Advanced Materials Research Vols. 97-101

Abstract: The semi-solid A356 alloy slurry is prepared by compound process, and the effect of the compound process on morphology and size of primary α-Al in A356 alloy is researched. The results indicate that the compound process remarkably affects the morphology and the size of primary α-Al. Primary α-Al with particle-like is distributed uniformly in A356 alloy, and there is no the transient area of change in structure morphology. Compared with the samples prepared by low superheat pouring and slightly electromagnetic stirring, the nucleation rate, morphology and grain size of primary α-Al in A356 alloy are markedly improved by the compound process. The mechanism of refining grain in the compound process is probed.

Abstract: The modification of liquid metal by electric pulse (EP) is a novel method for grain refinement. In this study, based on the reported structural heredity of EP-modified liquid aluminum, the structure tests of EP-modified liquid aluminum were conducted by using high temperature X-ray diffractometer. The results show that the EP-modified liquid structure also tends to be slack and unordered with increasing temperature, which is similar to that of the unmodified. However, the quantitative characterization denoted by the liquid structure parameters exhibits their discrepancy. At the modifying temperature of 750°C, the order degree of EP-modified liquid aluminum is remarkably strengthened and the value of average atomic number per cluster changes from 119 (no EP) up to 174 (EP) by an increase of 46%. These tests experimentally testified Wang’s electric pulse modification (EPM) model that was built only by phenomenology and hereby the mechanism of grain refinement resulting from EPM is further elucidated.

Abstract: Hot tear is one of the main defects during the aluminium casting process, and it usually occurs during solidification. In this work, the low frequency electromagnetic field was applied to study the effects of low frequency electromagnetic field on hot tear during the conventional hot-top casting process. The results show that under the effect of the low frequency electromagnetic field, the structure of the ingot is refined, the flow direction is changed, and the depth of the sump becomes shallow. All these factors can decrease the hot tear susceptibility.

Abstract: A single crystal Ni-base superalloy was prepared with five withdrawal rates: 2, 4, 6, 7 and 10 mm/min. Microstructures including dendrite arm spacing, element segregation, and porosity of the as-cast superalloy were investigated. The results showed that as the withdrawal rate increasing, the primary and secondary dendrite arm spacing decreased markedly, the γ/γ′ eutectic became smaller and more dispersive. Meanwhile, when withdrawal rate was higher, W, Ti, Ta and Al segregated in comparatively larger extent. Furthermore, as the withdrawal rate increasing, the amount of alloy microporosity increased, though the size of which decreased gradually. It can be concluded that the withdrawal rate of 4 mm/min and 6 mm/min are optimal for the experimental alloy.

Abstract: Ductile iron, Heavy section, Cooling curve, Microstructure, Cooling rate. Abstract. Two 250×250×250 mm cubic ductile iron castings solidified in sand and insulation mould were fabricated. The effect of cooling rate on graphite and matrix microstructure of heavy section ductile iron together with their cooling curves were evaluated. Results show that increasing the cooling rate leads to fine graphitization and favors spheroidal graphite formation. The matrix structure is fully ferrite structure at the edge of both castings, while pearlite can be seen near the eutectic cell boundaries at the center of two castings. Furthermore, the amount of pearlite increases with increasing solidification time. Cooling curves confirm that cooling rate affects solidification time of the eutectic transformation and characteristic temperature points on the cooling curves remain unchanged. Low cooling rate appears to significantly increase the eutectic plateau length. Besides, cooling curves show that eutectic temperature remains constant (about 1160°C), which allows for spheroidal graphite formation. Undercooling and inoculation fading during the long time eutectic solidification lead to pearlite formation in the center of cubic ductile iron castings.

Abstract: The influence of sulfur on the microstructure and properties of austenite grey iron with 14-16% Ni was studied. The results showed that the crystallizing temperature of primary austenite increases and the the eutectic temperature does not changes greatly with addition of sulfur. When the content of sulfur is lower than 0.046%, the undercooled graphite existed mainly, the hardness of austenite grey iron and the sensibility of cross section become higher. When the sulfur increases to 0.08%, the A type of flake graphite distributed mainly, and the hardness and the sensibility of cross section reaches the lowest. When the sulfur is over 0.11%, the dentritic primary austenite developed largely, the space interval of austenitic dentrite reduced, and a large amount of interdentritic graphite formed, and the hardness and the sensibility of cross section increased again. In additon, the surface roughness of machined casting is influenced by the morphology of flake graphite.

Abstract: Titanium and its alloys are promising materials in the dental field. However, they react easily with investment materials during casting, forming the reaction layer, resulting in changes of their mechanical property, and limiting their use. So it is necessary for us to study the surface reaction layer between titanium and investment materials and how to decrease the reaction layer. This paper studies the composition and microstructure of the surface reaction layer of titanium castings with Al2O3-based investment materials by SEM, EPMA and TEM. The result reveals that there were two layers on the surface of titanium castings, with outer layer being oxide layer including Al2O3, MgO, Al2TiO5(Al2O3•TiO2) and TixOy, and inner layer being alloy layer including Ti3Al and Ti. The formation mechanism of the surface reaction layer of titanium castings is that when molten titanium is cast into Al2O3 mold, Al2O3 dissolves into molten titanium by the form of atomic Al and O, Ti3Al and TixOy are separated out when the melt solidifies.

Abstract: Based on the non-uniform distribution of magnetic field within the ingot caused by its interactions during the dual-ingot low-frequency electromagnetic semi-continuous casting process, a three-dimensional finite element model was constructed. This model was meshed and calculated with the ANSYS software, and the distribution of magnetic field in low-frequency semi-continuous casting mould was obtained. The influence of the distance and current directions between two coils on magnetic field distribution in the ingot was studied. Calculated results showed that, during dual-ingot low-frequency electromagnetic semi-continuous casting process, whether the current was in the same direction or the reverse, magnetic field density on the remote end should be greater than that on the proximal of the same ingot; when the current directions of nearby coils were reverse, the magnetic intensity on ingot was higher than that with the same directions; as the distance between coils increased, the distal and proximal difference of magnetic flux density declined. Based on the results, a casting mould for dual-ingot electromagnetic semi-continuous casting ø152mm 7075 aluminum alloy was designed and produced. The experimental results showed that when the current directions of nearby coils were reverse, the as-cast macrostructures were a little bit better than that with the same directions. Moreover, when the current directions were reverse, the heterogeneity of the as-cast inner structure could be almost ignored.

Abstract: The green sand containing hot-box resin sand was reclaimed by the process of calcination followed by mechanical reclamation. The reclaimed sands were reused in the hot-box process. The grain size distribution, the shape factor, the clay content and the acid demand value were determined. The results show that the acid demand value of the reclaimed sand is higher than that of the base sand. Compared with the base sand, the grain size of the reclaimed sand is almost no difference. It can also be observed that the tensile strength of the molding sand is influenced by the acid demand value and clay content, but the reclaimed sand can still meet the casting process needs. In addition, the reclaimed green sand is satisfactory for hot-box process.

Abstract: Aluminum alloy A357 can be heat treated in order to obtain optimum mechanical properties. Moreover, SDAS (Second dendrite arm spacing) is an important structure feature in cast aluminum alloys. The effects of SDAS on the homogenization of Al-Si-Mg alloy are investigated through measurement of SDAS and hardness analysis. The results show that the homogenization time increases as the value of SDAS increases and the homogenization temperature decreases in the homogenization process. The homogenization kinetics equation is: . In addition, the hardness after homogenization treatment at 540°C for 6 hours is advanced with the reduction of SDAS.