EP1587965B1 - Aluminium alloy with increased resistance and low quench sensitivity - Google Patents

Abstract

The alloy comprises aluminum metal with production contaminants which individually constitute not more than 0.05 wt% and in total not more than 0.15 wt%. Other metals included in the alloy are 4.6-5.2 wt% Zn; 2.6-3.0 wt% Mg; 0.1-0.2 wt% Cu; 0.05-0.2 wt% Zr; not more than 0.05 wt% Mn; not more than 0.05 wt% Cr; not more than 0.15 wt% Fe; not more than 0.15 wt% Si; not more than 0.10 wt% Ti. Preferred amounts of the metals are: 4.6 wt% Zn; 2.6-2.8 wt% Mg; 0.10-0.15 wt% Cu; 0.08-0.18 wt% Zr; not more than 0.03 wt% Mn; not more than 0.02 wt% Cr; not more than 0.12 wt% Fe; not more than 0.12 wt% Si; not more than 0.05 wt%Ti. Independent claims are included for: a) a process for manufacturing plates up to 300 mm thick in the claimed alloy in which: A) the alloy is extruded to form bars not less than 300 mm thick; B) the bars are heated at not more than 20 degrees C/hr from 170-410 degrees C to 470-490 degrees C; C) the bars are homogenized for 10-14 hrs at 470-490 degrees C; D) bars are hot rolled to form plates; E) the plates are cooled to 400-410 degrees C to not more than 100 degrees C; F) plates are cooled to room temperature; G) plates are hardened: b) a similar process in which hot rolling to form plates is omitted and the final hardened bars are used as plates.

Description

The invention relates to an aluminum alloy with high strength and low quenching sensitivity. In the context of the invention is also a method for producing thick plates of the aluminum alloy.

Particularly in the automotive industry, there is an increasing demand for large plastic components, such as e.g. integral bumpers. To produce the correspondingly large injection molds, plates are required whose thickness very often exceeds 150 mm and in some cases even exceeds 500 mm.

The document FR-A-2 341 661 discloses an aluminum alloy whose composition is Zn 4.0-6.2 wt.%, Mg 0.8-3.0 wt.%, Cu 0-1.5 wt.%, Zr 0.05- 0.30 wt.%, Fe 0-0.2 wt.%, Si 0-0.15 wt.%, Mn 0-0.25 wt.%, Ti 0-0.1 wt. %, Remainder Al, contains.

For the construction of injection molds having a thickness of, for example, 50 to 300 mm, hot-rolled and hot-hardened plates are usually used today. Larger molds with a thickness of more than 300 mm were either made from forged blocks or even directly from continuous cast ingots.

A major disadvantage of the aluminum alloys used today for mold making is their high quenching sensitivity. In order for the ingots or slabs to reach the level of strength required for plastic injection molding in hot curing, the cooling rate must be increased from the homogenizing or solutionizing temperature with increasing slab thickness. As a result of the high temperature gradients occurring between the surface and the core of the ingots or slabs, the damaging internal stresses increase, so that limits are already set for this reason for a further increase in the cooling rate and thus the level of strength ultimately achievable.

The invention has the object of providing a suitable for the production of thick plates with high strength level aluminum alloy with less To provide quench sensitivity.

Another object of the invention is to provide a suitable method by which the aluminum alloy can be processed into thick plates having sufficiently high strength over the entire plate thickness.

• 4,6 bis 5,2 Gew.-% Zn
• 2,6 bis 3,0 Gew.-% Mg
• 0.1 bis 0,2 Gew.-% Cu
• 0,05 bis 0,2 Gew.-% Zr
• max. 0,05 Gew.-% Mn
• max. 0,05 Gew.-% Cr
• max. 0,15 Gew.-% Fe
• max. 0,15 Gew.-% Si
• max. 0,10 Gew.-% Ti

und Aluminium als Rest mit herstellungsbedingten Verunreinigungen, einzeln max. 0,05 Gew.-%, insgesamt max. 0,15 Gew.-%.For the inventive solution of the problem leads with an aluminum alloy

• 4.6 to 5.2% by weight of Zn
• 2.6 to 3.0% by weight Mg
• 0.1 to 0.2% by weight of Cu
• 0.05 to 0.2% by weight Zr
• Max. 0.05% by weight of Mn
• Max. 0.05 wt.% Cr
• Max. 0.15% by weight of Fe
• Max. 0.15% by weight of Si
• Max. 0.10% by weight of Ti

and aluminum as balance with production-related impurities, individually max. 0.05% by weight, in total max. 0.15% by weight.

