Microstructure-based fatigue modeling of cast A356-T6 alloy

High cycle fatigue (HCF) life in cast Al-Mg-Si alloys is particularly sensitive to the combination of microstructural inclusions and stress concentrations. Inclusions can range from large-scale shrinkage porosity with a tortuous surface profile to entrapped oxides introduced during the pour. When shrinkage porosity is controlled, the relevant microstructural initiation sites are often the larger Si particles within eutectic regions. In this paper, a HCF model is introduced which recognizes multiple inclusion severity scales for crack formation. The model addresses the role of constrained microplasticity around debonded particles or shrinkage pores in forming and growing microstructurally small fatigue cracks and is based on the cyclic crack tip displacement rather than linear elastic fracture mechanics stress intensity factor. Conditions for transitioning to long crack fatigue crack growth behavior are introduced. The model is applied to a cast A356-T6 Al alloy over a range of inclusion severities.     

Multiaxial fatigue behaviour of A356‐T6

Aluminum alloy A356‐T6 was subjected to fully reversed cyclic loading under tension, torsion and combined loading. Results indicate that endurance limits are governed by maximum principal stress. Fractography demonstrates long shear mode III propagation with multiple initiation sites under torsion. Under other loadings, fracture surfaces show unique initiation sites coincidental to defects and mode I crack propagation. Using the replica technique, it has been shown that the initiation life is negligible for fatigue lives close to 10⁶ cycles for combined loading. The natural crack growth rate has also been shown to be comparable to long cracks in similar materials.     

A356‐T6 wheels: Influence of casting defects on fatigue design

Rotating bending fatigue tests were executed on A356‐T6 wheels. The tests were performed both on components containing only the casting defects and on wheels in which artificial holes were machined. Results show the detrimental effect of sub‐superficial porosities on fatigue endurance. A predictive model was proposed. It resulted in a useful tool for the designer because knowing the applied stress, it is possible to estimate the number of cycles for crack nucleation considering the size of the defect, whereas knowing the endurance target requested it is possible to find the maximum defect size acceptable considering the applied stress.     

Characteristics of defect bands and their formation mechanisms in A356 wheel fabricated by horizontal squeeze casting

Based on the opinion that a decreasing solid fraction gradient in the cross-section develops during the intensification stage, the shear stress in the solidifying alloy was analysed, and a mechanism for dilatant band formation was discussed. That is, when the shear stress at one position of the stable grain network reaches a critical value, the network collapses and the band forms. The characteristics of defect bands at different locations of an A356 wheel fabricated by horizontal squeeze casting were then studied. Three types of defect bands, i.e. skin related band, dilatant band and separate band, were observed. Combining the process and conditions of defect band formation, several types of defect bands in the casting were discussed. Furthermore, the influence of the structural characteristics of castings on the defect bands, including thickness, geometry and location, was summarised.     

Influence of residual fatigue damage on the fracture toughness parameters of an AA6082-T6 aluminium alloy

The present investigation has been carried out in order to study the influence of the previous accumulated fatigue damage induced during high cycle fatigue (HCF), on the fracture toughness parameters of an AA6082-T6 aluminium alloy. The results show that previous fatigue damage accumulated in HCF does not affect the tensile static mechanical properties of the material, but gives rise to a significant debit of the toughness properties on this aluminium alloy. The fracture toughness results have shown that the crack opening displacement at a crack extension of 0.2 mm (COD_0.2) decreases in the range of ∼18 to 36% whereas the value of the non-linear fracture mechanics parameter  J _0.2, decreases in the range of ∼11 to 25% at applied maximum stresses of 200 and 275 MPa, respectively. Optical microscopy observations conducted on the surface of the specimens subjected to HCF damage indicate the existence of microcracks ∼15 to 25 μm long nucleated along the grain boundaries of the material. Also, the scanning electron microscopy (SEM) observations of the fracture surfaces after the tearing tests show the predominance of a ductile fracture mechanism for the material prior to residual fatigue damage, whereas a mixed ductile–brittle fracture mechanism and the presence of flat facets were observed on the fracture surfaces of the specimens with a fatigue damage of 0.70.     

