Method of manufacturing pressure vessel liners
A method of manufacturing a liner for use in a pressure vessel includes a cold drawing process that shapes a transition zone ( 20 b ) part of an aluminum alloy liner shell ( 10 ). The transition zone ( 20 b ) links a thinner walled drum part ( 18 ) and a thicker walled end region ( 20 a ), which is at an open top of the shell. The transition zone ( 20 b ) may be shaped by the cold drawing process such that it reduces susceptibility of the shell to fracture during removal from a cold drawing press and also redistributes shell material in preparation for a subsequent process in which a threaded neck is formed from the end region ( 20 a ). This process is particularly suited to processing harder aluminum alloy materials such as AA7060 or AA7032.
1. A method of manufacturing a liner for use in a pressure vessel, the method comprising the steps of:
(a) providing a cup-shaped shell ( 10 ) of aluminium alloy, with closed crown end ( 16 ) and open top, the shell having a drum region ( 18 ) with thinner walls, an end region ( 20 a ) at the open top with thicker walls and an intermediate transition zone ( 20 b ) of tapering thickness;
(b) cold drawing the shell using a punch ( 24 ) with generally cylindrical walls and a shaped upper region ( 24 a ) that, during the drawing operation, meets with the transition zone ( 20 b ) of the shell;
(c) forming the end region ( 20 a ) and transition zone ( 20 b ) to close the open top and to form a neck ( 32 ) extending therefrom; and
(d) cutting a thread into the neck.
2. A method according to claim 1 wherein the shaped upper region ( 24 a ) of the punch is generally inwardly extending and the step of cold drawing includes the steps of:
(i) pushing the punch ( 24 ) and shell ( 10 ) into a die of diameter smaller than at least an outer diameter of the end region ( 20 a ) of the shell; and
(ii) withdrawing the punch from the shell ( 10 ) such that the cylindrical walls of the punch further forms the transition zone ( 20 b ) of the shell.
3. A method according to claim 2 wherein step b(ii) results in a flattening of an inner surface of the transition zone ( 20 b ) of the shell.
4. A method according to claim 1 wherein the shaped upper region ( 24 a ) of the punch comprises a first inwardly curved portion of convex radius of curvature R1 adjacent a second inwardly curved portion of concave radius of curvature R2 and the step of cold drawing includes the steps of:
(i) pushing the punch ( 24 ) and shell ( 10 ) into a die of diameter smaller than at least an outer diameter of the end region ( 20 a ) of the shell; and
(ii) withdrawing the punch from the shell ( 10 ) such that the cylindrical walls of the punch further forms the transition zone ( 20 b ) of the shell.
5. A method according to claim 4 in which R1 is larger than R2.
6. A method according to claim 4 in which R1=2×R2±20%.
7. A method according to claim 4 in which R1 and R2 are set by the equations
R
1
=
1515
×
(
Tfm
-
FTT
)
A
R
2
=
675
×
(
Tfm
-
FTT
)
A
where the end region ( 20 a ) of the shell, after drawing, has thickness Tfm, the transition region has a wedge part, after drawing, with maximum thickness FTT and A is a constant of value between 11 and 60.
8. A method according to claim 2 in which the die has a diameter smaller than that of the drum region ( 18 ) of the shell.
9. A method according to claim 1 in which Step (a), the step of providing the cup-shaped shell, includes the steps of:
(i) providing the cup-shaped shell ( 10 ) with closed crown end ( 16 ) and open top; and
(ii) machining the cup-shaped shell to produce the shell with drum region ( 18 ) with thinner walls, the end region ( 20 a ) with thicker walls and the transition zone ( 20 b ) of tapering thickness.
10. A method according to claim 9 in which the cup-shaped shell ( 10 ) is formed by extruding the aluminium alloy.
11. A method according to claim 1 in which the Step (c) is carried out by spin forming.
12. A method according to claim 1 wherein the aluminium alloy is a strong alloy such as one selected from the AA7xxx or AA2xxx series.
13. A method according to claim 1 wherein the aluminium alloy is selected from the AA7xxx series.
14. A method according to claim 1 wherein the aluminium alloy is AA7060.
15. A method according to claim 1 in which the step of cold drawing involves cold working the shell an amount between 15 and 38%.
16. A method according to claim 15 in which the cold working is an amount between 25 and 38%.