Advanced Materials Research Vols. 264-265

Abstract: This study investigates the formability of AZ31 magnesium alloy for bearing cover with inner cavity under hot forging. Firstly high speed metal test machine of China steel Co. Ltd. is used to carry out the compression tests under different forming temperatures and strain rates to obtain the stress-strain curves. Then, the stress-strain data obtained from compression test under different dies are applied to analyze the formability of magnesium alloy for bearing cover of the car under forging by commercial package DEFORM. Besides, hot forgings of magnesium alloy for bearing cover are carried out to study the formability of magnesium alloy, and to find the best forging condition. Meanwhile, from the measured result of hardness and metallographic observation of forged part, the influence of forming temperatures on the strength and microstructure of magnesium alloy under forging of bearing cover are evaluated. Finally, the Artificial Neural Network (ANN) is applied to learn the data obtained from experiments and to predict the experimental result under new combination of process parameters. Also, confirmatory experiment is carried out to prove the usefulness of the ANN model.

Abstract: The flow behaviour of material is strongly influenced by the microstructure evolution during hot deformation processes. In this work, a comprehensive mathematical modelling of heat transfer and plastic deformation was carried out employing finite element analysis based on rigidviscoplastic formulation. Semi-empirical models of dynamic recovery and recrystallization were utilized to develop the microstructure dependent constitutive equations. They were then integrated into the finite element code to simulate stress-strain curve of API-X70 steel during hot deformation process. Hot torsion tests were carried out at various deformation conditions for characterization of microstructure equations and model validation. The good agreement between experimental data and simulation results were achieved. The model predicts work hardening, dynamic recovery and recrystallization simultaneously and it considers their effects on the flow stress of the material during hot deformation.

Abstract: Microstructural evolutions of AZ91 magnesium alloy, which applied to homogenizing annealing treatment, hot extrusion and ageing treatment respectively, are investigated in this paper. Results exhibit that the massive β-Mg17Al12 phase appearing in the initial structure dissolves to eliminate in α-Mg matrix mostly due to the homogenizing annealing treatment; dynamic recrystallization and consequent grain refinement occur during extrusion; banded structure, in which the β-Mg17Al12 phase precipitates parallel to the extrusion direction in α-Mg matrix, are observed in the ageing treated specimen. Furthermore, the effects of temperature, keeping time of homogenizing annealing treatment and ageing treatment, and the extrusion processing parameter on the microstructural evolution of this alloy are also discussed according to the experimental results in details. In addition, the various mechanisms of the morphology and quantity of α-Mg and β-Mg17Al12 phase are analyzed corresponded to the various states. As for the banded structure appearing in the ageing treated specimen, it can be attributed to the banded segregation which remained in the extruded one and resulted from the casting dendrite and subsequent extrusion.

Abstract: The process of lateral extrusion of gear-like components with radial tooth profile has been studied in this paper. The material flow in extrusion die cavity has been simulated using the SuperForge of FVM simulation package. The results of simulation have shown that the material fills the die cavity in two stages: Parallel movement to the end of toothed die cavity and fills the rest of shaped die cavity. Each stage of deformation has been analyzed by using slab method of analysis and the extrusion load values for each punch stroke have been estimated. Finally, comparisons between present theoretical results and experiments of other researchers’ work have been carried out to illustrate the validity of this proposed model.

Abstract: This paper proposed a novel method to fabricate micro-cylindrical lens array (μ-CLA) by using wire electrical discharge machining (WEDM) and precision glass molding (PGM) technique. Micro slits fabricated on the surface of tungsten carbide mold material with outline dimensions from 0.3mm to 2.1mm, the width and depth of 1 mm. The PGM processing parameters including width of slits, molding temperature, molding force and stroke were discussed. Moreover, the characteristics of μ-CLA were measured by contact profilometer. In addition, the relationship between the dimensions of the slit on tungsten carbide mold and the characteristics of μ-CLA also discussed.

