Environmental Effects on Fatigue Crack Growth in 7075 Aluminum Alloy

The fatigue behavior of aluminum alloys is greatly influenced by the environmental conditions. In this article, fatigue crack growth rates were measured for 7075-T651 Al alloy under ultrahigh vacuum (UHV, ~10^?10?Torr), dry air, and water vapor. Standard compact tension (CT) specimens were tested along the L-T orientation under various load ratios of 0.1, 0.5, and 0.8. Fracture surfaces and crack morphologies were studied using scanning electron microscopy and crack deflection analysis. The crack growth behavior under vacuum was affected by friction and possible rewelding of crack surfaces, causing an asymmetry in the crack growth behavior, from load shedding to constant load. The enhancement of crack growth at higher moisture levels was observed and is discussed in terms of moisture decreasing friction between the crack faces. The effect of crack deflection as a function of R ratio and environment is also presented.     

Effect of Operation Parameters on Diffusion Bonded AA5083, AA6082 and AA7075 Aluminum Alloys

Rolled plates of 5 mm thick AA5083, AA6082 and AA7075 aluminum alloys Joints were fabricated by diffusion bonding at different temperatures. The microstructure evolution of AA5083, AA6082 and AA7075 aluminium alloys were characterized by transmission electron microscopy. Metallurgical investigations and mechanical tests were also performed to correlate the microstructural investigations with the mechanical properties of the produced diffusion bonded joints. It was observed that the bonding and shear strength increased with the increase in bonding temperature due to the diffusion of micro-constituents in the interface. Higher temperature enhanced the uniform distribution of secondary phase particles, which further improved the reduction in pores/defects in the bonded joints.     

Effect of Thermomechanical Fatigue on Precipitation Microstructure in Two Precipitation-Hardened Cast Aluminum Alloys

Thermal loading induces modifications of the precipitation microstructure of Al–Si–Cu–Mg alloys. This study focuses on the effect of deformation on precipitation microstructure during thermomechanical loadings. Several specimens were thermomechanically cycled while others were exposed to the same thermal cycles without any mechanical loading. The nature and morphological characteristics of the precipitation microstructure of the thermomechanically cycled specimens are compared to those of the thermally aged ones, using transmission electron microscopy (TEM), in order to assess the effect of deformation on the precipitation microstructure and especially on the kinetics of precipitate growth. The absence of any significant effect of superimposed straining during thermal cycling is discussed. Implications for the prevision of yield strength degradation during service operation are briefly presented.

     

Calorimetric Studies of Precipitation and Dissolution Kinetics in Aluminum Alloys 2219 and 7075

Differential scanning calorimetry (DSC) was used to study the kinetics of precipitation and dissolution of metastable and stable phases in aluminum alloys 2219 and 7075. A comparison of DSC scans obtained at heating rates of 1, 5, 10, and 20 K per minute showed that, during a DSC scan, the rates of precipitation of θ′ and θ in 2219 and η′ and η in 7075 were limited by their reaction kinetics. Likewise, the rates of dissolution of GP zones, θ′ and η′, were found to be dominated by kinetics. In contrast, the dissolution of θ and η was dominated by the thermodynamic equilibrium between these phases and the matrix. Analysis of the kinetically dominated reaction peaks and their dependence on heating rate and particle size showed that the GP zone dissolution reaction could best be described by a three-dimensional volume diffusion limited rate expression with an activation energy equal to that for diffusion. The rate of formation of θ′ was best described by an Avrami expression withn = 1.1, indicating that nucleation was not the rate controlling step. A pronounced dependence of the θ′ formation rate on prior plastic deformation was observed and ascribed to the influence of the matrix dislocation density on diffusivity.     

Effect of Proximity and Dimension of Two Artificial Pitting Holes on the Fatigue Endurance of Aluminum Alloy AISI 6061-T6 Under Rotating Bending Fatigue Tests

This work deals with the study of the two artificial pitting holes effects, caused by their dimensions and proximity, on the fatigue endurance of aluminum alloy AISI 6061-T6 under rotating bending fatigue tests. Stress concentration induced by artificial pitting holes is analyzed and correlated with the experimental fatigue life. It is found that the stress concentration increases exponentially when the two pitting holes approach, and this induces an important reduction in the fatigue life. Concerning the diameter variation of one pitting in regard to the second, no important influence was observed on fatigue life for a given separation between them; this implies that the separation between the two artificial pitting holes and the associated stress concentration is the principal parameter on the fatigue life under these conditions. Finally, results are discussed and conclusions are presented involving the fatigue life, proximity, and dimension of pitting holes, stress concentration factor, and fracture surfaces where the failure origin is identified.     

