COMPACT ALUMINIUM ALLOY HEAT TREATMENT METHOD

Abstract

The invention relates to a method for the heat treatment of a moving aluminium alloy strip, the aluminium strip has an upper-surface and a lower-surface, the method comprising moving the aluminium strip over at least two rotating heating rolls, wherein the heating rolls comprises an outer-surface, such that a surface of the aluminium strip is in heat-transfer contact with the outer-surface of the heating rolls to induce heat into the aluminium strip to heat the aluminium strip at an annealing temperature, and comprising moving the aluminium alloy strip over a first rotating heating roll followed by moving the aluminium strip over a second rotating heating roll such that alternating the upper-surface and the lower-surface of the aluminium strip are in heat-transfer contact with the outer-surface of the rotating heating rolls.

Claims

1. Method for the heat treatment of a moving aluminium alloy strip, the aluminium strip has an upper-surface and a lower-surface, the method comprising moving the aluminium strip over at least two rotating heating rolls, wherein the heating rolls comprise an outer-surface, such that a surface of the aluminium strip is in heat-transfer contact with a part of the outer-surface of the heating rolls to induce heat into the aluminium alloy strip to heat the aluminium alloy strip at an annealing temperature, and comprising moving the aluminium alloy strip over a first rotating heating roll followed by moving the aluminium strip over a second rotating heating roll such that alternating the upper-surface and the lower-surface of the aluminium strip are in heat-transfer contact with the outer-surface of the rotating heating rolls.

2. The method according to claim 1, wherein the outer-surface of the heating rolls is coated with a composite diamond coating.

3. The method according to claim 1, wherein the outer-surface of the heating rolls is coated with a ceramic coating, preferably selected from the group consisting of titanium nitride, tungsten carbide, and chromium nitride.

4. The method according to claim 1, wherein the aluminium alloy strip is moving while one surface is in heat-transfer contact with a rotating heating roll and heat-loss of the other surface of the aluminium alloy strip is modulated by the presence of a screen.

5. The method according to claim 1, wherein the aluminium alloy strip following the heat treatment is quenched to below 100° C.

6. The method according to claim 1, wherein the aluminium alloy strip following the heat treatment is quenched to below 100° C. by moving the aluminium alloy strip over at least one rotating cooling roll, wherein the rotating cooling roll comprises an outer-surface, such that a surface of the aluminium alloy strip is in heat-transfer contact with the outer-surface of the rotating cooling roll to remove heat from the aluminium alloy strip to cool the aluminium alloy strip at a temperature below 100° C.

7. The method according to claim 1, wherein the annealing temperature is in a range of 400° C. to 590° C.

8. The method according to claim 1, wherein the heating roll has a diameter in a range of 1 to 3 meters.

9. The method according to claim 1, wherein the heating roll is made from a metal, preferably selected from the group of cast iron, steel, stainless steel, copper, copper-based alloy, and aluminium alloy.

10. The method according to claim 1, wherein the aluminium alloy strip has a thickness in a range of 0.3 mm to 4.5 mm, and preferably of 0.7 mm to 4 mm.

11. The method according to claim 1, wherein the aluminium alloy strip has a composition within the AA2XXX-, AA5XXX, AA6XXX- or AA7XXX-series aluminium alloys.

12. Facility for implementation of the method according to claim 1, characterised in that it comprises: a heating-section comprising two or more rotatable heating rolls adapted to move or transport in use an aluminium alloy strip while in heat-transfer contact with the outer-surface of the rotatable heating roll to induce heat into the aluminium alloy strip to heat the aluminium alloy strip at an annealing temperature; and a quenching section for rapid cooling or quenching of the aluminium alloy strip from the annealing temperature to below 100° C.

13. Facility according claim 12, further comprising one of more screens positioned to face the side of the aluminium alloy strip that is not in heat transfer contact with the outer-surface of a rotatable heating roll to modulate the heat-loss of a moving aluminium alloy strip.

