EP1441041A1 - Aluminium alloy with high strength and low quenching 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 Quench. A method is also within the scope of the invention for the production of thick plates made of aluminum alloy.

In the automotive industry in particular, there is an increasing need for large ones Plastic components, e.g. integral bumpers. To make the correspondingly large injection molds are required, the thickness of which very often exceeds 150 mm and in some cases is even more than 500 mm.

For the construction of injection molds with a thickness of, for example, 50 to Today 300 mm are usually hot rolled and hot hardened Plates used. Larger shapes with a thickness of more than 300 mm were either made from forged blocks or directly from Continuous cast ingot manufactured.

A major disadvantage of the aluminum alloys used today for mold making is their high sensitivity to quenching. So that the ingot or Sheets in hot curing that required for plastic injection molds Reach the strength level, the cooling rate of the homogenization or solution annealing temperature with increasing plate thickness increase. Due to the high temperature gradients that occur between the surface and the core of the ingot or plate the harmful residual stresses, so for this reason alone a further increase in the cooling rate and ultimately that achievable strength levels are limited.

The invention has for its object one for the production of thick Sheets of high strength suitable aluminum alloy with low To provide quench sensitivity.

Another object of the invention is to provide a suitable method with which the aluminum alloy to thick plates with sufficiently high Strength can be processed over the entire board thickness.

An aluminum alloy is used to achieve the object according to the invention 4.6 to 5.2% by weight Zn 2.6 to 3.0% by weight mg 0.1 to 0.2% by weight Cu 0.05 to 0.2% by weight Zr Max. 0.05% by weight Mn Max. 0.05% by weight Cr Max. 0.15% by weight Fe Max. 0.15% by weight Si Max. 0.10% by weight Ti and aluminum as the rest with production-related impurities, individually max. 0.05% by weight, total max. 0.15% by weight.

According to the invention, the composition of the alloy is selected such that it has a very low sensitivity to quenching and is nevertheless extremely good has a high level of strength. Thick cross sections can therefore with forced air cooling and through precipitation hardening to a high level Strength level are brought.

The following preferred ranges apply to the individual alloy elements: 4.6 to 4.8% by weight Zn 2.6 to 2.8% by weight mg 0.10 to 0.15% by weight Cu 0.08 to 0.18% by weight Zr Max. 0.03% by weight Mn Max. 0.02% by weight Cr Max. 0.12% by weight Fe Max. 0.12% by weight Si Max. 0.05% by weight Ti

For the use of the alloy according to the invention as a material for mold construction, an isotropic distribution of the residual stresses in the cross section of the plate is to be aimed for. The grain size and the shape of the grain in the plate are of importance for the reduction of the internal stresses. The finer and more uniform the crystals are, the better the internal stresses in the cross-section of the plate can balance. The grain boundaries act as sinks for dislocations when local stress peaks are reduced. As explained further below, the addition of zirconium can achieve a fine grain structure in the plate by selecting the rate at which the ingots heat up to the homogenization or solution annealing temperature in such a way that the distribution of submicron precipitates of Al ₃ Zr in the structure is as homogeneous as possible arises.

Are suitable for the production of plates from the alloy according to the invention in particular the following two methods, depending on the desired thickness of the Form into a hot-rolled and thermoset plate or one heat cured ingots used as a plate.

A. Stranggiessen der Aluminiumlegierung zu Barren mit einer Dicke von mehr als 300 mm,

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,

C. Homogenisieren der Barren während einer Zeitdauer von 10 bis 14 h bei einer Temperatur von 470 bis 490°C,

D. Warmwalzen der homogenisierten Barren zu Platten,

E. Abkühlen der Platten von einer Temperatur von 400 bis 410°C auf eine Temperatur von weniger als 100°C,

F. Abkühlen der Platten auf Raumtemperatur,

G. Warmaushärten der Platten.

The process for producing boards with a thickness of up to 300 mm is characterized by the following steps:

A. Continuous casting of the aluminum alloy into bars with a thickness of more than 300 mm,

B. heating the bars with a heating rate of max. 20 ° C / h between 170 and 410 ° C to a temperature of 470 to 490 ° C,

C. homogenizing the ingots over a period of 10 to 14 hours at a temperature of 470 to 490 ° C,

D. hot rolling the homogenized ingots into sheets,

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

F. cooling the plates to room temperature,

G. Heat curing the panels.

A. Stranggiessen der Legierung zu Barren mit einer Dicke von mehr als 300 mm,

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,

C. Homogenisieren der Barren während einer Zeitdauer von 10 bis 14 h bei einer Temperatur von 470 bis 490°C,

D. Abkühlen der Barren auf eine Zwischentemperatur von 400 bis 410 °C,

E. Abkühlen der Barren von der Zwischentemperatur von 400 bis 410°C auf eine Temperatur von weniger als 100°C,

F. Abkühlen der Barren auf Raumtemperatur,

G. Warmaushärten der Barren,

H. Verwenden der warmausgehärteten Barren als Platten.

For the production of plates with a thickness of more than 300 mm and in particular of plates with a thickness of more than 500 mm, a continuous cast ingot produced from the alloy according to the invention can be used directly as a plate. In this case, the procedure is characterized by the following steps:

