Yajun Yin

A three-dimensional sharp interface model for self-consistent keyhole and weld pool dynamics in deep penetration laser welding

A three-dimensional sharp interface model is proposed to investigate the self-consistent keyhole and weld pool dynamics in deep penetration laser welding. The coupling of three-dimensional heat transfer, fluid flow and keyhole free surface evolutions in the welding process is simulated. It is theoretically confirmed that under certain low heat input welding conditions deep penetration laser welding with a collapsing free keyhole could be obtained and the flow directions near the keyhole wall are upwards and approximately parallel to the keyhole wall. However, significantly different weld pool dynamics in a welding process with an unstable keyhole are numerically found. Many flow patterns in the welding process with an unstable keyhole, verified by x-ray transmission experiments, were successfully simulated and analysed. Periodical keyhole collapsing and bubble formation processes are also successfully simulated and believed to be in good agreement with experiments. The mechanisms of keyhole instability are found to be closely associated with the behaviour of humps on the keyhole wall, and it is found that the welding speed and surface tension are closely related to the formation of humps on the keyhole wall. It is also shown that the weld pool dynamics in laser welding with an unstable keyhole are closely associated with the transient keyhole instability and therefore modelling keyhole and weld pool in a self-consistent way is significant to understand the physics of laser welding.

An improved teaching-learning-based optimization algorithm and its application to a combinatorial optimization problem in foundry industry

Display Omitted We propose a novel improved teaching-learning-based optimization algorithm with the concept of historical population.Two new operators are designed in the proposed algorithm to achieve the balance of exploration and exploitation ability.24 benchmark functions are tested with other algorithms to verify the good exploration and exploitation ability of proposed algorithm.The proposed algorithm is applied to address a combinatorial optimization problem in foundry industry with the design of coding and decoding mechanism. Teaching-learning-based optimization (TLBO) algorithm is a novel nature-inspired algorithm that mimics the teaching and learning process. In this paper, an improved version of TLBO algorithm (I-TLBO) is investigated to enhance the performance of original TLBO by achieving a balance between exploitation and exploration ability. Inspired by the concept of historical population, two new phases, namely self-feedback learning phase as well as mutation and crossover phase, are introduced in I-TLBO algorithm. In self-feedback learning phase, a learner can improve his result based on the historical experience if his present state is better than the historical state. In mutation and crossover phase, the learners update their positions with probability based on the new population obtained by the crossover and mutation operations between present population and historical population. The design of self-feedback learning phase seeks the maintaining of good exploitation ability while the introduction of the mutation and crossover phase aims at the improvement of exploration ability in original TLBO. The effectiveness of proposed I-TLBO algorithm is tested on some benchmark functions and a combinatorial optimization problem of heat treating in foundry industry. The comparative results with some other improved TLBO algorithms and classic algorithms show that I-TLBO algorithm has significant advantages due to the balance between exploitation and exploration ability.

3D Quantitative Analysis of Graphite Morphology in Ductile Cast Iron by X-ray Microtomography

In this article, X-ray microtomography and color metallographic techniques have been used to perform three-dimensional quantitative characterization of graphite nodule morphology in a step-shaped ductile cast iron casting. Statistical analyses of the graphite nodule count, diameter, sphericity, and spatial distribution have been processed for three samples in detail. The results reveal that graphite nodules in ductile cast iron can be categorized into two categories. The first types are nodules located in eutectic cells (NIECs), and the other one refers to nodules located between the eutectic cells (NBECs). The NIECs possess a larger average diameter but smaller sphericity compared with the NBECs, and the sphericity decreases along with the increasing of diameter. The increasing casting thickness results in an increasing count and percentage of NBECs. In addition, most nodules are NIECs in thin walls instead of NBECs in thick walls. Nonuniform spatial distributions of graphite nodules caused by the existence of NBECs have been found to become more obvious along with the increase of cast thickness.

A Three-Dimensional Cellular Automata Model Coupling Energy and Curvature-Driven Mechanisms for Austenitic Grain Growth

AbstractA 3D cellular automata model is used to simulate normal austenitic grain growth in this study. The proposed model considers both the curvature- and thermodynamics-driven mechanisms of growth. The 3D grain growth kinetics shows good agreement with the Beck equation. Moreover, the growth exponent and grain size distribution calculated by the proposed model coincides well with experimental and simulation results from other researchers. A linear relationship is found between the average relative grain size and the grain face number. More specifically, for average relative grain sizes exceeding ~0.5, the number of faces increases linearly with relative grain size. For average relative grain sizes <0.5, this relationship is changed. Results simulated by the proposed model are translated to physical meaning by adjusting the actual temperature, space, and time for austenitic grain growth. The calibrated results are found to be in agreement with the simulation results from other research as well as the experimental results. By means of calibration of the proposed model, we can reliably predict the grain size in actual grain growth.

The Through Process Simulation of Mold filling, Solidification, and Heat Treatment of the Al Alloy Bending Beam Low-pressure Casting

The research on the simulation for the through process of low-pressure casting and heat treatment is conducive to combine information technology and advanced casting technology, which will help to predict the defects and mechanical properties of the castings in the through process. In this paper, we focus on the simulation for through process of low-pressure casting and heat treatment of ZL114A Bending beam. Firstly, we analyzethe distribution of the shrinkage and porosities in filling and solidification process, and simulate the distribution of stress and strain in the late solidification of casting. Then, the numerical simulation of heat treatment process for ZL114A Bending beam is realized according to the heat treatment parameters and the corresponding simulation results of temperature field, stress, strain, and aging performance are given. Finally, we verify that simulation platform for the through process of low-pressure casting and heat treatment can serve the production practice perfectly and provide technical guidance and process optimization for the through process of low-pressure casting and heat treatment.

Three dimensional simulation of the transient process of fiber laser keyhole welding of aluminium alloys

A comprehensive three dimensional transient model is developed for the streamline process of keyhole mode fiber laser welding. The physical effects of multi-reflections absorption, the Marangoni force, the recoil pressure, the surface tension and other physical and thermo-dynamical effects such as evaporation, melting and solidification etc. of the fiber laser keyhole welding process are rigorous considered in the present model. The self-consistent process of three dimensional keyhole instabilities and weld pool dynamics during keyhole mode fiber laser welding of ZL114 aluminium alloys is firstly reasonably modelled. The simulation results reasonably agree with in-situ high speed CCD imaging experiments.

Three-dimensional simulation transient keyhole evolution during laser keyhole welding

Laser keyhole welding process is not well understood because the related physics are too complex. Numerical simulation of transient keyhole evolution provides fundamental data needed to improve understanding of the mechanisms of porosity formation during laser welding. In this paper, a three dimensional mathematical model was proposed to simulate the keyhole formation process of laser welding. The coupled effects of multi-reflections Fresnel absorption of keyhole, heat conduction, heat convection, heat radiation and Knudsen jump boundary conditions on the liquid-vapor evaporation interface were considered in this model. A level set method and a fast sweeping method were developed to solve the model. The transient keyhole shape, the surface recession speed due to evaporation and the thermal distribution of keyhole wall were investigated. The time-varied surface tension force of keyhole wall was also calculated.