- Email: [email protected]
On the strain hardening behaviour of Al-Si-Mg casting alloys
Materials
Science
and Engineering
On the strain hardening
A234-236
(1997)
behaviour
1066109
of Al-Si-Mg
casting alloys
Q.G. Wang, C.H. Chceres * CRC,for
Alloy
and Solidijcation
Technology
(CAST),
Received
Department Brisbane
14 January
of Mining, Minerals QLD 4072, Austrulia
1997; received
in revised
and Materials
form
1 April
Engineering,
The University
of Queensland,
1997
Abstract The dependence of the strain hardening rate on the size and been studied using monotonic tensile testing and Bauschinger elongated Si particles and higher Mg content. At large strains, internal stresses increase rapidly at small strains, saturating elongated Si particles, small dendrites or higher Mg content. Keywords:
Strain
hardening:
Bauschinger
effect;
Dendrite
arm spacing;
1. Introduction The eutectic Si particles and dendrite arm spacing (DAS) have a strong effect on both the strain hardening and fracture behaviour of Al-Si-Mg casting alloys [l-4]. Plastic deformation results in the cracking of a large fraction of the Si particles, indicating that load is shed onto the particles. Thus, an understanding of the load shedding mechanisms is important to understand the micromechanics of fracture. In this work the influence of the different components of the microstructure on the strain hardening behaviour has been evaluated using tensile testing and Bauschinger effect experiments, for two Mg contents, 0.4 and 0.7%.
shape of the Si particles, dendrite arm spacing and Mg content effect. At low strains, the hardening rate is higher for material the hardening rate is higher for material with small dendrites. at large strains. The saturation value is higher for material 0 1997 Elsevier Science S.A. Al casting
has with The with
alloys
matrix of the casting alloys was tested for comparison. Tensile testing was performed on samples with gauge length 15 mm and 4 x 5 mm2 cross section, at a strain rate of 10~ 3 s-i. Bauschinger testing was performed on sampleswith a gauge region 18 mm long and 8 mm in diameter. A multi-loop technique [5] was used (see Fig. 1).
400,
,
,
I
,
I
,
I
,
.
,
.
,
.
,
loop
_
300 200 -
2. Experimental procedure
f b 22
Two commercial Al-7%Si-0.4/0.7%Mg alloys, (Australian alloys AA601/603, equivalent to the US A356/ 357, respectively) unmodified and Sr-modified were used [2]. All alloys were solution heat treated for 20 h at 540°C pre-aged 20 h at room temperature and aged 6 h at 170°C. An AlL0.8%Si-0.5%Mg wrought alloy (alloy AA6063) with Mg content close to that of the
co : 2
* Corresponding 33653888; e-mail: 0921-5093/97/$17.00
author. Tel.: + 61 7 33654377; [email protected] Q 1997 Elsevier
PII SO921-5093(97)00207-4
Science
S.A. All
fax:
rights
+
61 7
reserved.
100
-
O-100
-
-200
-
-300
-
single
----400 -0.01
’
’ 0.00
Fig. I. Comparison curves.
m
’ 0.01
of single,
*
’ ’ 0.02 true plastic
multi-loop
’ 0.03 strain
’
’ 0.04
and monotonic
monotonic n
’ 0.05
n
0.06
stress-strain
Q. G. Wang,
C.H.
C&eves
/Materials
Science
and Engineering
20
I
I
I
(1997)
I
I&-
I
107
109
,
I
,.’
, .-
1
4I
400 -
A234-236
16
I , , stra,n = 0.0015
-
0
-
,-i--
08 16 -
l @
=;
l
0
AA603-Sr
A A
@.-@y 14-
**603-“nl-
0
!
.o*
*ASO,-"FIl AA603-Sr
8
300 2 I : 2! z ii Z
12 -
* . __--
. 1
10 -
f
-----A
__--
A
s-'------
200 2.0 aspect
J
alloy 100 -
01
” 0.00
dv/df at strain 0.0015 (GPa) -
-
AA603-urn
16
-----
AAGOI-urn
11
--
AA6063
” 0.02
” ” ” 0.04 0.06 0.08 true plastic strain
Fig. 2. True stress-true plastic strain curves showing Si particles and Mg content on the flow behaviour (urn) alloys.
