【作者】 赵倩；

【导师】 张兴国；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2013， 博士

【摘要】 在金属凝固过程中施加电磁搅拌强迫熔体流动,对改变熔体中的传热与传质过程,控制凝固组织具有重要作用。本论文以常规条件下易产生成分偏析的低熔点Pb-Sn和Sn-Sb合金为目标金属,采用多功能电磁搅拌装置,在实验室条件下探讨不同磁场形式电磁搅拌对合金凝固过程、凝固组织、成分偏析及缩孔、疏松缺陷的影响,重点分析螺旋磁场的作用机理,结合温度场、电磁场数值模拟分析,优化电磁搅拌工艺参数,并将多种磁场与功率超声结合组成复合场应用于凝固过程中,研究其对凝固组织和宏观缺陷的影响及作用机理。论文得到的主要结果有：(1)旋转、行波和螺旋磁场的磁感应强度均随周向角度呈正弦分布,且随搅拌频率的增加而降低；旋转磁场在f=10Hz时达到磁感应强度最大值,螺旋磁场在户8-12Hz区间的磁场强而稳定。旋转和行波磁场中心最大磁感应强度出现在周向角度900和270°,而螺旋磁场出现在120°和180°。频率一定,磁感应强度随励磁电流的增加而增大。(2)在搅拌器轴线方向,磁感应强度呈现出“中间大、两端小”的分布特征；在搅拌器同一横断面上,边缘处磁感应强度最大而中心最小；螺旋磁场沿轴向分布较旋转磁场更为平稳,电磁搅拌覆盖范围更大；当R<60mm时,螺旋磁场沿径向衰减幅度更小分布更为平稳；行波磁场在搅拌器内壁处磁感应强度最大,但沿径向衰减程度也最大。测试和模拟结果均表明,螺旋磁场的磁感应强度大于旋转磁场和行波磁场。(3)数值模拟与实测结果均表明,螺旋磁场的电磁力使熔体主要产生切向和轴向上的大环流,而切向和轴向受力的分布对搅拌强度和效果起决定作用,随励磁电流的增强Fθ与Fz也显著增加,有利于金属熔体在横纵截面上的流动。熔体所受电磁合力Fsum随搅拌频率增加而增加,但幅度不大,在f=2～12Hz时电磁力的曲线几乎是重合的,但是沿半径方向逐渐增大。(4)螺旋磁场产生的感应热随着电流强度的增强而增大,沿着径向均匀分布；励磁电流较低时电磁搅拌对凝固组织起主要作用,当励磁电流达到一定值时感应热对凝固过程的影响占主导,导致冷却速度降低,冷却曲线斜率减小,共晶平台位置上移,共晶凝固时间延长。(5)励磁电流一定时,螺旋磁场频率较低时将产生反偏析,随着频率的增大反偏析逐步得到改善,在细化凝固组织及改善成分偏析方面合理的搅拌频率为f=10～13Hz,适宜的搅拌时间为15-20min,Pb-Sn合金晶粒尺寸由无磁场的175μm减至140μm,铸锭成分偏差值为1.5wt.%。(6)频率为f=10Hz时,螺旋磁场励磁电流越大,晶粒尺寸越细小,在改善成分偏析和凝固组织方面适宜的励磁电流为115-125A,平均晶粒尺寸最小为133μm,可以基本消除Pb-80%Sn过共晶合金的成分偏析。同时,螺旋磁场搅拌对合金共晶层片有明显的粗化作用,且共晶组织生长形态也发生从层片共晶向不规则共晶的转变。(7)螺旋磁场、旋转磁场和行波磁场电磁搅拌都可以有效的改善合金的凝固组织和成分偏析,综合对比不同形式的磁场搅拌对Pb-Sn和Sn-Sb合金内在质量的影响,可得到如下结论,在细化凝固组织的冶金效果是：螺旋磁场>旋转磁场>行波磁场,在改善成分偏析的冶金效果方面为：螺旋磁场>行波磁场>旋转磁场。(8)复合场在细化凝固组织和改善合金内部质量方面存在一个适宜的超声功率值和搅拌强度,获得较好的冶金效果。对比旋转磁场与超声场复合、螺旋磁场与超声场复合作用下合金凝固组织变化的结果发现,螺旋磁场切向分力扩大超声空化作用的覆盖面积,强化超声破碎枝晶、细化组织的作用；轴向分力与超声声流产生的轴向搅拌叠加,促进了功率超声在整个熔体内爆发生核,螺旋磁场与超声复合改善宏观缺陷和细化凝固组织效果最好。更多还原