The composition of the alloy according to the invention is chosen so that it has a very low quenching sensitivity and still has an extraordinarily high level of strength. Thick cross sections can therefore be brought to a high level of strength with forced air cooling and precipitation hardening.

• 4,6 bis 4,8 Gew.-% Zn
• 2,6 bis 2,8 Gew.-% Mg
• 0,10 bis 0,15 Gew.-% Cu
• 0,08 bis 0,18 Gew.-% Zr
• max. 0,03 Gew.-% Mn
• max. 0,02 Gew.-% Cr
• max. 0,12 Gew.-% Fe
• max. 0,12 Gew.-% Si
• max. 0,05 Gew.-% Ti

The following preferential ranges apply to the individual alloying elements:

• 4.6 to 4.8% by weight of Zn
• 2.6 to 2.8% by weight Mg
• 0.10 to 0.15% by weight of Cu
• 0.08 to 0.18 wt% Zr
• Max. 0.03 wt% Mn
• Max. 0.02 wt.% Cr
• Max. 0.12 wt.% Fe
• Max. 0.12% by weight of Si
• Max. 0.05% by weight of Ti

For the application of the alloy according to the invention as a material for mold construction, it is desirable to have an as isotropic distribution of the residual stresses in the cross section of the plate. Among other things, the grain size and the grain shape in the plate are important for the reduction of residual stresses. The finer and more uniform the crystals are, the better the internal stresses in the cross-section of the plate can be compensated. The grain boundaries act as sinks for dislocations in the degradation of local stress peaks. As explained below, by adding zirconium, a fine grain structure in the plate can be achieved by selecting the heating rate of the ingots to the homogenizing or solution annealing temperature so that a very homogeneous distribution of submicron precipitates of Al ₃ Zr in the structure arises.

For the production of plates from the alloy according to the invention, the following two methods are particularly suitable which, depending on the desired thickness of the mold, lead to a hot-rolled and hot-hardened plate or to a hot-hardened continuously cast ingot used as a plate.

1. A. Stranggiessen der Aluminiumlegierung zu Barren mit einer Dicke von mehr als 300 mm,
2. B. Aufheizen der Barren mit einer Aufheizgeschwindigkeit von max. 20°C/h zwischen 170 und 410°C auf eine Temperatur von 470 bis 490°C,
3. C. Homogenisieren der Barren während einer Zeitdauer von 10 bis 14 h bei einer Temperatur von 470 bis 490°C,
4. D. Warmwalzen der homogenisierten Barren zu Platten,
5. E. Abkühlen der Platten von einer Temperatur von 400 bis 410°C auf eine Temperatur von weniger als 100°C,
6. F. Abkühlen der Platten auf Raumtemperatur,
7. G. Warmaushärten der Platten.

For the production of sheets with a thickness of up to 300 mm, the process is characterized by the following steps:

1. A. Continuously casting the aluminum alloy into billets having a thickness of more than 300 mm,
2. B. Heating the ingot with a heating rate of max. 20 ° C / h between 170 and 410 ° C to a temperature of 470 to 490 ° C,
3. C. homogenizing the bars for a period of 10 to 14 hours at a temperature of 470 to 490 ° C,
4. D. hot rolling the homogenized billets into sheets,
5. E. cooling the plates from a temperature of 400 to 410 ° C to a temperature of less than 100 ° C,
6. F. cooling the plates to room temperature,
7. G. Hot curing of the plates.

1. A. Stranggiessen der Legierung zu Barren mit einer Dicke von mehr als 300 mm,
2. B. Aufheizen der Barren mit einer Aufheizgeschwindigkeit von max. 20°C/h zwischen 170 und 410°C auf eine Temperatur von 470 bis 490°C,
3. C. Homogenisieren der Barren während einer Zeitdauer von 10 bis 14 h bei einer Temperatur von 470 bis 490°C,
4. D. Abkühlen der Barren auf eine Zwischentemperatur von 400 bis 410 °C,
5. E. Abkühlen der Barren von der Zwischentemperatur von 400 bis 410°C auf eine Temperatur von weniger als 100°C,
6. F. Abkühlen der Barren auf Raumtemperatur,
7. G. Warmaushärten der Barren,
8. H. Verwenden der warmausgehärteten Barren als Platten.

