Materials Science Forum Vols. 620-622

Abstract: New cup-type diamond-grinding-wheels with hexagonal pattern have been developed. Grinding stone ratio, R is defined as the ratio between the hexagonal edge area containing abrasive grains and the total area of the wheel surface. In the present work, four kinds of hexagonal grinding wheels with different R (13 %, 19 %, 25 % and 36 %) and a conventional wheel (R: 100 %) were used to grind a light metals, which was represented by magnesium alloy AZ31B. Efficiency of abrasive grains and ground surface for machining a light metals were evaluated by calculating the number of abrasive grains which pass through a unit length of a sample surface for each grinding pass, Ng. The results show that surface roughness becomes smaller, i. e., smoother surfaces as Ng increases. Surfaces ground by the conventional wheel are rougher than those by using newly developed hexagonal grinding-wheels in spite of the larger Ng for the conventional wheel. Surface roughness data forms one curve in roughness vs. Ng graph for all hexagonal wheels, and forms another curve for the conventional grinding-wheel. The difference of two curves indicates that the number of effective working abrasive grains in hexagonal wheels is about 5 times higher than that of the conventional wheel. The similar results were obtained for machining sapphire according to our previous work. Hexagonal wheels show higher abrasive grain efficiency for machining not only hard-to-machine ceramics but also light metals such as magnesium alloys than conventional wheels.

Abstract: A novel method for preparing porous silicon carbide ceramics with high porosity had been developed by recrystallization of green bodies composed of α-SiC, β-SiC, remnant silicon and incompletely-reacted carbon. Fine microstructure and uniform pore structure of the resultant porous silicon carbide ceramics was obtained. The green bodies of porous ceramics were prepared by the precursor powder which contained α-SiC, carbon black and silicon powder. The precursor powder was sintered at 1600°C under Vacuum circumstance to obtain the green bodies; the sintering process is same with the reaction sintering silicon carbide. Then the green bodies were sintered to 2300°C for half an hour to recrystallization. The incompletely-reacted carbon was fully reacted with silicon. And the remnant silicon was excluded during the recrystallization process to create porous structures. The influence of composition of the precursor powder and the fabrication process (the moulding pressure) on the microstructure of sintering bodies was analyzed. X-ray diffractometry demonstrated the transformation of β-SiC to α-SiC during the recrystallization process. The density and the porosity of this material was 1.027g/cm3 and 67% respectively.

Abstract: In this paper a foam ceramic was prepared from clay, silicon carbide, ferric oxide, feldspar and other raw materials. The content of ferric oxide and the powder size of silicon carbide influenced the sinter behavior of the foam ceramics. The ferric oxide and feldspar are favorable to form of glass phase in the ceramics. The foam ceramics with the rate of moisture absorption is less than 0.05%, the bulk density is 0.3~0.9g/cm3, and compressive strength is over than 2MPa.

Abstract: This paper presents the mechanical properties of a collection of high density Polyethylene based foams. The produced materials are characterised by a reduction in density up to 60%, an excellent surface quality, cell sizes in the microcellular range (around 50 microns) and a multi-structured cellular structure (cranial structure) with dense skin and foamed core. The mechanical properties of these materials showed linear relationships between Young’s modulus and density for densities above 0.7 g/cm3. In addition, variations in the cell size did not influence the elastic properties.

Abstract: Porous metals with long cylindrical pores aligned in one direction were fabricated by unidirectional solidification using pressurized gas (hydrogen) method (PGM) and thermal decomposition method (TDM). The pores are evolved from insoluble gas when the molten metal dissolving the gas is solidified. In the conventional PGM, the hydrogen pressurized in a high-pressure chamber is used as the dissolving gas. However, the use of high-pressure hydrogen is not desirable because of inflammable and explosive gas, in particular, for scaling up to mass production of lotus metals. In order to overcome this shortcoming, the thermal decomposition method was developed as an alternative simple fabrication method. Gas-forming compounds were added into the molten metal to fabricate lotus metals. The porosity and pore size were controlled by the amount of gas-forming compounds. TDM was applied to fabricate porous copper and aluminium

Abstract: As a relatively new material, geopolymer concrete offers the benefits as a construction material for sustainable development. It utilized waste materials such as recycled concrete sludge, fly ash and etc. It has a very low rate of green house gas emission when compared to ordinary Portland cement. In this study, the component of geopolymer is concrete sludge, metakaolin and water glass, NaOH was used as alkalin activator. To improve the mechanical properties, the amount of NaOH and water glass were varied to obtain higher strength and the specimens were cured both in air and water, then their mechanical properties like compressive strength and bending strength were measured the microstructures were investigated.