Note: Descriptions are shown in the official language in which they were submitted.
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Aluminium alloy, method for producing an aluminium flat product,
aluminium flat product and use thereof
The invention relates to an aluminium alloy for superplastic aluminium flat
products,
a method for producing a superplastic aluminium flat product, and a
superplastic
aluminium flat product and its use.
In recent years, the production of components by superplastic forming has
become
increasingly important. In superplastic forming, a so-called superplastic
starting
material suitable for this process is heated to a forming temperature of
typically 450-
520 C in the case of aluminium alloys and shaped with high degrees of forming
of, in
some cases, several 100 %. By means of superplastic forming it is possible to
produce
even complex workpieces in only one forming step and with high dimensional
accuracy.
A typical method of superplastic forming is, for example, so-called blow
moulding, in
which a sheet-like starting material is forced by pressurising with a fluid,
in particular
a gas, into a die having a negative shape to the shape to be produced. While
superplastic forming is conventionally carried out at low strain rates of the
order of
magnitude of 10-4s-1, recent developments go in the direction of high speed
superplastic forming with very high strain rates.
The alloy AA 5083, for example, is known as a superplastic material, which can
be
used to produce products by means of superplastic forming.
Furthermore, in the article "Chronicling the development of a high strength
5xxx-
based superplastic aluminium alloy" by SP Miller-Jupp, Mat. Sci. For. 838-839
(2016)
pp. 208-213, the development of an AA 5456-based superplastic alloy is
described.
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The object of the present invention is to provide an aluminium alloy, a method
for
producing an aluminium flat product and an aluminium flat product having
improved
properties in superplastic forming.
This object is achieved according to the invention by an aluminium alloy, in
particular
for superplastic aluminium flat products, wherein the aluminium alloy has the
following composition:
Si 0.4 wt.%,
Fe 0.4 wt.%,
Cu 0.1 wt.%,
0.5 wt.% Mn 1.0 wt%,
4.7 wt.% Mg 5.5 wt.%,
0.05 wt.% Cr 0.25 wt.%,
Zn 0.25 wt.%,
Ti 0.20 wt.%,
unavoidable impurities individually 0.05%, in total 0.15%, the remainder
aluminium. The aluminium alloy can be used in particular to produce an
aluminium
product by superplastic forming of an aluminium flat product from the
aluminium
alloy.
The Na content of the aluminium alloy is preferably max. 2 ppm (i.e. max
0.0002
wt.%), more preferably max. 1.4 ppm, in particular max. 1.0 ppm. It has been
found
that the sodium content in the aluminium alloy must be kept extremely low,
since
otherwise edge tearing may occur during hot rolling of the ingot. This is
particularly
the case if the aluminium alloy has a high Mg content of 5.2 wt. % or more. In
order to
achieve the low aforementioned Na contents, a chlorine treatment of the melt
can, for
example, be carried out.
The aforementioned object is furthermore achieved according to the invention
by a
method for producing an aluminium flat product, in particular a superplastic
aluminium flat product, in which an aluminium melt is provided from the
aforementioned aluminium alloy, in which the aluminium melt is cast into an
ingot, in
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which the ingot is hot-rolled to form a hot strip, in which the hot strip is
cold-rolled to
form a cold strip, and in which the cold strip is levelled.
Furthermore, the aforementioned object is achieved according to the invention
by an
aluminium flat product, in particular a superplastic aluminium flat product,
which can
be produced or is produced by the afore-described method.
The aforementioned object is furthermore achieved according to the invention
by the
use of the aforedescribed aluminium flat product for producing an aluminium
product
by superplastic forming of the aluminium flat product, in particular by means
of blow
moulding.
It has been found that a superplastic aluminium flat product that is
particularly well
suited for superplastic forming can be produced by the aforedescribed method
and
the aforedescribed aluminium alloy. In particular, a heat treatment of the
aluminium
flat product during the heating to the forming temperature for superplastic
forming
leads to the formation of a fine microstructure, so that high degrees of
forming
without defects can be achieved. The forming temperature in the superplastic
forming
is preferably in the range of 450 C to 520 C. The total elongation during
superplastic
forming is preferably at least 100%.
As a superplastic aluminium flat product is understood in particular an
aluminium flat
dln(o) .
product that has a strain rate sensitivity m of at least 0.3, where m =
dln(E), is the
yield strength, and E. is the strain rate. A strain rate sensitivity m 0.3 is
typically
achieved only in a certain strain rate range, e.g. in the range of 10-4s-1- to
10-3s-1, in
which the aluminium flat product is superplastic.
The method is used to produce an aluminium flat product. The aluminium flat
product
may in particular be a strip or a sheet.
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In the method an aluminium melt of the aforedescribed aluminium alloy is
provided.
The provision of the aluminium melt is performed in particular by adjusting
the
composition of the aforedescribed aluminium alloy in an aluminium furnace by
melting primary aluminium, optionally scrap metal and other additives.
The produced molten aluminium is cast in the method into an ingot, especially
by DC
(direct chill) casting. The ingot is preheated for the hot rolling.
Alternatively, a
separate ingot homogenisation may be carried out prior to preheating, in order
to
obtain a more uniform structure.
The ingot is then hot rolled to form a hot strip, preferably at a temperature
in the
range of 280 C to 550 C, in particular a hot strip temperature (i.e. at hot
strip
thickness) in the range of 280 C to 350 C is set. The hot strip is then cold-
rolled to
form a cold strip.