Abstract: Electronic speckle pattern interferometry (ESPI) was applied to hydraulic bulging to analyze the entire deformation process, especially the instability evolution of Al 1060 sheet. Speckle patterns of the specimens were captured continuously to record the deformation fields. The development of full-field strain rate during bulging was represented by the fringe patterns real-timely. The emergence of the defect and its subsequent transformation into groove and crack were revealed clearly by the aberration region in the fringe patterns. The onset of diffuse necking and localized necking were determined by the strain rate distribution curves calculated based on the fringe patterns. Results indicate that ESPI is a satisfying method to analyze the instability and fracture of sheet metals even in three dimension deformation. The growth of micro-crack caused a banded weak region. Diffuse necking occurred in the weak region due to strain localization. A groove generated in the banded weak region and then extended along the length direction. The remarkable strain localization in the groove indicated the onset of localized necking. The thickness of the groove decreased quickly and finally caused fracture. The accurately and quickly determined necking strains would lead to a safer FLD.

Abstract: 1.5 mm, 0.7 mm and 0.3 mm thicknesses TiNP/2014Al composite sheets were obtained by hot rolling deformation carried out on as-extruded TiNP/2014Al composite rod. The effect of hot rolling deformation on high strain rate superplastic deformation behavior of the composite was researched by tensile experiment, OM, and SEM. Results show that 0.7mm thickness TiNP/2014Al composite sheet can gain the maximum elongation of 351% at 818 K and 3.3×10-1 s-1, and the m value is 0.43. The optimum strain rate increases with decreasing thickness of the TiNP/2014Al composite sheets. Flow stress and work hardening ability show contrary change tendency to optimum strain rate. The 0.7 mm thickness TiNP/2014Al composite sheet has medium flow resistance stress and shows excellent stability of plastic flow. Fracture surfaces show that the main superplastic deformation mechanism of the TiNP/2014Al composite includes in grain boundary sliding. Subgrain boundary sliding maybe another superplastic deformation mechanism.

Abstract: Considering the necessity of using light weight, high strength and corrosion resistant materials, automotive and aerospace industries need to use advanced production technologies. Hydroforming has been regarded as one of the new technologies in forming of aluminium and magnesium alloys. These alloys have very low formability at room temperature which will be improved at elevated temperatures. In this paper, AA1050 aluminium alloy tube is numerically and experimentally investigated at different temperatures. Thickness distribution in forming zone is studied under different thermal conditions. Numerical results have been verified by experiments and there is a good agreement.

Abstract: Hydroforming is an advanced process in automotive and aerospace industries to form metal tubes into desired shapes by high pressure fluid. The formation of a two layered tubes has been investigated both numerically and experimentally by a pressurized fluid fed into the internal tube through a nonlinear path with no axial feeding. The experimental setup including two units of die clamping system and pressure intensification system has been designed and built in Tarbiat Modares University. The internal and external layers of hydroformed tubes are used aluminum and copper alloys respectively. Effects of different friction conditions on tubes formability have been investigated. Finite element simulation is performed with LS-Dyna FE explicit code using ETA/Dynaform as the preprocessor. The simulation results show that the part can be formed successfully with the internal pressure of 61 MPa. The finite element results are in agreement with experimental results. It is also shown that imposing high friction condition for external tube and low friction condition for internal tube both will fail more likely.

Abstract: There has been a significant research interest in metal micro-forming for the last ten years. However, neither a methodology nor an exact micro mechanical property has been well determined. Hence, several experiments are carried out in this research to point out the influence of “size effect”. Based on the comparison between the experimental data and relevant parameters, the material characteristics in micro scale can be found. In order to find the mechanical properties in micro scale, this study shows the effect of specimen size and grain size on the micro tensile test of copper material. Firstly, five copper micro-sheets with different thicknesses have been chosen as experimental materials. Since the minimum standard test piece formulated in ASTM is still too big for this micro tensile test, this study decided to shrink the sample size of specimen shape to 1/2 . Then the flow stress decreases with decreasing specimen size. This way is able to get the important factor which influence the flow stress on the micro tensile test. Secondly, these samples of different grain sizes are made by heat treatment. Then these samples of different grain sizes are used to conduct the experiment. According to the experimental result, the change of flow stress, which is influenced by various grain sizes, can be found. Finally, the change in the mechanical behavior between specimen size and grain size is obtained.