SAFETY FREE LASER WELDING PASTE DESCRIPTION OF THE INVENTION The invention relates to a hard solder paste free of flux for welding copper and copper alloys. A significant industrial field of application for copper and copper alloy welding is the manufacture of radiators for internal combustion engines, especially for use in automobiles. Although aluminum has gained importance as a radiator material in the last 20 years, large copper radiators will also continue to be manufactured in the future. These types of radiators consist of the essentials of brass pipes that conduct refrigerants and are connected to heat conducting copper sheets. During the manufacture of the radiator, the construction parts are joined and welded together at the junctions. The use of soft solders of low melting point based on lead-tin alloys with high lead content is still required. In industrial series production the brass radiator tubes are sprayed with aqueous flux based on zinc chloride-ammonium chloride and then pre-soldered with soft solder. After assembly of the coated pipes and copper sheets, all of the REF: 119418 radiator is completely immersed in the flux solution and after drying in a passing oven, it is added. The flux residues must be removed with water after the melting process, since if there are no corrosive attacks to the radiators. This process of termination is no longer desirable from the economic and ecological point of view. The use of corrosive fluxes based on zinc chloride-ammonium chloride produces a large technical and financial expense in the treatment of waste water and the deposition of flux residues. Furthermore, during the welding process, hydrochloric acid emissions are introduced through the composition and hydrolysis of the flux, which must be removed from the waste gas from the furnace by means of a filter or washing facilities. In addition these radiators when they are out of service, due to the high content of lead, can hardly be recycled. An alternative to the fabrication of copper-brass radiators with soft solder is represented by hard welding with low-melting lead solder. For industrial use, copper-phosphorus solders with a melting range of 710-880 ° C or copper-phosphorus solders containing silver with working temperatures of approximately 700 ° C are usually used. To join the brass pipes and the copper sheets of the radiator, only those welding alloys can rarely be used, since the necessary welding temperatures soften the copper and a large fraction of its strength is lost. Previously for welding copper-brass radiators, suitable hard solders were especially developed. US Patent 5,178,827, US 5,130,090 and US 5,378,294 describe these hard solder alloys. These consist of copper-phosphorus alloys with additives reducing the melting point of nickel as well as eventually zinc and manganese. These liquefaction temperatures of the solder alloys are clearly below 700 ° C, which makes possible welding processes with temperatures below the temperature of softening of the copper. These solders, due to the phosphorus content, are self-extinguishing and can be used under protective gas atomospheres, preferably under nitrogen with low residual oxygen content, to join copper and brass free of fluxes. More information about these hard solders can be found in Adv. Mater. Precessses (1995), 147 (5), 33 and SAE Technical Paper 931076. However, these solders have the disadvantage that they are very brittle, and therefore can only be used in the form of rapidly cooled laminates.
(laminated by spinning of cast) or in the form of powder for the welding of radiators. Melt spin laminates are not considered for cost reasons in mass production. Also the application of films during the assembly of the components of the cooler. In addition, the application of the laminates during the assembly of the components of the cooling is very difficult to integrate during the finished in line. For the alternative use of that solder in powder form it is advantageous to prepare it in the form of a paste, in which the welding powder is dispersed in a binder system and can be applied in liquid or semi-liquid form on an object to be welded. Welding pastes with different solder alloys and binder systems are known and in use for a long time. Welding pastes in an organic binder system, and organic solvents, optionally also aqueous, in addition to the solder powder also contain flux additives. The essential criteria for the selection of the components of the binder system are that the paste remains stable and usable also after a long term and that the solder does not settle irreversibly, that it is easy to apply, without especially dripping by the vertical surfaces, and that the binder system does not damage the welding process. Especially in the case of hard welds with a high melting weight, the binder system must dissipate or burn without leaving residues, avoiding as much as possible the formation of toxic products or pollutants from the environment. An essential aspect in the application of the solder paste is the speed of application and the drying time. The binders commonly used in solder pastes when using high-boiling organic solvents have a drying time of several minutes. Very volatile solvents such as alcohols or ketones dry faster, however they emit very combustible vapors, which make expensive measurements necessary in installations for explosion protection. In the German publication documents DE 28 40 415 and DE 3012 925, formulations of pastes for certain soft and hard solders are described, whose binder systems have thermoplastic properties. The binder can be selected in such a way that the paste is solid at room temperature and at a temperature between 40 and 100 ° C, softens and melts. Here on the one hand the good storage capacity of the paste is given, since sedimentation in the welding powder can not occur. On the other hand a good result can be obtained by applying the paste in liquid molten state and subsequent rapid "drying" by means of cooling. In the aforementioned writings a large number of organic binders is mentioned, which can be selected from resins, waxes, oligomers and natural or synthetic polymers, of different nature and which can be used individually or also in combination with those solder pastes with thermoplastic characteristics. The formulation of the aforementioned low-melting copper-phosphorus alloys in the form of powder to thermoplastic welding pastes, however, caused unexpected problems. Here it was shown that the solder powder, due to its large surface and the specific alloying constituent parts, often reacts very rapidly with oxygen and air humidity, which is not prevented by means of the stuffing in the binder and by which, after a period of storage of the paste, worsens the wetting and fluidity characteristics. The high affinity of the oxygen welding powder