[0001] This invention concerns AA5000 series alloys with the addition of Cu that can be
retained in a solution treated condition after hot working, for example by hot rolling
on a hot mill or by hot extruding.
[0002] In the art AA5000 series alloys are usually regarded as non-heat treatable alloys
i.e. they are not regarded as age hardenable. The addition of Cu to these alloys renders
them age hardenable, as described in EP-A-0773303, EP-0616044 and EP-A-0645655. However
these known methods also require a formal solution treatment.
[0003] The novel feature of this invention is the discovery that for certain Cu - containing
AA5000 series alloys sufficient solution treatment occurs during hot working, for
example hot rolling, to render the alloys age hardenable without a further expensive
solution treating step. This gives a very significant economic advantage especially
for commodity products such as can end stock, automotive sheet products, or extruded
products such as structural sections.
[0004] EP-A-0605947 describes manufacturing can body sheet using two sequences of continuous
operations. The described additional steps of uncoiling the hot coiled sheet, quenching
the sheet without intermediate cooling, cold rolling and re-coiling the sheet are
required, but these additional steps are not needed in the method of the present invention.
[0005] WO-A-99/39019 describes a method for making can end and tab stock but annealing of
the sheet is required as a separate operation after hot rolling which is not needed
in the method of the present invention.
[0006] WO-A-98/01593 describes a process for producing aluminium alloy can body stock but
again a separate annealing step is required.
[0007] JP-A-100121179 describes aluminium alloy sheet for carbonated beverage can lids but
a formal solution heat treatment is required, which is not needed in the method of
the present invention.
[0008] US-A-5655593 describes aluminium alloy sheet manufacture in which the hot strip is
cooled rapidly to minimise the precipitation of the alloying elements. This teaching
of rapid cooling is contrary to that of the present invention.
[0009] US-A-3464866 describes a process for obtaining aluminium alloy conductors but again
teaches rapid cooling.
[0010] In accordance with the present invention there is provided a method of producing
an age-hardenable aluminium alloy comprising the steps of:
(a) casting an alloy of a composition comprising the following expressed in weight
percent:
Magnesium |
1.0 to 4.0 |
Copper |
0.1 to 0.6 |
Manganese |
up to 0.8 |
Iron |
up to 0.5 |
Silicon |
up to 0.3 |
Chromium |
up to 0.15% |
Titanium |
up to 0.15%, preferably up to 0.05% |
Boron |
from 0 up to 0.05, preferably up to 0.01 |
Balance |
Aluminium with incidental impurities |
(b) optionally homogenising the cast alloy,
(c) hot working the casting at an initial temperature of at least 400°C, to form an
intermediate product, wherein at least part of the hot working is carried out whilst
the casting is at a temperature above the solvus temperature of the alloy,
(d) cooling the intermediate product during hot working or in a subsequent step at
a rate of less than 5°C/min such that at least a partially recovered or recrystallised
structure is formed and that sufficient copper is retained in solid solution in the
alloy to cause an age hardening effect on the alloy if phase precipitation takes place
during the alloy's subsequent thermal history, and
(e) optionally allowing or arranging for phase precipitation to occur in the alloy.
[0011] Preferably after the said hot working step the intermediate product is generally
maintained at a temperature below the solvus temperature of the alloy, provided that
if the intermediate product is heated above the alloy's solvus temperature then cooling
thereof is effected at a rate less than 2°C/sec.
[0012] By the term "the solvus temperature of the alloy" is meant the temperature below
which under equilibrium conditions the copper begins to be removed from solid solution
to form a precipitate. However, as to the rate of copper removed that will depend
on the kinetics of the reaction.
[0013] The precipitation phase if formed is believed to be S phase (an AlCuMg phase) or
its metastable precursors.
[0014] The alloy may be cast by DC casting to form an ingot or by continuous casting, for
example in a belt caster or a twin roll casting machine, to form a sheet.
[0015] The cast and preferably homogenised alloy can be extruded but for the production
of can end stock it is generally hot rolled. After casting the preferred steps are:
optionally homogenising the casting at a temperature of at least 480°C, and preferably
500 to 600°C, so that substantially all of the magnesium and copper in the casting
are in solid solution,
optionally hot rolling the casting, optionally with re-heating of the casting to above
the alloy's solvus temperature, preferably at least 450°C, to take substantially all
of the magnesium and copper present into solid solution,
hot rolling the casting with a rolling mill entry temperature of the casting of at
least 400, and preferably from 450° to 580°C,
continuing rolling the casting to the desired thickness to form a sheet so that at
least part of the rolling reduction is carried out above the solvus temperature of
the alloy and cooling the alloy, either while rolling or subsequently, slow enough
so as to form at least a partially recovered or recrystallised structure but fast
enough to ensure that sufficient of the Cu is retained in solid solution to provide
an age hardening effect if a subsequent precipitation treatment is carried out,
optionally cold rolling the hot rolled sheet, and optionally age hardening the cold
rolled alloy, wherein preferably after the essential hot rolling step the rolled ingot
is always maintained at a temperature below its solvus temperature.