Fig. 3. The strain hardening coefficient as a function of the aspect ratio of the Si particles, measured at an applied plastic strain of 0.0015 (Sr indicates modification with strontium).
6 “. 0.10
0.12
the effect of the of unmodified
3. Results 3.1. Flow behaviour The influence of the Si particles and the Mg content on the flow behaviour is shown by Fig. 2. It is seen that the casting alloys show a higher yield stress as well as significantly more strain hardening at low strains than the wrought alloy. The effect of the aspect ratio of the Si particles on the strain hardening coefficient (defined as da/de, where c is the flow stress and E the strain) at low strains is shown in Fig. 3. For a given Mg content the work hardening rate increases linearly with the aspect ratio of the Si particles, a, while at a given aspect ratio the higher Mg content alloy has a higher hardening rate. The strain hardening rate at large strains is shown in Fig. 4, and it seen to increase (at constant Si particle aspect ratio) for the smaller DAS. 3.2. Bauschinger
2.2 ratio
effect
Metals with two or more phases often exhibit a decrease in the reverse yield stress when they are deformed cyclically (Bauschinger effect). The softening observed when the strain is reversed can be described by the Bauschinger stress parameter (BSP) [3],
where of is the maximum forward flow stress in each cycle and 0’ is the reverse flow stress (measured at a reverse offset strain of 0.002). The term c,, is called the back stress,and represents the stressthe elastic particles exert on the plastic matrix. 3.2.1. Effect of DAS and Si particles Fig. 5 shows the BSP for materials with similar Si aspect ratio but different DAS, as a function of the strain. It can be seenthat for strains of up to about 1% the BSP increasesrapidly for all alloys. Thereafter, the BSP continues to increase but at a slower rate, saturating at strains of about 0.02. The saturation value appears to be larger for the finer DAS, a result expected from Fig. 4. The influence of the aspect ratio of the Si 1000
8
Q
I
-
1
u
I strain
2
600 -
'
I
-
5 0.04-0.045
+
AA603
0
AA601
I 22 2 'I a, E 2
-
'EL+$
+ 'o-.;
600 -
0
0
0
+ * --__
&
--D-+
.s 2 a
+
4oo. 10
20
30
40 (run)
0
+ - - -+ -9. cl+.
_
0
50
60
70
DAS
Fig. 4. The strain hardening coefficient measured at applied plastic strains between 0.04 and 0.045, as a function of the DAS. Results for alloy AA601 and dashed line from [3]. Aspect ratio: 1.6-2.0.
108
Q.G.
Wang,
C.H.
Chceres / Materials
Science
0.4 1
0.3 -
i
0.2 alloy 0.1 AA6O3-wnlSr A.AGOl-urn 0.0 -
AA603.5, U 0
-0.1
A
’ 0.00
0
’ 0.02
3
’ 0.04
1.A.,.7
55-65
’
’ 0 0.06 plastic strain
’ 0.06
AAGO,.S, 8
’ 0.10
a
Fig. 5. The BSP as a function of plastic strain for materials similar aspect ratio of the Si particles but different DAS.
0.12
with
particles is shown in Fig. 6. As in Fig. 5, the BSP increases rapidly at small strains but gradually saturates at large strains ( > 0.02). Elongated Si particles (i.e. large a-values) result in higher saturation values.
