

Friction stir welding (FSW) has many advantages rather than fusion welding, but details of internal phenomena during its processes have not yet been clarified. In this study, a thermo-mechanically coupled process model was developed to investigate FSW phenomena inside a tool and workpiece. As a workpiece, 6061-T6 aluminum alloy was employed. The system of FSW process model includes several thermal boundaries. Among heat flows through these boundaries, heat transfers into the exterior of the system become more sensitive to tool and workpiece temperatures than heat transfers within the system. This paper especially focused on a heat transfer coefficient at a workpiece bottom, and optimized it through experiments and finite element method (FEM) analyses. The tool temperatures during FSW were measured with a special tooling system with imbedded thermocouples within a tool. As a result, an analysis model that is able to investigate details at a wide range of traverse speeds was developed for practical high speed welding. Then, the accuracy of developed FEM model was validated with them. Finally, the temperatures and stress distribution around workpiece/tool interfaces were investigated with the developed model.

Magnesium alloys are light weight and highly recyclable. To use magnesium alloys as parts of products such as automobiles, it is very effective make bosses by using plastic workings because this can reduce material loss and welding for producing parts. The bosses can be produced by a press cylindrical tool with rotation to the surface of the magnesium alloy sheet. To reduce the period of development of the tools in forming condition investigations, numerical simulation with FEM is very useful. FEM technologies have been developed for making bosses with plastic working. Because of the symmetry of the tool, the axisymmetrical FEM is applied as the first step. Bosses were found to increase in height as the friction coefficient increased. To investigate the friction between the tool and the alloy sheets, a ring compression test was conducted. The temperature of the alloy sheets increases as the bosses are formed. Therefore, the compression test was conducted at an elevated temperature. The heat transfer rate also affects the results of the forming simulation. To improve the shape of bosses from the simulation results, 3D simulation was performed. The shape of bosses in the 3D simulation is better than that in the 2D simulation by taking into account the traction at the contact area of the cylindrical tool.

During superplastic deformation (SPD) of tetragonal zirconia polycrystals containing 3 mol% yttria (3Y-TZP) at high strain-rates, a number of crack-like flat cavities having very narrow gaps lying along grain boundaries mostly normal to the tensile axis are produced in addition to conventional cavities. The formation and growth of these flat cavities are responsible for the strain softening that appears on the true stress versus true strain curves. The growth and coalescence of the flat cavities were a main cause of the degradation of elongation to fracture. We have found that a simple treatment, in which the superplastic deformation is temporally stopped, i.e., the cross-head movement is reversed and accordingly the applied load is removed, and then the specimen is kept at the test temperature for several minutes, is surprisingly effective to reduce the flat cavities. Carrying out this simple treatment repeatedly, after 30% nominal stain during the SPD, led to an increace in total elongation by about three times larger than that of a specimen not subjected to such a treatment. This treatment can also recover the strength and accordingly mechanical properties of the superplastically deformed 3Y-TZP to that of the undeformed state. This finding is believed to be quite significant for practical applications of superplasticity in 3Y-TZP, because the flat cavities can be closed very simply by keeping a product at the forming temperature after or during the superplastic forming process.

Dent resistance of AZ31 magnesium alloy sheets was examined under static and dynamic conditions. Dent tests were carried out for AZ31-O and AZ31-H24 sheets by using circular blanks and dent depth was measured and compared with that of aluminum alloys (1100-O, 5052-O, 5052-H34), copper alloy (C2801-O) and cold-rolled steel (SPCE) sheets. The critical punch load for initiation of the dent formation was not detected for AZ31 sheets in the punch load-stroke curves of static dent tests whereas it was clearly observed in steel sheet. In static dent tests, magnesium sheet with the lowest elastic modulus showed the largest spring back and consequently the smallest dent depth at the constant punch stroke. AZ31 sheet also showed smallest dent depth at the constant punch load. In dynamic dent tests, AZ31 sheet showed the highest dent resistance under various impact conditions. It is concluded that AZ31 magnesium sheets are suitable for various light weight panels with their excellent dent resistance as well as low density.

On the AZ31-O magnesium alloy plates of 20 mm in thickness, basal plane texture was studied for the samples taken from different layers. Compression tests were carried out in the rolling, width and thickness directions at room temperature. Tensile tests were conducted for the specimens taken from the different layers of the plates in different planar directions. The formability in deep drawing and stretch forming was evaluated for the thin sheet specimens taken from different layers of the plates and the results were discussed in relation to the texture and mechanical properties. The severity of the basal plane texture is higher at the surface layer than the inner layers. In tensile tests at room temperature, proof stress is higher for the surface layer than the inner layers, whereas elongation is lower and r-value is higher at the surface layer. In compression tests at room temperature, yield stress in the rolling and width directions is appreciably lower than in the thickness direction. At 573 K, anisotropic and non-uniform deformation behavior disappeared. Thin sheet specimens taken from inner layers of the plates showed higher formability than those from the surface layer in deepdrawing and stretch forming. It is concluded that the formability of magnesium alloy sheets can be improved by decreasing the severity of the basal plane texture. In rolling of magnesium and its alloys, the basal plane of the hexagonal lattice becomes aligned parallel to the sheet plane so that a strong basal plane texture is formed. Such textures are maintained even after recrystallization by annealing. Thus, rolled sheets of magnesium and its alloys generally show a strong anisotropy in plastic deformation behavior at around room temperature. Deformation accom- panying with thickness reduction is always difficult, that is, they show low limits of plastic deformation in cold rolling and in sheet metal forming. This is the main reason why the rolled sheets of magnesium and its alloys are not widely used in practice in spite of the recent high demands for the light- weight materials. However, high plastic anisotropy of magnesium alloy sheets becomes disappeared at tempera- tures above 473 K due to contribution of non-basal plane slip systems. At these temperatures, sheet rolling of magnesium alloys becomes much easier and the formability in deep drawing, 1-3) stretch forming, 4) bending 5) and forming limit curves 6) is appreciably improved. In thick plates of magnesium alloys, basal plane texture is considered to be less developed than in thin sheets due to lower reduction and higher rolling temperature. Additionally, mechanical properties in the thickness direction can be evaluated in thick plates. The anisotropy and non-uniformity of the basal plane texture and mechanical properties can also be discussed by testing specimens from the different layers of the plates. Obtained results are considered to suggest the desirable textures and mechanical properties for higher formability at room temperature. It has been reported in plane strain compression tests of AZ31 magnesium alloy plates that the compressive yield stress in the rolling direction is appreciably lower than in the other directions, which can be attributed to the highly developed basal plane textures. 7) However, very little has been known on the plastic behavior of magnesium plates. It is the purpose of this work to elucidate the anisotropy and non-uniformity of plastic behavior of the magnesium plates. In this work, the basal plane texture was studied for the samples taken from different layers of AZ31-O magnesium alloy plates of 20 mm in thickness. Compression tests were done in the rolling, width and thickness directions at room temperature. Tensile tests were conducted for the specimens taken from the different layers of the plates in different planar directions. Finally, formability in deepdrawing and stretch forming was evaluated for the thin sheet specimens taken from different layers of the plates and the results were discussed in relation to the texture and mechanical properties.