Influence of anodization on the fatigue strength of 7050-T7451 aluminium alloy

In recent years, with higher demand for improved quality and corrosion resistance, recovered substrates have been extensively used. Consequently residual stresses originated from these coatings reduce the fatigue strength of a component. Due to this negative influence occasioned by corrosion resistance protective coatings, an effective process like shot peening must be considered to improve the fatigue strength. The shot peening treatment pushes the crack sources beneath the surface in most of medium and high cycle cases due to the compressive residual stress field (CRSF) induced. The aim of this study was to evaluate the influence on the fatigue life of anodic films grown on 7050-T7451 aluminium alloy by sulphuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on the rotating and reverse bending fatigue strength of anodic films grown on the aluminium alloy is to degrade the stress life fatigue performance of the base material. A consistent gain in fatigue life in relation to the base material was obtained through the shot peening process in coated specimens, associated to a residual stress field compressive near the surface, useful to avoid fatigue crack nucleation and delay or even stop crack propagation.     

Flow stress and ductility of AA7075-T6 aluminum alloy at low deformation temperatures

The present investigation has been conducted in order to develop a rational approach able to describe the changes in flow stress of AA7075-T6 aluminum alloy with deformation temperature and strain rate, when this material is deformed at temperatures in the range of 123-298 K at strain rates in the range of 4 × 10⁻⁴ to 5 × 10⁻² s⁻¹. The constitutive formulation that has been advanced to accomplish these objectives represents a simplified form of the mechanical threshold stress (flow stress at 0 K) model developed at Los Alamos National Laboratory (Los Alamos, New Mexico, USA). Thus, it is assumed that the current flow stress of the material arises from both athermal and thermal barriers to dislocation motion. In the present case, the effect of three thermal barriers has been considered: solid solution, precipitation hardening and work-hardening. The first two effects do not evolve during plastic deformation, whereas the last one is considered as an evolutionary component of the flow stress. Such an evolution is described by means of the hardening law earlier advanced by Estrin and Mecking (1984) [20]. The law is implemented in differential form and is integrated numerically in order to update the changes in strain rate that occur during tensile tests carried out both at constant and variable crosshead speed. The extrapolation of the hardening components from 0 K to finite temperatures is accomplished by means of the model earlier advanced by Kocks (1976) [19]. The results illustrate that the constitutive formulation developed in this way is able to describe quite accurately both the flow stress and work-hardening rate of the material, as well as temperature and strain rate history effects that are present when deformation conditions change in the course of plastic deformation. The evaluation of the ductility of the alloy indicates that the changes in this property are mainly determined by deformation temperature rather by strain rate. When deformation temperature decreases from 298 to 123 K, ductility also decreases from ∼35 to 24%. However, despite these relatively small variations, significant changes in the fracture morphology could be observed on the fracture surfaces of the examined specimens, with the predominance of a mixed ductile-brittle mechanism at lower temperatures.     

Effect of microstructural variables on tensile behaviour of A356 cast aluminium alloy

Abstract

The effects of microstructural variables, including secondary dendrite arm spacing (SDAS), the size of primary α phase, the aspect ratio of eutectic Si particle and the thickness of eutectic wall structure, on tensile behaviour of A356 cast aluminium alloy, were quantitatively identified using linear regression analysis method. For systematic microstructural control of A356 specimen, directional solidification method was used with different solidification rates of 5, 25, 50 and 100 μm s^?1 respectively. The linear regression analysis suggests that each microstructural variable affects tensile strength and tensile elongation of A356 cast aluminium alloy in a similar fashion. The change in tensile behaviour with varying microstructural variables in A356 cast aluminium alloy is discussed based on fractographic and micrographic observations.     

基于遗传算法的低压铸造铝合金车轮工艺优化

为解决低压铸造铝合金车轮质量控制难度大的问题,采用遗传算法对工艺参数进行优化.基于铸造数值模拟结果,利用BP人工神经网络建立了铸造工艺参数与质量控制目标缩松缺陷和凝固时间的非线性关系,采用遗传算法实现了铸造工艺参数的优化.以某型低压铸造A356铝合金车轮为例,对浇注温度、上模温度、下模温度、侧模温度、模芯温度5个参数进行优化,得到的最佳工艺组合,可有效控制缩松缺陷和凝固时间.利用数值模拟结果、建立神经网络模型,采用遗传算法优化的方法,获得近似最优解,有助于优化低压铸造工艺.     