【Abstract】 Various laminar and turbulent flows caused by the application of electromagnetic field during the solidification could change the processes of heat and mass transfer in melt metal and proceed to affect the solidification structure and compositional distribution of alloy.With the application of the New-type multi-functional electromagnetic stirrer, low melting alloys Pb-Sn and Sn-Sb were selected as research object and the effects of different magnetic fields on the solidification process, composition segregation and defects such as shrinkage cavity, porosity of the alloy ingot were investigated.The action mechanism of the spiral magnetic field (SMF) emphatically was discussed, and stirring technologic parameters for EEMS was optimized by the numerical simulation analysis and experimental study, which would provide a guide for the practical application of the SMF stirrer in the factory. In addition, the study of compound field(combination of different kinds of electromagnetic field and power ultrasonic field) on the solidification process and the effects of compound field on the surface quality of the ingot, microstructures, composition segregation and macro-defects were studied. Moreover, the action mechanism of compound field on the microstructures of the metal was also analyzed.(1) The magnetic induction distribution of three magnetic fields exhibits sinusoidal relationship with the circumferential angle, and decreases with the increase of the stirring frequency. The magnetic field distribution under rotating magnetic field(RMF) is seriously affected by the frequency and reaches its maximum value f=10Hz, while under spiral magnetic field(SMF) at the range of f=8-12Hz is very stable. The magnetic induction reaches its maximum value in90°and270°under RMF and TMF, and under SMF is in120°and180°. The magnetic induction increases with the exciting current as well as increases with the frequency within limits.(2) The magnetic induction decreased gradually from the centre to the upper or lower part of the stirrer along the axial direction and from the inwall to the centre in the same height section along the radial direction; the coverage area of SMF was wider along the axial direction than the others and the decrease speed of rotating magnetic field was lower along the radial direction, though SMF showed a more stationary distribution when R<60mm. Experiments and simulation results show that magnetic induction intensity under the SMF is greater than the RMF or TMF. (3) The simulation calculation results show that:with the increase of exciting current, the electromagnetic force value on the metal melt increases along all directions. The electromagnetic force distribution acting on the metal melt is uneven, the electromagnetic force value near the stirrer lateral is greater than the value far away from the stirrer. The distribution of Fz and Fθ playes important role in stirring strength. And its value also increases with the increase of the excitating current, which is benificial to the metal melt flow along the longitudinal and cross section. The electromagnetic force Fsum increases with the increase of stirring frequency and its distribution is stable at frequency range f=2-12Hz and increases along the radial direction(4) The induction heat under the SMF increases with the increase of the exciting current and uniformly distributes along the radial direction. However, the influence of induction heat on the melt became more obvious while the exciting current is large. The cooling rate decreases, the position of eutectic plateau changes and eutectic reaction time is prolonged.(5) At the fixed exciting current, the alloy exhibits anti-segregation at lower frequency while segregation has been gradually improved at higher frequency, and the frequency of f=10-13Hz is the best value in the improvement of macro-segregation. The appropriate stirring time is15-20min, reasonable stirring frequency is10-13Hz has been obtained. The grain size of Pb-Sn alloy is reduced from175μm to140μm and composition deviation decreases to1.5wt.%.(6) when the frequency is10Hz, in a range of the exciting currents with the increase of its value, the grain size is finer. The minimum grain size is133μm at I=125A. To further increase the current, the grain coarsenes. When I=115A the SMF can basically eliminate segregation. So, the SMF has the appropriate exciting current in improving segregation and solidification structures of115-125A. Moreover, the eutectic lamellar spacing coarsened and the transition of eutectic structures morphology from lamellar to rod-like eutectic when the stirring strength is high enough under the SMF.(7) The SMF, RMF and TMF stirring can effectively improve microstructure and segregation of the alloy. Comparison results of effects on the intrinsic quality for Pb-Sn and Sn-Sb alloy under different magnetic stirring show that in the refinement of grain SMF is better than RMF, which is better than TMF; in the improving segregation as:SMF is better than TMF, which is better than RMF.(8) Compound field has the appropriate ultrasonic power value and the stirring intensity in improving solidification structure and internal quality for the surpose of better metallurgical results. Compared to the compound field of rotating magnetic with ultrasonic field, under spiral magnetic with ultrasonic field the results show that, the tangential component of spiral magnetic field force expands the coverage of ultrasonic cavitation area and strengthenes the role of refinement of microstructures. The supersition stirring, which is composed of axial component of electromagnetic force and an axial-flow generating by ultrasonic field, promotes nucleation in the all metal molten. Compound field of spiral magnetic and ultrasonic field achieves better effects in improving macro-defects and structure refinement.更多还原