To investigate the effects of rotation speed and shearing time on morphology of semisolid AZ91D alloy, experimental work was undertaken using a twin-screw slurry maker. The results show that increasing the rotation speed and reasonable time can give rise to substantial grain refinement during continuous shearing stage, which can be attributed to the increasing of effective nucleation rate caused by the extremely uniform temperature due to high shear rate and high degree of turbulence. Comparing with low rotation speed at the same thermal condition, the analysis indicates that the microstructures obtained at high rotation speed are homogenous spherical and fine grains instead of dendritic or rosette and exhibits uniform distribution in the eutectic matrix.

A computational model coupling an electromagnetic model with a macroscopic heat and fluid flow model in semi-solid aluminum alloy slurry preparation by annular electromagnetic stirring (A-EMS) was developed. Effects of A-EMS processing parameters, such as stirring current, stirring frequency and stirring gap width, on macroscopic transport phenomena during the solidification were analyzed by commercial software ANSYS 10.0 with corresponding experimental verification. The results show that the magnetic flux density and the melt velocity increase and the temperature difference decreases as stirring gap width and stirring frequency decrease or the stirring current increases. The slurry with the fine and uniform globular grain structure can be gained by adjusting gap width, electromagnetic frequency and current, such as under the conditions of 10 mm of gap width,10 Hz of electromagnetic frequency and 50 A of current. The calculated results are in reasonably good agreement with the measured ones.

Contaminated surfaces of the feedstock materials in aluminum alloy casting processes often produce various types of defects which can affect the tensile properties of the final products as well as their fatigue reliabilities. Semi-solid processing takes advantage of a much higher apparent viscosity of the die cast materials by limiting the risk of oxides formed at the free surfaces to become incorporated into the casting when the material is injected into the die. Most of existing semi-solid processes that use billets as feedstock material are however tied up with a different type of contaminated surface. During the injection phase, the external-skin on the periphery of the billet, which has been in contact with air and lubricant during the transfer in the shot sleeve, can be incorporated into the casting. When subjected to a heat treatment, the lubricant is decomposed and produces lens shape porosities. This might be a cause of reject for most structural parts. To avoid this kind of defects, the paths along which the billet skin evolves must be controlled during filling. In order to investigate the possibility of skin inclusion into cast parts during injection of the billet, a two-phase finite element mixture model is employed to model the metal flow. The formation of a skin on the periphery of the billet is modeled by setting an initial solid phase concentration profile in the radial direction. Microscopic observations of the real castings show that the approach is able to model the shear layers and to predict the paths along which the“lens porosity”defects could be formed. An Arbitrary Eulerian-Lagangian (ALE) method is also investigated and appears to be very promising to follow the skin movement in the casting.

The microstructure and rheological behavior of semi-solid Mg₂Si/AM60 magnesium matrix composite at steady state were investigated. The results show that the primary α-Mg phases are knapped by mechanical stirring and the Chinese script type reinforced Mg₂Si phases exist in liquid phase and grain boundary. The analysis of apparent viscosity indicates that the apparent viscosity of semi-solid Mg₂Si/AM60 magnesium matrix composite at steady state increases with increasing the volume fraction of Mg₂Si and solid fraction of primary α-Mg, but decreases with increasing the shearing rate and shearing time, and the apparent viscosity keeps stable when shearing time reaches 300 s.

Numerical simulation and experiment of thixoforming angle frame of AZ61 magnesium alloy were investigated. The results show that with the increase in punch displacement, cylinder billet firstly fills into die cavity of angle frame from feed inlet and plastic deformation occurs in touching region between billet and die cavity. After central thin wall of angle frame is created, semi- solid billet fills toward two edges. Lastly, complete plastic deformation occurs in billet, leading to complete filling of semis-olid billet. Effective strain, effective stress and billet temperature decrease with the increase in punch displacement. Effective stress decreases with the increase in billet temperature, die temperature and punch velocity. The optimal conditions decided by numerical simulation are as follows: die temperature of 450 ℃, billet temperature of 560 ℃ and punch velocity of 30 mm/s. Angle frame components with high mechanical properties such as yield strength of 225 MPa, tensile strength of 309 MPa and elongation of 21.8% and fine microstructure could be thixoformed successfully according to process parameters decided by numerical simulation.

