Hai Gong

Effects of rolling parameters of snake hot rolling on strain distribution of aluminum alloy 7075

Abstract The realization way of snake rolling was introduced. Flow velocity, strain and stress distribution of 7075 aluminum alloy plate during snake rolling and symmetrical rolling were analyzed in Deform 3D. Effects of velocity ratio, offset distance between two rolls and pass reduction on the distribution of equivalent strain and shear strain were analyzed. The results show that flow velocity and equivalent strain on the lower layer of the plate are larger than those of the upper layer because of the larger velocity of the lower roll and the gap is increased with the increase of velocity ratio and pass reduction. The shear strain of rolling direction in the center point is almost zero during symmetrical rolling, while it is much larger during snake rolling because of the existence of rub zone. The shear strain is increased with the increase of velocity ratio, offset distance and pass reduction. This additional shear strain is beneficial to improve the inhomogeneous strain distribution.

Comparison of three formulations for eddy-current and skin and proximity problems in a meander coil electromagnetic acoustic transducer

Three differential equations based on different definitions of current density are compared. Formulation I is based on an incomplete equation for total current density (TCD). Formulations II and III are based on incomplete and complete equations for source current density (SCD), respectively. Using the weak form of Finite Element Method (FEM), the three formulations were applied in a meander coil Electromagnetic Acoustic Transducer (EMAT) example to solve magnetic vector potential (MVP). The FEM results from frequency domain and time domain models are in excellent agreement with previously published works. Results show that the errors for Formulations I and II vary with coil dimensions, coil spacing, lift-off distance and external excitation frequency, for the existence of eddy-current and skin and proximity effects. And the current distribution across the coil conductors also follows the same trend. It is better to choose Formulation I instead of Formulation III to solve MVP when the coil height or width are less than twice the skin depth, due to the low cost and high efficiency of Formulation I.

Effect of non-uniform stress characteristics on stress measurement in specimen

Abstract There is a remarkable difference in stress distribution between a specimen and a plate removed from the specimen. The plate presents a uniform stress distribution whereas the specimen presents a non-uniform stress distribution. Firstly, the real stress distributions in plates with thickness of 30, 40 and 50 mm and then in the specimens were obtained through simulation and X-ray surface stress measurement. Secondly, in order to study the impact of specimens shapes and processing ways on the results accuracy, two irregular shapes (parallelogram and trapezoid) and two processing ways (saw and electron discharge machining (EDM)) were compared and analyzed by simulation and experiment using layer removal method, then the specimen effects on measurement results were evaluated. The results show that: 1) the non-uniform stress distribution characteristics of the specimen near the surface of the cut is significant, the range of non-uniform stress distribution is approximately one-thickness distance away from the cut, and it decreases gradually along the depth; 2) In order to ensure the stability in the results, it is suitable to take the specimen plane size 2-3 times of its thickness; 3) Conventional processing methods have little effect on experimental results and the average deviation is less than 5%.

Comparisons of Different Models on Dynamic Recrystallization of Plate during Asymmetrical Shear Rolling

Asymmetrical shear rolling with velocity asymmetry and geometry asymmetry is beneficial to enlarge deformation and refine grain size at the center of the thick plate compared to conventional symmetrical rolling. Dynamic recrystallization (DRX) plays a vital role in grain refinement during hot deformation. Finite element models (FEM) coupled with microstructure evolution models and cellular automata models (CA) are established to study the microstructure evolution of plate during asymmetrical shear rolling. The results show that a larger DRX fraction and a smaller average grain size can be obtained at the lower layer of the plate. The DRX fraction at the lower part increases with the ascending speed ratio, while that at upper part decreases. With the increase of the offset distance, the DRX fraction slightly decreases for the whole thickness of the plate. The differences in the DRX fraction and average grain size between the upper and lower surfaces increase with the ascending speed ratio; however, it varies little with the change of the speed ratio. Experiments are conducted and the CA models have a higher accuracy than FEM models as the grain morphology, DRX nuclei, and grain growth are taken into consideration in CA models, which are more similar to the actual DRX process during hot deformation.

Optimization of deformation parameters of dynamic recrystallization for 7055 aluminum alloy by cellular automaton

Abstract In order to simulate the microstructure evolution during hot compressive deformation, models of dynamic recrystallization (DRX) by cellular automaton (CA) method for 7055 aluminum alloy were established. The hot compression tests were conducted to obtain material constants, and models of dislocation density, nucleation rate and recrystallized grain growth were fitted by least square method. The effects of strain, strain rate, deformation temperature and initial grain size on microstructure variation were studied. The results show that the DRX plays a vital role in grain refinement in hot deformation. Large strain, high temperature and small strain rate are beneficial to grain refinement. The stable size of recrystallized grain is not concerned with initial grain size, but depends on strain rate and temperature. Kinetic characteristic of DRX process was analyzed. By comparison of simulated and experimental flow stress–strain curves and metallographs, it is found that the established CA models can accurately predict the microstructure evolution of 7055 aluminum alloy during hot compressive deformation.