For the production of sheets having a thickness of more than 300 mm, and in particular sheets having a thickness of more than 500 mm, a continuous casting ingot produced from the alloy according to the invention can be used directly as a sheet. The procedure in this case is characterized by the following steps:

1. A. continuous casting of the alloy into billets having a thickness of more than 300 mm,
2. B. Heating the ingot with a heating rate of max. 20 ° C / h between 170 and 410 ° C to a temperature of 470 to 490 ° C,
3. C. homogenizing the bars for a period of 10 to 14 hours at a temperature of 470 to 490 ° C,
4. D. cooling the ingots to an intermediate temperature of 400 to 410 ° C,
5. E. cooling the ingot from the intermediate temperature of 400 to 410 ° C to a temperature of less than 100 ° C,
6. F. cooling the bars to room temperature,
7. G. hot curing of the ingots,
8. H. Using the thermoset ingots as plates.

Preferably, the ingots are cooled from the homogenization temperature of 470 to 490 ° C to the intermediate temperature of 400 to 410 ° C in still air.

The cooling of the billets from the intermediate temperature of 400 to 410 ° C on the one hand should be done so quickly that the loss of strength is as low as possible. On the other hand, the cooling rate must not be too high, otherwise too high residual stresses are built up.

The cooling of the ingot from the intermediate temperature of 400 to 410 ° C to a temperature of less than 100 ° C is preferably carried out in forced air (forced air cooling) or in a water / air spray.

When choosing the cooling conditions, the bar thickness must also be considered. However, it is within the scope of the expert action to determine the optimum cooling conditions for a given ingot format by means of simple experiments.

The low heating rate in the temperature range between 170 and 410 ° C when heating the billets to the homogenization temperature is an essential feature of the inventive method. In the mentioned temperature range, which is also referred to as Heterogenisierungsintervall, the AlZnMg equilibrium phase (T-phase) is stable. The slow passage through the heterogenizing interval leads to a finely dispersed separation of the T phase, wherein the phase boundaries of the precipitated particles of the T phase form preferred nucleation sites for the precipitation of Al ₃ Zr particles beginning at a temperature of about 350 ° C. As the ingot is heated further to the homogenization temperature, the previously precipitated T-phase particles dissolve, leaving behind a uniform distribution of the fine, submicron Al ₃ Zr precipitates, which are preferably at the original T-phase particle boundaries and at sub-particle boundaries thus giving a homogeneous distribution. These fine Al ₃ Zr particles cause both a strong growth inhibition in the recrystallization of the plates in the solution annealing and in the homogenization annealing of ingot, and the result is the desired isotropic grain structure in the ingot. The grain-refining additional element Zr is thus used optimally.

Another essential feature of the inventive method is the combined homogenization and solution annealing with subsequent two-stage cooling, whereas in the conventional method according to the prior the technique for achieving a still acceptable in the center of the ingot strength a separate solution annealing with subsequent quenching at high cooling rate is required.

The term "cooling in agitated air" or "forced air cooling" is understood here to mean a fan-assisted air cooling, which leads to a heat transfer coefficient at the ingot surface of about 40 W / m ² K. Cooling in a water / air spray results in a slightly higher heat transfer coefficient at the ingot surface.

The alloy according to the invention has a low quenching sensitivity. In the production of thick plates, the loss of strength in the plate core is smaller than that of the prior art alloys, despite the relatively mild cooling conditions. It has also surprisingly been found that this effect is even more pronounced in plates made directly from continuous cast bars than in hot-rolled plates.

In the production of the thick plates, the two-stage cooling from the homogenization temperature to room temperature has proven to be particularly advantageous for achieving a structure with low residual stresses.

• 1 bis 30 Tage Lagerung bei Raumtemperatur,
• 6 bis 10 h Lagerung bei einer Temperatur von 90 bis 100°C,
• 8 bis 22 h Lagerung bei einer Temperatur von 150 bis 160°C.

