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• Fatigue performace of cast components is mainly affected by porosity. • An alternative fatigue procedure that accounts the effect of porosity was developed. • The model presented an average logarithmic error below 2% and absolute below 16%. • The required porosity parameters can be obtained by computed tomography. • In high series sectors, a statistical store with fracture surface data could be used. Porosity is one of the main parameters in the fatigue life of cast alloys as it affects both crack initiation and propagation phases. However, most fatigue assessments present a lack of accuracy as they only consider the effect of the initial pore on the fatigue limit. Therefore, in the present study, first, the effect of the main porosity characteristics on the crack initiation and propagation phases is evaluated. Then, an alternative fatigue assessment model is proposed and implemented for Low Pressure Die Casting (LPDC) A356-T6 aluminium alloy. The model accounts for the effect of the initial pore size and distance to surface on the initiation phase as well as fractographic porosity and average pore size on the fracture surface on the propagation phase. Finally, the model was validated by means of experimental tests. Results have shown that the proposed model is able to qualitatively predict the scatter trend of samples tested at the same load. In addition, the model presents high quantitative accuracy in life estimation with an average logarithmic error below 2% and an absolute average error below 16%. [ABSTRACT FROM AUTHOR]

The fatigue strength of cast aluminium alloys is known to be greatly affected by different defect types related to the manufacturing process, particularly microshrinkage pores created during the solidification phase of the casting process. Even if certain classification procedures are given in the standard ASTM E155-15 [1], the presence of defects is not readily related to the capacity of a component or a structure to meet the requirements of the mechanical technical specifications. The present study aims at establishing a clear link between certain microstructural features and the average fatigue strength. This is possible by looking for the average size of critical defects and using a relevant statistical analysis. More exactly, the Murakami approach based on the statistics of extremes is employed. The main originality of this work lies in the application of this approach to the case of a real structure submitted to high cycle fatigue damage: engine cylinder heads, used in the automotive industry. Indeed, both fatigue tests and microstructural characterizations are carried out on cylindrical specimens and real structures. The specimens are subjected to uniaxial and multiaxial loading conditions [2]. Original fatigue tests, developed by PSA to load in-service critical regions, are carried out on cylinder heads. Systematic analyses of fatigue failure surface are conducted to obtain the statistics of critical defects at the origin of the failures for both specimens and structures. In parallel, critical regions and the associated local loading mode in the structure are characterized by an appropriate high cycle fatigue analysis. The latter, combined with the fatigue test data and the statistical analysis of the critical defects, leads to a discussion about the size effect and an approach is proposed for a relevant fatigue design procedure. [ABSTRACT FROM AUTHOR]

Aluminium alloys can be strengthened significantly by nano-scale precipitates that restrict dislocation movement. In this study, the evolution of inhomogenously distributed trialuminide precipitates in two multi-component alloys was characterised by synchrotron small-angle X-ray scattering (SAXS). The appropriate selection of reference sample and data treatment required to successfully characterise a low volume fraction of precipitates in multi-component alloys via SAXS was investigated. The resulting SAXS study allowed the analysis of statistically significant numbers of precipitates (billions) as compared to electron microscopy (hundreds). Two cast aluminium alloys with different volume fractions of Al 3 Zr x V 1- x precipitates were studied. Data analysis was conducted using direct evaluation methods on SAXS spectra and the results compared with those from transmission electron microscopy (TEM). Precipitates were found to attain a spherical structure with homogeneous chemical composition. Precipitate evolution was quantified, including size, size distribution, volume fraction and number density. The results provide evidence that these multi-component alloys have a short nucleation stage, with coarsening dominating precipitate size. The coarsening rate constant was calculated and compared to similar precipitate behaviour. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the complex modification process of P and Sr for hypereutectic Al–30Si alloys was investigated. After the addition of 2.0% Al–3P and 0.7% Al–10Sr master alloys continuously added during the melt inoculation and solidification process, the primary Si can be significantly refined from more than 200.0μm decreasing to 32.8μm with homogeneous distribution and eutectic Si was obviously modified as fibrous simultaneously. The modification mechanism was also studied. It is found that the melt temperature has influence in the interaction between P and Sr. At about 1000°C, AlP is surrounded by Al4Sr phase to form encapsulation structure, which would react with Sr to form Sr3P2 at approximate 1300°C. However, P and Sr take the respective form of AlP and Al4Sr at 770°C, which just be the reason for the excellent complex modification of Si phase using continuous refining method. [Copyright &y& Elsevier]

