Viscoelastic Properties of Thixotropic Semisolid Alloy Relating the Microstructure

The subject of this work is to investigate viscoelastic properties such as loss modulus (G ́ ́), storage modulus (G ́), complex shear modulus (G*), complex viscosity (η*) and loss angle () at different temperatures by means of a small-amplitude oscillatory test. These properties allow to provide information about materials structure. For this purpose, we employed a tin-lead alloy (Sn-15%Pb) which exhibits a similar microstructure to aluminum alloys and is the classic alloy for semisolid thixotropic studies. It is interesting to note that the Sn-15%Pb alloy exhibits a slightly decrease in storage modulus (G ́) over the entire frequency (0.01-10Hz) at high temperatures, showing its viscoelastic behavior. In addition, a detailed analysis of master curves (oscillatory tests) was made to relate the semisolid microstructure (solid fraction) with the plateau modulus (GN0) which is directly related with both molecular weight or percolation threshold in polymer and gels science respectively.

A Study on Bar Drawing Process of A356 Alloy in Semisolid State

A bar drawing process of an aluminium alloy in semisolid state is presented in the work. The drawing process depends on various parameters such as temperature, die-angle, shear rate etc., accordingly a study is considered. The work involves development of a model to investigate the drawing process of A356 alloy in semisolid range. The rheology of the alloy in semisolid state shows a distinct behaviour and reduces energy requirement during the drawing process. In the context, a model suitably represents the rheology of the alloy is considered to perform a study of the process in details. An analytical and a numerical solutions are combined together to solve the governing equations. Finally, in the work, the distribution of velocity, viscosity variation and drawing power of the semisolid alloy under shear are predicted in the domain. It is found that the energy requirement is reasonably less in case of semisolid bar drawing process compare to a conventional bar drawing process. Finally, the drawing power required to deform a conventional solid A356 alloy is compared with that of the semisolid A356 alloy.

Research of Preparing Parameters in the Complex Process on Microstructure in Semisolid A356 Alloy Slurry with Micro-Addition of La

Based on the green and saving concept, a complex process preparing semisolid alloy slurry was developed, which was composed of the low superheat pouring and low frequency electromagnetic stirring. The semisolid A356-La slurry was prepared by the complex process, and the microstructure of the semisolid A356-La alloy was researched under the different preparing parameters in the complex process, such as the pouring temperature, electromagnetic stirring frequency, stirring time and micro-addition of La. The results indicated that it was feasible to reduce the addition of La and consumption of energy during the preparation of semisolid alloy slurry by optimizing the preparing parameters in the complex process. The suitable preparing parameters were obtained by the experiment, in which the pouring temperature was 630 °C, the frequency of electromagnetic stirring was 30 Hz, and the stirring time was 8 s. When semisolid A356 alloy slurry added 0.3 wt% La was prepared by the suitable preparing parameters in the complex process, the refining effects on microstructure in the semisolid A356-0.3La alloy was indistinguishable with that of the conventional addition amount of 0.6 wt% La in semisolid A356 alloy prepared by low superheat pouring.

Behaviour of Semisolid Slurry Flows through a Channel

In case of metal sheet forming of alloys in semisolid state, modelling of the process is very essential to predict flow behaviour, temperature distribution of the alloy etc. towards improvement of the product quality and to reduce manufacturing costs. Accordingly, the present work develops a model to predict the behaviour during metal sheet forming of an Al-alloy (A356) in semisolid state. The semisolid alloy passes through a rectangular channel having small depth and larger width. The alloy in semisolid state is cooled from the top at a controlled rate. In the model, the respective flow field is represented by the momentum conservation equation. The non-Newtonian behaviour of the semisolid slurry is incorporated considering the Herschel–Bulkley model. The agglomeration and de-agglomeration phenomena of the suspended particles under shear are represented using a time dependent structural parameter. The temperature field is predicted considering the transient energy conservation equation, and hence the fraction of solid is continuously updated. The solution considers an apparent viscosity of the semisolid alloy as a function of structural parameter, shear stress and shear rate. The governing equations are finally solved by finite difference method. The work predicts velocity, temperature and liquid fraction distribution of the semisolid slurry.

A Study on Bubbles in Semisolid Alloys

Bubble or blowhole is one of the most common defects in the workpiece produced by a semisolid alloy process. Except some bubbles are discharged out of the melt, many of the bubbles remained in the semisolid slurry will be deformed, enlarged or merged in the storage and transport process of the semisolid slurry, and be compressed, burst, flattened into crack which is called as gas induced crack in the further semisolid process. How to control and reduce the bubble defects is a key problem to give full play to the advantages of semisolid processing technology in industrial applications. In this paper, the behaviors of growing, floating, escaping and changing of the bubbles in semisolid alloys were theoretically explored during the smelting, filling and forming, and the mathematical models for predicting bubble dimensions and remained bubble ratio in the semisolid slurry were derived based on the theory of twophase flow and the principle of rheology. Moreover, the mechanism and critical conditions for forming the bubbles defects and gasinduced cracks defects in a workpiece were discussed by mechanics analysis. Finally, the relationships between blowhole defects, cracks defects and process parameters were built through kinetic analysis for the rheology behaviors of the semisolid slurry and bubble growing process. These mathematical models will provide a reference for controlling and preventing defects of the blowhole and cracks in the semisolid process.

Modelling of Extrusion Process for Aluminium A356 Alloy

In the present work, a model is developed to study extrusion process of A356 alloy in semi-solid state. The distinct rheology of the semisolid alloy reduces energy necessity during extrusion process. Accordingly, a proper rheological model of the alloy is considered in the model towards a detailed study of the process. A combination of analytical and numerical solutions is considered for solving the governing equations. The work finally predicts distribution of velocity and shear stress of the alloy under shear in the considered domain. It also predicts the energy requirement during the extrusion process. It is demonstrated that for semisolid extrusion, reasonably less energy is required as compared to a conventional extrusion process Keywords: Extrusion, semi-solid alloy, apparent viscosity, extrusion power

On the Filling of Semi-Solid during Thixocasting

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.

In Situ Study of the Altering Globule Packing-Density during Semisolid Alloy Deformation

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.