After cold rolling the cold strip is levelled. During levelling of the cold
strip, the cold
strip is fed through a plurality of mutually offset levelling rollers to
achieve a flatness
suitable for the superplastic forming.
Various embodiments of the aluminium alloy, the method, the aluminium flat
product
and its use are described hereinafter, wherein the individual embodiments
apply in
each case individually to the aluminium alloy, the method, the aluminium flat
product
and the use. Furthermore, the individual embodiments can also be combined with
each other.
In one embodiment the aluminium melt has a Si content of 0.03-0.10 wt.% and/or
an
Fe content of 0.05-0.15 wt.%. Silicon and iron are dispersoid formers and are
therefore basically advantageous for achieving a fine grain structure for the
superplastic forming. However, it has been found that silicon and iron can
form coarse
intermetallic phases, in particular AlSiFeMn phases, with a size of more than
20 lam or
even more than 30 [tm, which lead to pore formation during the superplastic
forming
and consequently in particular adversely affect the mechanical properties of
the
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aluminium product produced from the aluminium flat product. The Si content of
the
aluminium alloy is therefore preferably limited to 0.10 wt.% and the Fe
content of the
aluminium alloy preferably to 0.15 wt.%.
A silicon content below 0.03 wt.% or an iron content below 0.05 wt.% can be
achieved
only at great expense in industrial aluminium alloys, as a result of which the
production costs of the aluminium flat product and of the aluminium product
produced therefrom would be greatly increased. At the same time it was found
that
with silicon and iron contents in the aforementioned ranges, a fine grain
structure for
the superplastic forming can be achieved with still acceptably low pore
formation
during superplastic forming.
In a further embodiment the Cu content of the aluminium alloy is at most 0.05
wt.%.
In this way the corrosion resistance of the alloy is not adversely affected.
Furthermore, due to the low Cu content the yield strength at elevated
temperatures is
kept low, which has a positive effect on the superplastic forming.
In a further embodiment the aluminium alloy has an Mn content of 0.7 wt.% to
1.0
wt.%. It has been found that manganese in the aluminium alloy acts as a strong
dispersoid-forming agent, so that a large number or density of fine
dispersoids in the
aluminium flat product is produced by a relatively high content of manganese
of at
least 0.7 wt.%. It has been found that these manganese dspersoids hinder grain
growth, so that after the superplastic forming of an aluminium flat product
produced
from the aluminium alloy a fine-grain structure is present despite the high
forming
temperatures.
In a further embodiment the aluminium alloy has a magnesium content of 5.2
wt.% to
5.5 wt.%. It has been found that a stabilisation of the grain sizes can be
achieved by an
increased magnesium content of at least 5.2 wt. %, thereby further improving
the
superplastic properties of the aluminium flat product. Furthermore, an
improved
strength combined with still good rollability is achieved by the increased
magnesium
content in the specified range. In particular, the magnesium content in this
range
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improves the strength of an aluminium product produced from the aluminium flat
product after superplastic forming.
In a further embodiment the aluminium alloy has a zinc content of at most 0.06
wt.%
and/or a titanium content in the range of 0.015-0.03 wt.%. It has been found
that a
zinc content of up to 0.06 wt.% and a titanium content of up to 0.03 wt.% do
not
adversely affect the properties for the superplastic forming of the aluminium
flat
product. A titanium content is even desirable up to a limited extent as a
grain refiner,
in particular with a content of at least 0.015 wt.%.
In a further embodiment the aluminium alloy has a boron content of at most 50
ppm
(i.e. at most 0.005 wt.%) and/or a calcium content of at most 15 ppm (i.e. at
most
0.0015 wt.%) and/or a lithium content of at most 15 ppm (i.e. at most 0.0015
wt.%).
Titanium borides act as a grain refiner during casting and thus have a
beneficial effect
on the rolling process and also the homogeneity of the product, wherein a
boron
content of at most 50 ppm does not adversely affect the properties for the
superplastic
forming of the aluminium flat product. Calcium and lithium, like sodium,
promote the
formation of edge cracks and thereby adversely affect the rollability,
especially during
hot rolling.
In a further embodiment of the method the aluminium melt is provided by
melting
together a preliminary aluminium melt with additives so as to achieve the
composition of the molten aluminium to be provided, in particular the
aforedescribed
composition, wherein at least two of the alloying elements Cr, Mn and Ti,
preferably
all three alloying elements Cr, Mn and Ti, are charged separately from one
another.
In order to produce an aluminium melt with a specific alloy composition from
starting
material, for example primary aluminium and/or aluminium scrap, the starting
material is first melted in a melting furnace to form a preliminary aluminium
melt and
then - typically according to a pre-calculated charge make-up - is melted
together
with suitable additives, in particular of alloying metal, master alloys, scrap
and/or
suitable additives, so as to achieve the desired alloy composition.
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It has been found that with the simultaneous addition of several of the
dispersoid
formers Cr, Mn and Ti, coarse particles, in particular Al(Mn,Fe,Cr)Si
particles, can
formed, which may furthermore contain Mg, Ti and V, which accordingly lead to
coarse particles with a size of more than 20 lam or even more than 30 [trn in
the
aluminium flat product, so that defects and/or increased pore formation may
occur in
the superplastic forming of the aluminium flat product, which can adversely
affect the
mechanical properties of the aluminium product produced from the aluminium
flat
product.
