Solid State Phenomena Vols. 141-143

Abstract: Hypereutectic Al-Si based alloys are expected to be more wear resistant than the AlSi8Cu3Fe alloy, currently used in compressor connecting rods in die cast grades. Die casting of such alloys, however, is not straightforward. Semi-solid processing was thus considered to overcome the problems encountered in casting hypereutectic Al-Si alloys. Two experimental hypereutectic AlSiCuFe alloys were obtained by adding 8 to 12 wt% Si to the AlSi8Cu3Fe alloy. The ingots of these alloys were melted and cooled to very near their liquidus points, 625°C and 675°C respectively, before they were poured into a permanent mould in order to produce nondendritic feedstock for thixoforming. This low superheat casting process largely replaced α-Al dendrites with relatively smaller α-Al rosettes. The slugs machined from the ingots thus obtained were thixoformed after they were heated in situ in the semi-solid range for 5 minutes in a laboratory press. Several thixoformed parts were heat treated to the T6 temper with an accompanying increase in hardness from 91-96 HB to 130-131 HB.

Abstract: In the present paper, the precision forging of an impeller was studied by means of numerical simulation and test forming. Based on the structure and dimension of the impeller, the combination structure was used in the forging die to obtain the extrusion deformation. The forming processes were simulated with DEFORM-3D for different billet dimensions and processing parameters. The parameters, which could ensure the forming quality of the impeller, were determined by the calculations and analysis. The die structure and the billet dimensions were determined according to the simulation results, and the forging die was designed and manufactured. The billet with semi-solid microstructure was produced by means of the direct heating-isothermal treatment. The forming was conduced in an YX-315F hydraulic press, and the precision forgings of the impeller were produced successfully. Both of the simulation and the forming test show that the impeller forging can be formed with the combination structure die and the extruding forming stale satisfactorily. The ideal parameters to produce the precision forgings of the impeller are: billet temperature at 625°C, die temperature at 450°C and punch speed at 20mm/s. Under these conditions, forgings of the impeller can be produced with plump blades, smooth outer surface, and good flow line. This can match the requirements of the precision forging of impellers.

Abstract: The apparent viscosity model of the semi-solid A356 aluminum alloy, based on fitting of experimental data obtained by the Couette type viscometer, was developed in the paper. The commercial package CastSoft6.0 coupled with the model was used to simulate the mould filling of the semi-solid A356 aluminum alloy in the key-shaped component with iron cores. The simulation results showed that the position of the iron cores has an important effect on the filling of the semisolid slurry, and it is easy to obtain the completely filled key-shaped component when the iron cores were near to the inlet. The filling tests verified that the simulation results have good agreement with the experimental results. The fitting results indicated that the developed apparent viscosity model is practical and feasible and it can be used to simulate the mould filling process of the semisolid A356 aluminum alloy slurry. Also the parameters were optimized and the optimum parameters are as follows: the inject pressure is more than 15MPa, the inlet velocity is more than 1.73m/s and the forming temperature is over 585 °C.

Abstract: Computer base and simulation technique have been applied for modeling the semi-solid die filling and part of the solidification process of aluminum A356 alloy. A fairly simple one-phase rheological model has been implemented into a fluid flow finite element software Procast, to solve the partial differential equations. This model is purely viscous nature and is implemented in the power law cut-off model of Procast. The constitutive parameters of this model were determined for a rheocast A356 alloy. Using these parameters and comparing the simulation results with experimental data showed good correlation with the model prediction. The designed die for rheocasting was applied for the production of a small propeller with thin section.

Abstract: This paper illustrates investigations regarding the infiltration process of the thixotropic cast-alloy AlSi7Mg0,3 into laminated fibre woven fabrics by Fluid-Structure Interaction Analysis (FSI). As results of such FSI-Analysis on the one hand the kinematical behaviour of the reinforcement due to the infiltration process on the macroscopic and microscopic level on the other hand fluiddynamical effects of the regarded alloy are achieved. Thus in the run-up to timeconsuming and cost-intensive experiments, informative bases like fluidic optimizing of the cavity or the configuration and insertion of the reinforcement component can be numerically developed. Furthermore a reliable prediction of transient permeability of the fibre fabric is possible, which effects the infiltration process significantly. Numerical input data such as rheological parameters characterizing the behaviour of partial solidified alloys have been conducted. Therefor basic rheological tests of the aluminium cast-alloy AlSi7Mg0,3, like `HYSTERESIS TESTS´, `SHEAR RATE JUMP TESTS´ and detection of `STATIC´ and `DYNAMIC YIELD POINTS´ and the `DIFFERENTIAL STRUCTURAL PARAMETER´ have been conducted. Furthermore `EVOLUTION OF VISCOSITY´ has been correlated with thermodynamical calculations using ThermoCalc®. Finally the infiltration of a textile semi-finished part (carbon fibre fabric / canvas bonding) by A356 is discussed as an example to demonstrate the feasibility of FSI-Analysis, taking into account a two-way coupling between the interacting CSM- and CFD-Codes.