[0016] During cold rolling, the metal temperature generally rises to about 100-200°C as
it is passed through the mill. Conventionally after cold rolling, the metal is coiled
and being so massive the coiled metal takes a long time to cool down to room temperature.
Phase precipitation and hardening can occur during this cooling down period without
the need forcibly to cool the coil. Additional cooling can, however, be used if required.
If desired after cold rolling re-heating can be effected if desired, for example to
control the amount of cold work in the alloy. If this re-heating takes the alloy above
its solvus temperature then cooling is preferably effected at a rate less than 2°C/sec
to avoid distortion or to avoid the need for a separate quench stage.
[0017] As an alternative to batch DC casting, the alloy could be cast continuously by for
example belt casting or twin roll casting. These techniques allow thin strip to be
produced of a thickness of generally as low as 5mm, and sometimes as low as 2mm. Such
thin cast strip may or may not require homogenisation before hot rolling since it
tends to cool so quickly that the Cu and Mg present are likely to remain in solid
solution.
[0018] The casting could be extruded using direct or indirect extrusion. Preferably the
casting is homogenised as described above and then cooled to room temperature before
being re-heated to 400 to 500°C for extrusion. Alternatively the casting can be cooled
directly from its homogenisation temperature to the desired extrusion temperature.
[0019] The extrudate is cooled preferably with still air or with forced air. If desired,
the extrudate can be re-heated to above the solvus temperature of the alloy and then
cooled at a rate of less than 2°C/sec. This re-heating treatment may be needed for
texture and/or grain size control. After extrusion the extrudate is generally stretched
by about ½ to 2% and then aged.
[0020] The present invention has particular applicability for the production of can stock,
especially can end stock (CES) which possesses a combination of high strength and
formability. The combination of composition and process of the present invention overcomes
many of the manufacturing difficulties of the conventional AA5182 sheet currently
in use and is capable of producing CES at lower cost. It also improves the subsequent
performance of the can end, most notably its scoreline corrosion resistance. The invention
is particularly suitable for downgauging to produce lighter weight can ends, i.e.
gauges down to say 0.150mm.
[0021] For the production of can end stock, the preferred method is to cast an ingot, homogenise
it, and hot roll to, say, 2mm to form strip. A key aspect of the invention is that
the strip does not need an additional solution heat treatment step. Furthermore, even
if it does, the material does not need to be rapidly cooled, e.g. does not need to
be quenched into water; the cooling is generally air cooled (possible forced air).
The coil is then cold rolled to final gauge and lacquered.
The range (in weight percent) for the principal elements over which this invention
is operable is:
Magnesium: 1.0 - 4.0 wt.%, preferably 2.0 - 4.0, still more preferably 2.5 to 4.0%
Copper: 0.1 - 0.6 wt.%, preferably 0.2 - 0.5, still more preferably 0.2 to 0.4%
Manganese |
up to 0.8 wt.%, preferably up to 0.6, more preferably up to 0.5, still more preferably
up to 0.4%. For some alloys a minimum Mn content of 0.1% is preferred. |
Iron |
up to 0.5 wt.%, preferably 0.1 - 0.3% |
Silicon |
up to 0.3wt.%, preferably up to 0.2% |
Chromium |
up to 0.15%, preferably trace |
Titanium |
up to 0.15, preferably up to 0.05% |
Boron |
up to 0.05, preferably up to 0.01% |
Carbon |
up to 0.05, preferably up to 0.01% |
For grain refining of the casting either TiB
2 or TiC can be used, but generally not together.
[0022] The present invention will now be described in more detail with reference to the
accompanying drawings in which:
Figure 1 shows a thermodynamic calculation of the solvus temperature for S-phase precipitation
in Al-x%Mg-y%Cu-0.25Mn-0.2Fe-0.12Si,
Figure 2 shows the conductivity changes (%IACS) during isothermal annealing of an
Al-3Mg-0.4Cu-0.25Mn-0.2Fe-0.12Si alloy after solution heat treatment and cold water
quenching,
Figure 3 shows the conductivity changes (%IACS) during isothermal annealing of an
Al-3Mg-0.4Cu-0.25Mn-0.2Fe-0.12Si alloy after solution heat treatment, cold water quenching
and cold rolling, and
Figure 4 are curves showing the effect of time and temperature on the extent of recrystallisation
during isothermal annealing of an Al-3Mg-0.4Cu-0.25Mn-0.2Fe-0.12Si alloy after solution
heat treatment, cold water quenching and cold rolling.
[0023] The theoretical basis for the present invention is as follows:
The basic premise is to select an alloy composition which will enable solute to be
kept in solid solution during cooling from hot rolling temperatures (250°C to 400°C,
say). The strip is then processed to bring out a precipitation hardening phase which
provides extra strength. This precipitation forms preferentially on the dislocation
structure introduced during cold deformation In the case of CES this cold deformation
is cold rolling, for extrusions it is stretching, and for sheet it is during forming
of the sheet when it is fabricated into a component.
