Papers by Keyword: Rheology

Abstract: As to the time effect of prestress in bolt in geotechnical engineering, the stress distribution in fully-grounded MFRP bolt and its trend influenced by rock rheology is obtained by FEM. The Burgers body which is able to model the primary and secondary creep regions of the rock mass is applied to analyze the time effect of prestress in MFRP bolt. The results show that rock rheology has a great effect on MFRP bolt stress.

Abstract: Structural foams (SF) consist of a sandwich-like material composed by a cellular core and a solid skin. It is the ideal material solution for injection moulding of short production series of large size parts. The injection moulding of SF is a low pressure process, thus it is a viable alternative for light moulding tools. The right formulation of structural foams and the use of adequate processing conditions are important to optimize the performance of the plastics part. In this study the processing conditions (injection speeds and percentages of mould filling) and the amount of chemical blowing agent (CBA) added to polypropylene were assessed on injection moulding circular discs. A hybrid mould with epoxy/aluminium composite cavity was used to produce the parts. The structural foam performance was evaluated by rheological, morphological and mechanical tests.

Abstract: In order to investigate the influence of filler on the rheological properties of UV curable offset ink, ink samples with various varieties and contents of fillers were prepared, the flow curve, thixotropy, dynamic and viscoelastic properties have been tested, and the rheological properties UV offset ink samples with various varieties and contents of fillers have been discussed. The results indicated that: variety and content have great influence on the rheological properties of ink samples. Sample use SiO₂ as filler has the highest viscosity, the largest thixotropy and G’; Sample use SiO₂as filler has the lowest viscosity, the largest thixotropy, while it’s G’ has no big difference from the sample use CaCO₃. Along with the increase of filler content, the viscosity, thixotropy and G’ will increase.

Abstract: Rheometrical analysis of steel slurries in rotational rheometers requires a range of operating temperatures of about 1300 to 1600 °C, leading to a delicate device-related challenge with respect to the applied materials on the one hand and a suitable constructional set-up gaining unbiased measurements on the other hand. Accordingly, a new high-precision rotational rheometer for temperatures up to 1700 °C has been developed as an improvement on an existing rheometer previously developed at the Chair of Mechanical Process Engineering (AVT) of RWTH Aachen University. First measurements on the tool steel X210CrW12 in the mushy state with solid fractions of 20% to 40% demonstrate a remarkable improvement on the preciseness of torque gain which broadens the area of possible experimental operations on steel slurries in order to reliably characterize transient behavior of the material by e.g. creep, relaxation and oscillation experiments.

Abstract: Rheological properties of semi-solid alloys are closely knit to the solid-phase microstructure. Parameters such as particle size distribution are commonly determined by 2D cross section analysis. The determination of mechanisms such as particle deagglomeration with increasing shear rate however, requires information on the 3D spatial distribution. By means of synchrotron radiation tomography and SEM on AlCu samples, particle size distributions and the not yet microscopically observed interrelation of shear rate and particle agglomeration in thixo-material is investigated.

Abstract: The key to all semisolid processes is spherical and non-dendritic particles suspended in a liquid matrix. This special kind of microstructure causes the semisolid material to behave like a non-Newtonian fluid dependent on time and shear rate. Semi solid metal processing gives less shrinkage and porosity, non-macro segregation, lower flow stress, good formability and increased die life compared with conventional production methods. Therefore, semi solid processing introduces a good option for massive forming of high temperature alloys. Here, by partial remelting of M2 tool steel alloy under rapid compression test, deformation mechanism of steel alloy and its correlation to rheological properties were investigated. Flow stress for M2 tool steel alloy corresponding to solid fraction above 55% derived at constant strain rate and holding time. In order to investigate the parameter involved in this process, in the second stage of experiments the ram speed and holding time are variable. The analysis of the derived curves shows that the semisolid slurries exhibit a pseudo plastic behavior dependent on stain rate and temperature. The classical power law model used to describe the rheological and thixotropic dependence.

