METHOD FOR MANUFACTURING TANKS
The invention relates to the manufacture of tanks from one ore more metal plates using a friction stir welding process The metal plate or plates is first formed into a tubular shape with one pair of opposite edges facing one another to form a longitudinal joint line, the opposite edges then being friction stir welded together. At least a part of the friction stir welded region is cold worked and subsequently the tube is heat treated at a temperature above the recrystallisation temperature.
1 . A method of manufacturing a tank or liner for use in a pressure vessel, said method comprising the steps of:
a) forming one or more metal plates into a tube with one pair of opposite edges facing one another to form a longitudinal joint line; and
b) friction stir welding the opposite edges together along the joint line;
c) cold working at least a part of the friction stir welded region; and
d) heat treating the tube at a temperature above the recrystallisation temperature after cold working.
2 . A method as claimed in claim 1 , wherein said one or more metal plates comprises an aluminium alloy.
3 . A method as claimed in claim 1 , wherein the heat treatment step includes annealing within a range 350 to 475° C.
4 . A method as claimed in claim 1 wherein the heat treatment step comprises solution treatment whereby all or most of the soluble elements are taken into solution.
5 . A method as claimed in claim 4 wherein during the solution treatment the cylinder is heated to a temperature of between 400 to 545° C. and subsequently cooled at a rate to hold most or all of the soluble elements in solution.
6 . A method as claimed in claim 5 wherein the cooling includes quenching in air or water.
7 . A method as claimed in claim 4 wherein subsequent to solution treatment the tube is subjected to precipitation hardening at room temperature and/or one or more precipitation hardening operations at elevated temperature.
8 . A method as claimed in claim 7 wherein the elevated temperature lies substantially within the range 90 to 200° C.
9 . A method as claimed in claim 1 wherein the cold working is carried out at a temperature of less than approximately 100° C.
10 . A method as claimed in claim 9 , wherein the cold working operation includes a cold drawing and/or ironing operation.
11 . A method as claimed in claim 10 wherein the cold working operation reduces the thickness of the tube by more than 20%.
12 . A method as claimed in claim 9 , wherein one or more annealing operations are performed, the or each annealing operation being performed before, between or after one or more cold drawing operations.
13 . A method as claimed 10 wherein only the central part of the cylinder is subjected to drawing leaving the thickness of ends of the cylinder substantially unaltered or to a lesser degree than the central part.
14 . A method as claimed in claim 1 wherein the cylinder is formed from precipitation hardenable aluminium alloys.
15 . A method as claimed in claim 1 wherein the cylinder is formed from one or more alloys in the group including magnesium alloys, copper alloys, titanium alloys, nickel alloys or steels
16 . A method as claimed in claim 14 wherein the aluminium alloys are chosen from the AA2000, AA6000, AA7000 and AA8000 series.
17 . A method as claimed in claim 16 wherein the aluminium alloys are selected as AA6061, AA7032 and AA7475.
18 . A tank or liner for use in a pressure vessel manufactured from a method as claimed in claim 1
19 . A metal tank or liner for a pressure vessel having a friction stir weld along at least a portion of its length, at least a portion of the weld region having been subjected to cold working and heat treatment operations whereby said at least portion of the weld region has a higher burst strength than the parent metal surround the weld region.
20 . A metal tank or liner as claimed in claim 19 wherein the weld region has substantially the same thickness as the surrounding parent metal.