Papers by Keyword: Solidification Microstructures

Abstract: The effect of cooling rate on the solidification microstructure of Cu-Ni alloy is investigated here. The Cu-Ni alloy was prepared in a main frequency furnace with three different molds made of silica sand, graphite, and low carbon steel in the foundry. The simulation software of diathermanous-flowing-stress coupling PROCAST is used to simulate the Cu-Ni alloy solidification process about temperature field with the three molds. The microstructures of the alloys have been studied through the optical microscopy (OM), the scanning electron microscopy (SEM), together with the energy dispersive spectroscopy (EDS).The results show that the grain average size and the primary and secondary dendrite arm space were obviously reduced with the increase of cooling rate. The dendrite and interdendrite element segregation also decreases with increasing cooling rate.

Abstract: Alloys displaying positive enthalpy of mixing demix below a critical temperature. In Co-Cu and related ternaries the miscibility gap is metastable, i.e. it occurs at temperatures lower than the liquidus. In order to study the liquid phase separation high melt undercooling is necessary. This was obtained via rapid solidification techniques using melt spinning and casting in moulding devices, as well as high temperature DSC experiments with samples embedded in a flux. Results are given for Co-Cu, Co-Cu-Fe and Co-Cu-Ni systems. Phase diagrams were optimised using the DSC data. The mechanism of phase separation was investigated by comparing samples produced under different cooling conditions. The hierarchy of microstructures obtained was interpreted accounting for the processing technique and the phase diagram. They constitute a database useful for the interpretation of the thermal history of samples processed in microgravity.

Abstract: The ultrasonic melt treatment is highly efficient in controlling the size and morphology of the alpha-aluminum phase in the Al-Si cast alloys. However, the influence of this treatment on the other phases, namely: the eutectic Si and the Fe-intermetallic phases, has not been thoroughly investigated. This study was undertaken to investigate the effect of ultrasonic melt treatment on the morphology and size of these phases. Four Al-Si cast alloys, 384, 380, 356 and 356 (with 0.8% Fe), were considered in this study. The treatment temperatures were varied from about 100oC above the liquidus temperatures down to the Al-Si eutectic temperature, for different treatment times (4 to 54 s). The results showed that the Si particles are only affected by the ultrasonic vibrations when the eutectic reaction takes place under the ultrasonic field, where more compacted and large Si particles are formed. This limited effect was observed only near to the ultrasonic horn. For other investigated conditions, including treatment in the liquid and semisolid states, the Si particles were obviously unaffected by the ultrasonic treatment. On the other hand, the iron intermetallic phases changed their morphology from large plate-like particles to a more compacted globular form, by the application of ultrasonic vibrations at temperatures up to 10oC above liquidus. Treatments at higher temperatures have limited effect on the morphology of the Fe-intermetallic phases. These observations are general and apply to all the studied alloys.

Abstract: The strip casting of AZ31 were carried out by a Ø250×150mm of vertical twin roll caster at different casting speeds (9-15m/min) and in different casting temperatures (630-660°C). The solidification microstructure of the strips was examined. The experiment results showed that the crystal grain size of the casting was smaller than that of conventional ingot, and decreases while the casting speed is raised, or the casting temperature decrease. The casting temperature strongly affected the dendrite structure that changed into sphere-like when the casting temperature was lower. The appropriate casting temperature for AZ31 magnesium alloy is 640°C, nearby its liquidus temperature.

Abstract: Hydroxyapatite(HAP) has excellent osteoconductive properties. By controlling the Ca/P ratio better biphasic calcium phosphate ceramics can be produced than pure HAP ceramics. β- calcium pyrophosphate(β-Ca2P2O7) is a new biodegradable ceramic material and its biological response is quite similar to HAP. Obtaining HAP and other bioactive calcium phosphate ceramic coatings has been a popular research field in the past. In our research a new bioceramic composite coating was obtained by laser cladding with pre-depositing mixed powders of CaHPO4·2H2O and CaCO3 directly on the metal substrate. Its main constituents are HAP and β-Ca2P2O7. The microstructure of the coating consists of minute granular HAP that is distributed among the overlapped club-shaped or needle-like β-Ca2P2O7. The hardness distribution in the cladding layer is even and its value is much higher than that in the substrate. There is a bonded structure of the epitaxial planar growth between the substrate and cladding layer, and both a typical cellular microstructure in the middle and an equiaxed microstructure at the top of the cladding layer.

Abstract: Microstructure evolution was investigated during the solidification of succinonitrile-5at.% water transparent alloy and Sn–15 wt.%Pb alloy under mechanical stirring through in-situ observation and quenching, separately. The results showed that the evolution of primary microstructures under stirring experienced the growth of single grain particle and the successive agglomerating and coarsening of multi-particles when the particle size reached a certain value. The increase of stirring rate promoted the globular growth of solidification microstructure after it nucleated in the melt and increase the grain size. Thus, the microstructure during semi-solid processing could be refined by a controlled stirring and cooling process, which depended on the optimization among the stirring rate, cooling rate and temperature at which the stirring rate is changed.