Jinwu Kang

A study on the numerical simulation of thermal stress during the solidification of shaped castings

Abstract In order to study the development of thermal stress and to predict the hot tearing and residual stress of shaped casting, two models were used to carry out the stress analysis of the two stages of solidification. The rheological model [H]–[H|N]–[N|S] was used for the quasi-solid zone while the thermo-elasto-plastic model was used for the period after solidification. Coupling the thermal analysis based on the finite different method with the stress analysis based on the finite element method, a FDM/FEM integrated system of thermal stresses analysis during the solidification process was developed. After experimental verification, the system was put into practical application. The analysis results during the quasi-solid zone show that the visco-plastic strain is an important factor for the occurrence of hot tearing. The hot tearing of a case steel casting and the residual stresses and deformation of a hydro-turbine blade steel casting were analyzed and predicted using the system. The simulation and the practical results were basically in agreement.

FEM Modeling of Induction Hardening Processes in Steel

A modeling system for analyzing the integrated induction hardening processes was developed based on a general-purpose finite element program, with the capability to analyze the whole process from electromagnetic-induced thermal heating to final hardening. A coupled electromagnetic-thermal model was applied to study the induction heating process, which includes consideration of nonlinear material characteristics on temperature. Also, arrangement of AC current density distribution was conducted to simulate practical induction coil structure and magnetic concentrator effects to achieve desired heating patterns for later quenching and hardening analysis. Quenching analysis can provide cooling curve at any location in a heat-treated workpiece based on heat transfer principles. In hardening analysis, phase transformation was studied and an algorithm was developed to determine volumetric content of micro-structural constituents formed from austenitized phase in quenching process, based on analysis of the interaction between cooling curve and material time-temperature-transformation (TTT) diagram. Finally, hardness value was converted from martensite content based on a developed formulation. Validation was preliminary conducted based on comparison of hardening pattern of induction hardening of an automotive spindle with complex surface.

The effect of ultrasonic processing on solidification microstructure and heat transfer in stainless steel melt

The heat transfer in the ultrasonic processing of stainless steel melt is studied in this thesis. The temperature field is simulated when the metal melt is treated with and without ultrasound. In order to avoid the erosion of high temperature melt, ultrasound was introduced from the bottom of melt. It is found that the temperature of melt apparently increases when processed with ultrasound, and the greater the ultrasonic power is, the higher the melt temperature will be; ultrasonic processing can reduce the temperature gradient, leading to more uniform temperature distribution in the melt. The solidification speed is obviously brought down due to the introduction of ultrasound during solidification, with the increasing of ultrasonic power, the melt temperature rises and the solidification speed decreases; as without ultrasound, the interface of solid and mushy zone is arc-shaped, so is the interface of liquid and mushy zone, with ultrasound, the interface of solid and mushy zone is still arc-shaped, but the interface of liquid and mushy zone is almost flat. The simulation results of temperature field are verified in experiment, which also indicates that the dendrite growth direction is in accord with thermal flux direction. The effect of ultrasonic treatment, which improves with the increase of treating power, is in a limited area due to the attenuation of ultrasound.

The comparison of ultrasonic effects in different metal melts

The effect of ultrasonic treatment of the melts is mainly ultrasonic streaming and cavitation. In this paper, the ultrasonic streaming in water, aluminum and steel melts was numerically simulated and compared. And the simulated results of streaming in water were validated by experimental results. In the experiment, the ultrasonic booster was immersed vertically into water, the ultrasonic streaming phenomenon was observed by high-speed CCD (Charge-coupled Device) system, then the streaming velocity and streamlines were obtained. The cavitation area and threshold in aluminum and steel melts were compared. The results show that the effective streaming and cavitation area in steel melt is smaller than that in aluminum melt, and far smaller than that in water. A symmetrical vortex forms both in water and aluminum melt by the drive of downward ultrasonic streaming caused by the booster tip. However, in steel melt, a double-vortex structure, including a vortex in the upper part and a vortex with reverse cycling in the lower part appears in the flow field. As a result, inclusions and air bubbles may be trapped in steel melt. The density and viscosity of the fluids are the main factors influencing ultrasonic streaming and cavitation. The results provide references for the application of ultrasonic treatment in metal melts.

