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。     

铝合金铸造中冷铁作用效果的研究

通过试验与数值模拟相结合的方法,研究了冷铁厚度对T型铝铸件缺陷位置的影响,并根据T型铝铸件不同点处温度的变化来研究冷铁的有效作用距离.结果表明,当冷铁厚度增大到30 mm后,缺陷位置和冷铁的有效作用距离不再随着冷铁厚度的增加而改变.该研究对铝铸件的凝固过程中冷铁的应用具有参考作用.     

Microstructure–bifilm interaction and its relation with mechanical properties in A356

Cooling conditions and inclusions are of the most important factors that affect mechanical properties of cast aluminium alloys. This study investigated the effect of secondary dendrite arm spacing (SDAS) on the shape of pores (i.e. bifilm unravelling) and the mechanical properties of cast A356 alloy. Different cooling conditions were established by electrolytic copper chill, H13 steel chill and insulated ceramic that was placed in the mould cavity. SDAS and shape of pores were investigated by optical microscopy. The fracture surfaces of tensile test samples were analysed by SEM and EDX. Weibull two-parameter statistical method was used to assess the tensile properties. Results show that mechanical properties were dominantly affected by pore morphology that was formed by bifilms. Increasing the cooling rate (i.e. decreasing SDAS), the unravelling of bifilms delayed which decreased porosity formation significantly.     

Effects of Cooling Rate on High-temperature Mechanical Properties of A356 Alloys

High temperature mechanical properties of A356 alloy castings under different solidification cooling rates have been studied and the influence of cooling rates on secondary dendrite arm spacing (SDAS) and mechanical properties has been discussed. To get different cooling rates, three different types of mold—green sand, green sand with chill and permanent mold, were used to pour castings which would subsequently be machined into tensile test and metallographic specimens. The temperature curves of castings’ solidification in three different mold were recorded using thermal couples, which would be used to calculate their corresponding cooling rates. Tensile tests were carried out at 20, 200, 300, 400 and 500 ℃ and then mechanical properties, such as tensile strength, yield strength, elongation, of specimens from different mold types at different test temperatures were obtained. And SDAS of different specimens were measured using optical metallographic photos. From integrated analysis of all those results, following conclusions could be reached. The relationship between SDAS and cooling rates is negative, and the quantitative relationship has been obtained through data fitting analyzing. Generally speaking, tensile strength and yield strength decease as the temperature elevates while elongation behaves in the contrary trend. Through the regression analysis of SDAS, mechanical properties and temperature, the relationship among them is obtained, which makes quantitative prediction of A356 alloy’s mechanical properties at different temperatures with different solidification cooling rates be possible.     

Solidification structure simulation and casting process optimization of GCr15 bloom alloy

Based on the solidification heat transfer model and CAFémodel,the solidification structure of GCr15 bloom alloy was studied.Using nail shooting and acid etching experiments,the solidification models were verified.The secondary dendrite arm spacing(SDAS)model of GCr15 was obtained by simulation calculation and metallographic observation.With the increase of casting speed,the SDAS,equiaxed crystal ratio(ECR)and average grain size increase.With the rise of superheat,the SDAS increases in the 20-70 mm of thickness and decreases in the 80-160 mm of thickness.The ECR decreases and the average grain size increases with the increase of superheat.With the increase of specific water flow,both the SDAS and ECR decrease.The minimum average grain size is obtained when the specific water flow is 0.20 L·kg^-1.The central carbon segregation index is reduced from 1.11 to 1.075.     

Effect of melt convection on the secondary dendritic arm spacing in peritectic Nd–Fe–B alloy

Dendrites are one of the major microstructural constituents of peritectic alloys. In the present work, the effect of melt convection on the secondary dendritic arm spacing (SDAS) and volume fraction of properitectic α-Fe was investigated during solidification of stoichiometric Nd–Fe–B alloys using the forced crucible rotation technique. The resulting microstructure of the alloy in consideration of melt convection has been investigated using scanning electron microscopy and optical microscopy. The average SDAS was determined for each sample from the whole cross-section of the cylindrical test samples using image analyzing software LEICA QWIN. A detailed statistical analysis of the spacing distribution was performed on the basis of the variation of SDAS values, averaged from about 80 to 120 dendrites in different zones. The α-Fe volume fraction, measured by vibrating sample magnetometer (VSM), reduces with increasing crucible rotation frequency. Similarly, the SDAS values decrease with increasing rotation frequency. These results are explained from the viewpoint of a reduced melt convection state under steady forced crucible rotation leading to a reduced effective mass transfer coefficient.     

