# Study of damage mechanisms in A319 aluminium alloy by X-ray tomography and Digital Volume Correlation

Nora Dahdah, Nathalie Limodin, Ahmed El Bartali, Jean-Francois Witz, Rian Seghir, Eric Charkaluk, Jean-Yves Buffiere

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Nora Dahdah, Nathalie Limodin, Ahmed El Bartali, Jean-Francois Witz, Rian Seghir, et al.. Study of damage mechanisms in A319 aluminium alloy by X-ray tomography and Digital Volume Correlation. Photomechanics, May 2015, Delft, Netherlands. [hal-01729131](https://hal.science/hal-01729131v1)

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Distributed under a Creative Commons Attribution 4.0 International License
STUDY OF DAMAGE MECHANISMS IN A319 ALUMINIUM ALLOY BY X-RAY TOMOGRAPHY AND
DIGITAL VOLUME CORRELATION

N.Dahdah¹, N.Limodin¹, A.El Bartali¹, J.F.Witz¹, R.Seguir¹, E.Charkaluk¹ and J.Y.Buffiere²

¹LML, Ecole Centrale de Lille, Cité Scientifique, 59650 Villeneuve d'Ascq, France
(nora.dahdah@ec-lille.fr ; nathalie.limodin@ec-lille.fr ; ahmed.elbartali@ec-lille.fr ;
jean-francois.witz@ec-lille.fr ; rian.seghir@ec-lille.fr ; eric.charkaluk@ec-lille.fr)

² MATEIS, INSA Lyon, Avenue Jean Capelle 69621 Villeurbanne cedex, France
(jean-yves.buffiere@insa-lyon.fr)

**Key words:** Fatigue, Aluminum-silicon alloys, Digital Volume Correlation, synchrotron X-ray tomography

ABSTRACT:

In the cylinder heads produced by the Lost Foam Casting process, the microstructure consists of hard intermetallic phases and large gas and microshrinkage pores. In order to study the influence of this complex 3D microstructure on fatigue crack initiation and propagation, an experimental protocol using laboratory and synchrotron tomography, Finite Element simulation and 3D Digital Volume Correlation has been used. Tests performed at low temperatures (room temperature and 150°C) revealed the initiation of 3D cracks at large pores and a propagation along the hard inclusions towards the free surface. At temperatures characteristics of in-service conditions (above 200°C), an additional damage mechanism was observed: cracks were detected in silicon particles around the main pore that drove to failure but also in other areas of the specimen gauge length.

# 1. INTRODUCTION

In the automotive industry, the conventional die casting process is progressively being replaced by the Lost Foam Casting (LFC) process for the purpose of geometry optimization, cost reduction and consumption control. However, aluminium alloy automotive parts produced by the LFC process have a coarser microstructure and more porosity defects than parts produced with conventional casting processes at faster cooling rates. The cast Aluminium-Silicon alloy (A319) used for automotive cylinder heads has a multiscale microstructure: porosity with a size that ranges from small gas pores to large microshrinkage cavities, iron intermetallics, eutectic Al₂Cu and eutectic silicon. In order to study the influence of this microstructure upon the low cycle fatigue behaviour, an experimental protocol has been set up using X-ray tomography and 3D Digital Volume Correlation (DVC).

