超声—压力耦合的装置设计及其对Al-Cu合金微观组织的影响

发布时间：2018-04-03 03:10

本文选题：挤压压力　切入点：超声振动　出处：《华南理工大学》2016年博士论文

【摘要】：随着我国航空、航天、交通运输以及国防工业的发展,对高性能铝合金产品的需求越来越大,而采用先进的成形工艺是制备高性能铝合金的关键。目前,在铝合金铸造过程中常采用施加物理外场的方法来改善其凝固组织,即通过物理外场所产生的一系列机械、物理、化学效应来影响合金的形核和生长,达到破碎枝晶、细化晶粒等目的,最终有助于提升铸锭的性能和质量。目前在凝固过程中施加压力或者超声振动已在金属材料加工领域得到广泛的应用。为了克服单一物理外场的工艺缺陷以及进一步提升铝合金的性能,本论文提出一种在挤压铸造铝合金过程中同时耦合高能超声的新技术,设计并制造了一个独特的试验平台,能够实现超声振动与挤压压力的耦合,在此基础上对超声振动系统各组元以及挤压模具的关键参数进行设计和优化。随后以Al-5.0Cu合金为研究对象,对超声-压力耦合作用下的凝固组织进行数值模拟,并结合不同参数下的实验结果深入研究了耦合作用对合金凝固过程和微观组织、力学性能的影响规律,最后对超声-压力耦合场的作用机制进行探讨。论文得到的主要结论如下:(1)研制了一套超声-压力耦合铸造试验装置。考虑到超声-压力耦合铸造中超声振动系统所受温度、压力载荷不断变化,首先对超声波换能器、发生器分别进行阻抗匹配设计与谐振匹配设计,保证了换能器的工作效率和发生器的能量传输效率;然后基于理论尺寸设计的基础上,分别进行了普通条件和高温、高压下的不同形状(阶梯形、圆锥形以及指数形)变幅杆的模态分析和谐响应分析,发现阶梯形和指数形变幅杆具有较大的振幅放大比,但阶梯形变幅杆的尺寸突变会造成应力集中以及能量传输的损耗;为了得到更大的输出端谐振振幅,设计了一套具有不同过渡形状的多级复杂变幅杆,并进行了不同工况下的模态和谐响应分析,发现高温、高压下指数形过渡型变幅杆的振幅放大比最大,为144,同时高温、高压下的谐振频率为19.943 kHz,与理想谐振频率20 kHz最接近,最终选用指数形过渡段的复杂变幅杆为本文中的超声振动系统所用;为避免超声能量的损耗以及保证楔形密封效果,对挤压模具进行了关键参数的正交优化设计,当模具壁厚为25 mm,锥孔斜度为30°,孔内直线段长度为8 mm时,侧孔周围的平均变形量最小,可以充分避免变幅杆的振动能量的损失和楔形密封的失效。(2)对超声-压力耦合作用下Al-5.0Cu合金的凝固组织进行了数值模拟。首先基于ProCAST反计算模块建立了铸件-模具界面换热系数的反分析求解模型,对大量数据拟合得到不同工艺下的铸件-模具界面换热系数曲线;在此基础上,在CAFE宏-微观耦合计算模型中同时定义挤压力和超声声强,采用反分析法求解出的界面换热系数曲线对Al-5.0Cu合金凝固组织的进行模拟预测,通过定量分析柱状晶和等轴晶的比例、平均晶粒尺寸等结果,发现反分析界面换热系数曲线下模拟出的微观组织与实验结果的吻合度高于采用恒定界面换热系数下的模拟结果。本文继续通过数值模拟和实验结果对比的方法,优化了计算模型各参数的取值范围,最后通过对比不同超声-压力耦合参数下凝固组织的模拟结果和实验结果,进一步说明了耦合作用可显著降低平均晶粒尺寸,改善柱状晶的比例。(3)分别研究了挤压压力、超声振动以及超声-压力耦合工艺对Al-5.0Cu合金凝固过程和微观组织的影响规律。结果表明:挤压压力为75 MPa时熔体内部的冷却速度最大,挤压力基本消除了孔洞等铸造缺陷,同时还可以减小二次枝晶间距,但通过挤压铸造很难得到理想的细小均匀等轴晶组织;功率为1 kW的超声振动可以在变幅杆端面附近区域内形成一定程度上得到细化的微观组织,但随着超声能量的衰减,微观组织细化程度不均匀,远离变幅杆端面的试样微观组织细化程度较弱;超声-压力耦合作用(75MPa+1 kW)可以进一步减小晶粒尺寸,增加等轴晶组织的比例,并且各取样位置的细化程度较均匀,耦合作用还可以改善θ相形貌,并提高ɑ(Al)基体中的Cu元素含量,起到固溶强化的作用。(4)分析了超声-压力耦合场在Al-Cu合金凝固过程的作用机制。首先分别分析了单一挤压铸造工艺和超声振动工艺对Al-Cu合金凝固过程的影响。接下来基于空化模型,通过开发二次子程序UDF以及添加源项的方式分别在空化模型中导入挤压力和超声声流,分别模拟得到了单一超声和超声-压力耦合作用下Al-5.0Cu合金熔体内部的空化气泡体积分数、空化区域大小和强度以及熔体内部压力、流体速度等,发现耦合场会使得空化效应的强度和区域增大,还可以改变熔体内部压力分布和压力梯度差,从而导致熔体内部形成更强烈的对流,即超声-压力的耦合作用可以增强熔体内部的空化效应和声流效应,这会导致熔体内部出现更大过冷度,降低形核功,增加相变驱动力,促进更多的晶核产生和晶核游离,最终形成细小均匀的凝固组织。最后通过对不同工艺下的熔体内部温度分布以及宏、微观组织的定量分析验证了数值模拟的预测。
[Abstract]:With China's aviation, aerospace, transportation and development of the defense industry, the demand for high performance Aluminum Alloy products and the forming of advanced technology is the key to the preparation of high performance Aluminum Alloy. At present, in the Aluminum Alloy casting method applied physics field is often used to improve the solidification structure. A series of mechanical field generated by physical, physical and chemical effects to influence the nucleation and growth of alloy, crushing the dendrite, grain refinement to eventually help to improve the performance and quality of ingot in solidification process. The pressure or ultrasonic vibration has been widely used in metal materials in order to process processing field. To overcome the defect of single physical field and further enhance the performance Aluminum Alloy, this paper proposes a Aluminum Alloy in squeeze casting process and new technology of high intensity ultrasonic coupling, The design and manufacture of a unique experiment platform, can realize the coupling of ultrasonic vibration and extrusion