Although there is a thermodynamic driving force for the solute to be removed from
solid solution during hot working and subsequent cooling, the nucleation and diffusion
effects are such to keep a substantial amount of solute in solution, i.e. 'missing
the nose of the c-curve'. Accompanying Figure 1 shows a calculation of the solvus
temperatures for a range of Al-Cu-Mg alloys. This shows that the solute will stay
in solid solution above the temperatures indicated. Thus, the solute can not start
to come out of solid solution until the strip is at or below this temperature. It
should be noted, that even if the solute does start to come out of solid solution,
there may still be sufficient solute available to provide an appreciable strengthening
effect during subsequent processing.
[0024] The conductivity has been determined for a 3Mg-0.4Cu-0.25Mn-0.2Fe-0.12Si alloy (wt.%)
to demonstrate that for this type of alloy there is a barrier to nucleation and growth
of the precipitates which can be commercially exploited to provide an improved balance
of strength and formability. Accompanying Figure 2 shows the effect of isothermal
ageing on the conductivity of a full solution heat treated and cold water quenched
material subject to isothermal ageing. This shows that at temperatures below the solvus
the conductivity increases (indicating Cu along with Mg removed from solid solution),
but that at lower temperatures the precipitation becomes difficult. Thus, the solute
can be kept in solid solution if the strip can be cooled to these temperatures sufficiently
rapidly.
[0025] If there are dislocations present then the conductivity rise is more rapid, since
the precipitating phase is believed to be S-phase (an AlCuMg phase), or its metastable
precursors which is well-known to nucleate preferentially on dislocations. To demonstrate
this, a further set of isothermal ageing experiments have been performed on the same
alloy, but after solution heat treatment, cold water quenching and cold rolling. This
is shown in accompanying Figure 3. In this case the conductivity drop starts to occur
after a few seconds. This shows the importance of passing through this temperature
regime without large numbers of dislocation present, since if the phase nucleates
at these high temperatures it is likely to be relatively coarse and provide little
strengthening. The example shown is an extreme example since the strip was cold rolled
to introduce a high dislocation density prior to ageing. In hot deformation the dislocation
density is lower for a fixed level of macroscopic strain, thus providing fewer sites
for nucleation of the precipitates.
[0026] For the production of CES the hot rolling conditions are selected to ensure that
the hot rolled sheet recrystallises on or before coiling or very shortly thereafter.
Preferably the sheet is fully recrystallised resulting in a low dislocation density.
Recrystallisation is encouraged by arranging for the minimum temperature of the sheet
as it exits from the rolling mill to be 250°C, preferably 270°C and more preferably
300°C and/or arranging for the cooling rate of the sheet to be sufficiently slow to
allow time for the sheet to recrystallise when in its coiled form or during coiling.
In a conventional mill the coiling temperature is approximately the same as the exit
rolling mill temperature. Where additional cooling means are provided after the mill
the minimum coiling temperature should be in the range of minimum mill exit temperatures
mentioned above. In practice acceptable cooling rates are found to be of the order
of 0.1 and preferably 0.2 to 5°C/minute over the temperature range of 400-200°C. There
is no need to uncoil the sheet during cooling in order, for example, to quench it.
[0027] An indication of the time required to achieve recrystallisation has been determined
for a 3Mg-0.4 Cu-0.25Mn-0.2Fe-0.12Si alloy (wt.%). This material was solution heat
treated, cold water quenched and cold rolled 50%. Isothermal heat treatments were
performed to determine the extent of recrystallisation, as shown in Figure 4. This
shows that after this deformation, full recrystallisation is possible within a few
minutes at temperatures in excess of around 320°C. It should be noted that the precise
details of the recrystallisation kinetics will depend on the deformation conditions
and the material microstructure.
[0028] A high rolling mill exit temperature encourages precipitation of S phase or its precursors
while the strip or coil is cooling. Cooling more quickly can counter this and prevent
precipitation but if the exit temperature becomes too high, the cooling rate required
is too fast to be practically useful. To take maximum advantage of the rapid cooling
during hot rolling, the upper limit to the mill exit temperature, especially for the
alloys richer in Cu and Mg, should preferably be lower than the solvus temperature
of the alloy. Figure 1 gives an indication of the solvus temperature as a function
of the Mg and Cu contents. Preferably the maximum exit temperature should be between
340°C and 360°, although up to 380°C is possible for some alloys.
