Method of forming a component of complex shape from sheet material

Abstract

Method of forming a component of complex shape from an Al-alloy sheet or a Mg-alloy sheet. The method includes the steps of: a) heating the sheet to a temperature below the solution heat treatment (SHT) temperature for the alloy; b) forming the heated sheet between heated dies into or towards the complex shape; c) heating the sheet to at least its SHT temperature and substantially maintaining that temperature until SHT has been completed; and d) quenching the solution heat treated sheet between cold dies and at the same time completing the forming into the complex shape or maintaining that shape.

Claims

1. A method of forming a sheet component of multi-dimensional shape from an Al-alloy or Mg-alloy sheet, the method comprising the following sequence of steps in order: (a) heating an Al-alloy or Mg-alloy sheet to a temperature below the solution heat treatment (SHT) temperature for the alloy; (b) forming the heated sheet between heated dies into a shape whilst maintaining the sheet at a temperature below the solution heat treatment (SHT) temperature for the alloy; (c) solution heat treating the sheet by heating the sheet to its SHT temperature and substantially maintaining that temperature until SHT has been completed; and (d) quenching the solution heat treated sheet between cold dies and at the same time completing the formation of the sheet into a finished shape, wherein step (a) includes either: heating the Al-alloy sheet to a temperature between 430? C. and 493? C.; or heating the Mg-alloy sheet to a temperature between 400? C. and 480? C.

2. The method according to claim 1, wherein step (a) includes heating the sheet to a temperature below that at which inclusions in the alloy melt.

3. The method according to claim 1, wherein step (a) includes heating the sheet to a temperature at which formability of the alloy is greater than that at the SHT temperature.

4. The method according to claim 1, wherein step (a) includes heating the sheet to a temperature at which formability of the alloy is substantially maximized.

5. The method according to claim 1, wherein step (b) includes forming the sheet in hot dies arranged to minimize heat loss from the sheet.

6. The method according to claim 1, wherein, in step (b), the dies are at substantially the same temperature as that to which the sheet is heated in step (a).

7. The method according to claim 1, wherein, during step (b), the temperature of the dies is kept substantially constant.

8. The method according to claim 1, wherein the dies of step (b) comprise one or more heating elements.

9. The method according to claim 1, wherein the dies of step (d) are cooled.

10. The method according to claim 1, wherein the method includes the subsequent step of (e) artificially ageing the resulting component.

11. The method according to claim 1, wherein the sheet is an AZ31 or AZ91 Mg-alloy.

12. The method according to claim 1, wherein the sheet is a 2XXX series Al-alloy.

13. The method according to claim 12, wherein the 2XXX series Al-alloy is AA2024.

14. The method according to claim 9, wherein the dies of step (d) comprise one or more cooling elements, cooling channels, or both.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Specific embodiments of the invention are described below by way of example only and with reference to the accompanying drawing, in which:

(2) FIG. 1 is a representation of the variation of the temperature of an Al-alloy sheet with time during a method that embodies the invention.

SPECIFIC DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS

(3) With reference to FIG. 1, an embodiment of a method of forming a component of complex shape from an Al-alloy sheet will now be described.

(4) A sheet of AA2024 Al-alloy is firstly heated to a temperature of 450? C. in a furnace. This temperature of initial heating is below the typical solution heat treatment (SHT) temperature for AA2024 of 493? C. The sheet is then maintained at 450? C. for five minutes. This part of the method is illustrated by the line B in FIG. 1.

(5) The sheet is then transferred to a set of hot dies. In this embodiment, the dies are maintained at a temperature of below 400? C., specifically, in this embodiment, 350? C. by the operation of heating elements positioned in and around the dies. The sheet is transferred to the hot dies without delay in order to minimise cooling of the sheet during this transfer. The hot dies are then brought together to form the sheet into the shape of the complex component that is to be formed. This part of the method is represented by the line C on FIG. 1. In other embodiments, the hot dies may be such that they form the sheet towards the shape of the complex component such that some subsequent deformation is needed in order finally to achieve that component. This will be explained in more detail below.

