Study of shrinkage porosity in spheroidal graphite cast iron

This study aims at identifying the relationship between the shrinkage cavities and the solidification structure in spheroidal graphite cast iron. Cast samples specially designed to contain shrinkage cavities were used. The solidification macrostructure was revealed using the Direct Austempering After Solidification method, while the solidification microstructure was revealed by using colour etching. At the midsection of the pieces, the shrinkage cavities and the solidification structure were observed jointly. The study showed that the classification of shrinkage porosity found in literature does not correspond to the ductile iron solidification model recognized by most of the scientific community. Early solidification models, and therefore shrinkage formation mechanisms, were proposed in instances when there was not a thorough knowledge of the morphology of the solid phases during solidification. Nowadays, defects formation mechanisms can be described with higher accuracy. Therefore, an updated classification of shrinkage porosity for spheroidal graphite iron is proposed.     

影响薄壁球铁件缩孔缩松形成的几个因素

叙述化学成分，孕育工艺，铁液温度及浇注系统等因素对薄壁球铁铸件缩孔和缩孔形成的影响，对引起缩孔，缩松形成的部分原因进行了分析，并提出相应的防止措施，包括：确保补缩通道畅通，充分的孕育，较高的浇注温度。     

铸钢件缩孔缩松预测的试验研究

综合冶金和工艺的影响因素对铸钢件缩孔缩松进行了试验研究，确定了判据的临界值。用等效液面收缩量法模拟一次、二次缩孔的形成，用方法预测缩松的形成。结果表明，综合采用等效液面收缩量法和方法，可以准确地预测铸钢件内部缩孔缩松的形状、大小和分布范围。     

Prediction of Hot Tearing Using a Dimensionless Niyama Criterion

The dimensionless form of the well-known Niyama criterion is extended to include the effect of applied strain. Under applied tensile strain, the pressure drop in the mushy zone is enhanced and pores grow beyond typical shrinkage porosity without deformation. This porosity growth can be expected to align perpendicular to the applied strain and to contribute to hot tearing. A model to capture this coupled effect of solidification shrinkage and applied strain on the mushy zone is derived. The dimensionless Niyama criterion can be used to determine the critical liquid fraction value below which porosity forms. This critical value is a function of alloy properties, solidification conditions, and strain rate. Once a dimensionless Niyama criterion value is obtained from thermal and mechanical simulation results, the corresponding shrinkage and deformation pore volume fractions can be calculated. The novelty of the proposed method lies in using the critical liquid fraction at the critical pressure drop within the mushy zone to determine the onset of hot tearing. The magnitude of pore growth due to shrinkage and deformation is plotted as a function of the dimensionless Niyama criterion for an Al-Cu alloy as an example. Furthermore, a typical hot tear “lambda”-shaped curve showing deformation pore volume as a function of alloy content is produced for two Niyama criterion values.     

Influence of random shrinkage porosity on equivalent elastic modulus of casting: A statistical and numerical approach

Shrinkage porosity is a type of random distribution defects and exists in most large castings.Different from the periodic symmetry defects or certain distribution defects,shrinkage porosity presents a random "cloud-like" configuration,which brings difficulties in quantifying the effective performance of defected casting.In this paper,the influences of random shrinkage porosity on the equivalent elastic modulus of QT400-18 casting were studied by a numerical statistics approach.An improved random algorithm was applied into the lattice model to simulate the "cloud-like" morphology of shrinkage porosity.Then,a large number of numerical samples containing random levels of shrinkage were generated by the proposed algorithm.The stress concentration factor and equivalent elastic modulus of these numerical samples were calculated.Based on a statistical approach,the effects of shrinkage porosity's distribution characteristics,such as area fraction,shape,and relative location on the casting's equivalent mechanical properties were discussed respectively.It is shown that the approach with randomly distributed defects has better predictive capabilities than traditional methods.The following conclusions can be drawn from the statistical simulations:(1) the effective modulus decreases remarkably if the shrinkage porosity percent is greater than 1.5％;(2) the average Stress Concentration Factor (SCF) produced by shrinkage porosity is about 2.0;(3) the defect's length across the loading direction plays a more important role in the effective modulus than the length along the loading direction;(4) the surface defect perpendicular to loading direction reduces the mean modulus about 1.5％ more than a defect of other position.     