also prevents the use of conventional binders, such as cellulose, cellulose derivatives, polyethylene glycols, etc., since the welding powder during the heating process reacts with the decomposition products containing oxygen of the binder forming slag, which hinders the flow of the weld. It has also been shown that also using oxygen-free binders, such as hydrocarbon resin, etc., a good welding result can only be obtained when the welding powder has an oxygen content of less than 150 ppm. Higher oxide contents also lead in this case to incomplete welding. As a consequence of this property of the welding powder, the connection points between the tube and the sheets are only incompletely filled, which on the one hand causes considerable damage to the solidity and on the other hand a strong reduction of the thermal transfer rates. With this type of solder paste formulations can be obtained only with the addition, in itself undesirable, of flux, satisfactory welding results can be obtained. The task was therefore proposed to develop a thermoplastic organic binder system for oxygen-sensitive copper-phosphorus low-melting solder powder, which protects the weld from oxidation and also with the use of Welding powders with an oxygen content greater than 150 ppm without the addition of fluxes, allow welding to be obtained without the formation of slag. Additionally, the liquid binder-solder powder mixture should solidify as quickly as possible after application and decompose under a protective gas atmosphere without leaving residues. Surprisingly it was now found that these requirements are met by a binder system consisting of a mixture of polyisobutene with a relative molar mass of 50,000 to 5,000 and paraffin that melts in the range of 40 to 90 ° C preferably. The object of the invention is thus a solder paste free of flux, consisting of a fine particle welding based on a copper-phosphorus alloy with a working temperature not higher than 700 ° C and a thermoplastic organic binder system, which is characterized because the binder system consists of a mixture of polyisobutene with a relative molar mass of 50,000 to 500,000 and paraffin that melts in the range of 40 to 90 ° C. It has been shown that the binder system according to the invention protects the hard brazing powder from a low melting point phosphor-copper alloy, effectively and durably from oxidation, thus showing properties similar to that of the flux. in that it allows welding powder already oxidized superficially, with an oxygen content greater than 150 ppm, up to approximately an oxygen content of 300 ppm, is founded in the welding process without problems and without formation of slag, thus the binder system together the advantageous thermoplastic properties with the problem-free removal of waste in the heating process. In the solder paste according to the invention, the binder system consists preferably of a mixture of polyisobutene with a relative molar mass of 60,000 to 90,000 and paraffin which melts in the range of 40 to 60 ° C. Suitable solder alloys are those consisting of 10 to 20 atomic% phosphorus, 2 to 5% atomic nickel, 0 to 15 atomic tin, 0 to 5.5 atomic% manganese, and the rest copper . Such type of welding alloys, such as those described in the aforementioned US patents, are characterized by melting ranges that are at approximately 600 to 650 ° C, such that their working temperature does not exceed 700 °. C and can be used without problems to weld copper and copper alloys. The hard solder paste according to the invention advantageously contains 75 to 98% by weight of solder alloy in fine particles and 25 to 2% by weight of thermoplastic organic binder. It preferably contains from 80 to 95% by weight of solder alloy in fine particles and 20 to 5% by weight of thermoplastic binder. The qualitative and quantitative composition of the binder system is advantageously chosen such that when mixed with the brazing powder it gives a hard brazing paste which at room temperature is solid and from a temperature of 40 ° C is transformed into the liquid molten state. Advantageously, the thermoplastic organic binder system consists of 20 to 70% by weight of polyisobutene and 80 to 30% by weight of paraffin. Preferably it consists of
to 65 < % by weight of polyethobutene and 70 to 50% by weight of paraffin. Mixtures of 20 to 70% by weight of polyisobutene with a relative molar mass of 50,000 to 500,000 and 80 to 30% by weight of paraffin with fusions in the range of 40 to 90 ° C, can be advantageously used as thermoplastic organic binder systems for the Hard solder powder based on copper-phosphorus alloys. The solder pastes according to the invention are suitable for hard welding of copper and / or copper alloy parts. In particular, they can be used very advantageously in the manufacture of radiators for internal combustion engines. The method for welding parts of copper and / or copper alloys, especially in the manufacture of radiators for internal combustion engines, is carried out in a manner in which a paste is applied to the parts in the joints. hard welding according to the invention at a temperature between 40 and 90 ° C, optionally in an intermediate step by means of cooling at a temperature lower than 40 ° C the coating of solder paste is applied and then for welding it is heated until the Working temperature of the solder alloy, where during the heating process the binder system is eliminated without leaving residues. Example 1 Composition of the solder pastes: 90.0% by weight of solder powder Cu75Snl6P5Ni4 (oxygen content: 300 ppm) 6.0% by weight of paraffin, melting range 42-44 ° C; 4.0% by weight of polyisobutene, molar mass 60,000 (0ppanolmrB12, BASF AG) Viscosity (70 ° C, Brookfield RTV): 50 Pa.s. Example 2 Composition of solder pastes: 89.0% by weight of Cu75Snl6P5Ni4 solder powder (oxygen content: 300 ppm) 7.5% by weight paraffin, melting point 90 ° C; 3.5% by weight of polyisobutene, molar mass 85,000 (OppanolrarB15, BASF AG) Viscosity (70 ° C, Brookfield RTV): 35 Pa.s. Example 3 Use:
On a brass sheet, a solder paste is applied according to example 1 at 70 ° C and then, in order to fix it, it is cooled to room temperature. The welding is carried out in a pass oven under nitrogen at 650 ° C. A flat gloss welding layer is obtained. Example 4 Comparative Example: The Cu75Snl6P5Ni4 welding powder with an oxygen content of 300 ppm is dispersed in alcohol and applied to a brass sheet. After drying the solder layer, the solder is carried out in a pass oven under nitrogen at 650 ° C. The solder points have insufficient solder flux and black slag residues. EXAMPLE 5 Comparative Example: The solder powder is dispersed in a 3% aqueous solution of ethyl cellulose and then processed as in example 4. The solder points show black slag residues and no solder flux. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.