定向凝固法制备单晶高温合金一次枝晶间距研究

利用定向凝固技术和籽晶法相结合的方式制备了DD5单晶高温合金。通过正方形估算法和直接测量法研究了DD5单晶高温合金的不同凝固距离(50、150和250 mm)横截面样品的一次枝晶间距。结果表明,随着凝固距离的增大,DD5单晶高温合金的一次枝晶间距逐渐增大;正方形估算法和直接测量法统计的DD5单晶高温合金的平均一次枝晶间距差距不明显,说明正方形估算法的精确度较高,为了提高效率可以采用正方形估算法统计单晶高温合金的一次枝晶间距;不同凝固距离横截面的一次枝晶间距上限约为下限的6倍。     

Al-3．6％Cu合金定向凝固一次枝晶间距选择

采用Al—3．6％Cu单相固溶体合金,进行了较为全面的定向凝固淬火实验,考察了籽晶选取对合金凝固界面形态及一次枝晶间距的影响,并进一步分析了一次枝晶间距随凝固速度的演化规律。研究发现,籽晶的选取时于定向凝固的界面晶体取向和一次枝晶间距具有重要的影响,平均一次枝晶间距随着凝固速度的增大逐渐减小,且存在一个较宽的容许范围。实验结果与HL模型及LIN自洽模型计算获得了较好吻合,而KGT模型明显高估了实验值。     

镍基单晶高温合金薄壁试样的一次枝晶间距统计方法对比

分别采用面积测量法、三角形测量法和最小生成树法,对不同壁厚的镍基单晶高温合金试样的一次枝晶间距进行了统计。结果表明,面积测量法和三角形测量法对块状试样平均一次枝晶间距的统计结果基本相当,大于采用最小生成树法获得的数值。随试样壁厚降低,三角形测量法和最小生成树法统计的平均一次枝晶间距差距基本固定,一次枝晶间距的容许范围随着试样壁厚降低而缩小,而面积测量法在统计薄壁试样的平均一次枝晶枝晶间距时显示出明显的不稳定性。最后,讨论了3种方法统计镍基单晶高温合金薄壁试样一次枝晶间距时的特点和适用性。     

晶体取向对单晶高温合金一次枝晶间距的影响

在固液界面微单元热量平衡分析的基础上，建立了晶体取向对一次枝晶间距影响的理论模型。该模型表明，一维择优晶体取向与宏观定向凝固方向偏离越远，一次枝晶间距越小，对ＤＤ８单晶高温合金凝固组织尺度的实验研究结果表明，晶体取向对一次枝晶间距的影响趋势和理论模型相吻合，其影响程度和固液界面温度梯度及定向凝固速率有关。模型及实验都表明，提高固液界面温度梯度和定向凝固速率，晶体取向对一次枝晶间距的影响程度减弱。     

薄板试样定向凝固枝晶竞争生长

深入理解高温合金定向凝固杂晶演化具有重要的科学和工程意义,其中枝晶竞争生长是揭示晶界演化与淘汰的关键环节。定向凝固双晶竞争生长的实验研究证实了传统尖端过冷度理论的局限性,定量相场模拟的研究获得了大量统计规律,促进了人们对定向凝固双晶竞争生长的理解。本研究在薄板状试样的定向凝固过程中,同时获得了大量的不同取向竞争生长的枝晶,通过拼接金相显微组织同时获得多个晶界的取向演化规律。结果表明,枝晶列竞争生长主要通过晶界两侧不同取向一次枝晶臂及二次枝晶臂的交互作用实现。在汇聚生长过程中,择优枝晶一次臂阻挡非择优枝晶,并且择优枝晶自身无法形成三次臂,导致晶界方向沿择优枝晶生长方向。发散生长过程中,处于温度梯度方向一侧时,两列枝晶的竞争生长方式以非择优枝晶新生三次臂为主,晶界以沿择优枝晶方向生长为主;当两列枝晶处于温度梯度不同侧时,两列枝晶均有三次臂新生的概率,其晶界在两列枝晶取向之间。实验统计结果给出了晶界随两列竞争生长枝晶的取向的变化规律,与定量相场模拟结果吻合。商业软件ProCAST的CAFE计算晶粒定向竞争生长晶界演化统计结果与实验结果存在较大偏差,其主要原因是CAFE仅给出晶粒尺度的轮廓信息,未考虑溶质扩散及二次臂的竞争。本研究获得的薄板状试样枝晶竞争生长的统计规律,对认识高温合金定向凝固过程中晶界取向的演化具有重要的意义,对柱状晶叶片的制备具有指导意义。     

