超声作用下半连铸7050铝合金的偏析行为及形成机制

采用SPECTRO-MAXx立式直读光谱仪和Leica金相显微镜,研究超声功率对半连铸过程中7050铝合金微观组织及溶质Zn元素径向分布的作用规律。结果表明:在超声外场作用下铸锭组织明显细化,混晶减少,溶质Zn元素的径向逆偏析程度减弱,并随着超声功率的增加,晶粒细化程度及宏观偏析的弱化效果更好,溶质Zn元素的偏析比由未加超声时的1.190降低为240 W功率超声作用时的1.086;在超声外场调控下,铝合金熔体偏析弱化的主要原因是由于形核率增加,晶粒细化,两相区液固相相对运动速度减小;液穴变浅,由铸锭收缩导致的富集液相径向流动减弱,是径向偏析程度减弱的另一原因;在半连续铸造过程中,采用超声外场控制液相的流动是弱化铸锭径向宏观偏析的有效手段。     

电磁搅拌对镁合金铸锭宏观偏析的影响

研究了电磁搅拌对半连续铸造镁合金铸锭宏观偏析的影响。采用电磁铸造法制备了直径320 mm的AZ61镁合金铸锭,电磁搅拌施加电流为0~300 A,频率为15 Hz。利用化学分析法对比了不同电磁场强度下镁合金铸锭径向主要溶质元素的分布情况。结果表明,电磁搅拌可以有效改善镁合金铸锭主要溶质元素的分布,并且当施加电流强度为20~50 A时,铸锭径向元素偏析比明显减小,达到了改善镁合金铸锭宏观偏析的效果。电流强度超过100 A时,电磁场强度较大会引起熔体过搅现象,从而导致溶质元素偏析程度恶化。     

超声处理铝合金熔体对半连铸大铸锭组织与偏析的影响

在Ф1250 mm规格2219铝合金铸锭半连铸过程中,对结晶器中的熔体施加恒定功率的超声波处理,研究了超声处理熔体对大规格铸锭成分偏析的影响规律与作用机制。结果表明,在非对称超声场的作用下,铸锭整个断面的凝固组织均获得显著细化,晶粒尺寸减小且分布趋于均匀,溶质元素Cu的宏观偏析与显微偏析均得到了显著改善,与未加超声处理的铸锭试样相比,晶内相对固溶程度提高了13%,Cu元素的宏观偏析指数降低了24%。分析认为,超声的细晶作用以及超声声流、空化脉冲波促进的溶质元素扩撒对改善偏析具有积极影响。     

低频电磁场对半连续铸造Al-Mg-Si-Cu合金显微组织的影响

采用低频电磁场半连续铸造工艺制备Al-Mg-Si-Cu铝合金铸锭,研究了低频电磁场对Al-Mg-Si-Cu铝合金铸锭组织的影响。结果表明,施加低频电磁场可以细化Al-Mg-Si-Cu铝合金铸锭的晶粒组织,减轻晶内元素的偏析程度。与常规半连续铸造相比,施加频率为20 Hz、电流为120 A的交变电磁场后,铸锭中的等轴晶组织增多,枝晶状组织减少,晶粒尺寸变得更加细小,铸锭边部和中心平均晶粒尺寸由常规半连续铸造的170和259μm分别降至133和187μm,同时有利于提高溶质元素在晶内的分布,很大程度上减轻了溶质元素的偏析。此外,低频电磁场的电磁振荡使得液穴内部温度场和流动场更趋均匀,溶质元素分配系数增大,结晶区间变小,抑制了树枝晶的生长,促进了Al-Mg-Si-Cu合金半连续铸锭中非枝晶组织的形成和溶质元素的强制固溶,并且抑制了溶质元素的偏析。     

