Yanhong Wei

Numerical simulation of columnar dendritic grain growth during weld solidification process

Abstract A numerical model based on cellular automaton algorithm is developed to simulate dendrite growth at the edge of weld molten pool and the solute diffusion during grain growing process in weld solidification is analysed too. By means of two-dimensional square cells and von Neumann neighbourhood, the growing morphologies of the columnar dendritic grains with different cooling rates and different numbers of seeds are simulated. The growth of secondary dendrite arms, tertiary dendrite arms and their competitive growth are also presented. The results illustrate that the final primary dendrite spacing depends on the number of seeds that initially generated. With increasing cooling rate, the growing speed is increasing obviously. It is also indicated that competitive growth exists between different dendrite arms. The tendency of competitive growth in high cooling rate conditions is weaker than the one in relatively small cooling rate conditions.

Cellular automaton modelling of dynamic recrystallisation microstructure evolution during friction stir welding of titanium alloy

Abstract It is of great importance to study how the process parameters influence the continuum flow properties and mesomicrostructural features during friction stir welding (FSW) of titanium alloys. In the present paper, microstructure evolution of dynamic recrystallisation (DRX) was simulated by cellular automaton method based on metal plastic flow analysis. The established model can accomplish multiscale modelling of macroscopical plastic flow behaviour, mesostructural dislocation activities, microstructural grain growth dynamics and finally mechanical property of the FSW joint. The predicted flow stress curves are similar with that experimentally measured. Influences of strain rate and temperature on the evolution of the average dislocation density (equivalent to flow stress) as well as the final grain size are analysed, which are in good agreement with classic DRX theories. Final grain size results from the contrary effect of strain rate and temperature, while the strain rate with higher gradient in the welded joint has greater influence. According to the prediction result, a lot of options can be made on matching of marching speed and rotation speed to maintain refined microstructure.

A thermal–metallurgical model of laser beam welding simulation for carbon steels

A coupled thermal–metallurgical model is developed to predict the temperature fields and spatial distribution of volume fraction of phases during laser beam welding of 1020, 1045, and 1060 steels. The classical transient heat conduction model is used to calculate the temperature fields during laser beam welding. For phase transformation, the austenization, the austenite-to-pearlite/ferrite transformation, the austenite-to-bainite transformation, and the austenite-to-martensite transformation are modeled respectively. All of these transformation models are solved by the finite element method (FEM) based on the simulated temperature fields. The thermal properties of the three steels are determined by the linear interpolation base of the phase fractions, and thermal properties for each pure phase. The temperature fields and spatial distribution of phases are predicted by 3D finite element method (FEM) code which is developed by the authors to solve the thermal–metallurgical models. In addition, comparison between the coupled model and the pure conduction model without considering phase transformations is carried out to study the influence of phase transformation on temperature fields during welding. According to the comparison, the temperature of the coupled model is higher than the pure conduction model in the temperature region above 1000 °C, but the temperature profiles are very similar at the temperature region under 1000 °C. The predicted volume fractions of 1020 and 1060 steels are close to experimental results. However, there is an obvious difference between predicted and experimental results of the phase fraction of 1045 steels.

The Numerical and Experimental Investigation of the Multi-layer Laser-MIG Hybrid Welding for Fe36Ni Invar Alloy

Numerical and experimental investigations of multi-layer laser-MIG hybrid welding for Fe36Ni Invar alloy were presented in this paper. The multi-layer laser-MIG hybrid welding experiments with different parameters were conducted for the 19.5-mm-thick Invar plates. A finite element (FE) model was established to predict the temperature field, residual stress, and deformation distribution during and after welding. A plane-conical combined heat source model was used to simulate the laser-MIG hybrid welding process. Different numbers of welding layers were chosen to study the effect of welding layer on the temperature field, residual stress, and deformation distribution. It was found that the maximum residual stress of Invar plates after laser-MIG hybrid welding is 300xa0MPa and maximum deformation is 0.4xa0mm, so that laser-MIG hybrid welding can be used in actual manufacture of Invar moulds.

Stress distributions of welding joints in titanium-steel composite pressure vessel under working conditions

Abstract Titanium–steel composite plate finds its application to constructing large pressure vessels normally used for storage and processing petrochemicals. The present study aims to calculate the stress–strain distributions in different types of welding joints possibly used in a pressure vessel which is built with a TA2/16MnR composite plate. A finite element numerical investigation has been preformed. In this work, influences on stress–strain distributions caused by various factors such as work temperatures, working loads and joint structures, were studied, taking into consideration the welding residual stress caused by the welding process. It is found that working temperatures are the main factors that would cause a great effect on the final stress distribution. As to the different types of welding joints, the stress peaks and distributions present various patterns due to the different structures of the joints. Based on the simulation result, suitable welded joint types under different working conditions are proposed.