4. Discussion 4.1. Strain hardening Fig. 2 shows that the strain hardening rate at low strains is higher for the casting alloys. The strain hardening rate at a strain of 0.0015 is of the order of 16 GPa for alloy AA603, 11 GPa for alloy AA601 and only 6 GPa for alloy AA6063. Since the Mg content of the (single phase) wrought alloy is intermediate between the two casting alloys, the differences on the hardening
A
0.3 -
A
0.2 -
A2346236
(1997)
106-109
rate can only result from the effect of the Si particles [3]. In fact, a high hardening rate at low strains is normally observed when particles are added to a ductile matrix [5]. On the other hand, since the Si content of the two casting alloys is similar, the difference between them can only be ascribed to the difference in Mg content. Magnesium may also affect the content and nature of the Fe-rich intermetallics [6] in alloy AA603, but this possibility has been ignored in this work. To understand the effect of Mg on the hardening rate of the casting alloys at low strains, the different contributions to the strain hardening need to be considered. At low strains, plastic deformation results in the accumulation of Orowan loops around the reinforcing particles. This is called the linear hardening regime [3,7-91 and a rapid increase of flow stress with the applied strain is normally observed. At increasing strains the stressesaround the particles induce cross slip and secondary slip. Continued plastic straining results now in strain hardening due to the formation of a forest of dislocations near the reinforcing particles, but at a much reduced rate. This is called the parabolic hardening regime [7-91. The transition between the two regimes is likely to be controlled by the relative strength of the matrix [5]. Thus, for the stronger, higher Mg alloy, the onset of plastic relaxation will be delayed, increasing the contribution from the linear regime to the total hardening. At large strains, the dendritic cells behave like grains in a polycrystal [3] and thus the flow stresscan be expected to depend on (e/DAS)‘12 [9], as indeed observed (dashed line) in Fig. 4. 4.2. Bauschinger effect The results in Figs. 5 and 6 suggest that the back stress increases more or less linearly up to strains of about l%, gradually saturating afterwards. This behaviour is also consistent with inhomogeneous deformation at low strains, leading to a linear hardening at low strains, followed by low hardening rate at higher strains, when plastic relaxation limits the accumulation of internal stresses[3,7-91.
5. Conclusions
i aspect ratio, II
DAS (i/m)
0
2.2-2.4
55-65
AA603-urn
x
2.5-2.6
55-65
AABOl-urn
0
1.5-l
55-65
AA603.Sr
A
1.5-I .7
0.1 -
0.0 -x”
-8 -0.1 0.00
and Engineering
8
’ 0.01
m
’ 0.02
Fig. 6. The BSP as a function the same DAS but a different
*
’ n ’ 0.03 0.04 elastic strain
.7 ’
alby
55-65 AA60,-Sr ’ 8 ’ ’ 0.05 0.06
of the plastic strain for materials aspect ratio of the Si particles.
+
0.07
with
During uniaxial tension, the strain hardening of the materials is dependent on the Si particle morphology, dendrite arm spacing and Mg content. At low strains the Si particle aspect ratio and matrix strength dominate the work hardening, while at large strains the hardening rate depends on the DAS. The internal stressesincrease very rapidly at small strains, saturating at large strains. Elongated Si particles, fine DAS or higher Mg content result in a higher saturation value.
Q.G.
Wang,
C.H.
Chceres /Materials
Science
References [1] S.F. Frederick, W.A. Bailey, Trans. Metall. Sot. AIME 242 (1968) 2063-2067. [2] C.H. Clceres, C.J. Davidson, J.R. Griffiths, Mater. Sci. Eng. 97 (1995) 171-179. [3] C.H. CXceres, J.R. Griffiths, P. Reiner, Acta Mater. 44 (1996) 25-34.
and Engineering
A234-236
[4] C.H. CBceres, [5] S.F. Corbin, 1319-1327. [6] D.A. Granger, Foundrymen’s [7] L.M. Brown, [S] L.M. Brown, [9] M.F. Ashby,
(1997)
106-109
109
J.R. Griffiths, Acta Mater. 44 (1996) 15-24. D.S. Wilkinson, Acta Metall. Mater. 42 (1995) R.R. Sawtell, M.M. Kersker, Trans. Am. Sot. 92 (1984) 579-586. W.M. Stobbs, Philos. Mag. 23 (1971) 1185-1200. W.M. Stobbs, Philos. Mag. 23 (1971) 1201-1233. Philos. Mag. 21 (1970) 399-424.
