Flux for brazing

Abstract

The invention concerns a flux for brazing, a process for brazing metal parts employing said flux, a flux composition containing said flux, aluminum parts coated with said flux or said flux composition, a process for brazing and a brazed metal object obtainable by said brazing process. The flux is high in KAlF 4 and low in K 3 AlF 6 .

1 . A flux comprising a fundamental flux, wherein the fundamental flux comprises from 80 mol % to 100 mol % KAlF 4 , the content of K 3 AlF 6  in the flux is equal to or lower than 2 mol % including 0 mol %, the content of free KF in the flux is lower than 0.2% by weight including 0% by weight, and wherein the flux further comprises from 0.1 to 20 weight % relative to the total weight of the flux of at least one additive salt, wherein the at least one additive salt comprises at least one anion selected from the group consisting of F − , CO 3   2− , O 2− , nitrate, phosphate, borate, metaborate and oxalate, and at least one cation selected from the group consisting of earth alkali metal cations. 
     
     
         2 . The flux according to  claim 1  wherein the at least one additive salt is selected from the group consisting of CaF 2 , BeF 2 , MgF 2 , BaF 2 , SrF 2 , CaO, BeO, MgO, BaO, SrO, CaCO 3 , BeCO 3 , MgCO 3 , BaCO 3 , and SrCO 3 . 
     
     
         3 . The flux according to  claim 2  wherein the at least one additive salt is selected from the group consisting of CaF 2 , BeF 2 , MgF 2 , BaF 2  and SrF 2 . 
     
     
         4 . The flux according to  claim 1 , wherein the fundamental flux comprises one or more components selected from the group consisting of K 2 AlF 5 , K 3 AlF 6 , potassium fluorozincates, cesium fluoroaluminates, potassium fluorostannates and cesium fluorostannates, and hydrates wherein the one or more components are comprised in the fundamental flux from 0 to 20 mol %, adding up with the content of KAlF 4  to 100 mol %. 
     
     
         5 . The flux according to  claim 1 , wherein the flux further comprises at least one brazing additive selected from the group consisting of Si, LiOH, LiF, Li fluoroaluminates, lithium potassium fluoroaluminates, solder metals and solder metal precursors. 
     
     
         6 . The flux according to  claim 1 , wherein the at least one earth alkali metal additive salt is present in an amount of equal to or more than 1 weight %. 
     
     
         7 . The flux according to  claim 1 , wherein the at least one additive salt is present in an amount of from equal to or less than 15 weight %. 
     
     
         8 . The flux according to  claim 1 , wherein the fundamental flux comprises a molar content of equal to or higher than 98 mol % KAlF 4 . 
     
     
         9 . A flux composition comprising a flux according to  claim 1 , and at least one fluxing additive selected from the group consisting of solvents, binders, thickeners, suspension stabilizers, antifoaming agents, surfactants and thixotropic agents. 
     
     
         10 . Aluminum parts or aluminum alloy parts for brazing, coated at least partially with a flux according to  claim 1 . 
     
     
         11 . A process for brazing of metal parts made from aluminum or aluminum alloys with metal parts made from aluminum, aluminum alloys, copper, titanium or steel, which comprises the steps of
 a) coating metal parts at least partially with the flux according to  claim 1 ;   b) optionally drying the at least partially coated metal parts;   c) assembling the at least partially coated metal parts;   d) heating the assembled, at least partially coated metal parts to a temperature sufficiently high to braze the at least partially coated metal parts;   e) brazing the at least partially coated metal parts;   f) optionally cooling the brazed parts.   
     
     
         12 . The process according to  claim 11 , wherein step c) and/or d) are performed at a temperature equal to or higher than 540° C., and at a temperature equal to or lower than 650° C. 
     
     
         13 . The process according to  claim 11 , wherein step c) and/or d) are performed in the presence of a protective gas containing equal to or more than 75% by volume of at least one gas selected from the group consisting of helium, nitrogen, argon and xenon. 
     
     
         14 . A brazed metal object, obtained according to the process of  claim 11 . 
     
     
         15 . The brazed metal object according to  claim 14 , which forms part of a cooler for stationary or mobile refrigeration equipment or of a stationary heat exchanger. 
     
     
         16 . Aluminum parts or aluminum alloy parts for brazing, coated at least partially with a flux composition according to  claim 9 . 
     
     
         17 . A process for brazing of metal parts made from aluminum or aluminum alloys with metal parts made from aluminum, aluminum alloys, copper, titanium or steel, which comprises the steps of
 a) coating metal parts at least partially with a flux composition according to  claim 9 ;   b) optionally drying the at least partially coated metal parts;   c) assembling the at least partially coated metal parts;   d) heating the assembled, at least partially coated metal parts to a temperature sufficiently high to braze the at least partially coated metal parts;   e) brazing the at least partially coated metal parts;   f) optionally cooling the brazed parts.   
     
     
         18 . The process according to  claim 17 , wherein step c) and/or d) are performed at a temperature equal to or higher than 540° C., and at a temperature equal to or lower than 650° C. 
     
     
         19 . The process according to  claim 17 , wherein step c) and/or d) are performed in the presence of a protective gas containing equal to or more than 75% by volume of at least one gas selected from the group consisting of helium, nitrogen, argon and xenon. 
     
     
         20 . A brazed metal object, obtained according to the process of  claim 17 .