激光增材制造高性能大型钛合金构件凝固晶粒形态及显微组织控制研究进展

简要介绍高性能大型关键金属构件激光增材制造的技术特点,总结本团队在高性能大型关键钛合金构件激光增材制造过程中,对凝固晶粒形态和显微组织控制的主要研究进展:发现了钛合金构件激光增材制造过程中熔池底部外延生长和顶部异质形核两种主要形核生长机制,建立了基于增材制造工艺参数及凝固条件控制的全柱状晶、全等轴晶和柱状晶-等轴晶混合组织等凝固晶粒形态主动控制技术;发现了激光增材制造双相钛合金构件高性能特种双态显微组织新形态,并建立其固态相变理论及显微组织主动控制技术。     

基于CAFE法的单晶铜杆热型连铸过程晶体生长模拟

基于元胞自动机(CA)和有限元(FE)耦合法对单晶铜杆热型连铸过程中晶粒生长及演化过程进行了模拟,获得了稳定状态下铸棒内温度分布及液固相界面的位置和形态,模拟了连铸初期沿热流方向细小等轴晶快速合并形成柱状晶的过程,采用截面形态和极图法分析了定向凝固条件下各个生长时刻的晶粒形貌和晶粒生长取向以及晶粒竞争生长过程中的快速淘汰和慢速淘汰阶段,为热型连铸工艺优化提供了依据。     

Precipitation and growth of titanium nitride during solidification of clean steel

With the help of solidification kinetics, precipitation and growth of TiN have been studied theoretically and the effect of cooling rate on the growth of TiN has been discussed. Through induction electric furnace experiment and electroslag remelting experiment with different cooling rates, the distribution and particle size of TiN were investigated. It is proved that the higher cooling rate is, the smaller the final size of TiN is and the more TiN particles can be found in the steel. That TiN can restrain the growth of columnar crystal and enhance the proportion of equiaxed crystals to refine solidification structure as the heterogeneous nucleation sites of 8-ferrite is confirmed preliminarily. A new method to reduce macrosegregation in continuous cast steel is provided.     

TiA(l)-10Nb合金铸态组织演化规律的研究

研究Ti-(44～54)Al-10Nb合金铸态组织的演化规律,并确定全β凝固的TiAl-10Nb合金的成分范围.实验结果表明:Ti-(44～53)Al-10Nb合金的凝固组织为典型的柱状晶组织,Ti-54Al-10Nb合金的宏观组织为等轴组织;在其凝固组织(纵截面)上部存在较多的缩松,这主要是由于Nb元素的添加量加多,...     

Si对Ti-46Al-5Hf-0.6B合金凝固组织的影响

为研究Si对Ti-46Al-5Hf-0.6B(原子百分比)合金铸态组织的影响,将名义成分为Ti-46Al-5Hf-0.6B-xSi(x=0,0.5%,1.5%,3%)合金用电弧熔炼法在氩气保护下制成纽扣锭.合金宏观和微观组织的特征采用OM、SEM、XRD技术进行分析.结果表明:Si在合金中形成Ti5Si3相,Ti5Si3相的数量和形态随Si含量的增加而变化.当Si的原子数等于0.5%、1.5%、3.0%时,合金凝固组织分别为近片层的粗大柱状晶、细化的柱状晶和以块状和条状Ti5Si3为初生相的等轴组织.Ti5Si3为脆性相,Si的原子数应小于1.5%.     

Prediction of the solidification structure of casting and heterogeneous nucleation

The frequency of heterogeneous nucleation during the solidification of Al-Si binary alloy was estimated by comparing experimentally obtained macrostructures of castings with numerically simulated ones. A molten alloy was unidirectionally solidified from a water-cooled copper chill in an adiabatic mold. The location of columnar to equiaxed transition (CET) in the solidified alloy ingot was measured. A numerical simulation for grain structure formation based on the Monte Carlo method was carried out, and the frequency of heterogeneous nucleation in the alloy was evaluated by producing similar structure with the experimental one. The frequency of heterogeneous nucleation was expressed as a probabilistic function with an exponential form of undercooling that determines the probability of nucleation event in the simulation. The value of the exponent is regarded as the nucleation parameter. The nucleation parameter of Al-Si binary alloy varied with initial Si content.     

Microstructure of AISI 304 stainless steel strips produced by a twin-roll caster

By optical inspection of macro-etched metallography and electron back-scattered diffraction (EBSD) mapping,this paper analyzed the microstructure of anstenitic stainless steel strips produced with an equal-diameter twin-roll strip caster.The results indicate that the microstructure of the strips includes two columnar zones with highly compact dendrites and one equiaxed zone.The characteristics,such as grain size and growing direction of columnar grains and equiaxed grains,were investigated.An additional transitional area with many freer grains between the columnar zone and the equiaxed zone was found.As shown in EBSD analysis,small angle boundaries exist both in the columnar zone and the equiaxed zone,although they are especially more in the transitional area.Additionally,some <111> twin boundaries were found in the mierostructure of the strips.     