Science
and Engineering
On the strain hardening
A234-236
(1997)
behaviour
1066109
of Al-Si-Mg
casting alloys
Q.G. Wang, C.H. Chceres * CRC,for
Alloy
and Solidijcation
Technology
(CAST),
Received
Department Brisbane
14 January
of Mining, Minerals QLD 4072, Austrulia
1997; received
in revised
and Materials
form
1 April
Engineering,
The University
of Queensland,
1997
Abstract The dependence of the strain hardening rate on the size and been studied using monotonic tensile testing and Bauschinger elongated Si particles and higher Mg content. At large strains, internal stresses increase rapidly at small strains, saturating elongated Si particles, small dendrites or higher Mg content. Keywords:
Strain
hardening:
Bauschinger
effect;
Dendrite
arm spacing;
1. Introduction The eutectic Si particles and dendrite arm spacing (DAS) have a strong effect on both the strain hardening and fracture behaviour of Al-Si-Mg casting alloys [l-4]. Plastic deformation results in the cracking of a large fraction of the Si particles, indicating that load is shed onto the particles. Thus, an understanding of the load shedding mechanisms is important to understand the micromechanics of fracture. In this work the influence of the different components of the microstructure on the strain hardening behaviour has been evaluated using tensile testing and Bauschinger effect experiments, for two Mg contents, 0.4 and 0.7%.
shape of the Si particles, dendrite arm spacing and Mg content effect. At low strains, the hardening rate is higher for material the hardening rate is higher for material with small dendrites. at large strains. The saturation value is higher for material 0 1997 Elsevier Science S.A. Al casting
has with The with
alloys
matrix of the casting alloys was tested for comparison. Tensile testing was performed on samples with gauge length 15 mm and 4 x 5 mm2 cross section, at a strain rate of 10~ 3 s-i. Bauschinger testing was performed on sampleswith a gauge region 18 mm long and 8 mm in diameter. A multi-loop technique [5] was used (see Fig. 1).
400,
,
,
I
,
I
,
I
,
.
,
.
,
.
,
loop
_
300 200 -
2. Experimental procedure
f b 22
Two commercial Al-7%Si-0.4/0.7%Mg alloys, (Australian alloys AA601/603, equivalent to the US A356/ 357, respectively) unmodified and Sr-modified were used [2]. All alloys were solution heat treated for 20 h at 540°C pre-aged 20 h at room temperature and aged 6 h at 170°C. An AlL0.8%Si-0.5%Mg wrought alloy (alloy AA6063) with Mg content close to that of the
co : 2
* Corresponding 33653888; e-mail: 0921-5093/97/$17.00
author. Tel.: + 61 7 33654377; [email protected] Q 1997 Elsevier
PII SO921-5093(97)00207-4
Science
S.A. All
fax:
rights
+
61 7
reserved.
100
-
O-100
-
-200
-
-300
-
single
----400 -0.01
’
’ 0.00
Fig. I. Comparison curves.
m
’ 0.01
of single,
*
’ ’ 0.02 true plastic
multi-loop
’ 0.03 strain
’
’ 0.04
and monotonic
monotonic n
’ 0.05
n
0.06
stress-strain
Q. G. Wang,
C.H.
C&eves
/Materials
Science
and Engineering
20
I
I
I
(1997)
I
I&-
I
107
109
,
I
,.’
, .-
1
4I
400 -
A234-236
16
I , , stra,n = 0.0015
-
0
-
,-i--
08 16 -
l @
=;
l
0
AA603-Sr
A A
@.-@y 14-
**603-“nl-
0
!
.o*
*ASO,-"FIl AA603-Sr
8
300 2 I : 2! z ii Z
12 -
* . __--
. 1
10 -
f
-----A
__--
A
s-'------
200 2.0 aspect
J
alloy 100 -
01
” 0.00
dv/df at strain 0.0015 (GPa) -
-
AA603-urn
16
-----
AAGOI-urn
11
--
AA6063
” 0.02
” ” ” 0.04 0.06 0.08 true plastic strain
Fig. 2. True stress-true plastic strain curves showing Si particles and Mg content on the flow behaviour (urn) alloys.
