Solid State Phenomena Vols. 141-143

Abstract: By semi-solid aluminum alloy compression test, deformation mechanism of semi-solid aluminum alloy was investigated, the thixotropic strength existing in semi-solid aluminum alloy was verified and the influences of temperature and strain rate on the deformation mechanism and thixotropic strength were discussed. The results show that there are 4 stages in the process of compression deformation for semi-solid aluminum alloy: stress swift growth stage, stress drop stage, steady stress stage and stress gradual growth stage. The semi-solid billet has a thixotropic strength, which is sensitive to temperature and strain rate. The higher the temperature or the higher the strain rate is, the smaller the thixotropic strength of the semi-solid alloy.

Abstract: The behavior of semi-solid alloys is quite different in tension, compression and shear and depends strongly on the morphology of the micro-structure. This article outlines a generalized viscoplastic material model for semi-solid alloys which reflects this complex viscoplastic behavior. From the generalized model a number of well known yield functions and viscoplastic material models for semi-solid and solid materials can be reproduces. The general model is applied to describe the behavior of the semi-solid A356 alloy below the coherency temperature during equiaxed solidification.

Abstract: Thixo-extrusion processing could become an important technique to extend the range and complexity of extruded profiles. This work presents the results of thixo-extrusion process applied on aluminium alloy and they were carried out with both computer numerical simulation and experimental methods. The thixo-extrusion set-up was made. Backward extrusion tests were particularly studied and simulated using Forge 2005 software. The constitutive equation used for these simulations is based on a micro-macro model for the semi-solid evolution. The constitutive equation parameters were identified due to comparisons of the simulated load-displacement responses with experimental ones for backward extrusion tests on 7075 semi-solid aluminium alloy, for different temperatures.

Abstract: An analytical model of apparent viscosity was built up based on analysis of energy dissipation during rheocasting. By study the evaluation law of microstructure analyzed with a quantitative metallographic analysis system for semi-solid alloys, the variation law of apparent viscosity with time has been obtained. The model was verified in the experiment of A356 alloys with a coaxial double-bucket rheometer, and the results of simulated experiment and theoretical calculation are in a good agreement.

Abstract: The temperature fields during semi-solid magnesium alloy produced by casting-rolling technology has been simulated by finite element method on the basis of ANSYS. The temperature fields for different conditions were obtained, which is consistent with the experimental results. Results show that there is a high temperature field in the casting and rolling zone. The temperature fluctuates from the center to edge of the strip near the entry of the casting and rolling zone. but The temperature decreases gradually from the center to edge of the strip near the exit of the casting and rolling zone. There are some remarkable effects of the temperature of the casting and rolling, the velocity of the casting and rolling, the gap of two roll, the cooling of the roll and the diameter of the roll on the temperature field, which are in agreement with the experimental results.

Abstract: The semi-solid AZ61 wrought magnesium alloy was fabricated by mechanical stirring method. Thixo-forging set-up were designed and made. The traditional forging and thixo-forging processes of AZ61 magnesium alloy were studied with both computer numerical simulation and experimental methods. The constitutive model of semi-solid AZ61 alloy was established in our prior literature. The differences between traditional forging and thixo-forging processes were analyzed. Results indicated that smaller load and uniform strain & stress were acquired in thixo-extrusion process. Simulation results were good agreement with experimental ones.

Abstract: Equal channel angular extrusion (ECAE) is a technology of refining crystal grain. In this paper, ECAE was introduced into the preparation of semi-solid billet and the extrusion process was simulated using finite element method. The variation of stress (mean stress, max principle stress, effective stress), the distribution of strain, the lattice change were obtained. The results show that not only does the shear stress exist, but also the normal stress in the extrusion process; the stresses in die inner corner of channel and outer corner of channel are not the same. In the inner corner, besides the simple shear, there are compression, and in the outer corner, tension. The strain is non-homogeneous and appears a graded distribution.

Abstract: The pre and post treatment of the thixoformed parts in terms of heating conditions and controlled cooling has a great influence on the final microstructure and the properties of the parts and their homogenization. The paper presents recent investigations on metallographic analysis and hardness measurements on as thixoformed parts and post treated ones. For these tests, different steel grades are investigated and the liquid fraction of the slug before deformation is evaluated to 30 %.

Abstract: The process of thixoforming incorporates a series of forming processes in the semi-solid state, which can be categorized between the conventional processes of forging and casting and combines the advantages of these processes. Thixoforming of steels in the semi-solid state, requires round, solid particles (globulites) in a liquid matrix which is deformed with low forming forces. In order to achieve laminar material flow and to produce segregation-free components, the material must fulfil diverse criteria. First, the melting interval should be as large as possible for an easy temperature regulation. Next, low solidus and liquidus temperatures are advantageous regarding tool loading. Additionally, thixoformable steels should show a melting behaviour that is finegrained and globular. Furthermore, these steels should possess low contents of intraglobular liquid phase fractions. This paper gives a survey of the current state of steel Thixoforming and deals with the development of adaptive heat treatment strategies. Regarding the structure formation and the development of suitable heat treatment strategies, the once semi-solid state yields new structures that can be applied in ways not previously possible with conventional hardening processes. New microstructures and up to date unknown better mechanical properties can be adjusted with an optimised heat treatment strategy. By this, new fields of application for thixo-materials can be entered and also advanced procedures for special applications can be established. For example the steel X210CrW12 leads to a very hard material with high wear-resistance, which can be used at higher temperatures than the conventional hardened material. In general, new generic microstructures after thixoforming results in unexpected favourable mechanical properties. Problems arise with respect to segregation and pores which resulting in inhomogeneous property distributions.

Abstract: Steel thixoforging process combines the advantages of performing parts having highly complex shapes and good mechanical properties. These advantages result from the shear thinning flow behaviour of semi-solids due to microstructure changes during the material deformation. Many parameters such as steel grade, raw speed, slug temperature, tool temperature influence the steel thixoforging process. In this work, we are interested in analysing the influence of thermal conditions as well as ram speed on the flow behaviour and the microstructure of the final parts. To do so, thixo-extrusion tests on a high speed hydraulic press were performed on C38 steels using different ram speeds and different temperatures for the tools. The temperature is measured in the die close to the tool-material interface during the whole process. Correlations between the temperature measurements, the flow profile and the microstructure are discussed. Using the Finite Element code Forge2005, these thixo-extrusion tests are simulated. Using an inverse method, some parameters used in the thermal exchange modelling are identified by successive comparisons between calculated and measured temperatures within the die.