Investigation on Temperature Change of Cold Magnesium Alloy Strips Rolling Process with Heated Roll

Magnesium alloy strips are widely used in aerospace, automotive industry, etc., which are difficult to produce through cold forming process due to their poor deformation ability. In this article, we studied whether the rolling process with heated roll could be used to roll cold magnesium alloy strips. Thermal-mechanical finite element simulation of the rolling process, using heated roll and cold strips to produce the magnesium alloy strips, was carried out. Influences of roll temperature, rolling velocity, rolling reduction ratio, and initial strip thickness on the thermal field and the mean temperature of magnesium alloy strips were analyzed. Both the heated area in strips in rolling deformation zone and the mean temperature of strips at exit of rolling deformation zone increase with increasing the roll temperature and/or rolling reduction ratio, and/or with decreasing the rolling velocity and/or initial strip thickness. Finally, a formula was developed to predict the mean temperature of strips under different rolling conditions, which also could be used to calculate the critical value of parameters in rolling process.

Numerical simulation of deformation in large scale hydroturbine blade casting

Abstract Deformation is one of the most common defects during the casting of large scale hydroturbine blades because of their curved three-dimentional (3D) surface geometry. Inverse deformation is usually applied to the pattern for sand moulds to finally obtain proper shape. However, the value of inverse deformation is hard to be determined. In this paper, a method is presented to determine the inverse deformation values by cycling numerical simulation. The inverse deformation is determined by the criteria of the achievement of even and appropriate machining allowance of the whole casting. This method is applied to a large scale blade casting used in the Three Gorges Power Plant. The calculated inverse deformation is obtained after three cycles. By using the electronic coordinate determination system (ECDS), its surface is measured and the machining allowance values of some points are acquired. The measured and calculated results are in agreement.

Development of a Para-AMR algorithm for simulating dendrite growth under convection using a phase-field–lattice Boltzmann method

By combining adaptive mesh refinement and parallel computing, a high performance numerical algorithm was developed to simulate dendrite growth against convection using a phase-field–lattice Boltzmann method. Numerical tests on both 2-D and 3-D dendrite growth cases revealed that, by employing moderate amount of computing resources ( 10 1 –10 2 1 0 1 – 1 0 2 parallel processes), this algorithm, without compromising any accuracy, could improve the computational efficiency by 2–3 orders of magnitude, or for most cases shorten the overall elapsed simulation time by 95%, comparing with the normally applied explicit algorithm. Besides, the computational stability or convergence of the algorithm could be maintained even when the local volume fraction of solid approached ∼100% ∼ 100 % , which could not be achieved if other implicit algorithms like SIMPLE was employed.

Evaluation of distortion of castings

Abstract Displacement results are usually used as the criterion for distortion of castings during casting and heat treatment processes. However, the displacement results consist of both contraction and distortion. Contraction is a kind of uniform volume change, which can be solved by enlarging the castings by a contraction coefficient. However, the actual distortion has to be deleted by giving inverse distortion or by mechanical correction method. In this paper, algorithms are presented to evaluate the distortion of castings. One method is to subtract the uniform contraction from the displacement results. The other is to calculate the deflection angle between the normal direction of the discretised surface triangle of the original casting shape in stereolithography format and that of the simulated casting shape. The third one is the application of actual machining allowance in the evaluation of distortion. These methods are validated by three case studies, including a heavy hydroturbine blade casting.

Stress analysis and deformation prediction of a heavy hydraulic turbine blade casting during casting and heat treatment

Abstract Owing to their large and curved shape, blade castings, a key component for heavy hydro turbines, are susceptible to deformation during casting and heat treatment. In the present paper, the stress analysis of a blade casting during both casting and heat treatment is performed. The coupled thermo-stress and thermo-phase transformation stress models are used for casting and heat treatment respectively. Machining allowance distribution is used as the deformation criterion and an algorithm of inverse deformation determination is presented. The mechanical properties of the martensitic stainless steel ZG0Cr13Ni4Mo (13Cr–5Ni–1Mo) at different temperatures are measured under as cast and heat treated status. Finally, the inverse deformation of the blade during both casting and heat treatment processes is obtained, and a series of sections of the blade casting with inverse deformation design are given for pattern making. The calculated deformation results are compared to the measured one, and they are basically in agreement.

Thermal stresses in a cylinder block casting due to coupled thermal and mechanical effects

Thermal stress in castings results from nonuniform cooling. The thermal stress and the deformation can change the casting and mold contact conditions which then alter the heat transfer between the casting and the mold. The contact element method was used to study the interaction between a sand mold and a casting. The contact status was then fed back to the heat transfer analysis between the sand mold and the casting to re-evaluate the heat transfer coefficient based on the gap size or pressure between surfaces. The thermal and mechanical phenomena are then coupled in two directions. The method was applied to analyze stress in a stress frame specimen casting and a cylinder block. The results are more accurate than without consideration of the contact effects on the heat transfer.