pressure, based on the key parameters of the ultrasonic vibration system components and extrusion die design and optimization. Then using Al-5.0Cu alloy as the research object, numerical simulation on solidification structure of ultrasonic coupling under pressure and combined with the different parameters of the experimental results of in-depth study of the coupling effect on solidification process and microstructure, influence of mechanical properties, discussed the mechanism of ultrasonic pressure at the coupling field. The main conclusions are as follows: (1) developed a set of ultrasonic pressure coupling casting experimental device is considered. The ultrasonic vibration system of ultrasonic pressure coupling in the casting temperature, pressure load changing, the ultrasonic transducer, the generator impedance matching design and resonance The matching design, to ensure the efficiency of energy transfer efficiency of the transducer and generator; and then the size of foundation design based on the theory, are common and high temperature conditions, different shapes under high pressure (stepped, and conical horn shape index) modal analysis and harmonic response analysis, it is found that the amplitude stepped and deformation index horn with large magnification ratio, but the mutation of steppedultrasonichorn size will cause stress concentration and energy transmission loss; in order to get the output resonant larger amplitude, designed a set of different transition shape of multi-stage complex horn, and the modal and harmonic response analysis under different conditions, the discovery of high amplitude, high pressure index shape transition horn amplification ratio is 144, while the maximum temperature, the resonant frequency under high pressure is 19.943 kHz, the most close to the ideal resonant frequency of 20 kHz, the most The final selection of complex exponential transition horn is used for ultrasonic vibration system in this paper; in order to avoid the loss of ultrasonic energy and ensure the wedge-shaped sealing effect, optimum design of extrusion die for the key parameters, when the mold wall thickness is 25 mm, cone angle for 30 degrees, straight line length in the hole 8 mm, the average amount of deformation around the side hole of the minimum, can fully avoid the loss of failure and wedge vibration energy of the horn seal. (2) on the solidification structure of Al-5.0Cu alloy under the coupling effect of ultrasonic pressure are simulated. Based on the analysis model of anti ProCAST anti - mold casting calculation module is established the interfacial heat transfer coefficient, the large amount of data obtained under different technological conditions of the casting mold interfacial heat transfer coefficient curve; on this basis, the CAFE macro microcosmic coupling calculation and extrusion pressure and ultra sound definition model Strong, is solved by the inverse analysis method of interfacial heat transfer coefficient curves were simulated on solidification of Al-5.0Cu alloy, through quantitative analysis of columnar and equiaxed ratio, average grain size results agree with the experimental results found that the microstructure analysis of interfacial heat