By charging at least two of the alloying elements Cr, Mn and Ti, preferably
all three
alloying elements Cr, Mn and Ti, separately from one another, the formation of
these
coarse particles can be prevented. Separately charging two alloying elements
is
understood to mean that the additives to be added in order to adjust the
desired
content of one of the two alloying elements and the additives to be added in
order to
adjust the desired content of the other of the two alloying elements, are
added at
different times to the preliminary aluminium melt. If for example titanium
boride rods
are added to the preliminary aluminium melt in order to adjust the Mn content
and
pieces of an Mn-containing master alloy are added in order to adjust the Ti
content,
then the titanium boride rods and the master alloy pieces are preferably
melted
together with the preliminary aluminium melt at different times.
Preferably, a mixing of the aluminium melt in the melting furnace, in
particular by
means of stirring, takes place between the addition of the additives for a
first of the
alloying elements Cr, Mn and Ti and the addition of the additives for a second
of the
alloying elements Cr, Mn and Ti. Preferably, after addition of the additives
for the first
of the alloying elements Cr, Mn and Ti the preliminary aluminium melt is mixed
in the
melting furnace until an aluminium melt having a homogeneous composition has
been
obtained. The homogeneity of the preliminary aluminium melt in the melting
furnace
is sufficient if the chemical analysis of the melt matches the charge make-up
for the
first of the alloying elements Cr, Mn and Ti. The sampling to determine the
homogeneity preferably takes place in three different areas of the melting
furnace. In
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the above example a homogenisation of the preliminary aluminium melt by
stirring is
accordingly preferably carried out after the addition of titanium boride and
before
adding the pieces of the master alloy, until a matching Ti content is achieved
in three
different regions of the melting furnace.
When a first of the alloying elements Cr and Mn and a second of the alloying
elements
Cr and Mn (i.e. in each case the respective other alloying element) are
charged
separately, the content of the second of the alloying elements Cr and Mn
during the
charging of the first of the alloying elements Cr and Mn in the preliminary
aluminium
melt is preferably at most 0.05 wt. %. If for example Mn is charged first
followed by Cr,
the Cr content in the aluminium melt during the charging of Mn is preferably
at most
0.05 wt. %. This has proved to be advantageous to counteract the formation of
coarse
particles.
The temperature of the preliminary molten aluminium during the charging of Cr
is
preferably more than 740 C, especially at least 750 C. In this way Cr can be
distributed very uniformly in the aluminium melt.
Mg is preferably charged only after Cr, Mn and/or Ti, preferably as the last
element.
Furthermore, the temperature of the preliminary molten aluminium during the
charging of Mg is preferably less than 740 C, in particular at most 730 C.
In this way
the desired Mg content can be better adjusted since the Mg content can be
reduced at
elevated temperatures or premature addition by melting loss.
In a further embodiment a scrap content of less than 5 wt. %, preferably less
than 1
wt.%, in particular less than 0.1 wt.% is used to provide the aluminium melt.
It has
been found that even small amounts of certain accompanying elements and
impurities
from the scrap fraction can lead to the formation of large particles in the
aluminium
melt and in the aluminium flat product produced therefrom, which, being
nucleating
agents, contribute to pore formation and thus to damage during superplastic
forming.
The scrap content in the production of the aluminium melt is therefore
preferably
kept as low as possible or the addition of scrap is preferably completely
dispensed
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with. Accordingly, the aluminium melt is preferably produced in particular in
that
essentially primary aluminium is melted, optionally with additives, to obtain
the
desired composition.
In a further embodiment the degree of rolling in cold rolling in total is
generally in the
range of 70% to 80%. In a corresponding embodiment, the aluminium flat product
is
in the H18 state, preferably in the H19 state, according to DIN EN 515. Due to
the high
degree of forming in cold rolling, a high dislocation density is introduced
into the
material. This results in that the material of the aluminium flat product when
heated
for the superplastic forming spontaneously recrystallises with a very fine
microstructure, which is advantageous for superplastic forming.
The cold rolling is carried out in particular without intermediate annealing.
If an
intermediate annealing is however performed, the aforementioned degree of
rolling in
cold rolling refers to the total degree of rolling after the last intermediate
annealing.
The final thickness of the cold strip is preferably in the range of 1 - 3 mm.
In order to
achieve the advantageous high degree of rolling during cold rolling, the hot
strip
thickness is preferably in the range of 3 to 15 mm, in particular in the range
of 4 to 12
mm.
The degree of rolling in the last cold rolling pass is preferably less than
33%. The H18
and H19 states can thus be produced without having any adverse effects on the
superplastic forming. In addition, surface defects, in particular chatter
marks, are
avoided by limiting the degree of rolling in the last pass.
In a further embodiment, the levelling of the cold strip is performed by
levelling
rollers with a diameter of more than 60 mm. It has been found that by using
larger
levelling rollers undesirable surface defects after the superplastic forming
can be
avoided.
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In a further embodiment, the cold strip is cut into sheets after levelling
without
intermediate coiling. In this way, the flatness of the strip achieved by
levelling is not
made worse again, so that a second levelling process can be dispensed with.
This is
particularly advantageous if the cold strip was levelled with levelling
rollers with a
diameter of more than 60 mm and thus with a reduction or even avoidance of
surface
defects. The possible introduction of surface defects in a second, possibly
customer-
side levelling process, can be avoided in this way.
