Papers by Author: Jie Zhou

Abstract: In this research, recrystallization of AA7020 aluminum alloy after hot compression testing was predicted using a framework being a combination of physical modeling and Monte Carlo simulation. Stored energy was calculated as a function of subgrain size related to the Zener Hollomon parameter. The as-deformed grain structure was mapped into the Monte Carlo simulation from experimental results. Calculated stored energy was assigned to the mapped structure, considering the length scale of the simulation. Results were validated by comparing the microstructures obtained from the model predictions with those from experimental results and a reasonable agreement was reached. The predicted grain size was found to be 15 % smaller than the experimental values. Predicted fractions recrystallized showed a similar trend to the experimental results. However, a discrepancy between the model predictions and experimental results in terms of recrystallization kinetics was found, which was attributed to neglecting the effect of subgrain growth and resulting reduction of the stored energy during recovery on the recrystallization kinetics in the present simulation.

Abstract: In the present study, the evolution of the grain structure of a Mg-Al-Ca-based alloy during hot extrusion was simulated with the cellular automation method. The Laasraoui-Jonas microstructure model was used to describe the dislocation evolution inside crystallites during dynamic recrystallization. The parameters in the Laasraoui-Jonas model, such as the hardening parameter, recovery parameter and material constants, were determined from the flow stress-strain data obtained from hot compression tests using a Gleeble-1500 thermomechanical simulator. The extrusion process was simulated using a DEFORM 3D FEM code. The influence of ram speed on grain structure evolution was analyzed. It was found that the average grain size increases with increasing ram speed. Good agreements between the predicted and observed grain structures were achieved.

Abstract: Four variants of AA7020 aluminum alloy having different Zr and Cr contents were investigated aiming at reaching high recrystallization resistance during and after hot deformation. Isothermal homogenization treatments were performed at temperatures of 390-550 °C for 2 to 48 hours. The uni-axial hot compression tests were conducted at 450 °C and strain rate of 10 s-1 at a strain of 0.6. Thereafter, the samples were annealed at 550 °C for 10 min. It was found that the samples with the highest Zr and Cr contents showed the lowest volume fraction of recrystallized grains which was attributed to the highest volume fraction of Zr- and Cr-containing dispersoids formed during homogenization. The optimum homogenization treatment to achieve highest recrystallization resistance for these samples was 470 °C for 24 hours.

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: Wide, thin-wall profiles exiting simultaneously from a multi-hole die during aluminum extrusion tend to have different velocities and deflect from the straight line. The pockets in front of the die orifices are often used to balance the metal flow and equalize the velocities. In practice, the effect of a pocket die design on metal flow becomes known, only after the die is manufactured and a trial extrusion run is completed. The present research was intended to demonstrate the feasibility of using FEM simulation to predict metal flow, thereby reducing or hopefully eliminating trial extrusion runs. The extrusion of U-shaped profiles with different wall thicknesses through a multi-hole pocket was taken as an example to show the scope of adjusting the die pocket to regulate metal flow. The effect of pocket shape on metal flow was evaluated. It is clear that 3D FEM simulation can indeed be effectively used to optimize die design, before the die design is finalized.

Abstract: The effect of process parameters on the creep-fatigue behavior of a hot-work tool steel for aluminum extrusion die was investigated through a technological test in which the specimen geometry resembled the mandrel of a hollow extrusion die. Tests were performed on a Gleeble thermomechanical simulator by heating the specimen using joule’s effect and by applying cyclic loading up to 6.30 h or till specimen failure. Displacements during the tests at 380, 490, 540 and 580°C and under the average stresses of 400, 600 and 800 MPa were determined. A dwell time of 3 min was introduced during each of the tests to understand the creep behavior. The results showed that the test could indeed physically simulate the cyclic loading on the hollow die during extrusion and reveal all the mechanisms of creep-fatigue interaction.

Abstract: A novel extrusion testing method, double action extrusion (DAE), to highlight the effect of friction at the die bearing in aluminum extrusion was developed. It was found that the lengths of the extrudates and extrusion force were indeed sensitive to the die bearing length and thus to the friction. FEM simulations of DAE were carried out to evaluate the shear and Coulomb friction models over a wide range of friction factors/coefficients from 0.2 to 1. The full sticking friction appeared to represent the interfacial contact between hot aluminum and die the best. The friction factor values in the shear friction model over a range of 0.3 to 0.6 commonly used to describe the contact at the billet-die interface in FEM simulation appeared to be too low. The comparison between the experimental and simulation results indicated that the shear friction model at m = 1 predicted the extrusion force the best, while the Coulomb friction model at µ = 1 predicted the extrudate lengths the best. Of the existing friction models and friction factors/coefficients, it is recommended to use the shear friction model at m = 1 to describe the friction at the billet-die interface in FEM simulation.

Abstract: Recently, blast furnace slag, fly ash and limestone powder are increasingly used as blending materials in producing concrete. The use of these materials not only has economical and environmental advantages, but also improves the mechanical properties, durability and workability of concrete. In this paper, the results of experimental investigations on the evolution of hydration heat and the development of microstructure of Portland cement blended with blast furnace slag, fly ash or limestone powder are presented. These results show that three blending materials accelerate the hydration of Portland cement, but result in less heat release during the first 72 hours. The Portland cement with blast furnace slag has a denser pore structure than the others.

Abstract: The plasma nitriding (PN) process in the duplex surface treatment was controlled to create nitrided diffusion layers with depths of 0, 5, 15 and 80 m in the substrate of the DIN 1.2367 hot-work tool steel with the maximum microhardness values of 600, 700, 820 and 1000 HV, respectively. The scratch properties, i.e. the critical loads of cohesion (LC1), adhesion (LC2), breakthrough (LC3) and worn out (LC4), of the PACVD TiBN coating (boride, 5-7 at.%) on these substrates increased linearly with the maximum hardness of the PN diffusion layer. Instead of the composite hardness, the peak scratch hardness was used to describe the load-carrying capacity of the TiBN coating and PN substrate. Deep tensile cracks in the PN substrate with a hardness value of 1000 HV formed during the scratch test at a load as low as 90 N, indicating the low fracture toughness of the substrate. Therefore, an optimum balance between the scratch properties of the coating and the good fracture toughness of the nitrided substrate must be achieved through exercising the control of the PN and PACVD duplex process.