Small specimens were cut out from automotive cylinder heads. A preliminary characterization by laboratory X-ray microtomography was performed at a medium resolution (voxel=4.5µm) in order to:
1. select the most suitable specimens,
2. characterize in 3D the population of defects in the specimen gauge length,
3. obtain a 3D mesh of the porous aluminium matrix in order to compute the associated strain fields using finite element software (Abaqus).
These preliminary steps help focusing the scanned volumes to an observation region where cracks are more likely to initiate.
Low Cycle Fatigue tests were then realized under synchrotron X-ray tomography at the European Synchrotron Radiation Facility (ID19 beamline) using an in-situ test rig developed at MATEIS laboratory to study crack initiation and growth in bulk at room temperature and at high temperatures that are representative of the in-service loadings of cylinder heads, i.e. 150, 200 and 250°C. Images were acquired in pink beam mode (E=35 keV) with a CMOS PCO Dimax detector (2048 pixels²). The scan duration was 45 s (2000 radiographies) with a 2.75 µm voxel size; it was short enough to avoid creep/relaxation effects at the temperatures investigated (T<250°C). Seven fatigue specimens with a 2\*2 mm² cross section were tested: 2 at 150°C, 2 at 200°C and 3 at 250°C. Uniaxial fatigue tests at constant stress amplitude were performed with a load ratio of 0.1 and a maximum stress of the order of 150% of the yield stress at the corresponding temperature. Tests were conducted until crack initiation or failure with periodic interruptions to acquire tomographic scans under minimum and maximum load of the considered cycle in order to detect and follow fatigue crack initiation and growth. All the 3D images were reconstructed by using both the classical filtered backprojection algorithm [1] and the Paganin reconstruction to have a better contrast between the different phases including eutectic Silicon [2]. The grey level images were binned to reduce the amount of data and the time needed to process them; these results are in an effective voxel size of 5.5µm. Synchrotron tomography allowed visualizing the eutectic Al-Al₂Cu, iron based intermetallics phases and above all eutectic Si, which could not be distinguished with laboratory tomography; these constituents were proved a suitable natural speckle for DVC.

The 3D displacement field was measured with the DVC technique [3]. In the present study, a 3D image correlation platform recently developed at LML laboratory (Lille, France) has been used. This platform is based on C++ and it has been optimized to process large 3D volumes in a limited time. For a 600x600x1008 voxels image the calculation lasts about 40min with an isotropic element of 16 voxel edge on a 32Go RAM laptop. The correlation calculation is a sequence of optical flow integrated and finite element methods using regularization with a median filter (window of 3 pixels) [4]. The displacement field measurements allowed identifying and tracking the development and localization of deformation, and allowed identifying the initiation sites of microcracks during the low cycle fatigue test in relation with the observed microstructure. 3D observations give access to 3D information on the cracks and on the strain field but are limited to coarse image resolution as compared to 2D optical observations.
## 2. RESULTS AND DISCUSSION

<figure><img src="image_2.png" /><figcaption>Figure 1- Tomography images showing cracks localization at (a) 100°C (N<sub>cycles</sub> = 10 000, σ<sub>max</sub> = 90MPa), (b) 250°C (N<sub>cycles</sub> = 30, σ<sub>max</sub> = 75MPa)</figcaption></figure>

At any temperature, the main parameter that influences crack initiation is the porosity. Crack always initiated close to a pore, and often at the subsurface (Fig.1a & b).

The results obtained at room temperature [5, 6] at different numbers of fatigue cycles allowed the observation of crack initiation close to the largest pores and of crack propagation along the hard inclusions towards the free surface.

At 150°C, the mechanism of propagation seems similar: cracks nucleated at hard inclusions in a high stress concentration zones between pores, then cracks propagated between these pores along the hard inclusions towards the free surface.

At temperatures above 200°C, an additional damage mechanism was observed: cracks were detected in silicon particles (Fig.2b) around the main pore that drove to failure but also in other areas of the specimen gauge length (Fig.2c). Micro-cracks seem to be in majority in the silicon particles although some are also visible in the hard second phases.

<figure><img src="image_3.png" /><figcaption>Figure 2- Cracks observation of the specimen at 250°C, 610µm below the surface, σ<sub>max</sub> = 75MPa, at (a) before failure (40th cycle) in the (b) failure zone and (c) far away from the principal crack</figcaption></figure>

3D images obtained by laboratory tomography were used to generate microstructurally realistic meshes of the microstructure within the pristine samples (pores + matrix, no intermetallic second phase); those meshes were used to perform Finite Element (FE) simulation of the strain/stress distribution during monotonic loading (elasto plastic calculation), i.e. at the maximum load of the first half-cycle. An example of these simulations is shown in figure 3: a large strain concentration can be seen close to the pore cluster where crack initiation was indeed observed. Figure 3 also shows the strain field calculated by DVC along the loading direction at the first cycle between the minimum and maximum load; the image of the microstructure was superimposed to this field to allow comparison of the crack path with local deformation. The DVC results are in good agreement with the FE simulation regarding the strain concentration
around the pores; however a large strain accumulation close to clusters of hard second phase particles is also detected by DVC but not by the FE simulation which is based on a simplified mesh (see arrows).