The semi-solid filling-plastic flowing integrated forging process of semi-solid 6061 Al alloy was simulated by commercial finite element software DEFORM-3D. Temperature, fluid and stress-strain fields were considered in numerical simulation. The simulation results show that the plastic deformation of billet of the ends is higher than that of billet in the straight cylinder. The value of plastic deformation varies with loading mode and plastic deformation fields at the stage of increasing pressure to constant value. When the thixoforging experiments were performed at 590 ℃, 15 mm/s of punch velocity and 46 MPa of pressure side urn, it gets the filling wholly and dense internal organization of semi-solid thixoforging parts is gotten. Finite element analysis results are compatible with experimental ones.

Preparation of semisolid slurry using a cooling slope is increasingly becoming popular, primarily because of the simplicity in design and ease control of the process. In this process, liquid alloy is poured down an inclined surface which is cooled from underneath. The cooling enables partial solidification and the incline provides the necessary shear for producing semisolid slurry. However, the final microstructure of the ingot depends on several process parameters such as cooling rate, incline angle of the cooling slope, length of the slope and initial melt superheat. In this work, a CFD model using volume of fluid (VOF) method for simulating flow along the cooling slope was presented. Equations for conservation of mass, momentum, energy and species were solved to predict hydrodynamic and thermal behavior, in addition to predicting solid fraction distribution and macrosegregation. Solidification was modeled using an enthalpy approach and a volume averaged technique for the different phases. The mushy region was modeled as a multi-layered porous medium consisting of fixed columnar dendrites and mobile equiaxed/fragmented grains. The alloy chosen for the study was aluminum alloy A356, for which adequate experimental data were available in the literature. The effects of two key process parameters, namely the slope angle and the pouring temperature, on temperature distribution, velocity distribution and macrosegregation were also studied.

The influence of temperature on the flow behavior and rheological characteristics of an A356 alloy in the semi-solid state was investigated using backward extrusion process. Experiments were performed at 5 temperatures and 4 different wall thicknesses. Viscosities were determined using the force-displacement graphs obtained form back extrusion tests. As observed experimentally, at a constant temperature, the increase of shear rate results in the decrease of alloy viscosity exponentially. Raising the temperature increases the liquid fraction hence reduces the semi-solid alloy viscosity. Metallographic and image analyses show that, because of low forming speed, liquid has time to escape from solid phase forward the sample wall. This condition is the main reason for the segregation phenomenon seen in the base and walls. Vickers hardness test on samples reveals that the hardness increases with the decrease of temperature and wall thickness.

Extended finite element method (XFEM) is proposed to simulate the discontinuous interface in the liquid-solid forming process. The discontinuous interface is an important phenomenon happening in the liquid-solid forming processes and it is difficult to be simulated accurately with conventional finite element method (CFEM) because it involves solid phase and liquid phase simultaneously. XFEM is becoming more and more popular with the need of solving the discontinuous problem happening in engineering field. The implementation method of XFEM is proposed on Abaqus code by using UEL(user element) with the flowchart. The key is to modify the element stiffness in the proposed method by using UEL on the platform of Abaqus code. In contrast to XFEM used in the simulation of solidification, the geometrical and physical properties of elements were modified at the same time in our method that is beneficial to getting smooth interface transition and precise analysis results. The analysis is simplified significantly with XFEM.

The main goal of this work is to analyze the semi-industrial process of steel thixoforming. The process was carried out using industrial equipment. This equipment consists of a heating device, industrial robots and a hydraulic press. The globular microstructure ensuring thixotropic properties was obtained using the SIMA method. It is one of the simplest and cheapest methods which could be easy applied in the case of steel alloys. In this work, the hot forged rods, commercially produced from 100Cr6 steel, were used. The first part of the work concerned the determination of the proper temperature range, for thixoforming of 100Cr6 steel. Next, some heating tests were carried out in order to obtain as uniform temperature distribution as possible. Heating process was executed using inductive heating. Microstructure analysis of heated samples reveals globular particles surrounded by liquid phase. At last, the thixoforming process was carried out using closed-die forming technique. Completely filled die cavity and good microstructure of the part show that applied process parameters were properly selected.

