冷铁对铝合金铸件二次枝晶间距的影响

研究了冷铁厚度、冷铁的有效作用距离、铸件二次枝晶间距以及铸件的模数之间的关系。结果表明,随着冷铁厚度的增加,试样的二次枝晶间距减小;试样的直径和冷铁的厚度显著影响冷铁的有效作用距离,随着试样直径的增加,必须相应增加冷铁的厚度。根据研究结果,绘制了铸件模数、铸件热节厚度及应加冷铁厚度之间的关系诺模图。还研究了冷铁厚度与局部凝固速度及铸件二次枝晶间距之间的关系,并给出了局部凝固速度及铸件二次枝晶间距之间关系的数学表达式,对于任何模数的A357合金铸件,只要合理控制其局部凝固速度,即可得到需要的二次枝晶间距。     

Influence of dendrite arrangement on coarsening during solidification of high-solute Al alloys

Abstract

Coarsening of dendrite arms during continuous solidification is usually characterised by measuring the secondary dendrite arm spacing (SDAS). Images obtained in-situ from X-ray microscopy studies during solidification were used to study SDAS development. Local coarsening and growth kinetics were studied during the solidification of high-solute content aluminium alloys (i.e. Al–30 Cu and Al–20 Cu (wt-%)). Downward and upward solidification conditions were imposed on the sample alloys in order to study the effect of those on coarsening and growth kinetics. The dendritic arrangement, direction of growth and growth fluctuations influence solute-rich liquid distribution which in turn affects solute gradients changing undercooling and thus coarsening and growth kinetics.     

金属型重力铸造铝合金缸盖关键部位二次枝晶间距研究

文章重点分析了同一生产条件下同一发动机缸盖的不同关键位置的二次枝晶间距值,其结果表明,缸盖的金属型铸造工艺是符合发动机性能设计要求的,探讨了二次枝晶间距与铸造工艺、产品性能设计要求之间的关系。分析了整个缸盖毛坯铸件的凝固顺序,从而推断铸造工艺的合理性。     

铸件模数Mc对A357合金二次枝晶臂间距及致密度的影响

研究试样直径和铸件模数Mc与铸件二次枝晶臂间(SDAS)的关系，通过测试A357合金不同凝固条件下铸件不同部位的SDAS，得出砂型铸造凝固条件下SDAS与铸件模数M之间的关系式。研究了铸件模数等对铸件致密度的影响，确定了获得高致密度铸件的合理的凝固条件。     

铝合金凝固过程中传热系数的数值求解及二次枝晶间距预测

以A357合金为研究对象,利用反运算,即试验温度曲线与模拟冷却曲线有限元对比耦合的方法,计算了在空冷和水冷条件下铸件/铸型间传热系数与冷却时间的关系,获得了精确的边界条件。在此基础上,对合金凝固过程中的冷却行为以及二次枝晶间距的分布进行模拟,并与试验结果进行对比。结果表明二者相符合。     

The correlation between dendritic microstructure and mechanical properties of directionally solidified hypoeutectic Al-Ni alloys

Al-Ni hypoeutectic alloys were directionally solidified under upward transient heat flow conditions. The aim of the present study is to set up correlations between the as-cast microstructure and the resulting mechanical properties of these alloys. The dependence of primary and secondary dendrite arm spacing on the alloy solute content and on solidification thermal parameters is also analyzed. The results include transient metal/mold heat transfer coefficient, tip growth rate, cooling rate, dendrite arm spacing, ultimate tensile strength, yield tensile strength and elongation. Expressions relating dendrite spacing to solidification thermal parameters and mechanical properties to the scale of the dendritic microstructure have been determined. It was found that the ultimate tensile strength and the yield tensile strength increase with increasing alloy solute content and with decreasing primary and secondary dendrite arm spacing. In contrast, the elongation was found to be independent of both alloy composition and dendritic arrangement.     