Shrinkage porosity and its relation to solidification structure of grey cast iron parts

Abstract

The purpose of this work was to investigate the relation between shrinkage porosity and the macrostructure. Based on results from an earlier investigation regarding shrinkage porosity and its characteristic features, a test casting was developed. The macrostructure was preserved using direct austempering after solidification heat treatment. Shrinkage porosity was found in regions where it was designed to occur. It was also found that this type of porosity is connected to the atmosphere via defects located on the surface, confirming earlier findings. Beneath the surface, it extends as a three-dimensional network through the casting. The shrinkage porosity was, in some cases, found along the boundaries separating primary crystals, and in some cases, it encircled separate primary crystals. A shrinkage porosity formation mechanism is proposed based on these observations, discussing the importance of a rigid columnar zone, movement of a local thermal centre and how pressure differences developing during solidification influence the formation of shrinkage porosity.     

应用均衡凝固理论和压边浇冒口改进主滚轮的工艺

分析了主滚轮传统铸造工艺的缺点，提出采用均衡凝固理论和压边浇冒口的工艺方法，防止铸件产生缩孔和缩松缺陷，效果显著。     

Modeling on the Solidification of 1J51 Fe-Ni-Based Alloy Ingot Under Vacuum Conditions

In this article, a numerical simulation model on the solidification of 1J51 Fe-Ni based alloy under vacuum conditions was established using ProCAST software (ESI Group, Paris, France). The calculated temperature profiles of the mold and the solidification profile of the ingot were compared with the measurement, showing a reasonable agreement. The validated model was then used to study the effects of the insulation condition, filling rate, and maximum filling fraction on the solidification state and porosity distribution of the ingot. It was indicated that the shrinkage cavity position of ingot with zirconia fiber was lower than that without zirconia fiber, and the likelihood of centerline shrinkage porosity was independent of the insulation method. The position of macroporosity varied little when the filling rate was from 2.17 kg/s to 5.17 kg/s, while serious solidification defects occurred in the body of ingot when the filling rate was lower than 4.17 kg/s. Increasing the maximum filling fraction had a positive impact on the rise of shrinkage cavity position, but it was hardly advisable because the metal yield decreased with the increase of the maximum filling fraction. Considering the above parameters discussed in this article, it was proposed to remove the zirconia fiber layer for the industrial practice.     

真空自耗电弧熔炼TC4铸锭的凝固组织和缩松缩孔的模拟

建立三维轴对称有限元模型,采用移动边界法模拟了真空自耗电弧熔炼( VAR)TC4合金铸锭凝固过程的温度场分布,并基于元胞自动机和有限元耦合法(CAFE法)计算了VAR过程中TC4铸锭的凝固组织、缩松和缩孔的形成.结果表明,模拟结果与试验观察在晶粒结构、柱状晶生长方向,柱状晶-等轴晶转变及缩松和缩孔等方面相符合.     

Influence of shrinkage porosity on fatigue performance of iron castings and life estimation method

Shrinkage porosity exists more or less in heavy castings, and it plays an important role in the fatigue behavior of cast materials. In this study, fatigue tests were carried out on the QT400-18 cast iron specimens containing random degrees of shrinkage porosity defect. Experimental results showed that the order of magnitude of life scattered from 103 to 106 cycles when the shrinkage percentage ranged from 0.67% to 5.91%. SEM analyses were carried out on the shrinkage porosity region. The inter-granular discontinuous, micro cracks and inclusions interfered with the fatigue sliding or hindering process. The slip in shrinkage porosity region was not as orderly as the ordinary continuous medium. The shrinkage porosity area on fracture surface(SPAFS) and alternating stress intensity factor(ASIF) were applied to evaluate the tendency of residual life distribution; their relationship was fitted by negative exponent functions. Based on the intermediate variable of ASIF, a fatigue life prediction model of nodular cast iron containing shrinkage porosity defects was established. The modeling prediction was in agreement with the experimental results.     

挤压铸造ZA27合金大高径比铸件

通常挤压铸造仅适用于高径比小于3.5的铸件,研究大高径比铸件的挤压铸造将扩大这种先进铸造技术的应用范围.用试验和计算机模拟的方法,研究了高径比为7的ZA27合金铸件的力学性能、凝固过程中铸件内的温度、压力以及缩松的分布.结果表明,增加挤压铸造的压力不能有效地减少大高径比铸件的缩松缺陷;合理控制浇注温度和铸型预热温度能有效地消除大高径比铸件内的缩松,获得力学性能高且均匀的铸件.     

出口铸钢件支座的工艺设计及铸造生产

对质量要求高、出口铸钢件140T支座的结构及铸造工艺性进行了分析,针对铸件厚大热节处易出现缩松、非加工表面粗糙度及尺寸精度要求高的特点,提出了采用浇口通过保温冒口、加冒口补贴及保温覆盖剂强化补缩、倾斜浇注等工艺措施,设计了具体工艺参数,实践表明,有效地防止了裂纹和缩松的产生,生产出了合格铸件.     