[0029] It is important to note that the location of the nose of the c-curve for these alloys
when recrystallised varies with the composition of the alloy. For example, for the
alloy referred to in Figure 2, the nose of the curve is located at a time of around
100 to 1000 seconds. For dilute alloys the nose is moved to longer times whilst for
more concentrated alloys the nose is moved to shorter times. The time indicated in
Figure 2 compares with times of between 1 and 100 seconds for conventional age hardening
systems such as AA7075, AA2017, AA6061 and AA6063. For the alloys described in the
present invention, this provides longer times at temperatures below the solvus temperature
in which to cool the strip and still maintain the Cu (and Mg) in solid solution. For
this preferred alloy of Figure 2 it has been found that a cooling rate of 5°C/min
is sufficient substantially to miss the nose of the c-curve and provide a substantial
age hardening response during subsequent processing. This cooling rate can be achieved
by, for example, forced air cooling of a coil. Previous art regarding solution heat
treatment of these Al-Mg-Cu alloys teaches that, not only is a separate solution heat
treatment stage required, but that the strip must be quenched with a cooling rate
of 2°C/second or faster. For the present invention it has been found that neither
of these steps need to be used, thereby providing a lower cost manufacturing route
for these alloys. Likewise no separate annealing step is needed after the hot working
step and before the cooling step.
[0030] This solute is then used to give a significant precipitation hardening effect during
subsequent thermomechanical processing. During subsequent cold (or warm) deformation
of the strip an increased dislocation density is introduced giving enhanced nucleation
sites for the strengthening phase. This deformation may not be needed for all applications
of this invention, since for these compositions it is known that the precipitation
can also occur in the absence of dislocations, albeit at slower rates. The precipitating
phase is believed to be S-phase which can form as needles or rods on the dislocation
structure. In the case of CES this precipitation could occur during a separate ageing
step or during the thermal history which the material would experience during deformation
in, for example, strip rolling.
[0031] As shown above, it may be important to achieve rapid recrystallisation in order to
remove the dislocations from the material as it cools. Mn can be added as a strengthening
element and to control grain size and is therefore desirably kept as high as possible.
However, Mn inhibits recrystallisation after hot rolling or during annealing, and
so a maximum Mn content of 0.4% may have to be set in order to achieve full recrystallisation
for some alloys under certain conditions. For many of the alloys to assist in controlling
the grain size of the recrystallised sheet, it may be desirable to have a minimum
of at least 0.05%Mn and preferably at least 0.1%Mn present in the alloy. Recrystallisation
may also be important for crystallographic texture control in CES, but this may not
be necessary if the can end tooling is modified to take significantly higher levels
of earing into account. Crystallographic texture control can also be important for
automotive sheet formability; another potential application of this invention.
[0032] Another feature of the composition used in the present invention is the importance
of having low Fe and Si in the alloy, since this will prevent the presence of excessive
numbers of coarse constituent particles in the sheet. These form during solidification
and cannot be fully dissolved during homogenisation of the ingot. Although they break
up during rolling, their presence is sufficient adversely to affect formability. Since
this invention has been found to produce improved formability over existing AA5182
CES, the strip may be able to tolerate higher levels of these elements, thus reducing
cost. Tolerance of higher levels of Si and Fe may allow greater use of recycled aluminium
scrap and this is another important aspect of this invention. Up to 0.5%Fe may be
tolerated in the alloy and preferably up to 0.3%Fe. The minimum amount of Fe present
will be dictated by cost and there is unlikely to be less than 0.1Fe. Silicon up to
0.3% may be present, preferably up to 0.2%.
[0033] Another advantage over conventional AA5182 CES is that the lower Mg content will
also make the can end less susceptible to stress corrosion cracking (SCC), which can
lead to catastrophic failure of the end under the stressed conditions which are encountered
in the pressurised can. The invention described here will make the end less sensitive
to these conditions, since the lower Mg content reduces beta-phase precipitation,
which has been linked to SCC. Avoidance of SCC is also important in many other applications
including car body sheet.
[0034] CES is currently made from AA5182 and gets its strength predominantly from a combination
of solute hardening and strain hardening. This makes it difficult to roll and gives
a relatively high manufacturing cost.
[0035] The alloy used in the present invention has lower strength during the rolling operations,
but develops its strength during subsequent thermal exposure during fabrication. Thus
there is the benefit of rolling a lower strength sheet, but still enabling the desired
sheet properties to be obtained ultimately. It is also possible to produce a higher
strength sheet suitable for downgauging without a reduction in rollability (higher
rolling loads, more difficulties in performing the rolling operation) encountered
in higher Mg containing alloys such as AA5182 and AA5019A.
[0036] The present invention is also applicable to production of low cost automotive sheet
where the material could be used in the hot rolled condition (Direct Hot Roll to Gauge),
thereby potentially avoiding the need to solution heat treat the sheet. Alternatively,
the sheet could be cold rolled to gauge, as for CES, with a final continuous anneal
to impart the formability required for this application and to take the solute into
solution. Cooling after annealing should be sufficiently rapid to retain substantially
all of the solute in solution. Ageing could be carried out in a separate operation
before or after forming, for example during the paint bake stoving of the automotive
part.