<figure><img src="image_4.png" /><figcaption>Figure 3- Tomographic slices (a) & (b) of fatigue cracks in AlSi7Cu3 specimen initiated from a subsurface pore after 40 cycles at 250°C, 335 µm below the surface, σmax= 75MPa with (c) the corresponding strain field measured with DVC and (d) the strain field computed by FE simulation during monotonic loading (elasto plastic calculation), i.e. at the maximum load of the first half-cycle of fatigue.</figcaption></figure>

The 3D microstructure observations are compared with the DVC analysis to better understand the fatigue mechanisms. The different figures illustrated the displacement field, the strain field and the microstructure at 250°C after 5 cycles (Fig.4), and 40cycles loading, just before the final failure (Fig.5).

After 5 cycles loading, strain localization begins to be visible, especially in the area where a crack initiated (see the red circle in Fig.4c). Crack propagation is observed in Fig.5, indeed displacement discontinuities are clearly visible in the crack zone and they are in accordance with the strain localization and the crack location. The most striking phenomenon observed is the silicon cracking (Fig. 5c).

<figure><img src="image_5.png" /><figcaption>Figure 4- Slice view at 477 µm below the surface; (a) 3D displacement field (in voxels; 1 voxel=5.5µm) along the loading direction z, (b) εzz strain, and (c) the crack location for a fatigue test at 250°C, Ncycles=5, σmax= 75MPa</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5- Slice view at 477 µm below the surface : (a) 3D displacement field (in voxels; 1 voxel=5.5µm) along the loading direction z, (b) εzz strain, and (c) the crack location for a fatigue test at 250°C, N<sub>cycles</sub>=40, σ<sub>max</sub> = 75MPa</figcaption></figure>

The efficiency of the experimental protocol using laboratory and synchrotron tomography, FE simulation and 3D DVC to study the influence of the casting microstructure upon the mechanical properties of an Al-Si alloy has been proved. The DVC analysis of the 3D images recorded in situ during fatigue tests helps to understand the relations between initiation sites and crack path and the local microstructural features: crack initiation is porosity driven while propagation is correlated with the presence of hard intermetallic phases. It also gives some very interesting information on the degree of complexity which has to be taken into account in so-called "realistic" microstructures to reproduce actual strain development within the samples.

ACKNOWLEDGMENTS

The authors wish to thank the ANR (Agence Nationale de la Recherche) MatetPro project INDiANA (ANR-12RMNP-0011) for funding the study on Al-Si aluminum alloys. The ESRF (European Synchrotron Radiation Facility) is also acknowledged for providing beamtime on the ID19 beamline; Dr. Elodie Boller and Dr. Vincent Fernandez from ESRF are particularly acknowledged for their help and advices with the Synchrotron tomography experiment.

REFERENCES

1. A.C. Kak, M. Slaney, "Principles of Computerized Tomographic Imaging (Classics in Applied Mathematics)" Paperback – July 1, 2001

2. D. Paganin, S.C. Mayo, T.E. Gureyev, P.R. Miller, S.W. Wilkins, "Simultaneous phase and amplitude extraction from a single defocused image of a homogeneous object", Journal of Microscopy, Vol.206, 2001.

3. J. Réthoré, N. Limodin, J-Y. Buffiere, F. Hild, W. Ludwig, S. Roux, "Digital volume correlation analyses of synchrotron tomographic images," Journal of Strain Analysis for Engineering Design, 46 (2011), 683-695.

4. P. Cachier, E. Bardinet, D. Dormont, X. Pennec, N. Ayache. "Iconic feature based nonrigid registration: the PASHA algorithm." Computer Vision and Image Understanding, 89(2003): 272–298, 2003. ISSN 1077-3142

5. N. Dahdah, N. Limodin, A. El Bartali, J.F. Witz, R. Seguir, L. Wang, E. Charkaluk, J.Y. Buffiere, "Influence of the lost foam casting microstructure on low cycle fatigue damage of A319 aluminium alloy", 3DMS 2014, Annecy, France.

6. L.Wang, Influence of the casting microstructure on damage mechanisms in Al-Si alloys by using 2D and 3D in-situ analysis, Thesis 2015.