内置电磁熔体处理对大规格2219铝合金铸锭组织的影响

针对传统大规格2219铝合金铸锭存在的组织粗大不均,宏观偏析严重及第二相粗大等问题,采用内置电磁熔体处理半连续铸造方法制备了?630 mm 2219铝合金铸锭,通过光学显微镜、光谱仪、SEM等测试方法对铸锭进行了凝固组织与成分检测,研究了内置电磁熔体处理对大规格2219铝合金铸锭凝固组织细化的影响,揭示了溶质元素的分布规律。结果表明,相比普通半连续铸造,使用内置电磁熔体处理法制备的铸锭沿径向上晶粒细小,平均晶粒尺寸在200～300μm之间,组织整体上分布均匀,金属间化合物尺寸明显细化,晶间第二相呈断续网格状分布,Cu分布更均匀。     

电磁振荡法半连铸7075合金的微观组织及溶质元素分布

研究了电磁振荡对7075铝合金半连续铸锭微观组织及溶质元素宏观和微观分布的作用规律。研究结果表明：存在一个最佳电磁场频率范围10～30Hz，在此范围内，随着交变磁场感应线圈电流增大，铸锭中近球形组织增多，蔷薇形组织减少，晶粒尺寸变得更加细小和均匀；同时，溶质元素晶内含量显著增加，宏观偏析现象在很大程度上得到了抑制和消除；此外，电磁振荡使得液穴内部结晶核心增加，温度场和含量场更趋均匀，初凝壳高度和液穴深度降低，溶质元素分配系数增大和结晶区间变小，对枝晶生长的抑制作用加强，从而促进了电磁振荡法半连续铸锭中非枝晶组织的形成和溶质元素的强制固溶，并且抑制了宏观偏析。     

电磁搅拌与超声作用对6082铝合金成分偏析的影响研究

消除宏观偏析是制备高品质6082铸坯的重要环节。本文研究了电磁搅拌、超声场和添加细化剂等熔体处理手段对6082铸锭宏观偏析的影响规律。结果表明：施加电磁搅拌和超声场均能减少Mg和Si的偏析,作用效果随电磁搅拌电流强度和超声作用时间的增加而增强。当磁场电流强度为20 A时,Mg和Si的偏析比由未施加电磁搅拌的1.113、1.224减小至1.037、1.048,分别降低6.8%和14.3%。施加超声作用150s时,Mg和Si的偏析比减小至1.041和1.047,相较未施加时,降低了6.5%和14.5%。同时,电磁搅拌和超声作用有效促进了元素在横截面径向上的均匀分布。作为对比,加入Al-Ti-B细化剂可大幅减小6082的平均晶粒尺寸,但对元素偏析作用不明显,电磁搅拌和超声作用对偏析的改善作用主要源自物理搅拌。     

钛合金TC4真空自耗熔炼工艺参数对宏观偏析影响

采用Fluent软件模拟了钛合金TC4真空自耗熔炼过程中温度场、流场和溶质场相互作用,研究了与铸锭直接相关的3个工艺参数(熔速、铸锭上表面温度和冷却强度)对铸锭宏观偏析的影响规律。结果表明：不同熔炼条件下,在铸锭1000 mm高度处的铁元素径向偏析均呈钟形分布,即铸锭芯部为正偏析,表面区域为负偏析,且负偏析程度均大于正偏析。熔炼速度对铸锭温度场和宏观偏析的影响最为明显：当熔炼速度由0.15 mm/s增加到0.18 mm/s时,铸锭达到稳定熔炼阶段时的高度由1200 mm增加到1600 mm,熔池深度由494 mm增加到738 mm。当距铸锭中心距离小于130 mm时,偏析随熔炼速度增加而减小,在熔炼速度为0.15 mm/s时达到最大值,为3.36%;当距铸锭中心距离大于295 mm时,偏析随熔炼速度增大而增大,在熔炼速度为0.21 mm/s时达到最大值6.23%。铸锭上表面温度和冷却强度对宏观偏析和熔池深度的影响不明显。通过正交分析得到3个主要工艺参数对宏观偏析影响程度为：熔炼速度>冷却强度>铸锭上表面温度,并得到最优工艺参数为熔炼速度0.15 mm/s、铸锭上表面温度21...     