[0037] Some embodiments of the present invention will now be described by way of example:
Example 1
[0038] An alloy of the following composition was cast as a 225mm x 75mm cross section DC
ingot;
Magnesium |
3.0 wt.% |
Copper |
0.4 wt.% |
Manganese |
0.25 wt.% |
Iron |
0.20 wt.% |
Silicon |
0.12 wt.% |
Balance |
aluminium with incidental impurities. The ingot was not grained refined during casting
and as a consequence the Ti level was 0.0018% and B was less than 0.0001%. |
[0039] This was homogenised for 2 hours at 540°C (50°C/hr heating rate), followed by laboratory
hot rolling to 6mm. During this rolling stage the temperatures were only about 100-200°C,
so the strip was re-solution heat treated to bring about full recrystallisation and
to put the solute back into solid solution. This reproduces solute levels more like
those which would be found during rolling on an industrial hot line (but prior to
coiling).
[0040] Different heat treatments were then applied at this gauge. The strip was either solution
heat treated (SHT) (5 minutes at 550°C) and cold water quenched (CWQ) or it was solution
heat treated and then air cooled to temperatures in the range 300 to 340°C and then
cooled at 1°C/min. Conductivity was measured at this stage to determine how much solute
remained in solid solution. These conditions were selected to simulate the conditions
which might be expected to exist during commercial use of this invention. Until the
strip temperature drops below the solvus temperature for the alloy the S phase therein
cannot precipitate and therefore the Cu (and Mg) would be substantially in solid solution.
The strip could then be quenched at the end of hot rolling or, preferably, cooled
after coiling. During this process the starting temperature could be in the range
300 to 340°C and a typical initial cooling rate would be 1°C/min. The temperature
range between the solvus temperature (about 390°C for the alloy) and the coiling temperature
is passed through very quickly since this is when the strip might typically be in
the hot tandem mill and, hence, there is lubricant applied to the strip which acts
as a coolant. This phase was simulated using the air cool from the solution heat treatment
temperature.
[0041] The strip was then cold rolled to 0.24mm and given a simulation of a coil cool down
to ambient temperature from 150°C at 0.4°C/min. It was then given a simulation of
a lacquer curing cycle for 3 minutes at 205°C. Tensile testing was performed at each
stage of the treatment and the results compared with results on conventional AA5182
CES materials processed in the laboratory.
[0042] The effect of strength development was also studied at various stages of the laboratory
simulation of the CES production route. An example is given below for this alloy which
has been solution heat treated at 2mm and rolled to 0.20mm gauge. This is compared
with AA5182 rolled in the laboratory using a simulation of the commercial route for
that alloy. The 0.2% yield strength is shown in Table 1 below. The as-rolled strength
was found to be lower than AA5182, indicating easier rolling, and the strength drop
during coiling and lacquer stoving simulation was less, showing the benefits of precipitation
hardening. In addition, in AA5182 CES the softest direction is usually at about 45°
to the rolling direction of the sheet (softer by about 10-20 MPa) and this is believed
to control the buckle pressure of the sheet. In this invention the levels of cold
reduction needed to generate the desired strength level are lower and thus the weakest
direction is likely to be this longitudinal value. Hence, at its best, the combination
of the composition and processing route of the present invention is capable of producing
a strength level approximately 45 MPa stronger than existing AA5182.
Table 1:
Comparison of properties with conventional CES |
Condition |
5182 CES |
This alloy |
As-rolled at final gauge |
430 MPa |
399 MPa |
As-rolled and coil annealed |
358 MPa |
386 MPa |
As-lacquered |
345 MPa |
370 MPa |
[0043] Conductivity results are shown in Table 2 below. This shows that the conductivity
at the solution heat treatment stage is capable of being increased from 33.1 to 35.0
if the solute is allowed to be removed from solid solution, but that if the material
is cooled to ambient temperature at 1°C/min from 300°C there is only a fraction of
the increase in the conductivity (0.3% versus 1.9%). This implies that a significant
amount of the solute is kept in solid solution, even at these cooling rates.
Table 2:
Conductivity after different heat treatments at 2mm gauge |
Condition |
Conductivity (% IACS) |
Solution heat treated and CWQ |
33.1 |
Solution heat treated and Fast Air Cooled |
33.1 |
Solution heat treated and cooled from 340°C |
33.9 |
Solution heat treated and cooled from 320°C |
33.8 |
Solution heat treated and cooled from 300°C |
33.4 |
SHT, cold worked and aged 14 hours at 320° |
35.0 |
[0044] The strength developed in these materials at final CES gauge after lacquer stoving
is shown in Table 3 below. In this case the sheet has been rolled to 0.24mm. This
shows that sufficient solute remains in solid solution still to give an appreciable
strength CES. Bend testing has also been performed and indicates an improvement in
the amount of bending which can be performed prior to failure when compared with conventional
AA5182 CES.
