Metal treatment – Compositions – Fluxing
Reexamination Certificate
1998-07-15
2003-12-02
Thibodeau, Paul (Department: 1773)
Metal treatment
Compositions
Fluxing
C228S248100, C228S224000, C419S036000, C419S065000, C428S654000
Reexamination Certificate
active
06656290
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a hard-soldering paste for bonding together and/or coating working parts made of aluminum or aluminum alloys.
In the sate of technology today, in addition to the techniques of welding, more and more often, hard-soldering processes are used to bond work pieces made of aluminum or aluminum alloys. In this regard the work pieces are soldered together so they are mechanically secure and interlocking by heating metallic soldering materials and with the use of fluxing agents, which can be in solution, suspension, or paste form.
In order to be able to solder together parts made of aluminum or aluminum alloys, soldering alloys are required that have a lower melting point than that of the parts that are to be bonded together. Usually aluminum alloys with additives that impart low melting points such as silicon, copper, or zinc are used in this connection.
The fluxing agents have the task of providing a good moistening effect during the soldering process by dispersing the troublesome oxide layers on the parts and soldering materials. Mixtures of fluorides and chlorides of alkali metals and alkaline earth metals or complex alkali aluminum fluorides are usually used as fluxing agents for soldering aluminum.
During furnace brazing, it is advantageous to use fluxing agents containing complex alkali aluminum fluorides because they are not corrosive. However, with these fluxing agents additional cost-intensive cleaning procedures are needed to remove the fluxing agent and its reaction products from the surface of the parts after the soldering process.
To simplify the production process, particularly in mass production of a large number of pieces, hard-soldering pastes that consist of a prefabricated mixture of solder powder, fluxing agent, and binding agent are currently preferred for soldering aluminum. The parts to be bonded together are coated with the hard-soldering paste and then, for example, soldered together in a tunnel kiln. The soldering operation can be carried out in air. However, in order to avoid oxidation of the parts and the soldering powder, the soldering process often is carried out in a protective gas atmosphere, as for example, nitrogen/hydrogen.
In addition to the two component parts, namely, the soldering powder and the fluxing agent, an organic binding agent has great significance with respect to storage performance, its ability to be metered or dispensed, and the application of the soldering paste. The organic binding agent primarily has the task of holding the soldering and fluxing agent powders in suspension in a finely divided form so that the cementing of the dispersed particles cannot take place and a problem-free dispensing thereof is possible even after a lengthy storage time. Moreover, the rheological properties of the binding agent must be coordinated in such a way that the portions of dispensed paste needed for the job, referred to herein as depots, do not run during the drying and heating phase. Usually the binding agent in the soldering paste consists of a solution of various polymers in an organic solvent. Depending on the conditions for use and soldering, usually cellulose derivatives, hydroxypropyl cellulose, or polymethacrylate in heavy hydrocarbons, alcohols, glycols, or polyalkylene glycols are used.
At present, extremely high demands are made on these hard-soldering pastes in automated mass production systems.
The application of the soldering pastes is carried out via an automated metering or dispensing system in most cases. In so doing, one must find a way that during storage and dispensing of the paste, a separation into the individual components does not take place and the viscosity remains constant over a long period of time.
After dispensing the soldering paste the work piece goes through a zone of preparatory heating in the oven during which the soldering paste dries. In this initial phase of the soldering process it is necessary that the paste remains on the desired site. That is, it should not run off the component part while being heated due to the lowered viscosity. Particularly when depositing the paste on vertical surfaces a shortage of solder can occur at the solder surface because of runny soldering paste, which leads to defective soldering and thus an increase in the portion of rejects.
Furthermore, one must be able to achieve a system so that the binding-agent system of the soldering paste is completely vaporized or burned out before ever reaching the soldering temperature so that the effectiveness of the fluxing agent is not impaired and the soldering flux is not hindered. Due to optical and technical testing concerns it is also desirable that after the soldering process is completed no black residue remains on the soldering site, which impairs the visual inspection of the soldering connection.
In addition, for economic and ecological reasons it is desirable that the hard-soldering paste be water soluble so the barrel and metering device can be cleaned with water. Expensive and less environmentally compatible organic solvents that cannot be mixed with water should be avoided in production.
In EP 0 512 489 an aluminum hard-soldering paste is described whose binding agent consists of a liquid polyisobutylene polymer. The binding agents of these pastes do not, however, display thixotropic behavior so the metered paste deposit runs off the component part before the actual soldering process takes place. Moreover, these soldering pastes cannot be thinned with water and washed away.
EP 0 475 956 describes a soldering paste for hard-soldering aluminum the binding agent of which consists of a very pure monopropylene glycol without higher molecular proportions. This solvent is very expensive because it is very pure. In addition, the soldering pastes prepared from such binding agents do not display any thixotropic behavior and this results in the paste separating into its component parts while in storage because the soldering powder precipitates.
It is an object of the present invention to avoid the problems of the prior known solder paste formulations used in the hard-soldering of aluminum and aluminum alloys.
Another object of the present invention is to be able to solder parts made of aluminum and/or aluminum alloys by using automatic solder paste metering devices while avoiding run off during the final heating process, and without leaving behind any black decomposition products from the binding agent after the soldering process is completed.
Still a further object of the present invention is to develop a water-miscible, hard-soldering paste for the soldering of parts of aluminum and/or aluminum alloys that is storage stable and adapted for use in the soldering processes described therein.
SUMMARY OF THE INVENTION
The above and other objects of the invention can be achieved by an aluminum hard-soldering paste that contains 25 to 35 percent by weight of a water-miscible organic binding agent on the basis of aliphatic glycols, 25 to 45 percent by weight of an aluminum hard-solder, and 25 to 45 percent by weight of an aluminum fluxing agent, and as an additive to influence the rheological properties 0.01 to 0.2 percent by weight (with respect to the total weight) of at least one fatty acid containing 10 to 20 C atoms and/or an ammonium salt thereof.
A feature of the present invention is, furthermore, the use of one or more fatty acids, each acid having 10 to 20 C atoms and/or their ammonium salts in a proportion of 0.01 to 0.2 percent in weight based on the total weight of the solder paste to influence the rheological properties thereof and, in particular, to raise the thixotropy and/or structural viscosity of such water-miscible, aluminum hard-soldering pastes.
DETAILED DESCRIPTION OF THE INVENTION
In the soldering paste according to the present invention all types of aluminum hard-solders that have a lower melting temperature than the base metals to be joined or coated can be used as a hard-soldering powder for coating and joining aluminum and aluminum alloys. These materials are w
Koch Jürgen
Kohlweiler Wolfgang
Staab Leander
Starz Karl-Anton
Degussa - Aktiengesellschaft
Kalow & Springut LLP
Kruer Kevin R
Thibodeau Paul
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