搅拌磁场强度对Ti-1023合金凝固组织和Fe偏析的影响

研究了工业化制备的超大规格Ti-1023合金铸锭在不同搅拌磁场强度下的纵向凝固组织和Fe元素分布。结果表明:当磁场强度为0.001和0.003 T时,铸锭组织主要由粗大柱状晶,竹节状的小柱状晶和等轴晶组成;当搅拌磁场强度为0.002 T时,凝固组织转变为由粗大柱状晶和细小等轴晶组成。Ti-1023合金铸锭中Fe元素在不同磁场强度下遵循正偏析规律,沿铸锭长度方向Fe元素含量从底部到顶部逐渐增加,沿径向从边缘到中心逐渐增加。随着磁场强度从0.001 T增加到0.003 T,合金元素Fe的宏观偏析率呈现先减小后增大的趋势。     

磁场强度对Ti-1023合金凝固组织和Fe偏析的影响

研究了工业化制备的超大规格Ti-1023合金铸锭在不同磁场强度下的纵向凝固组织和Fe元素成分分布,并对其影响进行了探讨。结果表明：当磁场强度为10Gs和30Gs时,铸锭组织主要由粗大柱状晶,竹节状的小柱状晶和等轴晶组成;当磁场搅拌强度为20Gs时,凝固组织转变为由底部粗大柱状晶和细小等轴晶组成。Ti-1023合金铸锭中Fe元素在不同磁场强度下遵循正偏析规律,沿铸锭长度方向Fe元素含量从底部到顶部逐渐增加,沿径向从边缘到中心逐渐增加。随着磁场强度从10Gs增加到30Gs,合金元素Fe的宏观偏析率先减小后增大。     

Effect of Rotating Magnetic Field (RMF) on Segregation of Solute Elements in CuNi10Fe1Mn Alloy Hollow Billet

The effect of rotating magnetic field (RMF) on macro/microsegregation of solute elements is investigated experimentally, and a comprehensive three-dimensional mathematical model is built, and the finite element package ANSYS^® is employed to calculate the distribution of temperature and liquid fraction along the radial direction in horizontal continuous casting of CuNi10Fe1Mn alloy hollow billet. The results show that RMF makes the temperature field and liquid fraction uniform. The original inhomogeneous columnar grain macrostructure turns into homogeneous equiaxed grain structure, and the macro/microsegregation of Ni, Fe, and Mn elements are restrained effectively with the application of RMF. Moreover, the action mechanism of RMF is discussed to explain its effect on improving the distribution of solute elements.     

行波磁场作用下空心管坯的两相凝固数值模拟

利用等轴球晶两相凝固模型计算了行波磁场作用下空心管坯的凝固过程.分析了施加不同强度的行波磁场时,Sn-3.5%Pb管坯凝固过程的温度梯度、冷却速率、溶质浓度分布以及晶粒大小的变化情况.结果表明:金属液在行波磁场的作用下产生纵截面上的大环流,随着搅拌速度的加大,金属液凝固过程的冷却速率逐渐降低,熔体内的温度梯度减小,使管坯内部的晶粒得到细化,改善了铸坯凝固组织,但是强制流动导致管坯内产生宏观偏析,且搅拌速度越大,宏观偏析越明显,因此,在生产过程中应该严格控制搅拌速度的大小.     

