神经元网络反演LY2铝合金焊接过程热导率

结合数值模拟与温度测量,通过神经元网络,反演了LY2铝合金焊接过程热导率随温度变化情况。对热导率分段线性化后,在稳定状态下热导率数据的±20%范围内,将热导率分段线性值同时变化1%时的数据分为一组,利用有限元模型,计算对应每组热导率数据的焊接温度场;以温度为输入,热导率为输出,代入神经元网络进行训练,直至收敛,将实测温度代入训练好的网络,网络输出即为反演的热导率数值。分别对焊接加热和冷却两个过程的热导率进行了反演,结果表明,温度变化方向对铝合金热导率的影响是显著的,即热导率会随材料的加热或者冷却,出现不同程度的“滞后”现象。     

超声波焊接金属界面温度仿真及实验研究

针对超声波焊接金属界面温度难以实时测量的问题,从能量角度建立了超声波焊接二维瞬态传热模型。利用ABAQUS计算了超声波焊接铝箔表面不同点处的温度历程,红外热像仪测量了焊接过程中铝箔表面温度历程曲线,铝箔表面最高温度计算值与实验值对比误差在5%以内,证明该模型具有良好的计算精度。再利用该模型计算了不同参数组合下超声波焊接铝箔界面的温度场,研究表明,超声波焊接铝箔界面最高温度不超过金属熔点的50%;得到了焊接界面最高温度与速度、声极振幅和法向压力的关系,其中,焊接最高温度随焊接振幅的增加而增加,随焊接速度的增加而减小。本研究结果对于揭示超声波焊接成型热过程机理及热机耦合应力过程的研究具有重要意义。     

电子束焊接温度场实时监测系统设计

焊接温度场是影响焊接质量和生产效率的主要因素,因此对焊接过程中的温度场分布进行监测就显得极为重要。利用普通彩色CCD构成了焊接温度场实时测量系统;基于工件热辐射随温度变化单调上升特性建立了温度场测量模型;采用最小二乘法对实测的温度数据进行分析,得到了温度场测量模型的最佳参数。试验结果表明了该监测系统的可行性。     

不锈钢焊接温度场的三维无网格对称粒子法模拟

提出了模拟焊接温度场的无网格对称粒子法,编制了相应的计算程序。建立了不锈钢钨极氩弧焊的三维数值模型,应用无网格对称粒子法进行求解,得到了焊件的温度分布和焊接熔池的形状、尺寸,将模拟结果与有限元法的结果和试验结果进行对比,结果表明无网格对称粒子法与有限元法的精度相近,而前者的效率更高。建立并应用渐进变化的非均匀无网格粒子模型对焊接温度场进行了模拟,计算效率得到了进一步提高。在此基础上,研究了高斯热源模型和双椭球形热源模型对模拟结果的影响,数值结果表明基于后者能给出与试验更相符的熔池形状。     

薄板T型接头TIG焊温度场的研究

运用大型有限元分析软件ABAQUS,建立了适当的模型对薄板T型TIG焊接接头的温度场进行了数值模拟。模拟中考虑了随温度变化的材料性能,考虑了模型的对流和辐射换热以及潜热的影响。采用热电偶测量了典型位置的温度场以验证模拟结果,结果表明,建立的模型能较准确地模拟薄板TIG焊的温度场。     

正交接管焊接残余应力的三维有限元模拟

对正交接管焊接接头的温度场和应力场进行了数值模拟。选用三维实体单元,考虑材料物理性能随温度的变化和相变的影响,采用内部热生成和振幅曲线的加载方法模拟焊接热源的移动,运用单元生死技术模拟多道焊过程。获得了焊接温度场和应力场的动态变化过程,并对计算结果进行了分析。     

摩擦焊接过程的热力耦合有限元分析

采用热力耦合有限元分析方法,综合考虑摩擦焊接过程中不同的产热机制、随温度变化的材料性能、飞边形成及摩擦系数随速度、温度、压力的变化规律,直接由焊件材料的性能参数及设定的焊接规范参数(轴向压力与转速随时间的变化曲线)对焊接全过程的焊接参数(摩擦转矩、轴向缩短量及平均温度)、物理参量场(温度场、应力场、变形场)及焊合区金属晶粒尺寸的变化规律进行了数值模拟,得到了焊接过程中摩擦表面上切应力、加热功率、温度及轴向压应力的分布及变化规律,实现了对焊合区金属晶粒尺寸的数值模拟。     