For thermosetting, a room temperature storage, a first heat treatment at a first temperature and a second heat treatment at a second temperature higher than the first temperature are preferably carried out successively, for example

• 1 to 30 days storage at room temperature,
• 6 to 10 hours storage at a temperature of 90 to 100 ° C,
• 8 to 22 h storage at a temperature of 150 to 160 ° C.

It is particularly preferable that the thermosetting is for the heat treatment state T76.

The field of application of the alloy according to the invention and of the thick plates produced therefrom results from the property spectrum described above. The plates are particularly suitable for mold making, i. for the production of plastic injection molds, but also generally for machine, tool and mold making.

• Fig. 1 die Verteilung der Brinell-Härte über einen Teil des Querschnitts eines Stranggussbarrens mit einem Querschnitt von 440 mm x 900 mm nach Ventilatorkühlung.
• Fig. 2 den gemessenen Temperaturverlauf bei einem Stranggussbarren mit einem Querschnitt von 440 mm x 900 mm an der Oberfläche und in der Mitte bei Ventilatorkühlung;
• Fig. 3 den berechneten Verlauf der inneren Temperaturgradienten beim Temperaturverlauf von Fig. 2;
• Fig. 4 den berechneten Temperaturverlauf bei einem Stranggussbarren mit einem Querschnitt von 1000 mm x 1200 mm an der Oberfläche und in der Mitte bei Ventilatorkühlung;
• Fig. 5 den berechneten Verlauf der inneren Temperaturgradienten beim Temperaturverlauf von Fig. 4;

Further advantages, features and details of the invention will become apparent from the following description of preferred embodiments and from the drawing; this shows schematically in

• Fig. 1 shows the distribution of Brinell hardness over a part of the cross section of a continuous casting ingot with a cross section of 440 mm x 900 mm after fan cooling.
• 2 shows the measured temperature profile in a continuous casting ingot with a cross section of 440 mm × 900 mm at the surface and in the middle with fan cooling;
• FIG. 3 shows the calculated course of the internal temperature gradients in the temperature profile of FIG. 2; FIG.
• 4 shows the calculated temperature profile in the case of a continuous casting ingot with a cross section of 1000 mm × 1200 mm at the surface and in the middle with fan cooling;
• FIG. 5 shows the calculated profile of the internal temperature gradients in the temperature profile of FIG. 4; FIG.

example

An alloy having the composition (in wt.%): 0.040 Si, 0.08 Fe, 0.14 Cu, 0.0046 Mn, 2.69 Mg, 0.0028 Cr, 4.69 Zn, 0.017 Ti, 0.16 Zr, balance Al, became a continuous casting billet on an industrial scale Cast with a cross section of 440 x 900 mm. The bars were heated to a temperature of 480 ° C within 30 h, being careful that the heating rate in the range between 170 and 410 ° C less than 20 ° C / h. The homogenization of the billet to compensate for the solidification-related Kristallseigerungen carried out by holding the billet for 12 hours at 480 ° C.

The homogenized billet was cooled in a first stage in still air from the homogenization temperature to an intermediate temperature of 400 ° C and then in a second stage with fans from 400 ° C to 100 ° C. The further cooling to room temperature was again in still air.

The billet was after 14 days of storage at room temperature for 8h at 95 ° C and then cured for 18h at 155 ° C to the overheated state T76 warm.

Brinell hardness was determined over the ingot cross-section at samples of the thermoset ingots cut out perpendicular to the bar longitudinal direction. The areas of equal hardness shown in FIG. 1 clearly show the low hardness or strength loss in the barrel core relative to the bar surface.

In Fig. 2, the temperature-time curves calculated for the surface (O) and the core (K) of a billet having a cross section of 440 × 900 mm at a fan cooling and in Fig. 3 are the gradients derived therefrom between the temperature T _K in the barrel core and the temperature T _{o shown} on the bar surface. For comparison, Figs. 4 and 5 show the corresponding curves for a billet with a cross section of 1000 x 1200 mm. The results show that billets produced with the method according to the invention, with a thickness of up to 1000 mm, are still likely to fulfill the requirements imposed on plates for the production of plastic injection molds in terms of mechanical strength.