Modelling the mean stress effect on fatigue life of fibre reinforced plastics

The mean stress influence on fatigue life of carbon and glass fibre reinforced plastics is investigated in detail. A new phenomenological approach is presented to model the mean stress effect in various material systems and fibre dominated stacking sequences. The model is calibrated to fatigue data via a developed fitting-routine that is based on least squares method. The calibration input data is one Woehler curve at R = 0.1 and the ultimate static strengths in tension and compression loading. The characterization effort is reduced by this significantly. Finally the method is verified successfully by fatigue data of several material systems.     

The effects of thermo-mechanical parameters on the microstructure of Thixocast A356 aluminum alloy

In general, semi solid Thixocast (A356) alloys are consisted of extensive globular -Al regions, which are surrounded by eutectic (β) phase. The better formability, higher toughness and structure free porosities, gas cavities and shrinkages are the important advantages of this globular structure. In addition the thermo-mechanical processing is known as one of the most effective processing techniques to control the final mechanical properties. Accordingly, in present work the effects of strain rate and the deformation temperature on the microstructure (morphology of Si phase) of Thixocast aluminum (A356) alloy have been studied. In this regard, hot compression tests at 450, 500 and 540 °C with strain rate of 0.0001, 0.0005, 0.001 and 0.01 s⁻¹ have been performed. The results showed an extensive change in the morphology of eutectic Si fibers through breaking and spheroidization processes.     

剪切停留对液固温区A356合金组织及瞬态流变化为的影响

采用自行研制的流变装置进行瞬态流变试验，研究了剪切停留对液固温区不同初生α相形态A356合金的组织变化及其瞬态液变化为的影响，结果表明，在相同的剪切条件下，随剪切停留时间的增加，瞬时峰值表观粘度增加，说明切停留期间半固态合金初生α相形态发生了变化；对半固态合金初生相形态观察发现，剪切停留促使初生相发生了“团聚”或“合并”，而且初生相形态变化的程度取决于剪切停留时间和初生相的退化程度，对试验结果进行曲线拟合，得到了描述半固态合金峰值表观粘度和剪切停留时间的瞬态流变方程。     

Investigation of the grain refining performance of Al-5Ti-1B master alloy on the recycling process of A356 alloy

In this paper, the grain refining performance of Al-5Ti-1B master alloy on the recycled A356 alloy is investigated using the macrostructure examination and chemical analysis. Results show that Al-5Ti-1B is composed of the dispersion of blocky TiAl₃ particles and mixtures of small TiAl₃ and TiB₂ particles. Both particles refine grain structures of A356 alloy. As the initial recycling process proceeds, A356 alloy still exhibits fine structures. However, during the subsequent recycling process, the grain size of A356 alloy become larger. The concentration of titanium and boron decreases with increasing the number of recycling, especially in the subsequent recycling process. It is proposed that recycling of refined A356 can best be conducted in the initial recycling process and then additional grain refiner needs to be added to maintain the grain refining performance for the continuous recycling process of A356 alloy.     

On the relationship between statistical distributions of defect size and fatigue life in 7050-T7451 thick plate and A356-T6 castings

The relationship between the distributions for the size of fatigue-initiating defects and fatigue life of 7050-T7451 thick plate and A356-T6 alloy castings reported previously in the literature were analyzed. Results showed that (i) the size of fatigue-initiating defects in all four datasets follow the Gumbel distribution, (ii) the fatigue life model based on the Paris-Erdoğan law for crack propagation provides respectable fits to fatigue life data, and (iii) the statistical distribution for fatigue life based on the Gumbel distribution of defect size and the fatigue life model provides excellent fits to all datasets, (iv) this statistical distribution for fatigue life performs better than the lognormal and Weibull distributions commonly used to model variability in fatigue life of aluminum alloys.     

Effect of mean stress on fretting fatigue of Ti-6Al-4V on Ti-6Al-4V

Fretting fatigue tests of Ti‐6Al‐4V on Ti‐6Al‐4V have been conducted to determine the influence of stress amplitude and mean stress on life. The stress ratio was varied from R=−1 to 0.8. Both flat and cylindrical contacts were studied using a bridge‐type fretting fatigue test apparatus operating either in the partial slip or mixed fretting regimes. The fretting fatigue lives were correlated to a Walker equivalent stress relation. The influence of mean stress on fretting fatigue crack initiation, characterized by the value of the Walker exponent, is smaller compared with plain fatigue. The fretting fatigue knockdown factor based on the Walker equivalent stress is 4. Formation of fretting cracks is primarily associated with the tangential force amplitude at the contact interface. A simple fretting fatigue crack initiation metric that is based on the strength of the singular stress field at the edge of contact is evaluated. The metric has the advantage in that it is neither dependent on the coefficient of friction nor the location of the stick/slip boundary, both of which are often difficult to define with certainty a priori.     