transfer coefficient curve under the simulation of the degree is higher than that of the constant interface for simulation results under thermal coefficient. In this paper, through the method of comparison to numerical simulation and experimental results, the optimal range of calculation of the parameters in the model, the simulation results and experimental results by comparing the different ultrasonic pressure coupling parameters of coagulation of tissue, further explains that the coupling effect can significantly reduce the average grain size and improve the columnar crystal ratio. (3) the extrusion pressure of ultrasonic vibration and ultrasonic pressure coupling process on Al-5.0Cu alloy solidification process and microstructure. Sound rules. The results show that the cooling rate of the melt extrusion maximum internal pressure is 75 MPa, pressure casting defects such as voids basically eliminated, but also can reduce the two primary dendrite spacing, but by squeeze casting is difficult to get the desired uniform fine equiaxed grains; the power ultrasonic vibration of 1 kW can be formed the microstructure of a certain extent can be refined in the area near the horn tip, but with the attenuation of ultrasonic energy, microstructure inhomogeneity, away from the horn end of the sample microstructure is weaker; ultrasonic pressure coupling (75MPa+1 kW) can further reduce the increase of grain size, and equiaxed grains. The proportion and degree of refinement of each sampling location is uniform, the coupling effect can also improve the morphology and improve the theta, alpha (Al) Cu content in the matrix, the solid solution strengthening function (4) is analyzed. Ultrasonic pressure coupling field in the mechanism of the solidification of the Al-Cu alloy. Firstly analyses the single effect of extrusion casting technology and technology of ultrasonic vibration on the solidification of the Al-Cu alloy. Then based on cavitation model, through the development of two UDF and add subroutine source terms into the pressure and the flow in the ultrasonic cavitation model respectively. Simulated single ultrasound and ultrasound - pressure coupling internal Al-5.0Cu alloy melt cavitation bubble volume fraction, cavitation region size and strength and melt the internal pressure, fluid velocity, it is found that the coupling field makes the cavitation intensity and the area increases, but also can change the internal melt pressure distribution and pressure gradient, resulting in inside the melt convection formed more strongly, the coupling effect of ultrasonic cavitation effect and acoustic pressure can enhance the melt flow inside this effect. Will lead to more internal melt undercooling, reduce nucleation work, increase the transformation driving force, promote the nucleation and crystal nucleus generated more free, and ultimately the formation of fine and uniform structure. By the end of the different process of internal temperature distribution of melt and macro, quantitative analysis of the microstructure of the verified numerical simulation forecast.

【学位授予单位】：华南理工大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146.21

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