【作者】 张永；

【导师】 丁宏升； 陈瑞润；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2011， 博士

【摘要】 在电场、磁场、超声波声场和微重力场等外场作用的金属凝固技术范畴内,电场凝固技术以其设备相对简单和操作比较灵活的特点而得到铸造研究和工程技术领域的重视。与传统的凝固过程采用化学方法处理相比较,电场以其特有的外场能量形式同样可以达到细化铸造晶粒和改良组织结构的目的,而且对合金熔体无成分改变、对环境和人体健康也不会造成损害。虽然已有研究表明电场可对多种铸造合金材料进行晶粒细化、组织结构调整和析出相形态控制,但在电场作用下可影响的合金种类在深度和广度上仍存在问题,尤其是在电场作用方式、电场作用液相流动机理和电场作用凝固机理需要开展深入而系统的研究。本文综合运用数值模拟、相似性物理模拟和合金凝固实验的方法,研究了电场影响的合金液相流动及其凝固结晶生长过程。首先采用有限元方法,利用APDL（ANSYS Parametric Design Language）参数化设计语言,以Ti-（46-50at%）Al合金为例分别计算了在交流电场、脉冲电场及稳恒电场作用下,合金液相在型腔宽厚比分别为1:1、2:1、3:1、4:1、5:1及9:1铸型内电磁场及流场的分布规律,并对不同型腔中固/液界面前沿液相流动进行了数值模拟。其次根据相似性物理模拟准则,自行建立了用于研究电场影响的液相流动与结晶生长的多功能动态观测装置,分别选择NH4Cl-89.43at%H₂O、SCN-8.38at%ETH为单相类合金模拟物、NPG-90.45at%SCN为亚共晶类合金模拟物、AMPD-19at%SCN为过包晶类合金模拟物,观察了电场对其结晶生长的影响规律。最后采用型腔宽厚比为5:1的矩形铸型,通过直接通入电流,开展了电场影响下Al-9at%Si和Ti-50at%Al合金的铸造实验,并利用OM、XRD、SEM、EDS等分析测试手段分别对比分析了有无电场影响时两种合金的凝固组织、抗拉强度及维氏显微硬度变化。本文通过上述研究获得以下主要结果:（1）在三种电场作用方式下,与其它铸型型腔宽厚比相比,5:1型腔内合金液相的流动趋于均衡,而且通过与交流电场或者脉冲电场比较,由稳恒电场所驱动的合金液相流动更加平稳和持久。电场对枝晶前沿液相区流场所产生的影响表现为,在胞、枝晶尖端的一侧形成漩涡,形成所谓的流动上游区。（2）稳恒电场作用于单相合金模拟物NH4Cl-89.43at%H₂O时,由于其固、液两相的电导率差别大,Joule热效应将起主要作用,对其游离晶重溶和等轴树枝晶、柱状树枝晶的生长形态产生影响,而采用单相类合金模拟物SCN-8.38at%ETH时,电场的Pinch效应及TEMHD（Thermoelectric magnetohydro-dynamic）效应促进了“平-胞”转变并加快了胞晶生长速度,由于TEMHD造成胞晶尖端前沿过冷度增大,导致胞晶出现分叉现象,此两种效应会造成柱状树枝晶迎流倾斜生长。（3）稳恒电场作用于亚共晶类合金模拟物NPG-90.45at%SCN结晶生长时,电场的Pinch效应会导致初生NPG相出现冲断和重熔,并导致NPG/SCN共晶相中NPG相出现分叉现象。（4）稳恒电场作用于过包晶类合金模拟物AMPD-19at%SCN时,发现电场使得包晶β相由长柱状生长转变为颗粒状生长,这是由于电场改变了初生相及包晶相的化学位,抑制了包晶相变的发生,从而降低了包晶β相的产量,导致其不易向发达的长柱状生长。（5）在电场影响的Al-9at%Si和Ti-50at%Al合金浇注与凝固实验中,建立了同质复合电极电流施加方法,并导出矩形铸型型腔内电流作用合金液相流动均匀性的判断准则,即（D/W）/（1/7/） ,（电极接线柱半径R≥0.01m,两极板间作用区长度L≤10m）,当铸型型腔尺寸和电极尺寸满足该式时,可使电场影响的型腔内的液相流动效果最佳。电场对Al-9at%Si和Ti-50at%Al合金宽厚比5:1板坯试样的凝固晶粒组织均产生一定程度的细化,这验证并支持了现有关于电场细化铸造合金组织的结果。在Al-9at%Si的共晶组织中出现了共晶Si相分叉的现象,而在Ti-50at%Al合金的凝固组织中也出现了包晶相含量相对偏少的现象,即在电场的作用下,其包晶相的生长受到抑制。本文最后从凝固热力学与凝固动力学角度分析和阐明了电场影响合金凝固的作用机制,这为促进电场凝固技术在铸造方面的应用奠定了一定的技术理论基础,同时也对Al-Si类合金和Ti-Al类合金在航空、航天和汽车等领域的进一步应用将产生积极影响。更多还原