A. Continuous casting of the alloy into bars with a thickness of more than 300 mm,

B. heating the bars with a heating rate of max. 20 ° C / h between 170 and 410 ° C to a temperature of 470 to 490 ° C,

C. homogenizing the ingots over a period of 10 to 14 hours at a temperature of 470 to 490 ° C,

D. cooling of the bars to an intermediate temperature of 400 to 410 ° C,

E. cooling of the bars from the intermediate temperature of 400 to 410 ° C to a temperature of less than 100 ° C,

F. cooling the bars to room temperature,

G. hot hardening of the bars,

H. Use the thermoset bars as plates.

The ingot is preferably cooled from the homogenization temperature from 470 to 490 ° C to the intermediate temperature of 400 to 410 ° C at rest Air.

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

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

The bar thickness must also be taken into account when choosing the cooling conditions become. However, it is within the scope of professional action for a given ingot format based on the optimal cooling conditions easier attempts to determine.

The low heating rate in the temperature range between 170 and 410 ° C. when the bars are heated to the homogenization temperature is an essential feature of the process according to the invention. In the temperature range mentioned, which is also referred to as the heterogenization interval, the AlZnMg equilibrium phase (T phase) is stable. The slow passage through the heterogenization interval leads to a finely dispersed separation of the T phase, the phase interfaces of the separated particles of the T phase forming preferred nucleation sites for the precipitation of Al ₃ Zr particles starting at a temperature of about 350 ° C. When the ingot is further heated to the homogenization temperature, the previously separated particles of the T phase dissolve and what remains is an even distribution of the fine, submicron Al ₃ Zr precipitates, which are preferably due to the original particle boundaries of the T phase and to subcom boundaries and so that there is a homogeneous distribution. These fine Al ₃ Zr particles cause a strong growth inhibition in the recrystallization of the plates in solution annealing as well as in the homogenization annealing of cast ingots, and the desired isotropic grain structure in the ingot results. The grain-refining additional element Zr is thus optimally used.

Another essential feature of the method according to the invention is that combined homogenization and solution annealing with subsequent two-stage cooling, whereas in the usual processes according to the state the technology to achieve an acceptable level in the middle of the bar Strength a separate solution heat treatment with subsequent quenching high cooling rate is required.

The term “cooling in moving air” or “forced air cooling” is understood here to mean an air cooling which is usually supported by fans and which leads to a heat transfer coefficient on the bar surface of approximately 40 W / m ² K. Cooling in a water / air spray leads to a somewhat higher heat transfer coefficient on the bar surface.

The alloy according to the invention has a low sensitivity to quenching on. When producing thick boards, there is a loss of strength in the board core despite the relatively mild cooling conditions, smaller than the Alloys according to the state of the art. It has also surprisingly emphasized that this effect is produced directly from continuous cast ingots Plates is much more pronounced than with hot-rolled plates.

During the production of the thick sheets, the two-stage cooling of the homogenization temperature to room temperature is particularly advantageous to achieve 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 heat curing, 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 in succession, for example

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

Heat curing to the heat treatment state is particularly preferred T76.

The field of application of the alloy according to the invention and of this The thick plates produced result from the above Spectrum of properties. The plates are particularly suitable for mold making, i.e. for the production of plastic injection molds, but also in general 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 result from the following description of preferred exemplary embodiments and from the drawing; this shows schematically in

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

example

An alloy with the composition (in% by weight): 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 on an industrial scale to a continuous cast ingot with a cross section of 440 x 900 mm potted. The ingot was placed on a bar within 30 hours Temperature of 480 ° C heated, taking care that the heating rate in the range between 170 and 410 ° C less than 20 ° C / h scam. The homogenization of the ingot to compensate for the solidification Crystal segregation was carried out by holding the ingot for 12 hours 480 ° C.

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

The ingot was stored for 8 hours after 14 days of storage at room temperature at 95 ° C and then for 18 hours at 155 ° C to the over-hardened state T76 hardened warm.

On samples of the thermoset that have been sawn out perpendicular to the longitudinal direction of the bar The Brinell hardness was determined over the bar cross-section. The areas of the same hardness shown in FIG. 1 clearly show the low Loss of hardness or strength in the bar core compared to the bar surface.

