Papers by Author: Yasuhiro Uetani

Abstract: Both rapid and stable semi-solid slurry preparation of hypereutectic Al-14.8%Si-4.5%Cu-1.1%Mg alloy (in mass%) with fine solid granules as well as finely dispersed primary silicon particles was tried using a simple method newly developed, where the low-superheat melt passes through a gap between a rotating regular octagonal rotor and a circumscribed chill block. By examining effects of melt temperature running out from tundish, gap distance and rotational speed of rotor on solidification structure of the semi-solid slurry manufactured, the optimum making condition of the targeted semi-solid slurry was found out. Mean sizes of solid granules and primary silicon particles within the slurry, which had been made quickly with the good reproducibility, were 45μm and 20μm, respectively. Subsequently the obtained semi-solid slurries were rheocast into a cylindrical shape cavity by the use of a high pressure casting machine. It has been shown that the rheocast samples have peak hardness value between the gravity casting and the hot-extrusion, but a good age-hardenability somewhat higher than that of gravity casting.

Abstract: Semi-solid slurry of phosphor bronze with fine solid granules for rheocasting was tried to make by passing the low-superheat melt through an opening between a rotating regular octagonal rotor and a circumscribed chill block. The effect of rotational speed on the structure of a solidified small ingot was mainly investigated. The ingot structures were changed from the dendrite of rotor-free condition to the rosette at the rotational speed of 50rpm, and the granular with a mean size of about 50μm at higher than150rpm. The slurries made at 150rpm were rheocast to a cylindrical shape cavity within a die kept at different temperatures from 573 to 1173K by the use of a high pressure casting machine. Microstructures of rheocast cylindrical shape products were granular at the die temperatures lower than 773K and changed to grained one as like recrystallized at higher than 973K. Together with the microstructure change, poor tensile ductility less than 3% at the lowest die temperature was improved up to about 60% at 973K.

Abstract: Semi-solid slurry of phosphor bronze with fine solid granules for rheocasting was tried to make by passing the low-superheat melt through an opening between a rotating regular octagonal rotor and a circumscribed chill block. The effect of rotational speed on the structure of a solidified small ingot was mainly investigated. The ingot structures were changed from the dendrite of rotor-free condition to the rosette at the rotational speed of 50rpm, and the granular with a mean size of about 50μm at higher than150rpm. The slurries made at 150rpm were rheocast to a cylindrical shape cavity within a die kept at different temperatures from 573 to 1173K by the use of a high pressure casting machine. Microstructures of rheocast cylindrical shape products were granular at the die temperatures lower than 773K and changed to grained one as like recrystallized at higher than 973K. Together with the microstructure change, poor tensile ductility less than 3% at the lowest die temperature was improved up to about 60% at 973K.

Abstract: The effect of Al-addition on the mechanism of phase decomposition from β₁ to α-phase in Cu-40at.%Zn alloy (base alloy) has been investigated using hardness measurement and HRTEM observation. The peak hardness during annealing and the number density of α-phase in Al-bearing alloy were the same as those in the base alloy. Many striations which have been considered as the periodicity of 9R structure were observed in the α-phase of the Al-bearing alloy. There were a lot of striations in the α-phase at the early annealing time, and the number of these striations decreased with increasing annealing time. The rate of decreasing of this striation in the α-phase of the Al-bearing alloy was the slower than that of the base alloy. According to the analysis of the selected area electron diffraction (SAED) patterns obtained for these α-phase in those alloys, transition time from the 9R structure to fcc structure t (9R-fcc) of has been detected by the changing of angles in SAED patterns. The t (9R-fcc) of α-phase in the Al-bearing alloy showed the slower than the base alloy.