Abstract: The solute nanostructures formed in the primary α-Al grains of a semi-solid metal cast Al–7Si–0.6Mg alloy (F357) during ageing at 180°C, and the age-hardening response of the alloy, have been systematically investigated by transmission electron microscopy, atom probe tomography and hardness testing. A 120h natural pre-ageing led to the formation of solute clusters and Guinier–Preston (GP) zones. The natural pre-ageing slowed down the precipitation kinetics 6-fold during 1h ageing at 180°C, but this effect diminished after 4h when the sample reached the same hardness as that without the pre-ageing treatment. It reduced the number density of β″ needles to approximately half of that formed in samples without the treatment, and postponed the peak hardness occurrence to 4h, four times that of the as-quenched sample. A hardness plateau developed in the as-quenched sample between 1 and 4h ageing corresponds to the growth of the β″ precipitates and a significant concurrent decrease of solute clusters and GP zones. The average Mg:Si ratio of early solute clusters is <0.7 while that of GP zones changes from 0.8 to 0.9 with increasing size, and that of β″ needles increases from 0.9 to 1.2. β″ needles, GP zones and solute clusters are important strengthening solute nanostructures of the alloy. The partitioning of solutes and precipitation kinetics of the alloy are discussed in detail. [Copyright &y& Elsevier]

Abstract: The three-dimensional (3-D) architecture of rigid multiphase networks present in AlSi10Cu5Ni1 and AlSi10Cu5Ni2 piston alloys in as-cast condition and after 4h spheroidization treatment is characterized by synchrotron tomography in terms of the volume fraction of rigid phases, interconnectivity, contiguity and morphology. The architecture of both alloys consists of α-Al matrix and a rigid long-range 3-D network of Al7Cu4Ni, Al4Cu2Mg8Si7, Al2Cu, Al15Si2(FeMn)3 and AlSiFeNiCu aluminides and Si. The investigated architectural parameters of both alloys studied are correlated with room-temperature and high-temperature (300°C) strengths as a function of solution treatment time. The AlSi10Cu5Ni1 and AlSi10Cu5Ni2 alloys behave like metal matrix composites with 16 and 20vol.% reinforcement, respectively. Both alloys have similar strengths in the as-cast condition, but the AlSi10Cu5Ni2 is able to retain ∼15% higher high temperature strength than the AlSi10Cu5Ni1 alloy after more than 4h of spheroidization treatment. This is due to the preservation of the 3-D interconnectivity and the morphology of the rigid network, which is governed by the higher degree of contiguity between aluminides and Si. [Copyright &y& Elsevier]

Abstract: The mechanical properties of aluminium–silicon casting alloys containing Cu and Mg are known to be improved by heat treatment. Over 60 papers are reviewed here in order to clarify the sequences of microstructural changes which occur during heat treatment, and their influence on the mechanical properties. It is shown that the changes occurring during solution treatment are relatively well understood, and that the equilibrium phase diagram can be used to predict the stability of phases at the solution treatment temperature. The influence of quench rate and natural ageing on subsequent artificial ageing needs to be studied further, but some conclusions can be drawn. These include: (1) An increase in quench rate above 4°C/s gives a small increase in yield strength after ageing, while the concomitant influence on elongation is more complicated and depends on the alloy. (2) Natural ageing is shown to have a large influence on subsequent artificial ageing response of Al–Si–Mg alloys, while there is a significant lack of knowledge for Cu-containing alloys. Artificial ageing of Al–Si–Mg alloys in the temperature range 170–210°C gives peak yield strengths of the same level, while Cu-containing alloys show a decrease in peak yield strength with increasing ageing temperature. The precipitation sequences in Al–Si–Mg and Al–Si–Cu alloys are relatively well known. In Al–Si–Cu–Mg alloys several precipitation sequences are possible, which need further investigation. [Copyright &y& Elsevier]