合金定向凝固一次枝晶间距模拟

建立二元合金树枝晶生长的二维元胞自动机模型,模拟丁二腈2.5%(质量分数)乙醇定向凝固枝晶生长和一次枝晶间距选择过程。模拟结果表明:在给定的凝固条件下,定向凝固一次枝晶间距可在一个范围内变化,其具体取值与凝固历史具有相关性。在相同的温度梯度和不同冷却速度下,模拟给出的一次枝晶间距上、下限与实验结果吻合较好,详细分析影响定向凝固一次枝晶间距上下限的因素。结果表明:在给定凝固条件和合金系条件下,液相中无对流,影响一次枝晶间距上下限的主要因素是界面能和溶质扩散系数。     

Al-7Si-0.36Mg合金定向凝固一次枝晶臂间距实验和模拟

通过Al-7Si-0.36Mg合金定向凝固实验和元胞自动机模型,开展定向凝固枝晶形貌演化和一次枝晶臂间距选择过程的实验和模拟。结果表明:在给定的凝固条件下,一次枝晶臂间距范围是一个连续的变化区间。在恒定温度梯度和不同凝固速度条件下,测得Al-7Si-0.36Mg合金一次枝晶臂间距上限值(λmax)、下限值(λmin)和平均值(λave)以及生长速率之间的关系,且上限值和下限值的比值接近3。模拟结果与实验结果的吻合程度明显优于Hunt-Lu等解析模型的预测结果,表明CA模型在枝晶定向凝固过程枝晶形貌演化模拟和枝晶臂间距预测等方面的准确性。结合模拟研究和文献调研分析影响定向凝固一次枝晶臂选择的因素,包括抽拉速度v、温度梯度G、界面能大小、溶质扩散系数DL、枝晶生长取向与热流方向的偏离角度θ等。     

Al—4.5Cu单晶定向凝固一次枝晶间距研究

采用Ａｌ－４．５Ｃｕ单晶进行台阶变速定向凝固实验，考察了金属合金中一次枝晶间距随凝固生长速度变化的动态响应规律，结果表明，对于确定的凝固控制参数，稳态一次枝晶间距选择一个较宽的容许分布范围，同时，平均一次间距的选择也是凝固系统所经历的历史显著地相关，实验结果很好支持了Ｈｕｎｔ－Ｌｕｅ提出了枝晶生长一次间距的下限模型。     

《001》和《011》取向DD407单晶高温合金枝晶间距和微观偏析

采用液态金属冷却(LMC)高温度梯度定向凝固炉制备<001>和<011>取向的DD407单晶高温合金,研究了晶体取向对枝晶间距和微观偏析的影响.结果表明,晶体取向对单晶高温合金的枝晶间距和微观偏析有明显的影响.枝晶间距随着<001>取向偏差角的增加而增加,但随<011>取向偏差角的增加而减小,且<011>取向的枝晶间距...     

脉冲电流作用下TiAl合金凝固组织演变及形成机理

利用OM和TEM研究了脉冲电流作用下Ti-48Al-2Cr-2Nb合金的凝固组织,并分析了其微观组织演变及形成机理。结果表明,脉冲电流细化了TiAl合金的一次枝晶臂间距、柱状晶尺寸和片层间距。未加载电流的TiAl合金凝固的初生相为α相,TiAl合金的片层取向与柱状晶生长方向夹角较大,甚至垂直于生长方向。脉冲电流作用导致枝晶发生熔断和破碎,促进了β枝晶相的析出及增多,片层取向与晶体生长方向夹角较小或成45°生长的片层进一步增多。脉冲电流降低了固-液相之间的自由能及原子扩散激活能,减少形核位垒及晶核的形核功,从而在一定程度上促进原子扩散,增大了形核率,细化一次枝晶臂间距及柱状晶;初生相的转变析出及其特殊的位向关系是片层取向变化的主要原因。     

Influence of directional solidification variables on primary dendrite arm spacing of Ni-based superalloy DZ125

The primary dendrite morphology and spacing of DZ125 superalloy have been observed during directional solidification under high thermal gradient about 500 K/cm. The results reveal that the primary dendrite arm spacing decreases from 94 μm to 35.8 μm with the increase of directional solidification cooling rate from 2.525 K/s to 36.4 K/s. The regression equation of the primary dendrite arm spacings λ1 versus cooling rate is λ1=0.013(GV)-0.32. The predictions of Kurz/Fisher model and Hunt/Lu model accord reasonably well with the experimental data. The influence of directional solidification rate under variable thermal gradient on the primary dendrite arm spacing has also been investigated.     