Fig. 3. The strain hardening coefficient as a function of the aspect ratio of the Si particles, measured at an applied plastic strain of 0.0015 (Sr indicates modification with strontium).
6 “. 0.10
0.12
the effect of the of unmodified
3. Results 3.1. Flow behaviour The influence of the Si particles and the Mg content on the flow behaviour is shown by Fig. 2. It is seen that the casting alloys show a higher yield stress as well as significantly more strain hardening at low strains than the wrought alloy. The effect of the aspect ratio of the Si particles on the strain hardening coefficient (defined as da/de, where c is the flow stress and E the strain) at low strains is shown in Fig. 3. For a given Mg content the work hardening rate increases linearly with the aspect ratio of the Si particles, a, while at a given aspect ratio the higher Mg content alloy has a higher hardening rate. The strain hardening rate at large strains is shown in Fig. 4, and it seen to increase (at constant Si particle aspect ratio) for the smaller DAS. 3.2. Bauschinger
2.2 ratio
effect
Metals with two or more phases often exhibit a decrease in the reverse yield stress when they are deformed cyclically (Bauschinger effect). The softening observed when the strain is reversed can be described by the Bauschinger stress parameter (BSP) [3],
where of is the maximum forward flow stress in each cycle and 0’ is the reverse flow stress (measured at a reverse offset strain of 0.002). The term c,, is called the back stress,and represents the stressthe elastic particles exert on the plastic matrix. 3.2.1. Effect of DAS and Si particles Fig. 5 shows the BSP for materials with similar Si aspect ratio but different DAS, as a function of the strain. It can be seenthat for strains of up to about 1% the BSP increasesrapidly for all alloys. Thereafter, the BSP continues to increase but at a slower rate, saturating at strains of about 0.02. The saturation value appears to be larger for the finer DAS, a result expected from Fig. 4. The influence of the aspect ratio of the Si 1000
8
Q
I
-
1
u
I strain
2
600 -
'
I
-
5 0.04-0.045
+
AA603
0
AA601
I 22 2 'I a, E 2
-
'EL+$
+ 'o-.;
600 -
0
0
0
+ * --__
&
--D-+
.s 2 a
+
4oo. 10
20
30
40 (run)
0
+ - - -+ -9. cl+.
_
0
50
60
70
DAS
Fig. 4. The strain hardening coefficient measured at applied plastic strains between 0.04 and 0.045, as a function of the DAS. Results for alloy AA601 and dashed line from [3]. Aspect ratio: 1.6-2.0.
108
Q.G.
Wang,
C.H.
Chceres / Materials
Science
0.4 1
0.3 -
i
0.2 alloy 0.1 AA6O3-wnlSr A.AGOl-urn 0.0 -
AA603.5, U 0
-0.1
A
’ 0.00
0
’ 0.02
3
’ 0.04
1.A.,.7
55-65
’
’ 0 0.06 plastic strain
’ 0.06
AAGO,.S, 8
’ 0.10
a
Fig. 5. The BSP as a function of plastic strain for materials similar aspect ratio of the Si particles but different DAS.
0.12
with
particles is shown in Fig. 6. As in Fig. 5, the BSP increases rapidly at small strains but gradually saturates at large strains ( > 0.02). Elongated Si particles (i.e. large a-values) result in higher saturation values.