The strip temperature between the cold rolling and the cutting into sheets is
kept in
the range below 200 C, preferably below 50 C, in particular at room
temperature of,
for example, about 20 C. In this way a premature recovery by reducing the
dislocations introduced by the cold rolling in the aluminium flat product is
avoided, so
that a strong recrystallisation effect with a fine microstructure can occur
only when
heating the sheet for the superplastic forming.
In a further embodiment the aluminium flat product after a heat treatment for
30
minutes at 500 C has a yield strength R/30.2 of at least 160 MPa, in
particular at least
170 MPa, and a tensile strength Rm of at least 310 MPa, in particular at least
320 MPa.
11/30,2 and Rm are each to be determined in the tensile test according to DIN
EN ISO
6892-1:2017. Additionally or alternatively, the aluminium flat product
preferably has
a porosity of less than 1.5%, in particular less than 1%, after a superplastic
forming at
a forming temperature of 515 C, a strain rate of 2.5 x 10-4s-1 and a total
elongation of
100%.
It has been found that, by means of the method described above, in particular
by an Fe
content of at most 0.15 wt.% and an Si content of at most 0.10 wt.%, and also
by the
preferably separate charging of Mn, Cr and preferably also Ti, the formation
of coarse
particles in the aluminium flat product can be avoided, which lead to pore
formation
during the superplastic forming. Aluminium flat products can thereby be
produced
with the method, which have a very low porosity after superplastic forming.
The low
porosity after superplastic forming, in particular combined with an Mn content
of at
least 0.7 wt.% and a Mg content of at least 5.2 wt.%, leads moreover to very
good
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mechanical properties of the aluminium flat product after a heat treatment
caused by
the typical forming temperatures of a superplastic forming. Aluminium products
with
very low porosity and very good mechanical properties can thus be produced
from the
aluminium flat products by superplastic forming.
In a further embodiment, the aluminium flat product has after a heat treatment
at 500
C for 5 minutes a mean grain diameter of at most 15 lam. The mean grain
diameters
are to be determined according to ASTM E112. It has been found that an
aluminium
flat product can be produced by the method described above, which after a
short heat
treatment by setting a typical forming temperature for the superplastic
forming has a
correspondingly fine microstructure. This is achieved especially at the
preferred Mg
content of at least 5.2 wt.%, the preferred Cr content between 0.12 and 0.18
wt.%, the
preferred Si content of at most 0.10 wt. %, the preferred Fe content of at
most 0.05
wt.%, by the separate charging of Mn, Cr and/or Ti and by the preferred H19
state of
the aluminium flat product.
In a further embodiment, the superplastic forming is carried out with a strain
rate of
more than 10-3s-1, in particular of at least 10-25-1. Typically, the
superplastic forming
occurs at strain rates in the range of 10-4 to 10-3s-1. It has been found that
the
aluminium flat products produced by the described method can be
superplastically
formed at significantly higher strain rates without the material constricting
during
forming. This is achieved in particular by a strain rate sensitivity m 0.3
even at
higher strain rates of above 10-3s-1. In a corresponding embodiment the
aluminium
flat product at a strain rate of more than 10-3s-1, in particular of at least
10-2s-1, for
example at least up to 5 x 10-2s-1, has a strain rate sensitivity m of at
least 0.3,
determined by means of the incremental strain rate tests according to Lederich
(Lederich et al. "Superplastic Formability Testing" Journal of Metals Vol. 34
Issue 8, pp.
16 to 20, 1982) using the strain rates 5 x 10-45-1, 1 x 10-45-1, 5 x 10-45-1,
1 x 10-35-1, 5 x
10-3s-1, lx 10-25-1, 5 x 10-4s-1 and lx 10-1s-1 as well as an ISO 20032:2007
compliant
testing machine and sample geometry. The higher strain rates in superplastic
forming
allow shorter forming times and therefore higher forming cycles, whereby
production
costs can be reduced.
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Further features and advantages of the method, of the aluminium flat product
and its
use emerge from the following description of exemplary embodiments, wherein
reference is made to the respective drawing.
In the drawing
Fig. 1 shows an exemplary embodiment of the method,
Fig. 2 shows an exemplary embodiment of the use of the aluminium flat
product
produced by the method,
Fig. 3 shows a second exemplary embodiment for the use of the aluminium
flat
product produced by the method of Fig. 1,
Fig. 4 shows a micrograph of an aluminium flat product with coarse Cr-
containing particles,
Figs. 5-7 show micrographs of aluminium flat products before a heat treatment
(Fig.
4), after 1 minute (Fig. 5) and after 60 minutes' heat treatment at 500 C
(Fig. 6)
Fig. 8 shows a diagram with experimental results for the forming
temperature-
dependent strain rate sensitivity m, and
Figs. 9-10 show diagrams with test results for the yield strength Rp0,2 (Fig.
9) and
tensile strength Rm (Fig. 10) at room temperature after 30 minutes' heat
treatment at various temperatures.
Fig. 1 shows an exemplary embodiment of the method for producing a flat
aluminium
product in a schematic representation.
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In a first step 2 of the method a preliminary aluminium melt is firstly
prepared, in
which primary metal 4 and alloying additives 6 are added to an aluminium
melting
furnace 8 and melted there. The use of aluminium scrap is preferably largely
dispensed with for the production of the preliminary aluminium melt 10.
In the second step 12 the preliminary aluminium melt 10 is homogenised in the
melting furnace 8, which is illustrated in Fig. 1 by the schematically
illustrated stirrer
14.