有关球铁铸件缩孔缩松形成及预防的文献综述

综述评述有关球墨铸铁凝固特性、凝固过程体积变化和缩孔、缩松形成机理的文献资料。笔者还结合本人的工作实践就缩孔、缩松的预防提出了看法。     

Modeling and Experimental Validation of Ductile Iron Castings During Solidification

Ductile irons are still important engineering materials due to their high strength and toughness, and relatively low price. In the foundries, ductile irons suffer from shrinkage porosity formation during solidification, which is detrimental to the mechanical properties. In order to minimize porosity formation, large risers are normally used in the design, which reduces porosity level sometimes but leads to a low yield. In order to better understand the shrinkage behavior of ductile iron during solidification, a micro model was developed to simulate the microstructure formation. The density change during solidification and the room temperature mechanical properties can be calculated based on the microstructure. The simulation has been compared with the experimental results and found to be in good agreement.     

铝合金铸件缩孔缩松预测新技术

在铝合金铸件三维温度场计算的基础上,提出了采用动态多熔池等效液面收缩量法对缩孔进行预测;并综合质量守恒方程、动量方程和氢含量守恒方程,建立了数学模型对缩松进行预测.通过实验验证了数学模型的正确性.     

铸件缩孔缩松数值模拟方法的进一步定量化

通过采用构造插值函数以及对数值模拟过程中局部网格的进一步细化,一方面把修正的等液面收缩量法与网格细化进行结合,有效地提高了缩孔的预测精度,另一方面G/T~(1/2)判据与网格细化的综合运用,一定程度上实现了缩松预测中量的概念,是对铸件缩孔缩松数值模拟方法的有效补充。     

利用数值模拟解决灰铸铁制动盘缩松的问题

运用数值模拟软件对DISA线生产灰铸铁制动盘3种不同工艺方案的铸造过程进行模拟。通过对充型温度场和热节点分布进行对比分析,发现仅仅加冒口或者局部设置冷铁不能够彻底解决缩松问题,需要同时加冒口和局部设置冷铁,局部冷铁的厚度要按照经验公式L/H3来决定,这样可以彻底解决制动盘的缩松问题。利用同时加冒口和冷铁配合的方法生产一批铸件后,经X射线探伤机检测的11件铸件,全部未发现缩松缺陷存在,表明对缩松缺陷的预测是准确的。     

Internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio

Steel ingot with a large height-to-diameter ratio is utilized to produce multiple products by one stock in practice. Water cooling is a usual way to enhance production efficiency. However, the combination of the two factors will generate internal defects, such as shrinkage porosity and hot crack. The characteristic of internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio was examined by an ultrasonic test. A slice was sectioned from the ingot middle part where billets containing star-like crack were further extracted. The billets were examined by X-ray high energy industrial CT, and the compactness was reconstructed in three dimensions. Microstructure near the crack was observed using scanning electron microscopy, and the solidification process and grain size were studied by high temperature confocal microscopy. Moreover, thermomechanical simulation and post-processing were carried out to analyze the formation of shrinkage porosity and hot crack. A new criterion considering mushy zone mechanical behavior in brittle temperature as well as grain size distribution was proposed to evaluate hot cracking potential in the ingot. The results show that a deep shrinkage porosity band easily forms in the center line of such an ingot with a large height-to-diameter ratio, and water-cooling further generates excessive tensile stress tearing the liquid films around the porosities. Then, hot cracks begin to propagate along grain boundaries. The grain size in the upper and center of the ingot is large, which leads to an inverted cone shape defects zone in the ingot center.

     

半固态过共晶高铬铸铁浆料在离心力作用下的凝固组织

对于倾斜冷却体法制备的半固态过共晶高铬铸铁浆料，采用离心铸造法将其制备成环形铸件，并观察和分析该铸件组织。结果表明，首先在半固态组织中初生碳化物出现偏析现象，且存在明显的偏析分界线，分界线位于距离环形铸件径向外侧面约6mm处。其次，在本试验条件下，离心力的作用只能部分消除半固态过共晶高铬铸铁组织中的缩松倾向，而且在组织中也存在缩松出现的分界线，即小于距环形铸件径向外侧面约6mm的径向区域内无缩松产生，大于距环形铸件径向外侧面约6mm的径向区域产生缩松。     

大型风电机架工艺设计及质量控制

介绍了德国DEWIND公司2 MW机架铸件的材料要求.采用侧立分型、高温慢浇等工艺,利用三维Solidworks建模和MAGMA计算机模拟验证了厚大部位与交接处不会出现缩松缺陷,同时验证了冒口的溢流排渣是有效的.实际生产中,通过对型砂强度、型砂可使用时间、抛砂质量、涂刷质量、熔炼质量的控制,机架铸件及附铸试块各项指标均符合客户要求.