Patent Application
20230372986
The invention relates to a plate made of a rolled aluminum alloy and a method for producing said plate.
Plates made of a rolled EN AW-6082 aluminum alloy are known. Plates of this kind can achieve a yield strength (Rp0.2) of around 260 MPa in the T6 state.
In order to achieve a finer grain structure in the recrystallized state in plates made of a rolled Al—Mg—Si aluminum alloy, it is known (EP1614760A1) to add from 0.1 to 0.4 wt % zirconium (Zr) to the alloy. The yield strengths (Rp0.2) of the plates in the T4 state are substantially the same with or without the Zr content.
The object of the invention, therefore, is to improve the strength, more particularly the yield strength (Rp0.2), of a plate made of an Al—Mg—Si aluminum alloy. Another object of the invention is to create a reproducible method for achieving this.
According to the invention, a plate made of a rolled aluminum alloy includes the following alloy components:
from 0.7 to 1.5 wt % silicon (Si),
from 0.5 to 1.3 wt % magnesium (Mg)
from 0.05 to 0.6 wt % manganese (Mn),
from 0.1 to 0.3 wt % zirconium (Zr),
and optionally
up to 0.5 wt % copper (Cu),
up to 0.7 wt % iron (Fe),
up to 0.1 wt % chromium (Cr),
up to 0.2 wt % titanium (Ti),
up to 0.5 wt % Zink (Zn),
up to 0.2 wt % tin (Sn),
up to 0.1 wt % strontium (Sr),
up to 0.2 wt % vanadium (V),
up to 0.2 wt % molybdenum (Mo),
a balance aluminum, and
inevitable production-related impurities, with at most 0.05 wt % of each and at most 0.15 wt % collectively.
The plate has a partially recrystallized structure with a degree of recrystallization of less than 25%, and a non-recrystallized structure region of the structure is in a recovered state and has an average subgrain size of less than 10 μm in a rolling direction.
A method for producing the plate includes the following steps in the indicated sequence:
casting a rolling ingot with the aluminum alloy,
multistep homogenization of the rolling ingot with subsequent accelerated cooling to room temperature, wherein the multistep homogenization includes at least a first homogenization at a first temperature in a range from 300° C. to 400° C. and after the first homogenization, a second homogenization at a second temperature in a range from 500° C. to 10° C. below a solidus temperature of the aluminum alloy,
hot rolling the homogenized rolling ingot into the plate and subsequent heat treatment, including solution annealing of the plate with subsequent accelerated cooling to room temperature, natural aging of the solution annealed plate, possibly with a cold deformation with a degree of deformation in a range between 0.5 and 10%, and subsequent artificial aging of the plate.
If the aluminum alloy contains from 0.7 to 1.5 wt % silicon (Si), from 0.5 to 1.3 wt % magnesium (Mg), from 0.05 to 0.6 wt % manganese (Mn), and from 0.1 to 0.3 wt % zirconium (Zr), then the requirements can be met for an increased strength, preferably the yield strength (Rp0.2). With this composition, it is possible, namely by taking into account the elevated Zr content in comparison to other rolled 6xxx alloys, to establish a particular structure of the plate—namely a substantially recovered structure, i.e. a structure with a low percentage of recrystallized grains. For this purpose the plate has a partially recrystallized structure with a degree of recrystallization of less than 25%, which can ensure an increased strength if in addition, the non-recrystallized structure region of the structure is in the recovered state and has an average subgrain size of less than 10 μm in the rolling direction. This led to the surprising discovery that because of the composition of the aluminum alloy with Zr, comparatively finely dispersed intermetallic Zr-containing particles such as (Al,Si)3Zr or Al3Zr particles form in the structure, which results in a pinning of the subgrain boundaries for a comparatively low degree of recrystallization while achieving comparatively small subgrain sizes. With this particular microstructure or structure of the plate, it is possible to increase the yield strength (Rp0.2) of the plate significantly.
