Rheological Investigation of Semisolid AlSi7 Alloy by Means of Oscillation Experiments

Dynamic Mechanical Analysis (DMA) of semisolid aluminum alloys was performed in a rheometer of Searle type. DMA was applied on a binary AlSi7 alloy to demonstrate the advantages of the method for the investigation of the behavior of alloys in semi-solid state and it was compared to classical shear experiments. Frequency sweeps, amplitude sweeps as well as constant condition experiments (CCE) were performed. It became obvious that elastic properties are getting more dominant with increasing resting time without shearing. The shift from a more viscous to more elastic nature of the material can be quantified. Interestingly, it was found that the semi-empirically based Cox-Merz rule, usually applied for polymers, holds for the semisolid material as well. This allows investigating the shear viscosity under different relevant conditions - important to improve material models for die-filling simulation of semisolid alloys.

Influence of Thermal Aging Parameters on the Characteristics of Aluminum Semi-Solid Alloys

The current study aimed at analyzing the response of semisolid A357 aluminum alloys to unconventional thermal treatment cycles of T4/T6/T7 conditions. The mechanical, electrical, and microstructural characterizations of such semisolid alloys were investigated. The microstructure evolutions of Fe-intermetallic phases and strengthening precipitates were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The mechanical failure of such semi solid A357 aluminum alloys, used for suspension automotive parts, is mostly related to cracking issues which start from the surface due to hardness problems and propagate due to severe load variations. For these reasons, the multiple thermal aging cycles, in this study, are applied to enhance the mechanical properties and to have compromised values compared to those obtained by standard thermal treatments. The results obtained in this work indicate that the heat treatment of this alloy can be optimized. The results showed that the optimum characteristics of A357 semisolid alloys were obtained by applying thermal under-aging cycle, interrupted thermal aging cycles and a T7/T6 two steps aging treatment condition. The electrical conductivity and electron microscopy were applied in this study to analyze the characteristics of hardening phases formed due to different aging cycles applied to the alloys investigated.

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.

Semisolid Metals: A Suspension with Non-Newtonian Liquid Matrix

The improvement of mathematical models for semisolid alloy flow properties requires profound understanding of the underlying physical nature. To date, it is commonly accepted that the shear thinning behaviour of these suspensions is caused by the solid phase microstructure, while the liquid phase is assumed to be Newtonian with a viscosity in the lowermPas-range. Recent measurements however, demonstrate non-Newtonian behaviour of fully liquid metals with pronounced shear thinning and high viscosities (multiplePas) in the low shear-rate range. By gathering and analysing rheological measurement data of various alloys (Sn14.2%Pb, A356 and X210CrW12), the relevance of the new findings for semisolid metals is investigated. The results indicate that the previously unexamined non-Newtonian flow behaviour of the liquid matrix has, besides the solid fraction, the most dominant influence on the shear thinning behaviour of semisolid alloys. The influences of shear-rate and solid fraction are nearly independent of each other which allow the construction of master-curves; a general flow curve for the suspension where the solid fraction is considered by a scaling factor. Consequently, a modelling approach is suggested in which the dependency of solid fraction is considered independently of the shear-rate.

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.

MODELING OF SEMISOLID STRUCTURE FORMATION IN CONTROLLED NUCLEATION METHOD

Modeling the semisolid structure formation is of significance in both understanding the mechanisms of the formation of such structure and optimization of the solidification conditions for the required structure. A modified cellular automaton (mCA) model has been developed, which is coupled with macroscopic models for heat transfer calculation and microscopic models for nucleation and grain growth. The mCA model is applied to Al - Si alloys, one of the most widely used semisolid alloys. It predicts microstructure morphology and grain size during semi-solid solidification, and determines the effects of poring temperature and mould temperature on the final microstructure. The simulated results are compared with those obtained experimentally. The resulting simulations give some insight into the mechanisms about the semisolid structure formation in Controlled Nucleation process.