A356合金熔体调控对流动性的影响机理研究

【作者】 起华荣；

【导师】 史庆南；

【作者基本信息】 昆明理工大学 ， 材料学， 2009， 博士

【摘要】 流动性是衡量合金铸造性能的重要指标,对于精密铸造和近终成形技术尤为重要,但是由于流动性测量受到凝固特性、非稳态流动、非稳态传热等因素影响,使流动问题变得十分复杂。为了弄清合金元素变化、细化变质等熔体调控手段对流动性的影响,本文从流动性测试方法研究入手,通过实验对比分析,设计制造了新的流动性测试模进行流动性测量,并以A356合金为研究对象,全面探讨了粘度、氧化物、细化变质、熔体热处理等因素对流动性的影响,采用数值模拟的方法进一步研究了理想状态下合金元素变化对流动性的影响,获得了铝合金流动性变化的一些基本规律,初步建立了粘度计算和流动性计算的数学公式,对于铝合金生产和研究具有一定的参考价值。本文首先通过分析各种流动性测试方法的利弊,设计了新的金属型流动性测试模对流动性进行测试,解决了流动性研究的方法问题。新设计采用感应加热保温方式保持熔体温度和成份均匀；设计新的浇杯结构,解决了金属型测试模分型难的问题；配合特殊的涂层涂覆与干燥方法,保持流道参数稳定；在浇口处设置过滤,除去夹杂,平稳浇速。实验研究证明,该测试装置制作简单,重现性好,具有一定的新颖性,适用于测量各种有色合金流动性,以该测试模测试为基础,对影响流动的各主要因素逐一进行研究,取得了良好的效果。通过对合金熔体粘度的理论分析,认为合金液相线附近的粘度与流动性存在很大的关联。合金凝固过程中,原子从无序运动状态转变为绕固定晶格点振动状态,原子振动频率和振幅降低,原子间距缩短,原子之间的引力增大,宏观表现为熔体粘度的增大。熔体粘度增大的速度和程度与合金元素在熔体中的扩散势垒高低有关。论文研究提出了合金粘度计算的数学公式：该公式表达了一种以基体金属粘度为基础进行二次修正获得合金粘度的新思想。合金熔体粘度与合金元素之间的振动频率差,半径差、质量差有一定关系,并且与相变势垒、扩散激活能、熔体过热温度、合金溶质质量分数有关,并遵守基本的二次函数规律,经验证计算,该公式计算值与Al-Si合金熔体粘度随成份变化实验值基本一致。对熔体进行过热处理或者是添加熔体调控元素,将会改变熔体的结构以及氧化物夹杂的数量和尺寸,从而对流动性产生影响。熔体中氧化物的增多会使熔体表面氧化膜厚度增加,降低熔体与模壁的润湿,能够微弱提高流动性,但是强烈降低铸件质量；对合金熔体进行适当的热处理可以细化熔体内的准晶团簇,加强熔体过冷能力,能提高流动性。本文讨论的新的熔体温度处理+电磁搅拌工艺,在实际A356合金生产过程中既能保证合金质量,又能提高了合金流动性,获得了良好的效果。熔体细化与变质通过改变熔体过冷度,改变固相分散度、固相形状系数,改变粘度来影响流动性。细化与变质处理改变了熔体的过冷能力,使准晶团簇变多变小,延长长大时间,使枝晶搭接时间后延,提高流动性。但是实验发现继续添加细化剂,晶粒继续细化但流动性反而下降。分析认为,随着细化加剧,异质颗粒使熔体形核温度上升,粘度增大,同时细化剂会造成固相分散度增大,使流动变得困难,从而使流动性降低。因此,细化变质对合金流动性的影响是延迟枝晶搭接的正面效应与增加固相分散度使粘滞力增加的负面效应相互作用的结果。数值模拟研究发现,微量Si、Mg、Ti、Cu、Fe元素能够使A356流动性发生显著改变,微量Pb、Mn、Sn、Zn对A356流动性影响不明显；微量Si、Mg、Ti、Cu、Fe影响流动性主要表现出了指数函数和二次函数两种规律。数值模拟同时还发现流动性指标属于一种统计平均值而并非恒定不变的常数值,这将有助于对流动性评价指标的重新思考和定位。数值模拟还发现了Al-Si合金中Si增加流动性提高的原因不是Si结晶潜热的释放使熔体温度升高,共晶Si释放大量潜热的时候流动已经停止。在合金加热熔解时需要吸收了大量热能用于熔化Si,使得熔体中Al团簇被大量熔解,自由原子增多,这可能是流动性提高的主要原因。根据本文的研究,按照物理学定义流动性与粘度的强烈关联性,本文在粘度计算公式的基础上,综合考虑影响流动性的铸造模数、充型压头、过热度、固相分散度、固相形状系数等参数的相互作用,遵循元素影响流动性的二次函数规律和指数函数规律,构建了新的元素影响流动性的理论计算公式：该公式对于合金流动性的控制与提高具有一定的参考价值。更多还原