Solidification structures of high niobium containing TiAl alloys

To understand the effect of alloy stoichiometry on the microstructural development and mechanical behavior of γ-TiAlbased materials, it is necessary to have a determination of the phase relationships for the TiAl alloy system near the γ phase field.Cast structures and phases of Ti-(43-47)Al-8Nb-(1-2)Mn (at%) alloys have been studied by using scanning electron microscope and X-ray diffraction. Their solidification path and microstructure development during the solidification were analyzed. The experimental results show that the alloys with different Al contents form different macrostructures and microstructural morphologies. This indicates that the solidification paths are different with different Al contents. The alloy with 43Al forms equiaxed grain structure, and the solidification path is as follows: L → L β→β→α β→α β cores →α2 γ B2 cores. Whereas the alloy with 47Al forms columnar grain structure, and the solidification path is as follows: L → L β→α β L →α γ β cores →α2 γ B2 cores. The β phase is their primary solid phase and can be retained to ambient temperature. The alloy with 43Al solidifies completely into β phase. The peritectic reactions L β→α and L α→γ appear when the Al content increases to 47Al.     

Microstructure evolution of cast Al-Si-Cu alloys in solution treatment

To develop a software to predict the evolution of microstructure and the development of mechanical properties during the heat treatment of cast aluminum alloys, we modeled the redistribution of solute during the solution treatment of multicomponent alloys. The predictions of solidification simulation software or the results of experiment provided the initial microstructure and solute distribution for simulation of heat treatment. Binary through quinary aluminum alloys with silicon, copper, magnesium, and iron were modeled. The basic model assumed local equilibrium （no undercooling due to nucleation or growth） and computed diffusion in the solid constituents during solidification. The evolution of microstructure during solution treatment was followed by qualitative and quantitative metallography. The results of simulation for the ternary alloy Al-7%Si-3.5%Cu were compared to experimental observation.     

Roles of titanium-rich precipitates as inoculants during solidification in low carbon steel

The effect of titanium on the as-cast structure and the growth form of titanium precipitates, and the effect of cooling rate on the size and distribution of titanium precipitates were studied. It is shown that Ti-rich precipitates acting as heterogeneous nucleation sites play an important role in refining the grain size and increasing the equiaxed grain ratio. Cooling rate has a great effect on the size and distribution of precipitates. The number of precipitates increases and the size decreases with the increase of cooling rate. Ti-rich particles acting as het-erogeneous nucleation sites at the onset of solidification are observed in the experiment. This result suggests that TiN nucleated on Ti2O3 is an effective inoculant for δ-ferrite during solidification in low carbon steel.     

Microstructure and penetration behavior of electroformed copper liners of shaped charges during explosive detonation deformation

The microstructure in the electroformed copper liners of shaped charges prepared with different electrolytes was studied by Scanning Electron Microscopy （SEM） and Electron Backscattering Kikuchi Pattern （EBSP） methods. SEM observations revealed the existence of columnar grains in electroformed copper liners of shaped charges formed by electrolyte without any additive and the average grain size is about 3 μm. When an additive is introduced to the electrolyte, the grains formed in the copper liners become equiaxed and finer. EBSP results show that the columnar grain grown during electroformation has the most preferential growth direction, whereas a micro-texture does not exit in the specimen prepared by electrolyte with the additive. Further, explosive detonation deformation experiments show that penetration depth is dramatically improved when the electroformed copper liners of shaped charges exhibit equiaxed grains.     

Effect of M-EMS on the solidification structure of a steel billet

Effects of mold electromagnetic stirring (M-EMS) on the solidification structure of 45# steel billet were investigated by examination of interdendritic corrosion. The results show that the primary and secondary dendrite arm spacings increase from the edge of the billet to the center and decrease obviously with increasing electromagnetic torque, which will be beneficial to refine the solidification structure and enlarge the equiaxed crystal zone. The ratio of equiaxed crystal increases by 15.9% with the elec...     

