Solid State Phenomena Vols. 192-193

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.

Abstract: In the present work, the thixotropic property of a semisolid aluminium alloy (A356) under deformation is investigated numerically where the Couette flow between two parallel plates is considered. The flow field is represented by momentum conservation equations where the non-Newtonian behavior of the semisolid material is represented by the Herschel-Bulkley model. The agglomeration and the de-agglomeration phenomena of the suspended particles under shear are represented using a time dependent structural parameter influenced by the rate of strain and shear stress. The simulation predicts the flow field, rate of strain and apparent viscosity of the semisolid materials under transient and steady state conditions. It is found that the apparent viscosity shows a transient nature during sudden change in the shear rate, and its value decreases with increasing shear rate and vice-versa. It is also found that the present prediction shows a good agreement with prior work.

Abstract: In the present work, a cooling channel is employed to produce semi-solid A356 alloy slurry. To understand the transport process involved, a 3D non-isothermal, multiphase volume averaging model has been developed for simulation of the semi-solid slurry generation process in the cooling channel. For simulation purpose, the three phases considered are the parent melt, the nearly spherical grains and air as separated but highly coupled interpenetrating continua. The conservation equations of mass, momentum, energy and species have been solved for each phase and the thermal and mechanical interactions (drag force) among the phases have been considered using appropriate model. The superheated liquid alloy is poured at the top of the cooling slope/channel, where specified velocity inlet boundary condition is used in the model, and allowed to flow along gravity through the channel. The melt loses its superheat and becomes semisolid up to the end of cooling channel due to the evolving -Al grains with decreasing temperature. The air phase forms a definable air/liquid melt interface, i.e. free surface, due its low density. The results obtained from the present model includes volume fractions of three different phases considered, grain evolution, grain growth rate, size and distribution of solid grains. The effect of key process variables such as pouring temperature, slope angle of the cooling channel and cooling channel wall temperature on temperature distribution, velocity distribution, grain formation and volume fraction of different phases are also studied. The results obtained from the simulations are validated by microstructure study using SEM and quantitative image analysis of the semi-solid slurry microstructure obtained from the experimental set-up.

Abstract: Thixoforming is a new forming technology which has been studied by many researchers during the last years. Mold filling is one of the most important steps for casting engineers which should be controlled to have a sound part. In the semisolid alloy forming die design, viscosity variations during forming and temperature decreasing, solid fraction plus globularity are the main parameters which affect final product. In this study, a stepped die was designed and some billets in different weights were injected into the die. This process helps to characterize the flow pattern during mold filling. The effect of two phase flow was also studied by microstructural investigating. Numerical simulation is the second method which is used in this study. The ProCast software was used to indicate the flow pattern of the fluid in the mold. Some comparisons were also done to show that this software is a suitable simulating software to predict the flow behavior of semisolid alloys. Also, the step casting method shows the exact pattern of flow in different parts of mold and is a reliable method for researchers to investigate the fluid pattern.

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: 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: 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: The SEED technology, a rheocasting process based on the slurry-on-demand approach, is an emerging technology that was developed in the mid-2000s. Many publications with regard to the process and to alloy development using this technology were made since, and several industrial units are operated worldwide. Moreover, the process is still actively developed and is fully supported by a team of scientists, engineers and technicians. With a global objective of addressing the user requirements and the industry needs, works were conducted toward optimization of the process and equipment. At first, the focus was on developing a simplified version of the SEED process to eliminate the so-called “drainage” phase while preserving the prime quality of the slurry produced. Improvement of some system components and integration of new features were also targeted to secure the overall equipment efficiency (OEE) and increase the process reliability. This work, backed with the optimization of process parameters and comprehensive techniques adapted for semi solid casting, led to the consolidation and even improvement of the properties of the parts produced for the common foundry alloys 356/357 and 319. Furthermore, the non-drained SEED version was applied to the validation of the process capabilities for uncommon cast alloys with works on 6061 wrought alloy, Duralcan metal matrix composite, and others. The results confirmed that the SEED process can efficiently be used in non-drained mode and achieve the same quality of slurry as the drained version originally developed. It is now proven in the industrial scale and actually integrated in the updated industrial equipment. Moreover, the capabilities of the process for special alloys and applications are still the subject of active development works.

Abstract: A new method of helical curve duct (HCD) to prepare semi-solid slurry was investigated. The semi-solid slurry of A356, AZ91, Cu-2.21wt%Ca (0.45wt%Ce) alloys were prepared through this method of HCD. Comparison of the microstructures of slurry prepared through the helical curve duct with different structure parameters and process parameters are made and the effects of structure and process parameters on microstructures are analyzed. The results show that the melt is stirred effectively by flowing through the helical curve duct and the semi-solid slurry of alloys could be prepared through the helical curve duct process; Spherical microstructures of alloy slurry were obtained through controlling or adjusting the process parameters to control solidification conditions.

Abstract: Recently, semi-solid slurry making method using metallic vessels, called "Cup-method", have been studied by the authors. In this method, semi-solid slurry with globular and fine structure is obtained only by pouring molten metal at certain temperature into a metallic vessel which is designed for desired slurry volume and solid fraction of the slurry. The method provides us shorter slurry making time and finer microstructure of α grain with around 50 μm in its size. However, the cup-method requires precise control in the slurry making condition; therefore it is difficult to obtain the homogeneous microstructure in the slurry, especially for the larger slurry. In this study, mechanical vibration was introduced to enhance homogeneity and fine microstructure generation. As a result, mechanical vibration provides the fine and homogeneous distribution of microstructure for wider slurry making conditions. Fine and non-dendrite structure of the slurry could be obtained even by using higher superheated melt, while the conventional method without vibration showed coarse and dendritic microstructure in the similar conditions.