Furthermore, the aluminum alloy can also optionally contain one or more of the following listed elements, each with the content indicated below: up to 0.5 wt % copper (Cu); up to 0.7 wt % iron (Fe); up to 0.1 wt % chromium (Cr); up to 0.2 wt % titanium (Ti); up to 0.5 wt % Zink (Zn); up to 0.2 wt % tin (Sn), up to 0.1 wt % strontium (Sr), up to 0.2 wt % vanadium (V), and/or up to 0.2 wt % molybdenum (Mo).
Preferably, the plate is made of a rolled aluminum alloy of the 6xxx series.
Preferably, the degree of recrystallization is lower in order to achieve an increased percentage of recovered structure. This is particularly true if the degree of recrystallization is less than 15%. Advantageously, the degree of recrystallization is less than 5% in order to be able to ensure a high percentage of recovered structure in the overall structure for high strengths.
The above can be further improved if the average subgrain size is less than or equal to 5 μm in the rolling direction.
The strength of the plate can be further increased if it is in the T6 state, for example the T651 state.
Based on the T6 state, the plate can have a yield strength (Rp0.2) of greater than 350 MPa among other things.
The plate can be further improved if the aluminum alloy is further adjusted with regard to one or more of the elements listed below:
Si: If the aluminum alloy contains from 0.9 to 1.3 wt % silicon (Si), then this can further increase the strength, more particularly if the aluminum alloy contains from 1.0 to 1.2 wt % silicon (Si).
Mg: If the aluminum alloy contains from 0.75 to 0.95 wt % magnesium (Mg), then an optimum of soluble Mg in the aluminum alloy can be achieved and the strength can be further increased through the presence of Mg- and Si-containing phases.
Mn: With a content of 0.3 to 0.5 wt % manganese (Mn) in the aluminum alloy, the percentage of Mn-containing and Zr-containing particles can be increased in order to further increase the strength of the plate, particularly in the T6 state.
Zr: A further increased content of zirconium, namely from 0.15 to 0.25 wt % zirconium (Zr), can further increase the strength of the plate. This is true, for example, because Zr inhibits the recrystallization of the structure in an improved way and achieves an increased density of particles. Thus by means of the increased Zr content, it has been possible to establish a comparatively thermally stable subgrain boundary hardening, whose activity persists even after a heat treatment of up to 570° C. The above is further improved if the aluminum alloy contains from 0.18 to 0.22 wt % zirconium (Zr).
Cu: If the aluminum alloy contains from 0.1 to 0.5 wt % copper (Cu), then this can further increase the strength of the plate. In this connection, the upper limit of 0.5 wt % copper (Cu) contributes to minimizing the corrosion susceptibility of the plate.
Si+Mg+Cu: Si and Mg (for example adjusted up to the maximum solubility) in combination with Cu can make a quite particular contribution to increasing the volume fraction of precipitates.
Fe: An iron (Fe) content of up to 0.7 wt % can also contribute to increasing the strength. For example, the Fe content can be at least 0.1 wt %.
Preferably, the intermetallic phase of the aluminum alloy has Zr-containing particles with an average particle size of at most 100 nm (nanometer), wherein the quantity of Zr-containing particles is greater than or equal to 1×106 particles/mm2. Based on such a particle size and particle quantity, it is possible to improve the pinning of the subgrain boundaries and it is thus possible to further increase the percentage of recovered and non-recrystallized structure. In addition, this can further reduce the average subgrain size of the recovered structure region, which can further increase the strength of the plate.
The above can be further improved if the average particle size of the Zr-containing particles is in the range from 30 nm to 100 nm.
It can also be advantageous if the quantity of Zr-containing particles is less than or equal to 100×106 particles/mm2.
It can also be advantageous if the quantity of Zr-containing particles is greater than or equal to 5×106 particles/mm2.
More particularly, the plate can be suitable for machine construction.
By using a multistep homogenization of the rolling ingot with a subsequent accelerated cooling (quenching) to room temperature, in comparison to other known methods, it is possible to reproducibly achieve a substantially recovered structure with a comparatively low degree of recrystallization and with a comparatively small subgrain size. This is accomplished in that a first homogenization is carried out at a first temperature in the range from 300° C. to 400° C. and after this, a second homogenization is carried out at a second temperature in the range from 500° C. to 10° C. below a solidus temperature of the aluminum alloy.
