A HIGH STRENGTH ALUMINIUM ALLOY FOR HIGH PRESSURE DIE CASTING
Shouxun Ji, Feng Yan, Zhongyun Fan
BCAST, Brunel University London, Uxbridge, Middlesex, UB8 3PH, UK
Keywords: Aluminium alloys; High pressure die casting; Mechanical property; Microstructure
Abstract
In order to achieve the mechanical properties of thin-wall
components made by high pressure die-casting at a level of
300MPa of yield strength, 420MPa of UTS and 3% of elongation,
the composition of the new aluminium alloy and casting
parameters have been optimized to reduce casting defects in final
components. In the present paper, we describe the alloy
development in an industrial scale HPDC system.
Introduction
Aluminium alloys are being increasingly used as lightweight
materials in transport since weight reduction has been proved as
one of the most effective approaches to improve fuel economy
and reduce CO2 emissions [1,2]. High pressure die casting
(HPDC) is a widely used process to make thin-wall aluminium
components for structural applications. The solution and ageing
heat treatment have been used to enhance the mechanical
properties of castings [3]. However, the existing aluminium alloys
for HPDC process are mainly based on Al-Si, Al-Si-Cu, and AlMg-Si systems. Most of these alloys are not particularly
developed for property enhancement by heat treatment, especially
not suitable for solution heat treatment because of the unsatisfied
mechanical properties for industrial application [4,5]. Therefore,
the development of high strength aluminium alloys becomes
significant to promote the application of aluminium alloys where
an improved strength and lightweight are essential.
In the present paper, we describe the alloy development including
the effect of Mg, Mn and Zn in the Al-Mg2Si system on the
mechanical properties and microstructure of the cast alloys. The
effect of solution and ageing heat treatment on the mechanical
properties and microstructure was also studied
The castings were produced using a 4500 kN cold chamber high
pressure die casting machine, in which most of the operation
parameters including injection speed and intensification pressure
are monitored. One set of die was used to cast standard tensile
samples, during which three 6.35mm round samples and three flat
tensile samples at 2mm, 3mm, and 5mm were made in one shot.
The mechanical properties were obtained from 6.35mm round
tensile samples. All parameters had been optimized for the HPDC
machine during casting.
The tensile tests were conducted following ASTM standard B557,
using an Instron 5500 Universal Electromechanical Testing
System equipped with Bluehill software. All tensile tests were
performed at ambient temperature. The gauge length of the
extensometer was 25 mm and the ramp rate for extension was 2
mm/min. The microstructure of each alloy was examined using a
Zeiss optical microscope with quantitative metallography, and a
Zeiss SUPRA 35VP SEM.
Results
1. Effect of extra Mg in Al-Mg2Si alloys
Fig. 1 shows the equilibrium phase diagrams at Al-rich side for
Al-Mg2Si system. The pseudo-binary Al-Mg2Si system behaves
like a normal binary eutectic system with a eutectic point at 13.9
wt.% Mg2Si. It shows that a narrow three-phase region for L+αAl+Mg2Si co-existence between 591 °C and 578 °C. With extra
Mg in the Al-Mg2Si system, the shape of equilibrium phase
diagram exhibits some changes. The eutectic point is shifted
towards a lower Mg2Si concentration and the area of co-existence
for the three phases is enlarged. This means that, for a given
Mg2Si content, an extra Mg addition results in a decrease of the
volume fraction of the primary α-Al phase and an increase of the
volume fraction of Al-Mg2Si eutectic phase.
Experimental
Commercial purity aluminium, magnesium, zinc, Al-20wt.%Mn,
and Al-50wt.%Si master alloys were used as base materials to
make the aluminium alloys. 10kg melt were made each time using
a clay-graphite crucible in an electrical resistance furnace. During
manufacturing, Al, Si and Mn were melted first and 15 ppm Be
was added to the melt 10 minutes prior to the addition of Mg
ingots. The melt was degassed at 750ºC using a rotatory degassing
equipment with a rotating speed of 500 rpm for 5 minutes. After
degassing, the melt was maintained at a temperature of 700ºC. A
mushroom casting with a bottom flat of 60×10mm was made by
a steel mould for composition analysis. The casting was cut 5mm
off from the bottom and ground down to 800 grid sand SiC paper.