(6) Returning to the present embodiment, once the sheet has been formed between the heated dies, it is heated in another furnace to its SHT temperature of 493? C. and maintained at that temperature for 15 minutes such that SHT of the formed sheet is completed. This part of the method is represented by the line D on FIG. 1.

(7) Immediately after the SHT has been completed, the sheet is transferred to cold dies. In this embodiment, the cold dies are of exactly the same shape as the hot dies (although they may differ in other embodiments, as will be described below). The cold dies are then brought together such that the formed sheet is maintained in the shape of the component, or such that the shape is recovered in the event of any distortion thereof during the SHT, and such that the sheet is simultaneously quenched. In this embodiment, the cold dies are maintained at a temperature below 150? C. This is done by the provision of coolant channels in and around the cold dies to convey a coolant therethrough. Once the sheet has been quenched, it is removed from the cold dies. This part of the method is represented by the line E on FIG. 1.

(8) Finally, the sheet, which is now formed into the component of complex shape is artificially aged in a conventional way. This part of the method is represented by the line F on FIG. 1.

(9) It has been found that the formability of AA2024 at its SHT temperature of 293? C. is even lower than its formability at room temperature. Further investigations revealed that this alloy contains large Al.sub.20Cu.sub.2Mn.sub.3 inclusions which melt at between 470? C. and 480? C. (that is, below the SHT temperature), depending on the heating rate. As a result, these inclusions become liquid at the SHT temperature, which results in the formation of voids in the microstructure of the sheet. This causes the formability to be low. For this reason, the sheet is heated to a temperature below the SHT temperature in the first step of the method. It has been found that AA2024 exhibits maximum formability at 450? C., and so this temperature is used. Similar characteristics have been found in other Al-alloys. In particular, it is envisaged that embodiments of the method may also be used to form components of complex shape from AA5XXX and AA6XXX series alloys, with appropriate changes in temperatures and durations.

(10) Forming the heated sheet between hot dies minimises heat loss from the sheet such that it can be formed at or near isothermal conditions. The forming process need not therefore be carried out as quickly as in WO 2008/059242 or with such large forming forces. Thus, less expensive forming equipment may be used and longer tool life may be expected.

(11) The remainder of the method is similar to that described in WO 2008/059242, but with the exception that no deformation of the sheet is carried out during the quenching between the cold dies (although, in other embodiments, some deformation, such as a small deformation, may occur). The main purposes to this part of the method are to quench the alloy after the SHT and to minimise distortion of the formed component during rapid cooling. In embodiments where further forming is carried out in this part of the method, the shape of the component is further refined into the finished shape and further features of the component may be added.

(12) As already mentioned, in other embodiments, the sheet may not be fully formed into the desired component between the hot dies. Instead, there may be some additional forming between the cold dies. In such embodiments, it is envisaged that the hot and cold dies will not be of exactly the same shape.

(13) As disclosed above, it has also been found that this method works well with Mg-alloys. In a further embodiment, this method is therefore used to form a component of complex shape from Mg-alloy, which in this embodiment is AZ31. The forgoing description of the method described with reference to and shown in FIG. 1 applies, in principal, equally to this embodiment. Certain of the temperatures and durations are, however, varied to take account of the different alloy. These differences are described below.

(14) The sheet of AZ31 is initially heated to 413? C., and maintained at this temperature for approximately 3 minutes. Again, this part of the method is illustrated by line B in FIG. 1. The part of the method illustrated by line C is as before. In the part of the method illustrated by line D, the sheet is heated to its SHT temperature of 480? C. and maintained there for, as before, 15 minutes. The part of the method illustrated by line E is as before, but with the cold dies being maintained below 50? C. Finally, the artificial ageing represented by line F is, as before, done in a conventional way.