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.

Prediction and improvement of shrinkage porosity in TiAl based alloy

The present research has developed a novel investment casting process for ingot production of TiAl alloys through forming a small vertical temperature gradient on the mold.The advantage of this process is to guarantee that the castings solidify sequentially from bottom to top.The analysis of numerical simulation and experimental results showed that the shrinkage porosity of Ti-47Al-2Cr-2Nb alloy was significantly improved by forming a vertical temperature gradient of 3 oC/mm on the mold,while the increase of pouring temperature and pressure on the molten alloys had no apparent effect on the reduction of shrinkage porosity.The critical value of the Niyama criterion that can reliably predict the shrinkage porosity in Ti-47Al-2Cr-2Nb alloy was identified by the comparison of experimental and simulated results.

Effect of shrinkage porosity on mechanical properties of ferritic ductile iron

Casting defects could largely affect the mechanical properties of casting products. A number of test pieces made of ductile iron (EN-GJS-400-18-LT) with different levels of shrinkage porosity were prepared and then tensile and fatigue tests were performed to investigate the impact of shrinkage porosity on their mechanical properties. The results showed that the tensile strength decreases linearly with increasing of the shrinkage porosity. The tensile elongation decreases sharply with the increase of the shrinkage porosity mainly due to the non-uniform plastic deformation. The fatigue life also dramatically declines with increasing of the porosity and follows a power law relationship with the area percentage of porosity. The existence of the shrinkage porosity made the fatigue fracture complex. The shrinkage pores, especially those close to the surface usually became the crack initiation sites. For test pieces with less porosity, the fatigue fracture was clearly composed of crack initiation, propagation, and overloading. While for samples with high level of porosity, multiple crack initiation sites were observed.

To develop a quantitative method for predicting shrinkage porosity in squeeze casting

In order to secure high strength and high elongation of suspension parts,it is critical to predict shrinkage porosity quantitatively.A new simulation method for quantitative prediction of shrinkage porosity when replenishing molten metal has been proposed for squeeze casting process.To examine the accuracy of the calculation model,the proposed method was applied to a plate model.

Influence of pressure on shrinkage porosity prediction

Most of the shrinkage porosity criteria only consider internal thermal conditions, but ignore external conditions, such as pressure. Therefore, to some castings with a relatively high molten metal hydrostatic pressure or atmosphere pressure, the shrinkage porosity prediction will not be accurate enough. In order to solve this problem, this work studies and improves a known shrinkage porosity criterion ‘Xue Criterion’. The pressure is introduced into the criterion function and a relationship between ‘Xue Criterion’ and the pressure is established. The modified ‘Xue Criterion’ function can be expressed as p0Gτ/τ. Compared with the previous experimental work, the prediction result from the new shrinkage criterion has a good agreement with the experimental result, and it is more accurate than the original ‘Xue Criterion’.

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.

Reduction of shrinkage porosities in aluminum alloy castings by external pressure fluctuation under gravity field

A novel method to improve the feeding capacity of ZL205 A alloy castings by pressure fluctuation during its solidification process under gravity field was proposed. The experiments were done in the graphite mould by applying the fluctuation pressure at the top of the riser. Results of the X-ray inspection of castings and simulations of flow velocity of alloy melt and temperature field show that the new method can effectively improve the feeding capacity and significantly reduce the shrinkage in the castings. The mechanism of improving the feeding capacity by pressure fluctuation is that the fluctuating pressure can make the ZL205 A alloy melt form a vibration wave in the melt. The vibration wave can intensify the convection of the melt, and therefore, break down the barrage from the feeding channel which forms during the solidification process of the ZL205 A alloy, improving the feeding capacity.

THEORETICAL DERIVATION OF CRITERION G／FOR PREDICTION OF SHRINKAGF CAVITIES AND POROSITIES IN CASTINGS

The criterion for shrinkage cavity and porosity prediction of casting, G/T<K,is an empirical formula used widely in practice. However, its physical meaning is not evident and lacks proper dimensional relation.The empirical criterion does not include the factor of alloy element segregation. The criterion for the prediction of shrinkage cavity and porosity defects in castings has been derived as: G/T<A /DL or G/R<B. △T/DL. In addition, its physical meaning as well as dimension is discussed. It is worth noting that by using this theoretical criterion the influence of section thickness on the critical value K is clarified.

Theoretical method for calculating porosity and permeability under self-regulating effect

In order to study the effect of stress-sensitivity and matrix shrinkage on coalbed methane production,equivalent matrix particle model is proposed considering the process of adsorption and desorption.Calculating mathematical models for calculating porosity and permeability which considered matrix shrinkage by combining diameter model and desorption were established.The calculations of porosity and permeability under self-regulating effect were obtained by combining traditional stress-sensitivity equations.The changes of porosity and permeability in different reservoirs were calculated and analyzed through a variety of basic parameters.The results show that high coal rank reservoir has the biggest range ability of porosity and permeability under the same pressure difference conditions,followed by the middle rank and the low rank.The research observed the positive relationship between stress-sensitivity and declining period of porosity and permeability in low rank coal reservoir,and the inverse relationship between matrix shrinkage and declining period of porosity and permeability.The stronger the matrix shrinkage is,the earlier declining period and rise period will occur.