The composition was obtained from a well-calibrated optical mass
spectroscope, in which at least five spark analyses were tested and
the average value was taken as the chemical composition of the
alloy.
To confirm the calculated results, a series of experiments were
performed with different amounts of extra Mg in the Al-Mg2Si
alloys. The representative microstructures are shown in Fig. 2.
The primary phase presented in the as-cast microstructure was
different. With 1.9wt.% extra Mg, the alloy showed a typical
hypoeutectic microstructure, consisting of dendrites or fragmented
dendrites of primary α-Al phase surrounded by Al-Mg2Si eutectic
phase. However, the microstructure was much different when the
extra Mg was 5.8 wt.%. The coarse primary α-Al phase almost
disappeared and the microstructure was dominated by fine
primary α-Al phase and Al-Mg2Si eutectics. In general, the
experimental results were consistent with the calculation results,
although the Al-Mg2Si alloys without extra Mg cannot be
obtained in high pressure die casting because of the severe die
soldering problem.
1
700
(a)
α-Al
Liquid
Al-Mg2Si
o
Temperature ( C)
650
600
α-Al
L+β-Mg2Si
L+α-Al
α-Al
550
L+α-Al +β-Mg2Si
Al-Mg2Si
Al-Mg2Si-6Mg
(b)
500
0
4
8
12
16
20
Mg2Si (wt.%)
α-Al
Fig. 1 Equilibrium phase diagrams of Al-Mg2Si pseudo-binary
system and Al-Mg2Si-Mg ternary system with 6wt.%Mg.
The yield strength, UTS and elongation of the as-cast Al-Mg2SiMg alloys can be varied by the content of extra Mg. The yield
strength was increased with increasing Mg content, but showed no
significant difference with increasing the Mg2Si phase.
Conversely, the elongation was decreased with the increase of Mg
and Mg2Si in the alloys. The UTS were different for the alloys
with different extra Mg contents. The decrease of the UTS was
significant for the Al-9.9wt.%Mg2Si and Al-12.7wt.% Mg2Si
alloys with the increase of extra Mg in the alloys. However, it is
noted that the yield strength is much lower than the target of
300MPa.
2. Effect of Mn in Al-Mg2Si-Mg alloys
Fig. 3 shows the typical microstructure of the intermetallics in the
Al-7.8wt.%Mg2Si- 5.8wt.%Mg-0.59wt.%Mn alloy with 0.12
wt.% Fe. Intermetallics with two different sizes were observed
and the volume fraction of the intermetallics increased with the
increase of Mn addition. The quantitative analysis showed that the
average size of the fine intermetallics were about 0.8μm. An
obvious increase of the volume fraction of the Mn-rich
intermetallics could be seen for the increasing Mn content. The
composition of the Mn-rich intermetallic phase was measured
with different amounts of Mn additions and the results confirmed
that the Mn-rich intermetallics were α-AlFeMnSi phase. The
measured constituent of the intermetallics could be described by
Al42-46(Fe,Mn)3.5-Si7. The Mn-rich intermetallics are essentially
associated with Fe in the die cast alloys.
The increase of Mn content in the alloys resulted in a slight
increase of the yield strength and UTS, but a slight decrease of the
elongation for the Al-7.8wt.%Mg2Si-5.8wt.%Mg alloy under ascast condition. The elongation was decreased from 6.9% to 6.5%
when the Mn content was increased from 0.19wt.% to 0.59wt.%,
which is the level of the commonly used in the die-cast Al-Mg-Si
alloy.
Al-Mg2Si
Fig. 2 Optical micrographs showing the effect of excess Mg on
the as-cast microstructure of Al-7.8 wt.%Mg2Si alloy with (a)
1.9wt.%Mg, (b) 5.8wt.%Mg.