形变热处理对7075铝合金组织和性能的影响

研究了形变热处理对7075合金组织和力学性能的影响。结果表明：形变热处理工艺可大幅提高合金板材的强度,且合金板材保持了较好的塑性。金相观察发现,预时效析出的沉淀相可为合金板材冷变形后的终时效强化相均匀析出的优先成核提供条件;形变处理引入大量的位错,促进了终时效时高密度均匀细小过渡强化相η/的析出。     

The Effect of Holding Liquid Aluminum Alloys on Oxide Film Content

Melts of commercially pure liquid aluminum, and an Al-7Si-0.3Mg alloy, were cast into molds designed to produce entrainment of oxide film defects. The melts were held for periods of up to 20 minutes to investigate whether changes in the oxide film defects in the melt could occur, once sufficient time had elapsed for consumption of their internal atmosphere. The alloys were characterized by the determination of their Weibull modulus, examination of fracture surfaces under scanning electron microscopy (SEM), and determination of their porosity characteristics. The Weibull moduli of the ultimate tensile strength (UTS) values of the Al-7Si-0.3Mg alloy were reduced initially by holding in the liquid state for 10 minutes, but then the values increased after holding for 20 minutes. This high Weibull modulus was found despite oxide films being observed on the fracture surfaces. In the case of the commercial purity Al, the UTS Weibull moduli increased only slightly with holding for 20 minutes. The results suggested that holding of Al alloys in the liquid state influenced the scatter of mechanical properties by influencing the porosity content of the castings, which was related to their oxide film content. Some evidence for healing of a double oxide film defect with time was also found in the commercial purity Al alloy.     

The Surface Tension of Pure Aluminum and Aluminum Alloys

The surface tension of high purity and commercial purity aluminum in vacuo was determined using the sessile drop method and the results were found to compare favorably with published data. The effects of holding atmosphere, substrate, and “surface fracture” of the sessile drop on the measured surface tension values were investigated together with the effects of different solute elements commonly present in commercial aluminum alloys. The results obtained suggest that the nature of the surface oxide film formed on the droplets (affected by alloy composition and atmosphere) and the rupture of this film are the dominant factors influencing the surface tension values obtained. Changes in surface tension values of up to 60 pct were observed. The possible effect of this variable surface tension on practical casting processes, such as direct chill casting, is suggested.     

稀土在铝及其合金中的作用和应用

稀土元素作为微量元素加入铝及其合金中,不仅有细化晶粒的变质作用,还有净化熔体、减少气体含量及氧化夹杂的精炼效果,从而可显著改善和提高铝及其合金的综合性能。讨论了稀土在铝及其合金中的作用机理。论述了稀土在铝及其合金中的应用现状及发展前景。     

Corrosion Fatigue Study of 6061 Aluminum Alloy: The Effect of Coatings on the Fatigue Characteristics

Metallurgical and Materials Transactions A - This study proposes a method for protecting 6061 aluminum alloy from corrosion fatigue. Corrosion fatigue can reduce fatigue strength at the endurance...     

Effect of Surface Mechanical Attrition Treatment on Fatigue Lives of Alloy 718

The effect of surface mechanical attrition treatment (SMAT) and its duration on fatigue lives of alloy 718 has been studied. The SMAT process was carried out in vacuum (?0.1?MPa) with SAE 52100 steel balls of 5?mm diameter for 30 and 60?min at a vibrating frequency of 50?Hz. SMAT resulted in surface nanocrystallization, higher surface roughness, higher surface hardness, higher compressive residual stress, higher tensile strength, reduced ductility and superior fatigue lives. The enhancement in the fatigue lives of treated samples can be attributed to the positive influence of nanostructured surface layer, compressive residual stress and work hardened layer, which surpassed the negative effect of increased surface roughness. There was no significant difference between the fatigue lives of samples treated for 30 and 60?min.     

Detection and Influence of Shrinkage Pores and Nonmetallic Inclusions on Fatigue Life of Cast Aluminum Alloys

In the current study, test bars of cast aluminum alloys EN AC-AlSi8Cu3 and EN AC-AlSi7Mg0.3 were produced with a defined amounts of shrinkage pores and oxides. For this purpose, a permanent mold with heating and cooling devices for the generation of pores was constructed. The oxides were produced by contaminating the melt. The specimens and their corresponding defect distributions were examined and quantified by X-ray computer tomography (CT) and quantitative metallography, respectively. A special test algorithm for the simultaneous image analyses of pores and oxides was developed. Fatigue tests were conducted on the defective samples. It was found that the presence of shrinkage pores lowers the fatigue strength, and only few oxide inclusions were found to initiate fatigue cracks when shrinkage pores are present. The results show that the pore volume is not sufficient to characterize the influence of shrinkage pores on fatigue life. A parametric model for the calculation of fatigue life based on the pore parameters obtained from CT scans was implemented. The model accounts for the combined impact of pore location, size, and shape on fatigue life reduction.     

