A METHOD AND A DEVICE FOR JOINING WORK PIECES BY BRAZING OR SOLDERING

Abstract: This invention relates to a method for joining work pieces by soldering or brazing whereby an induction coil is brought to a position where it at least partly surrounds the work pieces which by induction are heated to a suitable temperature for effecting the soldering or brazing operation after which solder is fed to the soldering or brazing are. In order to prevent creation of oxides a gaseous flux is blown to the soldering or brazing area in a free atmospheric enviroment before and/or when the solder is fed to soldering or brazing area. The invention also relates to a device for accomplishing the method.

Description: This invention relates to a method for joining work pieces by brazing or soldering in which an induction coil is brought to a position where it at least partly surrounds the work pieces which by means of induction are heated in a free atmospheric environment to a suitable temperature for the brazing or soldering operation after which a solder is fed to the soldering or brazing area. The invention also relates to a device according to the preamble of claim 6. Such a method, and a device having the features set forth in the preamble of claim 6, are known f.e. from EP-A-244 382. Soldering or brazing are well-known joining methods and are used for fixing two metal parts to each other by means of the so called solder which usually is fed to the soldering or brazing area in wire or powder form. The composition of the solder differs from, and has a lower melting point than, the material in the work pieces. During the soldering or brazing operation the joint is heated but is never allowed to melt. In order to achieve a binding between the solder and the joining surfaces the surfaces must be free from oxides. Such a clean surface is usually created by a grinding operation or by applying some kind of chemical agent, a so called flux, before or during the soldering or brazing operation. The flux usually is applied as a liquid, a paste or as a powder and is sometimes mixed into the solder. When soldering or brazing in an oven - see EP-A-0070909 - it is also common to use a protective gas in order to prevent the creation of oxides< Usually brazing and soldering are referred to as different methods where brazing is effected between 600 - 900°C and soldering below 300°C. Brazing normally gives very good strenght and can also be used for power transmitting joints whereas soldering gives comparatively low strenght values and therefore primarily is used for sealing purposes and for electric joints. In order to heat the work pieces different kinds of heating means are used for the brazing as well as the soldering operation. Thus, i.e. gas burners, different types of electric heating and heating in ovens are used. One often used heating method, particularly for automatic manufacturing, is the inductive heating method where the work pieces are placed in a coil which is influenced by a high frequency alternating current. One example of the last mentioned method appears from EP-A-244.382. According to this patent publication the solder is fed to the work pieces after having been pre-heated to a suitable temperature whereby the flow out of the solder on the joining surfaces is promoted. One difficulty when using the last mentioned method in an automatic process is that there is a demand for fluxes according to what has been said above. These fluxes must be applied manually or by providing the equipment with means by which the brazing or soldering surfaces can be dipped into the fluxes. When the flux then is heated this means development of a heavy smoke. Sometimes flux remains in the joint which can cause joints which are not tight. When the soldering or brazing has taken place there also is a need for washing the joint by a so called devitrification operation. It has also been suggested to use equipment for soldering and brazing purpose where the soldering or brazing area is embedded in a gaseous flux without being placed in a large chamber surrounding the complete equipment. This type of equipment, see DD-A1-216878 and DD-B-208099, comprises a tool which is arranged to cooperate with the work pieces for forming a small chamber in which the brazing or soldering takes place and to which the gaseous flux is distributed. The purpose of this invention is to achieve a method and a device which is suited for automatic production where applying the flux is facilitated and where the additional drawbacks referred to above are eliminated. This is achieved because the invention has the characteristics described in claims 1 and 6. An embodiment of the invention will now be described with reference to the attached drawing, the Figure of which is a partly broken perspective view of a device according to the invention. The Figure shows an arm 10 which is the movable part of an industrial robot, the arm being controlled by a control means not shown. The arm supports a holder 11 for an induction heating unit 12 to which the supply of current is effected via a cable 13. The unit 12 is via a fastening means 14 connected to an induction coil 15 in which the work pieces which are intended to be joint are inserted. The coil has an upper, in plan view seen, U-shaped part 16 and a lower, in plan view seen, fork-shaped part 17 which are connected to each other via two vertical parts 18. The fastening means 14 further supports a solder pre-heating device 19 which comprises a tube with a front part 20 with a comparatively small outer diameter and rear part 21 with a somewhat larger outer diameter the rear part having several cooling flanges 22. Through the solder pre-heating device 19 runs a wire-shaped solder 22 which is fed through an envelope 23 by means of a solder feeding equipment not shown. The solder feeding equipment is controlled from the previously mentioned control means. The fastening means 14 also supports a fiberoptical element 24 which is connected to an IR-detector 25 for measuring the temperature of the heated joining surfaces. The detector 25 via a cable 26 is connected to the control means not shown. Within the coil there also is a nozzle 27 for cleaning by blowing. This nozzle has an opening 28 which is directed towards the outer end, i.e. the eye, of the fibre optical element. The other end of the nozzle is via a hose 29 connected to a gas-source not shown by means of which gas under pressure can be blown on the eye of the fiber optical element. As appears from the Figure there is a curved chamber 30 between the upper and the lower part of the coil. The chamber is limited by an inner wall 31 and an outer wall 32 and there are sealed side walls extending between these walls and there also is a top and a bottom element. The walls of the chamber are made from a material which is mainly not heated during the induction phase. The chamber 30 via nipples 23 is connected with hoses 34 which are connected to a source supplying a gaseous flux. The inner wall 31 has several holes 35 through which the flux flows to the soldering or brazing area. The size of the holes and the pressure of the gas in choosen so that a laminar flow of the gas is achieved at the brazing or soldering area. Thereby it is prevented that the surrounding atmospheric air follows and is mixed into the gas flow. In order to achieve a sufficient flow of flux about the complete work piece the outer vertical rows of holes can be replaced by a slot 36 or the like. A mixture of 10% hydrogen and 90% nitrogen has been used as a flux with very good results. During these conditions the gas flow has been about 4 l/min. Of course it is possible to use other gases and gas mixtures and other flow conditions in order to achieve the desired effect. It should be observed that the gaseous flux is applied in a free atmospheric enviroment and at the atmospheric pressure which is prevailing in the working premises that is without using a surrounding chamber. The device also is provided with a scanning device 37 which, without getting in touch with the work pieces looks, for the joints within a pre-determined area and guides the coil to such a position that the soldering or brazing operation can be effected. The scanning device 37 is fastened to the arm 10 and is via a cable 38 connected to said control means. The device operates in the following way. The scanning device is programmed to automatically scan certain areas, i.e. at the rear of a combined refrigerator and freezer, where several tubes shall be joint. When the device has found the joint in question the coil is brought to it and is moved in a proper position for accomplishing the brazing or soldering operation. Then the induction coil is switched on and the gas and the flux are supplied through the nozzle 27 and the holes 35, 36. This means that the joining surfaces are heated up, the solder is pre-heated in the solder pre-heating device 19 by means of the induction field and soot particles if any and other contaminates which are created during the heating are blown away from the eye of the fibre optical element 24. When the IR-sensor 25 of the fibre optical element senses a pre-determined temperature the solder feeding device is actuated which means that the solder 22 is fed an touches the joining surfaces which have been heated and flows out between these surfaces. Then the current through the induction coil 15 is switched off, the supply of flux and gas ceases after which the joint is allowed to cool. The coil simultaneously is moved to a new soldering or brazing area. All functions are controlled by said control means.