Influence of Al-B grain refiner on porosity formation of directionally solidified Al-Si alloys

This work aims to present a perspective for porosity formation in three different alloys:A356,A413 and A380.1 by taking into account the addition of Al-B grain refiners:AlTi5B1 and Al3B.The directional solidification method was used,and microstructural changes of the alloys and its correlation with porosity formation were investigated.Pore size,number of pores,average pore length and distribution of pores were statistically analyzed.Also,external shrinkage was examined,and the volume of external shrinkage was calculated.It was found that there was a relationship between external shrinkage and the size and number of pores.As the size and number of pores internally decrease,external shrinkage increases.Additionally,porosity is decreased in all the three Al-Si alloys when Al-B grain refiners are used.The distribution of pore diameters is low when AlTi5B1 is used.Grain refiners have a different effect on porosity formation of Al-Si alloys with regard to their solidification morphology.

Holes Effects in Plane Periodic Multilayered Viscoelastic Media

This work deals with the study of the reflection and transmission properties of plane periodic structures composed of N periods (1 ≤ N ≤ 3) in the MHz frequency range. The period consists of two bounded plates presenting a high acoustic impedance contrast one of which is in aluminum, the other is in polyethylene. The longitudinal and transversal attenuations are considered in polyethylene and neglected in aluminum. We take into account the case of emerging holes in the polyethylene layer. Simulations are based on the stiffness matrix method (SMM) developed by Rokhlin. When attenuation is considered in polyethylene, the reflection coefficients are different depending on the insonification side. The comparison of results without or with holes configurations are performed and showed that throughout holes allow the rapid observation of forbidden bands. The attenuation of the whole multilayer is also determined.

基于柱孔收缩理论的含气土盾构施工引起地表沉降研究

依托杭州地铁塘青线盾构穿越含气地层的实际工程,对柱孔收缩理论进行修正,首次提出基于柱孔收缩理论的含气地层盾构施工引起地表沉降理论计算公式,并结合实际工程参数,分析隧道埋深、开挖半径、基质吸力等因素对含气地层盾构施工引起地表沉降的影响。研究表明:柱孔收缩理论计算结果与实测数据较为吻合,能较好地反映出含气地层盾构掘进引起的地表沉降特征;与饱和土工程不同,双线盾构穿越含气地层时,先行线引起的地表沉降更大;含气土等非饱和土中基质吸力通过改变土体内摩擦角和黏聚力进而影响土体特性,但较之于土体黏聚力,土体内摩擦角的变化对盾构穿越含气地层引起地表沉降的影响更大。

250 mm×250 mm轴承钢大方坯压下工艺优化及质量提升

建立了250 mm×250 mm轴承钢大方坯的凝固传热模型,并根据凝固平方根定律验证了模型的准确性。讨论了拉速和过热度对凝固终点的影响,并制定新的压下制度。运用不同压下制度进行了工业试验,试验结果表明:优化后的压下制度基本消除大方坯的中心缩孔,使得中心偏析比由1.1降低至1.0,同时使得V形偏析线上最高碳含量由1.391%降至1.184%,V形偏析线上和非偏析线上的平均碳含量极差由0.113%降至0.07%。

汽车机油泵体压铸工艺数值模拟与优化

为满足汽车铝合金机油泵体压铸件高气密性和高强度的要求,解决该铸件缩松、缩孔缺陷问题,进行压铸工艺设计和优化。首先对铸件进行工艺分析,根据经验分别设计浇注排溢系统并初步选取工艺参数;再运用田口正交试验法设计了5因素4水平压铸工艺参数方案,并使用Procast进行数值模拟。将16组正交试验结果基于信噪比进行极差和方差分析,结果表明,模具温度对缩松缩孔的影响最为显著,5因素的最优工艺参数为:浇注温度650℃、模具温度240℃、慢/快压射距离200 mm/60 mm、快压射速度为3.0 m/s、慢压射速度为0.2 m/s。数值模拟结果显示,该工艺参数组合下铸件缩松、缩孔体积为1.067 cm^(3),较优化前降低了26.5%。试模结果表明,铸件外观完好,关键部位X射线探伤显示无明显缩孔;采用金相显微镜(OM)和扫描电镜(SEM)对铸件进行组织观察,发现铸件各区域组织致密;力学性能测试表明,压铸件显微硬度大于HV85,同等工艺下试棒平均拉伸强度为253.36 MPa,铸件满足使用要求。