AlMg
α-Al
Al-Mg2Si
AlFeMnSi
Fig. 3 The morphology of Mn-rich intermetallic phase and AlMg
intermetallic phase in the Al-7.8 wt.%Mg2Si-5.8 wt.%Mg0.59wt.%Mn alloy with 0.12wt.%Fe.
2
4. Effect of solution and ageing in Al-Mg2Si-Mg-Zn-Mn alloys
3. Effect of Zn in Al-Mg2Si-Mg-Mn alloys
(a)
500
15
YS
UTS
El
12
400
9
300
6
Elongation (%)
Strength (MPa)
The typical microstructure of Al-Mg2Si-Mg-Zn-Mn alloy is
shown in Fig. 4. An AlMgZn phase was observed, which were
characterised by irregular shapes. The Zn-containing intermetallic
compounds were located at the boundaries of the primary α-Al
phase or between the eutectic cells and the primary α-Al phase.
The measured volume fractions of the AlMgZn phase were
increased significantly with increasing the Zn addition. The
quantitative measurement confirmed that the intermetallic
compounds contained 25±1 at.% Mg, 10±0.7 at.% Zn, which
corresponded to a formula of Al13Mg5Zn2. Therefore, the Zn-rich
intermetallic phase was essentially the same compound in the
experimental alloys.
200
3
(a)
100
0
0
1
2
3
4
Zn (wt.%)
(b)
(b)
Fig. 5 (a) Mechanical properties with different Zn contents in the
Al-7.8wt.%Mg2Si-5.8wt.%Mg-0.59wt.%Mn and (b) backscattered
SEM microstructure of the alloy with 3.51wt.%Zn alloy under a
condition of solution treated at 490oC for 30 minutes and
subsequent aged at 180oC for 90 minutes.
Fig. 4 Backscattered SEM images showing (a) the microstructure
and (b) the morphology of Zn- containing intermetallics in the Al7.8wt.%Mg2Si-5.8wt.%Mg-0.59wt.%Mn alloy with 3.51wt.% Zn.
The yield strength was increased but the elongation was decreased
significantly with increasing the Zn content in the Al-7.8wt.%
Mg2Si-5.8wt.%Mg-0.59wt.%Mn alloy under as-cast condition.
When Zn content was 3.51wt.%, the elongation was still higher
than 2% and the yield strength was at a level of 250MPa and the
UTS was at a level of 350MPa.
Solution and ageing treatments were performed at 490oC and
180oC, respectively. Fig. 5(a) shows the effect of Zn on the yield
strength, UTS and elongation of the alloys after solution and
ageing heat treatment. Significant increases in yield strength and
UTS and a decrease in elongation were observed in the alloy with
increasing the Zn content. The tensile properties can be easily
achieved at a level of 340MPa of yield strength, 430MPa of UTS
and 3.5% of elongation when Zn is 3.51wt.%.
The typical microstructures after solution at 490oC for 30 minutes
and subsequent aged at 180oC for 90 minutes are shown in Fig.
5b. During solutionising, the AlMgZn intermetallics were
gradually dissolved into the matrix. However, no obvious change
was observed for the AlFeMnSi intermetallics and eutectic Mg2Si
phase after the solution treatment. The TEM examination revealed
that a large numbers of fine MgZn precipitates were formed in the
α-Al phase after solutioning and ageing, which provide effective
precipitates strengthening.
3
Discussion
In the Al–Mg2Si–Mg–Zn alloy, the as-cast microstructure
contains several phases formed during solidification, including
primary α-Al, eutectic Al–Mg2Si, β-AlMgZn and α-AlFeMnSi
phases.
