Characterization of the microstructure evolution of a semi-solid metal slurry during the early stages

A key success of semi-solid metal forming by the rheocasting route is having an efficient and effective technique to prepare semi-solid slurries. To achieve that it is necessary to be able to characterize the microstructure during the early stages at different solid fractions as the slurry is being cooled. This present work applied the rapid quenching method to study microstructure evolution during the early stages in a rheocasting process. Nine stages of microstructure evolution were characterized by applying rheocasting times of 5, 10, 12, 15, 20, 30, 35, 40 and 45 s during cooling. Analysis to evaluate the solid fraction, particle density, particle size, particle shape factor, and particle distribution was performed. The results show that the relationship between the solid fraction and the rheocasting time can be well fitted to a quadratic equation. For particle density, the relationship is better fitted to a linear equation. The change in particle size with time can be modeled using a ripening model with an upper bound value for the ripening coefficient. In addition, the particle distribution may be quantified by the dilation and counting technique to determine different levels of particle clustering. The results from this study suggest that this new characterization method can be used as a process and quality control tool in the rheocasting process. It can also be used to optimize the process and to study the mechanism of the rheocasting technique.     

Parameter Estimation in the Error‐in‐Variables Models Using the Gibbs Sampler

Least squares and maximum likelihood techniques have long been used in parameter estimation problems. However, those techniques provide only point estimates with unknown or approximate uncertainty information. Bayesian inference coupled with the Gibbs Sampler is an approach to parameter estimation that exploits modern computing technology. The estimation results are complete with exact uncertainty information. The Error‐in‐Variables model (EVM) approach is investigated in this study. In it, both dependent and independent variables contain measurement errors, and the true values and uncertainties of all measurements are estimated. This EVM set‐up leads to unusually large dimensionality in the estimation problem, which makes parameter estimation very difficult with classical techniques. In this paper, an innovative way of performing parameter estimation is introduced to chemical engineers. The paper shows that the method is simple and efficient; as well, complete and accurate uncertainty information about parameter estimates is readily available. Two real‐world EVM examples are demonstrated: a large‐scale linear model and an epidemiological model. The former is simple enough for most readers to understand the new concepts without difficulty. The latter has very interesting features in that a Poisson distribution is assumed, and a parameter with known distribution is retained while other unknown parameters are estimated. The Gibbs Sampler results are compared with those of the least squares.     

Process-based cost modelling for gas induced semi-solid-processed below-knee prosthesis

Below-knee prosthesis is much needed in Thailand and the world over because it serves as a close substitute for a missing body part (e.g. a device to aid people with paraplegia). Production of these components by gas-induced semi-solid process is relatively new, and investors need to be assured of success before committing themselves into a venture. Most cost models, such as the activity-based model, have flexibility limitations, and hence more input-sensitive – the process-based cost model (PBCM) – is alternatively employed in analysing more simple work process, appropriate production volume and unit cost, and was utilised in this research. An array of production volumes (5000–100,000 sets) and a number of daily work shifts (1–3) were investigated. From detailed breakdowns, analysis results suggested an optimal annual production volume in the range of 10,000–40,000 sets employing any shifts per day. Below 10,000, unit cost escalates very sharply, and beyond 40,000 it decreases only marginally. Production for export purposes is feasible since the annual domestic requirement in Thailand is only a fraction of this volume. PBCM had also proven an effective tool in this case.     

Grain refinement behavior of an aluminum alloy by inoculation and dynamic nucleation

Grain refinement offers several benefits in aluminum casting applications. Two methods are normally used to achieve a grain-refined microstructure: inoculation and dynamic nucleation. Inoculation is widely applied in industry, but is not an efficient process. Dynamic nucleation, achieved by application of localized forced convection with rapid cooling, is an alternative process. However, a deeper understanding of dynamic nucleation is required if this process is to be used commercially. This study aims to understand the grain refinement behavior of an aluminum alloy under the influences of inoculation and dynamic nucleation. A rapid quenching method was used to investigate the combined effects of inoculation and dynamic nucleation on the solid fraction, particle density and particle size of the secondary nuclei. In addition, the effects of the particle density of the secondary nuclei on the final cast microstructure were studied. The rapid quenching results show that dynamic nucleation by application of forced convection with localized cooling to the melt yields an increased solid fraction and particle density of secondary nuclei. The solid fraction and particle density are further increased by inoculation. This study also shows that increasing the convection level in an inoculated melt held at a temperature slightly above the liquidus temperature increases the effectiveness of dynamic nucleation, which consequently yields a finer microstructure of the final cast samples. The findings suggest that grain refinement can be effectively achieved by applying forced convection with localized cooling to create a low fraction solid of secondary nuclei in the melt prior to pouring and casting.     