Abstract: Rheo-extrusion of hypereutectic Al-14.8%Si-4.5%Cu-1.1%Mg (in mass%) alloy which is considerably difficult to hot-extrude was tried using semi-solid slurry with fine solid granules and almost finely dispersed primary Si particles, made by a newly developed rotor process. Trials of rheo-extrusion at the constant extrusion ratio of 36 using this semi-solid slurry were carried out at the different conditions of the extrusion temperature and the ram speeds ranging from 520 to 560oC and from 5 to 20mm/s, respectively. Under the all rheo-extrusion conditions, this semi-solid slurry could easily be rheo-extruded to the round bars of 6mm diameter with sound surface at much lower extrusion force than that of hot-extrusion at very low extrusion ratio. In addition, the characteristic age-hardening behavior which is thought to be essential for the rheo-extrusion process has been found out, namely the peak hardness values of aged bars rheo-extruded at 520 and 540oC were considerably higher, 30HV at maximum, than that of a gravity casting.

Abstract: Semi-solid slurry with fine solid granules for rheocasting was made from lead-free bismuth bronze by passing the low-superheat melt through an opening between a rotating regular octagonal rotor and a circumscribed chill block. We mainly investigated the effect of rotational speed on the structure of a solidified small ingot. The ingot structures changed from the dendrite of rotor-free condition to the rosette at the rotational speed of 50rpm and to the granular with a mean grain size of about 50m at higher than100 rpm. If the slurry made at 100rpm was rheocast to a disk shape by the use of a high pressure casting machine, the disk had almost uniform granular structures from the center to the edge regions.

Abstract: The purpose of study is to investigate aging behavior, crystal structures of metastable phase and relative frequency of metastable phases in aged Al-Mg-Ge and Al-Mg-Ge-Si alloys using high resolution transmission electron microscope (HRTEM), energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectroscopy (EELS). Every alloy included rod-shaped precipitate which is the same as the typical metastable pahse, ’, in Al-Mg-Si alloy. Except to Mg-rich alloys, the Type-A precipitate, which is a typical metastable phase in the excess Si type Al-Mg-Si alloys and popular at over aged condition, was confirmed as a large rod-shaped precipitates in those alloys. This behavior is probably depends on the ratio of Mg/Ge. Also, Mg, Si and Ge were detected from the ’-phase in Al-Mg-Ge-Si alloy by EELS. This means that the ’-phase in Al-Mg-Ge-Si alloy consists of these 3 elements including Si, not just Ge to form metastable Mg2Ge.

Abstract: It is known that the phase-decomposition process of 60/40 Cu-Zn alloy is so-called the bainitic transformation, and decomposition of α-phase from the β’-phase is as follow: β’ → α9R → αfcc. In this work,decomposition of α-phase from the β’ single phase of Cu-40.26at.%Zn alloy has been investigated by high-resolution transmission electron microscopy (HRTEM) to understand the phase transformation of this alloy. Especially, striations in the α-phase has been focused on the special feature for the change of the structure and hardening of this alloy during annealing. The result of a comparison between this alloy and the Si added alloy is also reported.

Abstract: In order to rheocast using Al-Si foundry alloy with higher concentration of iron, semi-solid slurries were tried to manufacture by two different processes. The former process made the slurry with granular solid phase and iron containing intermetallic compound β-phases of average length of 60μm. In the latter process, morphology of the β-phases became thinner, although no difference in their lengths. These slurries were rheocast to a disk shape with diameter of 80mm and thickness of 5mm at slurry temperature of 873K. Ductility of the disk rheocast using the latter slurry was slightly improved in comparison with that of the former slurry. However these levels of ductility were still much lower than that of the slurry without β-phases.

Abstract: It is very popular to improve mechanical properties of Al-Mg-Si alloys by adding other element. For example, it is well known that addition of transition metals is useful because the refinement of grain size can be achieved, and with the addition of Cu to Al-Mg-Si alloy can increase the strength and hardness of the materials. Also, there are a few reports about the addition of Ag, which is particularly good for elongation. However, even now it has not been clarified yet why Cu and Ag are useful in improving alloys elongation. In this work, we have investigated the tensile behavior of excess Si-type Al-Mg-Si alloys, which contain Cu. The tensile test and hardness measurement were carried out. Scanning electron microscope (SEM) was used to investigate the fracture surfaces and individual crystallographic orientation. After small additional deformation, the crack was observed by the interface between higher Schmid Factor (S.F.) and lower S.F. grains. It was thought that this crack influenced fracture from fracture surface.