Fatigue and Fracture Mechanisms of Aluminum Lithium Alloy Containing Cerium

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Bifilm Defects in Ni-Based Alloy Castings

The Ni-base superalloys, which are normally melted and cast in a vacuum, entrain their surface-oxide film during turbulent pouring of the melt; unfortunately at this time, this process is universally practiced for investment castings of these materials. The entrained film becomes a bifilm crack automatically, so that cast alloys have a large population of cracks that controls their failure behavior. The problems of the growth of single crystals and the welding of polycrystalline alloys are reviewed to illustrate the central role of bifilms in the cracking of turbine blades, the heat-affected zones of welds, and the reliability of properties. It has been demonstrated that improved gravity pouring systems can reduce these problems significantly, but only countergravity filling of molds is expected to result in defect-free castings. Recent cases in which turbine blades failed in service are examined, and the central role of bifilm defects in these failures is discussed.     

Experimental Damage Criterion for Static and Fatigue Life Assessment of Commercial Aluminum Alloy Die Castings

Metallurgical and Materials Transactions A - Defects, particularly porosity and oxides, in high-pressure die casting can seriously compromise the in-service behavior and durability of products...     

Fracture Surface Facets and Fatigue Life Potential of Castings

Fatigue potential has been studied in cast aluminum alloys with regard to the fatigue crack initiation mechanism at the casting defects, particularly surface and subsurface defects. The significance of facets is interpreted as the presence of defects in the interior of castings. Furthermore, two varieties of facets have been identified, one originating as a dendrite-straightened bifilm (type I facet) and the other originating from a slip plane mechanism around casting defects (type II facet). The fatigue life potential of castings is reexamined based on the involvement of defects during the formation of both types of facets. It is proposed that the true fatigue life potential of defect free castings has yet to be observed, i.e., if castings can be produced without defects, then their fatigue performance will be significantly higher than even the best performances observed so far.     

板坯形状缺陷的研究与控制

结合济钢中厚板厂炼钢工序的实际生产工艺过程,对连铸板坯主要形状缺陷(鼓肚、凹陷)的产生原因进行了研究分析,并针对每种形状缺陷提出了相应的预防控制措施,实施后取得了良好效果。     

Detection of a Pool in Semi-Continuous Castings Made of Heat-Treatable Aluminum Alloys

Various products (sheets, sections, etc.) manufactured by metal forming (rolled products, forged pieces, etc.) from semi-continuous castings are widely used in the aerospace industry. The so-called pool, which is the conical volume of a liquid metal, exists at the top of the liquid metal. Experience demonstrates that the geometry, the depth, and the shape of the pool substantially affect the structure formation in a casting and its quality. The application of a titanium nitride nanopowder, which is introduced in a melt in the volume of a rod, as a modifier allowed us to find the exact geometry of the pool.     

Thermal Analysis for Shrinkage Prediction in Commercial Ductile Iron Castings

Abstract

Results from two complementary projects concerning shrinkage prediction in ductile iron castings are integrated: the first, a statistical approach to predictive modeling for process control using plant data; and the second, the development of a finite element model of ductile iron solidification. Each project is a distinct application of thermal analysis. Our integrated approach to the shrinkage problem provides a direct link between computer-aided design based on numerical simulation, and process control in the foundry based on thermal analysis.

Résumé

Cet article intègre les résultats de deux projets complémentaires sur la prédiction des contractions dans les fontes ductiles. Le premier est une approche statistique servant à modéliser de facon prévisionnaire le contrôle de procédé en utilisant les données de l'usine. Le second est le développement par analyse des éléments finis, d'un modèle de solidification des fontes ductiles. Chaque projet est une application particuliére de l'analyse thermique. Notre approche intégrée au probléme de contraction permet d'effectuer un lien direct entre le design assisté par ordinateur, basé sur des simulations numériques, et le contrôle de procédé dans la fonderie, basé sur l'analyse thermique.     

冷轧铝板带材生产的板形控制

阐述了冷轧板形的定义和控制方法，研究了铝铸轧坯料板形、轧制油、轧辊粗度、道次加工率的分配、张力分配、弯辊控制、热凸度等因素对冷板板形的影响机理和相应的控制方法，通过控制达到改善冷轧板形的目的。     

含铝废水的资源化利用

含铝废水是一种铝含量高、浊度高的工业废水。本文介绍了几种代表性的工业含铝废水的资源化综合利用方法。并在此基础上指出，含铝废水的综合利用具有良好的发展潜力，对于实现可持续发展战略有重要意义。     

