METHOD OF BRAZING ALUMINIUM OR ALUMINIUM ALLOY PARTS

Abstract: A method of brazing aluminium or aluminium alloy parts, using a flux which is made from an eutectic of BaCl₂, NaCl, and KCl as a base, and one or more fluorides as an activating agent. Because the flux is not deliquescent, the brazing method can be carried in an atmosphere which is free from oxygen. Flux residues are easily removable in this method.

Description: This invention relates to a method of brazing aluminium or aluminium alloy parts, i.e. a brazing method for joining components parts of aluminium or alloys thereof. Fluxes presently employed for joining aluminium parts with a brazing metal may be classified into two groups, viz., those which are soluble in water and those which are insoluble in water. A water-soluble flux is generally made from a mixture of chlorides such as chlorides of alkali metals or alkaline earth metals, while a water-insoluble flux is usually a fluoride such as a fluoroamine-potassium-complex. Almost all of the water-soluble fluxes which are made from chlorides are deliquescent, and when the become moist, they have severe corrosive actions against aluminium materials and brazing apparatuses. On the other hand, residues of fluxes containing fluorides which have adhered to surfaces of aluminium parts after brazing, can be removed by washing only with difficulty. For example, in a conventional water-solute flux which is made from a mixture of chlorides such as NaCl, KCl, LiCl, ZnCl₂, CaCl₂, and fluorides are extremely deliquescent. When such a flux is applied to a brazing sheet, and heated in a furnace, it is decomposed and evaporates as indicated in the following formula, whereby its efficiency as a flux considerably decreases ZnCl₂ + H₂O → ZnO + HCl Meanwhile, the flux which has evaporated adheres to the furnace and its fixtures and jigs, absorbs moisture, and corrodes them. This means that it is inappropriate to carry out a brazing operation with the employment of a deliquescent flux under an atmosphere having a low oxygen content and a low dew point such as one having an oxygen content lower than 50 ppm and a dew point lower than -50°C, since the flux itself is hydroscopic. Therefore, the operation is generally conducted under atmospheric conditions. Since such conditions cannot be relied on to prevent the oxidation of component parts of aluminium or alloys thereof, an excessive amount of flux has to be used for the brazing operation in order to remove metal oxides produced during the operation. Further, residues of flux which have adhered to the components have to be thoroughly washed out, since they are corrosive against the components in the presence of moisture. The aforementioned water-insoluble fluxes have been developed so that it is not required to remove them after brazing. They are mixtures of KAlF₄ and K₃AlF₆ which are insoluble in water even after brazing, and are not corrosive even in the presence of moisture. However, this kind of flux has a drawback that its residues adhere firmly onto aluminium components after brazing, and they can be removed only with difficulty. In particular, when the components are to be subjected to a surface treatment after having been brazed, the residues become barriers to the treatment. Also, when brazed aluminium components are employed in processing food, scattering of flux residues into the food is unavoidable, though this should be completely prevented. The present invention provides a method of brazing aluminium or aluminium alloy parts, characterized by applying onto the parts to be joined a flux of an eutectic of 55-47 weight % of BaCl₂, 18-22 weight % of NaCl, and 27-33 weight % of KCl, to which is added 1-20 weight % of one or more fluorides, and heating the parts to a temperature higher than 540°C so that brazing material(s) at the parts melt and join in conjunction with the flux. This invention therefore provides a brazing method for joining component parts of aluminium or alloys thereof, wherein a flux which has been left on the parts after brazing can be readily removed by washing for example, wherein the flux and its residues are not deliquescent, and wherein the brazing operation can be conducted under an atmosphere having a low oxygen content and a low dew point, effectively preventing the oxidation of the component parts. More specifically, the method in accordance with the invention employs as a flux, an eutectic mixture of BaCl₂, NaCl, and KCl, which is not deliquescent and which is activated by the addition of one or more fluorides thereto. The invention also relates to a method of applying said specific flux in conjunction with a specific atmosphere. The eutectic point of BaCl₂ 54.7%, NaCl 18.0 %, and KCl 27.3 % is 540°C, which temperature seems appropriate for brazing aluminium or aluminium alloy component parts, the brazing temperature of which is usually 580-615°C, and in which aluminium hard solders employed are generally Al-Si systems having a melting point of 561-580°C. Although the above-mentioned eutectic mixture or alloy is therefore good as a flux as it is, one or more fluorides are added thereto in order to make it chemically more active. Even when fluorides are added in an amount of 1-20 weight %, increase of the melting point of the mixture is only about 20°C. That is, the melting point of the eutectic mixture or alloy is limited to 560°C which is still appropriate for the intended brazing in accordance with this invention. This means that the mixing ratio of constituents of said mixture may be varied, taking into consideration increase of its melting temperature by the addition of fluorides, viz., about 10-15°C. The mixing ratio is 55-47 weight % for BaCl₂, 18-22 % for NaCl, and 27-33 % for KCl. It shall be noted that a mixture or eutectic alloy of BaCl₂, NaCl, and KCl is not deliquescent, and that a flux made from said mixture with the addition of one or more fluorides is also not deliquescent. In practice, a flux employed in the method in accordance with this invention is preferably made as a fine powder of size less than 50 µ.. The powder is prepared as a suspension of a desired density by the addition of a solvent, and then applied to aluminium parts to be brazed. The solvent is one which does not contain water when a water-soluble fluoride is used, since such fluoride reacts with an eutectic mixture of BaCl₂, NaCl, and KCl, and changes the composition of the mixture. When the fluoride is water-insoluble, the solvent may contain water or may be water. In view of the fact that aluminium or alloys thereof easily form chemically strong oxidized surface films on account of oxygen or steam even under a comparatively low temperature, and that such films lower brazing efficiencies, the method in accordance with this invention which can be carried under an atmosphere of low oxygen content and a low dew point is very advantageous. It is also very advantageous in this invention that a minimum amount of flux is required. The invention will be further described with reference to the following illustrative Examples. Example 1. An eutectic base of BaCl₂, NaCl, and KCl in mixing ratios by weight of 54.7, 18.0, 23.3 % was ground to obtain a powder of size less than 50 µ. To this powder, there was added 10 weight % of ZnF₂ to obtain a flux. This flux was made into a suspension by adding thereto a solvent which does not contain water, viz., perchloro-ethylene or xylene at a ratio of 1,000 ml per 50 g of the flux. The flux thus prepared as a suspension was sprayed onto brazing portions of aluminium component parts, and dried at 200°C. The parts were subjected to a brazing operation in a furnace of N₂ atmosphere, the oxygen content of which was less than 10 ppm, and the dew point of which was less than -50°C. The parts were kept in the furnace for about 4 minutes at 600-610°C, cooled down to less than 400°C in the furnace and then drawn out from the furnace to the air. The brazing produced fillets which were sufficient to ensure strong joints. Flux residues of little amount were removed easily by washing them in warm water at about 70°C. Example 2. A eutectic powder similar to that employed in Example 1 was added with 10 weight % of CaF₂, a water-insoluble fluoride. The powder was added with water at a ratio of 1,000 ml per 50 g to obtain a flux suspension. The suspension was sprayed onto aluminium alloy parts in exactly the same manner as in Example 1, and dried at 300°C. The parts were subjected to a brazing operation, similarly to Example 1. Reliable joints were obtained, and flux residues on and about the joints were easily removed by washing them in warm water at 70°C. Example 3. In experiments conducted under the conditions of Example 1 or 2 and with the addition of other fluorides to the eutectics, brazing efficiencies were compared with each other with respect to joints obtained and easiness of removal of the flux residues. The following results were obtained. Excellent: ZnF₂, CaF₂, MgF₂, AlF₂, LiF; Better: NaF, NiF₂, BaF₂; Good: CrF₂, MnF₂, CuF, KF.