The homogenised preliminary aluminium melt 10 in the aluminium melting furnace
8
has the following composition:
0.03 wt.% Si 0.10 wt.%,
0.05 wt.% Fe 0.15 wt.%,
Cu 0.05 wt.%,
0.7 wt.% Mn 1.0 wt.%,
Mg 1 wt.%,
Cr 0.05 wt.%,
Zn 0.06 wt.%,
0.015 wt.% Ti 0.030%,
Na 1.0 ppm,
unavoidable impurities individually up to a maximum of 0.05 wt.%, in total not
more
than 0.15 wt.%, remainder aluminium. The low Na content can be achieved, for
example, by a chlorine treatment of the melt.
In the third step 16 chromium-containing material 18 is added to the
preliminary
aluminium melt 10 and the resulting (still preliminary) aluminium melt 22 is
in turn
homogenised in the fourth step 20 (as illustrated by the stirrer 14). The
homogenised
aluminium melt 22 has the following composition:
0.03 wt.% Si 0.10 wt.%,
0.05 wt.% Fe 0.15 wt.%,
Cu 0.05 wt.%,
0.7 wt.% Mn 1.0 wt.%,
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Mg 1 wt.%,
0.12 wt.% Cr 0.18 wt.%,
Zn 0.06 wt.%,
0.015 wt.% Ti 0.030%,
Na 1.0 ppm,
unavoidable impurities individually up to a maximum of 0.05 wt.%, in total not
more
than 0.15 wt.%, remainder aluminium.
A separate charging of Mn or Ti and Cr is achieved with the described steps 2,
12 and
16. In the first step 2 the Mn content and the Ti content are firstly
adjusted, while the
material 18 to be added for the adjustment of the desired Cr content is added
separately only in the third step 16 after a homogenisation of the preliminary
melt 10
in step 12. In a similar way Ti can also be charged separately from Mn. In
addition to
the content of Mn and Ti, in the present example the contents of further
alloying
elements (in particular Si and Fe) are also adjusted in the first step 2. The
charging of
these alloying elements (in the present example, in particular Mn, Ti, Si and
Fe) can be
carried out simultaneously or also separately from one another.
In the fifth step 23 magnesium-containing material 24 is added to the
preliminary
aluminium melt 22, and the resulting aluminium melt 25 is in turn homogenised
in the
sixth step 26 (as illustrated by the stirrer 14). The homogenised aluminium
melt 25
has the following composition:
0.03 wt.% Si 0.10 wt.%,
0.05 wt.% Fe 0.15 wt.%,
Cu 0.05 wt.%,
0.7 wt.% Min 1.0 wt.%,
5.2 wt.% Mg 5.5 wt.%,
0.12 wt.% Cr 0.18 wt.%,
Zn 0.06 wt.%,
0.015 wt.% Ti 0.030%,
Na 1.0 ppm,
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unavoidable impurities individually up to a maximum of 0.05 wt.%, in total not
more
than 0.15 wt.%, remainder aluminium.
In this way, Mg is charged only after Mn/Ti and Cr, preferably as the last
alloying
element of the aluminium melt, in order to prevent the melting loss of Mg. To
this end
the temperature of the aluminium melt during the charging of Mg is also
preferably
less than 740 C, in particular at most 730 C. On the other hand, the
temperature of
the aluminium melt when charging Cr is preferably more than 740 C, in
particular at
least 750 C, in order to disperse Cr uniformly in the aluminium melt.
The aluminium melt 25 is cast in the following step 27 by direct chill casting
into an
ingot 28. To this end the aluminium melt 25 is poured, for example from a
crucible 29,
into a cooled and downwardly open frame mould 30 and solidified by spraying
with
water 31, resulting in the formation of the ingot 28.
In the following step 32 the ingot 28 undergoes ingot homogenisation and/or
ingot
preheating in a homogenising furnace 34 and in the following step 36 is hot
rolled in a
reversing hot rolling stand 38 for example, to form the hot strip 40,
preferably at a
temperature in the range of 280 C to 550 C, wherein in particular a hot
strip
temperature of 280 C to 350 C is adjusted. Due to the low Na content of the
aluminium alloy of the ingot 28 no edge cracks are formed during hot rolling,
despite
the high Mg content.
In the following step 42 the hot strip 40 is cold rolled in multiple passes
without
intermediate annealing on one or more cold rolling stands 44, so that finally
a cold
strip 46 with a final thickness in the range of 1 to 3 mm is formed. The
overall degree
of rolling in the cold rolling is at least 70%, the degree of rolling in the
last pass being
less than 33%.
In the following step 48 the cold strip 46 is guided through a levelling
system 50 with
multiple levelling rollers 52 arranged offset to one another and is thereby
levelled.
The levelling rollers 52 each have a diameter of 60 mm, so that the formation
of
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surface defects is avoided during levelling. After levelling, the cold-rolled
strip 46 is
cut directly into sheets 56 by means of a cutting device 54, without an
intermediate
coiling to a coil. This in turn avoids a unilateral buckling or elongation of
the cold strip
46.
The aluminium sheets 56 produced by the method described in Fig. 1 are
particularly
well suited for further use in a superplastic forming process.
Fig. 2 shows an embodiment for a use of an aluminium sheet 56 produced with
the
method of Fig. 1 for producing a component 66 by means of superplastic
forming.