14. Facility according to claim 12, wherein the outer-surface of the heating rolls is coated with a composite diamond coating.

Description

DESCRIPTION OF THE DRAWING

[0054] The invention shall now be described with reference to the appended drawings, in which:

[0055] FIG. 1 is a schematic representation of the principle of this invention;

[0056] FIG. 2 is a schematic representation of an exemplary method and apparatus;

[0057] FIG. 3 is a schematic representation of another exemplary method and apparatus; and

[0058] FIG. 4 is a schematic representation of another exemplary method and the apparatus.

[0059] FIG. 1 is a schematic representation of the principle of the method according to the invention. An aluminium alloy strip 1 is moving or transported in the direction of the arrows and is heat treated by bringing it in heat-transfer contact with the outer-surface of in this case three rotatable heating rolls 6,7,8. The required diameter of the rotatable heating rolls can be first order estimated by the following guidelines where:

[0060] v is the speed (m/sec) of the moving aluminium alloy strip;

[0061] Tc is the time (sec) of the required contact between the aluminium alloy strip 1 and the outer-surface of the heating rolls to heat-up the aluminium alloy strip to the required annealing or solution heat-treatment temperature and the required soaking time at this temperature. This is aluminium alloy dependent and can be established by simple experiments or thermodynamic computer modelling calculations by the skilled person;

[0062] Lc is the total length (m) of the required contact;

[0063] Di is the diameter (m) is heating toll number i;

[0064] Ki is the contact factor (dimensionless) of heating roll number i. Depending on the relative positions of the heating rolls and the position of the aluminium alloy strip, Ki is the ratio between the perimeter of heating roll i in contact with the strip divided by the total perimeter of said heating roll;

[0065] N is the number of rolls;

Lc=v.Math.Tc or Lc=Di.Math.Ki.Math.π;

[0066] If it is assumed for this model calculation that all rolls have the same Ki and the same diameter, then this can be simplified to:

Lc=N.Math.D.Math.K and consequently D=Tc.Math.v/(N.Math.π.Math.K)

To provide a first order of magnitude where Tc is 15 sec, v is 1 m/sec, N is 3 and K is 0.75, this would result in a heating roll diameter of 2.12 meter for each of three heating rolls.

[0067] The skilled person will immediately recognise that this concerns a mere model calculation and that various variations are possible or required while using the same principles.

[0068] In practice the diameter of a set of heating rolls can be varied within this set, but typically the diameter of each heating roll is in a range of about 1 meter to 3 meters.