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。     

Improving chill control in iron powder treated slightly hypereutectic grey cast irons

Recent studies revealed that in eutectic to slightly hypereutectic grey irons (CE = 4.3%-4.5%) the presence of austenite dendrites provides an opportunity to improve the cast iron properties, as a high number of eutectic cells are "reinforced" by austenite dendrites. An iron powder addition proved to be important by promoting dendritic austenite in hypereutectic irons, but was accompanied by adverse effect on the characteristics of potential nuclei for graphite. The purpose of the present paper is to investigate the solidification pattern of these irons. Chill wedges with different cooling moduli (CM = 0.11 - 0.43 cm) were poured in resin bonded sand and metal moulds.Relative clear / mottled / total chill measurement criteria were applied. Iron powder additions led to a higher chill tendency, while single inoculation showed the strongest graphitizing effect. The various double treatments show an intermediate position, but the inoculant added after iron powder appears to be the most effective in reducing base iron chill tendency, for all cooling moduli and chill evaluation parameters. This performance reflects the improved properties of (Mn,X)S polygonal compounds as nucleation sites for graphite, especially in resin bonded sand mould castings. Both austenite and graphite nucleation benefit from a double addition of iron powder + inoculant,with positive effect on the final structure and chill tendency.     

Relation between SDAS and eutectic cell size in grey iron

Abstract

The solidification of cast components is a complex and important process as this is the moment when the final properties are established. For hypoeutectic grey iron, solidification starts with nucleation and growth of the primary austenite followed by the eutectic reaction forming eutectic cells. In this work, the microstructure and significance of the different constituents formed during solidification has been examined. It was found that the size of the eutectic cells is a function of secondary dendrite arm spacing (SDAS). The SDAS, on the other hand, was found to depend on the solidification time and hence the growth rate of the dendrites. The effect of chemical composition on SDAS and eutectic cell size was found to depend on cooling rate. It is suggested that the relationship between the eutectic cells and dendrite arm spacing is based on segregation effects and the nucleating capacity of the melt.     

Effect of Cooling Rate on Microstructure and Mechanical Properties of Sand-Casted Al-5.0Mg-0.6Mn-0.25Ce Alloy

This study examines the relationship among cooling rate, microstructure and mechanical properties of a sand-casted Al-5.0Mg-0.6Mn-0.25Ce (wt%) alloy subjected to T4 heat treatment (430 °C × 12 h + natural aging for 5 days), and the tested alloys with wall thickness varying from 5 to 50 mm were prepared. The results show that as the cooling rate increases from 0.22 to 7.65 K/s, the average secondary dendritic arm spacing (SDAS, λ₂) decreases from 94.8 to 27.3 μm. The relation between SDAS and cooling rate can be expressed by an equation: $\lambda_{2} = 53.0R_{\text{c}}^{ - 0.345}$. Additionally, an increase in cooling rate was shown not only to reduce the amount of the secondary phases, but also to promote the transition from Al₁₀Mn₂Ce to α-Al₂₄(Mn,Fe)₆Si₂ phase. Tensile tests show that as the cooling rate increases from 0.22 to 7.65 K/s, the ultimate tensile strength (UTS) increases from 146.3 to 241.0 MPa and the elongation (EL) increases sharply from 4.4 to 12.2% for the as-cast alloys. Relations of UTS and EL with SDAS were determined, and both the UTS and EL increase linearly with (1/λ₂)^0.5 and that these changes can be explained by strengthening mechanisms. Most eutectic Al₃Mg₂ phases were dissolved during T4 treatment, which in turn further improve the YS, UTS and EL. However, the increment percent of YS, UTS and EL is affected by the cooling rate.     

靶基距对C/Cr复合镀层厚度影响的研究*

以四靶闭合场非平衡磁控溅射方法制备C／Cr镀层，用球痕法测量镀层厚度，研究了镀层厚度及均匀性随靶基距的变化规律，并探讨了靶基距影响镀层厚度的机理。结果表明：靶基距对镀层厚度有明显影响；镀层厚度随靶幕径向距增大而显著减小，随靶基轴向距的增加变化不大；试样放置位置越靠近真空室中心区域，镀层厚度均匀性越好；磁场空间分布和粒子迁移中的散射碰撞是导致靶基距影响镀层厚度的主要原因。     

Effect of casting speed on surface quality of horizontal direct chill casting 7075 aluminum alloy ingot

7075 aluminum alloy ingot with the diameter of 100 mm has been produced by hor-izontal direct chill casting in different casting speed. The effect of the casting speed on the ingot surface and subsurface layer was studied by surface observation and subsurface structure analysis. It was found that increasing the casting speed results in the adding of segregation knots in the ingot surface. The thickness of the dendrite microstructure layer in the subsurface reduces with increasing the casting speed. And the ...     

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。     

Heat transfer and solidification analysis in the mould region of a horizontal copper alloy billet continuous caster

Abstract

Mathematical modelling has been widely used as a powerful tool for process design and optimisation of the continuous casting process. A three-dimensional heat transfer model was developed to simulate heat transfer and solidification in a horizontal billet continuous casting system. In this model, the air gap formation and its effect on heat extraction from the billet was also modelled and considered. The developed model was run to simulate the heat transfer and solidification for an industrial billet casting machine. The predicted temperature distribution within the mould and billet was compared with those measured on an industrial caster and good agreement was obtained. Parametric studies were carried out to evaluate the effects of different parameters on the temperature distribution and solidification profile within the cast brass billet. Finally, the secondary dendrite arm spacing (SDAS) was determined experimentally and a semi-empirical correlation between measured SDAS and corresponding calculated cooling rate was proposed for continuously cast brass billet.     

