DFG-Projekt Schm 746/64-2
The effect of interdendritic solidification pores on the mechanical
properties of AlSi-cast alloys
Project begin: 01.02.2012
Project end: 31.01.2015
Aims
This proposal is the extension of the joint project DFG Schm 746/64-1 and AP 196/11 „The effect of interdendritic solidification pores on the mechanical properties of Al7%Si-0.3%Mg-Alloys“. Our aim is now to combine and extend the computational
models developed in the previous period to allow a comprehensive simulation of
microstructure and porosity formation together with the analysis of the mechanical
properties on the relevant length scales. To this end, the phase field method for the
microstructure predictions, including porosity formation, finite element simulations of
fracture behaviour based on the embedded cell model (real microstructure) and
cohesive zone elements, the element elimination technique for the prediction of the
mode of fracture and the crack path propagation, taking into account the particular
microstructure, will be combined. We will concentrate on the effect of interdendritic
“micropores” (d  20 µm) rather than bigger (up to d  400 µm) intragranular pores
Methods
For the investigations of the influence of microporosity the following methods are
used: metallographical investigations, tensile testing and fracture experiments as well
as phase-field method simulations for microstructure formation and FEM simulation
for simulations of mechanical behaviour:
Tomographical investigations
.
Metallographical investigations
Both tomographical and metallographical surface characterization testifies to
the existence of small pores (diameter around 20 mm) and big pores (200-400
mm).
Tensile test experiments
With heat-trtreatment
Without heat-trtreatment
140
250
100
Stress, MPa
Stress, MPa
120
Casting 4(Porosity 0,62%)
Casting 2(Porosity 1,62%)
Casting 6(Porosity 1,32)
80
60
40
200
150
Casting 2(Porosity1,62%)
Casting 4(Porosity 0,62%)
Casting 6 (porosity 1,32 %)
100
50
20
0
0.000
0.005
0.010
0.015
0.020
0
0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014
0.025
Strain
Strain
Tensile test experiments show that after heat treatment the strength significantly
increases but ductility decreases, the stress-strain curves differ by failure strain
FEM-simulations of crack propagation in microstructure with
Element Elimination technique
Evolution of crack propagation in the cut-out of1.1Al-7%Si-0.3%Mg casting alloy
A=1.41, B=1.035
1.0
0.9
pl
0.8
failure
0.7
0.6
0.5
0.4
(c, pl,c)
no failure
0.3
0.2
0.1
0.0
0.4
0.6
0.8
1.0
1.2
200
1.4

1.6
1.8
2.0
2.2
2.4
without pores
Stress, MPa
150
with pores
100
50
0
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
Strain
Porosity effects the crack propagation in microstructure deviating the crack
from its initial path at the later stages of simulations and the stress-drop on
stress-strain curve is clearly pronounced in the microstructure with porosity.
G. Lasko, M. Apel, A. Carre, U. Weber and S. Schmauder, Adv. Eng. Mat. 14, pp. 236247 (2012)
Combined simulations of C(T)-test on macro- and micro-level
For the investigations of the size of micropores on the crack path in cut-out of
microstructure the cut-out of microstructure with the same volume fraction of
porosities but different pore sizes have been considered, by embedding the
piece of microstructure in macro- compact tension test specimen and
simulations on both scale levels have been perfo Micro-level l=0.001rmed in one
FE-simulation run.
Macro-level l=0.1 mm
Fracture
Energy,
N/mm
3.6
Cohesive
strength,
MPa
150
1400
1200
Force, N
1000
800
600
Big pores
Small pores
400
200
0
0.00
0.04
0.08
0.12
COD, mm
F-COD curve matching the
instance of crack entering the
cut-out of microstructure
Micro-level
Cohesive
stiffness,
MPa
70000
l=0.001
The results obtained within the project have been presented at the following
conferences
1. ICCE-21 Tenerife, Spain July 21-27, 2013
2. International Conference ‘Hierarchically built systems of orgenic and
inorganic nature, September 9-13, 2013
Acknowledgement
The investigations are performed in the framework of German Research
Foundation, Schm 746/64-2. The financial support is highly appreciated.
Contact persons
Dr. rer. nat. Galina Lasko
Institute for Material testing, Material Science and Strengt hof Materials (IMWF)
University of Stuttgart
Pfaffenwaldring 32
70569 Stuttgart
Tel.: +49 / 711 685-62559
E-Mail: [email protected]
Prof. Dr. rer. nat. Siegfried Schmauder
Tel.: +49/711/685-62556
Fax: +49/711/685-62635
E-mail: [email protected]
Dr.-Ing. Ulrich Weber
Tel.: +49 / 711 685-63055
Fax: +49 / 711 685-62635
E-mail: [email protected]