after 4 thermal cycles: micro-cracks parallel to sample length due to compression stresses after 100 thermal cycles: cracks perpendicular to sample length due to tensile loading shortly before failure: further growth and accumulation of cracks preferentially perpendicular to

sample length failure: main crack propagates along pre-existing damage and rigid phases

High volume fractions of highly interconnected rigid phases can lead to a high tensile strength of the alloy at elevated temperatures, however the consequentially low ductility limits the TMF-resistance

Closely packed primary Si and intermetallic phases facilitate crack formation and propagation more refined structures have a positive effect on the TMF resistance

Damage mechanisms and evolution during thermomechanical fatigue of cast near eutectic Al-Si piston alloys

K. Bugelnig1, H. Germann2, T. Steffens2, F. Wilde3, E. Boller4, G. Requena5,6

1Institute of Materials Science and Technology, TU Vienna, Karlsplatz 13, 1040 Vienna, Austria 2KS Kolbenschmidt GmbH, Karl-Schmidt-Straße, 74172 Neckarsulm, Germany 3Institute of Materials Research, Helmholtz-Zentrum Geesthacht, D-21502 Geesthacht, Germany 4ESRF-The European Synchrotron, CS40220, Grenoble Cedex 9, France 5German Aerospace Centre, Linder Höhe, 51147 Cologne, Germany 6Metallic Structures and Materials Systems for Aerospace Engineering, RWTH Aachen University, Aachen 52062, Germany

Acknowledgements This work is part of the “K-Project for Non-Destructive Testing and Tomography Plus" supported by the COMET-Program of the Austrian Research Promotion Agency (FFG) as well as the Provinces of Upper Austria (LOÖ) and Styria, Grant No. 843540. The ESRF and DESY are acknowledged for the provision of synchrotron facilities at the beamlines ID19 and P05, respectively. Contact: katrin.bugelnig@tuwien.ac.at

Motivation Cast near eutectic Al-Si alloys are used in automotive pistons due to their high strength-to-weight ratio as well as excellent castability and heat conductivity. Particularly, the piston bowl rim in modern diesel engines must be able to withstand thermo-mechanical fatigue (TMF) conditions in a temperature range between 25 - 380 °C with thermal cycles of a few seconds during service. Topological and morphological changes in the microstructure owing to chemical compositions, thermal heat treatments and thermo-mechanical conditions during service can have a crucial effect on performance. Damage mechanisms and evolution during TMF of two Al-Si piston alloys have been studied with regards to the effect of 3D microstructural features.

Alloy Composition [wt.%]

Al Si Cu Ni Mg

AlSi12Cu3Ni2 bal. ~ 12 3 2 < 0.3

AlSi12Cu4Ni2Mg bal. ~ 12 4 2 1

Materials Methods

• gravity die casting • heat treatment: T5

primary Si

eutectic Si

aluminides

primary Si

eutectic Si aluminides

3D hybrid network of rigid phases

Microstructure in initial condition t [sec]

T [°C]

380

80

25 ε = 0 %

dT/dt = 15 K/sec

3 sec

10 sec

1. Gleeble 1500

piston bowl rim area

1 cm

plastic deformation [mm]

of the piston due to thermo-mechanical

loading

50 µm 50 µm

AlSi12Cu3Ni2 AlSi12Cu4Ni2Mg 2. Synchrotron tomography

Results

0 40 80 120 160 200 240 280 320 360

-200

-150

-100

-50

0

50

100

150

200 first 10 cycles

half of cycles to failure)

last 10 cycles

str

ess [

MP

a]

T [°C]

compression range

tensile range

Conclusions preferential damage initiation sites during TMF: micro-cracking through primary Si particles in Si clusters at junctions of coalesced primary Si

particles

AlSi12Cu4Ni2Mg AlSi12Cu3Ni2

Volume (primary Si) > 34500 µm³

Cu-rich alloy is prone to form large primary Si clusters during

solidification

AlSi12Cu3Ni2 AlSi12Cu4Ni2Mg

• high TMF resistance (822 ± 107 cycles) • comparatively higher ductility and lower

strength due to reduced Cu and Mg contents • comparatively small homogeneously

distributed primary Si particles (~ 8-10 µm) • nearly no primary Si clusters present

resulting in less sites for damage initiation and accumulation

• lower TMF resistance (about 40%) • lower ductility and higher strength due to

high volume fraction of intermetallic phases • larger primary Si particles (~ 10 - 15 µm)

• primary Si clusters provoke earlier formation

and higher accumulation of damage in these regions

initial condition micro-cracks through large, irregular primary Si (parallel to sample length)

after 4 cycles

micro-cracks through primary Si (parallel & perpendicular to sample length)

after 100 cycles

after 350 cycles post-mortem (504 cycles)

σ

100 µm

100 µm 100 µm

40 µm

10 µm

σ

primary Si

eutectic Si

aluminides 4 cycles 100 cycles 350 cycles post-mortem

200 µm

σ

200 µm 200 µm 200 µm

primary Si

micro-cracks

10 µm

σ

5. 3D visualization of damage evolution after selected TMF cycle numbers for AlSi12Cu4Ni2Mg

6. tomographic slices of the microstructure of AlSi12Cu4Ni2Mg after several TMF-cycles

4. Damage initiation preferentially in shape of micro-cracks parallel to sample length through large, irregular primary Si in clusters

1. cyclic stress evolution during TMF

10 µm

P05 ID19

Energy [keV] 23 19

Voxel- Size [µm³] (1.18)3 (0.33)3

Sample-to-detector distance [mm]

30 13

Exposure time [msec/proj] 1200 100

Nr. of proj. 900 5969

total scan time [h] 1.5 0.2

Beamline Parameters

Beamlines

Storage ring

Booster synchrotron Linear

accelerator

0

1.5x10-6

3.0x10-6

4.5x10-6

0 100 200 300 400 500 600 700 800 900

0

0.03

0.06

0.09

0.12

num

be

r d

en

sity

of vo

ids [µ

m-1]

AlSi12Cu3Ni2

AlSi12Cu4Ni2Mg

AlSi12Cu3Ni2

AlSi12Cu4Ni2Mg

vo

lum

e fra

ctio

n

of vo

ids [%

]

number of cycles to failure

0

200

400

600

800

1000

nu

mb

er o

f cyc

les

to f

ailu

re

AlSi12Cu3Ni2 AlSi12Cu4Ni2Mg

3. evaluation of damage during TMF

2. number of TMF-cycles to failure

Source: KS Kolbenschmidt GmbH

0.207

0.429

0.254

0.186 0.175

0.171

0.203

0.416

0.323

0.247