稀土在铝合金中的作用及新型含钪铝合金

本文介绍了稀土元素对铝合金熔体净化、变质处理及微合金化的作用，着重阐述了新型含钪铝合金的优异性能及其在尖端工业中的广阔应用前景。     

Healing of Double-Oxide Film Defects in Commercial Purity Aluminum Melt

The possibility of the formation of bonding between the two layers of a double-oxide film defect when held in a commercial purity liquid Al alloy was investigated. The defect was modeled experimentally by maintaining two aluminum oxide layers in contact with one another in a commercial purity Al melt at 1023 K (750 °C) for times ranging from 7 minutes to 48 hours. Any changes in the composition and morphology of these layers were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The results showed that the oxide layers started to bond to one another after approximately 5 hours, and the extent of the bonding increased gradually by the holding time. The bonding is suggested to form because of the transformation of γ- to α-Al₂O₃. A complete bonding formed between the layers only when the oxygen and nitrogen trapped between the two layers were consumed, after approximately 13 hours. The results also confirmed that the nitrogen within the atmosphere of an oxide film defect reacts with the surrounding Al melt to form AlN at the interface of the defect and the melt.     

Shape Factor Effect on Inclusion Sedimentation in Aluminum Melts

Metallurgical and Materials Transactions B - The quality requirements of aluminum products are steadily increasing, and the presence of non-metallic inclusions have a large impact on the quality of...     

Analytical Prediction of Fatigue Crack Propagation in Metals

An analytical model for fatigue crack propagation of long cracks in metals and metal alloys is presented. The key features of the model are an extension of Griffith’s theory of fracture to include fatigue, a dislocation model for the crack tip opening displacement, and cyclic plasticity-induced closure. The net cyclic stretch of the process zone at the crack tip plays a major role in the fatigue crack propagation under cyclic loading. Only constant amplitude loading is considered in this paper. The model predictions utilize only the readily available material properties, such as Young’s modulus, yield strength, threshold stress intensity factor, and the fracture toughness. There are no empirical fitting constants. The model predictions are validated by an extensive amount of published fatigue crack growth studies. The agreement between the model predictions and the experimental data is good.     

颗粒增强铝基复合材料疲劳断裂研究

对粉末冶金法制备的碳化硅颗粒增强铝基复合材料进行了旋转弯曲疲劳试验研究。采用金相显微镜和扫描电镜分别观察了疲劳试验后复合材料纵向显微组织和疲劳断口。通过金相显微镜,观察了增强体颗粒在疲劳循环应力水平下可能的损伤形式。通过疲劳断口观察,分析了断面上不同区域的疲劳裂纹传播特征。结果表明,增强体的加入有效地提高了复合材料的屈服强度、弹性模量和疲劳性能,使复合材料高周疲劳极限提高到约250MPa（1×10^7循环周次）。复合材料的疲劳损伤随机分布于试样内。断口分析还表明复合材料疲劳同样遵循裂纹萌生,长大,失稳断裂规律,其裂纹起源于铝基体内。加入SiC颗粒减弱或遮盖了疲劳裂纹传播时的晶体学特征,使得复合材料高周疲劳断面没有发现常见的疲劳辉纹。     

Low-Cycle Fatigue of Ultra-Fine-Grained Cryomilled 5083 Aluminum Alloy

The cyclic deformation behavior of cryomilled (CM) AA5083 alloys was compared to that of conventional AA5083-H131. The materials studied were a 100 pct CM alloy with a Gaussian grain size average of 315 nm and an alloy created by mixing 85 pct CM powder with 15 pct unmilled powder before consolidation to fabricate a plate with a bimodal grain size distribution with peak averages at 240 nm and 1.8 μm. Although the ultra-fine-grain (UFG) alloys exhibited considerably higher tensile strengths than those of the conventional material, the results from plastic-strain-controlled low-cycle fatigue tests demonstrate that all three materials exhibit identical fatigue lives across a range of plastic strain amplitudes. The CM materials exhibited softening during the first cycle, similar to other alloys produced by conventional powder metallurgy, followed by continual hardening to saturation before failure. The results reported in this study show that fatigue deformation in the CM material is accompanied by slight grain growth, pinning of dislocations at the grain boundaries, and grain rotation to produce macroscopic slip bands that localize strain, creating a single dominant fatigue crack. In contrast, the conventional alloy exhibits a cell structure and more diffuse fatigue damage accumulation.     

Equiaxed Grain Count in Aluminum Alloy Castings: Theoretical Background and Experimental Verification

In this work, a theoretical model is proposed for heterogeneous nucleation on substrates the size distributions of which can be described by the Weibull statistics. In particular, the proposed model suggests that the size distribution of the various nucleation sites is exponential in nature. It is found that the nuclei density can be given in terms of the maximum undercooling. Measurements of grain count were carried out on single-phase Al-1.3 Si and Al-5.0 Cu (wt pct) alloys inoculated using an Al-5Ti-1B (wt pct) master alloy. In the single-phase alloys, the area of equiaxed dendritic grains was estimated using EBSD analysis and by stereological means on polished and etched surface sections. In addition, maximum undercoolings were determined by thermal analysis. The experimental outcome indicates that the volumetric grain density can be properly described by an exponential expression. Finally, the magnitudes of the nucleation parameters were experimentally determined in this work.