【作者】 杨夏炜；

【导师】 朱景川；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2013， 博士

【摘要】 Al-Si系是铸造铝合金中用途最广的合金系，其中ZL114A合金以其优良的铸造性能、良好的耐腐蚀性及较高的比强度，广泛应用在航空航天及汽车产业等领域。Al-Cu系中的ZL205A合金是另外一种被广泛应用在航空航天等领域的铸造铝合金。航空航天等尖端技术的快速发展使铸造铝合金构件的大型化、整体化及复杂化成为必然趋势，热处理作为铸造铝合金大型复杂构件的最后一道热加工工序就显得尤为重要。预测和控制构件的淬火残余应力及淬火变形一直是航空航天领域迫切需要解决的问题。影响铝合金大型复杂构件淬火残余应力及变形的因素主要有构件的结构特点和尺寸大小，构件材料的热物性能和力学性能，淬火介质的冷却性能，构件进入介质的方式，边界条件及人为因素等。但目前针对大型复杂构件的淬火变形问题尚未有系统和全面的研究。本文首先通过差示扫描量热分析和金相分析的方法，确定了ZL114A和ZL205A合金的固溶温度，分别为535-550℃和540-550℃。通过热模拟实验得到了两种合金的应力-应变曲线。应变速率不变时，应力随着变形温度的升高而降低。当温度区间为300-500℃时，温度不变时，应力随着应变速率的升高而增加；但当温度小于300℃时，应变速率对应力的影响不明显。建立了温度为350-500℃时这两种合金的本构方程。本构方程计算的应力和实验值非常吻合。将变量SYIELD, HARD (1), HARD (2)和HARD (3)的表达式写入ABAQUS子程序UHARD中，本构模型在ABAQUS平台上得以实现。基于淬火介质冷却特性曲线，采用反传热法求得了介质的动态换热系数。通过对比实验结果和模拟计算值，验证了介质动态换热系数的精确性。介质换热系数是温度的函数，在整个温度区间内会出现峰值。温度对水和5%UCON-A溶液的换热系数影响很大，但对20号机械油换热系数的影响不明显。基于FLUENT软件分别建立了两类淬火槽的介质流场模型。随着入口速度的增大，未置搅拌系统的淬火槽其介质流速呈现增大的趋势，但曲线峰值没有规律性。对于置搅拌系统的淬火槽来说，导流筒出口圆角过渡可明显减小介质流速损耗。介质流速随着搅拌器螺旋桨转速的增大而增大，但介质流动均匀性会随之降低。螺旋桨的直径越大，介质流速越大。均流板可用来调节介质流速及其流动均匀性。在介质流场模拟精度得到验证的基础上，将FLUENT模拟计算与人工神经网络方法结合起来，建立了以螺旋桨的转速、均流板的位置及槽内x的坐标位置为输入，以介质y向速度为输出的网络。网络预测值与FLUENT模拟值的平均相对误差为4.24%。可知网络具有高精度的预测性能，由此成功实现了流场预测模型化向数字化的转变。基于ABAQUS有限元软件，根据铝合金轴对称件的结构特点，对其进行边界条件、初始条件的设置及网格的划分，建立了轴对称件热处理过程的热力耦合模型。模拟计算了轴对称件加热、淬火转移及淬火冷却三个过程中的温度场，分析了淬火残余应力的大小。将轴对称件热处理后的应力场作为拉伸变形初始态，建立了拉伸去应力的有限元模型。通过建立与文献中三个铝合金长方体块相同的淬火及淬火后拉伸变形的模型，并与文献中的实验和模拟结果进行对比，验证了铝合金热处理过程热力耦合的模拟精度及模拟结果的可靠性，也验证了金属小变形有限元模拟的可行性。对铝合金大型复杂构件热处理过程进行多场耦合模拟计算，研究了边界条件、固溶温度、介质及其温度、构件的壁厚、构件淬入介质所需时间及介质状态等因素对构件淬火残余应力和变形的影响规律。边界条件是大型构件淬火变形的敏感因素。固溶温度是构件淬火变形的不敏感因素。构件淬火残余应力随着水或5%UCON-A溶液温度的升高而降低。壁厚对构件最大残余拉应力的影响比最大残余压应力的明显。构件淬入介质所需的时间对其残余应力和变形均有影响。构件x和y向淬火残余拉应力的最大值随着螺旋桨转速的增大而增大。螺旋桨转速为0-600rpm时，转速对构件z向残余拉应力最大值的影响不明显，当转速为900rpm时，构件z向残余拉应力的最大值较其它几个转速下的小。最后验证了实际大型复杂铝合金构件淬火变形的模拟结果。水温为25℃和80℃时，构件1的两个特征部位的淬火变形实测值与模拟结果的平均相对误差为13.4%。对于构件2，水温为25和45℃时，特征部位变形实测值与模拟结果的平均相对误差为7.6%。对于构件3，水温为60℃时特征部位变形实测值与模拟结果的相对误差为8.8%。对于构件2和3来说，构件变形的模拟值和实验值的绝对误差在0.5mm左右。综上，本文建立的大型复杂构件热处理变形数值模型具有很高的预测精度。更多还原

