10
R & D, innovations
Alloy composition and property
profile of Al foundry alloys
In 2008, important steps were taken with regard to the cementing of the position of AMAG casting GmbH as a premium
supplier of Al secondary casting alloys. These moves included the use of advanced software packages for alloy simulation, which are utilised during the development of new materials and for a better understanding of the influences and
interactions of alloying elements. The know-how accumulated by AMAG and its research partners flows directly into customer consulting, which in particular is an indispensable part of customer support, especially with regard to technical and
business management process optimisation and the launch of new products.
100 mm
Test casthouse
Demands on cast components
It is expected that modern cast components can be produced in near-net or
even net shape. Moreover, that they can
be cast with large area, thin walls, with
strong wall thickness transitions, demonstrate high levels of strength, but nonetheless high levels of ductility and weldability, and can be joined using rivets. This
frequently means that parts are designed
that go to the limits of casting feasibility
and in the case of only minimal fluctuations in individual, process parameters, result in rejects. Increased casting speeds,
melt and die temperatures extend the flow
path, but generally reduce product quality
and the mould life time during gravity die
and high pressure die casting. Local increases in wall thickness can also improve
mould filling, but raise component weight
and material costs, or lead to additional,
expensive, machining.
Influence of fluidity
Therefore, the fluidity of an alloy plays an
important role in the selection of suitable
Casting spiral
process parameters and the resultan properties of the cast components. The most
important Al alloys have a tolerance window
for the respective alloying elements, which
is determined in standards. As a rule, the
considerable price pressure during alloy
sourcing means that within a given tolerance band, the apparently cheapest alloy
is selected. This is only apparently the
cheapest, because a reduction in expensive alloying elements, e.g. copper is not
without an effect on the property profile
of the alloy. It is not infrequently the case
that the supposed savings with regard to
alloy costs lead to increased expenses in
the foundry. These assume tangible form
in an increased number of rejects.
The tolerated fluctuations in alloy composition can exert a major influence on fluidity
and component properties. Earlier work by
Kaufmann et al [1,2] with regard to the
aluminium alloys, AlSi9Cu3 and AlSi7Mg,
demonstrate the sizeable degree to which
fluidity can be influenced by alloy composition, whereby the only aspects examined
related to alloy variations with all the alloying elements at the respective upper and
lower tolerance transfer limits.
Influence on mechanical properties
A recently published work [3] uses these
tests as a basis and in addition to the influence of alloy composition also describes
the impact of fluidity on the attainable mechanical properties. One example presents
the correlations of the alloy variations of
the A226* secondary casting alloy, which
is used frequently in both gravity and high
pressure die casting.
The casting tests were completed in
AMAG’s casting test centre and these
demonstrated that as a result of an understanding of the interconnections, customised alloys can be produced for differing
applications and component requirements.
For the foundrymen, it makes sense to
integrate a competent alloy supplier as a
partner into the launch of a new product
development or an optimisation process at
the earliest possible point in time.
R & D, innovations
Influence of iron
For example, under the applied testing parameters and without the use of grain refining and modification, suitable combinations
of alloying elements in the A226 alloy facilitate yield points approaching 200 MPa,
which represents a doubling of the standard
requirement for the AlSi8Cu3 alloy pursuant to EN-46200. Alloys with elongation
at fracture of up to 4% can also be produced and this represents a quadrupling of
the minimum requirements stipulated in the
standard. However, while in the case of the
suitable selection of alloy composition the
standard requirements can be clearly exceeded, it is also possible that an unfavourable composition will lead to a failure to fulfil
minimum requirements.
Iron plays an important role as an alloying
element in secondary casting alloys. It is frequently the case that users require that the
iron content be limited, which due to the limitations thus placed on usable scrap, creates
problems for the supplier relating to alloy
Fig. 1 Mechanical characteristics and flow distances in the case of the systematic variation of
the chemical composition of the A226 alloy within the alloying limits (blue: 0.4% Fe, red: 0.8
% Fe. green: 1.2 % Fe).
Elongation at fracture > 1 %
Elongation at fracture > 1 %
850
650
Flow distance [mm]
800
Flow distance [mm]
Phenomenological evaluation
The prescribed tolerance band for all the
alloying elements contained in the A226
secondary casting alloy opens up extensive room for the targeted selection of alloy
compositions to meet specific requirements
with regard to both the AlSi8Cu3 chill gravity casting version and the AlSi9Cu3 high
pressure die casting version.
Should the correlations be unknown, this
room for manoeuvre brings with it the danger of considerable fluctuations in castability, or the resulting product properties.
Fig. 1 provides an illustrative summary of
the attainable mechanical properties comprised by the yield point and elongation to
fracture, as well as the flow distances in
connection with the systematic variation of
the chemical composition. This clarifies the
adjustable range of the properties of A226
within the alloying limits, whereby a rough
allocation to the alloy sub-groups with differing Fe content was undertaken in order
to account for the significance of Fe in
secondary alloys.
11
750
700
650
600
550
600
550
500
450
400
500
1.2 % Fe
0.8 % Fe
450
110
120
130
140
150
160
170
180
Yield point Rp 0.2 [MPa]
350
110.00
130.00
150.00
170.00
Yield point Rp 0.2 [MPa]
Fig. 2 Yield point and flow distance of selected alloys containing 0.8 % Fe (left) and 1.2 % Fe
(right); all alloys with an elongation at fracture of over 1 %. Elongation at fracture > 1 %
production. This situation derives from the
fact that there is a widespread opinion that a
high iron content is generally negative.
Fig. 2 shows that even when the iron
content is close to the upper limit within
the standard range of 0.8% for gravity
casting and 1.2% for high pressure die
casting, depending on all the other alloying additives, yield points of approx. 120
MPa to approx. 180 MPa and flow distances of 400-800 mm can be set in the
casting spiral.
Conclusion
A forward-looking prediction concerning
the actual casting behaviour of the alloy and the properties of the component
can therefore be shown to be impossible
purely on the basis of the consideration
of the iron content. Instead, interactions
must be subject to a holistic approach and
this is precisely the method adopted by
AMAG during customer consulting. All the
presented fluctuations in mechanical characteristics resulting from alloy fluctuations
are determined on samples in a cast temper. The room for influencing properties
increases further when proper heat treat-
ment, grain refining and modification are
applied. AMAG will be reporting on these
topics in future.
Bibliography
[1] H. Kaufmann, W. Fragner and P. J.
Uggowitzer: “Influence of variations in alloy composition on castability and process
stability. Part 1: Gravity and pressure casting Processes”, Int. J. Cast. Metals Res.
18, (2005), p. 273-278.
[2] H. Kaufmann, P. J. Uggowitzer:
“Metallurgy and Processing of HighIntegrity Light Metal Pressure Castings”,
ISBN-13:978-3-7949-0754-0, Schiele &
Schön, (2007) p. 215-251.
[3] P. Pucher, J. Knaack, H. Böttcher, H.
Kaufmann, H. Antrekowitsch, P.J. Uggowitzer: “Einfluss der Legierungszusammensetzung auf die mechanischen Eigenschaften und das Fließvermögen der Sekundärlegierung” A226 (AlSi9Cu3), Giesserei
Praxis, Issue 3 (2009) p. 71-78
*) Although officially the old VDS 226 alloy (DIN 1725)
has been replaced by the somewhat more extensively
documented 46200 alloy (EN1706), most of the casting plants in Germany and Austria continue to produce
within the analysis tolerances of the old 226.
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