Tailoring Microstructure and Microsegregation in a Directionally Solidified Ni-Based SX Superalloy by a Weak Transverse Static Magnetic Field

A weak transverse static magnetic field (WTSMF, 0-0.5 T) is applied to the directional solidification process of a DD3 Ni-based SX superalloy, aiming to tailor the microstructure and microsegregation of alloys. The mechanisms of microstructural refinement and microsegregation distribution caused by a WTSMF during directional solidification are discussed. It is shown that the primary dendrite arm spacing is rapidly reduced from 181 to 143 μm, and the average size of γ′ phase is significantly refined from 0.85 to 0.25 μm as the magnetic field increases from 0 to 0.5 T. At the same time, the volume fractions of γ/γ′ eutectic and the segregation coefficient are also gradually decreased. The 3D numerical simulations of the multiscale convection in liquid phase show that the modifications of the microstructure and microsegregation in DD3 are mainly attributed to the enhanced liquid flow caused by thermoelectric magnetic convection (TEMC) at dendrite/sample scale under the WTSMF. The maximum of the TEMC increases with increasing the magnetic field intensity. This work paves a simple way to optimize the microstructure and microsegregation in directionally solidified Ni-based SX superalloys without changing the processing parameters and composition.     

Phase field modeling the selection mechanism of primary dendritic spacing in directional solidification

Selection mechanisms of primary dendritic spacing in directional solidification are investigated by the phase field method. Results show that the lower and upper limits of primary spacing are determined by the interdendritic solutal interactions and the interdendritic undercooling respectively. The upper limit of primary spacing resulting from overgrowth of the tertiary arm could be about four times as large as the lower limit. The microstructural evolution from the onset of planar instability during directional solidification with a constant pulling velocity can be divided into three stages: an initial competition stage, a submerging stage and a lateral adjustment stage. Simulation results also demonstrate that the final primary spacing with a constant pulling velocity is very close to the lower limit due to the dendrite submerging mechanism.     

A model of solidification microstructures in nickel-based superalloys: predicting primary dendrite spacing selection

A combined cellular automaton-finite difference (CA-FD) model has been developed to simulate solute diffusion controlled solidification of binary alloys. Constitutional and curvature undercooling were both solved to determine the growth velocity of the solid/liquid interface. A modified decentered square/octahedron (in two or three dimensions) growth technique was implemented in the cellular automaton to account for the effect of crystallographic anisotropy. The resulting model is capable of simulating the growth of equiaxed and columnar dendritic grains in 2D and 3D, with the <100> directions either aligned or inclined with the grid. The algorithm used can also be used on coarser grids, with a concomitant loss in resolution, allowing simulation of sufficiently large numbers of dendrites in 3D to investigate the distribution of spacings, as well as average behavior.Simulations were performed for directional solidification with a range of withdrawal velocities and nucleation conditions, but a constant thermal gradient. The simulations capture the full microstructural development and primary spacing selection by both branching and overgrowth mechanisms. The model illustrates that there is a range of possible stable spacings, and that the final spacing is history dependent. It was also found that a minimum deviation from the steady state dendrite spacing is required before the spacing adjustment mechanisms are activated. The influence of perturbing the withdrawal velocity upon the stability of the spacing was also investigated. It was found that perturbations significantly reduce the range of stable primary dendrite spacing.     

EFFECT OF LIQUID FLOW ON PRIMARY ARM SPACING OF CONSTRAINED COLUMNAR CRYSTALS

An experimental apparatus,which has a convection generator and an aid-heater,is developedfor the study of the effect of stable laminar liquid flow on the directional solidification processby the use of transparent alloy SCN-2wt-% Ace.The flow is perpendicular to primary arms.By in-situ observation and photographing at different specific moments,it has been found thatsuch a flow can cause a great change in primary spacings of constrained columnar crystals:forcells,the spacings become smaller;but for dendrites,they become larger.The former is mainlydue to the tilted growth of upstreamside branches,while the latter is mainly due to the coup-ling effect of liquid flow with solutal field around dendrite tips.The faster the liquid flows,thefurther smaller the cell spacing and the further larger the dendrite spacing.     