Crystallization,mechanical properties and thermal stability of cockleshell-derived CaCO<Subscript>3</Subscript> filled polypropylene

This study discusses the potential of utilizing waste cockleshell derived-CaCO₃ (CS) as filler in polypropylene (PP). Mineral fillers were prepared from cockleshell-derived CaCO₃ and used to fill polypropylene. The composites were prepared by melt blending and fabricated by injection and compression molding techniques. The effects of filler on crystal structure, crystallization and thermal degradation characteristics of filled polypropylene composites were elucidated. The cockleshell filler promoted the formation of the β-crystalline phase in PP, which improved the rigidity and toughness of the composites. However, stearic acid treatments on the filler would significantly affect the nucleation process and therefore hindered crystallization. Acceleration in thermal degradation of PP was also noted with increasing filler loading.     

Dynamic vulcanization of acrylic rubber‐blended PVC

Poly(vinyl chloride) was blended with an acrylic rubber at a variety of blending ratio using a twin‐screw extruder. The acrylic rubber was compounded with sulfur and sodium stearate in a two‐roll mill prior to the blending. Dynamic vulcanization was performed in a compression mould at 170°C. Mechanical properties of the blends were determined by using a tensile testing machine. Scanning electron microscope was used to examine morphology of these blends. Degree of crosslinking of acrylic rubber in the blends was evaluated by using a differential scanning calorimeter. It was found that the normal blends are miscible regardless of the blending variables. By performing dynamic vulcanization, however, the blends became immisicible, showing a typical dispersed particle morphology, which was accompanied by a remarkable improvement of tensile properties. The screw‐rotating speed was an important parameter affecting particle size and crosslink density of the rubber phase, which in turn controlled the tensile toughness of the blends. On the one hand, tensile toughness increased with the speed because of the decreasing particle size. On the other hand, the toughness decreased with the speed because of the decreasing crosslink density of the rubber. As a result, there was an optimum speed for each blend ratio, which corresponded to the maximum toughness. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2657–2663, 2003     

Characterization of flow behavior of semi-solid slurries containing low solid fractions in high-pressure die casting

Understanding the flow behavior of semi-solid slurries containing low solid fractions is key to the success in applying this process in the die casting industry. With these low initial solid fractions, the flow behavior of semi-solid slurries is quite complicated, making it difficult to model accurately. This present work developed and studied characterization methods for the flow behavior of semi-solid slurries at low solid fractions in high-pressure die casting. A new parameter, the ratio of gate speed to initial solid fraction (V_g/f_s), was proposed to be correlated to the normalized flow interface length, blister area and tensile properties. Results from the flow pattern analysis suggest that the flow behavior can be controlled to achieve laminar flow by varying the initial solid fraction. Blister test results show the trend that slurry die casting conditions with high V_g/f_s values exhibit high blister areas. Die casting conditions with excessively high gate speeds and insufficient solid fractions result in turbulent flow patterns and high levels of blister defect. The results of tensile test and fracture surface analysis are consistent with other analysis results. The samples formed by liquid die casting and slurry die casting with high V_g/f_s values have gas porosity due to turbulent flow pattern during die filling. On the other hand, the samples formed by slurry die casting with too low V_g/f_s values contain shrinkage porosity. This is because of insufficient time for shrinkage feeding due to a combination of a high solid fraction and a low gate speed. This study has demonstrated that die casting with slurries containing low initial solid fractions gives die casters another process parameter to adjust, which can help reduce and control the gas and shrinkage porosities.     

Characterization of Flow Behavior of Semi-Solid Slurries with Low Solid Fractions

Semi-solid slurry casting is a metal-forming process that involves transforming liquid metal into slurry having a low solid fraction and then forming the slurry into solid parts. To successfully apply this slurry-forming process, it is necessary to fully understand the flow behavior of semi-solid slurries. This present work applied the rapid quenching method and the modified gravity fluidity casting to investigate the flow behavior, which involves characterizations of the initial solid fraction, fluidity, and microstructure of semi-solid slurries. Three commercial aluminum alloys were used in this study: 383 (Al-Si11Cu), 356 (Al-Si7MgFe), and 7075 (Al-Zn6MgCu) alloys. The results show that the initial solid fractions can be controlled by varying the rheocasting time. The rapid quenching mold can be used to determine the initial solid fractions. In this method, it is important to apply the correcting procedure to account for growth during quenching and to include all the solid phases. Results from the fluidity study of semi-solid slurries show that the fluidity decreases as the initial solid fraction increases. The decrease is relatively rapid near the low end of the initial solid fraction curves, but is quite slow near the high end of the curves. All the three alloys follow this trend. The results also demonstrate that the slurries that contain high solid fractions of up to 30 pct can still flow well. The microstructure characterization results show that the solid particles in the slurries flow uniformly in the channel. A uniform and fine microstructure with limited phase segregation is observed in the slurry cast samples.