Semi-liquid slurry consists of globular primary solid phase and remaining liquid phase can be obtained directly from molten metal without stirring technique by pouring low temperature melt with superheated within 30 K, into the vessel and cooling the slurry at the rate of not more than 0.15 K/s. The reasons why shape of primary crystals become globular during solidification, not dendritic, are considered as follows; 1) large number of nuclei are produced during pouring process and those nuclei survive for a long term because of low superheat degree, so there are small space between growing crystals, 2) distribution of solute element and temperature gradient around solid-liquid interface are uniform, 3) supercooling degree is small. High pressure casting fabricated using this process show high tensile strength and large elongation. Prevention of mingling oxide inclusions, which forms at the upper surface of the slurries, by reversing vessel when injecting slurries into the shot sleeve and the fine eutectic microstructure give rise to the good mechanical properties, especially large elongation.

View full abstract

TiAl-based intermetallic alloys with a lamellar microstructure and β containing duplex microstructure were cyclically tensile deformed in various environmental media as a function of temperature. Environmental media used in this study were vacuum, air and a mixture gas of H₂ gas+Ar gas. The environmental embrittlement, i.e., reduction of number of cycles to failure was observed when deformed in air as well as in Ar gas including H₂ gas. The severe environmental embrittlement was observed not only at room temperature but also at ~873 K. Fracture patterns and the associated fracture mode were not affected by the environmental media. The possible species causing the environmental embrittlement is suggested to be hydrogen atoms decomposed from water vapor (H₂O) or hydrogen gas (H₂). The feature observed in the present study was similar in both TiAl-based intermetallic alloys and, also to the previous observation, i.e., the feature of the environmental embrittlement observed in simply tensile deformed specimens.

View full abstract

Influences of homogenization process and Cu addition on age-hardening behavior of an Al–0.93%Mg₂Si alloy were investigated. According to variation in electrical resistivity converted from the IACS%, Mg and Si solute concentration was decreased by the ingot homogenization at 723 K for 7.2 ks. The resistivity of a 0.5%Cu added alloy was higher than that of the base alloy and their difference was maintained almost constant for three homogenization temperatures, 723 K, 773 K and 823 K. Therefore, it is considered that almost all of the added 0.5%Cu was dissolved in matrix solid solution at all temperatures of the homogenization. The size and number density of secondary phase particles of intermetallic compounds obtained by SEM image analysis elucidated that volume fraction of Mg–Si particles was increased by the homogenization at 723 K, though no change was observed in that of Al–Fe–Si particles. After the homogenization treatment, ingots were rolled to 1.0 mm thickness. Tensile specimens sampled from the rolled sheets were held at 803 K for 15 s and quenched into water. Yield strength of 723 K homogenized specimens after T4, natural aging for more than one week, and final artificial aging at 443 K or 473 K was lower than that of specimens homogenized at 773 K or 823 K. To ensure enough high age-hardenability in the present alloys, it is necessary to homogenize ingots at temperature above 758 K, the complete dissolving temperature of Mg and Si. The Cu addition up to 0.7% increased yield strength after the artificial aging in all homogenizing and aging conditions.

View full abstract

The purpose of this study is to clarify the heat transfer coefficient between the molten magnesium alloy and roll, the temperature transition of molten metal and roll and the cooling rate of produced strip at single roll melt drag process. We applied the one dimensional heat transfer solidification analysis to melt drag process. Heat transfer coefficient was estimated from 1.42×10⁵ to 8.95×10^{4 W/m}2·K at roll speed of 5 to 90 m/min by comparing between analysis and experimental results such as strip thickness. The molten metal temperature at roll surface was estimated that getting to 336°C from 780°C in the nozzle, and the roll surface temperature was estimated that getting to 235°C from 20°C at a roll speed of 30 m/min. The cooling rate of strip was estimated from 1.4×10³ to 1.0×10⁴ K/s by Cα method.

View full abstract

Recently, the reduction of exhausted carbon-dioxide has been seriously required to avoid global warming. Naturally, this is an issue of great interest in the automobile industry. To achieve a sufficient reduction, lightening the autobody by converting the conventional steel sheets to aluminum alloy sheets is regarded as an effective and feasible method. Some Al–Mg–Si alloys have already been applied to a part of autobodies because these alloys have hardenability during paint baking and do not cause a stretcher strain pattern. However, their bendability is significantly poor compared to that of steels. In this study, the bendability has been assessed in terms of the macroscopic and microscopic appearance of the tension-side surface and measured surface roughness, as a function of the alloy composition, i.e., amount of Mg₂Si, amount of excess Si and addition of Cu and Cr. Specimens were solutionized, air-cooled or water-quenched and then naturally aged for 7 d. Before the bending test the specimens were stretched by 10%, since such straining is frequently carried out in the actual autobody sheets prior to the bending process. The results obtained were compared with those obtained in the previous report where stretching prior to bending had not been performed. The effects of the amounts of Mg₂Si and excess Si and of the Cu addition were similar to those in the specimen series without prior stretching, that is, bendability was decreased when the amounts of Mg₂Si and excess Si were increased, while it was slightly improved by the Cu addition of 0.34%. The effect of Cr addition of 0.21% did not coincide with that in the previous report, that is, Cr addition deteriorated surface roughness and tended to cause cracking despite an improvement in the bendability in terms of macroscopic appearance assessment in the present study, while it totally improved the bendability in the previous study.

View full abstract

Anisotropy in deformation and fracture strength of an aluminum beverage can body is investigated using a servo-controlled tension-internal pressure testing machine. The yield surface and forming limit strain and stress diagrams of the can body are successfully determined using linear strain-path tests. We conclude as follows:(i)The initial yield surface of the can body is in good agreement with Yld2000-2d yield criterion with degree of 10;(ii)fracture always occurs at θ=0° under linear strain-paths with ε_θ/ε_φ>1, where θ is the circumferential position on the can body measured from the rolling direction of the blank sheet, and ε_θ and ε_φ are circumferential and axial strain components, respectively. We found that ε_θ is significantly lower at θ=45° than at θ=0° and 90°;(iii)The fracture strength of the can body under plane-strain tension with ε_φ=0 is minimum at θ=0°and maximum at θ=45°; this accords with the result of(ii);(iv)In order to evaluate the fracture strength of the can body accurately, the stress state of the can body in the test should be as close as possible to the one that occurs in a can body in a real service condition.

View full abstract

Fatigue crack growth (FCG) properties were determined for aluminum alloy 6061-T6 and 7075-T6 plates exposed to a humid (relative humidity RH=90%) air or dry nitrogen gas, and the effect of hydrogen on the FCG process was discussed together with a fractographic study of fracture surfaces. The 6061-T6 shows a superior FCG resistance to the 7075-T6, resulting in no increase in FCG rate with a reduction of frequency from 10 Hz to 0.1 Hz and also with a change of cyclic loading wave from sine to trapezoid. This response is attributed to the effect of hydrogen to assist localized slip deformation, leading to a crack-tip bunting. According to applying the present FCG data to design a high-pressure container liner for automobile on a basis of the engineering standard JARI S001, it is found that the 6061-T6 can possess a higher allowable stress than the 7075-T6.

View full abstract