Papers by Author: Tetsuo Sakai

Abstract: Although forming of porous metal is demanded for industrial applications, the deformation characteristics have not been investigated sufficiently. In this study, lotus-type porous copper is processed by multi-pass cold rolling. At the early stage of rolling, the elongation of the porous copper in the rolling direction is small, and the porosity decreases almost linearly with the total reduction in thickness. It is found that pass schedule with small rolls and with small reduction per pass is effective to suppress pore closure. Hardness of the porous copper increases almost linearly with total reduction. If the effective total reduction is considered, the hardness change is similar to that of a nonporous copper.

Abstract: 1050 aluminum sheet was cold-rolled by 50% with mineral oil. Hardness was measured on longitudinal section through the thickness of the rolled sheet. The overall hardness statistics of the as-rolled sheet followed the normal (Gaussian) distribution. Though the sheet was rolled under good lubrication, considerable redundant shear deformation was introduced beneath the surfaces by friction. The layers beneath the surfaces showed higher hardness than the center. The hardness subset statistics in one layer in the thickness also followed normal distribution. Isothermal changes in Vickers hardness statistics and in tensile properties during annealing at 548K were investigated. Beneath the surfaces, the recrystallization initiated and completed earlier than the center. The fully recrystallized sheet also shows the normal distribution of hardness, however the standard deviation is much smaller than that before annealing. The layers beneath the surfaces show lower hardness than the center after recrystallization. The partially recrystallized sheet shows bi-modal distribution of hardness. The partially recrysrallized sheet shows slightly better tensile-elongation balance than both the as-rolled sheet and the sheet-fully recrystallized.

Abstract: Magnesium alloys are expected to be used widely as structural materials because of their lowest density (1.8g/cm3) among all practical alloys and superior specific strength. However, magnesium alloys exhibit poor ductility due to its hcp structure and inactiveness of non-basal slip systems below 523K. Accordingly, magnesium alloy sheets had to be rolled at elevated temperature to avoid edge cracking and fracture during rolling. The present authors succeeded in single pass large draught rolling of AZ31 magnesium alloy sheets below 473K without heating rolls by rolling at the speed higher than 1000m/min. The rolled and quenched sheets had fine recrystallized microstructure and exhibited excellent mechanical properties. It was found that the high speed rolling is a promising method not only for increasing productivity but also for controlling microstructures and improving mechanical properties. If the above mentioned advantages of high speed rolling can be drawn from the rolling at the speed lower than 1000m/min, it is possible to mass-produce magnesium alloy sheets having superior mechanical properties at lower cost. In this study, we tried to determine the lower limiting rolling speed at which we can obtain advantages of high speed rolling. We revealed that the thickness could be reduced about 60% by single pass operation even at 250m/min without heating rolls. The rolled and quenched sheets had equiaxed fine recrystallized microstructure. For example, the mean grain size of 2.1m was obtained in the AZ31B sheet rolled at 250m/min at room temperature to the reduction of 60%.

Abstract: The present authors have succeeded in single pass large draught rolling of AZ31 and ZK60A magnesium alloy sheet below 200°C without heating rolls by raising the rolling speed above 1000m/min. Maximum reduction attained in single pass rolling was 60%. Among magnesium alloys, AZ31 is known as the most ductile alloy. It remains uncertain whether the high limiting reduction by high speed rolling can be attained in other magnesium alloys that are less ductile but stronger than AZ31. In this study, AZ80A (Mg-8.1%Al-0.63%Zn) sheets with the thickness of 2.7mm cut from the extruded sheets were used. Rolling temperature was varied from RT to 350°C. Rolling speed was 1000m/min. The limiting reduction in thickness increases with rolling temperature, and the maximum reduction of 52% is obtained at 250°C. The fracture surface of sheet rolled at 100°C shows ductile fractured surface, while it shows brittle fracture surface at 350°C. This difference in fracture mode is attributed to the precipitation of -particles at grain boundaries during holding at 350°C before rolling. From this result, high speed rolling can also be an effective tool for improving the rolling deformability of AZ80 sheet. The hardness of the rolled sheets measured on the transverse plane increases with increasing temperature and reduction. The variation of hardness with rolling temperature and reduction indicates the occurrence of dynamic recrystallization (DRX). The sheet rolled at 200°C with the reduction of 50% shows the tensile strength of 353MPa and the elongation of 29%, which is an excellent strength-ductility balance. By applying high-speed rolling process to AZ80 magnesium alloy, we can obtain a remarkable improvement in the material characteristics as well as rolling deformability.

Abstract: It is known that well developed <111>//ND texture increases Lankford value (r-value) of not only bcc metals but also fcc metals and alloys. However, <111>//ND texture cannot be formed in fcc metals by conventional rolling and annealing processes. The <111>//ND orientation is one of the major components of shear texture. Accordingly, this orientation develops in aluminum sheet when shear deformation is introduced. Al-Mg-Si alloy 6016 sheet was processed by two-pass differential speed rolling at room temperature under a high friction conditions. The rolling direction of the second pass was so selected that the direction of shear deformation introduced in the second pass was either similar (unidirectional shear rolling) to or opposite (reverse shear rolling) to that in the first pass. The roll speed ratio was 2.0. Large shear strain was successfully introduced through the thickness uniformly by the differential speed rolling. The shear texture with major components of {001}<110> and {111}<110> were developed throughout the thickness. Though large reduction in thickness of 75% was applied to the sheets by the rolling, conventional rolling texture such as {112}<111> or {123}<643> orientation was not detected in any part of the thickness. By solution treatment after the rolling, intensity of shear texture weakened and grain size decreased. It has been found that r-value is improved by the differential speed rolling subsequently followed by solution treatment.