淬火介质因素对7075铝合金厚板残余应力的影响

就淬火介质因素(不同介质浓度、温度)对7075铝合金厚板残余应力的影响进行研究。结果表明：淬火残余应力随着介质浓度和温度增加而减少。即在浓度30％、温度40℃的AQ25l聚合物和乳液介质中淬火，与同温度下水淬相比，残余应力分别降低64．2％和83．5％，而板材显微硬度只降低5％和7％。     

Effect of gravity level on grain Refinement in Aluminum Alloys

Metallurgical and Materials Transactions A -     

铝及铝合金晶粒细化剂的研究

综述了铝及铝合金晶粒细化剂的发展历史，阐述了晶粒细化剂的细化机理，对各种细化剂进行了比较，并着重介绍了新一代Al—Ti—C晶粒细化剂。     

The Effect of Aluminum Additions on the Microstructure and Thermomechanical Behavior of NiTiZr Shape-Memory Alloys

The microstructure, thermal cycling, and mechanical behavior of Ni_48.5Ti_31.5?x Zr₂₀Al _x (x?=?0, 1, 2, 3) alloys were studied in the solution-treated and aged condition using microscopy techniques, differential scanning calorimetry, and compression tests. The microscopy techniques used include optical, scanning, and transmission electron microscopy, and three-dimensional, atom?Cprobe microscopy. The results of this study indicated a strong dependence of the transformation behavior on alloy chemistry and thermal cycling. The aluminum additions served to decrease transformation behaviors from 351?K to 596?K (78?°C to 323?°C) and reduce thermal stability. Additionally, aluminum was shown to increase the plateau stress in the aged condition, whereas the formation of coarse-grained intermetallic phases caused the embrittlement of the microstructure, reducing its ductility. The addition of Al resulted in the refinement of the coarse, lenticular precipitates identified as Ni₄(Ti,Zr)₃.     

不同磁场条件对双流铸造7075铝合金组织的影响

成功研制了双流低频电磁半连续铸造铝合金的设备,使用该设备实现7075铝合金连续稳定的铸造,为工业实现多流低频电磁半连续铸造提供实验基础.利用此铸造设备,对比研究双流铸造中不同磁场条件对7075铝合金组织的影响.研究发现:相邻线圈电流方向相反时铸锭宏观组织略微好于线圈电流方向相司时;随着电流强度的增大,晶粒逐渐得到细化,当电流强度大于160A后,磁场作用基本达到最佳效果;励磁电流频率小于等于10 Hz或者大于等于30 Hz均无法改善合金组织,当励磁电流频率为20 Hz的时候,铸锭晶粒相对细小,分布均匀,目宏观偏析得到了显著抑制.     

Multistage Fatigue Modeling of Cast A356-T6 and A380-F Aluminum Alloys

This article presents a microstructure-based multistage fatigue (MSF) model extended from the model developed by McDowell et al.[1,2] to an A380-F aluminum alloy to consider microstructure-property relations of descending order, signifying deleterious effects of defects/discontinuities: (1) pores or oxides greater than 100 μm, (2) pores or oxides greater than 50 μm near the free surface, (3) a high porosity region with an area greater than 200 μm, and (4) oxide film of an area greater than 10,000 μm². These microconstituents, inclusions, or discontinuities represent different casting features that may dominate fatigue life at stages of fatigue damage evolutions. The incubation life is estimated using a modified Coffin–Mansion law at the microscale based on the microplasticity at the discontinuity. The microstructurally small crack (MSC) and physically small crack (PSC) growth was modeled using the crack tip displacement as the driving force, which is affected by the porosity and dendrite cell size (DCS). When the fatigue damage evolves to several DCSs, cracks behave as long cracks with growth subject to the effective stress intensity factor in linear elastic fracture mechanics. Based on an understanding of the microstructures of A380-F and A356-T6 aluminum alloys, an engineering treatment of the MSF model was introduced for A380-F aluminum alloys by tailoring a few model parameters based on the mechanical properties of the alloy. The MSF model is used to predict the upper and lower bounds of the experimental fatigue strain life and stress life of the two cast aluminum alloys. This article is based on a presentation made in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.

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双级时效对7075铝合金力学性能的影响

本文研究了7075铝合金固溶处理后双级时效热处理工艺,分析了双级时效工艺对7075铝合金力学性能的影响.合金成份一定的7075铝合金挤压型材,经470℃2h固溶后,再经120℃×6h+165℃x5h双级时效时能获得较好的综合性能,为7075铝合金时效热处理工艺参数的确定及优化提供参考依据.