【作者】 李凤仙；

【导师】 刘允中；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2012， 博士

【摘要】 喷射轧制是一种金属半固态近净成形加工新技术，与传统的带材制造工艺相比，具有节能节材、短流程、制备高性能的板带材等优势。对该工艺及其原理进行深入研究有助于在一步工序内从液态金属直接制备具有均匀细晶组织和优异综合性能的近终形板带材。本文将喷射轧制过程分为雾化阶段、沉积阶段、轧制阶段三个部分，主要针对喷射轧制过程中的密度、沉积物形貌、温度的变化进行了数值模拟，并对相关实验研究进行了讨论和分析，为获得最大的制备效率，稳定连续制备带材，并获得较好的组织性能提供了理论依据。建立了喷射轧制过程中熔滴在雾化阶段的热力学和动力学行为的模型。采用非线性方程描述了熔滴与气体之间的相对速度与工艺参数之间的关系。分析关键工艺参数对熔滴和气体的动力学行为和热力学行的影响。模拟结果表明喷射轧制过程中不同工艺参数（如喷射距离、气体初始速度、过热度、熔滴质量流率）对熔滴的尺寸、熔滴的速度、传热系数、温度、固相分数有较大的影响。基于熔滴在雾化时的动力学和热力学行为，对喷射轧制过程中沉积阶段的沉积材料的相对密度进行了建模和理论分析，研究了主要工艺参数（如熔滴喷射距离、气体初始速度、过热度和熔体质量流率）对喷射轧制7050铝合金沉积时的平均固相分数和相对密度的影响。采用坐标追踪的方法，建立了喷射轧制7050铝合金过程中熔滴沉积到轧辊表面的形貌模型。该模型考虑了熔滴的反弹、二次沉积现象以及轧辊表面的几何特征，可预测不同工艺参数下的沉积层的厚度，并能对咬入角进行定量的分析。分析了工艺参数对沉积层厚度和咬入角的影响。建立了喷射轧制过程中沉积阶段的温度场模型，与沉积物形貌进行耦合，对沉积阶段的温度分布进行了研究。模拟结果与实验结果较吻合。分析了工艺参数对沉积层温度分布的影响。结果表明沉积阶段的沉积物内部形成孔隙主要是由于液相不足引起的。当熔滴沉积时的平均固相分数为60～70%时，沉积材料具有较高的相对密度。轧辊的直径、轧辊转速、参考位置处最大质量通量、轧辊间距、喷射距离对沉积层厚度和制备效率有较大的影响。在此基础上采用正交优化法对喷射轧制的主要工艺参数进行了优化。沉积物沿轧辊径向的降温速率较切向的降温速率大。工艺参数主要通过改变沉积物厚度和能否为体系提供能量来影响温度的分布。为体系提供能量（如提高接触时的初始温度，对轧辊表面预热等），增加沉积层的厚度（如增大雾化锥中心的最大质量通量、减小轧辊转速等），可以使沉积材料在沉积阶段具有较低的降温速率，获得均匀的冷却和凝固条件。对喷射轧制过程中带材的相对密度、平均固相分数和压下量之间的关系进行了研究。在轧制阶段考虑了温度对材料性能的影响和变形对温度产生影响，将温度场与应力场进行耦合，并采用更新的拉格朗日参考描述和网格重划分方法对其进行求解。为了对其显微组织进行分析，在相同的材料体系、熔滴尺寸、固相分数和轧制条件下，采用半固态粉末轧制和沉积坯的半固态轧制模拟喷射轧制条件制备出带材。评估了半固态轧制工艺的可行性。分析了加热温度和预热时间对生带材组织演变和力学性能的影响，并分析了生带材在烧结过程或者后处理过程中的显微组织的变化情况。实验结果与模拟结果吻合。结果表明平均固相分数为60～70%时，较小的压下量就能制备出较高密度的带材。喷射轧制过程中的致密化机制主要是以消除孔隙或者消除原始颗粒的边界为主。工艺参数如雾化锥中心的最大质量通量，轧辊预热温度和熔滴沉积初始温度对轧制变形区域的温度场有较大的影响。提高熔滴沉积时的初始温度，增加沉积层的厚度、对轧辊表面进行预热，可以使沉积材料在轧制变形区获得均匀的冷却条件。将气体雾化制备的粉末和喷射成形制备的材料加热到半固态温度区间，二次相和液相的数量发生了较大的变化。加热温度对颗粒边界的变化、孔隙的消失、溶质元素的扩散以及晶粒的长大有重要的影响。加热温度越高，制备的材料内MgZn2相的数量越少。随半固态粉末加热温度的升高，显微组织的演变机制由孔隙率的消失转变为以粗化为主。采用“预烧结冷压变形再烧结”工艺制备的带材相对密度达92.6%，显微硬度可达到310HV。喷射沉积态7050铝合金晶粒长大的激活能值为70.5KJ/mol，其粗化速率较低，最优的半固态加热温度为590。喷射沉积坯在半固态轧制时，形变的主要作用是使材料致密化、破碎晶粒、促进动态再结晶的发生。沉积坯在580下轧制的带材延伸率可达13.31%。沉积坯在550轧制后经固溶处理15h后，再经时效处理后，获得较高的屈服强度533.1MPa和较高的抗拉强度555.6MPa。更多还原