In a first step 68 the aluminium sheet 56 is heated to a temperature in the
range of
450 C to 520 C. The heating can be carried out for example as exemplified in
Fig. 2, in
a chamber furnace or a continuous furnace 70. Additionally or alternatively,
the
heating of the aluminium sheet 56 can also take place directly in a forming
tool 78 for
forming the aluminium sheet 56. In this case a separate furnace 70 can in
particular be
dispensed with.
Owing to the high dislocation density introduced into the material during the
cold
rolling step 42 of Fig. 1, when heating the aluminium sheet 56, for example in
the
furnace 70 or in the tool 78, a spontaneous recrystallisation of the aluminium
sheet 56
occurs with formation of a very fine microstructure, which has an advantageous
effect
on the subsequent superplastic forming. In contrast to a chamber furnace, in
particular the heating in the tool or in the continuous furnace favoured the
superplastic forming, since the transfer and residence times during which the
material
is exposed to high (forming) temperatures are minimised, and grain growth
before the
actual forming is thereby further minimised.
In a second step 72 the aluminium sheet 56 is arranged between a first die
half 74 and
a second die half 76 of the forming tool 78 for the superplastic forming,
unless this has
already happened beforehand for heating the aluminium sheet 56 in the forming
tool
78. The first die half 74 has in Fig. 2 by way of example a concavity 80 and
the second
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die half 76 has a bulge 82 corresponding thereto. However, the two die halves
74, 76
may instead also have more complex contours for producing a more complex
shaped
component.
In the next step 84 the two die halves 74, 76 are brought together, wherein
the
aluminium sheet 56 is superplastically formed. In particular, the degree of
forming of
the aluminium sheet 56 is locally in part 100% or more. On account of the good
properties of the aluminium sheet 56 for the superplastic forming, in
particular the
fine and uniform microstructure, the aluminium sheet 56 does not constrict or
crack
despite the high degree of forming. After the two die halves 74, 76 have been
separated a damage-free finished component 66 can thus be removed from the
forming die 78 in the last step 86. In addition, the component 66 produced in
this way
also has a high surface quality without noticeable surface defects.
The properties of the aluminium sheet 56 enable the superplastic forming to be
carried out very quickly. In particular, the bringing together of the two die
halves 74,
76 can be effected within a few minutes, preferably in at most 5 minutes. The
production time of the component 66 can thus be shortened and the cycle rate
of the
forming operations can be increased.
Fig. 3 shows a further exemplary embodiment for the use of an aluminium sheet
56'
produced according to the method of Fig. 1 by means of superplastic forming.
In the first step 90 of the method an aluminium sheet 56', for example as
illustrated by
way of example in Fig. 3, is heated in a chamber furnace, a continuous furnace
or a
furnace of other type of construction to a temperature in the range of 450 C
and 520
C, so that a fine grain distribution is formed. Additionally or alternatively,
the heating
can also take place directly in a forming tool 98.
In contrast to heating in the chamber furnace, heating in the tool or in the
continuous
furnace favours the superplastic forming since the transfer and residence
times
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during which the material is exposed to high (forming) temperatures is
minimised,
thereby further minimising grain growth before the actual forming.
The aluminium sheet 56' is then positioned in step 92 between a first tool
half 94 and
a second tool half 96 of the forming tool 98 for the blow moulding, unless the
aluminium sheet 56' has not already been arranged there for heating in the
forming
tool 98. The first tool half 94 has by way of example a concavity 100
corresponding to
the target shape of the component to be produced. The illustrated shape of the
first
tool half 94 is merely by way of example and can be significantly more complex
in
practice. In the second tool half 96 a channel 102 is provided for blowing in
a gas.
In the next step 104 the first and second tool halves 94, 96 are brought
together and a
gas 106 is blown at a pressure of for example 2 bar through the channel 102 in
the
region of the concavity 100 against the aluminium sheet 56', so that the
aluminium
sheet 56' is superplastically formed until it abuts the contour of the
concavity 100. The
degree of forming of the aluminium sheet 56' is locally in part 100% or more.
Because of the good properties of the aluminium sheet 56' for superplastic
forming, in
particular the fine and uniform microstructure, there is no constriction or
tearing of
the aluminium sheet 56' despite the high degree of forming. After the two
mould
halves 94, 96 have been separated, a damage-free finished component 110 can
thus be
taken out from the forming tool 98 in the last step 108. Furthermore, the
component
110 produced in this way also has a high surface quality without conspicuous
surface
defects.
The properties of the aluminium sheet 56' enable the superplastic forming to
be
carried out very quickly. In particular, the gas 106 can be introduced at such
a
pressure through the channel 102 that the aluminium sheet 56' adopts the
contour of
the concavity 100 within a few minutes, preferably in at most 5 minutes. The
production time of the component 110 can thus be shortened and the cycle rate
of the
forming operations can be increased.
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In experiments, the formation of coarse particles in the aluminium melt was
investigated depending on the charging of the dispersoid formers Cr, Mn and
Ti.
Si Fe Cu Mn Mg Cr Zn Ti Na B Ca Li Al
0.060 0.126 0.001 0.576 4.282 0.185 0.004 0.017 <0.0001 0.001 0.0001 <0.0001
remainder
Table 1 (all data in wt.%)
For this purpose an aluminium melt A of the composition mentioned in Table 1
was
first produced by melting primary aluminium in an aluminium melting furnace
and
simultaneously adding to it additives to achieve the desired Mn, Mg and Cr
contents.