Claims: 1. A method for joining work pieces by soldering or brazing in which an induction coil (15) is brought to a position where it is separated from and at least partly surrounds the work pieces which by means of induction are heated in a free atmospheric environment to a suitable temperature for the soldering or brazing operation after which a solder is fed to the soldering or brazing area, characterized in that when, or before and when, the solder is fed to soldering or brazing area a laminar flow of gas is supplied to the soldering or brazing area. 2. Method according to claim 1, characterized in that the gas is a mixture of gases i.e. hydrogen and nitrogen. 3. Method according to any of claims 1 or 2, characterized in that feeding of the solder is initiated by the temperature of the work pieces at the soldering or brazing area. 4. Method according to claim 3, characterized in that the temperature is sensed optically. 5. Method according to claim 4, characterized in that the optical measuring device is cleaned by blowing a gas on it. 6. Device for accomplishing the method mentioned in claim 1 which comprises an induction coil (15) which is brought to a position where it is separated from and at least partly surrounds the work pieces which are to be joined in a free atmospheric environment, a solder feeding device by means of which a solder is fed to the soldering or brazing area, characterized in that blow openings (35, 36) are placed in close vicinity of the coil (15) for supplying a laminar flow of gas to the soldering or brazing area. 7. Device according to claim 6, characterized in that the blow openings (35, 36) communicate with a chamber (30) which is connected to a gas source. 8. Device according to claim 7, characterized in that the chamber (30) is U-shaped whereby the openings which are situated close to the tips of the legs might be slot-shaped. 9. Device according to claim 8, characterized in that the temperature of the soldering or brazing area is sensed by means of an optical sensing device (24, 25) the device comprising one or several nozzles (28) through which gas is supplied for cleaning by blowing of the optical sensing device.

Inventors: PETERSON, CHRISTER CALEB INGEMAR
Applicants: AKTIEBOLAGET ELECTROLUX
Application year: 1989
Main classification: B23K1
IPC codes: B23K1, B23K3