Abstract: TMF loading in diesel cylinder heads made of cast aluminium alloys is expected to steadily become more severe in the future. This increasing loading condition will lead to an early crack initialization and eventually determine component life. Against this background several cast aluminium alloys have been studied with respect to their thermal and mechanical properties. In addition to standardized specimens, near component-shaped samples were tested under thermomechanical fatigue conditions in a testing device which represents near-service conditions, in order to characterize life until a long crack is formed. The sample geometry tries to represent the valve bridge of a diesel cylinder head, which is known to be the most thermomechanically loaded and hence critical part in this component. The results of the TMF tests with this new type of sample were compared with the results obtained on standardized specimens. The experimental results formed a sound basis to improve the thermomechanical life prediction methodology for cast aluminium alloys. Two different life prediction models were used to assess the TMF life of the near-component samples. Furthermore, an aging model for cast aluminium alloys was derived from the experimental results including the pronounced change of the mechanical properties which result from the combined effects of aging and cyclic plastic deformation. The comparison of the experimental and predicted results clearly shows that the new model leads to an improvement of the TMF life prediction. [Copyright &y& Elsevier]

Abstract: A hot shear spinning process of cast aluminium alloy parts was developed to eliminate casting defects and to obtain a desired distribution of wall thickness. The forming load of the shear spinning is comparatively low even for large deformation requisite for eliminating the casting defects. Dendrites and shrinkage cavities in the cast aluminium alloys were successfully eliminated without occurrence of a fracture by the hot shear spinning. Surface cracks around the corner of the mandrel caused by large shear deformation were reduced by decreasing the feed rate of the roller and by increasing the tip radius of the roller. It was found that hot shear spinning is effective in improving the mechanical properties of cast aluminium alloys parts. [Copyright &y& Elsevier]

Abstract: Out-of-phase thermal–mechanical fatigue (TMF) tests with superimposed high-cycle fatigue (HCF) loading were performed at the cylinder-head alloy AlSi6Cu4. For continuously superimposed HCF loading during TMF cycles with maximum temperatures of 250 and 300°C a threshold of the amplitude of the superimposed HCF loading exists below which the number of TMF cycles to failure is not significantly affected. Above this threshold the TMF life shows a strong decrease with increasing HCF loading. This characteristic lifetime behaviour is also found in the case that HCF loading is only superimposed while tensile stresses are induced in the specimen during the TMF cycle. However, in the case that the HCF loading is superimposed only during the compressive phase of the TMF cycle, the number of TMF cycles to failure is not significantly affected. [Copyright &y& Elsevier]

Abstract: Out-of-phase thermal–mechanical fatigue (TMF) tests with superimposed higher frequency mechanical fatigue (HCF) loading were performed at three cast Al–Si cylinder head alloys. All materials show for maximum temperatures of the TMF cycle between 200 and 250°C a threshold of the amplitude of the superimposed HCF loading below of which the number of TMF cycles to failure is not significantly affected by the superimposed HCF loading. Above this threshold the TMF life shows a strong decrease with increasing HCF loading. Extensive microscopic analyses revealed that independent from the HCF loading first cracks are initiated at eutectic Si-particles detached from the surrounding α-Al matrix. Crack propagation occurs at small HCF amplitudes along the eutectic regions. At higher superimposed HCF loadings an accelerated crack propagation along slip bands within the α-Al matrix is observed. [Copyright &y& Elsevier]