Furthermore, an aluminium melt B of the same composition was produced, wherein
Mn and Cr were charged separately, i.e., the Cr-containing additives to
achieve the
desired Cr content were added only after adjusting the desired Mn content and
then
homogenising the aluminium melt by stirring. The Cr content in the preliminary
molten aluminium during the adjustment of the desired Mn content and during
the
subsequent homogenisation of the melt was thereby less than 0.05 wt.%, and was
only
later adjusted to the target value.
From each of the two aluminium melts A and B produced in different ways,
ingots
were cast and strips were produced by hot and cold rolling. The strips
exhibited
coarse particles both on the surface and in the interior, the composition of
which was
analysed by WDX analysis (wavelength dispersive X-ray spectroscopy). The
following
Table 2 shows the results of the WDX analysis on six different coarse
particles (Nos. 1-
6) of a strip from the aluminium melt A, of which the particles Nos. 1-4 were
on the
surface and the particles 5 and 6 were in the interior of the strip:
Particle No. Mg Al Ti Cr Mn Fe
1 607 58061 354 5232 2909 223
2 5890 57001 3339 4806 3086 280
3 7729 51707 185 4339 1356 --
4 7194 54343 403 4607 1167 --
5 445 58683 313 5020 3342 300
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6 499 57399 332 5084 3089 240
Table 2
The numbers given in Table 2 are in each case pulse numbers of the WDX
analysis for
the respective elements. The numbers are approximately proportional to the
content
of the elements in the respective particle.
In addition, a microsection was prepared from a piece of the strip produced
from the
aluminium melt A. Fig. 4 shows an image of this polished and barked
microsection.
The micrograph clearly shows a coarse Cr-containing phase. The phase has a
size of 46
p.m x 210 lam in the microsection.
The aforedescribed WDX analyses show that the strips from the aluminium melt A
had
significant fractions of high-melting and sparingly soluble Cr-containing
phases, in
some cases also with certain proportions of Ti and Mg. Such phases (cf. Fig.
4) re-
dissolve - once formed - only with difficulty and form coarse, brittle
particles in the
strip, which adversely affect the superplastic properties of the strip or
sheet produced
therefrom.
The strips from the aluminium melt B exhibited practically no coarse particles
or
phases, i.e. only very fine but practically no coarse Al(Mn,Fe,Cr)Si phases
have formed
owing to the separate charging of Mn and Cr in the melt.
The investigated alloy with the composition of Table 1 has a lower Mg content
than is
envisaged according to the present teaching. For alloys with a Mg content of 5
wt.-%
and otherwise identical composition as in Table 1 similar results are found
however,
with the formation of coarse Cr-containing phases with co-charging of the
alloying
elements Mn and Cr and only slight or in some cases without the formation of
coarse
Cr-containing phases with separate charging of Mn and Cr. The separate
charging of Ti
has also proved to be beneficial in order to prevent the formation of coarse
phases.
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In further experiments, an aluminium melt C was produced having the
composition
shown in Table 3 below, wherein (as in the previously described aluminium melt
B)
Mn and Cr were charged separately from one another, with intermediate
homogenisation of the melt.
Si Fe Cu Mn Mg Cr Zn Ti Na B Ca Li Al
0.057 0.136 0.009 0.805 5.282 0.136 0.013 0.025 <0.0001 0.001 0.0004 <0.0001
remainder
Table 3 (all data in wt. %)
The aluminium melt C was cast into an ingot by direct chill casting. The ingot
was
preheated, and by subsequent hot and cold rolling without intermediate
annealing a
cold strip having a thickness of 1.5 mm was produced, with an overall
thickness
reduction in the cold rolling of 75%. The cold strip was then levelled by
levelling
rollers with a diameter of more than 60 mm in each case and cut into sheets.
Some of these sheets were then subjected to a heat treatment at 450 C for
various
durations in order to investigate the formation of the fine grain distribution
important
for superplastic forming. Fig. 5 shows an image of a polished and barked
microsection
of one of the sheets in the hard-rolled H19 state, i.e. before the heat
treatment. The
grains elongated by rolling are clearly visible.
Micrographs were taken of the heat-treated sheets and the average grain
diameters in
each case were determined according to ASTM E112. Fig. 6 shows an image of a
polished and barked microsection of a sheet heat-treated at 450 C for 1
minute. The
fine-grain microstructure with grain sizes between 5 and 15 lam and an average
grain
diameter of 7 m can readily be seen. This shows that the fine-grain
microstructure
important for superplastic forming is achieved practically instantaneously on
heating
up to the temperature for superplastic forming (typically 450 C-520 C). Fig.
7 shows
an image of a polished and barked microsection of a sheet that was heat
treated at 450
C for 60 minutes. The microstructure is just as fine-grained as in Fig. 6,
with a mean
grain diameter of also 7 m. This shows the stability of the fine
microstructure over
time at the superplastic forming temperature. This stability is achieved in
the
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investigated metal sheets in particular by the contents of Mn and Cr and their
fine
distribution in the aluminium matrix, in particular by the separate charging
of Mn, Ti
and Cr, which permanently prevent the growth of the aluminium grains.
Furthermore, the metallographic investigations show that the sheets have no
coarse
particles that would lead to pore formation during superplastic forming. This
is
achieved in particular by the low contents of Fe and Si as well as by the
separate
charging of Cr. The micrographs in Figs. 6 and 7 show that the sheets exhibit
a fine-
grain microstructure at the forming temperature, which has a very stable mean
grain
diameter even at high forming temperatures.