抽拉速率跃迁对定向凝固单晶高温合金DD3一次枝晶间距和微观偏析的影响

采用液态金属冷却高温度梯度定向凝固设备.研究了抽拉速率跃迁对单晶高温合金DD3一次枝晶间距和微观偏析的影响.结果表明,单晶高温合金枝晶一次间距表现出明显的历史相关性.当抽拉速率(或生长速率)分别从600和300μm/s跃迁到100μm/s和直接以100μm/s速率生长时,一次间距分别为56.5,86和111.5μm.而生长速率分别从50和100μm/s跃迁到300μm/s和直接以300μm/s速率生长获得的一次间距分别为109,93和70μm.一次间距值与Hunt-Lu模型预测的基本吻合,但实验结果证明其上限与下限的比值λ_1~(max)/λ_1~(min)>2,与Ma Sahm的预测结果一致.成分分析表明,相同稳态条件下枝晶的一次间距变小.偏析也有所降低.     

Microstructural evolution and microsegregation in directional solidification of hypoeutectic Al−Cu alloy: A comparison between experimental data and numerical results obtained via phase-field model

The objective of the work is focused on predictions of microsegregation, solidification speed, dendritic arm spacings and dendrite morphology by phase-field model. The numerical results were compared with experimental data. The experimental values for cooling rates and effective partition coefficient were adopted during calculations. The results of microsegregation through phase-field model show excellent agreement with the experimental data. Such excellent agreement is because cooling rates, effective partition coefficient and back-diffusion of solute are considered in the model. For solidification speed, the calculation results show good agreement with the experimental data. Tertiary dendritic arm spacing calculated with phase-field model is compared with experimental data. The results show good agreement between them. The dendrite arm spacing varies with position because high cooling rates are responsible for the refinement effect on microstructure. Finally, two-dimensional simulation produced a dendrite that is similar to that found in the experiment.     

Thermal Modeling of Spot Welds

Solidification structure and austenite decomposition in some spot welded 0.05% C-0.3% Mn steels have been studied with the aid of a numerical thermal model. The model is a one dimensional finite difference formulation, and accounts for melting, temperature dependent thermal properties, effective heat transfer in the liquid, and contact resistances. The model was used to predict solidification conditions during resolidification of the nugget. These solidification conditions were correlated with the primary dendrite spacing using available primary dendrite spacings modeling. The model was also used to predict cooling rates in the temperature regime for austenite decomposition. These cooling rates were used to interpret the underlying microstructure.     

电子束焊热作用对DD407单晶熔凝区裂纹形成的影响

使用电子束焊对DD407单晶高温合金进行焊接,采用光学显微镜和扫描电镜观察分析了熔凝区裂纹形态及分布特征,采用EDS测定了试样枝晶干与枝晶间区域的化学成分,计算了元素的偏析比并分析了裂纹形成原因。结果表明：在电子束热作用下,DD407单晶熔凝区中主要产生纵向裂纹、横向裂纹、弧坑裂纹、晶间裂纹,当其它工艺参数不变时,熔凝区凝固裂纹含量随着电子束流的增大而先增大后减小;DD407单晶在焊接凝固过程中,杂晶区合金元素偏析严重,导致低熔点共晶组织聚集并形成低熔点液态薄膜,在凝固收缩拉应力作用下,使得杂晶区常形成凝固裂纹。     

熔体过热对AISI 304不锈钢亚快速凝固薄带组织的影响

采用水冷铜模薄带铸造方法研究了熔体过热对AISI 304不锈钢亚快速凝固薄带组织的影响.结果表明:AISI 304不锈钢亚快速凝固薄带由外层的胞状奥氏体组织、次外层的柱状铁素体枝晶组织和中心的等轴铁素体枝晶组织组成;随着熔体过热度增加,奥氏体胞晶间距和柱状铁素体二次枝晶间距随之增加,残余铁素体含量亦降低.过热度的增加降低了熔体过冷度和冷却速率,造成薄带凝固组织中枝晶间距的增加和残余铁素体含量的降低.