EFFECT OF Ce ON SOLIDIFICATION STRUCTURE OF LOW SULPHUR 16 Mn STEEL

采用定向凝固工艺考察了Ce对低硫16Mn钢一次结晶的影响.结果表明,Ce作为溶质元素的合金化作用,使胞晶间距增大,二次枝晶臂间距变小,柱状晶区缩短,等轴晶粒细化,并使Mn的显微偏析减少.采用此工艺可以排除晶体固一液界面附近的夹杂物对凝固形貌的影响.     

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.     

Simulation of microstructural evolution in directional solidification of Ti-45at.%Al alloy using cellular automaton method

The microstructural evolution of Ti-45 at.%Al alloy during directional solidification was simulated by applying a solute diffusion controlled solidification model.The obtained results have shown that under high thermal gradients the stable primary spacing can be adjusted via branching or competitive growth.For dendritic structures formed under a high thermal gradient,the secondary dendrite arms are developed not very well in many cases due to the branching mechanism under a constrained dendritic growth cond...     

二次枝晶臂间距的研究

研究枝晶二次臂间距对于提高铸件力学性能有重要意义.本文中概述了3种二次枝晶臂间距理论模型,介绍了对Cu-Zn合金二次臂间距的实验结果.针对Cu-Zn合金,对理论模型的计算结果与实验结果进行了比较.结果表明,在冷却速度为0.06～130K/S、局部凝固时间为0.35～750S时,3种模型的计算结果与实验结果的相对误差平均值分别是11.3%、10.5%和11.5%.     

Heat transfer and solidification microstructure evolution of continuously cast steel by non-steady physical simulation

The heat transfer and solidification microstructure evolution during continuous casting were experimentally studied in this work. A new approach to physically simulate the steel solidification behavior during continuous casting was developed. Six steel grades with different solidification mode were introduced to elucidate the carbon equivalent dependent mold heat flux, prior austenite grain size and secondary dendrite arm spacing. It is found that the non-steady mold heat fluxes in the experiment against time for all steel grades are comparative to that versus distance in practical continuous casting. Due to the occurrence of L→L+δ→δ+γ→γ transformation with the largest amount of volume contraction in hypo-peritectic steel, it shows the lowest mold heat flux among these six steel grades. It is also demonstrated from the solidification microstructure results that the prior austenite grain size and secondary dendrite arm spacing in the physical simulation are in good agreement with those in continuously cast strand. In addition, the steel with a higher temperature for the onset of δ→γ transformation reveals the larger prior austenite grains resulted from the higher grains growth rate in the post solidification process.     

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.     

Ti-Al合金定向凝固过程中胞/枝晶间距选择的计算机模拟

利用溶质扩散控制模型对Ti-Al合金定向凝固初始阶段变速冷却过程中胞/枝晶转变过程进行了数值模拟。在给定的冷却速率下,枝晶臂间距大于胞晶臂间距,而在过渡区,枝晶间距达到最大。另外,模拟结果也显示,晶核数量对柱状晶间距产生影响,随着植入晶核数量的增加,柱状晶间距非均匀化程度明显减小。出现过渡区的原因与枝晶生长所引起固/液界面前沿成分波动有关。模拟与试验结果吻合较好。     

冷却速度对铸件二次枝晶臂间距影响的模拟研究

设计了45#钢阶梯凝固试验并进行了有限元分析,研究了不同冷却速度对二次枝晶臂间距尺寸的影响。根据模拟结果拟合了冷却速度与二次枝晶臂间距经验公式,为判断铸件冷却温度、建立高温扩散退火预判机制、降低铸件偏析和避免裂纹的产生提供理论依据。     

A three-dimensional cellular automaton model for dendritic growth in multi-component alloys

A three-dimensional (3-D) cellular automaton model for dendritic growth in multi-component alloys is developed. The velocity of advance of the solid/liquid (S/L) interface is calculated using the solute conservation relationship at the S/L interface. The effect of interactions between the alloying elements on the diffusion coefficient of solutes in the solid and liquid phases are considered. The model is first validated by comparing with the theoretical predictions for binary and ternary alloys, and then applied to simulate the solidification process of Al-Cu-Mg alloys by a coupling of thermodynamic and kinetic calculations. The numerical results obtained show both the free dendrite growth process as well as the directional solidification process. The calculated secondary dendrite arm spacing in the directionally solidified Al-Cu-Mg alloy is in good agreement with the experimental results. The effect of interactions between the various alloying elements on dendritic growth is discussed.     