On the sheets as described above produced from the aluminium melt C,
superplastic
forming tests according to Lederich were carried out by means of the
incremental
strain rate test (Lederich et al. "Superplastic Formability Testing" Journal
of Metals
Vol. 34 Issue 8, pp. 16-20, 1982) under in each case successive use of the
strain rates 5
x 104 s-1, ,1 x 104 s-1, 5 x 1045-1, 1 x 10-3s-1, 5 x 10-35-1, 1 x 10-2s-1, 5
x 10-4s-1 and 1 x
10-1-s-1 and an ISO 20032:2007 compliant testing machine and sample geometry
for
determining the strain rate sensitivity m at four different forming
temperatures (450
C, 475 C, 500 C and 525 C). The aforementioned strain rate sequence was
thus run
through for a first sheet sample at a forming temperature of 450 C, for a
second sheet
sample at a forming temperature of 475 C etc. In this connection the strain
rate 5 x
1045-1 in the above-mentioned strain rate sequence was used a total of three
times in
each case to detect any hardening or softening due to the high-temperature
forming.
In order to determine the strain rate sensitivity m dependent on the strain
rate, in
each case the values for the yield stress a measured for a sheet thickness at
the
different strain rates were plotted double-logarithmically against the
associated strain
rates and the function F(E) = 1n(ay , which is dependent on the strain
rate, was
ln(t)
determined by fitting a second degree polynomial to the measured values. The
derivative of the function F() i.e. dF(E) din(a) or the derivative of the
polynomial
de dln(t)
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fitted for it, respectively, then corresponds to the strain rate sensitivity
m(E) as a
function of the strain rate E.
The results of the forming experiments are shown in the diagram in Fig. 8, in
which
the forming temperature T of the superplastic forming in C is plotted on the
X-axis
and the (dimensionless) strain rate sensitivity m is plotted on the Y-axis.
Here in Fig. 8
the values of the previously described determined function m(E) for the strain
rates 1
x 10-45-1 (+ symbols), lx 10-35-1 (x-symbols), lx 10-25-1 (o-symbols) and lx
10-15-1
(squared symbols) are plotted for each forming experiment. The lines plotted
in Fig. 8
connect the m-values of the four forming experiments, which were each
determined
respectively for the same strain rate.
As Fig. 8 shows, at the forming temperatures in the range 450 C - 520 C
typical for
superplastic forming, strain rate sensitivities m> 0.3 were achieved not only
at the
typical forming rates 10-4s-1 to 10-3s-1 but also at higher forming rates, in
particular
forming rates of 10-2s-1 or higher. Accordingly, the sheets are suitable not
only for
superplastic forming at conventional strain rates, but also for high-speed
superplastic
forming with very high strain rates, as a result of which the forming times
can be
significantly reduced and thus higher production rates can be achieved.
To investigate the porosity and the mechanical properties after superplastic
forming,
sheets produced as described above from the aluminium melt C were
superplastically
formed at a forming temperature of 515 C. with an ISO 20032:2007 compliant
testing
device in a uniaxial tensile test, in which the sample geometry was based on
the
abovementioned Standard (ISO 20032:2007 S-Type sample shape). The strain rate
was 2.5 x 10-4s-1 and the total strain E at the end of the forming was 100%.
On some of these sheets that were superplastically formed at a forming
temperature
of 515 C, the porosity was determined by means of the metallographic
microsection
and cutting test in accordance with the VDG-Merkblatt (VDG leaflet) P201. The
tested
sheets showed a very low porosity in the range of 0.3% to 0.7%.
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Furthermore, tensile tests were carried out on some of the sheets in order to
determine the yield strength Rpo.2 and the tensile strength Rm according to
DIN EN ISO
6892-1:2017, wherein the test was carried out transversely to the rolling
direction.
The tensile tests were carried out in each case after heating the sheets in
order to
achieve the desired microstructure for the superplastic forming. The sheets
were not
superplastically formed before the tensile tests.
The results of the tensile tests are shown in the diagrams in Figs. 9 and 10,
in which
the superplastic forming temperature T in C is plotted on the X-axis, and the
yield
strength Rp0.2 and the tensile strength Rm, in each case in MPa, are plotted
on the Y-
axis. As the test results show, the sheets had a yield strength Rp0.2 of more
than 160
MPa over the entire forming temperature range investigated, and even a yield
strength Rp0.2 of more than 170 MPa at a forming temperature of 500 C. The
tensile
strength of the sheets was significantly above 310 MPa, even above 320 MPa,
over the
entire forming temperature range investigated. The good mechanical properties
after
superplastic forming are due in particular to the advantageous Mn content of
at least
0.7 wt. %, the advantageous Mg content of at least 5.2 wt. %, and the separate
charging of Cr and Mn.
.. Due to the Mn content of at least 0.7 wt.% and the separate charging of Cr
and Mn, in
particular the formation of coarse particles and thereby a pore formation
adversely
affecting the mechanical properties is also reduced or even prevented in the
superplastic forming. In superplastic forming there is therefore virtually no
further
softening over and above the softening induced by the heating, so that the
measured
values for 11/30,2 and Rm shown in Figs. 9 and 10 are achieved by the sheets
after the
heating and also the superplastically formed sheets.
Date Recue/Date Received 2020-07-15