挖掘机动臂板焊接温度场模拟及影响因素分析

利用有限元方法对挖掘机动臂平板对接焊温度场进行了分析,采用Goldak椭球热源模型,应用DFLUX子程序定义热源模型的位置、大小、热输入以及焊接速度等参数来模拟瞬态温度场的分布及其变化,在此基础上通过改变焊接热输入、热源模型参数和焊接速度来分析其对温度场分布的影响规律.研究结果表明,由于焊接热源具有集中移动的特点,焊接热源模型参数以及焊接速度对焊缝区和热影响区的温度分布影响比较明显,热输入的变化与最高温度的变化大致呈线性关系.运用有限元法,选取Goldak椭球热源模型通过DFLUX子程序可有效进行挖掘机动臂平板焊接的数值模拟.     

搅拌摩擦焊接的热力耦合分析模型

随着数值模拟技术在搅拌摩擦焊接研究中的应用日益广泛,对模型本身的准确程度要求越来越高,因而针对数值分析模型的研究显得更有意义.通过分析搅拌摩擦焊接热力耦合计算方面的相关资料,结合实际开展的搅拌摩擦焊接试验以及试验过程中对部分物理量的测量和分析,建立更加完善的搅拌摩擦焊接数值模拟模型.对生热过程、材料模型、夹具约束以及搅拌头机械载荷作用都进行细致分析和探讨,在新模型中采用被焊材料的剪切极限作为生热驱动力,考虑被焊材料的力学性能随温度和温度历史发生变化,建立夹具和试板之间的接触关系,并在力学分析模型中将搅拌头机械载荷简化考虑.利用新建立的数值分析模型对铝合金薄板搅拌摩擦焊接过程进行模拟,得到和试验结果吻合较好的温度场、残余应力和变形结果.     

单元组装式太阳能储热器的Fluent蓄热性能研究

研制一种单元组装式太阳能储热器,采用熔盐作为蓄热材料,利用Fluent中的凝固/融化模型对相变材料在储热器的蓄热、相变过程进行数值模拟研究,得到了相变材料固-液相变过程不同时刻的温度场分布,通过与实验数据对比验证了模拟方法的可行性;分析了传热介质入口温度以及流速对储热器储能过程的影响,计算了一定工况下,储能过程温度随时间的变化情况,得到之间的关系表达式以及对流传热系数与Re的关系,对太阳能储热器的设计、模块化生产及应用具有重要现实意义。     

固体吸附制冷吸附床数值分析及强化传热技术

建立了固体吸附制冷吸附床的物理模型和数学模型,通过数值分析得到床层温度分布及其各点温度随时间的变化情况,并分析吸附剂的物性参数、换热流体流速、温度和床层初始温度参数变化对床层温度分布的影响.结果表明,吸附剂的物性参数对床层温度分布影响显著;换热流体流速、温度和床层初始温度对床层温度影响不大,但影响换热效率和传热速率.在径向上床层温度存在着较大的温度梯度,但在轴向上温度分布比较均匀.在吸附剂的物性参数中,导热系数影响最大,其次是密度和比热容.因此为了提高传热效率,应增大吸附剂的导热系数,选取具有较小密度和比热容的吸附剂.     

混流式水轮机转轮简化模型焊接过程的数值模拟

在确定混流式水轮机转轮简化模型、解决分段焊焊接热源加载的问题及解决各独立实体间接触边界节点不重合的热传导问题的基础上,对转轮模型焊接过程的温度场与应力场进行了数值模拟。得到了焊接过程中各起弧点的热循环曲线、纵向应力的演变规律与焊后应力场的分布情况。结果表明:焊后应力的峰值出现在叶片与上冠或叶片与下环接触面附近的叶片上。这个结果可以较好地解释水轮机转轮叶片疲劳裂纹的产生和扩展的失效行为。     

中厚板熔化极气体保护焊的数值模拟及实验验证

在建立中厚板GMAW温度场数值仿真模型的基础上,用ANSYS进行了数值模拟,并通过红外热像仪进行了实验验证。实验结果与模拟结果对比分析表明,温度场热循环曲线的形状、峰值温度及加热速度比较接近实测数值,冷却区段偏差稍大,证明所建立的数值仿真模型可以较好地模拟中厚板GMAW的温度场,为应力应变分析等其他焊接过程的数值分析打下了良好的基础。     

A model for prediction of the effective thermal conductivity of granular materials with liquid binder