4. Discussion 4.1. Strain hardening Fig. 2 shows that the strain hardening rate at low strains is higher for the casting alloys. The strain hardening rate at a strain of 0.0015 is of the order of 16 GPa for alloy AA603, 11 GPa for alloy AA601 and only 6 GPa for alloy AA6063. Since the Mg content of the (single phase) wrought alloy is intermediate between the two casting alloys, the differences on the hardening
A
0.3 -
A
0.2 -
A2346236
(1997)
106-109
rate can only result from the effect of the Si particles [3]. In fact, a high hardening rate at low strains is normally observed when particles are added to a ductile matrix [5]. On the other hand, since the Si content of the two casting alloys is similar, the difference between them can only be ascribed to the difference in Mg content. Magnesium may also affect the content and nature of the Fe-rich intermetallics [6] in alloy AA603, but this possibility has been ignored in this work. To understand the effect of Mg on the hardening rate of the casting alloys at low strains, the different contributions to the strain hardening need to be considered. At low strains, plastic deformation results in the accumulation of Orowan loops around the reinforcing particles. This is called the linear hardening regime [3,7-91 and a rapid increase of flow stress with the applied strain is normally observed. At increasing strains the stressesaround the particles induce cross slip and secondary slip. Continued plastic straining results now in strain hardening due to the formation of a forest of dislocations near the reinforcing particles, but at a much reduced rate. This is called the parabolic hardening regime [7-91. The transition between the two regimes is likely to be controlled by the relative strength of the matrix [5]. Thus, for the stronger, higher Mg alloy, the onset of plastic relaxation will be delayed, increasing the contribution from the linear regime to the total hardening. At large strains, the dendritic cells behave like grains in a polycrystal [3] and thus the flow stresscan be expected to depend on (e/DAS)‘12 [9], as indeed observed (dashed line) in Fig. 4. 4.2. Bauschinger effect The results in Figs. 5 and 6 suggest that the back stress increases more or less linearly up to strains of about l%, gradually saturating afterwards. This behaviour is also consistent with inhomogeneous deformation at low strains, leading to a linear hardening at low strains, followed by low hardening rate at higher strains, when plastic relaxation limits the accumulation of internal stresses[3,7-91.
5. Conclusions
i aspect ratio, II
DAS (i/m)
0
2.2-2.4
55-65
AA603-urn
x
2.5-2.6
55-65
AABOl-urn
0
1.5-l
55-65
AA603.Sr
A
1.5-I .7
0.1 -
0.0 -x”
-8 -0.1 0.00
and Engineering
8
’ 0.01
m
’ 0.02
Fig. 6. The BSP as a function the same DAS but a different
*
’ n ’ 0.03 0.04 elastic strain
.7 ’
alby
55-65 AA60,-Sr ’ 8 ’ ’ 0.05 0.06
of the plastic strain for materials aspect ratio of the Si particles.
+
0.07
with
During uniaxial tension, the strain hardening of the materials is dependent on the Si particle morphology, dendrite arm spacing and Mg content. At low strains the Si particle aspect ratio and matrix strength dominate the work hardening, while at large strains the hardening rate depends on the DAS. The internal stressesincrease very rapidly at small strains, saturating at large strains. Elongated Si particles, fine DAS or higher Mg content result in a higher saturation value.
Q.G.
Wang,
C.H.
Chceres /Materials
Science
References [1] S.F. Frederick, W.A. Bailey, Trans. Metall. Sot. AIME 242 (1968) 2063-2067. [2] C.H. Clceres, C.J. Davidson, J.R. Griffiths, Mater. Sci. Eng. 97 (1995) 171-179. [3] C.H. CXceres, J.R. Griffiths, P. Reiner, Acta Mater. 44 (1996) 25-34.
and Engineering
A234-236
[4] C.H. CBceres, [5] S.F. Corbin, 1319-1327. [6] D.A. Granger, Foundrymen’s [7] L.M. Brown, [S] L.M. Brown, [9] M.F. Ashby,
(1997)
106-109
109
J.R. Griffiths, Acta Mater. 44 (1996) 15-24. D.S. Wilkinson, Acta Metall. Mater. 42 (1995) R.R. Sawtell, M.M. Kersker, Trans. Am. Sot. 92 (1984) 579-586. W.M. Stobbs, Philos. Mag. 23 (1971) 1185-1200. W.M. Stobbs, Philos. Mag. 23 (1971) 1201-1233. Philos. Mag. 21 (1970) 399-424.