【Abstract】 Al-Si series are the most widely used casting aluminum alloy. ZL114A alloybelongs to Al-Si series and it has been widely applied in aerospace, automotiveindustries and other aspects because of its excellent castability, good corrosionresistance and high strength-to-weight ratio. ZL205A belongs to Al-Cu series and itis another alloy that has been widely applied in aerospace industries. The castingaluminum alloy workpieces are large, integration and complex due to the rapiddevelopment of aerospace industries. Heat treatment is very important because it isthe last operations for hot processing of metals. Prediction and control thequenching residual stress and distortion of large complicated workpieces areurgent problems that need to be solved in aerospace industries. The factors thatinfluence the quenching residual stress and quench distortion include the structurefeatures and size of workpieces, the thermo-physical properties and mechanicalproperties of materials, the cooling performance of the quenching medium, thepatterns of workpieces immerge into quench tank, boundary conditions andhuman influence, etc. But nowadays a comprehensive systematic research on thequenching distortion of large workpieces is seldom reported.The solid solution temperature of ZL114A and ZL205A alloy was determinedusing differential scanning calorimetry analysis and metallographic analysis. Thesolid solution temperature of ZL114A and ZL205A alloy are535-550℃and540-550℃, respectively. Stress-strain curves of these two alloys were obtainedthrough thermal simulation deformation experiments. At constant strain rate, theflow stress decreases with increasing the deformation temperature. For thedeformation temperature range of300-500℃, at constant temperature, the flowstress increases with increasing the strain rate. While at low temperatures(＜300℃),at constant temperature, the influence of strain rate on flow stress is not obvious.According to the stress-strain curves, the constitutive equations of these two alloyswere established at temperature range of350-500℃. Explicit expression for yieldstress was given and four variables (SYIELD, HARD (1), HARD (2) and HARD (3))were programmed in the subroutine UHARD with ABAQUS. The constitutivemodel was complied on ABAQUS platform.Based on the quenching medium cooling characteristics curve, using inverseheat conduction method, the heat transfer coefficients of the medium were obtained. Through compare the simulation data and experimental results, the accuracy of theheat transfer coefficients was verified.The heat transfer coefficients of the mediumare the function of temperature, and the peak coefficients appear in the entiretemperature range. Temperature is the sensitive factor for heat transfer coefficientsof water and5%UCON-A, but it is not sensitive for No.20machine oil.Based on the FLUENT software, the simulation models of two types of quenchtank were established. As for the tank without mixing system, the velocity ofmedium in middle zone of tank increases with increasing the inlet velocity. But thecurve peak does not appear regularity. As for the tank with mixing system, roundedtransition in the outlet of draft tube can significantly reduce the loss of medium flowvelocity. The medium flow velocity increases with increasing the rotation speed ofstirrer propeller, but the uniformity of flow field decreases with increasing therotation speed. The flow velocity increases with increasing the propeller diameter.The flow velocity and flow field uniformity can be adjusted through moving theposition of guide plate. On the basis of verification for accuracy and reliability ofquenching tank modeling and simulation accuracy, the methods of FLUENTsimulation combine with artificial neural network (ANN) was proposed. Thepropeller speed, the position of guide plate and the position of x-coordinate inquench tank were used as the input variables, while y-velocity of medium was takenas the output. The results show that the mean relative error between the predictedy-velocity of the ANN model and the FLUENT simulation data is4.24%, whichdemonstrates that the ANN model is able to predict the y-velocity of medium withhigh precision.Based on ABAQUS software and according to the structural features ofaluminum alloy typical axsymmetric part, the boundary and initial conditions wereset, and the finite mesh elements were divided. The coupled thermo-mechanicalsimulation of axsymmetric part during heat treatment process was established. Thetemperature field of axsymmetric part during heating, quench transfer andquenching stage were simulated. The residual stresses of axsymmetric part afterquenching were analyzed. Through using the quenched stage of axsymmetric part asthe initial stage of tensile deformation, the finite element (FE) model of tensiledeformation was established. The coupled thermo-mechanical models of threealuminum blocks from publications were established to simulate the quenchingprocess of blocks, then the FE model of tensile deformation of the quenched blockswere established. By comparing the simulation results from this paper and thepublications data. The accuracy and reliability of coupled thermo-mechanical for simulation of heat treatment of aluminum alloy was verified. The feasibility of FEmodel for simulation of metal small deformation was verified.The heat treatment process of aluminum large workpieces was calculated usingmulti-field coupling simulation method. Influences of some factors on quenchedresidual stress and distortion were investigated. These factors include boundaryconditions, solid solution temperature, wall thickness, quench medium and theirstemperatures, time of workpiece immerse into quench tank and stage of quenchmedium. Boundary conditions are sensitive factor for quenched distortion of largeworkpieces. Solid solution temperature is not a sensitive factor for quencheddistortion. The quenched residual stresses of workpieces decrease with increasingthe quench medium (water or5%UCON-A) temperature. The influences of wallthickness on maximum residual tensile stress are more obvious than the maximumresidual compression stress. Time of workpiece immerse into quench tank is asensitive factor for quenched residual stress and distortion of large workpieces. Themaximum residual tensile stress of x-and y-component increase with increasing thestirrer rotation speeds. When the rotation speeds is0-600rpm, the rotation speed isnot a sensitive factor for the maximum residual tensile stress of z-component, whilethe rotation speed is900rpm, the maximum residual tensile stress of z-component islower than the other rotation speed conditions.Finally, the simulation quenched distortion of actual large complicatedaluminum workpieces were verified by the actual measurement. For the1st actualworkpiece, when water is25and80℃, the average relative error between thesimulated distortion and measured values on two characteristic positions ofworkpiece is13.4%. For the2nd actual workpiece, when water is25and45℃, theaverage relative error between the simulated distortion and measured values ofcharacteristic position is7.6%. For the3rd actual workpiece, when water is60℃,the relative error is8.8%. As for the2nd and3rd actual workpiece, the absoluteerror between the simulated distortion and measured values is approximate0.5mm.In summary, the numerical simulation model of large complicated aluminumworkpieces during heat treatment process is able to predict the quench distortionwith high precision.更多还原