Software package for simulation and prediction of welding solidification cracks

Abstract In the present work, a software system is developed that can simulate and predict welding solidification cracks, based on previous work on welding solidification crack simulation in stainless steels. First, it assists users in discretising the welding workpiece, calculating welding heat input distribution, inputting material properties, and generating input data cards. Second, the system runs commercial finite element calculation software on the basis of input data cards. Third, the system performs data treatment to provide the user with simulated results in the form of contours, three-dimensional plots, and featured curves such as welding temperature cycles and strain - stress cycles. Next, the system carries out regression analysis on experimental data from transverse Varestraint testing to determine the resistance of the material to welding solidification cracking. Finally, the system calculates the curves of strain evolution with temperature at the trailing edge of the mushy region in the weld pool to obtain the driving force for welding solidification cracking. Consequently, the system can predict welding solidification cracking by presenting users with the driving force and resistance curves in one figure. The software package is developed mainly using Visual Basic and its graphical functions are achieved with the assistance of the Matlab software packages.

A three-dimensional cellular automaton model of dendrite growth with stochastic orientation during the solidification in the molten pool of binary alloy

A three-dimensional (3D) cellular automaton model is developed for the prediction of dendrite growth with stochastic orientation during solidification process in the molten pool of binary alloy. An angle-information transfer method is proposed for improving cellular automaton technique to simulate the growth of the dendrites whose preferred growth direction owns stochastic misorientation with respect to the direction of the coordinate system. Dendrite morphologies and solute distributions of single dendrite growth and multi-dendrite growth are able to be obtained by the simulation using present model. The model is also employed to study the difference between two-dimensional simulation and 3D simulation on solute segregation and dendritic growth. Using the established model, 3D multi-columnar dendrites with stochastic crystallographic orientations can be obtained efficiently, and the competitive growth and impinging of dendrites can be reproduced in practice. The simulation results agree well with the experimental results.

A phase field investigation of dendrite morphology and solute distributions under transient conditions in an Al-Cu welding molten pool

A quantitative phase field model is developed to simulate dendrite morphology and solute distributions of the Al–4u2005wt-% Cu alloy in Gas Tungsten Arc Welding (GTAW) welding molten pool under transient conditions. The functions of temperature gradient and travel velocity are used to obtain transient conditions of the welding molten pool. Time evolutions of the dendrite morphology, solute distributions of different positions and interfaces are obtained. The dendrite growth process can be divided into four stages, namely linear growth, non-linear growth, competitive growth and short-term steady growth. The solute concentration near the primary dendrite tip region is the smallest, while solute concentration is larger in the front of plane crystals growth interface and the solute concentration in the liquid region among the primary dendrites is obviously the largest, where the solute segregation forms readily. For the given welding parameters, the dendrite morphology and the initial instability of solid/liquid interface agree well with the experimental result.

Cellular Automaton Study of Hydrogen Porosity Evolution Coupled with Dendrite Growth During Solidification in the Molten Pool of Al-Cu Alloys

Welding porosity defects significantly reduce the mechanical properties of welded joints. In this paper, the hydrogen porosity evolution coupled with dendrite growth during solidification in the molten pool of Al-4.0 wt pct Cu alloy was modeled and simulated. Three phases, including a liquid phase, a solid phase, and a gas phase, were considered in this model. The growth of dendrites and hydrogen gas pores was reproduced using a cellular automaton (CA) approach. The diffusion of solute and hydrogen was calculated using the finite difference method (FDM). Columnar and equiaxed dendrite growth with porosity evolution were simulated. Competitive growth between different dendrites and porosities was observed. Dendrite morphology was influenced by porosity formation near dendrites. After solidification, when the porosities were surrounded by dendrites, they could not escape from the liquid, and they made pores that existed in the welded joints. With the increase in the cooling rate, the average diameter of porosities decreased, and the average number of porosities increased. The average diameter of porosities and the number of porosities in the simulation results had the same trend as the experimental results.

Simulation of primary dendrite arm spacing in an Al–Cu welding molten pool

Primary dendrite arm spacing in tungsten inert gas welding pool for Al–4wt%Cu alloys is predicted by a quantitative phase-field model. Transient conditions are obtained by functions of thermal gradient and solidification rate. When the same welding power is given by 3500u2005W and different welding velocities are given by 1.5, 2.0 and 2.5u2005mmu2005s−1, the primary dendrite arm spacing obtained by simulation results is the largest under the welding velocity of 2.0u2005mmu2005s−1. When the same welding velocity is given by 2.5u2005mmu2005s−1 and different welding power is given by 3500, 4000 and 5000u2005W, the primary dendrite arm spacing acquired by simulation results is the largest under the welding power of 5000u2005W. Primary dendrite arm spacing and morphology obtained by simulation results agrees well with experimental findings.