Table 3:
Strength developed after processing to 0.24mm after various thermal treatments at
2mm 'hotband' gauge. |
Condition |
0.2% Proof Stress (MPa) |
Solution heat treated and CWQ |
350 MPa |
Solution heat treated and cooled from 340°C |
327 MPa |
Solution heat treated and cooled from 320°C |
329 MPa |
Example 2
[0045] An alloy of the following composition was DC cast for processing within an industrial
plant:
Magnesium |
2.9 wt.% |
Copper |
0.4 wt.% |
Manganese |
0.1 wt.% |
Iron |
0.20 wt.% |
Silicon |
0.08 wt.% |
Balance |
aluminium with incidental impurities. The ingots were cast with additional grain refiner. |
[0046] The ingot were homogenised at 540°C and hot rolled on a single stand reversing mill
to a thickness of 38mm at which point the temperature was around 480°C. The strip
was then hot rolled through a 3-stand hot tandem mill to a gauge of 2.5mm. The conditions
were adjusted to give two different coiling temperatures in order to show the effects
at opposite extremes of this invention. In both cases the coils were forced-air cooled,
giving a cooling rate measured on the outer laps of the coil of around 0.7°C/min.
[0047] The cooler coil was processed to give a sidewall temperature of 280-290°C. In this
instance the microstructure of the strip was largely unrecrystallised. As a consequence
the solute was easily removed from solid solution on the pre-existing dislocation
structure from the hot deformation. The conductivity of this strip is shown in Table
4, showing that the %IACS value is similar to that in which all of the precipitation
has been allowed to occur. Also in Table 4 is presented the conductivity obtained
by using a still-air cool on strips of the 2.5mm thick metal at the end of the hot
rolling (approximately 60°C per minute), showing that at these cooling rates a significant
amount of the solute can be kept in solid solution.
[0048] The hotter coil was processed to give a coil sidewall temperature of 330-340°C. Table
4 shows that in this case the forced air cooling leaves more solute in solid solution
as a consequence of the fully recrystallised grain structure achieved with the higher
coiling temperature. The amount of solute in solid solution with the faster cool is
even higher and approaches that of the conventional solution heat treated (SHT) and
cold water quenched (CWQ) material. This shows that a cooling rate of 0.7°C/min is
able to keep some of the copper in solid solution, but that more rapid cooling leaves
more copper in solid solution and yet is still fully recrystallised. Thus, cooling
the coil with forced-air from a temperature lower than 330°C will achieve a similar
effect (i.e. more solute in solid solution), since the c-curve will be substantially
missed in that case too. The forced-air cooled coil was cold rolled to 0.216mm and
the as-rolled tensile yield strength measured as 347 MPa.
[0049] Between these two limits of cooling temperature there will be even more solute in
solid solution at the end of hot rolling and thus even higher strength sheet can be
produced.
Table 4:
Conductivity after different thermomechanical treatments in an industrial plant |
Condition |
Conductivity (%IACS) |
Solution Heat Treated and CWQ |
35.4 |
SHT, CWQ + 24 hrs. at 310°C |
36.8 |
|
Forced-air cooled coil from 280-290°C |
36.9 |
air cooled strip from 280-290°C |
36.1 |
|
Forced-air cooled coil from 330-340°C |
36.4 |
air cooled strip from 330-340°C |
35.9 |
1. A method of producing an age-hardenable aluminium alloy comprising the steps of:
a) casting an alloy of a composition comprising the following expressed in weight
percent:
Magnesium |
1.0 to 4.0 |
Copper |
0.1 to 0.6 |
Manganese |
up to 0.8 |
Iron |
up to 0.5 |
Silicon |
up to 0.3 |
Chromium |
up to 0.15 |
Titanium |
up to 0.15 |
Balance |
Aluminium with incidental impurities |
b) optionally homongenising the cast alloy,
c) hot working the casting at an initial temperature of at least 400°C to form an
intermediate product, wherein at least part of the hot working is carried out whilst
the casting is at a temperature above the solvus temperature of the alloy,
d) cooling the intermediate product either during hot working or in a subsequent step
at a rate of less than 5°C/min such that at least a partially recovered or recrystallised
structure is formed and that sufficient copper is retained in solid solution in the
alloy to cause an age hardening effect on the alloy if phase precipitation takes place
during the alloy's subsequent thermal history, and
e) optionally allowing or arranging for phase precipitation to occur in the alloy.
2. A method as claimed in claim 1 wherein the alloy has the following composition expressed
in weight percent:
Magnesium |
2.0 to 4.0 |
Copper |
0.2 to 0.5 |
Manganese |
up to 0.6, preferably up to 0.5 |
Iron |
0.1 to 0.3 |
Silicon |
up to 0.2 |
Chromium |
up to 0.15 |
Titanium |
up to 0.05 |
Boron or
Carbon |
up to 0.01 |
Balance |
Aluminium with incidental impurities |
3. A method as claimed in claim 2 wherein the magnesium content is 2.5 to 4.0%.
4. A method as claimed in any one of claims 1 to 3 wherein the intermediate product has
a substantially fully recovered or recrystallised structure.
5. A method as claimed in any one of the preceding claims wherein the casting is homogenised
before hot working at a temperature of at least 480°C, preferably 500 to 600°C, so
that substantially all of the magnesium and copper in the casting are in solid solution.
6. A method as claimed in any one of the preceding claims wherein the casting is hot
worked, optionally with re-heating of the casting to above the alloy's solvus temperature,
and preferably at least 450°C, to take substantially all of the magnesium and copper
present into solid solution.
