Xiaohong Zhan

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.

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.

Virtual front tracking cellular automaton modeling of isothermal β to α phase transformation with crystallography preferred orientation of TA15 alloy

A virtual front tracking modified cellular automaton (CA) method is applied to simulate solid phase transformation with a specific crystallographic preferred orientation in a TA15 alloy, to eliminate the dependence of the traditional CA method on space meshing. Simulation results demonstrate the capabilities of the new model in growth anisotropy modeling at large space and time scales, as well as quantitative analysis and description of the precipitated new phase morphology relates to the solute diffusion space by comparison with the diagonal modeling method and rotation of cell sites technique. The isothermal phase transformation kinetics is analysed, which exhibits the desired agreement with the prediction result of the Johnson–Mehl–Avrami analytical equation, thereby a time–temperature-transformation curve is predicted. The crystallography characteristics are consistent with electron backscattered diffraction analysis data. Using the established model, microstructure evolution during the isothermal heat treatment of the TA15 alloy is simulated so that the microstructure heredity with thermal cycling is vividly reflected.

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.

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.

Simulated investigation on the deformation of double laser beam bilateral and simultaneous welding for aircraft panel

An experimental and numerical investigation of double laser beam bilateral and simultaneous welding is carried out in this paper. A dedicated heat source model is developed based on a combined heat source with coordinate transformation and mirroring. Several groups of heat source parameters are assigned, and the simulated results based on these heat source parameters are compared with the experimental results. The deformation after welding for a seven stringers welding panel is predicted by finite element analysis. To optimize the welding sequence, four welding schemes are applied to the simulation. Comparing the simulated results, the scheme 3 is thought to be the best weld sequence for the smallest welding distortion.

Comparative study on experimental and numerical investigations of laser beam and electron beam welded joints for Ti6Al4V alloy

This paper reports a numerical and experimental investigation of laser beam welding (LBW) and electron beam welding (EBW) for the Ti6Al4V titanium alloy. The microstructure of LBW and EBW welded joints is compared. A defect-free, fine-grain, and high-quality joint is produced using the EBW process, while undercut and some pores are observed in the LBW welded joint. The simulated temperature distribution and the cross section of the experimental specimen are compared to evaluate the model validity. The calibrated model is applied to determine the thermal cycles on three selected nodes for both LBW and EBW processes. It is observed that the energy density of the electron beam is higher than that of the laser beam, which causes the weld-seam width of the EBW to be larger than that of the LBW weld seam. Narrow weld-seam, fine grain, and no defect make the EBW a more suitable method to join Ti6Al4V titanium sheets compared with the LBW process.

Study on the Adhesive Joint Strength of 2060-T8 Al–Li Alloy Single Lap Joint with Different Adhesives

Bonding structures using in the new civil aircraft Al–Li alloy have to possess the characteristics of high safety, long service life, high fatigue strength, and good damage tolerance, which lead to a higher requirements for adhesive selection. Three kinds of typical adhesives were preliminarily selected according to requirements of 2060-T8 Al–Li alloy bonding structure. They are WD1001G, EA9394, and Permabond 910. The strengths of the adhesive joints with these adhesives were, respectively, evaluated with single lap shear (SLS) tests. The appropriate adhesive and the surface morphology in the Al–Li alloy bonding structure were selected. The shear strength of 29.9 ± 0.6 MPa was achieved on surfaces using WD1001G. The maximum shear strength can only reach 5% of the base material tensile strength. Shear strength of 18.2 ± 1.8 and 5.7 ± 1.6 MPa was achieved on surfaces using EA9394 and Permabond 910. Cohesive failure occurred in all the adhesive joints after the SLS tests. Wherein, adhesive failure mainly happened in the adhesive joints using WD1001G and EA9394 as well as interface failure mainly happened in the adhesive joints using Permabond 910.