Effect of Rapid Heating and Cooling Conditions on Microstructure Formation in Powder Bed Fusion of Al-Si Hypoeutectic Alloy: A Phase-Field Study.

Al alloy parts fabricated by powder bed fusion (PBF) have attracted much attention because of the degrees of freedom in both shapes and mechanical properties. We previously reported that the Si regions in Al-Si alloy that remain after the rapid remelting process in PBF act as intrinsic heterogeneous nucleation sites during the subsequent resolidification. This suggests that the Si particles are crucial for a novel grain refinement strategy. To provide guidelines for grain refinement, the effects of solidification, remelting, and resolidification conditions on microstructures were investigated by multiphase-field simulation. We revealed that the resolidification microstructure is determined by the size and number of Si regions in the initial solidification microstructures and by the threshold size for the nucleation site, depending on the remelting and resolidification conditions. Furthermore, the most refined microstructure with the average grain size of 4.8 µm is predicted to be formed under conditions with a large temperature gradient of Gsol = 106 K/m in the initial solidification, a high heating rate of HR = 105 K/s in the remelting process, and a fast solidification rate of Rresol = 10-1 m/s in the resolidification process. Each of these conditions is necessary to be considered to control the microstructures of Al-Si alloys fabricated via PBF.

Production of tagatose and talose through isomerization of galactose in a buffer solution under subcritical water conditions.

Galactose was isomerized in pure water or in 10 mmol/L sodium phosphate buffer at 160 °C under pressurized conditions. The isomerization of galactose to tagatose and talose in phosphate buffer resulted in 14% and 1.4% yields, respectively, which were significantly higher than those obtained in subcritical pure water (0.6% and <0.1%, respectively). The effect of the temperature on isomerization was examined between 100 and 160 °C. The most remarkable isomerization was observed at 120 °C or higher. The effect of the buffer solution type was also examined. The pH drop of the treated solution was lesser in MOPS and PIPES buffers than in the phosphate buffer; however, the isomerization was less likely to occur in MOPS and PIPES buffers. The relationship between the pH drop during the reaction in phosphate buffer and the yields of tagatose and talose revealed that the isomerization proceeded only when the pH was >6.3. Our results indicate that the galactose isomerization by Lobry de Bruyn-Alberda-van Ekenstein transformation rarely occurred at low pH due to the formation of organic acids.

Reaction Behavior of Glucose and Fructose in Subcritical Water in the Presence of Various Salts.

Glucose and fructose were treated in subcritical water in the presence of alkali or alkaline earth metal chlorides. All salts accelerated the conversion of saccharides, and alkaline earth metal chloride greatly promoted the isomerization of glucose to fructose. In contrast, alkali metal salts only slightly promoted this isomerization and facilitated the decomposition of glucose to byproducts such as organic acids. The selectivity of the glucose-to-fructose isomerization was higher at lower conversions of glucose and in the presence of alkaline earth metal chlorides. The pH of the reaction mixture also greatly affected the selectivity, which decreased rapidly at lower pH due to the generated organic acids. At low pH, decomposition of glucose became dominant over isomerization, but further conversion of glucose was suppressed. This result was elucidated by the suppression of the alkali-induced isomerization of glucose at low pH. Fructose underwent decomposition during the treatment of the fructose solution, but its isomerization to glucose was not observed. The added salts autocatalytically promoted the decomposition of fructose, and the reaction mechanism of fructose decomposition differed from that of glucose.