It should be generally noted that an accelerated cooling (often also referred to as quenching) can be understood to mean a faster cooling than one occurring at room temperature and in still air (see Friedrich Ostermann, Aluminum Application Technology [Anwendungstechnologie Aluminium], 3rd edition, published 2014: Cooling after the solution annealing).
Preferably, the first homogenization can take place with a first holding time of greater than or equal to 0.5 hours and/or up to 4 days and/or with a maximum heating rate of 5 K/min. It is thus possible to further increase the quantity of Zr-containing particles in the structure.
Preferably, the second homogenization takes place with a second holding time of greater than or equal to 0.5 hours and/or up to 24 hours in order to further reduce concentration differences in the structure.
The hot rolling of the homogenized rolling ingot can take place at a temperature that is 5° C. to 100° C. below the solidus temperature of the aluminum alloy in order to obtain a preferred deformation structure.
The solution annealing of the plate can take place at a temperature in the range from 460° C. to 580° C. The solution annealing of the plate can also take place with a holding time of 1 minute to 10 hours.
It should be noted in general that the solution annealing can be used to achieve as complete as possible a dissolution of the alloying elements involved in the tempering (see Friedrich Ostermann, Aluminum Application Technology [Anwendungstechnologie Aluminium], 3rd edition, published 2014, ISBN 987-3-662-43806-0, page 175).
For example, the natural aging can take place at room temperature and/or with a holding time of preferably up to 8 weeks. This can contribute to the further simplification of the method.
The artificial aging can take place at a temperature in the range from 130° C. to 210° C. and/or over a holding time of 1 to 24 hours in order to further increase the strength of the plate.
The above can be further increased if the heat treatment transforms the plate into the T6 state, more particularly the T651 state.
To verify the achieved effects, rolled semi-finished products, namely plates A and B, each with a plate thickness 6 mm (millimeter), were produced from one of the respective rolled aluminum alloys
as well as residual aluminum and inevitable production-related impurities, with at most 0.05 wt % of each and at most 0.15 wt % collectively. In general, a plate thickness of from 4 mm to 150 mm, more particularly from 6 mm to 40 mm, is conceivable for a plate.
The alloy of plate A is a standard EN AW-6082 alloy. Starting from this standard EN AW-6082 alloy, the content of the alloying elements Si, Mg, and Cu was increased. In addition to modified Si, Mg, and Cu contents, the plate B also has a Zr content and therefore constitutes the embodiment according to the invention.
The production method is schematically depicted in
Plates A and B then underwent the following method steps in the above-mentioned sequence, wherein the rolling ingot for plate A undergoes a different homogenization than is the case for the rolling ingot for plate B:
a. Homogenization (H) of a cast rolling ingot:
Rolling ingot for plate A: one-step homogenization (H2) at a temperature of 550° C. (degrees Celsius) for a holding time of 2 h (hours) and at a heating rate of 1 K/min (Kelvin/minute);
Rolling ingot for plate B: two-step homogenization with a first homogenization (H1) at 350° C. for a holding time of 16 h and at a heating rate of 1 K/min and with a second homogenization (H2) at 550° C. for a holding time of 2 h, and at a heating rate of 1 K/min, wherein the second homogenization (H2) immediately follows the first homogenization (H1), as is apparent from
b. Hot rolling (HR) of the homogenized rolling ingot at a temperature of 540° C. to a plate 6 mm thick, starting from an initial thickness of 40 mm (millimeter);
c. Solution annealing (SA) of the plate at a temperature of 570° C. for a holding time of 20 min (minutes) with subsequent accelerated cooling to a room temperature of 20° C. (RT) by means of water quenching;
d. Natural aging (NA) of the plate with a holding time of 14 days and subsequent cold forming through elongation of the plate with a degree of deformation of 2%;
e. Artificial aging (AA) of the plate at a temperature of 160° C. for a holding time of 14 h;
Plates A and B that underwent this process were tested by means of tensile testing (tensile testing in accordance with the DIN EN 10002-1 standard) with regard to the mechanical characteristic values 0.2% offset yield strength Rp0.2, tensile strength Rm, uniform elongation Ag, and elongation at break A.
