Metal treatment – Compositions – Fluxing
Reexamination Certificate
2000-10-25
2002-01-29
Jenkins, Daniel J. (Department: 1742)
Metal treatment
Compositions
Fluxing
C148S025000, C228S262600
Reexamination Certificate
active
06342106
ABSTRACT:
INTRODUCTION AND BACKGROUND
The invention relates to a flux-free brazing paste for the brazing of copper and copper alloys.
The invention relates to a flux-free brazing paste for the brazing of copper and copper alloys.
A significant industrial field of application of the soldering of copper and copper alloys is the production of radiators for internal combustion engines, particularly for use in motor vehicles.
Although aluminium has gained considerably in importance as radiator material in the last 20 years, radiators with relatively large dimensions will, in particular, also continue to be produced from copper in the future.
Radiators of this type consist substantially of brass pipes which conduct the cooling liquid and of copper fins which are connected to said brass pipes and which dissipate the heat. In the course of production of the radiators the components are assembled and soldered to one another at the points to be joined. In this connection the use of low-melting soft solders based on high-leaded lead-tin alloys is still widespread.
In industrial series production the brass pipes of the radiators are sprayed with an aqueous flux based on zinc chloride and ammonium chloride and are subsequently presoldered with the soft solder. After the assembly of coated pipes and copper fins the complete radiator is once again immersed in the flux solution and is presoldered after drying off in a continuous furnace. The flux residues have to be removed with water after the soldering process, since otherwise signs of corrosion appear on the finished radiators. From economic and ecological viewpoints this manufacturing process is no longer competitive nowadays. The use of corrosive fluxes based on zinc chloride and ammonium chloride gives rise to a high technical and financial effort in connection with the treatment of waste water and the disposal of the flux residues. In addition, as a result of decomposition and hydrolysis of the fluxes, emissions of hydrogen chloride occur during the soldering process which have to be removed from the waste gas of the furnace by means of filters or washing plants. In addition, when they are worn out these radiators can only be recycled with great difficulty, by reason of their high lead content.
An alternative to the production of the soft-soldered copper-brass radiators is represented by brazing with low-melting copper solders. For industrial applications, copper-phosphorus solders with a melting-range of 710-880° C. or argentiferous copper-phosphorus solders with operating temperatures around 700° C. are employed as a rule. However, these solder alloys can only be used to a limited extent for the purpose of connecting the brass pipes and copper fins of the radiators, since at the requisite brazing temperatures the copper softens and already loses a large part of its strength. Recently, therefore, brazing solders have been developed that are especially suitable for the brazing of copper-brass radiators. These brazing alloys are described in greater detail in Patent Specifications U.S. Pat. No. 5,178,827, U.S. Pat. No. 5,130,090 and U.S. Pat. No. 5,378,294. Said brazing alloys consist of copper-phosphorus alloys with additions of nickel which lower the melting-point as well as, optionally, tin and manganese. The liquidus temperatures of these solder alloys lie clearly below 700° C., as a result of which brazing processes with peak temperatures below the softening-temperature of copper are made possible. These solders are self-flowing by reason of the phosphorus content and can be employed in flux-free manner under protective-gas atmosphere, preferably under nitrogen with small contents of residual oxygen, for the purpose of joining copper and brass. Further information about these brazing solders is presented in Adv. Mater. Processes (1995), 147 (5), 33 and in SAE Technical Paper 931076. However, these solder alloys have the disadvantage that they are very brittle and therefore can only be employed in the form of rapidly cooled foils (melt-spinning foils) or in powder form for the brazing of radiators. For reasons of cost, melt-spinning foils do not enter into consideration for series production. In addition, the application of the foils during assembly of the radiator components can only be integrated into an on-line manufacturing process with great difficulty.
For the alternative use of these solders in powder form it is expedient to make them available in the form of a paste in which the solder powder is dispersed in a binding-agent system and which can be applied in liquid or semiliquid form onto the objects to be brazed. Solder pastes with an extremely wide range of solder alloys and binding-agent systems are known and have been in use for a long time. As a rule, solder pastes contain flux residues in addition to the solder powder in a system consisting of organic binding agent and organic—occasionally also aqueous—solvents.
The essential criteria in the selection of the components of the binder system are that the paste remains stable and usable also over a relatively long period and that the solder does not settle irreversibly, that it is capable of being applied well without running down, in particular also from vertical surfaces, and that the binder system does not impede the brazing operation. Particularly in the case of high-melting brazing solders the binder system has to be volatilised or burnt off without leaving any residue, as far as possible without forming toxic or environmentally hazardous products.
An essential aspect in connection with the application of the solder paste is the speed of application and the drying-time. The binding agents that are employed conventionally in solder pastes have a drying-time of several minutes when use is made of high-boiling organic solvents. More readily volatile solvents such as alcohols or ketones dry more quickly, but in the process emit readily combustible vapours which necessitate elaborate measures in the plants for the purpose of protection against explosion.
In German published patent applications DE 28 40 415 and DE 30 12 925, paste formulations for certain soft solders and brazing solders are described, the binding-agent system of which exhibits thermoplastic properties. The binding agent can be chosen so that the solder paste is solid at room temperature and softens or melts at elevated temperature, roughly between 40 and 100° C. On the one hand, good storability of the paste is ensured by this means, since no settling of the solder powder can occur. On the other hand, a good result can be achieved when the paste is applied in the molten state, followed by rapid “drying” as a result of cooling. In the stated documents a large number of possible organic binding agents are specified which may be selected from natural and synthetic resins, waxes, oligomers and polymers of the most diverse nature and which may be employed individually or in combinations in such solder pastes having thermoplastic behaviour.
However, the formulation of the aforementioned low-melting copper-phosphorus alloys in powder form into thermoplastic solder pastes with a relatively large number of selected thermoplastic binding agents has resulted in unexpected problems.
In this connection it has become evident that the solder powder reacts very quickly—obviously by reason of its large surface area and its specific alloy constituents—with oxygen and atmospheric moisture, reactions which are also not prevented by embedding in the binding agent and as a result of which the wetting behaviour and flow behaviour of the solder are impaired after just a short period of storage of the paste. The high affinity of the solder powder for oxygen furthermore prevents the use of conventional binding agents such as, for example, cellulose, cellulose derivatives, polyethylene glycols etc, since the solder powder reacts during the heating-up process with the oxygenous decomposition products of the binding agents, forming slag which hinders the flowing of the solder. Moreover it has become evident that even when use is made of oxyge
Koch Jürgen
Staab Leander
Staab Sylvia Anita
Wittpahl Sandra
Degussa - AG
Jenkins Daniel J.
Smith , Gambrell & Russell, LLP
Staab Sylvia Anita
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