高温合金单晶叶片定向凝固过程的宏微观数值模拟

基于有限元和Panda热动力学数据库建立了单晶叶片真空熔模铸造定向凝固过程的数理模型,对不同工艺下单晶叶片试样凝固过程中的温度场、糊状区演变及枝晶二次臂间距进行了仿真,研究了缺陷形成机理和规律。计算结果与实验吻合良好。计算结果显示,拉速大时二次臂细小,但杂晶产生的趋势加大;拉速小时杂晶不易形成,但二次臂增粗。对实际空心薄壁复杂单晶叶片定向凝固过程的模拟研究表明,二次臂间距在叶身部分分布比较均一, 3.5 mm/min抽拉时有可能在缘板处产生杂晶。采用变拉速工艺,不仅可避免杂晶缺陷,还能保证工件大部分枝晶细小,提高生产效率和成品率     

Influences of Y and Y-Rich Mischmetal Additions on Microstructure and Compressive Properties of As-Cast Al-Mg-Mn Alloy

The influences of Y and Y-rich mischmetal (Ym) additions on microstructural and compressive properties of as-cast Al-13Mg-0.8Mn alloy prepared by vacuum suction casting were investigated in this study. The average secondary dendrite arm spacing (SDAS) was decreased when adding Y and Ym additions. Moreover, the Al₂Y and Al₂Ym phases formed during the solidification were mainly distributed along the grain boundary. The mechanical results reveal that both Y and Ym additions are effective in increasing the compressive strength and hardness. The values of yield compressive strength, ultimate compressive strength, and Brinell hardness of the as-cast Al-13Mg-0.8Mn-0.8Y alloy are 357 MPa, 510 MPa, and 138, respectively. The improved mechanical properties are mainly attributed to fine SDAS and precipitation strengthening. A typical cleavage fracture mode is observed on the compressive fracture surfaces of the alloys.     

Casting AI sheets on a lab-scale,single-roll,continuous sheet caster

The research described here deals with the design and fabrication of a laboratory-scale single- roll continuous caster and then the casting of aluminum sheet under various operating conditions. The results indicate that the sheet thickness decreases with increasing rotational speed and melt superheat but increases with increasing stand-off distance and metal head in the tundish. The tundish opening size and the cooling conditions at the inner surface have only a marginal effect on the thickness. After the initial stages of solidification, the sheet thickness varies more or less linearly with the square root of the solidification time.     

重熔电流对ERNiCrFe-13镍基合金焊丝熔敷金属组织的影响

采用光学显微镜(OM)、扫描电镜及能谱仪(SEM,EDS)、JMatPro软件、Image-Pro Plus软件等手段研究了重熔电流对ERNiCrFe-13镍基合金焊丝熔敷金属不填丝重熔焊点组织的影响规律. 结果表明,随着重熔电流的增加,熔宽、熔深、二次枝晶臂间距(secondary dendrite arm spacing,SDAS)显著增加,符合关系λ₂ = 0.682 I 1/3,相关系数R2 = 0.95. 枝晶间γ/Laves共晶组织平均面积、最大面积尺寸随着重熔电流的增加而增大. 伴随着枝晶粗大,枝晶间偏析区域增大,共晶组织数量减少. 重熔电流对γ/Laves共晶组织的成分无显著影响. 随着重熔电流的增加,在γ/Laves共晶组织尺寸增大、应变加大的共同作用下焊点中结晶裂纹开裂加剧.     

Effect of chill cooling conditions on cooling rate,microstructure and casting/chill interfacial heat transfer coefficient for sand cast A319 alloy

A combination of experiments and numerical analyses were used to examine the cooling conditions, solidification microstructure and interfacial heat transfer in A319 cast in a chilled wedge format. Both solid copper chills and water cooled chills, with and without a delay in water cooling, were examined in the study. Various chill preheats were also included. The goal of the investigation is to explore methods of limiting heat transfer during solidification directly beside the chill and increasing cooling rates during solidification away from the chill. Within the range of conditions examined in the study, chill preheat was found to have only a small effect on cooling rates between 5 and 50 mm from the chill/casting interface, pour superheat a moderate effect and water cooling a significant effect. In comparison to the results for the solid chill, the solidification time at 5 mm with water cooling applied at the beginning of mould filling is reduced from 56 to 15 s and at 50 mm from 588 to 93·5 s. Furthermore, the average cooling rate during solidification is increased from 1·9 to 7·06°C s^?1 at 5 mm and from 0·18 to 1·13°C s^?1 at 50 mm. At 50 mm, for example, the increased cooling rate achieved with water translates into a reduction in secondary dendrite arm spacing from 40 to 25 μm or ～40%. Delaying the water cooling by 10 s facilitated slow cooling rates at 5 mm (similar to those achieved with a solid chill) and high cooling rates 50 mm from the chill. A temperature based correlation was found to be suitable for characterising the behaviour of the interfacial heat transfer coefficient in the solid shill castings, whereas a time based correlation was needed for the water cooled castings.