【Abstract】 Amongst the physical field assisting solidification techniques, including magnetic field, ultrasonic field and micro-gravity field as well as electric field solidification, the last has drawn much attention from many researchers and technicians due to its merits of easy facility, feasible operation and visible effect. In comprison to the normal solidification process modified by chemical additives, however, electric field can also result in refinement of the cast grains and improvement of the microstructures without changing alloy compisition, making surplus casting defects and damaging health and enviroment. In case of the advancement of casting and solidification assisted by the electric field, numerous results reveal that electric field is with the better abillity to refine grains, alter precipitate phase morphologies and distributions for a variaty of metals and alloys. Nevertheless, there are issues which should be resolved in terms of unstable chosen ways of electric current patterns, misunderstanding mechanisms of fluid flow and freezing and application of new generation materials when the electric field was intersected to solidification process.Aiming at the points mentioned above, a complex method which combined numerical and physical simulation and solidification experimental with and without electric field, was utilized subsequently to resolve the influence of electric current treatment on liquid phase flow and its crystal growth of alloys. In numerical simulation, FEM (Finite Element Method) and APDL(ANSYS Parametric Design Language) were chosen to calculate the intensity and distribution of electromagnetic field and flow field of alloy melt, taking Ti-(46-50at%)Al as an instance, inside mould cavities of width-to-thickness ratio in cross-section ranging from 1:1, 2:1, 3:1, 4:1, 5:1 to 9:1. The three excitation modes of electric field, e.g. alternate electric field, pulse electric field and steady electric field, were applied during the calculation respectively. The flow field in the front of the S/L interface was also simulated thereby. In physical simulation, a multi-functional and time-dependent dynamic observation appratus was set up according to the physical similarity laws. Metalloids or alloy-like binary solutions were selected to observe and examine the fluid flow and crystal growth under electric field. Mostly, NH4Cl-89.43at%H2O and SCN-8.38at%ETH were simulated to single phase and NPG-90.45at%SCN to hypoeutectic as well as AMPD-19at%SCN to hyperperitectic in their counterparts of alloys respectively. In experimental, the pouring and solidifying of binary alloys, Al-9at%Si and Ti-50at%Al has been investigated through exerting a steady electric field in which the alloys were cast into slabs with 5:1 in width-to-thickness ratio in cross-section. Finally, samples cutting from the 5:1 slab were analyzed through OM, XRD, SEM and EDS for observing structures and tensile test and microhardness test for measuring mechanical properties.Based on the above simulation and experiment, the results can be drawn into conclusion as followed. A width-to-thickness ratio of 5:1 mould cavity is appropriate for accommodating fluid because of the intensity and distribution of electromagnetic field are favorable in this case, where it leads to a more steady fluid flow in comparison with other mould cavity ratios under any electric field of the three modes. Moreover, the flow droven by steady electric field is much more stable and enduring than the others. Also, the fluid flow in the front of S/L interface presents turbulent votexes around dendrite tips at their same side which consolidates upstream flow in the fluid. Because NH4Cl-89.43at%H2O has large difference in electric conductivity of liquid phase and solid phase, Joule heating effect will play an important role to remelt the drifting crystals and surpress the growth of the second and third arms of equiaxed and columnar dendrites. For SCN-8.38at%ETH metalloid alloy, the Pinch effect and TEMHD (Thermoelectric Magneto- Hydrodynamic) effect simutaneously accelerate the planar to cellular transformation and speed up the cellular growth rate. Because TEMHD effect itself can arise increasing of the undercooling in the front of cellular tips, it leads to branching of cellular crystals. With their growth and further forcing the columnar grains grow inclinedly against the upstream flow in conjunction with Pinch effect. For NPG-90.45at%SCN metalloid alloy, Pinch effect causes the primary NPG phase to remelt and be broken down, and also makes the presented eutectic NPG phase in NPG/SCN eutectoid to branch. For AMPD-19at%SCN metalloid alloy, the steady electric field changes theβperitectic phase growth patern from columnar to granular due to the chemical potential varied by the electric field in both peritectic phase and primary phase, which means the peritectic phase transformation is being supressed, thereafter, the amount ofβproducts are reduced and growth of column grains are unable to develop in the next precipitate procedure. A method of inputing electric current into the mould cavity using electrodes with similar compisition to those of the alloys was created. In performance of the method, a criteria was also deduced for judging homogeneity of a curent conducting inside the melt, i.e. (D/W)/(1/7/) , where it needs to satisfy R≥0.01m, the radius of the binding post of electrode, and L≤10m the length of electric field acting zone. So far as the situation meeting the formula is satisfied, an optimal flow will be achieved consequently. The grain size of Al-9at%Si alloy and Ti-50at%Al alloy is decreased to some extent, which approves and supports the exsiting results on grain refininment of the cast alloys. In case of the eutectic colonies of Al-9at%Si alloy, the branching of eutectic silicon was also found. In the solidification microstrutures of Ti-50at%Al alloy a reduction of peritectic phase was observed, i.e. the peritectic phase was suppressd by elctric current under a electric field.In the end, this paper presents thermodynamic and kinetic interpretion on reseasons why the electric field affects solidification process of the casting alloys. It will make fundations for castings on the basis of electric field solidification technique. It is also expected to pave more ways on application of Al-Si alloy and Ti-Al alloy in aviation, aerospace and automobile industries.更多还原