【Abstract】 As a novel metallic semi-solid near-net-shape forming technology, spray rolling has manycompelling advantages over the traditional manufacturing processes, such as the energy andmaterial saving, the short production run and high-performance process for the metal industry.A comprehensive study on the technology of spray rolling and its process principles will helpto manufacture plates and strips with a fine grain size and excellent mechanical propertiesfrom liquid metal within one step.The process of spray rolling aluminum strips were dividedinto three different regions: atomization stage, deposition stage and semi-solid rolling stage.In order to provide a theoretical basis for achieving continue production with high productionrate and producing a high quality materials, the relative density model, shape evolution modeland heat transfer model during spray rolling have been established in this paper. Thecorresponding experimental discussions and analyses are conducted.The thermal history and dynamic behavior of droplets during atomization stage in sprayrolling have been established. A nonlinear quadratic is used to describe the relationshipbetween the gas/droplet slip velocity and the processing parameters. The qualitativeinfluences of the key processing parameters on the history of dynamic behavior and enthalpyof the gas and atomized droplets in spray rolling of7050aluminum alloy were investigated.Results show that the processing parameters such as spray distance, gas initial velocity,droplet diameter, metal mass flow rate have remarkable influence on process performanceincluding the heat transfer coefficient, droplet velocity, temperature and solid fraction.During deposition stage in spray rolling, the relative density of materials at the deposit/rollsurface during spray rolling was estimated and analyzed theoretically in detail based on themodel of dynamic behavior of droplets and solidification. This simulation study reveals therelationship among the processing parameters of spray rolling, average solid fraction, relativedensity of the deposit. The deposit shape evolution during spray rolling of7050alluminumalloy was investigated using mathematical models, tracing the coordinates of a growingdeposit with respect to time. Some important realistic scenarios, such as droplet reboundingand redeposition, the geometrical features of deposition surface were considered. Then thecalculated results of the shape evolution were utilized to predict the deposit thickness and thecorresponding entry angle. The effects of spray-rolling parameters on the deposit thicknessand drag in angle were studied and discussed. The heat transfer model was coupled with theshape model during deposition stage. By means of modeling, evolution of temperature filedunder stead state for the deposition stage during spray rolling have been analyzed and discussed. The simulated results are in good agreement with the experimental results. Theeffects of spray-rolling parameters on the evolution of temperature filed were studied anddiscussed. The porosity mechanism during spray rolling process corresponds to the coldporosity dominant regime. The solid fraction at the range of60～70%corresponds with ahigh density level of strips. Results show that the deposit shape and production rate aresensitive to processing parameters such as the roll gap, the roller diameter and rotationalspeed, the spray distance and the mass flux distribution in the spray cone. The optimumspray-rolling processing parameters were determined and presented by using an orthogonaldesign method. Results also show that the temperature of deposited materials decreases morequickly at the radial direction of the roll than that at the tangential direction of the roll. Thedeposit thickness and the enthalpy input from the spray of droplets are found to be thedominant influencing factors on the thermal profiles of the deposit. A suitable depositthickness, initial droplet temperature and preheating the roller surface will produce ahomogeneous deposit with uniform cooling and solidification condition.During semi-solid rolling stage in spray rolling, the relationship among the average solidfraction, relative density of the strip and thickness reduction ratio was investigated. A heattransfer model for the rolling stage during spray rolling has been established. This modelconcentrates on the effects of temperature on material properties and the effect of materialsdeformation on temperature. In addition, generic algorithms of coupling of temperature fieldand strain field based on updated Lagrangian model and mesh adaptivity methods weredeveloped. Strips have been prepared form gas atomized powders and spray depositedaluminum alloy by semi-solid rolling also taking account of materials system, droplet massmedia diameter, solid fraction and rolling condition. The feasibility of semi-solid rolling wasevaluated. The effect of heating temperature and holding time on the microstructuralevolution and mechanical properties of the rolled strips was investigated, as well as themicrostructural evolution of strips after sintering or post treating. The simulations agree withexperimental results well. The average solid fraction at the range of60～70%correspondswith a low thickness reduction rate and a high density level of strips. The densificationmechanism during spray rolling process corresponds to the removal of porosity dominantregime or the removal of prior droplet boundaries dominant regime. The main processingparameters, such as metal mass max flux, substrate pre-heating temperature and initial droplettemperature have a great influence on the thermal profiles during spray rolling. A suitabledeposit thickness, droplet temperature and preheating of the rollers before spray rolling will produce a homogeneous strip with uniform cooling and solidification condition. On reheatingthe gas atomized powders and spray deposited aluminum alloy into semisolid state, the phasechemistry and quantity of liquid were typically changed. As the heating temperature wasincreased, the disappearance of primary powder boundary and isolated pores, inter-diffusionof species and grain coarsening were accelerated during semi-solid powder rolling. Moreover,the amount of MgZn2phase decreases with increasing of heating temperature. During thesemi-solid powder rolling process, the mechanism of microstructure evolution changes fromthe densification dominant regime to the coarsening dominant regime with the increasingheating temperature. By means of―pre-sintering—cold deformation—terminal sintering‖, therelative density of the strip is92.6%and the microhardness is310HV. The activation energyof grain growth for the spray deposited7050aluminum alloy is about70.5KJ/mol. Spraydeposited7075aluminum alloy is resistant to coarsen due to the presence of particles. Theoptimum semi-solid heating temperature for spray deposited7050aluminum alloy is590.The rolling deformation has great influences on the elimination of pores, broken of grain anddynamic recrystallization. The elongation of the alloy semisolid rolled at580was thehighest up to13.31%. The tensile property of spray deposited alloy semi-solid rolled at550after solution treated for15h and annealed was the best with yield strength of555.6MPa, ultimate tensile strength of533.1MPa.更多还原