In addition, the degree of recrystallization, the average subgrain size, and the quantity and average size of the Zr-containing particles in the structure (which is calculated form the maximum Feret diameters of these Zr-containing particles) were determined for both of the plates. The degree of recrystallization was determined using a JEOL 7200F FEG-SEM EBSD detector with the aid of the two conditions (a) the misorientation within a 3rd order kernel of less than 0.5° determined across a grain with an increment of 0.6 μm and (b) the average band contrast of over 70% relative to the maximum measured band contrast. The values of the Zr-containing particles of plate B were determined with the aid of a scanning transmission electron microscope (HAADF images at 17,000 times enlargement, Talos F200X G2 S-TEM).
As can be inferred from Table 1, in comparison to plate A, plate B has significantly increased strength values Rp0.2 and Rm in the T651 state. But this is not due solely to the increased addition of Si, Mg, and Cu, which chiefly leads to an increase in the precipitation density and thus to a strength increase. Consequently, the strength of plate A is substantially due to precipitates, more particularly to β″ precipitates (Si, Mg), which form during an artificial aging, in combination with Fe-containing and/or Mn-containing particles, which stabilize the structure at higher temperatures.
By contrast, the significant increase of the 0.2% offset yield strength (Rp0.2) of plate B of 73 MPa in comparison to the 6082-plate A occurs substantially due to the hardness-increasing effect of Zr or more precisely, its Al3Zr particles. The structure therefore has an increased quantity of Zr-containing particles (Al3Zr), which stabilizes the structure region with the non-recrystallized deformation structure that forms in the structure as a result of the hot rolling. Subsequent heat treatments, for example the solution annealing at a comparatively high temperature of 570° C., essentially do not result in any recrystallization, but rather to a recovery of this structure region, which taking into account the low average subgrain size of 5 μm in the rolling direction, produces a significantly increased strength than could be achieved solely by the addition of Si, Mg, and Cu. The strength increase of plate B in comparison to plate A is also apparent in
Tests of the particles likewise show significant differences in the structure.
The intermetallic phase of the aluminum alloy of plate B has Zr-containing particles with an average particle size of 74 nm. The quantity of Zr-containing particles is 7.52×106 particles/mm2.
In comparison to this, the intermetallic phase of the aluminum alloy of plate A has only Al(Fe,Mn,Cr)Si-containing particles. These have an average particle size of 101 nm. The quantity of these Al(Fe,Mn,Cr)Si-containing particles is 1.2×106 particles/mm2. These particle values of plate A were determined with the aid of images from a scanning electron microscope (BSE images at 10,000 times enlargement, JEOL 7200F FEG-SEM).
The particles of plate A are therefore not only significantly larger, but their quantity is also several times lower than is the case for the Zr-containing particles of plate B, which also has these Al(Fe,Mn,Cr)Si-containing particles. This high quantity of the comparatively smallest Zr-containing particles of plate B pins subgrain boundaries in an improved way and can thus increase the percentage of recovered structure in the end state and ensure a further reduced subgrain size.
These effects lead to a particularly mechanically durable plate, which can be used, for example, in tool construction.
It was also possible to determine that due to the Zr content in the alloy, the introduced energy in the cold forming (elongation with a degree of deformation of 2%) is not canceled out by the subsequent artificial aging since the stabilizing effect of the Zr-containing particles comes into play here as well.
It should be noted in general that the German expression “insbesondere” can be translated as “more particularly” in English. A feature that is preceded by “more particularly” or “possibly” is to be considered an optional feature, which can be omitted and does not thereby constitute a limitation, for example of the claims. The same is true for the German expression “vorzugsweise”, which is translated as “preferably” in English.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20200645.8 | Oct 2020 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/077778 | 10/7/2021 | WO |