【Abstract】 Fluidity is one of important performances of casting alloy, but it is really complex because of the effect of solidify process, unstable flow and unstable heat transfer process etc. In order to make clear what will happen about fluidity with the change of elements, this article begin with a new fluidity measuring instrument design, based on experiment contrast, theoretical analysis and manufacture, and use this instrument to research the effect of viscosity, oxide, refine, modify and melt overheating treat on the fluidity on A356 alloy. Obtained the basic rule of the effect of elements variation on alloy fluidity, and mathematical formula of fluidity theoretical calculation also have been preliminary esTab.lished, it has reference value for manufacture and research.of aluminum alloy.After compared advantages and disadvantages of many traditional fluidity tester, this paper designs a new metal mould to test fluidity, use induction heating to heat preservation and it can also make melt uniform, then put forward new pouring cup structure, use feat coating and drying method, design filter at the entrance of pouring cup, remove inclusion, smooth flowing speed, this fluidity test mode is easy to make and results are repeatability. it can be used to fluidity test of all nonferrous metal, and this acticle is based on the datas obtained by this new fluidity measuring instrument.Melt viscosity is one of the important parameters of fluidity. During alloy solidification process, atoms change from the disordered movement to the fixed vibration around one crystal lattice point, atomic vibrations frequency reduce and atomic spacing curtail, in this process, the atomic force increase gradually, the macro is melt viscosity increased, the speed and scale of the increase of melt viscosity are relation with the measurement of diffusion activation energy. there is a relationship between viscosity and its components:This formula indicated a new thinking of fluidity calculation is twice revises on the base of pure Al viscosity. Melt viscosity is affect by vibration frequency, amplitude, radius, quality, diffusion activation energy, melt temperature, percentage of alloy elements, for Al-Si alloy melt, viscosity value abtained by calculation is basic agreement with the datas batained by experiment.Fluidity will be effected by serperheart treat or addition agent change, with the change of melt structure and oxide amount. With increased of oxide films the fluidity will increase a little bit, but the quality of casting will decrease severe; proper heat treatment will make quasicrystal cluster more thining, increasing surper-cooling ability of melt, then make fluidity invreasing. A new melt treat method which dicusse in this article with compined of heat treatment and electromagic mixing together has a good result in A356 alloy manufacture.Grain refinement and modification changed fluidity by change the nucleation and melt environment, silidoid rate and appearance. Grain refinement can increase fluidity with surpercooling incrasing and prolong dendritic contact point. But experiment find that increase addition of refiner continue, fluidity of melt will decrease. It suggest that with the incrase addition of refiner continue will lead to liquied temperature of melt acsend, which make viscosity increasing and fluidity reducing. Therefore, the influence of alloy refine and modify is a result decided by both positive effects of delay of dendrite contact and negative effects of increase viscous force with solid-phase increased.Numerical simulation found, a little addition of Si、Mg、Ti、Cu、Fe will change fluidity notability, but a little addition of Pb、Mn、Sn、Zn has little effect on fluidity.The change of fluidity with a little Si、Mg、Ti、Cu、Fe addition will abide by quadratic function principle. Bi-channel fluidity test modulus simulation suggests that different channel formed different length in a practical experiments, it also proved fluidity indexes should be an average value. And numerical simulation also found during A356 filling process, head of flow stops in a-Al nucleation process, a-Al nucleation ending when flow has stopped, therefore, at the time of Si separate out to form Al-Si eutectic structure flow had been stopped, this explanation that increasing Si addition in Al-Si alloy will lead to fluidity increase for the reason of latent need further discussion, because the Si phase formed when Al-Si eutectic, flow has stopped at this time. Numerical simulation also found quadratic function discipline and exponential function discipline of the influence of fluidity with ingredient change which are similar with experimental results.According to the research of this articl, there is a consuming relation between viscosity and melt fluidity,because on the viscosity caculating formulation, combine those factors obtained in experiments which effect on fluidity of casting alloy such as casting modulus, solidoid dispersion degree, filling pressure, degree of superheat etc, and fluidity change with quadratic functiondiscipline, exponential function discipline, a new formula to theory calculation of effects of alloy element on fluidity has been put forwarded:Although this formula needs to be completed, it also has reference value on fluidity control and inhance process.更多还原