Abstract: It is believed that the shear deformation superimposed on rolling deformation accelerates grain refinement. However, it has not yet been completely understood whether the grain refinement is due to the increase in amount of equivalent strain, or the change in strain path. In the present study, three different strain paths in plane strain - (1) simple shear, (2) compression and (3) the combination of simple shear and compression - are introduced into 1100 aluminum sheet. The recrystallization behaviours are compared. Plane-strain compression was achieved by a normal rolling, while the simple shear was achieved by a continuous ECAE (conshearing). The combined strain path was achieved by the conshearing subsequently followed by the rolling. The same amount of the equivalent strain of 1.28 was accumulated in the three paths. The ratio of shear strain to compressive strain was varied by three levels in the combined strain process. After heat treatment, the material processed by the combined strain path gave a finer recrystallized grain size than both of the monotonic strain paths at either annealing temperature. The finest recrystallized grain size was obtained at the shear strain ratio of 0.6 to the total equivalent strain. It was found that the change in strain path was effective for introducing more new high-angle grain boundaries.

Abstract: Magnesium alloy sheets had to be rolled at elevated temperature to avoid cracking. The poor workability of magnesium alloy is ascribed to its hcp crystallography and insufficient activation of independent slip systems. Present authors have succeeded in 1-pass heavy rolling of AZ31 magnesium alloy sheet below 473K by raising rolling speed above 1000m/min. Heavy reduction larger than 60% can be applied by 1-pass high speed rolling even at room temperature. The improvement of workability at lower rolling temperature is due to temperature rise by plastic working. The texture of heavily rolled AZ31 magnesium alloy sheet is investigated in the present study. The texture of sheets rolled 60% at room temperature was <0001>//ND basal texture. At the rolling temperature above 373K, the peak of (0001) pole tilted ±10-15 deg toward RD direction around TD axis
to form a double peak texture. The texture varied through thickness. At the surface, the (0001) peak tilted ±10-15 deg toward TD direction around RD axis to form a TD-split double peak texture. The direction of (0001) peak splitting rotated 90 deg from the surface to the center of thickness. Heavily rolled magnesium alloy sheets have non-basal texture. The sheets having non-basal texture are expected to show better ductility than sheets with basal texture.

Abstract: Nano-structured aluminum was fabricated by accumulative roll-bonding (ARB) process using different rolling methods. One is the ARB using conventional rolling (CR) in which the speed of two rolls (3.0m/min) was equal to each other. The other is the ARB using differential speed rolling (DSR) in which the speed of two rolls is different to each other. The roll peripheral speed of one roll was 2.0m/min and that of another roll was 3.6m/min. The roll speed ratio was kept at 1.8. The ARB was conducted up to 6 cycles at ambient temperature without lubrication. In both cases, the ultrafine grains were developed in the samples. The grains formed by the DSR-ARB were more equiaxed and finer than those produced by the CR-ARB. Tensile strength of the DSR-ARB processed sample was superior to that of the CR-ARB processed one. The elongation was not affected significantly by the number of ARB cycles in both cases. Texture analysis demonstrated that the shear strain, in the case of DSR-ARB, was introduced into the center of thickness. It was concluded that the DSR-ARB process was more effective for grain refinement and strengthening than the CR-ARB process.

Abstract: The preferential initiation of dynamic recrystallization (DRX) at triple junctions (TJs) in stainless steel polycrystals was investigated in compression at 1123 K to 1323 K at a strain rate of 2 x 10-4 s-1. Nucleation appeared at TJs at strains as low as 0.1. This strain is only about 1/5 to 1/2 of the peak strain at which DRX is conventionally believed to occur extensively. Furthermore, DRX nucleation was not observed to take place at grain boundaries or in the matrix at this strain. The probability of DRX nucleation at TJs increased monotonically with strain and temperature. It also depended on the angle, y, between the compression axis and the sliding boundary. That is, when the angle, y, approaches 45 degrees, the probability of DRX nucleation at TJs is higher. These results reveal the important role of grain-boundary sliding (GBS) on DRX nucleation at TJs. It should also be noted that more than 90% of the grains nucleated at TJs were twins. Such dynamic twinning suggests that the essential DRX nucleation mechanism is twinning.

Abstract: Large shear deformation was successfully introduced in 5182 aluminum alloy sheets by 2-pass differential speed warm rolling under a high friction condition. The roll speed ratio was varied from 1.0 to 2.0. When the roll speed ratio was smaller than 1.4, shear strain increased near the surface, but the strain decreased to zero at the mid-thickness. At a roll speed ratio larger than 1.4, shear strain was introduced even at the mid-thickness, and it increased near the surface. Thus the shear strain increased with the roll speed ratio. After 2-pass differential speed rolling, a large shear strain prevailed throughout the thickness. The rolling direction of the second pass was so selected that the direction of shear deformation introduced in the second pass was similar to (unidirectional shear rolling) or opposite (reverse shear rolling) that in the first pass. A shear texture with main components of {111}<110>, {112}<110> and {001}<110> prevailed throughout the thickness, and conventional rolling textures such as {112}<111> or {123}<634> orientation were not detected in any part of thickness. The rolling direction of the second pass had little effect on the deformation texture. After recrystallization annealing, the shear texture components were retained. The intensity of the shear texture components after recrystallization was almost similar to the deformation texture. The r-value of the annealed sheet was slightly increased and the planar anisotropy of the r-value was decreased by differential speed rolling. Differential speed rolling, by which shear deformation can be introduced throughout the thickness, was thus shown to be a promising process for improving the physical and mechanical properties of rolled and annealed aluminum alloy sheets by texture control.