7. A method as claimed in any one of the preceding claims wherein the hot working step
is carried out when the casting has an initial temperature of from 450°C to 580°C.
8. A method as claimed in any one of the preceding claims wherein the alloy is DC cast.
9. A method as claimed in any one of the preceding claims including the step of cold
rolling the hot worked casting, optionally with coiling.
10. A method as claimed in any one of claims 1 to 9 wherein the hot working is effected
by extrusion.
11. A method as claimed in any one of claims 1 to 9 wherein the hot working is effected
by hot rolling.
12. A method as claimed in any one of the preceding claims wherein the hot worked casting
is cooled at a rate of less than 1°C/min.
13. A method as claimed in any one of the preceding claims wherein if after the said hot
working step the temperature of the intermediate product exceeds the solvus temperature
of the alloy then cooling of the intermediate product to a temperature below the alloy's
solvus temperature is effected at a rate less than 2°C/sec.
1. Verfahren zur Herstellung von aushärtenden Aluminiumlegierungen, das die folgenden
Schritte umfasst:
a) Gießen einer Legierung, die das Folgende in Gewichtsprozent ausgedrückt enthält:
Magnesium |
1,0 bis 4,0 |
Kupfer |
0,1 bis 0,6 |
Mangan |
bis zu 0,8 |
Eisen |
bis zu 0,5 |
Silizium |
bis zu 0,3 |
Chrom |
bis zu 0,15 |
Titan |
bis zu 0,15 |
Rest |
Aluminium mit allfälligen Verunreinigungen |
b) wahlweise Homogenisieren der gegossenen Legierung
c) Warmumformen des Gussstücks bei einer anfänglichen Temperatur von mindestens 400
°C , um ein Zwischenprodukt zu erhalten, wobei mindestens ein Teil der Warmumformung
durchgeführt wird, während das Gussstück eine Temperatur oberhalb der Lösungstemperatur
der Legierung hat,
d) Kühlen des Zwischenprodukts entweder während dem Warmumformen oder in einem darauffolgenden
Schritt mit einer Geschwindigkeit von weniger als 5 °C/min , so dass mindestens eine
teilweise erholte oder rekristallisierte Struktur gebildet wird, und dass in der festen
Lösung in der Legierung genügend Kupfer verbleibt, um einen Aushärtungseffekt bei
der Legierung hervorzurufen, wenn während der weiteren thermischen Geschichte der
Legierung Phasenauskristallisierung eintritt, und
e) wahlweise Zulassen oder Herbeiführen des Eintretens von Phasenauskristallisierung
in der Legierung.
2. Verfahren nach Anspruch 1, wobei die Legierung die folgende Zusammensetzung, in Gewichtsprozent
ausgedrückt hat:
Magnesium |
1,0 bis 4,0 |
Kupfer |
0,2 bis 0,5 |
Mangan |
bis zu 0,6, vorzugsweise bis zu 0,5 |
Eisen |
0,1 bis 0,3 |
Silizium |
bis zu 0,2 |
Chrom |
bis zu 0,15 |
Titan |
bis zu 0,15 |
Bor oder Kohlenstoff |
bis zu 0,01 |
Rest |
Aluminium mit allfälligen Verunreinigungen |
3. Verfahren nach Anspruch 2, wobei der Magnesiumgehalt 2,5 bis 4 % ist.
4. Verfahren nach einem der Ansprüche 1 bis 3, wobei das Zwischenprodukt eine praktisch
vollständig erholte oder rekristallisierte Struktur hat.
5. Verfahren nach einem der vorhergehenden Ansprüche, wobei das Gussstück vor der Warmumformung
bei einer Temperatur von mindestens 480 °C, vorzugsweise 500 bis 600 °C homogenisiert
wird, so dass praktisch alles Kupfer und Magnesium in dem Gussstück in fester Lösung
sind.
6. Verfahren nach einem der vorhergehenden Ansprüche, wobei das Gusstück warmumgeformt
wird, wahlweise mit Wiedererhitzen des Gussstückes über die Lösungstemperatur der
Legierung, und vorzugsweise auf mindestens 450 °C, um praktisch alles vorhandene Magnesium
und Kupfer in die feste Lösung zu nehmen.
7. Verfahren nach einem der vorhergehenden Ansprüche, wobei der Warmumformungsschritt
durchgeführt wird, wenn das Gussstück eine anfängliche Temperatur von 450 °C bis 580
°C hat.
8. Verfahren nach einem der vorhergehenden Ansprüche, wobei die Legierung stranggegossen
wird.
9. Verfahren nach einem der vorhergehenden Ansprüche, das den Schritt des Kaltwalzens
des warmumgeformten Gussstücks, wahlweise mit Aufwickeln, umfasst.
10. Verfahren nach einem der Ansprüche 1 bis 9, wobei die Warmumformung durch Strangpressen
durchgeführt wird.
11. Verfahren nach einem der Ansprüche 1 bis 9, wobei die Warmumformung durch Warmwalzen
durchgeführt wird.