Claims: 1. A method of brazing aluminium or aluminium alloy parts, characterized by applying onto the parts to be joined a flux of an eutectic of 55-47 weight % of BaCl₂, 18-22 weight % of NaCl, and 27-33 weight % of KCl, to which is added 1-20 weight % of one or more fluorides, and heating the parts to a temperature higher than 540°C so that brazing material(s) at the parts melt and join in conjunction with the flux. 2. A method as claimed in claim 1, characterized in that the flux is applied to the said parts after mixing it with a solvent which does not contain water. 3. A method as claimed in claim 1, characterized in that said fluoride(s) is water-insoluble, and in that the flux is applied to the said parts after mixing it with water. 4. A method of any of claims 1 to 3, characterized in that the method is carried under an atmosphere having a low oxygen content and a low dew point. 5. A method of brazing aluminium or aluminium alloy parts, characterized by comprising applying onto portions of the parts to be joined a flux made from an eutectic of 55-47 weight % of BaCl₂, 18-22 weight % of NaCl, and 27-33 weight % of KCl, which is added by 1-20 weight % of fluorides, and heating the parts to a temperature higher than 540°C so that brazing materials at the portions melt and make joints in conjunction with the flux.

Inventors: TAKAHASHI, SUSUMU
Applicants: KANTO YAKIN KOGYO KABUSHIKI KAISHA
Application year: 1985
Main classification: B23K35
IPC codes: B23K35