Detailed material investigations of the fatigue behavior of two cast aluminium alloys used in combustion engines are presented. The network of intermetallic phases of both aluminium alloys is characterized by means of detailed energy dispersive X-ray spectroscopy. In order to investigate the temperature-dependent fatigue behavior of the materials, tensile, low cycle and thermomechanical fatigue tests are performed over a wide temperature and loading range. The influence of the temperature dependence on the experimental results is discussed. [ABSTRACT FROM AUTHOR]

In the present paper, the influence of locally varying microstructures in case of an AlSi12 cast aluminium alloy is investigated by means of extracting the test pieces from different removal positions and low cycle fatigue tests. The temperature-dependent damage mechanisms, the material specific defect types, sizes and their influence on the fatigue properties of two AlSi7 and AlSi12 cast aluminium alloys are studied. An extreme value statistics methodology is applied to predict maximum defect sizes expected in a critical surface volume from two-dimensional metallographic micrographs. A damage map for the AlSi12 cast aluminium alloy is presented explaining the influence of the temperature- and load-dependent damage mechanisms on the observed isothermal and thermomechanical lifetime behavior. [ABSTRACT FROM AUTHOR]

In this work, the influence of superimposed high cycle fatigue on the LCF/HCF and TMF/HCF lifetime is investigated for two cast aluminium alloys of the types AlSi7 and AlSi12. The replica technique is used to examine the short crack growth behavior under pure LCF and LCF/HCF loading. The observed short crack growth evolution explains the observed lifetime reduction with increasing HCF amplitudes. [ABSTRACT FROM AUTHOR]

• Effects of porosity on size effect and scatter of fatigue strength are investigated. • Two alloys with different degrees of porosity are used. • Magnitude of size effect and scatter are linked to the population defects. • High density and large size defects lead to minor size effect and less scatter. • Low density and small size defects induce large size effect and high scatter. Cast Al-Si alloys have been widely used in automotive applications with regard to their low density and excellent thermal conductivity. Many components made of these alloys are subjected to cyclic loads which can lead to fatigue failure. Furthermore, for these materials the well know size effect in fatigue, whereby the fatigue strength is reduced when the size is increased, can be significant and need to be properly evaluated. This paper analyses the role of casting defects on the fatigue strength's size effect sensitivity. A uniaxial fatigue testing campaign (R = 0.1) has been conducted using two cast aluminium alloys, fabricated by different casting processes (gravity die casting and lost foam casting), associated with the T7 heat treatment, and with different degrees of porosity. The fatigue response of different specimens (smooth and notched) with different stressed volumes has been investigated. The first part of this article is dedicated to the experimental characterization of the size effect in both alloys via the concept of the Highly Stressed Volume. The second part investigates the effect of the Highly Stressed Volume on the critical defect size via Kitagawa-Takahashi diagrams. The results show that the magnitude of the size effect and the experimental scatter are strongly linked to the characteristics of the defect population present in the alloy. It is revealed that the alloy B, with a high density of pore and a population of defects with relatively large size, shows non-significant size effect and less scatter in fatigue strength. In comparison, alloy A that exhibits a low density of pore and a population of defects of relatively small size manifests significant size effect and high scatter in fatigue strength. [ABSTRACT FROM AUTHOR]

Abstract: The influence of solidification rate on the solution treatment response for an Al–8Si–3.1Cu alloy has been investigated. The alloy was cast using the gradient solidification technique to produce samples with three different solidification rates. The samples were solution treated at 495°C for various times between 10min and 10h. The concentration of copper in the matrix was measured using the wavelength dispersive spectroscopy technique, WDS. The results show that the coarseness of the microstructure clearly affects the solution treatment time needed to dissolve particles and obtain a homogenous distribution of copper in the matrix. A short solution treatment time of 10min is enough to achieve a high and homogenous copper concentration for a material with a fine microstructure (secondary dendrite arm spacing, SDAS of 10μm), while more than 10h is needed for a coarse microstructure (SDAS of 50μm). A model was developed to describe the dissolution and homogenisation process. The model shows good agreement with the experimental results. [Copyright &y& Elsevier]