12. Verfahren nach einem der vorhergehenden Ansprüche, wobei das warmumgeformte Gussstück
mit einer Geschwindigkeit von weniger als 1 °C/min abgekühlt wird.
13. Verfahren nach einem der vorhergehenden Ansprüche, wobei, wenn nach dem Warmumformungsschritt
die Temperatur des Zwischenproduktes die Lösungstemperatur der Legierung überschreitet,
das Kühlen des Zwischenprodukts unter die Lösungstemperatur mit einer Geschwindigkeit
von weniger als 2 °C/sec durchgeführt wird.
1. Procédé pour produire un alliage d'aluminium durcissable par vieillissement comprenant
les étapes consistant à :
a) couler un alliage d'une composition comprenant les éléments suivants, les valeurs
étant exprimées en pourcentage pondéral :
Magnésium |
1,0 à 4,0 |
Cuivre |
0,1 à 0,6 |
Manganèse |
jusqu'à 0,8 |
Fer |
jusqu'à 0,5 |
Silicium |
jusqu'à 0,3 |
Chrome |
jusqu'à 0,15 |
Titane |
jusqu'à 0,15 |
Complément |
Aluminium avec des impuretés accidentelles |
b) éventuellement homogénéiser l'alliage coulé,
c) travailler à chaud la matière coulée à une température initiale d'au moins 400°C
afin de former un produit semi-ouvré, dans lequel au moins une partie du travail à
chaud est réalisée lorsque la matière coulée est à une température supérieure à la
température limite de solubilité de l'alliage,
d) refroidir le produit semi-ouvré soit pendant le travail à chaud, soit au cours
d'une étape ultérieure, à une vitesse inférieure à 5°C/min, de sorte qu'au moins une
structure partiellement récupérée ou recristallisée se forme et qu'une quantité suffisante
de cuivre reste sous forme de solution solide dans l'alliage afin d'entraîner un effet
de durcissement par vieillissement de l'alliage si une précipitation de phase se produit
lors de l'historique thermique ultérieur de l'alliage, et
e) éventuellement permettre ou faire en sorte qu'une précipitation de phase se produise
dans l'alliage.
2. Procédé selon la revendication 1, dans lequel l'alliage possède la composition suivante,
les valeurs étant exprimées en pourcentage pondéral :
Magnésium |
2,0 à 4,0 |
Cuivre |
0,2 à 0,5 |
Manganèse |
jusqu'à 0,6, de préférence jusqu'à 0,5 |
Fer |
0,1 à 0,3 |
Silicium |
jusqu'à 0,2 |
Chrome |
jusqu'à 0,15 |
Titane |
jusqu'à 0,05 |
Bore ou carbone |
jusqu'à 0,01 |
Complément |
Aluminium avec des impuretés accidentelles |
3. Procédé selon la revendication 2, dans lequel la teneur en magnésium va de 2,5% à
4,0%.
4. Procédé selon l'une quelconque des revendications 1 à 3, dans lequel le produit semi-ouvré
a une structure essentiellement entièrement récupérée ou recristallisée.
5. Procédé selon l'une quelconque des revendications précédentes, dans lequel la coulée
est homogénéisée avant le travail à chaud à une température d'au moins 480°C, de préférence
de 500°C à 600°C, de sorte qu'essentiellement tout le magnésium et le cuivre dans
la matière coulée soit sous forme de solution solide.
6. Procédé selon l'une quelconque des revendications précédentes, dans lequel la coulée
est travaillée à chaud, éventuellement avec un réchauffage de la coulée à une température
supérieure à la température limite de solubilité de l'alliage et de préférence à au
moins 450°C, afin d'obtenir essentiellement tout le magnésium et le cuivre présents
sous forme de solution solide.
7. Procédé selon l'une quelconque des revendications précédentes, dans lequel l'étape
de travail à chaud est réalisée lorsque la coulée est à une température initiale allant
de 450°C à 580°C.
8. Procédé selon l'une quelconque des revendications précédentes, dans lequel l'alliage
est une coulée semi-continue.
9. Procédé selon l'une quelconque des revendications précédentes, comprenant l'étape
de laminage à froid de la matière coulée travaillée à chaud, éventuellement avec cintrage.
10. Procédé selon l'une quelconque des revendications 1 à 9, dans lequel le travail à
chaud est effectué par extrusion.
11. Procédé selon l'une quelconque des revendications 1 à 9, dans lequel le travail à
chaud est effectué par laminage à chaud.
12. Procédé selon l'une quelconque des revendications précédentes, dans lequel la coulée
travaillée à chaud est refroidie à une vitesse inférieure à 1°C/min.
13. Procédé selon l'une quelconque des revendications précédentes, dans lequel si, après
ladite étape de travail à chaud, la température du produit semi-ouvré dépasse la température
limite de solubilité de l'alliage, alors un refroidissement du produit semi-ouvré
à une température inférieure à la température limite de solubilité est effectué à
une vitesse inférieure à 2°C/sec.