机械振动对焊接熔池金属凝固过程的影响

从力学的角度和能量的角度解释机械振动对焊接熔池金属凝固过程影响的机理。通过分析机械振动对焊接熔池金属的宏观作用和微观机理后认为 :机械振动改变了熔池中的结构起伏分布和能量起伏分布的状态 ,使晶体形核更为容易 ;机械振动使柱状树枝晶的枝晶折断 ,减小了成分过冷区的大小 ,抑制了柱状树枝晶的长大 ,被打碎的枝晶在熔池中独立长大成为等轴晶 ,等轴晶的数量增多 ;机械振动使熔池液体对流速度加快 ,紊流层宽度增大 ,焊缝的宏观偏析减弱 ,气孔和夹渣等缺陷减少。机械振动对焊接熔池凝固的这些影响对机械振动焊接焊缝组织和性能的改善起了重要的作用     

Morphological evolution model for unidirectional solidification of multicomponent alloys

Based on the nonlinear interaction of the different species and the calculation of phase diagram, a self-consistent model is developed to describe the interface morphology evolution during unidirectional solidification of multicomponent alloys. This model takes full account of the mutually coupled effect of temperature field, solute field, interface energy and interface attachment kinetics. In comparison with linearization analysis of multicomponent, it not only extends the convergent range of the analysis, but also greatly improves the applicability.     

Fabrication of directional solidification components of nickel-base superalloys by laser metal forming

Straight plates, hollow columns, ear-like blade tips, twist plates with directional solidification microstructure made of Rene 95 superalloys were successfully fabricated on Nickel-base superalloy and DD3 substrates, respectively. The processing conditions for production of the parts with corresponding shapes were obtained. The fabrication precision was high and the components were compact. The solidification microstructure of the parts was analyzed by optical microscopy. The results show that the solidification microstructure is composed of columnar dendrites, by epitaxial growth onto the directional solidification substrates. The crystallography orientation of the parts was parallel to that of the substrates. The primary arm spacing was about 10μm, which is in the range of superfine dendrites, and the secondary arm was small or even degenerated. It is concluded that the laser metal forming technique provides a method to manufacture directional solidification components.     

电脉冲孕育处理对ZL201合金凝固组织的影响

采用新型孕育处理方法—电脉冲孕育处理(简称EPM)技术,对ZL201熔体进行处理,研究其改善合金组织的能力。脉冲电场处理工艺对凝固组织影响的实验结果表明:EPM孕育处理技术可细化ZL201的凝固组织,增加等轴晶数量;合金熔体经EPM孕育处理后,凝固组织中α相之间的共晶组织量有所减少,共晶组织形态呈粒状化,分布均匀。并据此结果,对电脉冲孕育处理技术细化凝固组织及Al2Cu量减少的机理进行了分析。     

终烧温度对陶瓷型芯性能的作用

为了阐明终烧温度与陶瓷型芯性能的关系,利用高温抗折试验机和原位观测手段,研究了不同终烧温度下陶瓷型芯性能的变化.结果表明,对于非定向陶瓷型芯而言,随着终烧温度的提高,室温抗折强度保持稳定,在1 150℃条件下陶瓷型芯的高温抗折强度最高,且其变形抗力逐渐提高;而对于定向陶瓷型芯而言,随着终烧温度的提高,室温抗折强度和高温抗折强度均逐渐降低,且其变形抗力变化不大.根据上述实验结果,非定向陶瓷型芯的终烧温度确定为1 150℃,而定向陶瓷型芯的终烧温度确定为1 120℃.     

Q235小方坯连铸过程中凝固特性的研究

分析了Q235凝固时的结晶特性,柱状晶容易生长.数值计算了小方坯连铸机在2.6 m/min拉速、不同二冷配水下铸坯的温度场和凝固前沿温度梯度.在弱水冷却时,铸坯坯壳凝固前沿温度梯度比较小,得到细小的柱状晶和少量等轴晶.连铸坯中间裂纹主要与二冷水相关,合适的二冷配水可降低或消除中间裂纹.     

Mg含量对Al-Cu-Mg合金定向凝固组织的影响

建立模拟多元合金树枝晶生长的三维元胞自动机模型,耦合合金热力学和生长动力学计算,模拟预测Al-Cu-Mg合金定向凝固组织形成过程,考察Mg含量对定向凝固一次枝晶间距的影响。结果表明,在相同凝固条件下,定向凝固一次枝晶间距随着Mg含量的增加而增大,定向凝固二次枝晶间距随着Mg含量的增加而减小。     

Improving the solidification structure of commercially pure aluminium with electropulse acting on liquid metal

The effect of electropulse on the solidification structure of commercially pure aluminium was studied. The orthogonal array L9 was used to determine the effect of three process parameters of electropulse modification (EPM), which were pulse current intensity, pulse frequency, and treating time. For each factor, three levels were chosen to cover the experimental region. According to the ex-perimental results, the solidification structure of commercially pure aluminium was modified from large grains with columnar crystals to finer grains with equiaxed crystals, by allowing the electropulse to act on liquid aluminium. However, the solidification structures could be refined differently at different EPM parameters. Certain EPM parameters should be selected to get the optimum solidification structure. Among the three parameters, pulse frequency was the most important factor influencing the solidification structure, the secondary factor was current intensity, and treating time was the third one. The optimum parameters were the pulse frequency of 5 Hz, the current intensity of 68 A, and the treating time of 10 s.