Effect of harmonic magnetic field and pulse magnetic field on microstructure of high purity Cu during electromagnetic direct chill casting

The effects of two types of magnetic fields, namely harmonic magnetic field (HMF) and pulse magnetic field (PMF) on magnetic flux density, Lorentz force, temperature field, and microstructure of high purity Cu were studied by numerical simulation and experiment during electromagnetic direct chill casting. The magnetic field is induced by a magnetic generation system including an electromagnetic control system and a cylindrical crystallizer of 300 mm in diameter equipped with excitation coils. A comprehensive mathematical model for high purity Cu electromagnetic casting was established in finite element method. The distributions of magnetic flux density and Lorentz force generated by the two magnetic fields were acquired by simulation and experimental measurement. The microstructure of billets produced by HMF and PMF casting was compared. Results show that the magnetic flux density and penetrability of PMF are significantly higher than those of HMF, due to its faster variation in transient current and higher peak value of magnetic flux density. In addition, PMF drives a stronger Lorentz force and deeper penetration depth than HMF does, because HMF creates higher eddy current and reverse electromagnetic field which weakens the original electromagnetic field. The microstructure of a billet by HMF is composed of columnar structure regions and central fine grain regions. By contrast, the billet by PMF has a uniform microstructure which is characterized by ultra-refined and uniform grains because PMF drives a strong dual convection, which increases the uniformity of the temperature field, enhances the impact of the liquid flow on the edge of the liquid pool and reduces the curvature radius of liquid pool. Eventually, PMF shows a good prospect for industrialization.

     

Effects of casting speed on microstructure and segregation of electromagnetically stirred Aluminum alloy in continuous casting process

Recently, a semi-solid metal processing has been acknowledged as a cost-effective technique to be able to manufacture high quality product for the transportation industry.In this study a hypo-eutectic Al alloy was fabricated by means of an electromagnetic stirrer in continuous casting process and the microstructural change during solidification due to a fluid flow by electromagnetic stirring was examined.Due to the forced fluid flow during solidification a dendritic phase of primary α phase of Al alloy was turned into a globular phase, which can make the Al alloy get a thixotropic behavior in the semi-solid region.In order to establish the quantitative relationship between microstructure and the process parameters, the morphology shape, a silicon distribution and a size of primary α phase were observed according to casting speed in continuous casting machine.The primary α phase was turned into the degenerate dendrites approaching a spherical configuration with increasing casting speed.The fine-grained and equiaxed microstructure appeared at higher casting speed.A segregation behavior of Si element was declined with increasing casting speed and a very uniform distribution of Si element was observed on the billet at a casting speed of 600 mm·min-1.A thickness of the solidifying shell of the billet was shortened with increasing the casting speed.     

导流器对自孕育铸造法制备AM60镁合金半固态浆料的影响(Ⅲ)

采用自孕育铸造法制备AM60镁合金半固态坯料,研究导流器参数对半固态组织的影响规律及组织形成机理。结果表明:导流器能有效促进合金组织由树枝晶向等轴晶及颗粒状晶的转变;熔体处理温度和孕育剂加入量一定时,导流器的相关参数,如角度、长度、温度、混合段的凹槽宽度,均影响合金的传热及流动,进而影响其凝固组织;在导流器角度为30°～45°、长度为500 mm、混合段凹槽宽度为50 mm、且通水冷却时可以获得细小颗粒状或蔷薇状的半固态坯料,初生α-Mg的平均晶粒尺寸为37.5μm;减小导流器出口宽度有利于增强液体紊流,促进晶粒均匀分布;导流器促进了熔体非均质形核及激冷晶的形成,增强了晶粒增殖;熔体对流使合金的温度场和浓度场更均匀,晶粒生长以颗粒状生长、蔷薇晶根部颈缩熔断和磨圆熟化机制为主。     

齿轮钢大圆坯的铸态组织及轧制遗传性

基于φ500 mm大圆坯连铸连轧生产22CrMoH汽车桥齿钢的工艺,通过对铸坯低倍组织及高倍金相组织、轧后带状组织系统分析,开展了齿轮钢大圆坯铸态组织及轧制遗传性研究。研究发现,22CrMoH齿轮钢连铸大圆坯近表面细晶区、柱状晶区、中心等轴晶区的枝晶尺寸存在明显差异;自表面至芯部二次枝晶间距逐渐增加,退火状态下枝晶主干位置以铁素体为主,枝晶间以珠光体为主;轧后带状组织形态主要受铸态组织二次枝晶间距和轧制变形的影响;轧后退火态铁素体带宽自表面至芯部呈现增加趋势。     