In FIG. 2, the temperature-time curves for a fan cooling are calculated for the surface (O) and the core (K) of an ingot with a cross section of 440 × 900 mm, and in FIG. 3 the gradients between the temperature T derived therefrom _K in the ingot core and the temperature T _o on the ingot surface. For comparison, FIGS. 4 and 5 show the corresponding curves for an ingot with a cross section of 1000 x 1200 mm. The results show that bars produced with the method according to the invention with a thickness of up to 1000 mm are still likely to meet the mechanical strength requirements placed on plates for the production of plastic injection molds.

Claims (19)

Aluminum alloy with high strength and low quench sensitivity, with 4.6 to 5.2% by weight Zn 2.6 to 3.0% by weight mg 0.1 to 0.2% by weight Cu 0.05 to 0.2% by weight Zr Max. 0.05% by weight Mn Max. 0.05% by weight Cr Max. 0.15% by weight Fe Max. 0.15% by weight Si Max. 0.10% by weight Ti and aluminum as the rest with production-related impurities, individually max. 0.05% by weight, total max. 0.15% by weight. Aluminum alloy according to claim 1, characterized by 4.6 to 4.8 wt .-% Zn. Aluminum alloy according to claim 1 or 2, characterized by 2.6 to 2.8 wt .-% Mg. Aluminum alloy according to one of claims 1 to 3, characterized by 0.10 to 0.15% by weight of Cu. Aluminum alloy according to one of claims 1 to 4, characterized by 0.08 to 0.18% by weight of Zr. Aluminum alloy according to one of claims 1 to 6, characterized by max. 0.03 wt% Mn. Aluminum alloy according to one of claims 1 to 5, characterized by max. 0.02 wt% Cr. Aluminum alloy according to one of claims 1 to 7, characterized by max. 0.12 wt% Fe. Aluminum alloy according to one of claims 1 to 8, characterized by max. 0.12 wt% Si. Aluminum alloy according to one of claims 1 to 9, characterized by max. 0.05 wt% Ti. A method for producing plates with a thickness of up to 300 mm from an aluminum alloy according to one of claims 1 to 10, characterized by the steps A. Continuous casting of the aluminum alloy into bars with a thickness of more than 300 mm, B. heating the bars with a heating rate of max. 20 ° C / h between 170 and 410 ° C to a temperature of 470 to 490 ° C, C. homogenizing the ingots over a period of 10 to 14 hours at a temperature of 470 to 490 ° C, D. hot rolling the homogenized ingots into sheets, E. cooling the plates from a temperature of 400 to 410 ° C to a temperature of less than 100 ° C, F. cooling the plates to room temperature, H. Heat curing the panels. Process for the production of plates with a thickness of more than 300 mm from an aluminum alloy according to one of Claims 1 to 10, characterized by the steps A. Continuous casting of the alloy into bars with a thickness of more than 300 mm, B. heating the bars with a heating rate of max. 20 ° C / h between 170 and 410 ° C to a temperature of 470 to 490 ° C, C. homogenizing the ingots over a period of 10 to 14 hours at a temperature of 470 to 490 ° C, D. cooling of the bars to an intermediate temperature of 400 to 410 ° C, E. cooling of the bars from the intermediate temperature of 400 to 410 ° C to a temperature of less than 100 ° C, F. cooling the bars to room temperature, G. hot hardening of the bars, H. Using the thermoset bars as plates. A method according to claim 12, characterized in that the cooling of the bars from the homogenization temperature of 470 to 490 ° C to the intermediate temperature of 400 to 410 ° C takes place in still air. A method according to claim 11 or 12, characterized in that the billets are cooled from the intermediate temperature of 400 to 410 ° C to a temperature of less than 100 ° C in moving air (forced air cooling). A method according to claim 11 or 12, characterized in that the billets are cooled from the intermediate temperature of 400 to 410 ° C to a temperature of less than 100 ° C in a water / air spray. Method according to one of claims 11 to 15, characterized in that for thermosetting a room temperature storage, a first heat treatment at a first temperature and a second heat treatment at a second temperature which is higher than the first temperature are carried out in succession. A method according to claim 16, characterized by 1 to 30 days storage at room temperature, 6 to 10 h storage at a temperature of 90 to 100 ° C, 8 to 22 h storage at a temperature of 150 to 160 ° C. A method according to claim 17, characterized in that the heat curing takes place to the heat treatment state T76. Use of a with the method according to any one of claims 11 to 18 manufactured plate for machine, tool and mold making, in particular for the production of plastic injection molds.

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