Papers by Author: Luo Xing Li

Abstract: Artificial neural network (ANN) and inverse method were employed in modeling the rheological behavior of the AZ80 magnesium. The hot deformation behavior of extruded AZ80 magnesium was investigated by compression tests in the temperature 350-450 and strain rate range 0.01-50 s^-1. Investigation of flow stress curves and microstructure of the compression specimen illustrate occurrence of dynamic recrystallization. The inverse method of non-liner regression was used to determine the parameters of the suggested constitutive equation. The maximum relative errors at different temperatures and different strain rates between experimental and predicted flow stresses by ANN and inverse method were compared. The results show the ANN derives statistical models have better similar prediction ability to those of inverse method, especially at high strain rate. This indicates that ANN can be used as an alternative modeling tool for high temperature rheological behavior studies.

Abstract: Microstructures and creep properties of AM80 alloy with calcium and strontium additions have been investigated by using OM, X-ray diffraction, SEM and creep tests. The results indicate that the as-cast microstructure of the AM80 alloy consists of the α-Mg matrix, bones-shaped Mg17Al12 and lamellar second precipitation phase at grain boundaries. Calcium and strontium can refine the grain size and the secondary phases. Calcium addition results in the formation of a fishbone Al2Ca eutectic phase in AM80 alloy. With the increase of calcium, reticular Al2Ca phase distribute at the grain boundaries. The creep resistance of the AM80 alloy is significantly improved by a small amount of strontium and calcium addition due to the formation of a grain boundary network consisting of the high melting point Al2Ca phase. Microstructure observations performed on the sample after creep testing reveal that the phase is distorted during creep, reflecting its formation in the as-cast microstructure is unbeneficial to creep properties of the AM80 alloy. The creep resistance of the alloy at elevated temperatures was remarkably increased when calcium was added combined with strontium. The highest creep resistance was obtained from the alloy with xSr and y3Ca addition and its steady state creep rate reached as low as 3.941×10-8s-1, one order of magnitude lower than that of alloy AM80 without strontium and calcium additions.

Abstract: Accurate description of the material flow stress behaviour is an essential requirement for FEM simulation of metal forming processes. In the present hot compression tests of AZ80 magnesium alloy were performed on Gleeble 3500 at strain rates between 0.01-50s-1 and deformation temperatures between 300-450°C to determine the flow stress data of the AZ80 magnesium alloy. It was noticed that with increasing strain rate, deformation heating become more pronounced since there is no time for heat escaping during hot compression tests. Thus, a flow stress correction for deformation heating at high strain rates was carried out for the calculation of the constants of constitutive equation. Validation tests were then performed. Good agreements between the predicted and measured values in extrusion pressure were achieved.

Abstract: In the present study, the extrusion process for the AZ31B magnesium alloy was simulated using a DEFORM-3D software package to establish a database in order to provide input data for artificial neural networks (ANN). The network model was trained by taking extrusion ratio, ram speed, shape complexity and ram displacement as the input variables and the extrusion load and exit temperature as the output parameters. The data from FEM simulations were submitted for ANN as a training file and then ANN built were used to predict the target parameters. The ANN predicted results were found to be in agreement with the FEM simulated and experimental measured ones.

Abstract: The present case study addressed a practical problem of wall thickness attenuation during extrusion to produce a complex thin-walled hollow magnesium profile. A HyperWorks FEM software package was employed to aid in identifying the causes for the wall thickness attenuation. Recommendations were made to adjust the interspacing between the mandrels and the height of the welding chamber. The modified dies yielded much improved results in terms of velocity and hydrostatic pressure uniformity. The wall thickness of the extrudate predicted using FEM simulation was very close to experimental measurements. The case study demonstrated the feasibility of using FEM simulation as a useful tool to solve industrial problems encountered in the production of complex profiles.

Abstract: Isothermal extrusion is a very much desired technology. However, its implementation in the light-metal extrusion practice has, up till now, been technologically constrained. In an attempt to realise isothermal extrusion, a simulation model based on the PID control algorithms was developed to establish ram speed profiles that could prevent extrudate temperature from further increase after an initial rise during an extrusion cycle. With this simulation model, extrusion ram speed could be adjusted in real time according to the simulated exit temperature. A case study was conducted on the simulated extrusion of a magnesium alloy AZ31B into a hollow profile. The results showed significantly improved temperature homogeneity not only along the extrudate length but also on its cross section in the case of extrusion in the isothermal mode with a designed ram speed profile. In addition, die temperature varied over a narrower range and the force acting on the die face was more stable over the process cycle, in comparison with extrusion in the conventional iso-speed mode.