Method for manufacturing tanks

Abstract

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.