Method of joining heat-treatable aluminum alloy members by friction stir welding and joined product obtained by the method and used for press forming

Abstract

A method of joining heat-treatable aluminum alloy members by friction stir welding, including the steps of: a T4-treatment-performing step of performing a T4 treatment on heat-treatable aluminum alloy members so as to impart T4 temper to the heat-treatable aluminum alloy members; a joining step of joining the heat-treatable aluminum alloy members with T4 temper by friction stir welding to provide a joined product; and a reversion-treatment-performing step of performing a reversion treatment, the reversion-treatment-performing step being carried out prior to or after the joining step.

1 . A method of joining heat-treatable aluminum alloy members by friction stir welding, comprising the steps of: 
 a T4-treatment-performing step of performing a T4 treatment on heat-treatable aluminum alloy members so as to impart T4 temper to the heat-treatable aluminum alloy members;    a joining step of joining the heat-treatable aluminum alloy members with T4 temper by friction stir welding to provide a joined product; and    a reversion-treatment-performing step of performing a reversion treatment, the reversion-treatment-performing step being carried out prior to or after the joining step.    
   
   
       2 . The method according to  claim 1 , wherein the reversion-treatment-performing step is carried out prior to the joining step for placing the heat-treatable aluminum alloy members in a reversion state and the joining step is carried out such that the heat-treatable aluminum alloy members in the reversion state are joined together by friction stir welding.  
   
   
       3 . The method according to  claim 2 , wherein the reversion treatment is a heat treatment in which the heat-treatable aluminum alloys with T4 temper are heated at an elevated temperature of 150-350° C. for a time period of not longer than 300 seconds.  
   
   
       4 . The method according to  claim 2 , wherein each of the heat-treatable aluminum alloy members is formed of a 6000 series aluminum alloy and the reversion treatment is a heat treatment in which the heat-treatable aluminum alloy members with T4 temper are heated at an elevated temperature of 200-350° C. for a time period of not longer than 300 seconds.  
   
   
       5 . The method according to  claim 2 , wherein each of the heat-treatable aluminum alloy members is formed of a 2000 series aluminum alloy and the reversion treatment is a heat treatment in which the heat-treatable aluminum alloy members with T4 temper are heated at an elevated temperature of 150-300° C. for a time period of not longer than 300 seconds.  
   
   
       6 . The method according to  claim 2 , wherein each of the heat-treatable aluminum alloy members is formed of a 7000 series aluminum alloy and the reversion treatment is a heat treatment in which the aluminum alloy members with T4 temper are heated at an elevated temperature of 150-250° C. for a time period of not longer than 300 seconds.  
   
   
       7 . The method according to  claim 2 , wherein the joining step is carried out on the heat-treatable aluminum alloy members in the reversion state before an increase in tensile strength of the heat-treatable aluminum alloy members due to natural aging exceeds 10 MPa.  
   
   
       8 . The method according to  claim 2 , wherein the reversion treatment is performed using a heating means selected from a salt bath, an oil bath, an air-heating furnace, an iron, infrared heating and induction heating.  
   
   
       9 . The method according to  claim 2 , wherein the reversion-treatment-performing step is carried out such that the reversion treatment is sequentially performed using a suitable heating device along portions of the heat-treatable alloy members at which they are to be joined together by friction stir welding and the joining step is carried out such that the portions which have been subjected to the reversion treatment are sequentially joined together by friction stir welding.  
   
   
       10 . The method according to  claim 1 , wherein the reversion-treatment-performing step is carried out after the joining step so as to perform the reversion treatment on the joined product obtained in the joining step, before GP zones are formed at a stir zone of the joined product.  
   
   
       11 . The method according to  claim 10 , wherein the reversion treatment is a heat treatment in which the joined product is heated at an elevated temperature of 150-350° C. for a time period of not longer than 300 seconds.  
   
   
       12 . The method according to  claim 10 , wherein each of the heat-treatable aluminum alloy members is formed of a 6000 series aluminum alloy and the reversion treatment is a heat treatment in which the heat-treatable aluminum alloy members are heated at an elevated temperature of 200-350° C. for a time period of not longer than 300 seconds.  
   
   
       13 . The method according to  claim 10 , wherein each of the heat-treatable aluminum alloy members is formed of a 2000 series aluminum alloy and the reversion treatment is a heat treatment in which the heat-treatable aluminum alloy members are heated at an elevated temperature of 150-300° C. for a time period of not longer than 300 seconds.  
   
   
       14 . The method according to  claim 10 , wherein each of the heat-treatable aluminum alloy members is formed of a 7000 series aluminum alloy and the reversion treatment is a heat treatment in which the heat-treatable aluminum alloy members are heated at an elevated temperature of 150-250° C. for a time period of not longer than 300 seconds.  
   
   
       15 . The method according to  claim 10 , wherein the reversion-treatment-performing step is carried out on the joined product obtained in the joining step before a time period during which the joined product undergoes natural aging that occurs after the heat-treatable aluminum alloy members have been joined by friction stir welding exceeds 24 hours.  
   
   
       16 . The method according to  claim 10 , wherein the reversion treatment is performed using a heating means selected from a salt bath, an oil bath, an air-heating furnace, an iron, infrared heating and induction heating.  
   
   
       17 . The method according to  claim 10 , wherein the joining step is carried out such that the heat-treatable alloy members are joined together sequentially along portions thereof at which they are to be joined by friction stir welding and the reversion-treatment-performing step is carried out such that the reversion treatment is sequentially performed, using a suitable heating device, on the joined product which at least includes a stir zone formed by friction stir welding in the joining step.  
   
   
       18 . A joined product for press forming obtained by a method of joining heat-treatable aluminum alloy members by friction stir welding, comprising the steps of: (a) a T4-treatment-performing step of performing a T4 treatment on heat-treatable aluminum alloy members so as to impart T4 temper to the heat-treatable aluminum alloy members; (b) a joining step of joining the heat-treatable aluminum alloy members with the T4 temper by friction stir welding to provide a joined product; and (c) a reversion-treatment-performing step of performing a reversion treatment, the reversion-treatment-performing step being carried out prior to or after the joining step.  
   
   
       19 . A joined product for press forming with excellent ductility obtained by joining heat-treatable aluminum alloy members which have been subjected to a T3 treatment or a T4 treatment, wherein the improvement comprises: 
 a joint region of the joined product at which the heat-treatable aluminum alloy members are joined together being formed such that hardness of the joint region and a heat-affected zone located adjacent to the joint region is held in a range of 100-200 where hardness of base material portions of the joined product is 100.    
   
   
       20 . The joined product according to  claim 19 , wherein a total width of the joint region and the heat-affected zone on a surface of the joint product is held in a range of 1 mm to 100 mm.  
   
   
       21 . The joined product according to  claim 19 , wherein fracture occurs at the base material portions in a case where a tensile test is conducted using a rectangular specimen defined in ASTM B 557-94 that is obtained from the joined product such that the specimen includes, as a parallel portion thereof, the joint region and the heat-affected zone.