MONTE CARLO Simulation Of Grain Growth In Welded Joints For SUS316 And NIMONIC263

Microstructure evolution in the welded joints is complex because of local andinstantaneous heating and cooling process. The evolution includes the melting andsolidification in the weld bead, the solid-state phase transformation and grain growthin the Heat Affected Zone (HAZ), which strongly in?uences the materials physicalproperties. In some single-phase alloys, such as austenitic stainless steel SUS316 andnickel-based superalloy Nimonic263, there is no solid-state transformation in HAZ,so grain growth becomes the main microstructure evolution in HAZ during weldingprocess. In order to study how welding thermal process of single-phase alloys in?u-ences the grain growth in welded joints, three dimensional Monte Carlo(MC) modelsare built for simulating grain growth in both HAZ and Fusion Zone (FZ). Meanwhile,three kinds of models are built to simulate the ?uid ?ow and thermal fields of weldedjoints to get reasonable thermal cycles for MC simulation. Additionally, a three di-mensional grain growth simulation system is established to simulate the thermal pro-cess and microstructure evolution of welded joints for Tungsten Inert Gas (TIG) weld-ing, which can describe the process of grain growth and estimate the grain sizes. Thesoftware system will be helpful in optimizing the welding process parameters.By considering surface tension, electromagnetic force and buoyancy, the soft-ware PHOENICS is used to model heat transfer and ?uid ?ow process in TIG welding.The conduction model, the laminar model and the turbulence model are constructedto simulate the heat transfer process of Nimonic263 in TIG welding. The accuracyof simulated results and the conditions in which the models can be used are there-fore analyzed. It shows that the simulated results with turbulence model are closer tothe experimental ones. By using turbulence model, the thermal process of SUS316TIG welding is simulated, the distributions of effective viscosity and effective ther-mal conductivity are studied, and the heat transfer by convection and conduction ofturbulent liquid metal in welding pool are given too. The simulated results obtainedby turbulence model shows that, with the increase of traveling velocity the ability ofthermal conduction in the liquid metal is correspondingly enhanced, while the heattransfer with convection is relatively reduced because of viscosity increase. Accord- ingly, with the increase of welding heat inputs the instability of front plane increasesduring welding molten pool solidification, the grain growth in front plane of weld poolis non-planer.The heat and mass transfer process in Active ?ux TIG (A-TIG) welding is simu-lated with the heat transfer and ?uid ?ow model. The distribution of heat transfer and?uid ?ow is used to investigate the mechanism of the penetration increase in A-TIGwelding Nimonic263 plate. The simulated results show that the arc constriction workslittle on weld pool shape, and the following two factors contribute much more to thepenetration increase in A-TIG welding. For one thing, the active elements such as Oand S in the weld pool change the surface tension temperature coefficient from nega-tive to positive, and thereby the convection direction of the ?uid ?ow varies; Next, theliquid metal ?ow in A-TIG weld pool is more likely in laminar way, which is quitedifferent from conventional TIG welding.Monte Carlo (MC) technique is further employed to establish the grain growthmodel in HAZ. Based on Grain Boundary Migration (GBM) model and Experimen-tal Data Based (EDB) model, the relation between simulation iteration steps and thereal time-temperature history is obtained respectively and the grain growth process inHAZ of SUS316 is simulated. The results are much different from the actual graingrowth of HAZ in SUS316, so it can be deduced the following conclusions. Firstly,as GBM model is mainly designed to simulate the grain evolution of pure metal, thedifference between simulated result with GBM model and experimental result indi-cates that the grain growth in HAZ of SUS316 is quite different from the pure metal;Secondly, from the deviation between the results of EDB model and the experimentalones, it can be speculated that the grains boundary penetrate, such as M23C6, impedethe grains growth process, and because of the effect of welding thermal cycle, onlywhen the temperature is high enough to dissolve the precipitates does the grain beginto grow. Considered the in?uence of precipitates, the EDB model is modified, andthe experimental result coincides well with the simulated ones after its modification,which verifies the validity of the precipitates effect. The simulation of grain growthprocesses in TIG and A-TIG HAZ of Nimonic263 shows that HAZ grain growth ofNimonic263 is also in?uenced by the grain boundary penetration. Although the active?ux can change the shape of the weld pool significantly, it has little effect on the graingrowth in HAZ. By using MC technique, crystallization and solidification process in three-dimensional weld pool is also simulated and the microstructure evolution model dur-ing solidification process in weld pool is established. Based on the assumption thatthere is no heterogeneous nucleation and the solute is evenly distributed in the wholeweld pool, the liquid nucleation and growth in the weld pool are simulated. Accordingto grain growth kinetics, the temperature gradient, which in?uences the grain growth,is considered in simulating the dynamic process of the columnar grains formation inSUS316 TIG and Nimonic263 TIG/A-TIG weld pools. The simulation shows that inTIG/A-TIG welding of the plate, the circle or elliptic grains in the cross section of theweld bead are not always equiaxed grains, but the sections of columnar grains in therear of the weld pool. The simulated and experimental results show that the larger thewelding heat input the easier the large columnar grain forms.Finally, a grain growth system of singe-phase welded joints is established andthree-dimensional simulation of grain growth in welded joints is realized based onthe above models, they are the heat transfer and ?uid ?ow model, HAZ grain growthmodel and weld pool solidification model. The system can be used to simulate thegrain growth process in welded joints where no solid-state transformation occurs inthe alloys. It can dynamically replay the process of grain growth in welded jointsand provide an available way to understand the grain evolution of the welded joints inthree-dimensional scale.