Optimal design of spiral coil electromagnetic acoustic transducers considering lift-off sensitivity operating on non-ferromagnetic media

Abstract The generation efficiency of electromagnetic acoustic transducers (EMATs) dramatically reduces with increasing lift-off, thus restricting their applications. This paper aims to provide methods to decrease lift-off sensitivity. The process of generating a shear wave in a spiral coil EMAT is established based on a 2-D axisymmetric model using the finite element method. An equivalent excitation circuit, including an impedance-matching network and a limited power source, is presented for calculating the excitation current. Such an equivalent circuit has not been given adequate attention in previous publications in this field, but it has proven to be of great importance in determining the efficiency of ultrasonic generation. The predictions of static magnetic field, eddy current density and excitation current are validated by experimental results. The conversion efficiency and lift-off sensitivity of the EMAT are analysed with respect to impedance matching parameters, magnet dimensions, the presence of a copper backplate between the magnet and the coil, and backplate-to-coil distance. The results indicate that impedance matching parameters, magnet diameter and plate-to-coil distance have obvious effects on lift-off sensitivity. The backplate can be used to decrease lift-off sensitivity, and it can rearrange the source current density across the cross-sectional area of the coil conductor. Furthermore, compared with the original EMAT, the lift-off sensitivity of the optimised EMAT can be decreased by 7.9 dB at 2 mm lift-off.

Comparisons of flow behavior characteristics and microstructure between asymmetrical shear rolling and symmetrical rolling by macro/micro coupling simulation

Abstract A new rolling method of asymmetrical shear rolling is applied to solve heterogeneous strain and uneven microstructure distribution of thick plate preparation in symmetrical rolling. Coupled finite element models (FEM) and cellular automata (CA) models are established to compare strain and temperature characteristics and microstructure evolution during multi-pass rolling of two rolling ways. Different recrystallization mechanisms are taken into consideration in coupled models of multi-pass rolling. The results show that equivalent strain distribution is asymmetrical in asymmetrical shear rolling and the value at center point is 16% larger than that in symmetrical rolling due to accumulated shear deformation. The temperature at lower layer of the plate in asymmetrical shear rolling is higher, which results in faster metal flow and larger strain. The temperature variations in relation to time at different positions of the plate are fitted. Microstructure evolution for multi-pass rolling is studied and higher recrystallization fraction and finer grain size appear at center point of plate in asymmetrical shear rolling. Rolling experiments are conducted and the results of strain and microstructure agree with simulation results. The coupled models provide guidance for flow behavior and microstructure analysis at macro/micro level.

A modified model for simulating the effect of temperature on ultrasonic attenuation in 7050 aluminum alloy

Knowing propagating properties of an ultrasonic wave can enhance the non-destructive testing techniques in alloy materials field, such as the electromagnetic acoustic transducer techniques, and the piezoelectric ultrasonic transducer techniques. When temperature is taken into consideration, the ultrasonic propagating attenuation become very complex process. In this paper, a loss factor coefficient function with change in temperatures is established and the loss factor damping model with temperature term is coupled into the equations of elastic wave motion. A modified frequency domain model for calculating the ultrasonic attenuation due to temperature changes in 7050 Aluminum alloy is then developed. The model is validated experimentally using a high power pulse transmitter/receiver RPR-4000, a resistant high temperature electromagnetic acoustic transducer set-up and a 7050 Aluminum alloy sample. The simulation and the experimental results are determined to be in good agreement. The numerical model is used to calculate the ultrasonic-waves field, the ultrasonic attenuation, and the ultrasonic propagation directivity considering the temperature effect. The modeling results indicate that the ultrasonic energy attenuation is significantly affected by temperature. When the temperature increases from 20°C up to 480°C, the ultrasonic energy attenuates by 32.31%. It is also found that the length of near acoustic field increases with the increase in temperature. There is a common basic mode for the attenuation of ultrasonic waves, in which the attenuated mode cannot be affected by other factors. Increasing the temperature or the frequency, the ultrasonic propagation can obtain an excellent directivity. Results obtained from the present model will provide a comprehensive understanding of design parameter effects and consequently improve the design/performance in the non-destructive testing techniques.Knowing propagating properties of an ultrasonic wave can enhance the non-destructive testing techniques in alloy materials field, such as the electromagnetic acoustic transducer techniques, and the piezoelectric ultrasonic transducer techniques. When temperature is taken into consideration, the ultrasonic propagating attenuation become very complex process. In this paper, a loss factor coefficient function with change in temperatures is established and the loss factor damping model with temperature term is coupled into the equations of elastic wave motion. A modified frequency domain model for calculating the ultrasonic attenuation due to temperature changes in 7050 Aluminum alloy is then developed. The model is validated experimentally using a high power pulse transmitter/receiver RPR-4000, a resistant high temperature electromagnetic acoustic transducer set-up and a 7050 Aluminum alloy sample. The simulation and the experimental results are determined to be in good agreement. The numerical model is used...