[0069] FIG. 2 is a schematic representation of an embodiment of the method according to the invention and the apparatus employed therein. In this configuration, aluminium alloy strip 1 with a lower-surface 2 and an upper-surface 3 is being uncoiled from a coil 4 and moved or transported to a heating section comprising three cylindrical rotatable heating rolls 6,7,8 and followed by a rapid cooling section comprising of cylindrical three rotatable cooling rolls 9,10,11 and subsequently re-coiled into a coil 5. In the heating section, the upper-surface 3 of the aluminium alloy strip 1 is brought into heat-transfer contact with the outer-surface of heating roll 6 to induce heat into the aluminium strip to heat the aluminium strip to an annealing temperature. While progressively moving the lower-surface 2 of the aluminium alloy strip 1 is brought into heat-transfer contact with the outer-surface of heating roll 7, followed by bringing the upper-surface 3 of the aluminium alloy strip 1 into heat-transfer contact with the outer-surface of heating roll 8. Both heating rolls 7 and 9 are rotatable in a first direction, i.e. clockwise or counter-clockwise, and heating roll 8 is rotatable in an opposite second direction. By adjusting the transport speed or line speed of the moving aluminium alloy strip 1 it receives a heat treatment by soaking for a certain time at a pre-defined annealing temperature sufficient to achieve annealing required for the subject aluminium alloy. Soaking times are typically in a range of up to 1 minute, and preferably up to 30 sec. To enhance the heat input into the aluminium alloy strip 1, it can be pre-heated prior to being brought into contact with the first rotatable heating roll. The pre-heat can be achieved by various heating means, for example by using an inductive heating device 13. To assist in the temperature control of the aluminium alloy strip and to modulate heat-loss of the strip, heat shields 12 or screens 12 can be used. The screen is reflective at least on the side facing the heating roll and the moving aluminium sheet and reflects the infra-red radiation of the moving aluminium alloy strip or by adsorbing and re-emitting infra-red radiation. The thermal shield or screen also prevents or avoids uncontrolled temperature loss by preventing air currents around the moving aluminium sheet in the space or chamber defined by the rotating heating roll and the thermal shield or screen. Optionally, the heat shield or reflective screen can be provided further with active heating means (not shown). Following the heat-treatment, the aluminium alloy strip 1 is rapidly cooled or quenched in a quenching section by moving or transporting the aluminium strip over cylindrical rotatable cooling rolls 9,10,11, wherein the rotating cooling rolls comprises an outer-surface, such that a surface of the aluminium strip is in heat-transfer contact with the outer-surface of the rotating cooling rolls to remove heat from the aluminium strip and to cool the aluminium strip to a temperature below 100° C., and preferably to about ambient temperature. In this set-up, the aluminium alloy strip 1 is also further cooled by actively spraying water or water-mist via spray nozzles 14 onto either surface 2,3 of aluminium alloy strip to enhance the cooling rate of the strip material. Alternative approaches have been described herein.

[0070] FIG. 3 is a schematic representation of another embodiment of the method according to the invention and the apparatus used therein. In this configuration, aluminium alloy strip 1 is being uncoiled and via a cylindrical transfer roll or support roll 15 moved or transported to a heating section comprising three cylindrical rotatable heating rolls 6,7,8 of the same diameter and next transported to a rapid cooling or quenching section (not shown). All three rotatable heating rolls are provided with heat shields 12 or reflective screens 12. In this configuration, rotatable heating rolls 6 and 7 are heated via an external induction source 16, whereas rotatable heating roll 8 is heated by means of electric resistance heating (not shown).

[0071] FIG. 4 is a schematic representation of another embodiment of the method according to the invention and the apparatus used therein. Also in this configuration aluminium alloy strip 1 is being uncoiled and via a cylindrical transfer roll or support roll 15 moved or transported to a heating section comprising three cylindrical rotatable heating rolls 6,7,8 and next transported to a rapid cooling section (not shown). All three rotatable heating rolls 6,7,8 are heated by means of electric resistance heating. Optionally the moving aluminium alloy strip 1 can be pre-heated by means of induction heating using induction device 13. In this configuration the aluminium alloy strip 1 while in heat transfer contact with the outer-surface of the first heating roll 6 is further heated by means of induction heating using induction source 16. Such additional induction heating of the aluminium alloy strip 1 can be applied at one rotatable heating roll, but also at or near more of the rotatable heating rolls.

[0072] This kind of arrangement would be particularly suitable for processing high-strength alloys, for example but without limiting to these examples, aluminium strips from AA2XXX-series aluminium alloys for aircraft applications or AA7XXX-series aluminium alloys for aircraft or automotive applications, as these aluminium alloys contain high amounts of alloying elements requiring a longer soaking time at solution heat treatment temperature. With the current regular industrial equipment available for continuous annealing and quenching of aluminium alloy strips, this longer soaking time obliges to severely reduce the speed of the line (exit speed of the strip), typically by up to about 70% compared to the line speeds used for AA6XXX-series aluminium alloys, making these continuous annealing lines very costly to operate for manufacturing these high-strength aluminium alloys. In the approach of this invention, this can be done very much easier and more cost effective by increasing the diameter of the heating rolls or by adding one or more heating rolls while maintaining a high line speed of the moving aluminium alloy strip.

[0073] The invention is not limited to the embodiments described before, and which may be varied widely within the scope of the invention as defined by the appending claims.