考虑气隙分布的超大宽厚比钢锭凝固过程数值模拟研究

扁钢锭的设计及其质量控制是制约“冶炼-铸锭-轧制”方法生产特厚板的关键,大宽厚比扁锭可不经开坯直接轧制,从而简化生产流程,降低生产成本。采用数值模拟的方法研究了超大宽厚比扁锭的气隙生长、凝固进程以及缩孔缺陷的分布。同时,分析了不同宽厚比和预起拱参数对铸锭凝固质量的影响。结果表明,当大宽厚比扁锭凝固时,扁锭宽面中部与宽面1/4位置的气隙厚度差可达1 mm以上,进而在扁锭内部产生两个热中心。随着宽厚比的增加,双热中心现象更明显,导致缩孔缺陷增多。此外,随着预起拱高度的增加,双热中心现象变得不明显,缩孔体积减少,当拱高为80 mm时,超大宽厚比铸锭质量良好。

高镍奥氏体球墨铸铁涡壳的铸造工艺优化

介绍了高镍奥氏体球墨铸铁涡壳铸件的结构及铸造工艺,针对试制过程中出现的表面缩孔、缩松、显微缩松以及组织中的碎块状石墨、表面恶化层、碳化物等问题进行了工艺优化,最后指出:(1)高镍奥氏体球墨铸铁液态收缩量大、石墨化膨胀弱,冒口设计遵循顺序凝固原则是防止涡壳产生缩孔、缩松缺陷的有效措施;(2)涡壳厚处和舌尖易形成碎块状石墨,强化孕育可防止缺陷的产生;(3)表面恶化层超标,可通过较高的残余镁量和砂芯刷涂料进行防止;(4)由于冷却速度较慢,舌尖部位碳化物析出数量较多,良好的孕育可减少碳化物析出量,热处理也可改变碳化物体积分数及其形态。

基于田口法的离心铸造TiAl合金环形件组织调控工艺参数优化

为了获得TiAl合金环形件离心铸造过程中离心转速、浇注温度、浇注速度和模具预热温度对TiAl合金铸环凝固组织的影响规律,通过数值仿真和田口法,并利用晶粒尺寸和缩孔率的信噪比结果作为衡量铸件质量的指标,从而优化离心铸造工艺参数。试验结果表明:各工艺参数对晶粒尺寸影响因素大小依次为浇注速度、浇注温度、离心转速、铸型预热温度。各工艺参数对缩孔率影响因素大小则依次为浇注速度、离心转速、浇注温度、铸型预热温度。以晶粒细化程度最大、缩孔率最小作为优化目标时,最优工艺参数为离心转速600 r/min、浇注温度1620℃、浇注速度200 mm/s和型壳预热温度150℃。试验验证结果表明,该工艺参数制备出晶粒细小、组织致密的优质TiAl合金铸件。

灰铸铁机床底座铸件的铸造工艺改进

介绍了灰铸铁数控机床底座铸件的结构及技术要求,详细阐述了原铸造工艺及存在的缩孔、缩松及变形问题,通过增大球形冒口尺寸,在热节处增加冷铁进行激冷,使厚壁区域提前凝固,实现铁液的顺序凝固;通过提高废钢的加入比例,提高锰含量的控制中值,降低碳含量及硅含量的控制中值,保证珠光体的体积分数,从而保证铸件的硬度;通过增加加强拉筋,防止在凝固过程中铸件收缩变形。生产结果显示:连续生产的10件机床底座铸件顶面无缺陷,粗加工后未发现缩孔、缩松缺陷,变形量控制在2 mm以内,改进效果明显。

无缝钢管内表面缺陷的成因分析

圆坯经过穿孔、轧管加工的无缝钢管内表面存在缺陷,在手掌大小的区域内看到四处形貌不同的缺陷。利用金相显微镜、扫描电子显微镜和能谱分析仪对缺陷部位的表面样品和横、纵截面样品进行观察。在四个缺陷处均观察到高温氧化铁和高温氧化源,可以确定钢管内表面缺陷是在连铸的过程中产生的。通过对表面形貌的观察分析和纵截面缺陷深度的测量,确定形成缺陷的原因为连铸坯中心裂纹、中心缩孔和缩松,是经轧制变形所引起的。四个缺陷处都存在Na、Mg、Al、Si、P、K、Ca等元素,为穿孔过程中使用磷酸钠润滑剂的残留物。

70^(#)钢热轧盘条拉拔断裂的原因分析

70^(#)钢热轧盘条用于拉拔加工成多种钢丝产品,因此要求其具有良好的拉伸性能。但是,在实际生产中,某些因素通常会导致70^(#)钢热轧盘条抗拉强度降低,在拉拔过程中发生断裂,严重影响其产品质量,造成了巨大的经济损失。针对70^(#)钢热轧盘条在使用过程中出现拉拔断裂的问题,采用光学显微镜、扫描电子显微镜(SEM)、电子探针(EPMA)等检测设备分别从断裂试样的形貌、组织、成分等方面分析了拉拔断裂的原因。结果表明,在盘条中心处存在缩孔和疏松是造成拉拔断裂的主要原因。对连铸轻压下技术进行了优化,通过调整压下区间和压下量,提高了连铸坯的内部质量,消除了盘条中的缩孔和疏松,有效减少了盘条拉拔断裂现象。本研究为行业内相似问题的探究和解决提供了参考。