A model is developed to predict the effective thermal conductivity of a three-component system containing sand particles bonded with liquid binders. The effective thermal conductivities of the bonded and unbonded sands are measured by the line-heat-source method at room temperature. The parameters of a two-component model are determined from the measurements for unbonded sands. Finally the model for the three-component systems such as bonded sand is developed using the assumption of the coalescence of the liquid binder at the particle contact points. Comparison between the experimental results and the model predictions shows that the model developed in this study predicts well the effective thermal conductivities of fluid-saturated sand or sand bonded with liquid binders. Also, this model can be used to determine the effects of the parameters such as the thermal conductivities and the volume fractions of components and the geometrical characteristics of solid particles on the effective thermal conductivity of granular materials with or without the liquid binder.     

Validation of thermal and electrical model for induction motors using fiber Bragg gratings

This works presents the simulation and validation of the thermal, electrical and mechanical models of a three-phase induction motor (TIM). Fiber Bragg grating (FBG) sensors are used to measure stator temperature and validate the thermal model. The knowledge of the relationship between losses and temperature variation in the TIM makes a simulation of the motor possible. To determine losses in the TIM an equivalent electrical circuit in arbitrary reference frame is used, which combines a traditional model with the more usual modeling of losses in the stator iron. The thermal study of the motor is performed using an equivalent thermal circuit formed by thermal capacitances and thermal conductivities that are separately considered for the stator and rotor. The losses calculated with the electrical and mechanical models are the input parameters for the thermal model. The simulation of the electrical model produces an error of approximately 4.2% when determining the Joule effect losses in the motor when compared to the experimentally obtained results. The simulation of the mechanical model presents an error of 0.2% for the losses due to friction and ventilation. The stator and rotor temperature, obtained with the thermal model, presented a high correlation with the measured values. The thermal model presents a maximum error of 0.75 °C when one compares them to the average experimental values of temperature in the stator during the temperature transient behavior. When the temperature in the stator reaches steady state, the experimental and simulated results converge to the same values. The use of FBGs to measure temperature in the machine allowed a thermal model to be developed, which also uses the mechanical losses of the machine and is the main contribution of this work.     

Some studies on temperature profiles in AISI 304 stainless steel sheet during laser beam welding using FE simulation

The investigation of transient temperature profiles of a weld joint produced by the laser welding process is presented. A three-dimensional finite element model is developed using a commercial finite element code ANSYS in order to obtain the behavior of temperature field and molten pool shape during the welding process. A three-dimensional conical Gaussian heat source is employed as a heat source model for performing a non-linear transient thermal analysis. The temperature-dependent material properties of AISI 304 stainless steel sheet are taken into account, which has a great influence on the temperature fields indicated by the simulation results. The effect of latent heat and the convective and radiative boundary conditions are also included in the model. A series of laser welds are performed using a 2-kW continuous wave Nd:YAG laser welding system. The experimental trials are conducted by varying the laser input parameters namely beam power, welding speed, and beam incident angle to validate the model. The results show that there is a good agreement between the finite element simulation and the experimental observations.     

紫铜板搅拌摩擦焊接温度场有限元分析

FSW传热过程直接决定工件所经历的热循环,进而影响焊接接头的微观组织和力学性能。同时温度场的分析对于预测接头残余应力和变形,以及焊缝区硬度都具有重要意义。本文在工艺研究基础上,分析了FSW的产热过程;根据搅拌头形状与尺寸,建立了FSW三维传热有限元模型。使用Ansys有限元分析软件,结合有限几个测量点温度变化的实验数据,对6 mm厚度紫铜板FSW焊接过程的温度场进行了有限元分析和计算,获得了该焊接过程的温度场分布与变化规律。计算过程中考虑了工件下表面与支撑板接触热传导对温度场的影响,以及温度对紫铜材料热传导系数的影响,有限元计算结果与实验测量结果接近。     

A method for determining the thermal EMF and thermal conductivity

A method for measuring the thermal emf and thermal conductivity of conducting samples is described. After the temperature and potential differences are measured between the sample’s ends, on which metal contacts are deposited, at a specified direct current, the current is turned off, and instantaneous values of the integral thermal emf are measured directly at the metal contacts. Using the results from measuring the temperature and potential differences during passage of the direct current, the electrical and thermal conductivities of the sample are calculated. A specific feature of this technique is the use of additional thick metal layers being in contact with the surfaces of the metal contacts at the sample’s ends, which are aimed at stabilizing the temperature gradient along the sample when the current source is switched off.