Abstract: High quality magnesium alloy sheets with non-dendritic microstructure can be manufactured by the technique which incorporates semi-solid processing with continuous roll stripcasting. In particular, the process is believed to be cost-effective in mass production of magnesium sheet products. The processing parameters, such as roll speed and temperature, have significant influence on roll strip-casting process. In this paper, the commercial software, DEFORM-3D, for deformation simulation of bulk materials was used to carry out the simulation of roll strip-casting process for AZ91D magnesium alloy in semi-solid state, and the distribution of equivalent stress, equivalent strain and temperature was obtained. Based on the results, the optimization of processing parameters was discussed.

Abstract: In this paper, thixoforging of a magazine plate made of AZ91D magnesium alloy were investigated by means of numerical simulation and experiments. Numerical simulation results show that with increasing punch displacement, local bending, formation of a concave shell part and bulk plastic deformation occurs in billet continuously. Equivalent strain and stress increase and the temperature of the semi-solid billet decreases. When the temperature of the semi-solid billet or the die temperature is elevated, equivalent stain and stress decrease. Optimal technological parameters such as a billet temperature of 545°C, die temperature of 450°C and punch velocity of 15 mm/s were obtained by numerical simulation. Experimental results demonstrate that magazine plates with high mechanical properties such as tensile strength of 316.8 MPa, yield strength of 228.3 MPa and elongation of 12.6 % can be manufactured successfully when the optimal technological parameters selected according to the results of numerical simulation are applied.

Abstract: In order to model thixoforming processes, previous papers presented a thermomechanical one-phase modelling. This first version of constitutive model revealed several limitations: the model could not degenerate properly to pure solid or liquid behaviour neither to free solid suspensions. The aim of this paper was to propose solutions to overcome these limitations.

Abstract: Casting metal alloys in the semi-solid state is now becoming a well established manufacturing technique. But, the success of this technology necessitates a good understanding of the feedstock material behaviour. To obtain high quality components with semi-solid metal processing, a homogeneous distribution of phases must be maintained in the material during the die filling stage. Many parameters affect the process such as temperature, time and stress history, which influence the shape, size and connectivity of the particles that make up the slurry. The subsequent phase interaction mechanisms are quite complex and have direct effects on the flow and final micro-structure distribution of the cast part and thus, without any doubt, on its mechanical properties. Two-phase numerical models have been developed to account for the liquid-solid phase separation e.g. [1,2]. Several two-phase models have been elaborated on the basis of soil mechanics and consider that the phase interaction term is mainly due to the flow through a porous medium. Because of the difficulties of making direct measurements in an extremely hostile environment, there has been very little work done to validate these models. In order to fill this gap, a better understanding of the phase distribution and phase segregation mechanisms during the filling step is required. In this work, the post-solidification primary α-phase distribution inside an industrial semi-solid cast part has thus been investigated. A thorough metallographic analysis has been performed using an upright microscope coupled to a Clemex image-analysis software. The results were then processed to produce a map of the final α-phase distribution. Many different grain scales have been observed in the solidified part and their distributions seem to be closely associated to the velocity field. Contacts between moving particles seem to play an important role in the phase distribution and show many similarities to granular materials. This latter aspect should be considered in the development of new constitutive models for semi-solid slurries.

Abstract: In this work we will explore the use of thermochemical simulation methods (Calphad) to support alloy selection and processing in the semi-solid state. Semi-solid processing has been investigated extensively for aluminium alloys, in particular A356, but there is also an increasing interest in using semi-solid processing for steels, in particular high carbon steels. A key property for the semi-solid processing is the fraction of liquid phase as function of temperature. It is necessary to know the fraction of liquid phase in order to be able to control the process and in order to simulate the viscous flow during various forming operations. The approach used here is to calculate the fraction of liquid phase from thermodynamic (and diffusion) data, using equilibrium calculations, Scheil–Gulliver calculations and diffusion simulation. Normally only the solidification behaviour is considered, but during thixoforming also the melting behaviour is of importance. However, there is very little information on melting of alloys to be found in the literature. Here an attempt will be made to discuss also melting as it cannot in all cases be regarded as the reverse of solidification. In addition some further properties, such as enthalpy, heat capacity and density as function of temperature will be discussed. The materials treated are the aluminium alloy A356 and the tool steel X210CrW12. Interestingly they show fairly similar solidification behaviour.