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.sub.4 and low in K.sub.3AlF.sub.6.

Claims

1. A flux comprising a fundamental flux, wherein the fundamental flux comprises: from 80 mol % to 100 mol % KAlF.sub.4, and one or more components selected from the group consisting of K.sub.2AlF.sub.5, K.sub.3AlF.sub.6, potassium fluorozincates, cesium fluoroaluminates, potassium fluorostannates and cesium fluorostannates, and hydrates thereof, wherein the one or more components are comprised in the fundamental flux from 0 to 20 mol %, adding up with the content of KAlF.sub.4 to 100 mol %, wherein the content of K.sub.3AlF.sub.6 in the fundamental flux is equal to or lower than 2 mol % including 0 mol %, wherein 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 8 weight % relative to the total weight of the flux of at least one additive salt selected from group consisting of BaF.sub.2 and SrF.sub.2.

2. 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.

3. The flux according to claim 1, wherein the at least one additive salt is present in an amount of equal to or more than 1 weight %.

4. 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 8 weight %.

5. The flux according to claim 1, wherein the fundamental flux comprises a molar content of equal to or higher than 98 mol % KAlF4.

6. 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.

7. Aluminum parts or aluminum alloy parts for brazing, coated at least partially with a flux according to claim 1.

8. 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.

9. The process according to claim 8, 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.

10. The process according to claim 8, 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.

11. A brazed metal object, obtained according to the process of claim 8.

12. The brazed metal object according to claim 11, which forms part of a cooler for stationary or mobile refrigeration equipment, or of a stationary heat exchanger.

13. Aluminum parts or aluminum alloy parts for brazing, coated at least partially with a flux composition according to claim 9.

14. 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 6; 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.

15. The process according to claim 14, 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.

16. The process according to claim 14, 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.

17. A brazed metal object, obtained to the process of claim 14.

Description

EXAMPLE 1

(1) Nocolok Flux with 3% of BaF.sub.2 was homogenized thoroughly in a ball mill. An amount of 5 gram/m.sup.2 of this powder blend was evenly distributed on a cladded (4343) Aluminum 3003 coupon of 2.5 by 2.5 cm in size. The cladded Aluminum part was assembled with an aluminum angle. The assembled parts were placed in a lab brazing furnace (volume 2.5 L), with a controlled atmosphere of nitrogen gas (11 L/min). The brazing furnace containing the assembled parts was heated with a rate of 30 C./min. Upon reaching the holding temperature of 605 C., the furnace was held at 605 C. for 2 minutes. The heating was stopped and the setup cooled under nitrogen atmosphere. Overall brazing time was 22 minutes, cooling down was 4 minutes.

(2) The brazed coupondown to ambient temperaturewas then soaked 10 days at ambient temperature in a closed beaker containing 40 ml DI-water. (DI-water: deionized water).

(3) Fluoride content in mg/L of soaking water: Nocolok+3% BaF.sub.2: 6.5 Nolocok w/o additive: 7.5

EXAMPLE 2

(4) KAlF.sub.4 with 3%, respectively 1.5%, of BaF.sub.2 was homogenized thoroughly in a ball mill. An amount of 5 gram/m.sup.2 of this powder blend was evenly distributed on a cladded (4343) Aluminum 3003 coupon of 2.5 by 2.5 cm in size. The cladded Aluminum part was assembled with an aluminum angle. The assembled parts were placed in a lab brazing furnace (volume 2.5 L), with a controlled atmosphere of nitrogen gas (11 L/min). The brazing furnace containing the assembled parts was heated with a rate of 30 C./min. Upon reaching the holding temperature of 605 C., the furnace was held at 605 C. for 2 minutes. The heating was stopped and the setup cooled under nitrogen atmosphere. Overall brazing time was 22 minutes, cooling down was 4 minutes.

(5) The brazed coupondown to ambient temperaturewas then soaked 10 days at ambient temperature in a closed beaker containing 40 ml DI-water. (DI-water: deionized water).

(6) Fluoride content in mg/L of soaking water: KAlF.sub.4+3% BaF.sub.2: 4.5 KAlF.sub.4+1.5% BaF.sub.2: 4.0 KAlF.sub.4 w/o additive: 5.5