The microstructural characteristics of the alloy can be understood
from the solidification and phase formation process. According to
the equilibrium phase diagram of the Al–Mg2Si–Mg–Zn–Mn
system on the cross section of Al–8 wt%Mg2Si–6 wt%Mg–0.6
wt%Mn calculated by Pandat software [5]. It is seen that the
solidification sequence of the alloy is L-α-Al-Mg2Si- AlMgZnAlFeMnSi-AlMg when Zn is with in the calculation range. The
prior phase is α-AlFeMnSi, followed by the formation of other
phases with temperatures being reduced. However, the calculation
results are slightly different to the experimental results where no
AlMg phase was actually found in the experimental results. The
disappearance of the AlMg intermetallics is mainly because of the
existence of Zn in the alloy. Although the exact mechanism is still
not clear, the formation temperatures of both AlMg and AlMgZn
intermetallics is close to each other according to the equilibrium
shown in given non-equilibrium solidification, the AlMgZn phase
is formed before AlMg phase, resulting in suppression of the
AlMg phase.
As the microstructure in the diecastings compromises the primary
α-Al phase, Al–Mg2Si eutectics and intermetallic phases. Mg,Si,
Zn and Mn can provide solution strengthening to the α-Al phase.
The precipitates of intermetallics under as-cast condition can
strengthen the alloy by acting as pins to prevent dislocations from
sliding under stress. Therefore, the fine and compact α-AlFeMnSi
phase can reduce the detrimental effect of needle-shaped
intermetallics. However, the intermetallics themselves are brittle
and can initialize cracks, resulting in the reduction in ductility.
The further strengthening by solution and ageing heat treatment
results from the precipitation of MgZn phase in the α-Al phase.
The AlMgZn compounds decompose during solution and
subsequently precipitate during ageing. The fine precipitates
provide much more effective strengthening effect in the alloy,
during which the detrimental effect from irregular AlMgZn phase
is minimized and the elongation is thus improved. On the other
hand, during heat treatment, no significant change is observed for
the α-AlFeMnSi andMg2Si phase because of their high thermal
stability, indicating that the enhancement in mechanical properties
from the α-AlFeMnSi andMg2Si intermetallics is limited after the
heat treatment.
intermetallics are precipitated during the subsequent ageing
treatment.
Acknowledgement
The authors acknowledge the Engineering and Physical Sciences
Research Council (EPSRC), Technology Strategy Board (TSB)
and Jaguar Land Rover (JLR) in United Kingdom for financial
support.
References
[1] T. Zeuner, L. Würker. Process Optimisation of Cast
Aluminium Chassis Components WFO Technical Forum 2003,
p3.1-3.14.
[2] S. Ji, D. Watson, Z. Fan, M. White. Development of a super
ductile diecast Al–Mg–Si alloy Mat. Sci. Eng. A 556 (2012) 824–
833.
[3]S. Ji, F. Yan, Z. Fan, Casting Development with a High
Strength Aluminium Alloy, Materials Science Forum 828-829
(2015) 9-14.
[4] V. S. Zolotorevskii, N. A. Belov, M. Glazoff. Casting
aluminium alloys, first ed., Elsevier, London, 2007, pp. 386-396.
[5] S. Ji, F. Yan, Z. Fan, Development of a high strength Al–
Mg2Si–Mg–Zn based alloy for high pressure die casting,
Materials Science and Engineering: A 626, 165-174.
Summary
A high strength aluminium alloy for high pressure die casting has
been developed based on Al-Mg2Si-Mg-Zn system. The typical
alloying elements comprises of 8.0wt.%Mg2Si, 6.0wt.%Mg,
3.5wt.%Zn, 0.6wt.%Mn. The alloy consists of primary α-Al, AlMg2Si eutectics, β-AlMgZn, and α-AlFeMnSi phases in the ascast microstructure. The typical tensile properties of the die-cast
tensile samples are 300MPa of yield strength, 420MPa of ultimate
tensile strength, and 3% of elongation under solution and aged
condition. A quick solution and ageing is an effective way to
enhance the mechanical properties, by which the AlMgZn
intermetallics are decomposed into Al matrix and fine Zn-rich
4