Effects of rheocasting and heat treatment on microstructure and mechanical properties of A356 alloy

The effects of the rheocasting process and T5 heat treatment on microstructure and mechanical properties of A356 alloy were investigated. The results show that the temperature range for the solid-liquid state is roughly between 560 °C and 630 °C, and the solid fraction increases from 0% to 100% with decreasing temperature. The finer microstructure in rheocasting in comparison with the one in conventional casting was attributed to pressure breaking down the secondary dendrite arms, especially for specimens around 600-610 °C. It was proved that rheocasting specimens have improved mechanical properties over the conventional casting ones. Furthermore, the result shows that T5 heat treatment can strengthen A356 alloy, while the plasticity was reduced at the same time.     

Effect of friction stir processing conditions on fatigue behavior and texture development in A356-T6 cast aluminum alloy

Friction stir processing (FSP) was applied to A356-T6 cast aluminum alloy to modify the microstructure and to eliminate casting defects under two different tool rotational speeds. Plane bending fatigue tests had been conducted, revealing that FSP could enhance the fatigue strength where the lower rotational speed condition gave better results. The enhancement of fatigue strength was attributed to the elimination of casting defects. Crystallographic analysis by EBSD revealed that the texture induced by FSP had detrimental effect on growth resistance. The lower rotational speed condition resulted in the weaker texture, and consequently, further increase of fatigue strength was achieved compared with the higher rotational speed condition.     

Evaluation of three current methods for including the mean stress effect in fatigue crack growth rate prediction

The fatigue crack growth rates curves of engineering materials depend on two parameters. In addition to the dependence on the classical stress intensity factor (SIF) range ΔK, there is a dependence on the mean load (or mean SIF), mainly in the near‐threshold region. The present paper provides some useful suggestions and good practices for using three of the current available methods to reduce this second dependence through the use of tuning constants. The methods considered here are the Elber, Walker and Vasudevan (or unified approach). For each approach, multiple regression analyses are performed on experimental data from the literature, and the correlations in two and three dimensions are graphically analyzed. Numerical examples of crack growth analysis for cracks growing under nominal stresses of constant amplitude in single‐edge and notch/hole geometries are performed, assuming an identical material component to that of the available experimental data. The resulting curves of crack size versus number of cycles (a versus N) are then compared. All three models gave approximately the same (a versus N) curves in both geometries. Differences between the behaviors of the (a versus N) curves in both geometries are highlighted, and the reasons for these particular behaviors are discussed.     

Effects of directional grain structure on impact properties and dislocation substructure of 6061-T6 aluminium alloy

Abstract

The effects of the grain structure direction on the impact properties and dislocation substructure of 6061-T6 aluminium alloy are investigated under room temperature conditions and strain rates of 1×10³, 3×10³ and 5×10³ s^?1 using a split-Hopkinson pressure bar system. The impact tests are performed using specimens machined from rolled 6061-T6 plates in the longitudinal, transverse and through thickness directions respectively. The results show that for all specimens, the flow stress increases with increasing strain rate. Furthermore, for all strain rates, the highest flow stress occurs in the transverse specimen. For strain rates of 1×10³ and 3×10³ s^?1, the flow stress in the through thickness specimen is greater than that in the longitudinal specimen. However, at a strain rate of 5×10³ s^?1, the flow stress in the longitudinal specimen is higher than that in the through thickness specimen due to a greater dislocation multiplication rate. For all three grain structure directions, the strain rate sensitivity increases with increasing strain rate, but decreases with increasing true strain. The highest strain rate sensitivity is observed in the longitudinal specimen at strain rates of 3×10³ to 5×10³ s^?1. The dislocation density increases markedly with increasing strain rate. Moreover, the square root of the dislocation density varies as a linear function of the flow stress in accordance with the Bailey–Hirsch relationship. The strengthening effect produced by the increased dislocation density is particularly evident in the transverse specimen, followed by the longitudinal specimen and the through thickness specimen.