Simulation of microstructure evolution in directional solidification of Ti-45at.%Al alloy using cellular automaton method

The microstructural evolution of Ti-45 at.%Al alloy during directional solidification was simulated by applying a solute diffusion controlled solidification model. The obtained results have shown that under high thermal gradients the stable primary spacing can be adjusted via branching or competitive growth. For dendritic structures formed under a high thermal gradient, the secondary dendrite arms are developed not very well in many cases due to the branching mechanism under a constrained dendritic growth condition. Furthermore, it has been observed that, with increasing pulling velocity, there exists a cell/dendrite transition region consisting of cells and dendrites,which varies with the thermal gradient in a contradicting way, i.e. increase of the thermal gradient leading to the decrease of the range of the transition region. The simulations agree reasonably well with experiment results.     

In situ and real-time 3-D microtomography investigation of dendritic solidification in an Al–10 wt.% Cu alloy

The microstructural evolution of an Al–10 wt.% Cu alloy was investigated during solidification at constant cooling rate by in situ synchrotron X-ray microtomography with a resolution of 2.8 μm. Solidification of this alloy leads to a coarse dendritic microstructure which was fully characterized in terms of variation with temperature of the solid fraction, the specific surface area of the solid–liquid interface and the local curvatures of the solid phase. By analysing the evolution with solid fraction of individual dendrites, at least two coarsening mechanisms were clearly identified in addition to solidification growth. The first mechanism involves remelting of small secondary dendrite arms to the benefit of bigger adjacent arms. The second is the coalescence of adjacent secondary arms, with progressive filling of the inter-arm spacing and coalescence at the tips. Although this mechanism preferentially occurs at high solid fractions, these results show that the evolution of the dendritic microstructure during solidification is complex and involves the occurrence of various mechanisms operating concurrently. In situ X-ray tomography thus allows revisiting the various models which have been proposed to account for dendrite coarsening during solidification.     

Effect of sample diameter on primary dendrite spacing of directionally solidified Al-4%Cu alloy

The relationship between primary dendrite arm spacing and sample diameter was studied during directional solidification for Al-4%Cu (mass fraction) alloy. It is shown that primary dendrite spacing is decreased with the decrease of the sample diameter at given growth rate. By regressing the relationship between primary dendrite arm spacing and the growth rate, the primary dendrite arm spacing complies with 461.76v-0.53, 417.92v-0.28 and 415.83v-0.25 for the sample diameter of 1.8, 3.5 and 7.2 mm, respectively. The primary dendrite spacing, growth rate and thermal gradient for different sample diameters comply with 28.77v-0.35G-0.70, 23.17v-0.35G-0.70 and 23.84v-0.35G-0.70, respectively. They are all consistent with the theoretical model , and b1/a1=2. By analyzing the experimental results with classical models, it is shown that KURZ-FISHER model fits for the primary dendrite spacing in smaller sample diameters with weaker thermosolute convection. Whereas TRIVEDI model is suitable for describing primary dendrite arm spacing with a larger diameter (d＞2 mm) where convection should be considered.     

Computer Simulation of Microstructural Evolution in Columnar Castings

Abstract

A computer model has been developed which makes possible a prediction of the as-cast microstructure in unidirectional columnar castings. The primary and secondary dendrite arm spacings and dimensions, concentration profiles across the secondary dendrite arms, and the eutectic fraction can be predicted in every location of the casting, given the pouring temperature and the heat transfer conditions prevailing in the mould. The numerical results are presented for aluminium-4.5 copper alloy. These are shown to agree very well with previous experimental observations. The model is amenable to extensions which allow further predictions demonstrating equiaxed solidification ahead of the columnar front, columnar to equiaxed transition, and fluid flow through the mushy zone.