Claims (19)

1. Aluminium alloy exhibiting high strength and low quench sensitivity having

   4.6 to 5.2 wt.% Zn

   2.6 to 3.0 wt.% Mg

   0.1 to 0.2 wt.% Cu

   0.05 to 0.2 wt.% Zr

   max. 0.05 wt.% Mn

   max. 0.05 wt.% Cr

   max. 0.15 wt.% Fe

   max. 0.15 wt.% Si

   max. 0.10 wt.% Ti

   the remainder being impurities due to the manufacturing process, individually at maximum 0.05 wt.%, in total at maximum 0.15 wt.%.
2. Aluminium alloy according to claim 1, characterised by way of 4.6 to 4.8 wt.% Zn.
3. Aluminium alloy according to claim 1 or 2, characterised by way of 2.6 to 2.8 wt.% Mg.
4. Aluminium alloy according to one of the claims 1 to 3, characterised by way of 0.10 to 0.15 wt.% Cu.
5. Aluminium alloy according to one of the claims 1 to 4, characterised by way of 0.08 to 0.18 wt.% Zr.
6. Aluminium alloy according to one of the claims 1 to 5, characterised by way of a maximum concentration of 0.03 wt.% Mn.
7. Aluminium alloy according to one of the claims 1 to 5, characterised by way of a maximum concentration of 0.02 wt.% Cr.
8. Aluminium alloy according to one of the claims 1 to 7, characterised by way of a maximum concentration of 0.12 wt.% Fe.
9. Aluminium alloy according to one of the claims 1 to 8, characterised by way of a maximum concentration of 0.12 wt.% Si.
10. Aluminium alloy according to one of the claims 1 to 9, characterised by way of a maximum concentration of 0.05 wt.% Ti.
11. Process for manufacturing plates of thickness up to 300 mm out of an aluminium alloy according to one of the claims 1 to 10, characterised by way of the steps

    A. Continuous casting the aluminium alloy as an ingot with a thickness greater than 300 mm,

    B. Heating the ingot at a maximum heating rate of 20°C/h between 170 and 410°C to a temperature of 470 to 490°C,

    C. Homogenising the ingot for an interval of 10 to 14 h at a temperature of 470 to 490°C,

    D. Hot rolling the homogenised ingot to plate,

    E. Cooling the plate from a temperature of 400 to 410°C to a temperature of less than 100°C,

    F. Cooling the plate to room temperature

    G. Artificially age-hardening the plate.
12. Process for manufacturing plate with a thickness of greater than 300 mm out of an aluminium alloy according to one of the claims 1 to 10 is characterised by way of the steps

    A. Continuous casting the aluminium alloy as an ingot with a thickness greater than 300 mm,

    B. Heating the ingot at a maximum heating rate of 20°C/h between 170 and 410°C to a temperature of 470 to 490°C,

    C. Homogenising the ingot for an interval of 10 to 14 h at a temperature of 470 to 490°C,

    D. Cooling the ingot to an intermediate temperature of 400 to 410°C,

    E. Cooling the ingot from the intermediate temperature of 400 to 410°C to a temperature below 100°C,

    F. Cooling the ingot to room temperature,

    G. Artificially age-hardening the ingot,

    H. Use of the artificially age-hardened ingot as plate.
13. Process according to claim 12, characterised in that the cooling of the ingot from the homogenisation temperature of 470 - 490°C to the intermediate temperature of 400 - 410°C takes place in still air.
14. Process according to claim 11 or 12, characterised in that the cooling of the ingot from the intermediate temperature of 400 - 410°C to a temperature below 100°C takes place by forced air cooling.
15. Process according to claim 11 or 12, characterised in that the cooling of the ingot from the intermediate temperature of 400 - 410°C to a temperature below 100°C takes place in a water-air-mist spray.
16. Process according to one of the claims 11 to 15, characterised in that the artificial age-hardening is carried out, after storage at room temperature, in a first heat-treatment at a first temperature, followed by a second heat-treatment at a second temperature which is higher than the first temperature.
17. Process according to claim 16, characterised by way of

    - 1-30 days storage at room temperature,

    - 6 - 10 h at a temperature of 90 - 100°C

    - 8 - 22 h at a temperature of 150-160°C.
18. Process according to claim 17, characterised in that the artificial age-hardening is carried out resulting in a heat-treatment condition T76.
19. Use of a plate manufactured by the process according to one of the claims 11 to 18 for machine, tool and mould production, in particular for plastic injection moulding moulds.

Images