方坯软接触电磁连铸结晶器内三维电磁场的数值模拟

通过有限元法三维数值模拟，对方坯连铸软接触结晶器系统中的电磁参数进行了计算，得到磁感应强度，电磁体积力，感应涡流在分瓣结晶器系统中的分布规律，在切缝区域连铸坯表面磁感应强度较其他区域稍强，20000Hz时约为10%，由于电磁场透过切缝作用于连铸坯上，连铸坯表面受力的电磁体积力在横向较为均匀，当钢液面位于感应线圈中部时，沿拉坯方向钢液面以下附近电磁场的作用最强，然后逐渐降低。     

Time-Series Fluctuation Characteristics of Segregation Carbon Element Distribution Along Casting Direction in High Carbon Continuous Casting BilletEI北大核心CSCD

High carbon steel is prone to produce macroscopic/semi-macroscopic segregation, and C segregation along casting direction will have a serious impact on the quality of billet. Based on qualitative analysis (macroscopic quality rating and elemental macro content analysis) and relatively simple quantitative analysis (segregation index, mean square error), the existing technologies have judged the degree of segregation of billet in different levels, but with requirement of stricter quality stability standard, especially for the typical raw material of high-level rod-wire steel (e.g. 70 high carbon steel), it is very necessary to introduce a new method to measure the fluctuation characteristics more effectively. In this work, the ARMA (auto regressive moving average) model in the time series is firstly used to study the segregation degree of high carbon steel billet at different positions from the aspects of inherent period and damping rate in terms of the fluctuation characteristics (period and extremum) of C element along the casting direction. The size of the billet is 170 mmx170 mm and the sampling location is in the center longitudinal plane of the billet. The experiment is conducted by considering the effect of cooling strength (conventional cooling and strong cooling) on the fluctuation characteristics. Firstly, it is shown that the inherent period and the damping rate can quantitatively describe the inherent characteristics (periodic and extreme characteristics) of the C element along the casting direction. Secondly, strong cooling makes the average inherent period and the average damping rate of C element time series in the columnar grain region smaller, thus increasing the fluctuation degree and the segregation degree. However, strong cooling makes the average inherent period and the average damping rate of the C element time series in the equiaxed grain region larger, thus decreasing the fluctuation degree and the segregation degree. Finally, the period and the damping rate of the columnar grain region are respectively affected by the dendritic spacing and the temperature gradient, and the period and the damping rate of the equiaxed grain zone are respectively affected by the liquid flow in the solidification process (V-shaped segregation) and local cooling rate. By this research, a new theoretical basis may be supplied for delicacy control of element segregation and related quality fluctuation phenomenon.     

LY12铝合金摩擦焊接过程的电场效应

定量研究了外加强电场对LY12铝合金摩擦焊接头热影响区（HFZ）中动态再结晶区宽度与晶粒尺寸的影响，并初步探讨了其影响机制。实验结果表明，外加强电场使动态再结晶区晶粒细化、等轴性提高，超细等轴晶区亦加宽。此外，采用较高焊接压力时，电场的引入使焊接接头的动态再结晶区宽度增大10％－30％；采用中等摩擦压力施焊时，电场的作用不但使焊接接头动态再结晶区宽度明显增加，而且还影响其沿径向的分布形态，使近轴心线处的动态再结晶区宽度趋于均匀，可以推断，外加强电场可使摩擦焊接头更易发生动态再结晶，而且强电场使焊合区组织的细化与均匀化，有利于提高焊接接头性能。