Abstract: Wall slip of suspensions in confined flow is caused by segregation of a thin layer of liquid phase adjacent to the walls. This causes the bulk phase to slide along the walls, which means that the fluid flow velocities respective to the walls are not zero. In rheometers this affects the evaluation of the rheological properties. Despite the importance of understanding and controlling segregation effects, little research has been done on this subject area. Indeed in industrial casting, the die filling behaviour, and therefore the product quality, may depend on the segregation phenomena. It is important to understand the wall slip phenomenon’s correlation with experimental parameters, as a step towards casting process optimization. Two issues are handled in the present work, the first is the evaluation of different methods to investigate the wall slip effect, the second is the investigation of the wall slip effect dependency on the suspension parameters particle size and solid fraction, respectively. The suspensions employed for the investigations were the aluminium alloy A356 in semi-solid form and a “synthetic suspension” built up of glass spheres in silicon oil. As a result of the above described investigations, influence of suspension parameters are found, and a validated method to avoid the wall slip effect is suggested.

Abstract: Semi-solid processing is an attractive and an effective near-net-shape forming process to produce components with complex geometry and in fewer forming steps. Its key is the spheroidal solid particles suspended in a liquid matrix which in turn eventuate in a complex thixotropic behaviour of semi-solids. However, the consequences of such behaviour on the flow during semi- solid forming processes is still neither completely characterized and nor fully understood, especially for high melting point alloys like steels. Many attempts have been made for understanding the time- dependent flow behaviour of semisolid materials which is essential for the simulation of semi-solid processes. During processing, the structure of a semi-solid material changes with the processing history due to agglomeration and dissagglomeration of particles or particle–particle interactions. This kind of processing condition can be experimented using rotary rheometers. In the case of steel alloys, conventional rheometers cannot be directly used because of their high temperature processing conditions. For this purpose, a self-developed rotary rheometer has been employed to study the rheology of steels up to 1500 oC. Steel samples are melted in an Alumina cup and sheared with an Alumina rod in a resistance furnace under isothermal condition and also under argon controlled atmosphere. The torque and rotary speed signals are used to calculate the viscosity and shear rate data. For rheology investigations, shear rate jump experiments and rest experiments have been carried out using the rheometer for different steel alloys. The flow of SSM is modelled using the Herschel–Bulkley fluid model. The viscosity and the power-law index are assumed to be functions of the solid volume fraction and a structural parameter that changes with the processing history. The evolution of the structural parameter is described by a first-order kinetic differential equation. The model is in a good agreement with the experimental results which can then be used for simulation purposes.

Abstract: Synchrotron radiography experiments are overviewed that directly image semi-solid deformation at the globule-scale. Globular Al-15Cu at 50-60% solid was deformed in direct-shear at 10-2 s-1. Deformation is shown to occur by globule rearrangement without discernible deformation of the individual globules. Globules were found to translate and rotate as quasi-discrete bodies in response to forces acting at globule-globule contacts, similar to liquid-saturated granular materials such as water-saturated sand. Rearrangement caused the globule packing-density (the solid fraction) to adjust by local compaction and local dilation of the globule assembly, and deformation is highly inhomogeneous. During shear, there was a net dilation and strain began to localize into a shear band of decreased solid fraction by the end of the experiments.

Abstract: Rheological behavior of semi-solid slurries forms the backbone of semi-solid processing of metallic alloys. In particular, the effects of several process and metallurgical parameters such as shear rate, shear time, temperature, rest time and size, distribution and morphology of the primary phase on the viscosity of the slurry needs in-depth characterization. In the present work, rheological behaviour of the semisolid aluminium alloy (A356) slurry is investigated by using a high temperature Searle type Rheometer using concentric cylinders. Three different types of experiment are carried out: isothermal test, continuous cooling test and steady state test. Continuous decrease in viscosity is observed with increasing shear rate at a fixed temperature (isothermal test). It is also found that the viscosity increases with decreasing temperature for a particular shear rate due to increasing solid fraction (continuous cooling test). Thixotropic nature of the slurry is confirmed from the hysteresis loops obtained during experimentation. Time dependence of slurry viscosity has been evaluated from the steady state tests. After a longer shearing time under isothermal conditions the starting dendritic structure of the said alloy is transformed into globular grains due to abrasion, agglomeration, welding and ripening.