Based on analysis of the main forming methods for double-layer tube, a new short-term forming process called thixo-co-extrusion was put forward in producing double-layer tube by combining the semi-solid forming technology and multi-billet extrusion technology. By means of forward extrusion with shaft, a finite element model of thixo-co-extrusion with A356/AZ91 was constructed by ABAQUS FEM software. The distributions of temperature field and velocity field as well as the contact force during thixo-co-extrusion were studied. The diffusion on the interfaces between inner and outer metals was analyzed. The simulation results show that, in the beginning of thixo-co-extrusion, the uneven wall thickness can appear. To thickness ratio of 5:5, a double layer tube with good inner and outer wall combination can be realized if V_A356 is 0.12 m/s and V_AZ91 is 0.20 m/s.

A new technology, semi-solid casting (thixocasting) method, was used to replace the conventional hot forging process to form AISI420 stainless steel air-turbine blade. The power law cut-off (PLCO) material model in Procast software was used to simulate the thixocasting process. The thixocasting process was simulated. The results show that the reasonable technology parameters for air-turbine blade thixocasting process are obtained: billet temperature 1 483−1 485 ^oC, piston velocity 12−15 m/s and die temperature is about 400 ^oC.

There exist many problems in producing four-way valve of HPb59-1 alloy for an air-conditioner by traditional solid state hot forging, i.e. larger forming loads, lower material utilization, larger subsequent machining allowance and nonuniform microstructure. For this reason, based on the orthogonal test, the semisolid diecasting process of a certain type of four-way valve was simulated with FLOW-3D. The simulation results show that the optimized process parameters are: the pouring temperature of 897.25 ˚C, the shot velocity of 2.0 m/s and the preheated die temperature of 260 ˚C. The simulation results demonstrate that the cavity can be filled smoothly and completely, the surface defects are small, the temperature field and the pressure field are uniform, and the casting quality is satisfactory. The effectiveness and availability of applying this technology are well verified by the numerical simulation.

Simulating semi-solid metal forming requires modelling semi-solid behaviour. However, such modelling is difficult because semi-solid behavior is thixotropic and depends on the liquid−solid spatial distribution within the material. In order to better understand and model relationships between microstructure and behavior, a model based on micromechanical approaches and homogenisation techniques is presented. This model is an extension of a previous model established in a pure viscoplastic framework to account for elasticity. Indeed, experimental load−displacement signals reveal the presence of an elastic-type response in the earlier stages of deformation when semi-solids are loaded under rapid compression. This elastic feature of the behaviour is attributed to the response of the porous solid skeleton saturated by incompressible liquid. A good quantitative agreement is found between the elastic-viscoplastic predicted response and the experimental data. More precisely, the strong initial rising part of the load−displacement curve, the peak load and the subsequent fall in load are well captured. The effect of solid fraction on mechanical response is in qualitative agreement with experiments.

In long-term rheological shear experiments with semi-solid alloys, coarsening of the particles will falsify the interpretation of the experimental results. The coarsening is intensified by the shear induced convection and the mean size of the particles is changed significantly during the experiments. A simple model has been set up which takes the influence of the convection into account. The resulting growth law has been simplified for diffusion and convection dominated growth. The growth law was verified with shear experiments in a Searl-rheometer with A356 and tin-lead alloys. The experiments demonstrated that under convection the growth follows a linear time law and that the rate constant depends on the root of the shear rate. The correction of experimental results to gain the true viscosity function is demonstrated for a shear jump experiment with A356.

Pitting behavior of thixoformed A356 alloy, with different reheating temperatures, was evaluated. Linear sweep voltammetric tests were used to study the pitting behavior of thixoformed, rheocast and gravity-cast A356 alloy in a 3.5% NaCl solution. A simulation method was also used in order to identify local galvanic corrosion current density between local galvanic couples. The results obtained show that the resistance to pitting corrosion of the thixoformed samples formed at 600 °C is higher than that of the samples formed at 610 °C as well as rheocast and gravity-cast samples. These results could be explained by morphological aspects of silicon phase as well as the area effect as related to galvanic corrosion between silicon particles and eutectic aluminum phase.

Based on the research of modern electronic packaging materials, thixo-forming technology was used to fabricate electronic packaging shell. The process of thixo-extrusion with SiC_p/A356 composites was simulated by the finite element software DEFORM-3D, then the flow velocity field, equivalent strain field and temperature field were analyzed. The electronic packaging shell was manufactured by extrusion according to the results from numerical simulation. The results show that thixo-forming technology can be used in producing electronic package shell with SiC_p/A356 composites, and high volume fraction of SiC_p with homogeneous distribution can be achieved, being in agreement with the requirements of electronic packaging materials.