Primary Crystal Size Research Articles

Improving the Fatigue Strength of Aluminum Alloy Castings by HIP Treatment.

The influence of HIP treatment on the fatigue reliability of aluminum alloy castings was investigated. The main results obtained were as follows : (1) The fatigue strength of HIP-treated castings was improved by 20% ∼ 30% by the HIP treatment. (2) As the cooling rate of HIP-treated castings was increased, the microstructure was controlled fine. The fatigue strength of HIP-treated castings produced using a rapid cooling rate was greater than that produced using a slow coolling rate. (3) According to the S/δ0.2 N diagram, it could be understood that the fatigue strength of HIP-treated castings was mutually related to the yield strength. (4) The effect of stress gradient on 107 fatigue strength of HIP-treated castings was less than that on non-HIP-treated castings. (5) The plane facet induced by slip was always observed at the origin of the fatigue crack in HIP-treated castings. Hence it could be concluded that the fatigue strength of HIP-treated castings was improved because the porosities were reduced by HIP treatment. (6) From SEM fractographs, it is considered that the fatigue life is controlled by the plane facet size at the origin of the fatigue crack, that is, the primary crystal size of aluminum.

スラリー・溶湯混合法による半凝固過共晶Al-Si合金の初晶Si粒径の制御

The hypereutectic Al-Si alloy slurries with various mean sizes of primary silicon crystals were produced by the slurry-melt mixing process, in which the hypoeutectic alloy slurry and the hypereutectic alloy melt with various silicon contents were mixed and cooled under different environmental conditions. Two kinds of parameters were examined to evaluate the effect of the combination of the alloy composition on the primary silicon crystal size: one is related to the stability of the primary silicon crystals in the mixed alloy, and the other to the number of them which are to nucleate in the second alloy. It was elucidated that both of them provide a good guide on controlling the primary silicon crystal size. And it was also observed that the heat absorptivity of the mixing vessel plays a more important role in the refinement of the primary silicon crystals than the thermal condition of the atmosphere.

On the effect of liquid mixing rate on primary crystal size during the gas-liquid precipitation of calcium carbonate

An analysis based on gas-liquid mass transfer with chemical reaction coupled with equations describing the dynamic population balance model of crystallization and precipitation kinetics is used to predict the effect of liquid mixing rate on the transient mean crystal size of calcium carbonate precipitated in a small flat-interface gas-liquid reaction cell via the gaseous carbonation of lime water. Experimentally, small discrete calcite crystals (<1μm) are formed initially in the vicinity of the gas-liquid interface during the early stages of batch precipitation followed by subsequent primary crystal growth reaching ∼6μm in the bulk. The mean crystal size increases with increasing agitation rate consistent with model predictions due to the effect of liquid agitation rate on gas-liquid mass transfer.

鋳物用Ａｌ‐Ｓｉ系過共晶合金の熱衝撃性における初晶Ｓｉの挙動

Cylindrical specimens of Al-20%Si alloys containing primary silicon crystals of different sizes were prepared and their thermal shock behavior was examined. The specimens were quenched into iced water after heated at 550°C for 20 min., and this was repeated. After thermal shock cycles, macroscopic and microscopic examinations were made and the resistance to thermal shock was discussed in relation to the size of primary silicon crystals.1) No crack was observed on the surface of the specimens with refined primary silicon crystals after subjected to the thermal shocks for 400 times. However, some cracks were detected in the central part after about 100 cycles.2) Cracks initiated at the interface between primary silicon and α-aluminum and propagated into the silicon crystal.3) Thermal shock cycles resulted in the deformation of the specimens, but it was confirmed that the resistance to deformation was higher in the specimens with refined primary silicon crystals.4) With an increase in thermal shock cycles, needle-like eutectic silicon changed its shape to spheres, but little change was observed for primary silicon crystals.

凝固中に加えた圧力が鋼塊の内部性状におよぼす影響

There are earlier reports about the relations between the gas content of molten steel and the internal structure of the solidified ingot, but their mechanism was left unsolved. As an attempt to solve this problem, a series of comparative studies was made. The same heats were poured into two same ingot molds, one was solidified under high atmospheric pressure and another under ordinary pressure applied was chosen as 5, 10, 20 and 40kg/cm2 based on the calculation of the gas formation during solidification. Comparison of the internal structures of the two ingots from respective heats gave the following results.(1) The gas formation during solidification has influence on the inverse V segregation.(2) The inverse V segregations almost disappear at pressure of 5kg/cm2 in a chill mold and 40kg/cm2 in a sand mold.(3) The grain size of primary crystals is refined by high pressure.(4) The length of columnar crystals is unchanged.