【作者】 徐艳利；

【导师】 魏艳红；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 博士

【摘要】 焊接接头由于受到不均匀加热冷却过程的影响,其组织转变过程复杂,主要包括焊接热影响区内晶粒粗化、相变以及焊缝金属的熔化凝固。这些转变导致焊接接头性能发生变化。在一些单相合金如奥氏体不锈钢SUS316和高温镍基合金Nimonic263中,焊接热影响区内无相变过程,因此晶粒长大成为焊接热影响区组织演变的主要现象。为了深入研究无相变合金焊接热过程对焊接接头晶粒长大的影响规律,本文建立了焊接热影响区和焊缝三维MonteCarlo(MC)晶粒长大模型。同时,为了获得MC模拟所需要的焊接热循环曲线,建立了三种不同的流场温度场模型,对焊接接头的流场和温度场分布进行模拟。并进一步设计了焊接接头晶粒长大模拟系统,实现了钨极气体保护焊(TIG)热过程及接头组织演变过程的模拟,三维再现了焊接接头晶粒长大过程。该系统可预测TIG焊接头晶粒尺寸,为焊接工艺参数的优化提供参考。首先,综合考虑了表面张力、电磁力、浮力等驱动力的作用,采用流体计算软件PHOENICS建立了TIG焊流场温度场模型。分别采用导热模型、层流模型和紊流模型模拟了Nimonic263 TIG焊热过程,分析了这几种模型的模拟精度及适用范围。结果表明紊流模型的模拟结果与实验结果更为接近。进一步采用紊流模型模拟了SUS316不锈钢焊接热过程,通过研究有效粘度和有效导热的分布,揭示了熔池内液态金属紊流时导热与对流传输热量的变化规律。模拟结果表明,紊流模型中随着流速的增加,金属液体导热能力不断增加而对流散热能力则因粘度的增加而相对下降。根据模拟结果,对部分凝固参数进行分析得出,随着焊接热输入的增加,界面前沿的不稳定性也随之增加,导致熔池凝固界面前沿晶粒生长并不是以平面生长方式进行。在流场温度场模型的基础上,模拟了活性剂焊接(A-TIG)传热和传质过程,根据流场温度场分布分析了Nimonic263平板A-TIG焊接熔深增加机理。模拟结果表明:电弧收缩对熔池形状的改变影响很小,A-TIG焊接熔深增加机理主要体现在两方面:第一,表面活性元素O、S等元素的存在,改变了表面张力温度系数,当表面张力温度系数由负值变为正值时,熔池内液体的流动方向发生改变;第二,与传统TIG焊接不同,在A-TIG焊接中,熔池内液体更接近于层流流动。这两方面因素的综合作用是熔深增加的主要原因。进一步采用Monte Carlo技术,建立了焊接热影响区晶粒长大模型。分别通过晶界扩散模型(GBM)和基于实验数据模型(EDB)建立了SUS316不锈钢Monte Carlo时间步和真实时间温度的关系,模拟了SUS316不锈钢焊接热影响区的晶粒长大过程。这两种模型的模拟结果均与实际SUS316焊接热影响区晶粒长大存在较大差别,由此可以得出如下结论:首先,由于GBM模型主要针对纯金属晶粒长大过程的模拟,模拟与实验结果的差别说明SUS316焊接热影响区晶粒长大与纯金属的晶粒长大存在着较大的偏差;其次,由EDB模型的模拟与实验结果的偏差推测位于晶界处的沉淀相如M23C6等阻碍了晶粒的长大,受焊接热循环的作用,晶粒只有在这些晶界沉淀溶解后才开始长大。考虑到沉淀的影响,对EDB模型进一步修正,修正后的模拟结果与实验结果的吻合验证了这种假设的正确性。在分别对Nimonic263 TIG和A-TIG焊接热影响区的模拟结果显示,Nimonic263热影响区晶粒长大,同样也受到晶界沉淀相的影响,而涂敷剂的存在虽然可明显改变熔池形状,但对焊接热影响区晶粒尺寸变化没有明显影响。应用Monte Carlo法,对三维熔池凝固结晶过程的模拟进行了初步探索,建立了焊接熔池凝固组织转变模型。在假设熔池内无自发形核、溶质均匀分布的基础上,模拟熔池内液体形核生长过程。根据晶粒生长动力学理论,考虑温度梯度对晶粒生长的影响,动态地再现了SUS316 TIG焊接和Nimonic263TIG/A-TIG焊接熔池晶粒长大过程,并发现在中厚板TIG/A-TIG焊接时,通常在横断面看到的圆形或椭圆形的晶粒剖面并不一定是等轴晶,而是熔池尾部的柱状晶的一个横断面。模拟结果和试验结果均显示,焊接热输入越大时,焊缝越容易形成粗大的柱状晶。根据上述流场温度场模型、焊接热影响区晶粒长大模型和焊缝凝固结晶模型,建立了单相合金焊接接头晶粒长大模拟系统。该模型可适用于无固态相变合金的焊接接头晶粒长大过程模拟,使动态再现焊接接头晶粒长大过程、从三维尺度深入了解焊接接头的晶粒长大形态成为可能。更多还原

【Abstract】 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.更多还原